Floatover Installation Analysis and Its Application in Bohai Bay

Min He, Ruhua Yuan, Huailiang Li, Wentai Yu, Jianwei Qian, Alan M. Wang Installation Division, Offshore Oil Engineering Co., Ltd.,

Tanggu, Tianjin, China

ABSTRACT Nonlinear time-domain simulations are performed to analyze various floatover installation scenarios, including docking, mating, and undocking operations at different critical stages. Their findings are used to properly define the limiting environmental conditions, the dynamic behavior of the floatover barge, the movement of stabbing cones, as well as guide the design of LMUs, DSUs, and fender system, etc. This paper takes the 8,700Te integrated topsides of BZ34-1 CPP Platform as an example and presents the nonlinear time-domain mating analysis and its application to a typical floatover installation design in the shallow water and benign environment of Bohai Bay, China. KEY WORDS: Mating analysis; floatover installation; LMU; DSU. NOMENCLATURE AHTS = Anchor Handling Tow Supply tug CD = Chart Datum DSF = Deck Support Frame DSU = Deck Support Unit LSF = Loadout Support Frame LMU = Leg Mating Unit MSL = Mean Sea Level INTRODUCTION Floatover technologies have been gaining more and more popularity in recent years, particularly in the shallow water and benign environment, such as Bohai Bay, China. Since 2002 there have been eleven successful floatover installations performed in the Bohai Bay using conventional floatover method and strand jack lifting scheme, and many more floatover installations will follow thereafter. The floatover technology uses varied functions of floatover systems and lets large platform topsides be installed as a single integrated package without the use of a heavy lift crane vessel. This allows not only elimination of expensive day-rate derrick barges, minimization of offshore hookup, and maximization of onshore testing and commissioning, but also freedom of equipment layout within the deck compared to modular lifting designs. Nonlinear time-domain simulations are performed to analyze various

installation scenarios including docking, mating, and undocking operations at different critical stages, thus defining the limiting environmental conditions, the dynamic behavior of the floatover barge and the movement of stabbing cones. The limiting environmental conditions and the dynamic behavior of the barge defined by the mating analysis shall be monitored via an environmental measure system and a motion monitoring system, respectively, during floatover operations. The nonlinear time-domain analysis uses a coupled hydro-structure model to include jacket flexibility, fender gaps between barge and jacket legs, stiffness of mooring lines and docking winching soft lines, elastomeric behavior of leg mating units (LMUs) at mating and deck support units (DSUs) at separation, and so on. It is very important to correctly model the magnitude of elastomeric stiffness, gap size and effective inertial properties stated above. Therefore the design loads predicted accurately can be used to guide the design of LMUs, DSUs, fender system, spread positioning mooring system, soft-line docking system, as well as determine the critical fender gap size.

Fig. 1: Docking Operation of Barge Loaded with BZ34-1 CPP Topsides This paper takes the 8,700Te integrated topsides of Bozhong 34-1 Central Production Platform as an example, see Fig. 1, and presents a nonlinear time-domain mating analysis and its application to the floatover installation design in Bohai Bay. The findings indicate importance of the stiffness of elastomeric units and mooring lines, the inertial properties of barge and topsides, in particular the fender gap

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Proceedings of the Twenty-first (2011) International Offshore and Polar Engineering ConferenceMaui, Hawaii, USA, June 19-24, 2011Copyright 2011 by the International Society of Offshore and Polar Engineers (ISOPE)ISBN 978-1-880653-96-8 (Set); ISSN 1098-6189 (Set); www.isope.org

size between barge and jacket legs. The mating analysis also addresses the sensitivity study and put these variable quantities into perspective, and thereby being devoted to the analysis and optimization of floatover operations. BZ34-1 CPP PLATFORM The BZ34-1 CPP Platform is located at the west area of BZ34-1 oil field, south of the Bohai Bay, and was successfully installed in October 2007. Since 2005 six integrated topsides have been successfully installed onto the similar floatover configuration jackets in Bohai Bay, whose floatover weights of the topsides range from 6,500Te to 11,000Te. Liu et al. (2006) presented a successful installation of similar floatover design. As shown in Fig. 2, this is a typical floatover platform which has a 12-legged jacket and is widely adopted in the area of Bohai Bay. The platform is designed to have an open slot configuration which allows the docking and undocking of the installation vessel. The 12-legged platform is located in a water depth of 20.7m and has the opening slot in the longitudinal direction. The particulars of the topsides are given in Table 1.

