1

Prepared for the LL Working Group Meetingat 29-30 April 1997 in Shanghai, China.

Reprinted: 25-09-2001Website: www.shipmotions.nl

Report 1093-P, April 1997,Delft University of Technology,Ship Hydromechanics Laboratory,Mekelweg 2, 2628 CD Delft,The Netherlands.

COMPARATIVE MOTION CALCULATIONSOF FLOKSTRA CONTAINER SHIP MODEL

J.M.J. Journée

SUMMARY

The "Sub-Committee on Stability and Load Lines and on Fishing Vessels Safety" studies a revisionof the technical regulations of the 1966 International Convention on Load Lines (1966 ICLL).A "Working Group on Revision of the 1966 ICLL" and an "Inter-session Correspondence Group"assist in this field. Comprehensive research on freeboard requirements has been carried out inseveral countries. These investigations are based on calculated vertical relative ship motions, ineach country obtained by its own ship motions computer code.Among others, studies on theoretical approaches for load line definitions were carried out in Japan,Germany and China. In a previous report of the Dutch members of the "Correspondence Group"these approaches have been compared mutually for three ship types, using only one ship motionscomputer program. These data have been compared with the freeboards and bow heights requiredby the rules of the 1966 ICLL.For one of these ship types, a 270 meter container vessel, extensive model experiments on shipmotions in waves has been carried out last year by the China Ship Scientific Research Center(CSSRC). Comparisons with their strip theory predictions have been made too.In this report, a comparison is given between the experimental and theoretical data for this shipobtained in China and the results of the computer program which was used in the Dutchcomparative study. A fair agreement has been found.

IITRODUCTION

The "Sub-Committee on Stability and Load Lines and on Fishing Vessels Safety" studies a revisionof the technical regulations of the 1966 International Convention on Load Lines (1966 ICLL). A"Working Group on Revision of the 1966 ICLL" and an "Inter-session Correspondence Group"assist in this field. Comprehensive research on freeboard requirements has been carried out inseveral countries. These investigations are based on calculated vertical relative ship motions, ineach country obtained by its own ship motions computer code.Among others, these studies on theoretical approaches for load line definitions were carried out inJapan, Germany and China. In a previous report of the Dutch members of the "Correspondence

2

Group", these approaches have been compared mutually for three ship types, using only one shipmotions computer program. Also, these data have been compared with the freeboards and bowheights required by the rules of the 1966 ICLL. For this purpose, the strip theory computer codeSEAWAY of the Delft University of Technology - see Journée (1992, 1996) - had been adapted.The resulting new computer program SEAWAY-R includes the regulations of the 1966 ICLL andthe theoretical approaches of each of these countries.This program had been used for comparative purposes, as presented by Journée et al. (1997). For anassumed safe and seaworthy ship, freeboard and bow height have been determined according to therules of the 1966 ICLL. For this ship, the probabilities on shipping water at deck and at the bowhave been calculated from the vertical relative motions of this ship and long term weatherinformation as used in the theoretical approaches of the different countries, like for instance wavescatter diagrams. When using these approaches, these probabilities have been kept constant for alarge number of ships with a wide range of ship dimensions. This results in required freeboards andbow heights, by which the ship motions and the weather are accounted for. For the three approachesand for three ship types, these data have been compared with the freeboards and bow heightsrequired by the rules of the 1966 ICLL. Also, these theoretical approaches have been comparedmutually.

For one of the ship types in this study, a container vessel with a length of 270 meter and called herethe Flokstra ship, extensive model experiments on ship motions in waves have been carried out lastyear by the China Ship Scientific Research Center (CSSRC). Zhou et al. (1996) reports the results.Comparisons with strip theory predictions of CSSRC have been given in this report too.In the underlying report, a comparison is given between the experimental and theoretical data forthis ship obtained in China and the results of the computer program, used in the Dutch comparativestudy. A fair agreement has been found.

FLOKSTRA SHIP

In the past, the Flokstra ship was used by Flokstra (1974) of MARIN to verify his strip theorycalculations with comprehensive experimental data on ship motions and internal loads of a 1:55model of a container vessel. These model experiments were carried out at MARIN and reported byTan (1972).

Recently, Meier and Ostergaard (1996) of Germanische Lloyd have used this ship during relativemotion calculations for the "Working Group on Revision of the 1966 ICLL".

This ship was also subject of a comparative study of Journée et al. (1997) in the Netherlands on thetheoretical approaches for load line definitions of Japan, Germany and China.

Finally, extensive seakeeping experiments have been carried out with a 1:80 model of this ship byZhou (1996) of the China Ship Scientific Research Center (CSSRC) in Wuxi.

The principal dimensions of the Flokstra ship are given in Table 1 and the body plan of the ship -with a fictive depth of 24.85 - meter is presented in Figure 1..

