AN INNOVATIVE SYNTHETIC MOORING SOLUTION FOR AN …

1 Copyright © 2010 by ASME

Proceedings of the ASME 2010 29th International Conference on Ocean, Offshore and Arctic EngineeringOMAE2010

June 6-11, 2010, Shanghai, China

OMAE2010-20888

AN INNOVATIVE SYNTHETIC MOORING SOLUTION FOR AN OCTAGONAL FPSO IN SHALLOW WATERS

Mo FanCNOOC RC

Beijing, China

Da LiCNOOC RC

Beijing, China

Tuanjie LiuOcean Dynamics LLCHouston, Texas, USA

Alex RanOcean Dynamics LLCHouston, Texas, USA

Wei YeOcean Dynamics LLCHouston, Texas, USA

ABSTRACTAn octagonal FPSO has been proposed for marginal oil

and gas development in shallow waters. A shuttle tanker will bedeployed near the FPSO during offloading operations. This new concept simplifies the design and manufacturing processes, yet maintains full production, storage, and offloading functions of a conventional ship-shaped FPSO.However, design of the mooring system for this floating unitimposes technical challenges due to: 1) high environmental loads expected on this unit, 2) large dynamic offsets of the unit in shallow waters, and 3) inadequate performance of catenary mooring systems in shallow waters. Thus, development of a viable station keeping solution becomes a key issue to the new concept FPSO design.

In this paper, an innovative mooring system is designed to meet the challenges. The FPSO mooring system consists of pile anchors, bridle chains, anchorage buoys, and polyester ropes. Nine mooring lines are grouped into three bundles which evenly spread around the FPSO. The shuttle tanker is attached to the FPSO with a nylon rope hawser at the bow and secured to pre-installed anchorage buoys at the stern with two other nylon ropes. Analyses have been performed for the FPSO mooring system. It is concluded that the proposed mooring system is fully functional and effective.

1. INTRODUCTIONAn FPSO for marginal field can provide the capability to

store produced oil and gas for offloading to a shuttle tanker or export through a pipeline. Ship-shaped FPSOs, many of them conversions, still dominate the FPU category, whereas new builds and novel configurations are becoming more common in the recent years. Among others, round hull design, barge

shaped configuration, as well as other innovative non-ship shaped concepts, have broken the tradition of tanker conversions [1].

The new octagonal FPSO concept was proposed by CNOOC RC to exploit oil and gas resources from marginal reservoirs in shallow waters. Ocean Dynamics, LLC. has designed a synthetic mooring system for this new concept of floating unit. Synthetic moorings have become more commonplace since the first permanent use of polyester moorings in the GoM for BP’s Mad Dog truss spar [2]. Prior to use in the GoM, synthetic ropes were used offshore Brazil onP-27 semisubmersible in 1998 [3]. Most of the existing synthetic mooring applications are located in deep waters where mooring system weight is a big concern. However, overall mooring system stiffness, in other words, the resilience to the platform motions, poses increasing technical challenges for activities in shallower waters. In this paper, the design and analysis of aspread mooring system composed of synthetic ropes and anchorage buoys will be introduced. The mooring system hasbeen checked for strength and bottom clearance criteria. Time domain quasi-static method has been used in the mooring analysis for line strength check. Fully dynamic analyses of the most loaded lines are performed to assess the dynamic effects on the mooring line tensions.

2. FPSO HULL AND MOORING SYSTEMDESCRIPTIONThe octagonal FPSO concept was designed by CNOOC

RC for developments in 40 m waters offshore China. The platform consists of a 60 x 60 m hull, 66 x 66 m main deck, and the facilities and living quarter on the main deck. The hull displacement at the fully loaded draft is 49,000 tonnes and the


ballasted displacement is about 20,000 tonnes. Two heave skirts located at the lower hull were designed to provide additional hydrodynamic damping for motions in the vertical dimension. Studies done by Cozijn, et al. (2005) indicates that such skirts provide significant damping effects for heave, roll and pitch motions for deepwater calm buoy which shares similar geometry with the octagonal FPSO.