Table 1: Particulars of BZ34-1 Topsides

Particulars Values

Length Overall 60.0m

Width Overall 64.0m

Elevation of Upper Deck 28.0m

Elevation of Middle Deck 22.0m

Elevation of Lower Deck 14.0m

Elevation of Heli-Deck 43.30m

Floatover Weight 8,700Te

Fig. 2: BZ34-1 CPP Platform, a Typical Floatover Design Fig. 3 presents the typical 12-legged jacket configuration which has two panels interconnecting four main legs with braces on each side of the slot. There are two 4-legged wellhead towers placed on both sides of the main jacket structures and extending above the water. The pre-drilling operation often hinders the favorable jacket entry from the strong end of the 4-legged towers where there is a consequent interference between the pre-installed Christmas trees and the sump

deck structure. This mating analysis is based on the jacket entry from the weak end of the jacket structure. Table 2 lists the particulars of the jacket.

Table 2: Particulars of BZ34-1 Jacket

Particulars Values

Height Overall 26.7m

Top Elevation of Main Piles EL(+)6.0m

Width of Jacket Slot 37.975m

Length of Jacket Slot 40.0m

Topside Elevation of 1st Underwater Frame EL(-)7.543m

Elevation of Top Frame EL(+)5.000m

Elevation of 1st Underwater Frame EL(-)8.000m

Elevation of 2nd Underwater Frame EL(-)19.700m

Jacket Weight 1,800Te

Fig. 3: BZ34-1 CPP 12-Legged Jacket, a Typical Floatover Design An 8,000Te launch barge, named as Hai Yang Shi You 221, has been selected for the floatover installation of the 8,700Te BZ34-1 CPP Topsides in Bohai Bay. Fig. 4 illustrated the floatover barge arrangement including the two skidbeams spacing at 20.0m and the fender arrangement, including the stern jacket entry guides, sway fenders, and surge fenders. The main particulars of the launch barge are given as follows:

Table 3: Particulars of Launch Barge HYSY221

Particulars Values

Length Overall 142.0m

Breath Moulded 36.0m

Depth Moulded 9.75m

Floatover Design Draft 4.5m

Lightship Weight 8,592.5Te

Longitudinal Center of Gravity from bow (LCG) 70.19m

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Particulars Values

Transverse Center of Gravity from CL (TCG) 0.00m

Vertical Center of Gravity above BL (VCG) 4.953m

Fig. 4: 8,000Te Launch Barge HYSY221 with Skidbeams & Fenders It is important to correctly model the lateral and vertical stiffness of the local jacket leg combined with the flexibility of the rubber fenders and the elastomeric elements of the LMUs and DSUs. This will ensure the accurate prediction of the impact forces acting on the jacket structure, the fenders, the LMUs and DSUs when the barge builds up a series of rattling impacts during intermittent contacts between the jacket legs and barge fenders. Compared with the jacket, the topsides can be regarded as a rigid structure when evaluating the design impact loads. The barge structure and associated skidbeams are also treated as rigid structures in the mating analysis. MATING ANALYSIS Mating analyses are nonlinear time-domain simulations based on the equations of a multi-structures system coupled with hydrodynamic models and structural models. There are three types of structures to be taken into considerations. The floating vessel, subject to the environmental loads due to wind, waves, and currents, shall be represented by a hydrodynamic panel model of the wet hull in the coordinate system of the barge body, which may vary with the different ballast conditions. The dry structure, subject to wind loads only, shall be modeled by a lumped mass point with the inertial properties of the integrated topsides in the coordinate system of the topsides model. The fixed jacket substructure is modeled as contact points in the global coordinate system. The mating analysis is performed based on the coupled hydro-structure models with various mooring positioning elements and contact elements. The mooring positioning elements include primary mooring spread and secondary soft-line positioning hawsers which are modeled with weak nonlinear stiffness laws, as well as constant pull loads modeled for positioning tugs. The contact elements consist of rubber fenders, LMUs, and DSUs, which are modeled with high nonlinear stiffness. Contact law shall be applied during a very short duration if the rattling impacts occur during intermittent contacts, or be permanently active where only the nonlinear stiffness of the contact devices is applied. In the mathematical model, the barge and the topside are linked together through the DSUs which are modeled as bilinear springs in

