Sea Keeping Performance Investigation of Cylindrical Floating

Production Storage and Offloading (FPSO) on the Indonesian

Offshore Environment

Sarjito Joko Sisworo

, Aulia Windyandari

and Ahmad Fauzan Zakki

Naval Architecture Department, Engineering Faculty, Diponegoro University, Indonesia

Industrial Technology Department, School of Vocation, Diponegoro University, Indonesia

Keywords: Sea Keeping, FPSO, Natuna Island.

Abstract: The Natuna Islands have the largest natural gas reserves in the Asia Pacific region even in the World. To

explore the hydrocarbon reserves that are scattered in many points, the FPSO is one of the alternative

choices that investors are interested. The FPSO investment is more efficient than FSO, because the FPSO is

a mobile system that able to carry out a reliable production processes and operation. The technological

revolution in the FPSO has shown a significant improvement through the development of cylindrical shaped

FPSO instead of ship shaped FPSO. In this study, the research is focused on the numerical investigations to

obtain sea keeping performance of cylindrical FPSO which have an alternative bottom shape design in the

Indonesia sea environment. By considering the results of the analysis of the ship's FPSO motion

characteristics (Ship Shaped FPSO) and the JONSWAP wave spectrum, the operability analysis of the

FPSO cylinder was performed to determine the relationship between the FPSO cylinder motion

characteristics and the ability to operate at the sea, and concerning the specified operating criteria. Heave

and roll motion performance will be discussed in this study.

1 INTRODUCTION

FPSO (Floating Production Storage and Offloading)

is a facility on a floating building that is operated in

offshore oil and gas field that functions to receive,

process, store and distribute hydrocarbons that are

permanently tethered to where it operates and can be

moved from one place to another place. Based on

the shape of the building, the FPSO is divided into

two kinds of types that consist of a ship shaped and a

cylindrical shaped vessel.

The first time, the barge shaped FPSO was

dedicated by Arco, that is used in the Ardjuna field

and it is operated at a depth of 42.7 meters in the

Java Sea offshore in 1976 (D’Souza et.al, 1994).

Then, it is followed by the Castellon FPSO in the

offshore Spain which was using a tanker shaped

design that would be operated for a 10-years field

life in 1977. As the increasing demand for oil-gas

and for exploration cost efficiency, nowadays the

operating FPSO is designed to support the field

exploration with return period of waves for the

longitudinal strength design as 100 years (Paik and

Thayamballi, 2007).

In 2007, the first cylindrical FPSO in the world is

introduced as the Sevan Piranema FPSO. The

cylindrical FPSO is developed by Sevan Marine

which operated in the offshore Brazil with the

design of operations in ultra-deep waters, ranging

from 1000 m - 1600 m (Paik and Thayamballi,

2007)., see Fig. 1. Then it is followed by the

construction of other cylindrical FPSOs, including:

FPSO Hummingbird, FPSO Voyageur and FPSO

Goliath.

Although the FPSO have shown good

performance, however, in the last decades the

performance limitations was observed for FPSO

with the traditional ship shaped type and cylindrical

shaped FPSO (Wang and Feng, 2011; WU, 2012).

Ship shaped FPSO is significantly sensitive to the

wave direction. These kind of characteristics might

cause poor performance on FPSO heave and roll

motion in the oblique wave condition which is FPSO

usually being operated with the single point mooring

system. Since ship shaped have significantly large

Sisworo, S., Windyandari, A. and Zakki, A.

Sea Keeping Performance Investigation of Cylindrical Floating Production Storage and Ofﬂoading (FPSO) on the Indonesian Offshore Environment.

DOI: 10.5220/0008565901530158

In Proceedings of the 6th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management (ISOCEEN 2018), pages 153-158

ISBN: 978-989-758-455-8

153

longitudinal scale, the wave impact and green water

load might be occurred and possible to damage the

deck structure. Moreover, the turret structure also

vulnerable because of the excessive yaw motion

which is generated by the vane effect. The bending

moment of hogging and sagging may cause fatigue

failure and severe deformation in the ship shaped

FPSO. In the case of cylindrical shaped, the heave

motion response is significantly large because of the

natural period of heave motion is centralized in the

area of wave energy.