Figure 2-1 shows the side view of the Octagonal FPSO unit. The main particulars of the vessel are specified in Table 2-1.

Figure 2-1: Profile view of octagonal FPSO

Table 2-1: Particulars of the FPSO

Parameters Unit FPSO

Main Deck Dimension m 66 x 66

Hull Dimension m 60 x 60

Hull Depth m 28

DraftFully loaded m 16

Ballasted m 6.5

DisplacementFully loaded t ~49,000

Ballasted t ~20,000

KG overallFully loaded m 15.2

Ballasted m 20.5

Roll/Pitch Radii of Gyration

Fully loaded m 19.58

Ballasted m 17.57

Yaw Radii of Gyration

Fully loaded m 26.7

Ballasted m 26.7

The mooring system designed for the octagonal FPSO has nine mooring lines which are grouped into three bundles. The three bundles of mooring lines are evenly distributed around the FPSO hull. Three mooring lines within each bundle are separated by five (5) degrees in azimuth. Each mooring line consists of platform chain, polyester ropes, mid-rope buoy, pile

end buoy, pile chain, and driven pile. The chain is 120 mm in diameter, and the polyester rope has a nominal diameter of 200 mm. The minimum breaking load (MBL) of the chain and rope is approximately 1267 MT. The mid-rope buoys and pile-end buoys are used to keep the polyester rope off the seabed in slack conditions. The pile-end buoy is required to provide a net buoyancy of 30 MT, while the mid-rope buoy provides a net buoyancy of 3 MT.

The arrangement of nine polyester mooring lines is presented in the drawings in Figure 2-2 and Figure 2-3.Properties of the mooring components are presented in Table 2-2. Table 2-3 shows the mooring line azimuth angle and pretension.

Figure 2-2: FPSO mooring system, top view

Figure 2-3: FPSO mooring system, side view


Table 2-2: Mooring line propertiesPlatform Chain Polyester Rope Pile Chain Mid-rope

BuoyPile-end

BuoyType R4 Studless Parallel Laid

& JacketedR4 Studless Foam with

steel frameFoam with steel frame

Nominal Diameter 120 mm 200 mm 120 mm 1500 mm 3400 mmLength1 40.0 m 500 m x 2 ~40.0 mWeight in air 288 kg/m 30 kg/m 288 kg/m 0.7 MT 6.8 MTWeight in water 250 kg/m 7.9 kg/m 250 kg/m -3 MT -30 MTBreaking Strength 1267 MT 1270 MT 1267 MT - -EA 115000 MT Vary 115000 MT - -

Table 2-3: Mooring line orientation and pretensionMooring Group

Mooring line #

Azimuth Angle (degree)*

Pretension (MT)

11 55 752 60 753 65 75

24 175 755 180 756 185 75

37 295 758 300 759 305 75

Note: * Measured counter-clockwise from bow

3. DESIGN SAFETY FACTORS AND ENVIRONMENTAL CONDITIONS

3.1 Safety FactorsThe design safety factors as specified in API RP 2SK and

2SM for the mooring system are presented in Table 3-1.

Table 3-1: Mooring line design safety factorsSafety Factor

Intact One line Damaged

Quasi-static method 2.00 1.43

Dynamic method 1.67 1.25

3.2 Design EnvironmentsThe extreme environment for mooring strength design is

100-year return storm, as shown in Table 3-2. A 1-year returncondition is the limiting environment for offloading of the shuttle tanker. For the analysis it is conservatively assumed that the wind, wave and current are co-linear (travel in the same direction) and omni-directional (the same in all directions).