horizontal direction and linear compression-only springs in vertical direction during mating. The impact points between the barge fenders and the jacket legs have been modeled as non-linear compression-only springs. Similarly, the topsides and the jacket are linked together through the LMUs which are modeled as bilinear springs in horizontal direction and non-linear compression-only spring in vertical direction during load transfer. The topsides are defined as a rigid body subjected to external wind loads. Each body will have 6 degrees of freedom resulting in a coupled body system with 12 degrees of freedom which are represented by 12 coupled differential equations in which the fluid reactive forces are described by the convolution integrals. The mass properties act at the defined center of gravity of each body. The hydrodynamic interaction between the barge and the jacket has been assumed to be small due to the small jacket members and thereby being neglected. The time histories of the motion response of the barge and the topsides, as well as the impact loads have been calculated based on given sea states. The gaps between the LMU and the jacket leg, the DSU and the under deck structure, as well as the barge fender and the jacket legs have been considered. The impact loads have been calculated using the defined load and deflection relationship of each LMU and DSU. The impact loads are very sensitive to the fender gaps. The smaller the fender gaps, the less the dynamic amplification of the wave loads, and therefore the less the impact loads applied onto the jacket and the barge. Refer to Hamilton et al. (2008) for the sensitivity study of the fender gaps. The motions at the mating cones of the LMUs and the upper points of the DSUs have been calculated using the post-process module of MOSES. Post processing of the results has been carried out using this post process module. The reports, graphs and other types of information about the time domain simulations can be readily obtained. Fig. 5 illustrates the working relationship between floatover devices including the eight LMUs, the ten DSUs, the eight skid-shoe DSF, the sway and surge fenders at the mating location where the barge is located to achieve the final alignment between the mating cones and the LMU receptors. It should be pointed out that the topsides and the stabbing cones are omitted for illustrative clarification.

Fig. 5: Illustrative Working Relationship between LMUs, DSUs, DSF, Sway & Surge Fenders at Mating Location

Met-ocean Data: The mating analysis shall be performed based on the irregular sea states listed in Table 4, which is applicable to the BZ34-1 field site where the water depth is 20.7m referred to Chart Datum. The JONSWAP wave spectrum with a enhancement factor = 1.0 and

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long-crested seas are applied. There is no significant swell condition in the shallow water and benign environment of Bohai Bay.

Table 4: Irregular Sea States for Mating Analyses

Wave Heading Significant Wave Height Peak Period

Hs Tp Head Seas 1.00m 5.0 sec Quartering Seas 0.75m 5.0 sec Beam Seas 0.50m 5.0 sec

The one-minute average wind speed of 10m/sec, given at EL(+)10m, is considered in the time domain analysis. API wind spectrum was used to simulate the dynamic wind effect. The wind forces were calculated based on the API recommended block method. It is assumed that the wind is co-linear with the waves. The current was modeled as a steady current with a surface speed of 1.0m/sec. The current forces were calculated using the Morisons formula based on the relative velocity. Therefore the current forces also provide damping effect. The current is also assumed to be co-linear with the waves. Docking the barge into the jacket slot is only allowed when the tidal level is above MSL. All subsequent stages of the floatover operation are not restricted to any tidal phases. However the mating operation should be scheduled to account for the beneficial effects on load transfer during a falling tide. Therefore the mating analysis is performed based on the MSL, that is, 0.8m above Chart Datum. Loading Conditions: GL Noble Denton (2010) provides the general guidelines for the mating analysis which shall cover the loading conditions for each stage of docking, mating, and undocking operations. The details of the docking, mating, and undocking cases depend on the actual floatover operation procedures. Wang et al. (2010) provide a general description of the typical installation procedures. The loading cases applicable to the BZ34-1 floatover design can be defined in the Table 5. Table 5: Loading Cases Definition for Docking, Mating & Undocking

Cases Description

Docking

Five docking cases denote that the barge stern just passes different rows of jacket legs, that is,

D1: Just passed 2 meters through Row 1;

D2: Just passed 2 meters through Row 2;

D3: Just passed 2 meters through Row 3;

D4: Just passed 2 meters through Row 4;

D5: Arrived at the final mating position and engaged with surge fenders just before mating.

