,\ - ,"'"SNAME Tren ctions, Vol. 88, 1980, pp. 195-223

195 •.. , •.•..•.. .

Practical Design Approaches for the Analysis of BargePerformance in Offshore Transportation and LaunchingOperations

-Rubin Szajnberg,1 Member, William Greiner,1 Associate Member, Henry H. T. Chen," AssociateMember, and Philip Rawstron,1 Associate Member

The problems and solution techniques encountered in quantifying the safety factors involved in thetransportation of large offshore structures on deck cargo barges are discussed in this paper. Theprimary factors considered are environmental force prediction, stability, motion and strength, andtheir interaction, which forms the criteria for selecting an acceptable barge/jacket configuration fortowing and launching operations. The methodologies are presented and compared in light of thestate of the art in naval architecture and structural analysis. and practical implications on the designof the tiedown system and jacket reinforcement are discussed based on past expe;~nces.

I Senior industrialist specialist, design engineer, project staff engi-neer, and senior engineer, respectively, Brown & Root, Inc., Houston,Texas.

Presented at the Annual Meeting. New York, N. Y., November13-15, 1980, of THE SocIETY OF N AVAL ARCHITECT'S AND MARINEENGINEERS.

Transportation analysis techniques~he problems of analyzing the safety of a particular tow are

basically those of defining the interaction of the tow with itsenvironment. The tools and techniques available to the de-signer must be directed toward the two primary damage or loss

Practical Oesign Approaches for the Analysis of Barge Performance

Introduction1

I DUSTRY has made increasing demands over the past dec-ade on the engineering disciplines to develop new technologyand methods to transport and install deepwater structures inan e~ficient. a.n? safe manner. As the exploration and pro-duction activities of the offshore petroleum industry haveventured into deeper and more hostile waters, a number ofattempts have been made to respond to these demands.GEMI I [1]2 and HIDECK [2] are novel approaches appliedto the problem, and Side-launching and self-floating structureshave also been proposed for the transporting effort.

Yet transportation experiences in deepwater applicationsaround the globe indicate that often the most economicalmethod of transporting jackets or structures from fabricationyards to offshore locations remains the flat-deck cargo barge

I' approach. Once on site, a derrick barge either lifts off thestructure, or it is launched from the transporting oarge with a~l~ctive com~in~~ion of ballasting and winching. A so-called

third generation of barges was first designed and constructed_ in the early seventies [3] to perform the transportation task for

large deepwater structures.Today there exists a fleet of barges which are designed to

transport and launch deepwater- jackets and carry offshorestructure modules. These vessels have large deck spaces, andenough stability, strength and reserve buoyancy to carry deckloads in excess of 25 000 tons [22500 metric tons (t)]. Table1 is a partial list of the existing fleet, giving the vessels' maindesign particulars.

Desig~ featur:s ~or this class of cargo barge generally includehea~y skid and tilting beams for launching the large jackets. Atyplca~ launch barge arrangement is shown in Fig. 1. Quickballasting pumps rated in the region of 2000 metric tons perhour are also usually installed to trim the barge duringlaunch.

. Sink~ge and stability considerations determine the principaldimensions of these barges. The stability criteria used considera typical deck cargo which exhibits a high center of gravityabove deck and a large windage area. The forward and aftrake segments of the barge are then designed to minimize re-sistance and promote good seakeeping and coursekeeping be-havior.

Generally, engineering analysis in the past has assured therelative safety of tows by closely following ship practice indetermining environmental loads, stability and motion factors.However, demands for more-specific guidance criteria forcargo barge problems have grown as transportation practiceshave become more complicated. At the same time a similarpressure has been felt for design criteria for offshore structuresto keep up with the increasing technology required for deep-w~ter applications. This has resulted in various new guidelinesbeing developed for general offshore use which now exist in theform of rules and recommendations.

T~e r~se of .regulations did not automatically bring stan-dardization to Industry practice, however, and the burden forprodu~ing acceptabl~ designs to ensure safety during trans-portation and launching rests with naval architects and struc-tural engineers. Tiedown braces, reinforcing members, towingarrangements, and ballast procedures all must be reevaluatedbecause of the new and expanding applications in offshore use.The further responsibility of minimizing jacket structuraldamages due to fatigue and jacket slamming also rests with thedesigner.

Agencies which to date have issued regulations to guide de-signers include the U. S. Coast Guard (USCG), the Departmentof Energy U.K. (DOE), and the American Petroleum Institute(API). In addition, vessel regulations exist from classification

2 Numbers in brackets designate References at end of paper.

196

societ.ies such as Det norske Veritas (DnV), Lloyd's Register,Amencan Bureau of Shipping (ABS), and Bureau Veritas (BV),as well as from known consultant companies such as 1 obleDenton (ND) and United States Salvage Inc.

These various agencies have tended to set different criteriaor have chosen to leave the criteria to the contractor's discretionin the area of seakeeping and structural evaluations. Thesevari~tions exis~ particularly in the areas of predicting themaximum environmental conditions that will be experienced~nroute a~d in certain i?tact stability criteria. Damage toJackets dunng transportation and launch has been experiencedas a result of a combination of problems derived from the pre-ceding factors, and the loss of investment, apart from the lossof time, has sometimes run into the hundreds of millions ofdollars.

Besides a lack of agreement about rational methodologies furbarge stability'and structural analysis, there has also been,until recently, a lack of data available to carry out the necessarycalculations to satisfy the regulatory bodies or to evaluate thebarge/jacket transportation in terms of risk exposure from anowner's point of view. Yet the trend continues toward the useof steel jackets for deepsea oil production, and the safe trans-portation and launching of these jackets will remain a difficulttask for engineers for many years to come.

This paper addresses some of the critical problems in the areaof offshore transportation and, based on past experience. at-tempts to approach in a unified and systematic manner theevaluations necessary for adequate barge performances incarrying out successful transportation operations. The paperapproaches the task from the perspective of a naval architector structural engineer who must analyze the transportationpro?l.em under investigation and make appropriate designdecisions based on the critical inputs from practical consider-ations.

The design task is divided into two parts for discussion. Thefirst part reviews the various analytical echniques in the areaof environmental load estimations, stability evaluations, motionpredictions, structure analysis and risk assessment. It is in-tended to provide the reader with a review of the present in-dustry standards for offshore transportation analysis, and ex-pl~re the areas of uncertainty for future developments. Figure2 Illustrates a typical design spiral for applying availabletechniques in a transportation analysis.

~he seco~d par.t of the paper concentrates on the practicaldesl.gn considerations which must be made when using theavailable tools and procedures for transportation studies. Dueto the practical constraints such as barge availability, time, dataand resource considerations, various tradeoffs have to be made.In actual operations a designer is confronted with decisionsconcerning motions versus stability and stability versus strength,as well as decisions regarding the level of detail to be performed~nthe ana~ysis. These causal effects on the total safety of theJacket dunng a transportation operation are discussed in lightof past experiences.

It is hoped that these practical experiences will help designersbetter understand the complex interactions of a transportationand launch operation for an offshore jacket. Ultimately,how~ver, the goal of the paper is not only to assist designers inm~klng appropriate-decisions in a transportation analysis, butto Improve overall performance during the actual transporta-tion operation.

z

TiltL B 0 Tux Dlsp. DWT LIB BIT BID TID Beam

BARGE LenathNAME ft ft tt lL ~ "'.T. - - - - ft

Ft LTons LTons

Intennac 198.12 51.82 12.19 30.48650 650.0 170.0 40.0 3.82 4.25 100

"'leoperl 190.0 50.0 11.4 32.00M44 623.0 164 .0 37.4 3.8 4.4 105

Hl09 183.0 47.2 11.6 9.4 75920 57300600.0 155.0 38.0 30.8 74700 56398 3.9 5.0 4.6 .81

BAR 376 176.8 4B.8 11.0 8.06 84226 66680 30.48580.0 160.0 36.0 26.42 82900 65630 3.6 6.06 4.4 .73 100

Hll0 160.0 42.1 10.7 7.5 49570 39550 18.90525.0 138.0 35.1 24.6 48790 393~0 3.8 5.6 3.9 .70 62

Intennac 152.4 36.58 10.06 7.66 41790 31730 18.29600 500.0 120.0 33.4 25.13 41130 31230 4.2 4.8 3.6 .75 60

Oceanic 93 137.16 31.70 9.14 18.29450.0 104.0 30.0 4.32 3.5 60

BAR 398 121. 9 31. 94 7.62 8.87 27605 15281 15.24400.0 104.8 25.0 29.1 27170 15040 3.8 5.5 4.2 .76 50.

Golia! 10 121. 92 30.48 9.14 7.27 24600 20400400.0 100.0 30.0 23.85 24212 20079 4.0 4.2 3.3 .80

BAR 267 115.82 30.48 7.62 5.29 17607 12456 15.24380.0 100.0 25.0 17.36 17330 12260 3.8 5.8 4.0 . 69 50 .

Intennac 500 106.68 24.38 7.62 5.12 12294 9449 . 10.52350.0 80.0 25.0 16.79 12100 9300 4.4 4.76 3.2 .69 34.5

8AR 319 101.19 27.43 6.10 5.18 13930 11308 15.24332.0 90.0 20.0 17.01 13711 11130 3.7 5.3 4.5 .85 50.

Golia! 6 100.0 27.0 7.0 5.55 13868 13868328.0 88.6 23.0 18.25 13650 13650 3.7 4.85 3.85 .79-

BAR 362 91. 44 27.43 6.10 4.66 11176 8636 15.24300.0 90.0 20.0 15.29 11000 8500 3.3 5.9 4.5 . 77 50 .

Agano 89.92 29.87 7.01 4 ..88295.0 98.0 23.0 16.0 3.01 6.13 4.26 .70

BAR 396 92.35 27.43 6.70 5.42 12635 10626303.0 90.0 22.0 17.8 12436 10459 3.4 5.1 4.1 .81

Intennac 400 91.44 27.43 6.55 4.82 10818 8941 11 .89300.0 90.0 21. 5 15.8 10648 8800 3.33 5.7 4.19 .74 39

Gplia!3 77.42 24.0 6.19 5.0 9754 8230254 .0 78.8 20.3 16.3 9600 8100 3.22 4.83 3.88 .80

V BAR 271 76.2 21: 95 4.88 3.63 6195 5158250 .0 72.0 16.0 11. 92 6095 5075 3.5 6.04 4.5 .75

Intennac 250 73.15 21. 95 5.23 4.21 6248 5263 6.25240.0 72.0 17.16 13.82 6150 5180 3.3 5.2 4.3 .805 20.5

Table 1 Typical deck cargo and launch barge characteristics"-

Fig. 1 Typical launch barge arrangement

Practical DeSign Approaches for the Analysis of Barge Performance 197

TIL T 8EAM : PUMP :: ROOM : TK 2 . TK-.....T--._-- .-~;.., ,!.-"':~ _

13 o

WIRE BRIDLESFAIRLEAD

TOWING BRACKETo ollJDo----o;o--- 0:0 0:0 0.0 IIJl' 0

.' I I I , --e_.' , : : "s..C===== I -:--r _ ! :::oJ•• I' I I

-r - - - - - - - -- ...•-- -, - - - ~- -- -- - - --- •-- -_..----- ...J •. _ •.•. _ •. ~ _ •. L. _ - - --

: : ; : : PUMP :c"OOLIN"G!WINCH:o e ' 0 ~:C' 0:0 :>'0 0: ROOM I WATER, 0~_"""J._,. .•_ .•._,._ ••._._ •. _ •....._,_ •.. _ •• -. •.• •.._._ .-,,-_-1_"- _, _...•. _, " l ..•_•..• ..•_•._,..•.

o 0: 0 0:0 0:0 0:0 0: :CCX>LJNG~I I : I I ~ WATER WINCHI I.! ,0 0

•. "'" -.---- ---------.--- - -_. _.~- ------ •__.•• '1---". --- •• ---,..- .••.•---,-- -- .•-_ .•_ -- --.[=::::::=~=0 ._ I I I :cr===----- .....o.....- _.J==:: -=:::I:= =-::JI I I I I I~

c· 0 I 0 0:0 0:0 0:0 0;0 -e~

____________________________________ Nomenclature _

•.

A = projected windage areaAI = effective windage area of member

or surfaceA = area under righting-arm curve as

defined in Figs. 5 and 6ACR = dynamic stability criterion = 0.08

m-rad (15 ft-deg) for offshoreservice

IAIGLOM:' = acceleration matrix in globalcoordinate

IAILOCAL = acceleration matrix in local coor-dinate

B = barge beamB = area under wind heeling and

rightin~ arm curves as definedin Fi~> S and 6

C = area und ••r w ind heeling armcurve as defined in Figs. 5 and6

CA = added-ma . and viscous dampingcoeffi lent

C. = effective- hape coefficient foropen truss

CH = height coef licient

CHG = height and ~ust coefficientC, = shape coefficient for windage

areaCm = shape coefficient for member of

infinite lengthce = center of ~ra\ity

D = barge depthIT = translation RAO vector for surge,

swav. and heaveD = \eIOCii~ RAO's{) = acceleration RAO's

DCG = complex RAO at syst m ceDz = absolute vertical motion RAO

D(x,1j,z) = relative vertical motion RAO fromspecific location

D(H 1/3,P.) = cumulative fatigue damage perunit time for a specific sea stateand heading

DVQYAGE = expected fatigue damage duringtransportation

FWIND = wind forceIFI = generalized nodel force vector

IFwl = complex wave forces acting onbarge

IFml = complex motion-induced accel-eration forces

GM,GMT = transverse metacentric heightGZo = righting arm for KG = 0

H = vertical distance between centersof above and underwaterareas

H 0 = ccnstant reflecting intercept ofbest-fit line on Weibull plot

HI '3,H, = igrnficant wave heightif 1/3 = extreme design sea state associated

with a return period TR

Ho = wave height from visual observa-tion

lu = mass moment of inertia[K I = generalized stiffness coefficient

matrix

KG = vertical center of gravity abovebaseline

KGI = maximum allowable KG of bargewith respect to sce weathercriteria

KG2 = maximum allowable KG of bargewith respect to seedynamicstability criteria

KGA = maximum allowable barge KG forspecified stability criteria

KGLS = light ship center of gravityK.W = metacentric height above base-

lineKu = transverse radius of g} ration

L = duration of transportation opera-tion in days

L = length of bargeLT = long ton

[."11 = generalized mass coefficient ma-trix

'\1'w = wind heeling momentM ••• = teady wind heeling moment at

angle <I>

Mwc = gust wind heeling moment

.IV = number of wind area elementsN = number of sea state observations in

a day, = number of independent observa-

tions.(0') = expected number of cycles to

failure at stress level 0'

P = [SCe wind pressure= 0.053 + (LI 1330)2 l/m2

= 0.005 + (L/14 2(0)2 LT/ft2)P(nodamage) = probability of no damagePE(li 1/3) = encounter probability (probability

of one or more exceedances ofhazardous events)

Pj(O') = probability density function ofstress range

PI (Ii Id = probability of occurrence of seastate HI/3

P{N = m 1 = probability of hazardous eventsP(O' > iT) = probability of stress level exceed-

ing iT

R = return interval based on numberof voyages between recur-rence

R = vertical relative motion displace-ment RAO's

if = RAO vector for roll, pitch, andyaw

R. = Reyonlds numberRAO = response amplitude operator

RAO(w,v,p.) = RAO at frequency w, speed v,and headings p.