Table 3-2: Design environmentsExtreme (Design)

Operating Environment

Return period 100-yr 1-yr

Wind1-hr mean @ 10m 38.7 m/s 22.8 m/s

1-min mean @ 10m

46.4 m/s 27.4 m/s

Waves

Significant wave height

8.8 m 4.6 m

Maximum wave height

14.5 m 7.7 m

Zero upcrossing wave period

8.6 s 6.6 s

Peak wave period 11.8 s 8.7 s

CurrentSurface speed 1.82 m/s 1.08 m/s

Middle 1.40 m/s 0.78 m/sBottom speed 1.07 m/s 0.56 m/s

4. METHODOLOGYFrequency domain radiation/diffraction program was used

for the wave force, added mass and damping coefficient calculations. Time-domain quasi-static method and fully dynamic method were used in the analysis to predict the maximum mooring tension and the vessel motions in the design environments. Marine analysis program MOSES, developed by Ultramarine, was used for the quasi-static analysis. Six-degree-of-freedom finite element program Flexcom was used for the fully dynamic mooring tension analysis. The analyses methodology is described in the following sections.

4.1 Frequency Domain AnalysisPotential theory and panel method are used to predict the

hydrodynamic interactions between the waves and the FPSO and shuttle tanker in frequency domain. The hydrodynamic coefficients obtained from the analysis include added mass, wave damping, first order wave forces and second order mean drift forces on the vessel. Those coefficients will later be used to generate wave load time series in the times domain analysis. RAO (response amplitude operator) of the vessel were also computed, which is used to investigate the motion characteristics of the vessel but not used in time domain.


Figure 4-1 shows the mesh applied for the frequency domain analysis in WAMIT. A total of 100 wave periods, ranging from 3 second to 30 second to cover the range of period where wave energy exist, were used in the analysis. A total of 13 wave headings were selected in the analysis.

Figure 4-1: Panel model of the octagonal FPSO

Figure 4-2: MOSES coordinate system and environmental headings

4.2 Time Domain Quasi-Static AnalysisThe mooring strength and motion analyses of the mooring

system were performed based on the time domain quasi-static analysis method using MOSES. In the analysis, the wave forces, including linear wave force and second order slow drifting wave force (using Newman’s Approximation), wind force and current force were generated at each time step and the motions of the vessel was calculated for that time step. Added mass and wave damping were also considered in the time domain by using the convolution integral. Wind force was

calculated based on API guidelines, which means the shape of the structure, the variation of wind speed at different elevation, and the wind energy distribution (spectrum) were all taken into account. The current force on the vessel was calculated using Morison’s Equation.

Each simulation is performed for three-hour duration with a time step of 0.5 second, excluding 1200 seconds ramp time (to avoid numerical transient motions). Considering the symmetry of the FPSO hull and the mooring configuration, 9 environmental headings—0 (head sea), 30, 55, 65, 90, 120,150, 175 and 180 degrees were analyzed for the extreme conditions.

Statistical maximum values were calculated based on the calculated time series of motions and tensions. For each load case, ten 3 hour simulations each based on different realization of the random wave train were carried out. The simulations were conducted using two different polyester rope stiffnessesfor the purpose of predicting vessel motions and mooring strength, respectively. In one line damaged cases, the environmental heading which induces the maximum intact mooring line tension were identified first, then for that heading the second most loaded line was assumed to be damaged. This is a conservative method because it usually generates the highest tension in the most loaded lines.

Due to the nonlinear and visco-elastic characteristic of the polyester ropes, the following two kinds of axial stiffness of the polyester lines were used in the analysis.

A post-installation stiffness of 14 x MBL for the polyester line. This value was used in predicting the motions of the vessel.

A storm stiffness of 28 x MBL for the polyester line. This value was used in predicting the tensions in the mooring lines.

4.3 Time Domain Fully Dynamic AnalysisIn the quasi-static analysis, for each mooring line the

tension was calculated using centenary theory based on the fairlead and anchor positions, assuming the line is in equilibrium statically. This means that the dynamics of the mooring line and the wave forces on the line, which may affect the tension of the line and the vessel motions, are not considered. In order to assess the dynamic effects of the mooring system, fully dynamic analyses taking into account the inertia and hydrodynamic loads on the mooring lines were performed using Flexcom for the most critical cases.

