The Influence of Additional Equipment to the

Ultimate Strength of FPSO

Muhammad Zubair Muis Alie

, Juswan

, Wahyuddin Mustafa

, Kevin Gabriel Pangalinan

and Nurul Inda Pratiwi

Department of Ocean Engineering, Hasanuddin University, Jalan Poros Malino km. 6 Bontomarannu,

Makassar, Indonesia

KTU Shipyard, Project Department, Riau, Indonesia

Keywords: FPSO, Cross Section, FEM, Ultimate Strength.

Abstract: Nowadays, there have many ship conversions, for example, from double hull tanker to Floating Production

Storage Offloading (FPSO). The conversion is conducted due to the change function of the ship. On the

other hand, due to the change function of the ship, some items of the ship, such as equipment, need to be

evaluated because of those related to the ultimate strength of the ship. The objective of the present study is

to analyze the influence of additional equipment to the ultimate strength of FPSO under vertical bending

moment for hogging condition. The cross-section of FPSO is taken by considering one-frame space. The

numerical method is used, and the Multi-Point Constraint (MPC) is applied to both sides of the cross-

section, and it is assumed to have remained plane. The shell element type is used for modeling the cross-

section. The material properties are set to be constant. As a simple calculation, the ship’s cross-section is

analyzed in intact. The additional equipment is calculated and included in the analysis to know the influence

of it. The analysis of the ultimate strength, including their influence of additional equipment, is conducted

under longitudinal bending. To compare the ultimate strength obtained by the numerical method, the

analytical method is adopted. It is found that the ultimate strength gained by the numerical method is almost

identical to the analytical method. The behavior of the ship’s cross-section is also presented in terms of

stress distribution and deformation.

1 INTRODUCTION

The conversion of a ship is implemented in recent

years. This phenomenon is mostly conducted for an

offshore structure such as from Double Hull Tanker

to Floating Storage Offloading (FSO) or Floating

Production Storage Offloading (FPSO). The

conversion aims to change the ship function and

operation, including ship payload. In some years, the

conversion from tanker to FPSO is very significant.

Time-consuming is shorter than a new design; that is

one of the reasons. On the other hand, during

conversion, some equipment must be added not only

for the ship’s function but also for the ship’s

construction. Due to this reason, the analysis of the

ultimate strength of being converted considering the

additional equipment must be taken into account.

The analysis of the ship’s strength had been

introduced by some papers like; The residual

strength of an Aframax-class double hull oil tanker

damaged in the collision had been assessed by

(Parunov, Rudan, and Bužančić Primorac, 2017)

considering the influence of the rotation of the

neutral axis. The impact of nonlinear finite element

method models on the ultimate bending moment for

hull girder was studied by (Xu and Pan 2017). There

were two analyses performed; those were implicit

static analysis and explicit dynamic analysis. A

structural reliability analysis model based on a

Bayesian belief network was proposed by (Li and

Tang, 2019) for the hull girder collapse risk after

accidents. The Bayesian belief network was used to

represent random states of variable risk events after

accidents, as well as the dependencies between

activities, and the structural reliability analysis was

used to evaluate the failure probability hull girder

for each possible accident conditions. The incidence

of collision damage models on an oil tanker and bulk

carrier reliability was investigated by (Campanile,

Piscopo and Scamardella, 2018) considering the

Alie, M., Juswan, ., Mustafa, W., Pangalinan, K. and Pratiwi, N.

The Inﬂuence of Additional Equipment to the Ultimate Strength of FPSO.

DOI: 10.5220/0010056700830087

In Proceedings of the 7th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management (ISOCEEN 2019), pages 83-87

ISBN: 978-989-758-516-6

IACS deterministic model against GOLADS/IMO

database statistics for collision events, substantiating

the probabilistic model. Safety of an oil tanker in

intact condition was performed by (Campanile,

Piscopo, and Scamardella, 2017) to investigate the

incidence of load combination methods on hull

girder sagging/hogging time-variant failure

probability. The simplified approach to the ultimate

hull girder strength of asymmetrically damaged

ships was conducted by (Muis Alie, 2018),

considering the critical element under sagging

condition. The effect of symmetrical and

unsymmetrical configuration shapes on buckling and

fatigue strength analysis of fixed offshore nonlinear

finite element was conducted by (Muis Alie, 2016),

and damages were modeled simply by removing the

element on the damaged part. The progressive

collapse analysis of the local element was conducted

by (Muis Alie and Latumahina, 2019) and the cross-

section of the Ro-Ro under longitudinal bending.

In the present study, the hull girder strength

analysis caused by additional equipment on FSO

after being converted into FPSO under longitudinal

bending is conducted. The ship’s construction is not

changed, but additional equipment after conversion

is applied. For the simple calculation, the one-frame

space of FPSO’s cross-section is considered. The

cross-section is assumed to have remained plane.

The element type of shell 181 is used on the model.

The initial imperfections, cracks, and residual

welding stress are not taken into account. The

ultimate strength obtained by the numerical method

is compared with the analytical method, and the

behavior of the ship in terms of stress distribution

and deformation are also shown in this study.