3

Container ship

Length between perpendicularsppL (m) 270.00

Breadth B (m) 32.20

Draught d (m) 10.85

Block Coefficient BC (-) 0.597

Amidships section coefficient MC (-) 0.949

Center of buoyancy forward of 2/ppL ppCB LL / (-) -0.037

Length - breadth ratio BLpp / (-) 8.39

Breadth – draught ratio dB / (-) 2.97

Table 1 Principal dimensions of Flokstra ship

Figure 1 Hull form of Flokstra ship

4

EXPERIMENTS OF CSSRC

The model experiments have been carried out in the seakeeping basin of the China Ship ScientificResearch Center (CSSRC) in Wuxi. The dimensions of this basin are 69 x 46 meter, with a waterdepth of 4 meter.

A 1:80 ship model of glass reinforced polyester was constructed by CSSRC. The model has anintegral hull form including the hull form above the waterline and an integral deck form. During theexperiments, the model was equipped with bilge keels, propeller shafts, shell bossing, twopropellers and a rudder.

The principal dimensions of the ship and the model are given in Table 2.

Flokstra ship

Denomination Symbol Unit Ship Model

Length over all oaL (m) 284.000 3.550

Length between perpendiculars ppL (m) 270.000 3.375

Breadth B (m) 32.200 0.403Depth D (m) 18.662 0.233Draught d (m) 10.850 0.136Trim t (m) 0.000 0.000Volume ∇ (m3) 56097 0.110Block coeffficient

BC (-) 0.598 0.598CoG above base line KG (m) 13.490 0.169Metacentric height GM (m) 1.150 0.014CoG forward of station 10

CGL (m) -10.120 -0.126

Gyradius for roll xxk (m) 12.075 0.151Gyradius for pitch

yyk (m) 66.960 0.837

Natural roll periodφT (s) 24.900 2.780

Natural pitch periodθT (s) 8.600 0.960

Length of bilge keelsbkl (m) 47.000 0.588

Height of bilge keelsbkh (m) 0.480 0.006

Diameter of propellerspD (m) 6.560 0.082

Table 2 Principal dimensions of ship and CSSRC-model

During the tests, the model was self-propelled by two stock propellers and free to move with sixdegrees of freedom. It was kept on course by an auto-pilot, which controlled the rudder in such away that a straight course through the middle of the basin was maintained by small rudder angles.

5

The model was connected by a lightweight vertical rod in the center of gravity of the model to alow-mass and low-friction sub-carriage, so no appreciable forces were transmitted to the model.

The model experiments, used in this report, were carried out at the following environmentalconditions:• two Froude numbers: 0.10 and 0.22• a constant regular wave height: 70 mm (5.60 m)• a range of 15 regular wave lengths: 5.3/4.0 ≤≤ Lλ• three wave directions: 180, 135 and 90 degrees

The following measured quantities are used here:• model speed• wave motions• heave motions• pitch motions• roll motions• vertical accelerations at station 17• vertical relative motions at stations 5, 10, 14 and 17 on the weather side

To determine the roll damping at zero forward speed and at the two forward speeds, roll decay testsin still water have been carried out. The obtained non-dimensional roll damping coefficients κ atthe two forward speeds are:• 10.0=Fn : 0394.0=κ• 22.0=Fn : 1031.0=κ

To check their theoretical computer program, strip theory motion calculations have been carried outby CSSRC too.

In the report of Zhou et al. (1996) all experimental and theoretical results are plotted in graphs.These graphs have been scanned to bitmap files and these files were digitized on a computer screenafterwards.The obtained data are given here in Figure 2 through Figure 9. In these figures, also reference isgiven to the corresponding figure numbers of the original graphs in the CSSRC report.

COMPUTATIONS BY STRIP THEORY

The computer code SEAWAY of the Delft University of Technology was basis for the creation ofthe new computer program SEAWAY-R. A short description of the parent program is given here.

SEAWAY (see Journée (1992, 1996)) is a frequency domain ship motions PC program, based onboth the ordinary and the modified strip theory, to calculate among others the wave-induced loadsand motions with six degrees of freedom of mono-hull ships and barges in a seaway.When not accounting for interaction effects between the two individual hulls, these calculations canalso be carried out for twin-hull ships, such as semi-submersibles or catamarans. The program issuitable for deep and shallow water.The program requires two separate input data files: hull form data file and a variable input data file.The offsets of the cross sections of the fully loaded ship are input and have to be stored in the hullform data file. At any actual loading of the ship, new offsets will be calculated by the program from

6

these data by the actual amidships draught and trim, given in the variable input data file. A lineartransformation of the hull form can be carried out by three independent scale factors. A controlprogram displays the body plan of the ship, as stored in the hull form data file, on the screen and aninput-editor can be used to create the input data file.At choice, the two-dimensional hydrodynamic deep-water coefficients can be calculated by theLewis or the N-parameter close-fit conformal mapping method and the potential theory of Ursell-Tasai. Also the 2-D diffraction pulsating source theory of Frank can be used. Shallow watercoefficients can be determined with the Lewis conformal mapping method and the potential theorygiven by Keil.Special attention is paid to submerged sections and to the surge coefficients. The wave potentialswill be defined for the actual water depth. Linear and non-linear roll damping coefficients aredetermined by the Ikeda method, but they can be an input too. The linearisation will be carried outby the program.At choice, the uni-directional wave spectra can be defined by input spectra and various idealspectra, such as Bretschneider and JONSWAP spectra. These wave spectra can be defined by eitherthe spectral centroid period 1T or the zero-upcrossing period 2T .