Mating

Seven mating cases denote the different load transfer conditions during mating, that is,

M1: 0% topsides load transferred, initial contact, at draft of 5.875m;

M2: 5% topsides load transferred at draft of 5.89m;

Cases Description

M3: 25% topsides load transferred at draft of 5.94m;

M4: 50% topsides load transferred at draft of 6.01m;

M5: 75% topsides load transferred at draft of 6.075m;

M6: 95% topsides load transferred at draft of 6.081m;

M7: 100% topsides load transferred, initial separation, at draft of 6.10m.

Note: The intermediate drafts are based on the MSL of 0.8m above Chart Datum.

Undocking

Five undocking cases denote that the barge stern will pass different rows of jacket legs, that is,

U5: Still at the mating position and engaged with surge fenders;

U4: Remain 2m just before passing through Row 4;

U3: Remain 2m just before passing through Row 3;

U2: Remain 2m just before passing through Row 2;

U1: Remain 2m just before passing through Row 1 and fully clearing from the jacket slot.

MOSES MODELLING The software MOSES is used to perform the nonlinear 3-D time domain mating analysis. The analyses performed are time-domain non-linear analyses at the different stages of docking, mating, and undocking operations stated above, where the barge and topsides are modeled as rigid bodies with non-linear springs representing fenders, LMUs and DSUs associated with jacket stiffness and linear springs representing mooring lines and docking soft-lines, etc. Barge Coordinate System: Fig. 6 shows the coordinate system of the barge model in MOSES. The origin of X axis is located at the bow of the barge, positive towards stern. The origin of Y axis is located at the center line of barge, positive towards starboard. The origin of Z axis is located at the keel of barge, positive upward.

Fig. 6: Coordinate System of Barge Model

The inertial properties of DSUs, skid beams, fender system, tiedowns and other onboard equipment, etc., have been applied as miscellaneous weights in this study. The total weight is approximately 1,994.5Te with the center of gravity located at LCG = 78.09m, TCG = 0.12m and VCG = 12.32m. Topsides Coordinate System: In MOSES, the topsides module is modeled as a point mass representing its mass, centre of gravity and

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radii of gyration. Fig. 7 presents the coordinate system of the topsides model. The origin of X axis is located at the middle of Row 2 and Row 3. The origin of Y axis is located at the middle of Row A and Row B. The origin of Z axis is located at Chart Datum.

Fig. 7: Coordinate System of Topsides Model Jacket Coordinate System: There are three sets of contact points defined in the MOSES part ground system to predict the impact loads on the LMUs and mating cones, the DSUs and DSF, the fenders and jacket legs, respectively, that is, the global earth-bound coordinate system. Fig. 8 illustrates the global coordinate system where the origin of X axis is located at Row 1, the origin of Y axis is located at the middle of Row A and Row B, while the origin of Z axis is located at the MSL.

Fig. 8: Global Earth-Bound Coordinate System for Jacket Model Topsides Transportation Arrangement: Fig. 9 denotes the transportation arrangement of the topsides whose location on the deck of the barge is defined as the lumped mass point representing approximately 8,700Te floatover weight of the integrated topsides in the coordinate system of topsides model with the center of gravity located at LCG = -4.133m, TCG = 0.353m, VCG = 21.262m, and the radii of gyration RXX = 17.59m, RYY = 19.32m, RZZ = 23.75m. Inertial Properties of Barge & Equipment: Table A-1 presents inertial properties used in the mating analysis. The inertial properties include the barge lightship, ballast distributions, DSF weight, installation devices, structural items on barge deck, etc., where the radii of gyration are given with reference to the system COG of the barge and topsides. In reality the draft, ballast and deck weight transfer will change continuously during the operations. However, in order to keep each operational stage exposed to the environmental conditions for a long enough time to ensure a sufficient statistical basis, the analysis is performed in keeping these global parameters constant during each stage.