R..\ •..•AX = maximum righting arm

Q(H I 3) = probability of exceeding ignifi-cant wave height

S(w) = wave spectral density function

T = duration or periodT••= natural roll period of barge

TR = return period/average interarrivaltime

Tz = zero crossing periodTo = wave Period observed visually

[TI = transformation tensor from bargecoordinate system to jacketcoordinate system

TI = total exposure time to a particularsea state

T2 = 21r{mo/m2)1 2 (1.0 - 0.OSt}2V = relative", ind velocity

V IhrlO = wind velocity at to m above waterlevel averaged over one-hourperiod

Vc.w = current velocity due to windshear

vce = vertical center of gravityvcec = vertical center of gravity of cargo

above barge decka. = projected area of a structural

member or surface exposed towind

b, = truss block area

!(a) = spreading function for multidi-rectional sea state

g = gravitational accelerationh; = vertical distance from center of

wind pressure to center of un-derwater resistance

k = constant reflecting best-fit ex-trapolation line on Weibulldistribution plot

m = number of hazardous events oc-curring during period L

mo = mean square value of stress, equalto area under stress responsespectrum

m2,m4 = 2nd and 4th spectral momentsabove spectral density axis

Po = probability of stress level exceed-ing & = p(O' > &)

q = wind pressure

r = position vector from system ce toa specific location

lul,lul = generalized displacement andacceleration with respect tostructural coordinate system

v = velocity:r = r-coordinate or locationIj = Ij-coordinate or locationz = z-coordinate or location

::1 = z-coordinate of a member abovestillwater level

a = spacing ratio of open trussa = wind direction with respect to

member axis

a,{J = coefficients for CHC to definewind speed profile

f3 = aerodynamic solidity ratio11 = displacement

111.. = light ship displacement

e = spectral broadness factor1'/ = shielding effect coefficient

198 Practical Design Approaches for the Analysis of Barge Performance

·'.

(1)

199

o = pitch angle or incident anglelJ = pitch angular accelerationA = rate of arrival of hazardous

events11 = heading anglep = density of air "'" 1.225 kg!m3

(00765 Ib/ft3)a = stress level

aN = most probable extreme stresslevel

iJ = arbitrary threshold stress level oryield stress

cP = solidity ratiocP = heel angle

CPJ = first intercept angle on righting!heeling arm curve

CPd = lesser of downflooding or secondintercept angles on righting!heeling arm curves

CPm = angle of maximum righting arm(i, = roll angular accelerationf = yaw rotation or heading anglef = yaw angular accelerationw = frequency = 2. T

modes for the tow, namely, stability losses and structural fail-ures. First, stability of the barge must be assessed to insure thatthe barge will not capsize in the anticipated wind and waves,and secondly, the action of these waves on the barge and jacketmust be determined to define the slamming and inertia loadsto which the jacket and tiedowns will be designed.

The external forces and moments created by environmentalparameters such as wind and waves must be analyzed, on boththe basis of their probabilistic occurrence and on their inter-action with the barge/iacket system under tow. The state ofthe art of naval architecture and structural engineering providesa variety of methods applicable to these transportation analysistasks, and a discussion of these methods now follows.

wave parameters are briefly outlined in Table 2. An overviewof sources of data and an approach to predicting environmentalconditions during a tow are presented in the following.

Wind. Wind forces and moments are used in stability cal-culations to determine the magnitude of overturning loads. Ingeneral, wind forces which act on exposed structures may beexpressed as a function of wind speed, direction, projected areaand shape:

where

Environmental loadsEnvironmental loads used in the transportation analysis

comprise those oceanographic processes which will ultimatelyaffect the structural safety and integrity of the tow. Primarily,these forces include the wind, wave, and current loads as de-scribed in this subsection. Methods prescribed by several au-thorities for determining environmental extremes for wind and

p = density of airV = relative wind velocity

CHGf = wind speed coefficient of member which includesheight and gust effects

= O'(Z/lO)/I as explained in Appendix 1C, = windage area coefficient of member which in-

cludes shape, shielding, and solidification effectscaused by wind blowing from angle a

SYSTEMSEVALUATION ~<s->:

COST CONSIDERATIONS BARGE /JACKET

RISK / <, ~ /CONFIGURATIONASSESSMENTS DESIGN<, / MODIFICATION

PROB OF BALLASTINGNO DAMAGE FINAL TlE·DOWN

('-:::,E:,i~"~'" /,"\ !~:::.~'"

(CLIENT / CONTRACTOR S6:~~~ TUG SELECTION'-----

SPECIFICAliOHS AND TOW SPEED

STflUCTURE __ ~:~\iJe:R£c~CAL -- ~~:~~~ ••PRED 8ARGE ~)ANALYSIS DA••••GE (DETERMINISTIC) SELECTION

(PR08A8L1STI\l \ I••••x WIND SPEED

TRAJECTORY CURRENT, WAVESIMULATION HEIGHT a PERIOO___..

/ -

/

<, INTACT/DAMAGE SPECTRALACC.lMOTION _ STA81L1lY WAVE DATA _

PARTICIPATION CRIT ERIA ) ENVIROMENTALFACTOIIS \ DATA PREDICTION

MODEL

TEST~ FINAL

/

CHECK

LAUNCH - ~ \SIMULATION WAVE a MOTION

INOUCED LOADS HYDROSTATICSLAN PR£DICTION STABILITYI CALCULATIONS

MOTION a LOADANALYSIS

Fig. 2 Design spiral

Practical Design Approaches for the Analysis of Barge Performance

Table 2 Summary of wind and wave load requirements

~lHeO[ 25~ BV[ 24J DnV RULES [21] OOE [28] DnV GUIDELINES [18J N08LE DENTON USGS[ 23JA8S 26 [ 19]

DESCRIBE USCGf 27

NORMAl. ·70 knots ·Return period equal ·Return period equal • 70 knots for stabi I i- ·Return period eq. three -Re turnOPERATING . Return I month three times the ty calculations of times the expected dur- period ofCONDITIONS period eq. 'Wave Probable. of -6 duration of cperat- at i on of the opera ti on 10 years

50 years Occurrence 4 x 10 ion (except operat- 'Return period eq , 50for struct. 'Lacking weather ional phase of rig) for structure calc.

§ calc. predict ion: -For operationalCo. Es tab 11 shed wi nd - phase for ODU, theon • 35 kn. design period eq ,<> Gust • 50 kn. 100 yearsz: • DnV does not spec ify3

'"SEVERE 100 knots ·Return period eq. operational and ·100 knots in s tab i l t-

e STORM 50 years extreme conditions ty calculations<>

( EXTRE"'E

I·Wave probable. of separately ·Return period of 50

0 CONDITIC~S occurrence 10-8 years for structura 1s 'Lacking prediction: ca lc .Co.

IEstablished wind -

z: • 70 kn.'" Gust ·100 kn.=> I~ SHELTE=<EJ 50 knots ·Ha If val ues of the NOT SPEC I F I ED ·Return period equal NOT SPECIFIED'" I=> LOCA !~, S ( except operational condo to the length of thes USCG - tow, but never less::

Idoes not than one week (for

'" specify) towing within 48hours from she Iter

t area)

WIND ~OT Sustained - I hr. ONE MINUTE WIND NOTAVERAGi~G ISPECIFIED Gust - 10 ~ec SPECIFIEDPERIDD

-WAVE ~OT .75 SEE RULES T=IO sec NOTo'AIW1ETFS IjSPECIFIED H • 1.68 H Hs • Hma/l.i6 Ro11'20-25" SPECIFIED(OR BARSE s v Pitch'/()TION '~~I

.98 ~IOHs<T< J20Hs12.5-15"

T • . 82 Tv Heave ·2gz -ai = projected area of member or surface exposed to

wind

The estimation of total wind load acting on the barge/jacketcombination is calculated based on the summation of forcesacting on each individual member. Three methods of calcu-lation which can be used to estimate a total wind load aresummarized in Appendix 1.

Current. Estimates of currents are used together withmaximum wind and wave parameters to define the power re-quirements for the tug employed for a tow in stalling weatherconditions. Towline pull at the indicated current speed mustbe sufficient to overcome both the maximum wind forces onthe structure and the drag on the barge due to current andwaves.

The current velocity may be computed by combining globalcirculation and tidal currents, if applicable, with wind-inducedcurrents. In the absence of statistical data on maximum currentvelocities, the wind-induced current may be estimated basedon the following relationship:

Vc.w = 0.02 V IhrlO (2)

where Vc.w is the current velocity due to wind shear, andV IhrlO is the wind velocity averaged over one hour at 10-m (33ft) height.

Barge resistance curves may be calculated by using the bargeform series presented in references [4J and [5J.

Waves. Wave pressure forces produce the oscillatory heave,sway, surge, pitch, roll, and yaw motions of a vessel. Thesefirst-order motions induce significant inertia loads on the jacketand tiedown braces, all of which require careful attention inthe analysis. Second-order effects, such as wave drift forcesand add~ resistance to tow, may also require special consid-eration for particular towages.

The methods of obtaining wave loads on a barge are essen-tially the same as those for shiplike forms. However, modifi-cations are necessary in order to evaluate the added-mass anddamping coefficients because of the large B/T ratios of barges,

and a more appropriate technique should be applied (for ex-ample, Frank's close-fit method [61). Furthermore, it is nec-essary to modify some of the resulting hydrodynamic coeffi-cients in the equations of motion to account for three-dimen-sional effects due to the small L/B ratios of barges. Wave loadcalculations then proceed in the same manner as for ships[7J.

Besides the hydrodynamic pressures acting on a barge hull,wave-induced vibratory loads, such as slamming and springing,should also be considered. Although the theoretical predictionof these loads is still unresolved, empirical relationships havebeen developed for estimating the slam impact loads on thebarge as well as on overhanging jacket structures [8, 9J.

Environmental data. Environmental data for a transpor-tation study, unlike environmental criteria for fixed offshorestructures, which usually are provided by the owner, most oftenare the responsibility of the towing contractor. A contractorwill normally be required to provide the necessary design in-formation on wind and wave conditions, subject to approval bythe client and a cognizant regulatory agency or both.

The final choice of environmental conditions will dependdirectly on the towing route, time of the year the towing willtake place, ability to get to a sheltered area, and the assumedrecurrence period of environmental extremes. The predictedor assumed weather conditions used for developing the designloads for the transportation phase of an offshore constructionproject thus playa significant role in evaluating and designingbarge/jacket systems, especially with respect to the barge hy-drostatic stability, barge and jacket strength, and seafasteningdesign parameters.

Wind and wave data are presently available from threesources: direct measurements, hindcasting techniques and shipobservations.

1. Direct measurement at the location of interest will givea designer the most accurate form of environmental data. Thetypes of instruments most commonly used to measure wavesin spectral form may include wave staffs, wave buoys, andshipborne wave recorders.

200 Practical Design Approaches for the Analysis of Barge Performance

-«

'1

Due to the expense of maintaining an instrument at one lo-cation for long periods of time, measured data are not usuallyavailable for the site of interest, and when they are, generallythere is not a record of sufficient length. In the case of arransportation analysis, data are required at all points along theroute. This makes it extremely unlikely that direct measure-ment will be available for the entire route and duration.

2. A more readily available source of wind and wave datais the hindcasting technique. This procedure utilizes the dailysurface pressure charts for an ocean area, and estimates thesurface winds from this information. Finally, the wind fieldsare used to estimate the local waves, which are allowed topropagate from one area to another to build a complete pictureof the sea state at any desired point in space and time.

The earlier hindcasting methods developed by Pierson,Newman, and James (PNJ) [10] and Sverdrup, Munk, andBretschneider (SMB) [Ll ] are presently being superseded bytechniques known as spectral wave models. Spectral modelsconsider the generation, propagation, and decay properties ofindividual frequency components of wave specta [12] and,therefore, will provide more detail and perhaps more accuratedescriptions of the wave climate. Although no model yetavailable has been able to accurately predict daily events, it hasbeen proven that hind casting techniques do provide true sta-tistical data when long-term records are utilized [13]. Fortu-nately, long-term statistics are one of the basic requirements,for transportation analyses.

When using hindcast data it is valuable when possible tomake comparisons with other representative, independently-measured data. This procedure assures that the hindcasttechniques used will be suitable for the specific site under in-vestigation. Such factors as shoaling, local wind variations, gridspacing, swell, generating areas outside the assumptions of themodel, and the insufficient numbers of weather stationsavailable to build reliable pressure systems maps, may lead toerroneous hindcast results in certain locations.

3. Ship observations for most ocean areas based on a com-piled massive data source can also be used in a transportationstudy. However, these data should be used with caution sincethey are derived from data collected by merchant vesselspassing through areas randomly. It must also be recognizedthat the observations taken are made visually by untrainedobservers, and that it is extremely difficult to observe accuratelywave heights and periods from a moving vessel. Furthermore,it must be noted that merchant vessels also tend to avoid theworst storm conditions, and therefore a lack of storm conditiondata may bias the sample. These factors of bias are offset bythe availability and inexpensiveness of these data, which havebeen compiled into tabular format and published by variousauthorities, such as the U. S. National Climatic Center [14] andthe U. K. National Physical Laboratory [15]. Other sources ofsummarized ship observations can be found in reference[16].

Several attempts have been made to correlate visual waveobservations with measured wave data in order to overcomethe shortcomings of the observation technique. These attemptshave resulted if!a diversity of correction formulas for both waveheight and period, but the consensus seems to be that the visualobserver tends to underestimate the small waves and overesti-mate the larger waves.

Design environmental conditions. Design environmentalconditions which may occur during the passage must be pre-dicted once the most suitable source or sources of wave datahave been selected for the tow. Normally the data used inpredictions should cover all months during which the tow couldtake place plus one month before and one month after the towperiod. This procedure helps to smooth out the anomalieswhich are sometimes present in monthly wave statistics. For

long ocean tows it has been a practice to carry out a simulationwhich routes the tow through actual past weather conditions,with adjustments of speed as necessary. The accumulatedenvironmental statistics from many voyage simulations can thenbe used to develop predictions of significant and extreme en-vironmental conditions.