Finite element (FE) model of the mooring system was created using Flexcom-3D. The motion time histories of the fairleads calculated by MOSES were used as input data for the dynamic simulations. Identical wave components were used in both Flexcom and MOSES in order to generate exactly the same wave time traces in the two analyses. The maximum mooring tensions at the fairleads and under the buoy were obtained to verify whether the design criteria specified were met.

13 2

4

6

5

7

9

8

X

Y

5. RESULTS AND DISCUSSIONSThis section discusses the mooring line tensions of the

polyester mooring system in 100-year return environments. results presented in this paper are the average of the maximumvalues from analyses based on 10 different realizations of random wave series in order to achieve most probable responses in extreme events.

5.1 Frequency Domain Analysis Results for Fully Loaded DraftThe following figures show the motion RAO

frequency domain for the FPSO fully loaded condition.

It can be observed that the heave natural period of the vessel is about 14 seconds. The roll and pitch natural frequency is about 19 seconds. The natural periods are longer than the wave peak period, which is less than 12 seconds for the extreme conditions. Therefore, no significant resonance is expected for the fully loaded FPSO.

Figure 5-1: Motion RAO, heave, FPSO fully loaded

Figure 5-2: Motion RAO, roll, FPSO fully loaded

5

This section discusses the mooring line tensions of the year return environments. The

average of the maximumvalues from analyses based on 10 different realizations of random wave series in order to achieve most probable

Frequency Domain Analysis Results for FPSO at

RAOs calculated in frequency domain for the FPSO fully loaded condition.

It can be observed that the heave natural period of the vessel is about 14 seconds. The roll and pitch natural frequency is about 19 seconds. The natural periods are longer than the

period, which is less than 12 seconds for the extreme conditions. Therefore, no significant resonance is

: Motion RAO, heave, FPSO fully loaded.

: Motion RAO, roll, FPSO fully loaded

Figure 5-3: Motion RAO, pitch, FPSO fully loaded

5.2 Frequency Domain Analysis Results for Ballasted DraftThe following figures show the motion

calculated in frequency domain for the FPSO in ballasted condition.

It can be observed that the heave natural period of the vessel is about 10 seconds. The roll and pitch natural frequency is about 15 seconds. The heave natural period is in the near proximity of the peak wave energy spectrum, and thus, may cause resonance in heave motions. occurs, the heave damping is critical in determining their response amplitudes. The damping vortex shedding at the heave skirtsincluded in the motion RAO calculationanalyses the damping effects werebottom and heave skirts with Morison discs distribbottom of the FPSO. It should be noted that for both fully loaded and ballasted drafts, the roll and pitch RAOs have a minimum at about 10 seconds, where yr waves resides. It can be concluded from the result that this concept design is optimized for reduction of platform roll and pitch motions.

Figure 5-4: Motion RAO, heave, FPSO ballasted

Copyright © 2010 by ASME

: Motion RAO, pitch, FPSO fully loaded

Frequency Domain Analysis Results for FPSO at

the motion RAOs of the vessel calculated in frequency domain for the FPSO in ballasted

that the heave natural period of the vessel is about 10 seconds. The roll and pitch natural frequency is about 15 seconds. The heave natural period is in the near proximity of the peak wave energy spectrum, and thus, may cause resonance in heave motions. When resonant response occurs, the heave damping is critical in determining their response amplitudes. The damping effects that come from the

heave skirts and the FPSO bottom wereincluded in the motion RAO calculation. In the time domain

effects were introduced by modeling the with Morison discs distributed on the

It should be noted that for both fully loaded and ballasted drafts, the roll and pitch RAOs have a

at about 10 seconds, where the peak period for 100-yr waves resides. It can be concluded from the result that this concept design is optimized for reduction of platform roll and

Motion RAO, heave, FPSO ballasted

Figure 5-5: Motion RAO, roll, FPSO ballasted

Figure 5-6: Motion RAO, pitch, FPSO ballasted

5.3 Polyester Mooring Tension in Intact ConditionsFor the polyester mooring system, the calculated

maximum intact mooring line tensions in the 100environments are shown in Figure 5-7 and Figure loaded and ballasted conditions, respectively.