Fig. 9: Topsides Transportation Arrangement Mooring Positioning System: There are two types of the mooring positioning lines, that is, two catenary lines at the bow of the barge and two cross lines/two parallel lines at the stern of the barge. The cross mooring pattern is shown in Fig. 10. The properties of these four lines are shown in the Table 6. At the earlier stages of entry, the cross lines are used to pull the barge into the jacket slot, and the parallel lines are inactive. At the later stages of docking and deck transfer, the parallel lines are activated to keep the barge position, and the cross lines are slacked. The mooring lines are simulated as B_CAT and H_CAT type of connectors in MOSES.

line no.2 (wire)

line no.1 (wire)

line no.3(150m chain + 350m wire)

bowstern hysy221

line no.4 (150m chai n + 350m wi re)

Fig. 10: Mooring Arrangement & Definition

Table 6: Properties of Mooring Positioning Lines Type Diameter MBL Young's Modulus Weight in water

Unit [mm] [kN] E [kN/m2] [kg/m] Wire 52 1,420 6.0107 9.38 Chain 76 6,001 5.6107 109.6 Sway Fenders: The sway fenders are installed along both the gunwales of the barge, that is, 0.325 meter above the deck. The sway fenders are used to protect the jacket from contact with the barge hull during the docking operation. The fenders consist of rubbers and are simulated by horizontal non-linear compression-only springs with the equivalent stiffness that combines both the fender stiffness and the jacket structural flexibility at the impact elevation. The gap between the jacket

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legs and the fenders is approximately 100mm at the final mating position and 2.0m when the jacket entry guides approach to the jacket slot. The nonlinear stiffness of the sway fenders and the stiffness of the jacket legs are given in Fig. 11 and Fig. 12, respectively. Fender Stiffness of HY221 For BZ34-1Oil-field

050

100150200250

300350400450500

0.000 0.100 0.200 0.300 0.400 0.500

Compression(m)

Forc

e(t)

FenderStiffness

Fig. 11: Stiffness of Sway Fenders Jacket Leg Stiffness

0

1000

2000

3000

4000

5000

0.000 0.030 0.060 0.090 0.120 0.150

Compression(m)

Forc

e(K

N)

Row1Row2Row3Row4

Fig. 12: Stiffness of Jacket Legs Surge Fenders: The surge fenders installed on the barge is constructed with a horizontal hinged beam connected to a conical rubber fender and shimmed with adjustable wooden plates. The fenders are designed to have adequate stiffness withstand with deformation of pulling force exerted by the mooring lines in compliance with the installation clearances. When the barge is at the final alignment position, the mating cones of the topsides legs shall be located above the LMU receptors within the capture radius. The combined stiffness of the surge rubber cone and jacket leg deformation is assumed to be approximately 100Te/m. A pull of 20Te has been modeled on the barge to simulate the pre-tension of the mooring lines acting on both the surge fenders and the jacket legs at Row 1, that is, 10Te load on each leg. Separation between the surge fenders and the jacket legs has also been simulated in the analysis if there is no adequate longitudinal winching force, or via positioning tugs, applied to ensure contact at these locations. LMUs: The LMU is installed on the top of the jacket pile and is used to absorb the impact load between the stabbing cones and the LMU receptor during mating operation. The capture radius depends on the top radius of the receptors and the bottom radius of the stabbing cones. At the final mating position, the alignment will ensure the stabbing cones within the capture radius of the LMU receptors. The LMU and jacket leg system has been represented by vertical non-linear

compression-only springs and lateral bilinear compression springs. The actual load-deflection curve of the LMU unit has been modeled in combination with the jacket flexibility. The longitudinal stiffness for the jacket legs is 3Te/mm while the transverse stiffness for the jacket legs are 2.2Te/mm for Legs Row 1 & Row 2 and 6.0Te/mm for Legs Row 3 & Row 4. The horizontal and vertical stiffness for the LMUs are displayed in Fig. 13 and Fig. 14. Fig. 14 shows that the first section of the gently smooth curves up to 225m compression mainly indicates the stiffness of the LMUs while the second section of the sharply steep curves mainly indicates the stiffness of the jacket leg. Fig. 15 presents the pre-mating position where LMU clearance, fender gap, barge draft, under-keel clearance, etc., are defined for Case D5. Note that the LMU receptors and the stabbing cones have conic geometry which can be defined by the cone depth, top and bottom diameters. Both the contact mechanics of the stabbing cones and the receptors and the horizontal and vertical elastomeric stiffnesses of the LMUs shall be correctly modeled in the analysis.