The design sea state for the towing operation can be derivedfrom a data base which contains the percentage-of-exceedancestatistic for significant wave heights, and the ioint-probabilitvdistribution of both wind speed and wave height and period.The cumulative probability of each significant value is thenplotted on Weibull probability paper for extrapolation of seastates at a desired probability level. The joint height and periodprobability is used to determine the range of mean spectralperiods for the extrapolated significant wave height in de-scribing the design sea state. The extreme wind and sea statein terms of the Weibull distribution can be ascertained byapplying the techniques described later in the Risk Assessmentsubsection.

Experience has shown that the two-parameter Weibull dis-tribution fits most sea state statistics well. A true Weibull dis-tribution is represented by a straight line on the plot, as shownin Fig. 3. The best-fit straight line can be drawn through thedata by linear regression or by other methods such as maximumlikelihood estimation. Particular emphasis should be placedon the higher points, and a weighted square fit may be requiredto arrive at a reasonable answer.

Once a designer has derived an empirical model for theprobabilistic occurrence of particular sea states, he must thenselect a probability level to determine the extreme design seastate for the specified towing operations under analysis. Theprobability level may be selected according to risk level andencounter probability, or by the probability of zero damageoutlined in the Risk Assessment subsection. In the past, a simpleand intuitive approach has normally been adopted.

The desired probability level may be presented as a functionof return interval based on the number of voyages betweenrecurrences and the average tow duration. The probabilityof sea state less than the design sea state, H 1/3, is therefore givenby

• 1P(H 1/3) = 1 - R X L X N (3)

where

R = return interval based on number of voyages betweenrecurrences

L = duration of tow in daysN = number of sea state observations in a day

Practical experience indicates that a range between 100 to200 voyages is normally considered adequate. The significantwave height corresponding to probability level is then used asa design sea state description for motion and strength calcula-tions.

StabilityIn general, stability rules for barges are set to establish a factor

of safety against capsizing due to inadequate dynamic stability,and against sinking or capsizing due to inadequate cornpart-mentation. The criteria which these rules set up are based onboth the predicted environmental conditions and the statisticaldata on the survival of models and real ships in these predictedconditions [17].

The differences in hull form between offshore constructionbarges and ships result in basic differences in st~tical stabilitycharacteristics, as shown in Fig. 4. Compared with most othership forms, a barge has a high maximum righting arm and alarge area below the righting-arm curve. On the other hand,

Practical Design Approaches for the Analysis of Barge Performance 201

o.o

,0.

99999

."n5"9 f- - - - - -"950 /"' I I

._0 IV I

.t9000 iI"J I

JI

.ssoco I

1I I

.90000 I I

/

/ I I

Ieoooc

/ I.7""""

/II

.60000 I:/ I

.eoooo

I

.40000 I,- I

I

:v>nnnI

I!>OOO I

.2~I

I

15000. IIII

.10000Z 4 5 6 7 I 9 ,0. 20 30 40 50 60 70 '090'

Fig. 3 Sample plot of significantwave height distribution on Weibullpaper

o.e.

oo

o\oJoz::!o'"::><.)<.)00...o

~ 0::::;a;;:;0o'"Q. 0

\oJ>I-

~o::>:Ii::>u

0.

o

o

o00

17.0

SIGNIFICANT WAVE HEIGHT

due to its high B/D ratio, the barge's range of positive stabilityis often low, with corresponding low angles of downfloodingand of maximum righting arm. The regulations selected foruse with deck cargo barges are those which most closely con-form to barge characteristics.

Tables 3 and 4 represent a summary of intact and damagedstability rules, regulations and recommendations from a varietyof sources, and are not comprehensive lists. These rules maybe broken into three categories. First, there are rules which

have been developed specifically for deck cargo barges. Theseare the DnV guidelines [18], the ND guidelines [19], and theUSCG deck cargo barge rules [20]. Each of these containsexplicit or implied cautions for using their rules/guidelines forlarge overhanging structures and exceptional towages.

The second category includes rules for the design and con-struction of offshore structures, such as those published by DnV[211, DOE (U.K.) [22], the United States Geological Survey(USGS) [23], and Bureau Veritas [241. Each of these containsspecific requirements for loadout, transportation, launchingand upending of steel jackets. In general, they offer a moreuniform set of guidelines for determining environmental loadsand risk levels.

The third category contains rules developed for offshoremobile drilling units (OMDU's), and includes those issued bythe Intergovernmental Maritime Consultative Organization(IMCO) [251, the American Bureau of Shipping (ABS) [261, andthe U. S. Coast Guard. These rules are included because thereare similarities in the stability problems of a barge/jacket towand those of an OMDU in that they both exhibit high centersof gravity and large, often complex, wind heeling areas. In thisrespect, the OMDU rules have been predecessors of the othertwo categories of rules presented.

All of the stability criteria consider the features of the bargegeometry which have the greatest effect on stability,namely:

8m-Maximum Righting Arm Angle

80' - Downflooding Angle8m

HEEL ANGLES (DEG.l

Fig. 4 Comparison of righting-arm curve characteristics for bargesand ships

• characteristics of righting-arm curves,• wind heeling, and• meta centric height of loaded barge.

202 Practical Design Approaches for the Analysis of Barge Performance

..

.1

Table 3 Summary of Intact stability requirements

~yIMeo t 25 ] 6 ..•.(Z<4} ONV (24) I 'Ol ,28) I 0 •• NOBLE OENiON USGS (23) USCG [20)A8S (26)

ITEM USCG [271 ~UlER G•.rO[ I..IHE S {IS] GJIOELlN(S (l9]

WINO fORCE ·r,'/2,CS:-.V'A .ECOGNIZEO of:1I2fCV:'''S,1\6 I R[COG"" HO I NC,i 5PECTIfiEC 48S OR OTHER F=ll2fCs "':1..5 p\e f: PA

CALCU~A110N -TUNNEL TEST "E MOOS -RECOGNIZ£,: S""'I\j;',"~:"S coorsME 1 HQ:>S I TUNNEL 1ES" WtND TUNNEL

." -ExPERIENCE~ WINDAGE ARE A . PROFILE PROFlL.E WITM SIWILAR PROJECTED A,*' A:L{2VCGL +Z P.OJECTED A'EA PftOJiC rc AREA. S"RU-:''' URES ON THE PLANE O-T)W

I~ · ••••DE. DECK A.EA ON ~lAN(

, HORIoW. TO T>t£::> DUE TO TRIM NORfIo!~l T( . rORCE~cr AND HEEL WIN::' DIRECTION DIRECTION<1

0MINIMUM WINO '360 "'IS ·50 YEAR 'NINO, I ••INurE I hit NLrTE M NU [ I I h4INUTE SUSTAINEO WINO WIN:> PRESSURE

W fOR STABILITY NORMAL CONO I HOUR AVER SUSTAINED WINO SUS1A'NE: WINO SUSTAINED WIN0I WITH 10 YEAq RETURN EOUATIONcr EVALVATION ·514 MIS FOR SUSTAINED WI1H RE TURN WITM 10 YEAR w'TIi qtTURN PERIO~

p:: 0.53 +::> SEVE RE STORM WIND PERIOD [QUAL RE u"1H PERIOD P[r.tIOO EQUAl

IL1I3301' •0 2~ 7 MIS 10 SEe~ GUST THREE TI"'4£ S TI'1QEE TlholESW

SHEL TER i.oc LACKING THE L[NGTH OF THE LC:NGTooI OJ: 11/"'1crIE'ECPT USCGl OBSERVATIONS OPEQATION OPERATION~ 36.0 "IS (NOT LESS IN';)'!" LESS

Z NORholAL COND THAN I wH1(1 T..I."" I WEE"')W~ 51.4 MIS0 SEVE.E STO.",~ HAlf OF TH£

~ VALUE FOR

Z OPERA' !HG CONDo=:; IN SHELTER AREAWW

WINO HEELING COSINE NOT SPECIFIED TO BE COSINE NOT SPECIFIED TO 8E:r0

"OWENT FOR SHIP TYPE CALCULATED FOR SHIP l'fPE CALCULATEDVARIATION CONF IGURAT ION FOR SUFfiCIENT CONFIGURATION FOR SUFFICIENT --z

i NO OF HEEL NO or HEELANGLE S AHGLES

INITIAL G"T ·POSITIVE GMT~ 030M NOT SPECIFIEO G"T ~ 0.30 •• 'GWT ~ PAH/6TV8>-'USCG -AREA TO RA••.•..•~

~(f)GMT ~ 005 M }. 008 M-Rod

m~<tz MINIMUM RANGE SECOND NOT SPECIFIED 0·-'5· <0 DEGREE NOT SPECIFIED~W OF POSITIVE INTERCEPT OF LONG MOVESv>~

STATICAL RIGHTING AND 0·-20·W~cr STA81LIl Y HEELING ARM fiELD MOVES0- CURVE (LESS 12 h)<t::>~O -WINO rrr ec r IZW_cr St-iOU •..D NO'" BE I

INCLUDED

AREA RATIO GREATER OR EOUA t, TO 1.40 NOT SPECIFIEO

IHBlI(8+C) HE OOWNfL.OCXlfrffi RANGE OF

CALCULATED AS A.NGLE SHOULD LIB. BID 8 T10COMMENTS -- RATIO Of RIGHTING -- EXCEED 20· -- SPECIFIE.D

A.EA TO GUSTAR£AEXCLUDING ST£,tDYWIND HEEL

Table 4 Comparison of damage stability criteria

~IMCO BV DnV RULES USCG ABSII DOE DnV HOBLEDENTON USGS[25] [24) [21] [27] [26] [2B] [18] [19] [23]

Number of Compartments One or more. Depends on damage penetrations. One See IMCO One... Flooded at anyone time0 ...0- '" ~ I ~ IBeam Penetration 1.5m 1.5m 1.5m 1 . 'im~ ~ 1.5m

B -c ~ ~ ~ lonQitudinal 3.()n 2.3m 2.3m - Not Specified Not Spec ified0

~:!; IVertical From the bot too shell to the upper deck

General The fi ne l water line tak i ng into account sinkage. trim & heel. should be Deck - Oooe conpar tmentbe low the lower edge of the opening through which any progressive flooding Edge damage does notmight occur. Should caps i ze or sink

Not be structureSub-merged

V> Minimum Wind Speed 25.7m/s One hour sustained wind & 1/2 of predict- 25.7m/s Not 25.7m/s (or -0- 10 see gust with one month ed sustained Spec i- applied for~ of return period. lacking wind for return fied intact stab.'"... this: period eq. three or 20.6 for'":; Sustained Wind' 18.()n/s three times the inside. if~ Gust = 25.7m/s 1.ngth of the less)tx tow>-0-

-'- Heeling Moment Cosine Hot Specified Cosine Hot Specified'"« Variation0-V>... Initial Metacentri c Not Specified ) 0.3()n Not Specified'"~ Height w/o wind effect«0

Dynamic Stabil ity - - ) 1.40. calculated with Sufficient Stabil ity to Withs tand Suffi- ) 1.40. ---Area Ratio respect to gust & Wind cent area calc.

(A+B)/(B+C) established wind stabi- perfonned1ity to from theproceed new originsafely at angleto re- of heelpairlocat-ion.

Practical Design Approaches for the Analysis of Barge Performance 203

em - Maximum Righting Arm

90F - DownfloodinQ

Righting Arms(I)

0::....I-....::f

92- SecondIntercept

HEEL ANGLES (OEG.)

Fig. 5 Definition of area ratio criteria

In so doing, each criterion makes two implicit assumptions: (i)that the righting moments at sea are qualitatively representedby stillwater righting moments, and (iy that the assumed windspeed and heeling moments are representative of the envi-ronmental overturning moments.

The principal intact stability criterion used for a majority ofthese rules/guidelines is the area ratio criterion. This states thatthere must be a minimum of 40 percent reserve righting-armarea over wind heeling arm area to the lesser of either thedownflooding or second intercept angle (see Fig. 5). A stan-dard 51.4-m/s (100 knot) wind speed is generally assumed forworldwide applicability in the absence of values predictedbased on location and time of year.

The BV criterion for statical stability differs slightly from theforegoing. It requires a minimum of 40 percent reserverighting-arm area over a gust wind heeling area, excluding thesteady wind heeling arms from both areas (see Fig. 6). Lackingstatistical observations, an established wind speed of 36.1 m/s(70 knots) and a gust wind of 51.4 m/s (100 knots) are to be used.In addition, BV rules require that the area subtended by therighting-arm curve be greater than 0.10 m-rad (18.8 ft-deg),

The USCG weather criterion establishes a minimum GMTbased on the wind heeling moment due to barge and deckcargo. The wind pressure used is a function of bargelength:

RIQhtinQ Arms em

eOF

.,

Gust Wind(I) Heelingffi ArmsI-....::f

Steady WindHeelinQ Arms

HEEL ANGLES (OEG.l

Fig. 8 Definition of Bureau Veritas area ratio criteria

. PXAXH.GM T (required) = tl. (4)

X tan1>The USCG dynamic criterion, which is used in conjunction

with the weather criterion, requires that the area under therighting-arm curve up to the maximum righting arm be greaterthan or equal to 0.08 rn-rad (15 ft-deg). These two criteria areusually used to formulate a curve of maximum cargo VCGabove deck (VCGc) versus draft or cargo deadweight. Char-acteristically, the weather criterion limits the VCG at shallowdrafts while the dynamic criterion limits it at deeper drafts.

The maximum allowable KG of the barge and cargo can bedetermined for each of the criteria as described in the fol-lowing:

Area ratio criteria:

5o~d(GZo - 1.40 Mw) d1>KG

A= 0 tl. (5)

1- cos1>dBV criteria:

c ~d ( 140 ) 04s; GZo-~Mwc d1>-j;:Mws(1)d-1>tl

cos1>l- cos1>d

USCG weather and dynamic criteria:

KGl = KMT - P X A X H/(tl. X tane (7a)

50 <Pm GZO - ACR

KG2 = (7b)1 - cos1>m

where

KGA :! KGl or KG2, whichever is less (7c)

The maximum VCG of cargo above deck may then be cal-culated by

VCGc

= tl.. KGA - tl.LS• KGLS - D (8)tl. - tl.LS

where tl.LS and KGLS are barge operational light ship proper-ties.

The three criteria described were applied to a 91.4-m (300ft) deck cargo barge to obtain VCGc-versus-draft curves, asshown in Fig. 7. To calculate these curves, the windage areawas assumed to vary with VCGc, so that

A = L(2 . VCGc + D - T)

H = VCGc + D - T /2

Note that for the ABSand BV curves shown, the area ratioswere calculated based on the conservative assumption thatdownflooding will occur at a tank vent close to the barge's side.When the ratios were calculated to the second intercept, theallowable VCGc rose between 0 and 7 percent.

The area ratio and BV criteria curves are, of course, verysensitive to the wind velocities used. When an actual windspeed prediction is made, the comparison between these criteriamay be somewhat different. Figure 8 was developed basedon a wind speed of 30.9 m/s (60 knots) for the area ratio criteria,based on a 50-year return period and an averaging period ofone minute. For the same prediction, the BV criteria steadywind speed is 26.2 ta]«; (50.8 knots) or a one-hour wind, andthe gust wind speed becomes 33.6 m/s (65.3 knots), or a ten-second wind.

