Technical Overview of Structural Engineering Activities for

AGBAMI FPSO Hull and Topside Interface Structures

Seock-Hee Jun∗, Oe-Ju Hwang, Joong-Kyoo Kang, Je-Hyouk Woo

Structure R&D Team

Daewoo Shipbuilding & Marine Engineering Co., Ltd.

AGBAMI FPSO 구조엔지니어링에 대한 기술적 고찰

전석희∗, 황외주, 강중규, 우제혁

구조 R&D팀

대우조선해양(주)

요 약

AGBAMI FPSO 는 서아프리카 나이지리아 OPL 216 & 217 해역에서 하루 2 백만 배럴 이상의

원유를 생산 (production), 저장 (storage), 하역 (offloading) 하도록 설계되었다. 본 호선은 수심

1500 미터 해역에서 spread mooring 계류시스템으로 위치를 유지하며 설계수명은 약 20 년이다. 특

이사항으로는 FPSO 설계에 통상 적용되는 towing 및 on-site 조건 이외에도 North-Atlantic 20 년

극한파랑조건을 적용한 unrestricted seagoing 조건이 고려되었다. 또한 선체 및 해양 Appurtenances

의 구조설계 시 주문주의 optional requirement 에 따라 ABS 선급의 SH-DLA (Dynamic Loading

Approach), SFA (Spectral Fatigue Analysis) Notation 을 만족하도록 설계되었다.

본 논문에서는 AGBAMI FPSO 의 선체 및 해양인터페이스 구조물에 국한해서 DLA 개념이

적용된 전선구조해석, 국부해석 그리고 통계적 해석절차를 따르는 SFA 피로해석의 수행내용에

대해서 개괄적으로 소개하였다. 또한 FPSO 구조엔지니어링 초기단계에서부터 면밀히 검토되고

지속적으로 관리할 필요가 있는 중요한 구조엔지니어링의 기술적 사항들과 향후 신조 FPSO 프

로젝트 수행 시 사전에 검토되고 대비해야 할 주요 사항들에 대해서 다루고 있다.

Keywords: ※ AGBAMI FPSO, DLA (Dynamic Loading Approach), SFA (Spectral Fatigue Analysis)

Tel : 055-680-5536 ∗ 교신저자, E-mail: sheejun@dsme.co.kr

2008년도 한국해양과학기술협의회 공동학술대회

5월 29(목)~30일(금) 제주 국제컨벤션센터(ICC)

1. INTRODUCTION

AGBAMI FPSO is a floating vessel system for

production, storage and offloading of crude oil in the

operation site, AGBAMI field as shown in Figure 1.

The field is located in OPL (Offshore Prospecting

Lease) 216 & 217 offshore Nigeria in water depth of

abt. 1433 m.

Fig. 1 AGBAMI Field Location

The Nigerian offshore has unidirectional and swell-

governed sea environment, which is characterized by

multi-peaked spectra; one local wind wave and two

swells. AGBAMI FPSO is spread moored toward

south-southwest on site, which is 20 degree off the

head (STAR, 2003). It was designed to satisfy the

service life of 20 years. According to the Owner’s

optional request, ABS SH-DLA and SFA notation

were taken into account for both sea-going and on-

site condition. The main particulars are as follows:

• LOA 320.0 m

• LBP 320.0 m

• Length Scantlings 310.4 m

• Breadth Moulded 58.4 m

• Depth Moulded 32.0 m

• Draught Design 23.5 m

• Draught Scantling 24.0 m

• Block Coefficient 0.995

This paper introduces a technical overview of

structural engineering activities performed for

AGBAMI FPSO Hull and Topside interface

structures in accordance with ABS DLA and SFA

procedure (ABS, 2002). In addition, this paper deals

with some other issues than technical ones which

may have to be taken care of in the process of

performing the structural engineering works.