Fig. 13: Horizontal Stiffness of LMUs

Fig. 14: Vertical Stiffness of LMUs including Jacket Flexibility DSUs: The DSUs on the top of the DSF have been represented by vertical linear compression-only springs in vertical direction and bilinear springs in horizontal direction. The value of the structural stiffness between the topsides and the barge has been considered. The simplified sand-dish DSUs are the connection part of the topsides and the barge which are modeled with the 5Te/mm horizontal stiffness and the 20Te/mm vertical stiffness, respectively. Fig. 16 presents the separation position where DSU clearance, fender gap, barge draft, under-keel clearance, etc., are defined for Case U5.

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Fig. 15: Pre-Mating & LMU Gap at Final Alignment Position

Fig. 16: Separation & DSU Gap upon 100% Load Transfer Damping: Only wave damping is taken into consideration in the mating analysis. No structural damping or positioning element damping is assumed since they are much smaller than the wave damping. This may yield conservative results. ANALYSIS METHOD The mating analysis has adopted a snap shoot approach to the entire operation at different docking, mating, and undocking stages. The operations have been assumed to be halted at different stages. The exposure duration has been assumed to be 3 hours for different stages of the operation which will be long enough to simulate the whole installation procedure and thereby obtaining the rational statistic results. The time steps used in the analysis are 0.05 sec during the docking and undocking stages, and 0.02 sec during the mating stages where there is stronger geometric and material nonlinearity due to the LMUs and DSUs. A ramping time of 400 seconds is used in the time-domain simulation in order to avoid any unrealistic transient motion induced numerically. The results within the ramping time period are disregarded in the statistical analysis. Due to strong nonlinear nature, the existing statistics theory may not be accurate when predicting the design maxima. Therefore the maximum values observed in the time series are recommended to be used as the

design maxima. ANALYSIS RESULTS The main findings are summarized as follows: Docking Cases: The maximum impact loads acting on the sway fenders and the minimum dynamic clearance between the mating cone and the LMU receptor are summarized in the following table:

Case Sway Fender Load LMU Clearance Wave Heading D1 116.0Te @ Leg P1 0.760m Beam Seas D2 156.3Te @ Leg P1 0.761m Beam Seas D3 225.1Te @ Leg P1 0.760m Beam Seas D4 185.6Te @ Leg P4 0.757m Beam Seas D5 78.1Te @ Leg P4 0.765m Beam Seas

Some of important design maxima and findings are listed and elucidated as follows: 1) The maximum design load acting on the sway fenders is

about 225.1Te at the docking stage D3. 2) The mooring and cross lines are considered prior to jacket

entry. The maximum sway motion of the barge stern is 1.95m obtained from the wave frequency motion, the low frequency drift and static offset. The maximum allowable sway offset is 2.0m per design of the jacket entry guides. The maximum force in the mooring line No. 1 or No. 2 is 59.0Te while the maximum force in the mooring line No. 3 or No. 4 is 49.0Te, thus yielding a factor of safety is 2.5, greater than 1.67 per API RP 2SK Sec. 6.3.2.

3) In Case D1 the barge surge, sway and yaw motion is mainly dominated by slow drift motion with isolated impacts at the sway fender.

4) In Case D5, a pull of 20Te has been applied to simulate the pull of the parallel mooring lines by soft-line winches or by positioning tugs to ensure the surge fenders in contacting condition. The maximum impact load acting on the surge fender is 23.5Te in head seas. The maximum horizontal motion at the bottom of LMU stabbing cone is approximately 0.202m, thus ensuring the stabbing cone offset is within the capture radius of 0.43m. Eight sets of contact spring are used to model the sway fenders with a nominal gap of 100mm between the barge fenders and the jacket legs.

Mating Cases: The maximum impact loads acting on the LMUs and DSUs, as well as occurrence of premature separation at DSUs, are summarized in the following table:

Case Major Findings

M1

Max Vertical Load = 420.5Te @ LMU S1 in Beam Seas; Max Horizontal Load = 278.3Te @ LMU S3 in Beam Seas; Vertical Load = 562.7 1,235.2Te @ DSUs in Beam Seas; Max Horizontal Load = 186.8Te @ DSU P4 in Beam Seas; No premature separation at DSUs.

M2

Max Vertical Load = 511.5Te @ LMU S1 in Head Seas; Max Horizontal Load = 164.3Te @ LMU P3 in Beam Seas; Vertical Load = 542.0 1,129.1Te @ DSUs in Beam Seas; Max Horizontal Load = 118.6Te @ DSU S4 in Beam Seas; No premature separation at DSUs.