It should be noted that the tendencies shown here may nothold for barges which are significantly different from the one

204 Practical Design Approaches for the Analysis of Barge Performance

(6)

(9a)

(9b)

91.4", X 27.4 1ft X6.' 1ft

(300' X90'X 20')91.4mX 27.4m X6.lm

L~OO' X 90' X 20')DECK CARGO BARGEDECK CARGO ·BARGE

\

'\\

\ \\ --- - B.V. AREA RATIO CRITERIA

\\\' V. = 36.0 "'I, , V,= 51.4 "'I,\1 --- A.B.S. AREA RATIO CRITERIA'I

\ \\ V = 51.4 m/,

\ \\ '.

\1

\ '~\ ,~__ \. I.

~ ~\" \'

~" ~':"<,

• .0X•...Il.o.Jo<,){:.l 5.o

\\\

:r 6.0•...c,Wo"){u~5.0

-------- U.S.C.G. OYMAIIIIC CRITERIA- -- U.S. C.G. WEATHER CRITERIA

--- 8.V. 0.1••-"'4 CRITERIA

- - - - - -- U. S. C.G DYNAMIC CRITERIA--- U.S.C.G. WEATHER CRITERIA_.- B.V. O.lm-Rod CRITERIA

---- I.V. AREA RATIO CRITERIA

V, .26.2 mls , "- = 55.6 "'I,--- A I S AREA ItATIO CRITERIA

V .50.9 "'I,w>oal<l

.04.0C>a::<lU

w>oal<l4.0

oC>a::<lu...o 10C>u>

u,oC> 5.0u>o.J..Jal~Z.Oo:J<l

~::>~ 1.0

~~

o.J..J~ 2.0~o..J..J<l

:fi 1.0

X4(~

,

\~\~

\\,,,

O.O'L..- ••••••---------------O.OL------'---------;-'-----0.80.4 0.60.20.4 0.6 O.B0.2

DRAFT/DEPTH

Fig. 8 Comparison of stability for given wind conditionDRAFT/DEPTH

Fig. 7 Comparison of stability criteria for worldwide service

The velocity and acceleration RAO's are simply calculatedby differentiating displacement RA~'s:_J? -::s?t.J)~ W (; e'V J) (7\, C) , 21) ,

D = iwDe1wt (lla)

j fj = -w2De,wt (llb)

In order to facilitate the strength calculations, the absoluteacceleration has to be transformed into the jacket coordinatesystem (see Fig. 9). This global-to-local transformation isachieved by using a transformation tensor based on a roll-pitch-yaw sequence by

used. The curves are presented simply to show the range ofresults which can be expected for typical stability calcula-tions.

Motion calculations ./Wave forces are the single most important environmental

factor causing a vessel's dynamic motions. Consequently, aswas noted in the Introduction, stress on a jacket induced by thecombined jacket/barge system motion should be analyzed earlyon in the design process.

Ship motion programs have become the standard tool forsuch seakeeping analysis, and barge motion in six degrees offreedom can be readily calculated. BARMOT (barge motions)128], a computer program especially suited for barge motionanalysis, is one such program which provides a frequency do-main solution that has demonstrated good agreement withmodel test results. The program considers motions to be linear,harmonic, and small amplitude, and the nonlinear effect dueto viscous damping is taken into account in roll motion by aniterative procedure. The solution Ior regular wave excitationis in terms of a set of response amplitude operators (RAO's) andphase angles at the combined center of gravity (CG) of thejacket/barge system for different encounter frequencies and

• headings.Once the program obtains the motion RAO at the system's

CG, the motions in three orthogonal directions can be calculatedat any discrete location away from the combined center ofgravity. The frequency RAO in complex form can be trans-ferred to any specified location using the following relations:

. ~

ITj = [TjROLL X [TjPITCH X [TjVAw (12)

where

ARBITRARY

NODAL~t1~~k-~~POINT

~---~

Y'D(x,y,z) = Dee + R. X r (10)

e-. where D represents the translation RAO vector for surge, swayand heave; R is the rotational RAO vector for roll, pitch andyaw; and r is the position vector from combined CG to thespecific location.

z/Barge/jacket coordinate systemsFig. 9

205Practical Design Approaches for the Analysis of Barge Performance

· ..

lul,lul = generalized displacement and acceleration withrespect to a local coordinate system fixed on thestructure

IFI = a generalized nodal force vector

For complex structures such as a jacket, the matrices [K] and[M] may be readily generated using a number of existing fi-nite-element programs, such as the in-house DAMS package(design and analysis of marine structures) [30].

By treating the jacket; barge system as a whole, the gener-alized inertial acceleration due to barge motion and in termsof RAO's may be derived for each node as described in the

(14) Motion Calculation subsection. Ideally, a dynamic analysisshould be carried out to account for the contribution from high

The three linear accelerations, including gravity, in the local modes of jacket vibration. Since the high-frequency springingcoordinate system then become is a rare occurrence, however, a static analysis may suffice.

X = Ax - A;Y, + gO (I5a) It is important to note that when barge deflection is signifi-_ _/ cant due to high wave loadings, it is necessary to include theY = Ay + Azr/> + gr/> (I5b) barge in the finite-element model together with the hydrody-i = Az + Az8 _ Ayr/> . / (15c) namic loads. The forcing function now becomes

The angular acceleration, ¢, 8, :.;",remains the same as be- " IFI = IFml + IFw) (19)fore. J where IFwi is the complex wave force acting on the barge and

Predictions of relative motion between a jacket structure and IF mI represents the complex motion-induced forces. An in-the wave are critical in order to gain some insight into barge/ house program, SEALOAD [31], has been developed for thisjacket slamming, particularly where the jacket overhangs. The purpose.estimation of slamming loads on overhanging jacket members The inertia, gra vitational and wave loads at each frequency,is a difficult subject which has attracted many research efforts both in real and imaginary parts, are treated as a static load case(29). Though far from complete, theoretical derivation and in the structural analysis. The resulting solution for the systempreliminary results indicate that the slamming load is a function in terms of displacement in the inertial frame is then convertedof the relative motion that exists between the jacket member back to physical coordinates to obtain stress levels.and water surface, and the impact velocity entering into the When the stress distribution around a tubular joint is desired,water, which is similar to ship slamming. In frequency domain a stress concentration factor is applied. The desired stressthe RAO for relative vertical motion at any specified location, RAO's on the circumference of the tubular joint can then bex, y, z, is given by, determined using the stress concentration factors. Once the

R( ) D ( ) ['k( . () desired stress RAO's are obtained, the response statistic can bex,y,z = z x,y,z - exp t x cosu. + .Y SInJ-L)] 16 readily calculated by applying the well-known principle of

where R is the vertical relative motion displacement RAO's and superposition for linear systems.D;z; is the absolute vertical motion RAO. Given the spectral density function S(w) of the wave, and the

Relative velocity RAO's are readily developed as follows: RAO in regular seas, the response statistics in an irregular sea'\ can then be calculated in terms of its spectral momentso, = iwD:e!wt (17)

[T) = [cosO, cosy; cosO siny;sinl/>sinO cosy; - siny; cosr/> cost/! cos¢ + sinr/>sinO cost/!sinl/>siny; + cosl/>sinO cost/! cosr/>sinOsiny; - sino sint/!

where I/>is the roll rotation, 0 the pitch rotation, and y; the yawrotation.

Vessel acceleration in the local coordinate system is thengiven by the relation

IAILOCAL = [TIIAlcLOBALBesides the inertia accelerations induced by motion, an ec-

centric gravitational acceleration due to roll and pitch motionshould be accounted for. The component for acceleration dueto earth's gravity, which is basically nonharrnonic in nature, canbe resolved for small roll and pitch angles as follows:

IAI = IAlLOCAL + (-gk) X if .

The derived information can then be used to simulate thejacket member submergence and the impact velocity in thetime domain for slam investigations in conjunction with modeltests. Statistics on the probability of slamming can also becalculated using a theoretical formula [8].

Structure analysisAfter determining the motion characteristics of the barge/

jacket system, the designer can then calculate the stresses in-duced by the motion during transportation.

Classically, the equation of motion of an elastic undampedsystem subjected to arbitrary motion-induced loading may berepresented in matrix form as .

[M lIiil + [KlIul = IF)where

[M) = generalized mass coefficient matrix[K] = generalized stiffness influences coefficient ma-

trix

-sinO ]sine sinOcosr/>cosO

(13)

mj = So'" So211' w!RA02(w,v,J-L'S.w)f(a)dadw (20)

where RAO(w,v,J-L) is the RAO at frequency w, speed v, andheading J-L,and f(a) represents the spreading function.

Generally, the peak value of the stress follows a Rayleighdistribution for short-term predictions. The probability of thestress level being greater than a certain threshold value 0- isgiven by

Pia > 0-) = exp(o-z/2mo) (21)

(18)

where mo is the mean square value of the stress equal to the areaunder the stress response spectrum, and can be evaluated byequation (20). . n-.

Furthermore, the most probable extreme value of t~ A.J.JV.Msponses expected to occur once in N independent observations

'Cail6e estimated by the following asymptotic expression forlarge N [32]:

UN = ~ X [on N)I/2 + ~X 0.5722 (In N)-1/2 ... ) (22)

Practical Design Approaches for the Analysis of Sarge Performance

For a given response spectrum, N can be estimated by usingthe zero-crossing period Tz in seconds, and the duration ofexposure T in hours:

N = 3600 X T /T2 (23)

Tz = 27r(mo/mz)l/2(1.0 - 0.05 f)2 (24)

where f is the spectral broadness factor

E= (l - mz2/mo/m4)1/2

For long-term prediction, however, there normally existsuncertainty of the parameter mo due to random variation ofwave spectral shape. Thus, the probability of a exceeding athreshold level fr, and taking into account the parameter's un-certainty, is given by a combined Rayleigh-normal distribution.A detailed explanation of the procedure can be found in[33].

In fatigue damage assessmen s which consider the entirerange of stress, as well as the total number of stress cycles, themean period of the stress cycles has to be determined. Typi-cally the zero-crossing period T2 is used, which is defined inequation (24).

By using the well-known Palmgren-Miner rule, the cumu-lative fatigue damage per unit time for a particular sea state,H1/3, and heading, J.I., can be, estimated:

/ (25)

where Pj(u) represents the probability density function of thestress range, and N; (c) is the expected number of cycles tofailure at stress level a from a fatigue curve.

Hence, the total expected fatigue damage during the trans-portation can be estimated by

Dvoyage = L T; X D//

where T, is the total exposure in Area i encountered, and D, isthe cumulative fatigue damage, taking. into considerationjhe .'sea state and heading probabilities I~::~. aj S-eA-~

d -: ~ 1,. t-u-..cP-7o, = L L Pk(H 1/3)Pj(J.I.)D(H 1/3J.1.) (27)

k j

A special-purpose program, TPFA TIG (TransportationProbablistic Fatigue Analysis) [34], has been developed in-housefor the purpose of calculating fatigue damage and maximumstress during transportation.

Risk assessmentIn a transportation study the question the designer must

answer for the jacket owner is, "What will the level of risk bein transporting the jacket from fabrication yard to productionsite?"

One way of defining the risk level is to determine the returnperiod associated with the design environmental conditionswhich the barge/jacket may encounter during a specified pe-riod and tow route. These possible environmental extremesthen become the design sea state and wind conditions whichthe designer can use for stability, motion and structural analysisto ensure that the barge/jacket system will "survive" duringtransportation. Thus, the probability associated with the designsea state and extreme wind conditions indicates a measure ofrisk level during the tow.

In the long-term oceanographic statistics, each sea state isgenerally represented by its significant wave height, H1/3, anda characteristic wave period. Recorded data have shown thatH1/3generally follows a Weibull extreme probability densityfunction .

- ,.

Q(H1/3) = I - P(H1/3) = exp[- (~~3)klfor which

(2&).

(26)

H1/3 = Ho[-ln Q(H1/3)P/k (28b)

where Q(H 1/3) is the probability of exceeding the significantwave height, H1/3, and H0 and k are two parameters indicatingthe intercept and slope of the best-fit line on Weibull paper.

Once the design significant wave height has been established,a range of characteristic periods may be used in order to ensurean accurate and sufficient determination of the maximum re-sponse. The period range, usually in terms of mean spectralperiod T 1, may be obtained from a theoretical joint probabilitydensity function of H 1/3and T1,or, in the absence of data, thedesigner may use the value recommended by some classifica-tion society. For example, Dn V specifies the following rangeof wave periods [18]:

VlOH1/3 < T1 < V20H1/3 (29)

where H1/3 is the design significant wave height in meters.For fixed offshore structure designs, the probability level is

usually translated into a so-called maximum design wave heightwith N-year return period. This notion, however, is lessmeaningful for a towing operation which lasts days, or a max-imum of several weeks. Given a particular return period, itis not at all clear what risk is being taken for a specified towingoperation. A more meaningful criterion is therefore neededto convert the return period of hazardous events into mission-related statistics.

One way of arriving at a meaningful risk level for a specifiedtowing operation is to use the notion of encounter probability.Provided that the extreme sea state and wind speeds are rareevents during the transportation, the probability of these haz-ardous events occurring may be described by a Poisson model[35]: .

eLA(L'A)mP(m) = , (30)

m.where L represents the duration of transportation, 'A is the rateof arrival of hazardous events, and m is the number of haz-ardous events occurring during L.

The return period, or average interarrival time, TH, is givenby

(31)

The encounter probability, defined as the probability of oneor more exceedances of -tbe hazardous events during L timeunits of exposure, is given by

PECH1/3) = 1- exp( -L/T H) (32)

where H1/3 is the design sea state associated with a return periodTR. Notice that the encounter probability is a function oftowing time as well as the return period of the hazardous events,which can be extrapolated from a Weibull probability plot asshown in Fig. 3. ~

Furthermore, for a given design sea state H1/3, the peak stressfollows a Rayleigh distribution. Thus, the probability of themaximum stress exceeding the yield stress fr can be estimatedby equation (21). The probability of no damage during theentire voyage is given by

P(no damage) = 1 - PE(H1/31"o). = exp (- i:0) (33)

where Po is the probability pto > fr) as shown in equation(21).

Practical Design Approaches for the Analysis of Bar~e Performance 207

PR06ABILITY OF NO OA!!AGE FOR Po ( (T""> tr)10~~===========-I '" -O.O!!-0.1008 -0.2!!

-O.!!O

-0.632

ENCOUNTER PROBABILITY

0.2

00~----~0~Z----~0~4----~0~.6~--~0~.8~----71~0---!!ISSION PERIOO / RETURN PERIOO

Fig. 10 Risk levels as a function of mission period and design sea statereturn period

,.