2. DLA (Dynamic Loading Approach)

Global and local strength analyses by DLA were

done to verify whether the hull structural design

satisfied the ABS criteria for yielding and buckling

strength in the sea-going and on-site condition

(DSME, 2006). The analyses were carried out using

a DSME in-house system, D-STAS (DSME

STrength Analysis System), and two full ship FE

models and a number of local fine mesh FE models

were used to evaluate the structural strength of the

ship and offshore areas. As shown in Figure 2,

topside modules were incorporated in the global FE

model as beam structures.

Fig. 2 Complete Full Ship FE Model

A three-two-one fixation was applied at point A, B

and C of the FE model as shown in Figure 3, and

boundary nodal displacements from the full ship FE

analysis were applied to the local fine mesh model.

Fig. 3 Boundary condition

The lower nodes of bottom legs connected to

module support stool structures were constrained

with multi-point constraint to stool top plate. In the

local models, openings and bracket ends were

meshed as fine as one tenths of longitudinal stiffener

spacing. The following offshore areas, which have

additional designated local loads, were examined in

the local fine mesh analyses:

• Riser I-tubes and associated structures

• Module support stools and under-deck structures

• Mooring chain stopper and associated structures

• Off-loading line porch and associated structures

• Flare tower support structures

• Crane pedestal and associated structures

A lot of structural load cases were used to determine

critical stresses due to the combined effect of the

dominant load parameters and other accompanying

loads. Still water hull girder loads for each DLA

loading condition were checked along the vessel’s

length. As shown in Figure 4, ship motions and wave

loads were calculated by WASIM taking into

account both hull and free surface and incorporating

the vessel’s forward speed in the case of sea-going

and towing condition (DNV, 2005).

Fig. 4 Fig. 3 Hydro panel models

Mass models were made from the full ship FE model

calibrated by the adjustment of material densities in

order to increase the accuracy of the results of sea

keeping analysis and FE analysis. By tuning linear

roll damping such that the calculated maximum roll

amplitude matches the model test result in the beam

wave, equivalent linear roll damping coefficients

were determined. The roll motion RAO in the sea-

going condition was plotted in Figure 5.

.

Fig. 5 Plot of Roll Motion RAO

Extreme responses were calculated applying the

North Atlantic wave data for sea-going condition

and the metocean wave data for on-site condition. It

was noted that the calculated extreme VWBM and

VWSF were much bigger than the corresponding

rule values in the sea-going condition as summarized

in Table 1.

Table 1 Ratio between extreme value and rule value

The long-term extreme responses in sea-going

condition were determined for 20 years service life.

The short-term responses on site were evaluated at a

short time interval of 3 hours. In order to take

account of short crested waves in the real sea state,

cosine squared wave spreading functions in sea-

going and cos2/cos

4 wave spreading functions on site

were adopted.

The envelop curves based on long-term response

values of VBM and VSF were established. The

sectional loads of each DLP at a specific design

wave were compared with the corresponding long-

term values at each longitudinal location. In order to

minimize the unbalanced forces and moments, the

self-weight of the FE model was controlled by the

adjustment of gravity acceleration. The distribution

of external sea pressure and internal tank pressure

were checked as shown in Figure 6.

Fig. 6 External pressure distribution

Strength criteria and element mesh size were

considered depending on analysis step, loading

condition, material grade and ABS criteria. All the

engineering activities in DLA were successfully

finished and the proposed structural design was

found to well meet the FPSO design requirements.

3. SFA (Spectral Fatigue Analysis)

For the AGBAMI FPSO Hull and Topside

interface structures, fatigue strength was evaluated

using a component-based spectral analysis (DSME,

2006). In particular, a DSME in-house system, D-

SFAS (DSME Spectral Fatigue Analysis System),

was used to calculate the fatigue life. As shown in

Figure 7, the U.K. DEn S-N curve as a basic curve

was utilized and thickness effect was considered

according to the Owner’s design specifications.