M3

Max Vertical Load = 693.1Te @ LMU S1 in Head Seas; Max Horizontal Load = 146.6Te @ LMU P3 in Beam Seas; Vertical Load = 427.0 1,015.5Te @ DSUs in Beam Seas; Max Horizontal Load = 112.6Te @ DSU S4 in Beam Seas;

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Case Major Findings No premature separation at DSUs.

M4

Max Vertical Load = 1,073.3Te @ LMU S1 in Head Seas; Max Horizontal Load = 158.6Te @ LMU P3 in Beam Seas; Vertical Load = 280.7 731.4Te @ DSUs in Beam Seas; Max Horizontal Load = 121.6Te @ DSU P4 in Beam Seas; No premature separation at DSUs.

M5

Max Vertical Load = 1,924.9Te @ LMU S1 in Head Seas; Max Horizontal Load = 179.4Te @ LMU S3 in Beam Seas; Max Vertical Load = 558.7Te @ DSU S4 in Head Seas; Max Horizontal Load = 137.0Te @ DSU P4 in Quartering; DSU separation may occur.

M6

Max Vertical Load = 2,062.5Te @ LMU S1 in Head Seas; Max Horizontal Load = 216.8Te @ LMU P3 in Beam Seas; Max Vertical Load = 582.2Te @ DSU S4 in Head Seas; Max Horizontal Load = 142.0Te @ DSU P4 in Quartering; DSU separation may occur.

M7

Max Vertical Load = 2,550.0Te @ LMU S1 in Beam Seas; Max Horizontal Load = 226.1Te @ LMU P3 in Beam Seas; Max Vertical Load = 644.8Te @ DSU S4 in Head Seas; Max Horizontal Load = 204.4Te @ DSU P4 in Beam Seas; DSU separation may occur.

Fig. 17: Case M7: Time Series of Maximum Vertical Load on LMU Some of important design maxima and findings are listed and elucidated as follows: 1) At the final alignment position, the sway and surge fenders

are subject to 78.1Te and 23.5Te, respectively. The maximum vertical movement of the mating cone is 0.14m. The impact between the mating cone and the LMU receptor may occur at the barge draft of 5.265m when the tidal elevation is at MSL.

2) In the initial contact condition, the bottom of the cone of the deck leg is always within the LMU receiver horizontally even with the cone above the LMU receiver vertically.

3) When the 75% load transfer is completed, the separation between the topsides and the DSUs may occur.

4) The maximum vertical impact load acting on the LMUs is approximately 2,550Te when 100% load is transferred. There is no indication that the significant impact will occur at the earlier mating stages.

5) The maximum horizontal impact load acting on the LMUs is 278.32Te which may occur in the condition of the 0% topside load transfer condition.

6) During mating the topsides and the barge have been considered as two separated bodies. All the tiedowns between the

DSF and the topsides around the DSUs are assumed to be removed. The effect of fenders and mooring system is not considered since the topsides are restrained by the LMUs which have much larger stiffness than those of the mooring lines and the fenders.

7) Fig. 17 shows an example of the time series of the vertical load acting on the LMU at Leg S1, obtained from Case M7, where LMU S1 experiences the maximum vertical load during mating.

8) Fig. 18 shows another example of the time series of the vertical load acting on the DSU at Support S4, obtained from Case M1, where DSU S1 experiences the maximum vertical load during mating.

Fig. 18: Case M1: Time Series of Maximum Vertical Load on DSU Undocking Cases: The maximum impact loads acting on the sway fenders and the minimum dynamic under-keel clearances between the barge bottom and the jacket underwater horizontal frame, derived from the maximum vertical motion at the four corners of the barge bottom, are summarized in the following table:

Case Maximum Sway

Fender Load Minimum Under-Keel Clearance

Wave Heading

U1 122.8Te @ Leg P1 0.450m Beam Seas U2 168.8Te @ Leg P1 0.460m Beam Seas U3 240.1Te @ Leg P1 0.460m Beam Seas U4 180.9Te @ Leg S1 0.400m Beam Seas U5 89.6Te @ Leg P4 0.490m Beam Seas

Some of important design maxima and findings are listed and elucidated as follows: 1) In Case U5, a pull of 20Te has been applied to simulate the

pull of the parallel mooring lines by soft-line winches or by positioning tugs to ensure the surge fenders in contacting condition. The maximum impact load acting on the surge fender is 25.73Te in head seas. The maximum vertical motion at the top of DSU is approximately 0.17m, thus ensuring there is a minimum vertical clearance of 0.66m between the underside of the topsides and the top of DSU. Eight sets of contact spring are used to model the sway fenders with a nominal gap of 100mm between the barge fenders and the jacket legs.