Figure 10 shows a plot of encounter probability level andprobability of no damage at various Po versus the ratio of mis-sion period and return period. \'otice that if the most probableextreme value is used for the design criterion. there is a prob-ability of 0 6.3:2 that the extreme value will be higher than themost probable \ alue. Such a risk level is generally unacceptablein practice for intermediate and long voyages. This leaves thedesigner the choice between increasing the return period of thedesign sea state and using a design stress level with a lowerprobability of exceeding the yield stress.

Practical design considerationsThe preceding section has described the acceptable criteria

and the approaches to the essential elements needed to performa transportation study. The paper now addresses some of thepractical aspects a designer faces when analyzing an offshoretransportation operation.

Design decisions are affected both by the state of the art ofnaval architecture and structural analysis. and by the practicalfactors involved. such as time and data available and level ofanalysis requested. Thus. each transportation study performedwill vary its use of available methodologies. Consequently, thefinal determination for the level of effort in any transportationstudy is a result of weighing the anticipated gains (monetary)with choices about acceptable methodologies.

The basic criterion a designer must use to develop a rational,consistent .1Od practical methodology for transporting a struc-ture is that of safetv: the analvsis should insure that the vsternwill not capsize or' be broke~ up. The practical design con-siderations. therefore. center about the apparent tradeolfs be-tween motions and stability. and the effect these two factorshave on structural strength.

This section now turns to the practical options the de ignercan deploy to .eep the H" f'1.lIHI its cargo \\ ithin the pn-dictedrequirements for a particular mission.

Barge motions and stability

It has long been know n that .l vessel with :.I high dt'~rt'e ofstability wdl have stiff or jerk\ motions, because as the C.\I ofthe vessel b raised, the low er its natural period I)t'COIlH:'S. Thistradeoff betwe-en d y na mic ,tallilit) and motion accel.-rution

is the practical problem that confronts the designer in trans-portation analysis.

In standard ship practice the vessel KG can be altered to someextent in order to decrease stability to a nominal level. whilevarying the vessel's natural roll period in order to reduce mo-tion. and thus keep wear and tear on passengers. crews. cargoand machinery at a minimum level. Alternatively. bilge keels.fin stabilizers and antiroll tanks. and other active and passivedevices may be used to modify a ship's motion characteristics.However, proper ballasting and cargo allocation remain themost common means available for control of merchant shipmotions.

The position of the vertical center of gravity (KG) of a jacketis determined by the depth of the barge and the height of the"hI beams. Therefore. the KG of the system may only be al-tL-r,·d through ballasting. which may have little effect on thebarues GM, due to the variation of K,\l with draft. A designerab" has available several passive roll-limiting devices which canalter a barge's natural period. but the most practical solutionis etten the selection of a barge with characteristics that com-pliment the needs of the mission under study.

Barge selection can be particularly important because off-shore tructures are especially sensitive to transportation loadsdue to their great size and weight, and because jacket designconfigurations are optimized for their in-place loading ratherthan for towing conditions. It is also often true that a decreasein barge motions will result in a direct decrease in steel weightof the tiedown system and jacket and. therefore, in a reductionIn its acquisition cost. The designer's concern. then. is that ofmotion control rather than maximum stability.

The relationship between motion and stability is demon-strated in the following examples. Because the roll motionsoften cause the most severe loading on the cargo. the examplesconsider only the effect of the barge's roll motions in sea statesof varying wave heights.

The natural roll period of the barge. T <p. may be expresseda

T '= C 27rKn'" .-\. G '"V g LVIT

(3-t)

where

CA = added mass coefficient of barge= F{Kn/B. BIT)

Kxr = roll radius of gyration of barge/jacket system

The only means of changing the natural roll period for agiven barge and jacket is by adding or moving ballast. Addingballast lowers the KG and increases the draft. while the positionof the ballast affects the svsterns inertia.

To avoid roll resonance it is necessary to operate the bargeso that its natural period will not likely be in the range of themajority of a given sea state's energy. As an illustration of this.the period range for the maximum spectral energy density ofa Pierson-Moskowitz spectrum has been plotted for ignificantwave heights up to 9 m (:!9.5 ft) (see Fig. I l ). :\ote that withthis spectrum, less than LO percent of the total wave energy isat periods greater than the range indicated.

Assume that Barges [ and II are ballasted so that their naturalperiods are L.5.-t and 13.:2 seconds (s), respectively. It can thenbe seen that Barge [ will be in resonance in sea states with sig-nificant wave heights greater than .').9 m (19.:3 ft). while BargeII (the larger barge) wil] be in resonance for significant waveheights greater than -t.-t m (1-t.-t It). Therefore. from a rollmotions standpoint, the smaller. less table barge probablywould be more acceptable for tows where significant waveheights greater than -t.-t m are expected.

The idea of variuble-stubility (that is, variable wuterplane)burucs has resulted from the need to "tune" barges for partie-

208 Pracucal Oesiqn Approaches for he A.I arysis of Barge Per ormance

ular jacket tows. Through thf' use of modular barzr« such asFlexifloat 136;, or b~ variable free-flooding arrang. ·",'111., itmay be possible to suit a barge to the needs of a particular jackettowage operation This matching of barge and to« can beaccomplished b) reducing the water plane inertia of tilt' vessel,and thereby approach its minimum intact and damaged sta-bility limit while at the same time minimizing its anticipatedmotion responses. The ability to significantly varv a vessel':waterplane properties to increase its suitability for particularvoyage requirements is unique to barges, and provides all ef-fective means to control barge accelerations.

AntirolJ tanks, such as those used in ships, also have the ad-vantage of being able to be "tuned" to a specific frequency fora particular tow. However, they are usually only effective overa short range of wave periods, and would have to be quite largeto be of significant value. Thus, antirolJ tanks have seen littleapplication in deck cargo barges.

Once a barge has been selected for the towage under study,the use of bilge keels remains the most common method tofurther moderate barge roll motions. It is also usually the leastexpensive approach. In general, bilge keels will damp rollmotions over a wide range of barge drafts and wave periods byincreasing the added mass and viscous damping of the barge.The effects of bilge keels of varying sizes on a particular towagecan be seen in Fig. 12. It should be noted that the roll RAO'smay begin to increase again for bilge keels beyond a certainoptimum size.

Barge motions may, therefore, be minimized through the useof a variety of designer-controlled variables. These includethe selection of the barge and ballasting configuration, the useof variable-stability barges to "tune" for specific towages, andthe use of bilge keels and other motion-damping devices.

5.0

DECK CARGO BARGEI01.2mX274mX6.lm(332' X 90'X 20')

40

BARGE WITHOUT

X BILGE KEELS

O~-::I 3.0...J...J)(

0a::NBARGE WITH-

Q 09,. (30') BILGE KEELS•...~ 14,. (45') B'_GE KEELSa::w 20 I 8 •• (60') B,GE KEELSII)Z0CLII)wa::...J...J

0a:: 10

r:

10

BARGES

I 914M~27~MX61'"

(300' X 90'X 20')

II (158MX30SMX7.6M

(380'XI00'X2S')

/19

18

17

II)aZ 16ouw'" 15

a000-14_a::a::wwn.n. '" 13-'<f-'won.a:: 12

-'-'<f<fa::a:: •... 11::>u•...w<fn.Z"'IOI I

"..•s~ 9•...1- PERIOD RANGE fOOl

50% OF WAVE ENERGY(CENTERED ABOU T~[A,)

B

7L- ~ ~ ~ ~0.2 0.4 0.6 0.8 10

DRAFT / DEPTH

..o 6 8 104

Fig, 11

SIGNIFICANT WAVE HEIGHT (METERS)

Variation of natural roll period with operating draft and variationof spectral peak period with significant wave height

15 20

WAVE PERIOD

Fig, 12 Variation of roll RAO's for bilge keels of various lengths

Practical Design Approaches for the Analysis of Sarge Performance 209

ELEVATIONFRAMING-=----_,

JACKET LE

STBD

"

TUBULAR TIEDOWNFWD

LONG'L SHEAR PLATE

PLATE TIEDOWN

Fig. 13 Typical tiedown arrangement

."

StrengthSeveral levels of analysis are possible for determining loads

and stress levels in the barge/iacket system. depending mostimportantly on the designer's concern over the magnitude ofthese loadings. Obviously, for a tow of short duration in rela-tively protected waters, only a very simple and conservativeanalysis will be performed to reduce the overall loadings andthe required tiedown arrangements. It may even be deducedthat it is safe to use a standard tiedown size and an arrangementbased on successful usage in the past. Figure 13 shows a fewexamples of typical tiedown arrangements.

For tows of intermediate duration, where more exposure tohostile weather is anticipated, simple hand calculations are oftenperformed to size tiedowns. In this analysis certain conserva-tive assumptions are necessary due to the fact that tiedowns arenormally a redundant structure (that is, the problem is staticallyindeterminate). For a common tiedown arrangement, possiblythe most straightforward assumption is to replace the jacket bya set of lumped weights and lumped inertias at its tiedownpoints. This information may be easily available from previouscomputer analysis of the jacket structure, or may be hand-comp';!ted from drawings or estimated simply by knowing thejacket s length and center of gravity. This method is similarto that used for launching calculations [:371.

The next question that often arises is what sort of accelerationto use with this quasi-static system. The simplest method wouldbe to apply some standard maximum motions to the barge inthe manner described by one of several authorities [l9, 381.

The problem which arises in using this approach is in de-termining the interaction nf various. accelerations acting si-

multaneously on a given member. A common approach is toassume that the worst loading will come from beam seas whenthe roll angle and roll and heave accelerations are at theirmaximum. A number of possible worst loading conditions maythen be developed by alternating the directions of theseloads.

This simple approach is suitable for preliminary design andstress estimation of tiedown bracings. The worst-load conditionis less sensitive with respect to the relative signs of these accel-erations when the barge/jacket configuration is symmetric. Insome cases, the number of loading conditions may be reducedby simple inspection, though as the load size and complexityincreases for large structures, the ability to do so decreases.

As the jacket size increases, tiedown and jacket stress esti-mations become more important. Failure of jacket membersfrom fatigue damage during transportation isa genuine concernto the designer.

Normally the structural analysis of the jacket has been fo-cused about its in-place loads, with a certain amount of bracingadded to accommodate its lying on its side during constructionand transportation. When the transportation analysis is done,the finite-element model for the jacket is modified by alteringthe support points and adding the tiedowns and skid beam tothe structural model. Dynamic transportation loads may thenbe applied by transferring the linear and angular accelerationsto lumped masses at each node of the model. The resultingstress distribution is used then to determine whether the jacketstructure is adequate or whether some modification is required.The results are also used to size and position the tiedowns.

The question :lgaill arises as to the validity of the maximum

210 Practical Design Approaches for the AnalYSISof Barge Performance

Table 5 Expected participation factor matrix for 6-deg acceleration and angular motions

LINEAR HCELERATION DUE TO' 4NGUlt..~ t.."~~l[P.ATIO"l DUE TO' ~N4Ut ~~ ",rTION DUE TO'SURGE SWAY H[AVE YAW ~OLl PIlCH POLL- PIICHM

SU~GE 0.0 Y- o . a 0.0 0.0 0.0 0.0 0.0 0.0

SWAY 0.0 x 100 .00 x - 34 .89 x 0.0 Y- -64. 95 Y- 0.0 Y- 44 . ) 0 ~ o .0 x

HEAVE 0.0 x -30. 96 Y- 100 .00 Y- 0.0 % 39 .09 x 0.0 -45.24 0.0 Y-

YAW 0.0 x o .0 x 0.0 x 0.0 % 0.0 0.0 Y- 0.0 o .0 xR!l'lL 0.0 Y. -&8 22 x 40. 30 x 0.0 x 100. 00 % 0.0 x -100 .00 0.0 xPIlCH 0.0 ~ 0.0 x 0.0 0.0 x 0.0 0.0 Y- o .0 '0 0.0 xROLLM 0.0 Y- 88 22 x -40. 30 % 0.0 x -100. 00 x 0.0 x 100.00 ~ 0.0 %

PIlCHM o .0 x 0.0 Y. 0.0 % 0.0 % 0.0 x 0.0 x 0.0 0.0 x

••• SHORT TERM RESPONSE STATISTICS (SINGLE AMPLITUDE) PER UNIT HI/3 IN FT .•••

• UNIDIRECTIONAL LONG CRESTED SEA IS ASSUMED WITH HEADING ANGLE' 90.0 DEGREES• UNIT ISSC SPECTRAL FORMULATION IS USED'MEAN SQ. VALUE

R.M.S. VALUESIGNIFIC. VALUETl.PERIOD (SEC)BROADt~ESS (EPS)MPMAX. IH .5 HR

0.00.00.00.00.00.0

0.45)4E-020.6)63E-Ol0.1353E 000.8596E 010.4262E 000.2351E 00

MEAN WAVE PERIOD 10 .0 SECONDS0 .9806[-02 0.0 0 .H23t-02 0.0 0.2353E-OI 0 .00 .9902[- 01 o . 0 O. 3199E-OI 0.0 0.1534E 00 0 .0O. I?8OE 00 0.0 0 097E-OI 0.0 0.306~E 00 o. 00 .BI2lE 01 0.0 O. 1242E 02 0.0 0.1364E C2 0.00 .3809E 00 0.0 0 .3655E 00 0.0 0.2543E 00 0.00 .3458 E 00 0.0 O. lonE 00 0.0 0.5125E 00 0.0

NOTE MULTIPLY BY THE SIGNIFICANT WAVE HEIGHT TO ARRIVE AT THE CORRECT RMS. SIGNIFICANT AND THE MOST PROBABLE MAXIMUM RESPONSESFOR THAT SEA STATE.

loads as well as to determine the combination of accelerationsto be used in the jacket stress analysis. It is important to em-phasize that the maximum loading condition cannot always bedetermined by simple inspection as in the case of tiedown de-signs. Therefore, a more rational method is needed to find theso-called "participation factors" for each of the motion com-ponents when one of them reaches its maximum in a given seastate and duration.

The estimation of the participation factor is achieved byutilizing the notion of cofactors in random processes [39J. Theanalysis, using basic Six-degree motion RAO's and phase angle,determines the relative percentage values of its expectedmaximum when one of the responses (accelerations or angularmotion) is at its maximum. Table 5 gives an example of theparticipation factor from the CARGO (participation factors)computer program 140J matrix for a typical jacket/barge systemin unidirectional beam sea. The results have been verified withthe time history simulation using the same acceleration andmotion RAO·s. The expected maximum values of the accel-eration and motion using frequency domain analysis seem tobe in good agreement with the time domain simulation, exceptin the case of roll responses, as shown in Table 6. The "par-ticipation factors" expressed in terms of percentage of theirrespective motions may also be calculated by using the averagevalues of the time history runs.

After the participation factor of the acceleration and motionfor different headings and sea conditions has been determined,the jacket structure can be readily analyzed by transformingthe responses at the barge/jacket combined center of gravityto modal forces and moments in a local coordinate system tosolve for member stresses by using an existing finite-elementpackage such as DAMS.