The overall maximum tension for the intact condition is 716.9 MT. It gives a corresponding minimum safety factor of 1.77, which satisfies the design criteria for intact conditions. It can be seen that the maximum tension occurs at the 180wave heading when the environmental loads are in line with mooring line #5 in ballasted condition. The octagonal FPSO has a nearly axis-symmetrical hull hence the environmental loads from all directions are nearly at the same level. The maximum mooring line tension occurs when the environments are in line with one of the mooring line bundle when almost all the loads are taken by this bundle.

5.4 Polyester Mooring Tension in One Line Damaged ConditionFor the polyester mooring system, based on the results of

the intact mooring condition, the analysis for one line damaged condition is performed for 65 degree heading in the FPSO loaded condition and 180 degree heading in the ballasted

6

: Motion RAO, roll, FPSO ballasted

: Motion RAO, pitch, FPSO ballasted

Intact ConditionsFor the polyester mooring system, the calculated

maximum intact mooring line tensions in the 100-year extreme Figure 5-8 for the

maximum tension for the intact condition is 716.9 MT. It gives a corresponding minimum safety factor of 1.77, which satisfies the design criteria for intact conditions. It can be seen that the maximum tension occurs at the 180-degree

environmental loads are in line with mooring line #5 in ballasted condition. The octagonal FPSO

symmetrical hull hence the environmental loads from all directions are nearly at the same level. The

the environments are in line with one of the mooring line bundle when almost all

Polyester Mooring Tension in One Line Damaged

For the polyester mooring system, based on the results of ion, the analysis for one line damaged

condition is performed for 65 degree heading in the FPSO loaded condition and 180 degree heading in the ballasted

condition, where the maximum tensions occur. In the analyses the secondly loaded line in each group, licondition and #4 for ballasted condition, were assumed damaged. The maximum tensions in the damage conditions are summarized in the following table. It can be seen that the minimum safety factors meet the design criteria damaged conditions.

Figure 5-7: Maximum polyester mooring tensions in FPSO loaded condition

Figure 5-8: Maximum polyester mooring tensions in FPSO ballasted condition

Table 5-1: maximum mooring tension in 100damaged

LoadedEnvironment heading (deg)

65

Damaged line 2Most loaded line 3

Maximum tension (MT)

728.8

Min. safety factor 1.74Allowable safety

factor

Copyright © 2010 by ASME

condition, where the maximum tensions occur. In the analyses group, line #2 for fully loaded

4 for ballasted condition, were assumed damaged. The maximum tensions in the damage conditions are summarized in the following table. It can be seen that the minimum safety factors meet the design criteria damaged

: Maximum polyester mooring tensions in FPSO loaded condition

: Maximum polyester mooring tensions in FPSO ballasted condition

maximum mooring tension in 100-year storm -

Loaded Ballasted

180

45

852.2

1.49

1.25


5.5 Fully Dynamic Analysis of Mooring TensionIn order to investigate dynamic effects from the mooring

line and buoy to the mooring tension calculations, the most critical load case was identified and reevaluated using fully dynamic analysis method. According to the quasi-static line tension results presented before, the minimum safety factor of 1.77 occurs on line #5 for 180 degree heading when the tanker is ballasted and the mooring in system intact condition, which is considered most critical load case.

The results in Figure 5-9 show that the maximum tension from fully dynamic analysis is 727.4 MT, compared to the value of 716.9 MT from quasi-static analysis. The minimum safety factor from fully dynamic analysis is 1.75, compared to 1.77 in quasi-static analysis. The allowable minimum safety factor is 1.67, according to API RP 2SK. Therefore, the design criteria for line tension are satisfied.