Fig. 7 Design S-N curve (from U.K. Den)

A total of six loading conditions, one for towing, two

for sea-going and three for on-site conditions, were

considered in the fatigue analysis as per the Owner’s

requirements. The vessel was designed to provide

sufficient fatigue strength even for the sea-going

condition under North Atlantic environment. In the

case of on-site condition, three cargo loading

conditions (ballast, full-loaded, intermediate) were

considered. Considering that the towing of the vessel

from shipyard to AGBAMI site was expected to take

three months, the fraction of lifetime in the towing

condition was set to 1% when calculating the total

fatigue damage ratio

The Bretschneider wave spectrum and Walden wave

data complying with ABS requirement were taken

into account for the sea-going and towing conditions.

The wave scatter diagram was composed of three

parts of waves from the metocean data; wind wave,

primary swell and secondary swell, for which

appropriate wave spectra were defined. Jonswap

spectrum was assigned to the local wind wave and

Gaussian spectrum to the swells. The Gaussian

spectrum is expressed as follows:

⎥⎦

⎤⎢⎣

⎡ −−=

2

2

00

2

)(exp

2)(

xxS

ff

S

mfS

π

Where, 0

m is zeroth spectral moment

0f is peak frequency

xS is standard deviation of variable x

To obtain the cumulative fatigue damage for on-site

condition, the effect of wind wave and two swell

waves were combined using the spectral

combination method as shown below,

2

2

2

1

2

swellswellwavecSSSS ++=

Where,

Sc is standard deviation of combined spectrum

Swave is standard deviation of wind wave

Sswell1 is standard deviation of primary swell

Sswell2 is standard deviation of secondary swell

The global FE analysis was performed as many

times as the number of load components considered

in the component-based spectral fatigue analysis. As

shown in Figure 8, the fatigue models with the mesh

size of plate thickness at the crack-prone areas were

made to establish the hotspot stresses. The hotspot

stresses at weld toes were calculated by linear

extrapolation.

Fig. 8 Fatigue Model of Flare Tower Supports

It is essential to have a screening process to identify

the critical locations among a number of details and

ensure the fatigue strength over the hull structure.

The critical locations for the longitudinal stiffener

connections on side shell were simply screened by

referring to the results from ABS Safe-Hull Phase A,

i.e. simplified fatigue analysis based on Weibull

probability distribution for the on-site condition. On

the contrary, the other specific details including

topsides stools and I-tube connections were screened

based on the spectral fatigue analysis using coarse

mesh FE model with the mesh size of one

longitudinal stiffener spacing. The on-site condition

was chosen as a basis for screening analysis.

Through discussions with the Owner, the loading &

offloading sequence was determined. As shown in

Figure 9, the hull structure was designed to satisfy

the requirement of 1 Mbbls offloading operation

within 24 hours, including connection, ramping up

and topping off.

Fig. 9 Loading/Offloading Scenario for LCF

It is necessary to combine the interface loads from

the subsea parties such as OOL, risers and moorings

with the wave-induced loads. The fatigue loading

data were given in the form of load-cycle from the

results of the time domain analysis. For the offshore

details specified in FPSO, the fatigue life may be

mostly dependent on the offshore operating loads.

The fatigue strength of the AGBAMI FPSO hull and

offshore structures was verified through SFA for not

only the on-site condition but also the sea-going

condition, and it was found that the sea-going was

governing.

4. Outstanding Items

AGBAMI FPSO structural engineering works give a

number of useful feed-back experiences and makes

the successful engineering activities on time for the

new FPSO project. In order to minimize the errors

and losses that may arise from engineering activities,

the following outstanding items should be cautiously

controlled in the initial stage of engineering process.

Additional Works

Through a comparative study on the results of sea-

keeping analysis, DLA analysis and SFA done by

DSME and ABS respectively, it was confirmed that

the results from DSME were equivalent to those

from ABS.

Also, the comparison of dominant loads from towing

condition and sea-going ballast condition was made

to prove that the dominant loads in towing condition

are less than those in seagoing condition. From the

result, the DLA analysis in towing was waived.

As per the criterion for the selection of critical

locations referred to the Owner’s specifications, the

most critical locations were identified from the

analysis of DLA and SFA, which have been given

the corresponding notes in the structural drawings

and Safe-Hull Construction Monitoring Plan.