It should be noted, however, that although this approachprovides a more rational basis for treating the maximum mo-tion-induced stresses. it still involves rnanv simplified as-sumptions which may not realistically represent the actualconditions under tow. First, the calculated motion-inducedstresses are not rigorously derived, using a set of average valuesof participation factors for a given seastate and duration.Secondly, the effect of the barge's structural response on thejacket is not taken into consideration. Finally. the result cannotbe used for a rigorous fatigue analysis, which could l» criticalfor certain \\ pes of jackets.

The ultimate approach to the jacket/barge structural analvsisis to model tilt:' system as a whole by the finite-element method.Both h\'dro(hna'mic and hydrostatic loads can be applied to thebarge using the computer program SEALOAD. In this way,the effect, of barge/jacket structural interaction call be ac-counted for The results obtained by the TPFATIG post-processor an- ill term, of member stress RAO's, which can be

Table 6 Maximum heave, sway, roll acceleration and roll angle with their respective participation factors for a barge/jacket systemin beam seas from time history (ISse spectrum H 1'3 = 20 It; Tl = 10 sec)

Heave Accelerat ion. Sway Accelerat i,,~ . Roll Acceleration, Holl Anul«.ft/S~ ftlS" deg/S~ dp~

Heave acceleration 6.828 -1.579 o.sso -;1.151(lOO'}(j (33.4~' ) n~.9"') (39.9%).- Sway acceleration -2.233 4.727 - 1.19, 4.020(32.7<;() (l00"! ) (66.7rC) (50.9%)

Roll acceleration 1.78:2 ":'3.291 1.79;, - 7.424(26.1Ci) (80.:20, ) (100"; I (9·1.0%)... Roll angle -2.397 3.143 .-1.,8-1 7.898(35.1 ';cl (66.5%) (99.:16(", ) (lOOCi, )

NOTES: 1. The values indicated are t.he average values from six time-history simulat ion runs.2. The negative signs are used to indicat.e the opposite direction to the maximum responses.

Practical Design Approaches for the Analysis of Barge Performance 211

STAGE 0 - WINCHING

STAGE I - SLIDING

STAGE 2 - ROTATING

STAGE ~ - SLIDING e.

/

STAGE 4 - CLEARING BARGE

SKID BEA~

BARGE

Fig. 14( a) Launch stages for an offshore jacket

combined for maximum stress prediction and fatigue damagefrom an estimation using stress concentration factors. Asoutlined previously in the Structure Analysis subsection. thisapproach represents a more coherent methodology for ajacket/barge analv is. nfortunateiy, the volume of compu-tation may be prohibitive and justified in only a limited numberof ca es.

A barge structure, however, may be checked for adequacywith an alternative and simpler approach. First, the primaryhull bending tress is obtained by the traditional method ofcalculating the limiting stillwater and wave-induced bendingmoments, using the barge section modulus and the design waveheight. Bending stresses should be obtained for both tow andlaunch conditions based on the actual jacket and ballast con-figuration.

Second, for some barges the carrying capacity of the deckmay be in question due to the high local loads transferred frornthe cargo through the kid beam. These loads tend to peaksharply at the major framing elevations where loads are dis-tributed in from other parts of the jacket. Such loads may bein the region of l/~ to 1/6of the jacket's total weight. This loadis distributed by the skid beams to the deck frames and even-tually to the transverse bulkheads and side shell.

The third consideration is the barge's local strength in wayof the tiedown structure. Experience shows that this is wheremuch of the member failure occurs, especially fatigue failurefor members periodically in tension. The design must considerthe local strength of the deck relative to the maximum (com-pressive) load expected, and also consider the periodic tensileloads that will be present.

Jacket launch considerationsBarge requirements determined by launch considerations

will often have a significant effect on barge selection fortransportation. Barge SUitability for launch is defined byvarious parameters, starting with the overall strength of thebarge as defined by its maximum tilt pin reactions, the lengthand flexibility of the tilt beams, and the hull girder sectionmodulus. Other barge parameters include stahilitv L haruc-teristics at high trims, and compartmentation and hallast-ability.

The objective of the launch analysis is to define a method totransfer the jacket from the barge to the water in the srnoothcstand safest manner possible. This process involves minimizins;jacket and barge stresses and maximizing both barge and jac .etstability. A primary consideration is to minimize the launch's

212 Practical Desrqn Approaches for the Analysis of Barge Performance

•

sensinvity to small variations In the establi: hed configura-non

Launch dqnanncs The main concern in am launch analvsisCt'nters,about predicting the dynamic lx-havior of th Jacket andbarge. Therefore. design prediction must con ider tilt' d)-namic of the svslem, w hich are norrnallv derived from modeltest or from computer simulations which produce a time his-tory of the launch or from both 14J I. A typical stern launch canbe divided into S stages for evaluauon, a shown in FI~ J .1\a ).Figure 14(v) shows the launching of a large offshore jacket

The most critical stage for both jacket and barge is usuallyrotation of the jacket on the tilt beams ( tage 2) Stresses in thebarge are at a maximum due to both the unfavorable longitu-dinal distribution of weight, and the concentrated local loadingsat the tilt beam supports. Typically, launch barges are highlyreinforced at the launch end, and together with the tilt beamsare rated with a maximum reaction capacity. On the otherhand, the barge section modulus is normally dose to the nom-inal value required for all ocean service barges, and the hullbending stress must therefore be closely monitored.

Both the tilt beam and hull girder loadings may be moder-ated to some extent by the prudent positioning of ballast priorto launch (see Figs. ISa and ISb). Increasing the barge trimhas the effect of immersing more of the jacket early in thelaunch, which results in an increased buoyant force that reducesthe load on the barge. While a high trim tends to add to hullbending stress, placing the ballast near midship reduces theinherent hogging moment on the barge.

Jacket rotation is also a critical stage in the launch process dueto the fact that the jacket is supported only by a short span oftilt beam. In general, the more of the jacket that is immersed,the lower the jacket stress will be due to the buoyancy of thesubmerged section of the structure.

The barge's transverse and longitudinal stability will decreaserapidly if the barge is allowed to trim to such angles that eitherthe bow emerges or the stern submerge in the water. As anexample of the trim effect. d) namic stability has been calcu-lated for a range of trims for barges of various depths (Fig.16).

The use of a high trim angle produces another hazard, thatof jacket stalling. High trim increases the probability that thejacket will slow down on entering the water, and thi slowingeffect may cause the structure to stall, or "hang up," on thebarge. In this case, both the drag and buoy ant forces on thejacket act to prevent rotation and separation. When high trimangles are inevitabl in a particular launch, a light draft is oftenused to minimize jacket submergence prior to Stage 2.

~ hile the launch operation is of short duration and generallyperformed under good weather conditions, the possibility existsthat the jacket may hang up during launch and remain on thetilt beams for some period of time. A major concern is that thejacket may skew on the launch rails or launch in an unpre-dictable fashion. Either of these situations can lead to damageof the jacket or barge. The authors feel that an adequate cri-terion would be to require that the barge heel no more than theangle at which the jacket would begin to skew (2 to 5 deg) ina nominal beam wind. The value for wind speed used couldbe determined based on the maximum one-minute averagewind to develop in 24 hours from an initially calm sea state.

Two other items of importance addressed by a launch sim-ulation concern the maximum ubmergence of the jacket as itclears the barge ( tage 4), and the final attitude of the structurein the water. The trajectory of the jacket is normally governedby the initial draft and trim of thebarge, along with the typeof lubrication used on the jacket runners.

A designer can control additional items, such as extent ofadded weight (skirt piles, boat landings, etc.) and addedbuoyancy (flotation tanks) affixed to the jacket at launch. The

- -

Fig. 14(b) Launching of the 700-ft-long. 10 OOO-tonChevron "GardenBanks" jacket. The structure is being launched in the Gulf of Mexico

from the 66 OOO-dwt Brown & Root launch barge BAR 376

Practical Design Approaches for the Analysis of Barge Performance 213

AI'T•••--'-F.:.:.WD=---_ It •., •..2· %L

L=914" (~OO')B=27.4" (90')6=9150 T. (9000LT)

2 o IBALLAST LOCATION

~0.4a::I

2

2a::«0.3C)Z;::rC)

C;:0.2

2:J2X«200.1~«L.Ja::«

DEPTH= 7.6 ••

DEPTH = 5 I ••

IlL X 100'1.

Fig. 15(a) Variation of barge stresses with longitudinal position ofballast

oJ/

f

'">'" TIoJ

II)

'"'" T2or•...'":2-e~~oJ;:

TRIM

Fig. 16 Variation of dynamic stability with trim for various bargedepths

models during launching and upending can be easily inter-preted visually for comparison against analytically predictedresults. This method of verification is particularly useful formore sensitive analysis when' computation is lacking.

A few modeling problem will usually exist due to the com-plexity of a jacket structure. It is important that the modelshould represent closely the full-scale structure in ,weight. inertiaand shape. While the barge i usually simple to construct, theoff-the-shelf range of miniature tubing sizes may determinethe scale factor between the model and full-size structure.

Buoyancy calculations normally are performed to ensure thatthe buoyancy of each level and frame of the jacket is correct,so that even though some members are not exactly scaled, thefinal hydrostatics of the model and prototype will agree closely.Finally, both barge and jacket models must be accurately bal-anced and ballasted to the correct CG position and inertia.

By using the similitude relationships between the model andthe prototype based on Froude number scaling, the motion,force and time measurements can be transferred quantitativelyfrom the model to the prototype. It should be noted thatdrag-induced forces, which arise from the viscosity of the water,cannot be scaled to the same ratios as acceleration-inducedforces. The viscous drag will be slightly higher on the modelthan on the prototype.

The motions of a jacket on the barge in waves are one of themost important aspects of the model test. Regular wave results,in terms of amplitude and phases at different frequencies ofinterest, are recorded for deriving motion RAO's, which canthen be readily compared with theoretical results. Particularattention should be directed to getting the roll motion RAO'sin beam seas, where the nonlinear viscous damping andadded-mass effects are important. Several tests using differentwave slopes should be used to check linearity as umptions.Other motions and accelerations are often measured at thejacket CG and at those extreme locations where the highest localinertia loads tend to act.

For launching and upending tests it is extremely important.to closely simulate the actual properties of the barge's skid andtilt beams. Often. small variations in properties such as thesliding coefficient of friction between skid beam and launchrunner will have a strong influence on the launch procedure.Therefore, the sensitivity of launch and upending to initialbarge trim, friction coefficients. and variations in the centersof gravity and buoyancy of the jacket isoften tested. Standardprocedures have now been developed by most of the reputabletank facilities for these types of te ·ts.

214 Practical Design Approaches for the Analysis of Barge Performance

• FWD lit AFT.--~~----~~----~O------~;----~2~--"·%LBALLAST LOCATION

Fig, 15(b) Variation of tilt beam stresses with longitudinal positionof ballast-

designer will be evaluating all options in terms of their abilityto reduce jacket and barge stress as well as their potential toimpede the progress of the launch, so that the launch analysisminimizes the risks involved in the launch process.

Model testsFor practical consideration in a transportation study, scale-

model tests are often necessary, in addition to the analytical.methods, in order to confirm and verify barge selection. A·model test offers an analog representation of the true physicalcircumstances while analytical methods provide a quantitativeassessment of s stem d namics. A designer often must exhibita fair knowle ge 0 ,an experience in, practical operations inorder to combine the two results, given the experimental errors,or scale effects, in modeling and the simplifying assumptionsmade in theoretical models.

A model test program could consist of the two followingcomponents in order to meet the requirements of a towingtransportation study.

l. Towing testL~ resistance tests in calm water and waves

• seakeeping stability tests in severe sea states2. Launch tests:

• sea eeping tests during launch• launch and upending simulation

These test programs would enable the designer to confirmthe transportation design analysis by determining the barge/jacket stability, motion and acceleration. The behavior of scale

cedure has been proposed to assess the level of risk based on theencounter probability of the predicted maximum sea state,probability of no damage, and other mission-related statistics.It is hoped that further developments in this area will providea rational approach for evaluating on a common basis safetyfactors for a transportation operation.

Finally it should be stressed that a successful jacket deploy-ment operation involves many phases of careful planning fromload out, tow, and launching to jacket upending. The paperhas addressed some important aspects of the transportation andlaunching phases of the operation. As practical experience andresearch efforts continue to accumulate, the engineering dis-ciplines in the marine field can better respond to industry'sdemand for effective, safe offshore transportation and instal-lation procedures.

Summary and conclusionsThi paper has attempted to present, in a unified and sys-

tematic manner various analysis techniques involved in thedesign a~d eval~ation of an offshore jacket transport~tion op-eration. Appropriate discussions of rules and regulations havebeen included in view of the lack of industry-wide standardsfor the area.

The methodology outlined in the paper is not intended todetail the unique problems of a particular barge /jacket undertow. Rather, the procedures have been given as a generaloverview of the steps in the process, to be used as a guide forindividual planning in carrying out a transportation study. Inthis respect, examples based on past experiences have beenincluded to illustrate various tradeoffs between static stabilityversus dynamic loading, level of detail of analysis versus re-source availability, and so on.

The state-of-th~-art development of naval architecture andstructural analysis continues to provide more tools for designand investigation of the complex interaction between stability,motion, strength and risk levels in a transportation study, andvarious options are now available to designers to ensure thesafety of a jacket under tow. Based on past experiences incarrying out these studies, a summary of the conclusions andrecommendations for continued development efforts fol-lows.

1. Although the standard methods for wind force andmoment calculations differ somewhat in their detailed proce-dures, they are generally in agreement between various clas-sification societies. A more uncertain area, however, is de-termining the maximum design wind condition and sea statefor the tow. Various classification societies have specified 50-or lOO-year return periods similar to fixed offshore structuredesigns. It is felt that a more appropriate design criterionshould be established based on the risk levels, such as encounterprobability, which takes into consideration the voyage dura-tion.