To show the validity of the use of program SEAWAY-R, in Figure 2 through Figure 9, comparisonsare given between the calculated Response Amplitude Operator (RAO) data by SEAWAY and theexperimental and theoretical RAO data of CSSRC.For the cross sections aft with a skeg and the bulbous cross sections forward, Frank's pulsatingsource method has been used to calculate the 2-D potential coefficients. For the other crosssections, a close-fit conformal mapping to the unit circle with 10 mapping coefficients ( 1a through

19a ) and the potential theories of Ursell and Tasai have been used. The RAO's of the verticalrelative motions have been calculated in undisturbed waves, so the dynamic swell-up has not beenaccounted for.With all experimental data, a fair to well agreement has been found.

7

Figure 2 Measured and calculated RAO's of heave motions

Figure 3 Measured and calculated RAO's of pitch motions

8

Figure 4 Measured and calculated RAO's of roll motions

Figure 5 Measured and calculated RAO's of accelerations at station 17

9

Figure 6 Measured and calculated RAO's of relative motions at station 5

Figure 7 Measured and calculated RAO's of relative motions at station 10

10

Figure 8 Measured and calculated RAO's of relative motions at station 14

Figure 9 Measured and calculated RAO's of relative motions at station 17

11

As can be seen in Figure 4, the roll motions will be over-predicted by SEAWAY. The roll dampingdoes not cause this, because in all computations the measured roll damping by Zhou et al. (1996)was input. So 0394.0=κ and 1031.0=κ respectively, have been used as input for the twospeeds.When using the empirical method of Ikeda et al. (1978) to obtain the roll damping, higher rollmotions will be predicted. This has been demonstrated in Figure 10 for beam waves.

Figure 10 Effect of damping on RAO's of roll motions

Probably, the deviations are caused by the definition of the wave loads for roll. Experience,obtained with this type of calculations in the past, learned that using the diffraction wave loadsinstead of the relative motion approach in SEAWAY will increase accuracy of the results.

CONCLUSIONS

From these computations, the following conclusions may be drawn with respect to the accuracy ofthe prediction of the motions of the ship by the computer codes SEAWAY and SEAWAY-R:• The heave and pitch motions are well predicted.• The roll motions are somewhat over-predicted by SEAWAY. This is caused by the definitions of

the wave loads. Using the diffraction wave loads instead of the relative motion approach inSEAWAY will increase accuracy of the results.

• The vertical accelerations at station 17 are well predicted.• In head and bow-quartering waves, the vertical relative motions are fairly well predicted.

The deviations in beam waves are caused by the definitions of the wave loads for roll. Nodynamic swell-up has been included in the calculations.

12

Overall, it can be concluded that with all experimental data a fair to well agreement has been foundby SEAWAY. However, use of the diffraction wave loads will increase the accuracy of the results.

Also, it can be concluded that for this ship the CSSRC predictions are slightly better than thosemade by the present SEAWAY computer codes. This is mainly caused by a better prediction byCSSRC of the roll motions of the vessel.

REFERENCES

ICLL, 1966, “International Convention on Load Lines”, International Maritime Organisation,International Conference on Load Lines.

Flokstra, C., 1974, “Comparison of Ship Motion Theories with Experiments for a Containership”,International Shipbuilding Progress, Volume 21, pages 168-189.

Ikeda, Y., Himeno, Y. and Tanaka, N., 1978, “A Prediction Method for Ship Rolling”, TechnicalReport 00405, Department of Naval Architecture, University of Osaka Prefecture, Japan.

Journée, J.M.J., 1992, “SEAWAY-DELFT, User Manual of Release 4.00”, Report No. 910, March1992, Ship Hydromechanics Laboratory, Delft University of Technology, The Netherlands.

Journée, J.M.J., 1996, “The Behaviour of Ships in a Seaway”, ISBN 90-370-0142-4, Report No.1049, May 1996, Ship Hydromechanics Laboratory, Delft University of Technology, TheNetherlands.

Journée, J.M.J., de Kat, J.O. and Vermeer H., 1997, “Comparative Load Line Calculations, ReportNo. 1078, January 1997, Ship Hydromechanics Laboratory, Delft University of Technology, TheNetherlands.

Meier, H. and Ostergaard, C., 1996, “Systematic Calculation of Freeboard, Summary Report 1996”,Technical Report 40306/94, Germanische Lloyd.

Tan, S.G., 1972, “Wave Load Measurements on a Model of a Large Containership”, TechnicalReport 173-S, Netherlands Ship Research Centre TNO, Shipbuilding Department, Delft, TheNetherlands.

Zhou, Z.Q., Zhou, D.C. and Xie N., 1996, “A Seakeeping Experiment Research on FlokstraContainer Ship Model, July 1996, China Ship Scientific Research Center, Wuxi, China,Technical Report No. 4 of: "A Study on Reviewing Freeboards of ICLL (1)", China ClassificationSociety, Shanghai Rules and Research Institute, Shanghai, China.