2) During separation, the maximum vertical movement at the DSU is 0.17m. The findings indicate that no impact at the DSUs would occur when the barge is ballasted down at a draft greater

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than 6.47m. The corresponding vertical movement at the barge bottom is approximately 0.49m.

3) During undocking, when the draft reaches 6.90m, the maximum vertical movement at the four corners of barge bottom is 0.49m, less than the nominal clearance of 1.393m. Therefore no impact between the barge bottom and the jacket underwater horizontal frames will occur.

4) The maximum design load acting on the sway fenders is about 240.1Te at the undocking stage U3.

5) Fig. 19 shows an example of the time series of the lateral load acting on the sway fender at Leg P1, obtained from Case U3, where Leg P1 experiences the maximum lateral load during undocking.

Fig. 19: Case U3: Time Series of Max Lateral Load on Sway Fender

CONCLUSIONS This paper presents the nonlinear time-domain mating simulations and their major findings successfully applied in designing the installation devices and selecting the dominant design parameters, thus ensuring the successful execution of the floatover installation for the BZ34-1 integrated topsides. It is essential to correctly and accurately model the jacket flexibility, fender gaps, nonlinear fender stiffness, as well as the contact mechanism and high nonlinearities of the elastomeric elements in LMUs and DSUs, etc., and therefore obtain reliable and repeatable design maxima. ACKNOWLEDGEMENTS Several people have contributed to this work in many vital ways. Very special thanks to Mr. Wang Xinwei, Mr. Liu Bo and Mr. Su Jie for their enthusiastic support and drafting expertise. REFERENCES GL Noble Denton (2010). "Guidelines for Float-over Installations,"

0031/ND, Dec 6, 2010, Rev. 0, 36pp. Hamilton, J, French, R and Rawstron, P (2008). "Topsides and Jacket

Modelling for Floatover Installation Design," Offshore Tech Conf, Paper No 19227, 14 pp.

Liu, LM, Zhang, SF, Fang, XM, Chen BJ, Hao J, and Wang, AM (2006). "Floatover Installation Succeeds for Nan Bao 35-2 Topsides," J World Oil, Vol 227, No 7, pp 63-69.

Wang, AM, Jiang, XZ, Yu, CS, Zhu, SH, Li, HL and Wei, YG (2010). "Latest Progress in Floatover Technologies for Offshore Installations and Decommissioning," Proc 20th International Offshore and Polar Engineering Conference, Beijing, China, ISOPE, Vol 1, pp 9-20.

ANNEX A

Table A-1: Inertial Properties of Barge & Equipment

Case Description Draft Weight* LCG TCG VCG RXX RYY RZZ Unit [m] [Te] [m] [m] [m] [m] [m] [m]

D1-D5 Docking draft condition 4.500 13409.5 74.90 -0.24 5.18 11.60 40.23 41.31 M1 0% load transfer draft condition 5.875 20570.7 73.69 -0.16 4.53 11.13 41.05 42.14 M2 5% load transfer draft condition 5.890 21083.9 73.57 -0.15 4.44 11.21 40.58 41.70 M3 25% load transfer draft condition 5.940 23084.4 73.19 -0.11 4.35 11.43 38.86 40.12 M4 50% load transfer draft condition 6.010 25624.3 72.79 -0.06 4.06 11.29 37.44 38.72 M5 75% load transfer draft condition 6.075 28138.0 72.47 -0.03 3.99 11.20 36.24 37.56 M6 95% load transfer draft condition 6.081 29909.3 72.26 -0.01 4.05 11.13 35.50 36.84 M7 100% load transfer draft condition 6.100 30443.3 72.21 0.00 4.08 11.11 35.29 36.63

U1-U5 Undocking draft condition 6.900 34873.5 72.10 0.00 4.40 11.25 33.93 35.43 Note: The weight* = the total weight (displacement) - the weight of the topsides, which varies with ballast conditions at different mating stages.

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