2. Stability criteria for deck cargo barges have been largely Referencesderived from ship and offshore mobile drilling units. Further 1 Blight,G. J. and Tuturea, D. P.. "The GEMINI Methodof In-research into the actual mechanism of barge capsizing, in- stallingDeepwater Platforms," SNAME,Gulf Section, March 1978:eluding factors such as water on deck and restoring force from 2 Blight, G. J., "HIDECK," SNAME,Gulf Section West, Apnljacket member immersion, is necessary to determine an ade- 1978.

d d b I 3 Martin,M.R.,"What toExpectin the Wayof MarinePlatformsquate level of barge/jacket intact an amage sta i ity. to Corne" Offshore, Nov. 1972.3. Tradeoffs between static stability and dynamic loadings 4 Moss,J. L. and Townsend, G J., III, "Desig~,Considerations

induced by barge motion are possible for certain types of and Resistanceof Large Towed Sea-GoingBarges, SNAMET&Rbarge/jacket combinations. The designer is advised to inves- Bulletin 1-29, 1969.tigate various alternatives within the constraints of stability, 5 Blight,G. J. and Dai.B. Y.T., "Resistanceof Offshore Bargestaking into consideration the predominant wave excitation and Required Tug Horsepower," Offshore Technology Conference,

I d OTC Paper 3320, Houston,Texas,May 1978.periods Other design options, such as possible ro 1- am ping 6 Frank, W., "The Frank Close-Fit Ship Motion Computerdevices. should also be considered. Program," aval Ship Research and Development Center, Report

4. Local damage on overhanging jacket members due to 3289,1970.slamming isoften a concern in a tow, and the exact impact force 7 Kim,G H. and Chou,F., "Wave-ExcitingForcesand Moments

h on an Ocean Platform in Oblique Seas,"Offshore Technology Con-on the jacket is still an area under researc . Computer simu- ference, OTC Paper 1180, Houston,Texas,April 1970.lations and model tests should be performed to investigate the 8 Ochi, M. K. and Motter, L. E., "Prediction of Slammingdegree of seriousness of such impact, and to gain insight into Characteristicsand Hull Responsesfor ShipDesign,"TRANS. SNAME,the source of the dynamic effects of slamming. Vol.81, 1973.

5. Several levels of structural analysis for both the jacket 9 Miller,B.L., "Wave SlammingLoadson Horizontal CircularI d Elements of Offshore Structures," Trans. RINA, 1977.and barge have been outlined and their re ative merits is- 10 Pierson,W. J., NeumanG.:andJames,R.W., Practical Method

cussed The choice of technique may largely depend on ex- for Observing and Forecasting Ocean Waves by Means of Waveperiences with the type of barge/jacket for a similar tow route, Spectra and Statistics, PublicationNo. 603, United States avy Hy-and on available resources for the study. Generally, for a long drodynamics Office, Washington, D. G, 1955.voyage where barge as well as jacket structur.al damage i.s~f 11 Bretschneider,G L., "Revisionand Waves Forecasting,Deepcritical concern, it is recommended that a detailed probablistic and ShallowWater," Proceedings, SixthConference on CoastalEn-

I d be gineering, American Society of Civil Engineers Council on Waveanalysis of maximum stress level, fatigue and loca amage Research, 1958.carried out. For relatively short towing operations a less-de- 12 Cardone, V.J., Pierson,W. J., and Ward, E. G., "~indcasting,tailed, standard type of calculation may be adequate to back the Directional Spectra of Hurricane-Generated Waves, Journal oJup past experiences. Petroleum Technology, Vol.25, 1976, pp. 385-394.

6. In the area of risk assessment for a towing study, a pro- 13 Chen, H. T., Hoffman, D., and Chen, H. H., "The lmple-

Practical Design Approaches for the Analysis of Barge Performance 215

AcknowledgmentsThe authors are indebted to Brown & Root, Inc., whose

sponsorship made this paper possible, and they appreciatedeeply the encouragement and support given by Mr. J. C.Lochridge, vice president, the late Mr. W. A. Morgan, the latesenior department manager, and especially by Mr. DavidKummer, senior engineer.

This paper includes a large amount of information fromregulatory and consultive organizations. The authors wouldparticularly like to thank those at the U. S. Coast Guard, Detnorske Veritas, the National Maritime Institute (D.O.l., u. K.),and Noble Denton and Associates, Ltd. for their help in pro-viding the information.

Special thanks are also due Mrs. K. Fonda for her dedicationin typing the manuscript, and to Ms. M. E. Archer for her greatassistance as our technical writer and editor.

The opinions expressed in this paper are those of the authorsand do not necessarily reflect those of Brown & Root, Inc.

r·

50

enII:••••r-••••:J!z 40

..J

~(f)

.... 30>0CD«r-:I:<!)

20••••:I:

10

70

60

OL- ~ ~ ~ ~ ~~ ~~1.0 1.1 1.2 1.3 1.4 1.5 1.6

HEIGHT COEFFICIEN T CHG OR "rr;.;Fig. 17 CHG, height and gust coefficient, DnV; CH, height coefficient, IMea

mentation of 20- Year Hindcast Wave Data in the Design and Opera-tion of Marine Structures," Offshore Technology Conference, OTCPaper 3644, Houston, Texas, 1978.

14 Summary of Synoptic Meteorological Observations, U. S.Naval Weather Service Command, National Climatic Center, Ashe-ville, N. G, 1978.

15 Hogben, . and Lumb, F. E., Ocean Wave Statistics, NationalPhysical Laboratory, United Kingdom, 1967.

16 Hoffman, D. and Miles, M., ..Analysis of a Stratified Sample -of Ocean Wave Records at Station ZANDIA," SNAME, Panel H-7,1976.

17 Snyder, Eric D., "Capsizing of Deck-Loaded Barges in IrregularBeam Seas," Research Report 48104, Department of aval Architec-ture and Marine Engineering, The University of Michigan, Ann Arbor,Michigan, July 1974.

18 Towing Operations Guidelines and Recommendations forBarge Transportation, Det norske Veritas Report No. 78-404, Oslo,1978.

19 General Guidelines for Transport of Modules on Barges inNorthern European Waters, oble Denton & Associates,Ltd., London,June 1978.

20 Stability Criteria for Barges, U. S. Coast Guard, Technical NoteNo. 3-69, 1969.

21 Rules for Construction, Designing and Inspection of OffshoreStructures. Det norske Veritas, Oslo, 1977.

22 Guidance on the Design and Construction of Offshore In-stallations, Department of Energy, Her \\ajestys Stationery Office,London, 197-t.

23 Requirements for Verifying the Structural lntegritu of OCSPlatforms, Prefared by American Bureau of Shipping, ew York,U. S. Geologica Survey, 1978.

24 Rules and Regulations for the Construction and Classificationof Offshore Platforms, Bureau Veritas, Paris, 1975.

25 "Safety Measures for Special Purpose Ships," Code for theConstruction and Equipment of Mobil Offshore Drilling Units, DEXIX/6, I~ICO, 23 March 1978.

26 Rules for Building and Classijicatton=Ojjshore MobileDrilling Units, American Bureau of Shipping, 1973.

27 Requirements for Mobile Offshore Drilling Units, Departmentof Transportation, U. S. Coast Guard, 1978.

28 "BARMOT User's Manual: Barge Motion Computer Pro-gram," Marine Engineering Division Publication, Brown & Root, Inc.,Houston, Texas, 1979.

. '"29 Karlan, P. and Gilbert, M. ., "Impact Forces on PlatformHorizonta Members in the Splash Zone," Offshore Technology Con-ference, OTC Paper 2438, Houston, Texas, 1976.

30 "DAMS User's Manual Level I: Design and Analysis of MarineStructures," Marine Engineering Division Publication, Brown & Root,Inc., Houston, Texas, 1978.

31 "SEA LOAD User's Manual," Marine Engineering DivisionPublication, Brown & Root, Inc., Houston, Texas, 1979.

32 Longuet-Higgtns, M. S., "On the Statistical Distribution of theHeights of Sea Waves," Journal of Marine Research, Vol. 2, No.3,952.

\ 33 Chen, H. T., "Long Term Prediction of Offshore Vessel Re--sponses for Design and Operability Evaluations," Offshore TechnologyConference, OTC Paper 3800, Houston, Texas, 1980.

34 "TPFATIG User's Manual, Transportation Probabilistic FatigueAnalysis," Marine Engineering Division Publication, Brown & Root,Inc., Houston, Texas, 1980.

35 Borgman, L. E., "Risk Criteria," Journal of Waterways andHarbor Division, Proceedin¥.s, ASCE, Aug. 1963.

36 Robishaw, Paul A., . Flexifloat Construction Systems," Rob-ishaw Engineering, Inc., personal correspondence, Houston, Texas,Oct. 1978.

37 Andrews, Harrison B., "Launching," Principles of Naval Ar-chitecture, J. P. Comstock, Ed., 51 AME, 1967, pp. 752-781.

:38 "Notes on Transverse Stability on Floating Vessels, Freeboard,Bulwarks and Freeing Ports. Hatches and AccessOpenings," DynamicsASSociated u.it]: Rolling, United States Salvage Association. Inc., NewYork. N. Y., 1968.• 39 Hutchison, B. L. and Bringloe, J. T., "Application of Seakeeping

216~

Practical Desiqn Approaches for the Analysis of. Barge Performance

Analysis," Marine Technology, 'Vol. 15, o. 4, Oct. 1978, pp. 416-431

40 "CARGO User's Manual," Marine Engineering DivisionPublication, Brown & Root, lnc., Houston, Texas, 1980.

41 "FLAPS User's Manual. Flotation and Launching AnalysisProgram," Marine Engineering Division Publication, Brown & Root,Inc., Houston, Texas, December 1977.

Appendix 1

Wind moment assessment methodsThree levels of sophistication in wind moment assessment

are presented here. The simplest and usually the most con-servative is the USCG deck cargo barge method [20J, which isdependent only on barge particulars. It is most often used indetermining the maximum allowable VCGc for each draft.The second method is used by most OMDU rules and entailsbreaking down the windage area into component parts andapplying height and shape coefficients. The most sophisticatedmethod is found in design and construction (D&C) rules foroffshore structures, and entails a detailed member-by-membercalculation using height, shape and shielding effects.

In each case the computation may be broken into two parts,the effects and the area, shape and shielding effects. Note thatonly USCG [27J, IMCO [25J, and DnV [21J formulas are pre-sented; other approaches are similar in most respects.

Wind pressureIn the USCG rules [27J a constant wind pressure is assumed

over the entire windage area, and is dependent on barge length,as noted in Table 2 of the paper.

In OMDU rules a wind speed may either be a predicted valueor an appropriate assumed value prescribed by the rules, Thewind speed varies with height according to a tabular heightcoefficient based on the one-seventh power law:

where

q = wind pressure in kg/m2 (Ib/ft2) for memberk = constant = 0.623 (0.00338)

CH = height coefficient (from IMCO [25], Table 2)V = wind speed in m/s (knots)

The wind speed used in D&C rules is also either predictedor prescribed based on severe storm conditions. This windspeed is then modified to account for height, gust and angle ofincidence for each member or section of a projected area (Dn V[21 J), where

where

V1hrlO = wind speed, I-hour (h) averaging period, 10 mabove SWL

p = air density ~ 1.225 kg/m3 (0.0765 Ib/ft3)

CHS = height and gust coefficient= O'(Z;/1O),8, Z in meters= 0'(Zd32.8)B, Z in feet

(j = incident angleZi = height of member above water surface{3= height coefficient dependent on wind averaging

period (from DnV [21J, Table A.l)0' = gust coefficient based on wind averaging period

(from DnV [21J Table A.l)

Figure 17 compares CHG with the square root of IMCO

WInd Pressure111 • ~ (a (ZI )6 V1hr10)2 sin e

TOa and 6 from Table A.1

Spacing RatioQ & d/B

Area SolidityS • ~ • aShielding Factor" & 1.0TableB.1

No

Is Item - 1 Open Truss 2or 2 Single Member or Surface

Shape CoefficientCs • 0:: CooTables B.2. 8.3.B.4. and 8.5

(35)Yes

Fig. 18 Wind moment calculation by DnV method (tables mentionedrefer to reference [21))

(36)

values of CH for heights from 0 to 70 m (230 ft). From Fig. 17it appears that the averaging period to be used for OMDU rulesis greater than 1 h.

Area, shape and shieldingThe USCG rules [27] are generally used with the block

windage area assumptions as given by equations (9a) and (9b).These assumptions are best used when the solidity ratio of thedeck cargo approaches 1.

The OMDU rules commonly present a table of shape coef-ficients to be used in the area calculations. Component areasmay be calculated by thefollowing rule:

(37)

where

Aj = effective area of member or membersGj = projected area of member

Cs = shape coefficient (from IMCO [25J, Table 1)

In OMDU rules, shielding may be accounted for in truss-type

Practical Design Approaches for the Analysis of Barge Performance 217

"-.

structures by applying a shape coefficient of 0.3 to the block(or outline) area of each truss face.

The windage area calculations for D&C rules may be quitecomplex, due to the fact that this method normally applies todefining wind forces for structural loading and in situ over-turning moments. The basic equation for elemental area iseither (DnV [21]):

(38a)

where

ql,AI = as defined in equations (35-38a,b)hi = vertical distance from center of pressure to center

of resistanceN = number of area elements

The moment should be calculated at a sufficient number ofheel angles to define a heeling moment (arm) curve. Forvessels with ship-shape hulls the moment is assumed to varywith the cosine of the heel angle.

Appendix 2

(39)

Typical towing approval proceduresThe following is a list of typical calculations, drawings, and

procedures required for a towing approval.

• Loadout plan• Barge arrangement, capacity plan and ballast system

drawing• Main and emergency towing arrangement• Barge and jacket structural drawings, including seafas-

tening• Tug specification• Tug bollard pull calculation, including barge resistance

prediction• Ballasting and stability study (intact and damage for transit

and launch operation)• Weather and route trip prediction, including points of

shelter• Barge/iacket motion response• Study of loads and stresses in barge, jacket and seafas-

tening• Logistics procedure (command and communication sys-

tem, emergency procedure, methods of handling andsecuring jacket after launch and upending proce-dure)

• Crane barge specifications• ' Study of jacket behavior during launching, flotation and

upending

or

(38b)where

1/ = shielding factor from [21] (Table B.l) based on ex, {3= 1.0 for windward faces

Cs = shape coefficient from [21] (Tables B.2, B.3, B.4, andB.5)

= kCa>aj = projected area of member in direction of wind

Ce = effective shape coefficient from. 21] (Table B.6) basedon 8 and He

1> = solidity ratio = projected area of truss members dividedby block area of truss normal to wind

hi = block area of trussex = spacing ratio--distance between member centers di-

vided by least dimension of b,Re = Reynolds number

{3 = aerodynamic solidity ratio = 1>a from [25] (TableB.1)

A flow chart of these calculations is given in Fig. 18. The Dn Vwindage area calculations would normally be used only in thetransportation analysis when areas determined by either of theother methods appear overly conservative for stability pur-poses.