In the process of engineering, a number of RFI

(Request for Information) documents including

inquires and comments were transmitted among

three parties; the Owner, the Class and the Builder.

These documents could be a good reference to

understand the Owner’s intentions and Class

requirements very well.

Schedule and Man Power

During AGBAMI FPSO was constructed in shipyard

until the delivery stage after C/A, the engineering

schedule was changed many times. To minimize the

impact from frequent change in the schedule, the

following items should be considered:

• Sufficient understandings for target project

• Attentive feasibility study before C/A

• Quick establishment of design brief

• Sufficient contingency at initial design stage

• Close communication between three parties

• Consideration of fabrication sequence

• Rule application for Hull & Offshore areas

• Prudent selection of loading conditions

A number of members participated in the hull

strength evaluation by DLA, SFA and Accidental

Load Analysis because a lot of engineering activities

had to be carried out within a short period. In order

to obtain a lot of FE models as quickly as possible,

outsourcing modelling works was done through the

university and engineering company. For the future

project, it is recommended to settle down the

realistic action plan of schedule and man power.

External and Internal Interfaces

The FPSO consists of three parts; hull, topside and

subsea structures. The ship builder must provide and

receive the interface data for and from other parts as

well as sub-contractors in order not to have an

impact on engineering. Therefore, the external and

internal interface works play an important role for

the successful project execution.

To perform the structural analysis works in time, the

internal key information from the prerequisite works

such as T&S booklet, hydrodynamic analysis,

structure drawings, vendor data, etc. should be

provided as soon as possible. Even a slight delay in

the internal information might consequently be a big

impact on the accompanying engineering works.

In case of outsourcing of FE models, a typical FE

model specification needs to be published in

advance, which will be useful for the subcontractors

to avoid additional modification and/or time

consuming. The FPSO has a number of specific

interfaces such as topside, flare boom, mooring, riser

and so on. The external loads as tabulated in Table 2

were taken into account for the hull interface

structures. These data should be received from the

Owner, sub-contractors and equipment vendors in

order not to have an impact on design.

Table 2 External Interfaces in AGBAMI FPSO

Early Check Items

From the lessons-learned of AGBAMI FPSO, the

following items in view of strength and fatigue

should be carefully checked from the initial

engineering stage in order to prevent post design

impact.

• Hydrostatic equilibrium of barge shaped vessel

• Wave loads check in extreme wave condition

• Strength criteria and element mesh size

• Loading/unloading sequences for production

• FE model control in view of model outsourcing

• Interface data control such as hull deflection,

topside reactions and external operation loads

• Screening procedure to reduce check locations

• Stiffness and boundary constraints of Topsides

• Site specific multi-peak wave scatter diagram

• Operating loads from sub-contractors or vendors

A complete design brief shall be made as a proactive

measure for main demands from the Owners and the

Class. Some data check sheets can be produced in

order to minimize the engineering human errors and

to make the best communication between pre and

post processing engineers.

5. Conclusion

This paper presented a technical overview of

structural engineering activities performed for

AGBAMI FPSO Hull and Topside interface

structures in accordance with ABS DLA and SFA

procedure. Also, some useful outstanding items were

introduced as a practical guide for the successful

engineering process for the new FPSO project in the

future.

[References]

•ABS, 2002, “Guidance Notes on SafeHull-Dynamic

Loading Approach for FPSO System”

•ABS, 2002, “Guidance Notes on Spectral-based

Fatigue Analysis for FPSO System”

•DSME, 2006, “3-D F.E. Analysis by DLA,”

Document No.: AGB-DSM-FM-ANA-ST-0003.

•DSME, 2006, “Fatigue Analysis for AGBAMI

FPSO (I) and (II),” Document No.: AGB-DSM-FM-

ANA-ST-0032 and AGB-DSM-FM-ANA-ST-0034.

•DNV, 2005, “SESAM User Manual–WASIM”

•STAR, 2003, “AGBAMI PROJECT–Design Basis,”

Document No.: AGB-CVX-GN-DSG-GN-0001.