2 FINITE ELEMENT MODEL

In the present study, the numerical method to

analyze the hull girder strength due to load

equipment of change function from FSO to FPSO

before and after being converted is conducted. The

ship has 172 m, 30 m, and 18.4 m of length, breadth,

and depth of ship, respectively. The midship section

consists of two kinds of longitudinal stiffeners those

are flat-bar and angle-bar. The analysis of the

ultimate strength is performed under the hogging

condition. There is also the inner hull in the cross-

section. The element type is shell-181. The shell-181

element applied to all of the cross-sections, as shown

in figure 1.

Figure 1: Finite element model.

According to figure 1, there are two points

located at the neutral axis. These two points are

placed at both sides of the cross-section, and those

are used to place the MPC (Multi-Point Constraint)

for representing the behavior of the cross-section.

The ultimate strength analysis, including the effect

of change function from FSO to FPSO, is calculated

using the numerical method under sagging

condition. The rigidly linked corresponding with the

boundary condition where MPC is applied to both

sides of the cross-section, as shown in figure 2.

Figure 2: Rigid link and MPC.

According to figure 2, the applied MPC is placed

at the fore and aft side of the cross-section. This is

done to connect the MPC called master node and the

points of the all side’s cross-section. The applied

moment is placed at this MPC.

ISOCEEN 2019 - The 7th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management

3 RESULTS AND DISCUSSIONS

The behavior of the ultimate strength analysis is

described in terms of working stress distribution.

Figure 3 shows the working stress of FSO under the

hogging condition. The deck and bottom part are

under tension and compression since the hull cross-

section is under hogging condition.

Figure 3: Working Stress of FSO in Hogging.

Figure 4 shows the working stress of FPSO

under the hogging condition. It is observed that the

characteristic of the working stress of FSO and

FPSO is different from one another due to the

additional equipment after being converted.

Figure 4: Working Stress of FPSO in Hogging.

Figure 5 shows the moment-curvature

relationship of FSO under the hogging condition.

The ultimate strength and collapse stages are

represented by points A and B, respectively.

Figure 5: Moment-Curvature of FSO in Hogging.

Deformation of the FPSO at the ultimate strength

and collapse stage at point A and B under hogging

condition are shown in figures 6 and 7, respectively.

Failure takes place at some elements at the deck part

where tension happen under hogging condition.

Figure 6: Deformation of FSO at Ultimate Strength.

Figure 7: Deformation of FSO at Collapse.

0 0.1 0.2 0.3

Moment x 10

(Nmm)

Curvature x 10

-2

(mm

-1

)

The Inﬂuence of Additional Equipment to the Ultimate Strength of FPSO

Figure 8: Moment-Curvature of FPSO in Hogging.

Figure 9: Deformation of FPSO at Ultimate Strength.

Figure 10: Deformation of FPSO at Collapse.

Figure 8 shows the moment-curvature relationship

of FPSO at point A and B in hogging conditions.

Points A and B represent the ultimate strength and

collapse regime, respectively. Figures 9 and 10 show

the deformation at the ultimate strength and collapse

region of FPSO under hogging condition.

Figure 11: Moment-Curvature Relationship.

Figure 11 expresses the comparison of the moment-

curvature relationship after being converted from

FSO to FPSO under the hogging condition.

According to figure 11, the bending moment

capacity of FSO is larger than FPSO. This is because

the additional equipment is applied from FSO to

FPSO so that the capacity bending moment after

being converted is decreased.

Figure 12: Comparison of Moment-Curvature

Relationship.

For the comparison purpose for the ultimate strength

analysis, the analytical method is adopted. Figure 12

shows the comparison of the ultimate strength

obtained by the numerical method and analytical

method. The numerical method is represented by the

solid line and the dashed line for the analytical

method. It is found that the ultimate strength

obtained by the numerical method is larger than the

analytical method. This phenomenon happens due to

0.00 0.05 0.10 0.15 0.20

Moment x 10

(Nmm)

Curvature x 10

-2

(mm

-1

)

0.00 0.10 0.20 0.30

Moment x 10

(Nmm)

Curvature x 10

-2

(mm

-1

)

FPSO

FSO

0 0.1 0.2 0.3

Moment x 10

(Nmm)

Curvature x 10

-2

(mm

-1

)

FE Method

Smith's Method

ISOCEEN 2019 - The 7th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management

the redistribution of the stress concentration takes

place since the three-dimensional analysis is

considered. The bending stiffness is also changed

due to the additional equipment after being

converted from FSO to FPSO.

4 CONCLUSIONS

The ultimate strength analysis has been conducted

by using the numerical method under longitudinal

bending in hogging condition. The following

conclusions are; the additional equipment has a

significant influence on the ultimate strength after

being converted from FSO to FPSO. The bending

stiffness is different due to additional equipment

before conversion from FSO to FPSO. The ultimate

strength obtained by the numerical method for FPSO

is larger than FSO.

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Campanile, A., Piscopo, V. and Scamardella, A. (2017)

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Campanile, A., Piscopo, V. and Scamardella, A. (2018)

‘Conditional reliability of bulk carriers damaged by

ship collisions’, Marine Structures. Vol. 58, pp. 321–

341

Muis Alie, M. Z and Latumahina, S. I. (2019) ‘Progressive

Collapse Analysis Of The Local Elements And

Ultimate Strength Of A Ro-Ro Ship', International

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The Inﬂuence of Additional Equipment to the Ultimate Strength of FPSO