Wind momentsThe wind moment used in the USCG rules [27] is given by

the P X A X H defined in equation (4).The wind moments for both OMDU and D&C rules are

calculated by

Discussion. .;: ,

Robert Latorre, Member, Frederick Ashcroft. Membe~nd'Stuart Cohen, Member 'J- -

The authors are to be commended on their compr he~s.ivediscussion of the factors in selecting an acceptable ba~ge/jacketconfiguration for towing and launching offshore ttructures.Our questions concern another aspect which is brieHv men-tioned, the coursekeeping behavior of the towe((barge.Typically when towing such large structures as shown in thefront-is-piece photo two or more towing tugs may be employed.However, for the smaller launch barges in Table l, the bargemay be towed by a bridle and towing hawser attached to asingle t~g. With the large deck cargo the lorigtrudinal shift inthe tow s center of gravity could affect the yawing and swayingof the towed barge. Have the authors any experience in howthis has affected the course keeping ~rformance of the towedbarge? e ,

At the University of Michigan, re istance tests as well as thetowed barge coursekeeping performance model tests are rou-

tinely made. It is our experience that the coursekeeping per-formance can improve with properly designed skegs. How-ever, since the skegs add resistance to the barge hull there issome trade-off between the course stability and the added skegdrag [42,43,44] (additional references follow some discussions).The authors mention using bilge keels to reduce the bargerolling. We would like t? know what is the effect of the bilgekeels on the towed barge s coursekeeping performance?

We concur with the authors' statement that the designer'sconcern is that of motion control. It appears that while ex-cessive trim is undesirable, some trim by the stern may improvethe towed barge coursekeeping performance. To illustrate ourpoint, Fig. 19 shows the barge lines and detail of the skeg witha movable flap. This barge is a notched stern barge usedin a previous study [44]. Its particulars are summarized inTable 7. The trajectory of a light mounted at station 1 of thebarge was recorded by means of an optical tracker during thecoursekeeping test. Several skeg flap angles were used and thecorresponding trajectories are compared in Fig. 20. Starting

Practical Desiqn Approaches for the Analysis of Barge Performance2 8

--------- -------LOwER TANGENT

------------------------~~---L--~~--~CL

18 9 4

F:=:I! :C~!

~'r======='f==· ====,==:::::::/H~.c--+-cT -.------------ 14 SO'

- 17 CO' ~

17 16 15 1420 19

15 14

I 17

£.

~ OF RUOO£,q 5700<

I,

;3.CO· ,,,/

I

rI

/

skeg details showing movable skeg flap

8 7 o5 3 26

Fig. 19 Barge lines

from offset of 9 ft the barge's yaw and sway motions wereminimum at the lO-deg setting. In Fig. 21 it is clear that thetrim by the stern reduces the sway and yawing of the barge andimproves the coursekeeping performance. Have the authors

. considered using the ballast tanks to obtain a suitable trim forboth seakeeping and coursekeeping?

We are grateful to have this opportunity of discussing the

I-en

~II

FULL LOADenl EVEN KEEL

Vs 6 kts

- ~----'--'----'---'--~-'----'---'

TOWED BARGE MODEL TRAJECTORYEFFECT OF SKEG FLAP ANGLEKEY A-TESTS

1 0 DEG2 10 DEG3 15 DEG

START 48

Fig. 20

96 144DISTANCE, feet

Effect of skeg flap angle on towed barge model trajectory

192 240

coursekeeping performance of towed barges and congratulatethe authors again on their fine paper.

Additional references42 Latorre. R. and Ashcroft. F., "Recent Developments in Barge

Design, Towing, and Pushing," Marine Technology. Vol. 17. No.1.Jan 1981, pp. 10-21.

TOWE D BARGE MODEL TRAJECTORYEFF ECT OF BARGE TRIMKEY A-TESTS

1 NO TRIM2 TRIM BY STERN

I~~

lO,nches Full scal~ K /2 -V ~ :;:/

VFULL LOADV 15 DEG FLAP ANGLEV!, 6 kts

II I

Ia,

~I~(en

ISTART ~ % ~ 1~ 2~

DISTANCE, feetEffect of stern trim on towed barge model trajectoryFig. 21

Practical Design Approaches for the Analysis of Barge Performance 219

Table 7 Principal particulars of barge model

Name Symbol Model (X = 40) Barge

Length LWL 3.788 ft 351.50 ftBeam B l.500 ft 60.00 ftFull load Taft 0.:349ft 13.96 ftDraft Tfwd 0.349 ft 13.96 ftWetted surface S 13.531 ft2 21 649.00 ft2Displacement 'V 3.752 ft3

6 233.70 lb (68°F) 6861 LTSW (59°)Block Coefficient CB 0.816 0.816. otch length I" 1.5 ft 60.00 ft

:"JOTES: 1. Towing bridle with legs equal to one beam was con-nected at corner of barge head log.

2. Tracker light mounted at station 1 in Fig. 19.3. Monofilament line used for towing hawser material.4. Initial offset at 9 ft or six model beams.

-t.'3 Latorre, R., "Improvement of Barge Towin~; Translations ofSelected Japanese and Russian Technical Articles, ' Department of'.Hal Architecture and Marine Engineering Report, niversity of\lichigan .. Ann Arbor, Michigan, Report No. 226, May 1980.

l-t Latorre, R., Ashcroft, R., and Cohen, S., "Investigation of BargeTowing Performance, Phase I Experiments," Proceedings, 19thAmerican Towing Tank Conference, Ann Arbor, Michigan, July1':1. O.

:

W. P. Stewart, Member

This comprehensive paper emphasizes that barge roll motionis relatively lightly damped and tends to exhibit large-ampli-tude resonant response. At large amplitudes, however, lineartheory breaks down, added mass and inertia terms vary and thebuoyancy force becomes highly nonlinear. Hence, the responseamplitude is not necessarily a function only of damping atresonance. This problem is compounded when the vessel haslarge overhanging cargo which partly immerses during a rollcycle.

Depending upon the wave height and frequency, and cargogeometry, the cargo may pick up hydrostatic and hydrody-namic forces which are in phase with the diffraction roll mo-ment and consequently increase roll response amplitudes.Alternatively the phasing of the forces may be such that theyoppose the diffraction roll moment and roll response will bereduced. In a recent study using a 3-D time-history program,the nonlinear buoyancy forces acting on the cargo acted inphase with the primary forcing terms and resulted in a rollmotion amplitude of 30 deg. Reducing the cargo overhangchanged the phasing of the cargo-induced buoyancy forces andresulted in a roll amplitude of only 13 deg in the same waveswith the same mass distribution.

The paper rightly emphasizes the importance of model testswhich must be used to calibrate computer programs. It iscommon practice to adjust the roll damping in otherwise linearprograms so that peak resonant response predicted matches thatfound in the model test, assuming other parameters to be cor-rect. This may be highly erroneous especially where cargoimmersion takes place.

One of the primary reasons for marine deck cargo loss ordamage is towline failure. The prediction of towline tensionsis generally restricted to the calculation of the mean staticcomponent. and a safety factor of typically two times the tugbollard pull is used to take account of the dynamic compo-nent.

Recent research with a North Sea barge tow shows that evenin moderate weather (-t-m significant wave height) the dynamictension component in the line can result in peak tensions oft\\ ice the mean value and that the ratio of dynamic to meanforce increases with increasing wave height.

More offshore tow monitoring is required to give greaterinsight into barge hydrodynamics and the problems of towlinefailure. Monitoring enables response to multidirectional seastates to be measured and enables greater control of the towingoperation.

Bruce L. Hutchison, Member

The authors have presented an interesting and useful papercovering many different aspects of barge performance analysis.The presentation in Tables 3 and 4 of summarized barge intactand damaged stability requirements is a particularly usefulcontribution.

I would like to concentrate my remarks on the topics ofmotion calculations and strength as presented in this paper. Itshould be noted that equations (12), (lSa), (l5b), (l5c) and (16)in the preprint contain several printer errors. These errors havebeen noted in my correspondence with the authors and willpresumably be corrected in the TRANSACTIONS:

Some further comments are appropriate to the transforma-tion from earth to vessel coordinates. Equation (12) and theassociated transformation tensor imply sequential rotationsabout the z-axis (yaw), the y-axis (pitch) and the x-axis (roll).For rotations in reverse sequence the order of multiplicationof the individual transformation tensors would have to be re-versed and a different final transformation tensor would re-sult.

This points up the fact that the transformation tensor dependsupon the sequence of rotations. The transformation matricesare not cumulative and in general finite angles of rotationcannot be represented by vectors. Infinitesimal rotations canhowever be represented by vectors and this result can be viewedas acceptable since it falls within the assumptions underlyingmost ship motion analysis.

The assumption of small angles of rotation can be used tosimplify the transformation tensor by replacing trigonometricterms with their small angle approximations. Under thisscheme cosines are replaces by +1.0, sines are replaced by theargument angle, and products of sines are ignored. If this isdone the following skew-symmetric transformation tensor isobtained:

ITI = [-~ ~ -!]() -cJ> 1

This result would be obtained for small angles of rotationregardless of the sequence of rotation and therefore forms a veryuseful invariant basis for coordinate transformations.

Concerning equation (16) for the relative vertical motion,it should be observed that (following correction of the errorsin the preprint) care must be taken before applying this ex-pression to determine the precise expression for the incidentwave and the exact definition of the response phase angle.These factors may necessitate some modifications to equation(16) depending on the specific definitions.

The authors in their section on strength make the observationthat their utilization of the participation factors involves manySimplified assumptions and may not realistically represent theactual conditions under tow. In this assessment they are correctand I would like to offer the following observations on how theituation can be improved.

First I would observe that it is not necessary to restrict theanalysis to unidirectional seas, as we [391also derived the co-factors in a directional sea spectrum. Second, and most im-portant, the determination of the cofactors need not be re-stricted to the motions at the combined center of gravity. Thecofactors can be derived at whatever point is of interest (for

.::20 Practical Design Approaches for the Analysis of Barge Performance

example, at the location of a mass or structural element). In-deed, failure to do so may lead to serious errors. For example,Table 8 with this discussion, taken from program CARGO 139],shows the variation with location in the cofactor C:y for a 400-ftbarge in beam seas with TJ = 11.5 sec. The advantage tocombined load analysis of evaluating the cofactors at the pointof interest should be apparent.

Finally, the development of the cofactor concept does notlead to the maximum of a process consisting of a linear com-bination of correlated vector processes. To obtain the maxi-mum of such a process the cross cospectral moment matrix isrequired. This matrix is an invariant for a given load condition,speed, heading and spectral shape. Once this matrix is derived,the spectral moments and thereby the response statistics for anyprocess expressible as a linear combination of the base vectorprocesses can be easily determined. A short technical note onthis topic has been submitted to the Journal of Ship Re-search.

Dennis C, Perryman,3 Visitor

From a meteorologist's point of view, it is refreshing to seethe amount of effort injected into the collection and utilizationof environmental data as outlined in this paper. In an attemptto further the engineering science, the authors have shown arealization that related technology must also keep pace. Thisis evident from their statement that spectral wave models aresuperseding PNJ, 5MB manual hind cast techniques. Exceptionmight be taken, however, of the statement that "no model yetavailable has been able to accurately predict daily events .... "The discusser's firm, as well as the u.s. Navy, uses an opera-tional version of a spectral wave model to predict directionalwave spectra at various grid points over a given geographicregion. We have used one such model in the North Sea dailysince 1976. These models are highly accurate in 24-hr forecastsand are able to provide useful forecasts for up to 72 hours. Agiven model may contain limitations in its ability to makepredictions for some regions as a result of man's inability toprovide accurate wind data as input, but, on the whole, themodels perform very satisfactorily and with consistently ac-curate results.

Very often, ship reports constitute the only available database with which to simulate a towing operation. As the authorshave pointed out, ship reports must be used cautiously. Analternative to ship reports is the Spectral Ocean Wave Model(SOWM) data generated by the u.S. Navy. These data areoutput twice daily and are already gridded. The fair-weatherbias of ship reports is omitted in the SO\\'M data, and gridspacing is sufficient to yield representative profiles of wind andwave statistics. Using SOWM data contributes directly to themain advantage of a tow simulation-repeating the voyagethrough past weather conditions for a number of years. Theauthors have noted this technique as being useful in derivingdesign criteria and exceedance statistics.

The practical design criteria and related references as tab-ulated by the authors are highly commendable. If more in-vestigations of this type were to be carried out so that a sum-mary table would not be necessary, a real benefit to the offshoreindustry would be realized. Convincing marine surveyors toagree on the design criteria would be a monumental achieve-ment, second only to getting naval architects to agree on any-thing!

A reliable method of determining design criteria for thetowing operation is of utmost importance when considering thefact that many structures today are towed in much worse con-ditions than will ever occur at the launch site for the entire lifeof the structure. The authors have made a good case for put-

Table 8 Acceleration cotactors, Cry

Heig htAbove Y = -46.7 )" = 0.0 ). = 46.7

Deck, ft ft It ft

0 -0.H6;; -O.I;,!! -U.2:l:l1;. -O.HIlI -0.684 -0.061:lO -0.844 -0.;;96 0.04,

ting as much time and effort in the tow design as was expendedin the structure's design.

Kaare Lindemann,4 Visitor

Barge transportation is a matter of concern to regulatorybodies classification societies and marine survevors. Theprinciples and methods used by such bodies to e~·aluate thesafety of barge transportation are well described in the paper.The authors have also noted the important difference in limitsbetween ship and offshore design and barge transportation.

In ship and offshore design, the design philosophy is that thestructure should have a good chance to survive a lifetime op-eration. This means that the structure should survive all pos-sible loads which, it has a fair chance to be exposed to duringits lifetime. In popular terms this is often expressed as the20-year wave for ships and the 50 to 100-year wave for offshorestructures.

For barge transportation the philosophy is parallel. but inpractice the extreme condition used for barge transportationis different from that of ships and offshore structures. The timespent on the tow is an important parameter, often resulting inless strict criteria. In fact, barge transports may be permittedwith strict limits on permissible environmental conditionsprovided possible points of shelter can be reached before a stormrises.

The methods available, however, are not always completein their description of the problem, and some uncertainties areassociated with the results obtained. Among others I canmention barge roll motion, relative motion, slamming phe-nomena, fatigue analysis and barge vibratory responses(springing). In addition the operational limitations are notalways well defined. When tows are to be approved basedupon such analyses the limiting sea state is even more uncertain,resulting in large safety factors.

Much is to be gained in barge transport efficiency, economyand safety by concentrating efforts in obtaining more knowl-edge on limiting conditions and the phenomena which causethem and methods used to assess them. The state of the art inbarge transportation analysis techniques and hence the pro-cedures used to evaluate the safety of a transport is not satis-factory and further development is required.

A new dimension should also be added to the design andoperational evaluation, namely, the capabilities of the operatoror towmaster. A careful transport evaluation based on a soundtechnical analysis may be jeopardized by an operator who doesnot fully understand or who is not capable of assessing thelimiting conditions. Hence rules and regulations should bedeveloped further to take into account certain minimum re-quirements for operator education and training. Requirementsshould also be made for instrumental aids, putting the operatorin a position to better evaluate the conditions.

This is an area where marine research has been neglected,and more emphasis should be placed on such studies in the yearsto come.

r.

I.

, ,!

3 Oceanroutes, Inc., Palo Alto, California. 4 Det norske Veritas, Oslo, Norway.

Practical Design Approaches for the Analysis of Barge Performance 221

1 ~ __=- ~ __~ -, ~--~~~ /