According to the comparison, the difference between fully dynamic and quasi-static mooring tension is minimal for this FPSO mooring system. This agrees with our previous project experiences that the dynamic effects are very small for polyester mooring due to the fact that the rope is relatively light and stretchable (compared with chain or steel wire rope), and the effect of line dynamics is very small thus can be ignored in the design.

Figure 5-9: Fully dynamic result for polyester mooring tension in FPSO ballasted condition

5.6 Maximum Anchor LoadThe maximum anchor load is calculated using Flexcom.

The maximum anchor load is 858.2 MT when the most loaded mooring line reaches 852.2 MT at the fairlead in damaged condition.

6. MOORING SYSTEM INSTALLATION PROCEDURES It is assumed that the installation will be carried out using

a conventional anchor handing vessel (AHV) equipped with a drum winch, stern roll, chain jaws, etc,. The anchor piles (driven) may be either pre-installed or installed at the same time with the mooring lines. The platform chain, polyester ropes, buoys will be stored on the AHV prior to the installation. The polyester rope will be spooled from the transportation reel

on to the drum of the winch. The buoy is connected to the pile through the anchor chain prior to the installation. Storage and installation of anchor piles (driven pile) may be handled by another vessel. During the driving of anchor pile, the operation will paused when the pile padeye is half way in the water, and the buoy will be lifted and lowered into the water with the flooding valves open. The buoy will be flooded and descended to the sea bed. Then the pile driving will be resumed. After the installation of the pile is completed, the buoy will be deballasted and rise up to about 1 m under water. The AHV will approach the pile, pick up the loose end of the chain above the buoy and connect it to the polyester rope. After the installation of pile is completed, the polyester rope will be spooled off the winch and into the water from the stern of AHV, while the AHV is moving towards the FPSO. After the whole rope segment is spooled off the winch, the on-board end of the rope will be held by the grip device called “Chinese figure”, allowing the end of the rope (thimble) to be connected to the platform chain. Continue paying out the rope and chain into water until the AHV reach the FPSO. Then transfer the free end of the platform chain to the FPSO through a messenger wire on board of the FPSO. Pull in the wire until the chain reaches the chain jack. Then use the chain jack to tension up the line. Figure 6-1 and Figure 6-2 illustrate two selected stages of the whole installation procedures.

Figure 6-1: Mooring line installation procedure, step 7

Figure 6-2: Mooring line installation procedure, step 8

7. CONCLUSIONSIt can be concluded from the results that the new concept

of octagonal FPSO platform is feasible for operations in shallow waters conditions as discussed in this paper. The hull sizing and unique design of heave skirts effectively reduces the platform motions in the vertical dimensions.

It is also proved that the application of synthetic ropes for the mooring system is successful. The mooring line strength

5250 5350 5450 5550 5650

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Elem. no. 215; End node


has been checked using the 100-year extreme environment for maximum tension and bottom clearance. Safety factors specified in API 2SK and 2SM were used to check the mooring strength. Wind and current loads were calculated base on API guidelines. For the polyester mooring system, the calculated minimum tension safety factors are 1.75 and 1.49 in the intact and the one line damaged conditions, respectively. The mooring system meets the design criteria for both intact and one line damaged conditions.

8. REFERENCES

1. Craig D. Bloomer (2009), “Agbami Project: People and Partnership Delivering a World Scale Field Development”, OTC20249

2. Petruska, David, Geyer, Jeff, Macon, Rick, Craig, Michael, Ran, Alex, and Schultz, Neil (2004), “Polyester mooring for the Mad Dog spar—design issues and other considerations,” Ocean Engineering, November 2004.

3. Ma, W, Huang, K, Lee, MY, and Albuquerque, S (1999),“On the Design and Installation of an Innovative Deepwater Taut-Leg Mooring System”, OTC 10780.

4. Cozijn, H, Uittenbogaard, R, and ter Brake, E (2005), “Heave, Roll and Pitch Damping of a Deepwater CALM Buoy with a Skirt”, Proceeding of the 15th (2005) International Offshore and Polar Engineering Conference.

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