Based on the condition, this paper is focused on

the investigation of sea keeping performance of

cylindrical FPSO which influenced by the

modification of the geometry of bottom shape

design. The modification of the bottom shape

geometry is conducted by adopting the disc shaped

for the skirt design to improve the heave motion

characteristics. Furthermore, the sea keeping

characteristics of the proposed design is investigated

and compared with the sevan piranema performance.

Figure 1: The cylindrical shaped FPSO developed by

Sevan Marine.

2 MATERIAL AND METHODS

2.1 Literature Review

Recently, several new concepts design of the

offshore structures are proposed to improve the sea

worthiness and motion characteristics to support the

exploration activities on the wave environment.

Some literature could be found for the development

of marine structure design that was made to improve

the heave motion performance of the cylindrical

FPSO. Goncalves, et. al., (2009) was developed the

concept design of Mono column Production Storage

and Offloading System (MPSO) to reduce the

motion, to maintain the storage capability and have

the ability to facilitate the use of rigid risers. The

MPSO adopted an auxiliary apparatus which consist

of moon pool, beach, skirt and spoiler plates to

minimize the vertical motion. The developed MPSO

design provides the best solution for the exploration

field in the pre-salt area of Brazil. In the other study,

Goncalves, et al., (2010), develop the new concept

for MPSO by combining two functional as great oil

storage capacity and dry tree production capability.

The results show that the decreasing vertical motion

could be achieved by implementing the appropriate

appendages to the platform. The experimental

comparative study on Vortex Induced Motion of a

Mono column platform also investigated by

Goncalvez, et. al. for the design of MPSO in the

Gulf of Mexico (2012).

Besides the cylindrical FPSO of sevan

technology and MPSO, a sandglass type ocean

engineering floating structure is introduced by

Huang et. al. (2013). The sand glass type is provided

with an inversed truncated cone as the upper body,

and a regular frustum or truncated cone for the lower

body. Wang, et. al. was made a numerical and

experimental analysis on the motion performance of

the new sandglass-type floating body in waves

(Wang, et.al, 2016). The study was found that the

sandglass-type design able to improve the

hydrodynamic performance of FPSO.

Figure 2: The geometry of cylindrical FPSO: (a) original

sevan technology design; (b) modified design with the

Disc Shaped skirt geometry.

ISOCEEN 2018 - 6th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management

154

2.2 Modification of the Cylindrical

FPSO Hull Form

The modification of cylindrical FPSO is made by

modifying the bottom design of the existing hull

form design. Therefore the principal dimension was

adopted from the previous hull form design data that

might be seen on the Table 1. Since the principal

dimension was determined, the next step is to

modify the skirt design of the bottom of cylindrical

FPSO. The previous skirt design of the cylindrical

FPSO has the thin truncated frustum shape and the

flap fin is attached at the tip of the frustum, see Fig.

2(a). The flap and the frustum shape are subjected to

reduce the heave motion and vertical acceleration.

In the process of modification, the bottom shape

of the FPSO was changes with the adoption of the

thick disc shaped for the skirt design; see Fig 2(b).

The larger volume of the thick disc shape is

considered able to increase the added mass of the

cylindrical FPSO. Therefore it is expected that the

modification might reduce the vertical acceleration

induced by wave force in the offshore environment.

The influence of the design modification to the

motion performance would be investigated using

strip theory method for the sea keeping analysis. The

sea keeping analysis is performed with the wave

spectrum that is defined for the Indonesian offshore

environment.

Table 1: Principal Dimension of the Cylindrical FPSO.

Parameters Dimension

Hull Diameter 93 m

Bilge Box Diameter 124 m

Bilge Box Plate Diameter 138 m

Main Deck Diameter 103 m

Process Deck Diameter 109 m

Main Deck Diameter 42 m

Process Deck Diameter 48 m

Draft, Ballast 22 m

Draft Loaded 31 m

Constant Draft 27 m

Freeboard to Ballast 20 m

Freeboard to Loaded 11 m

2.3 Wave Spectrum and the Offshore

Condition

In this study the wave spectra used are the ITTC

wave spectra with 2 corresponding parameters in Eq.

1. The significant wave variations (Hs) used were 3

m, 2.5 m, 2 m, and 1.5 m with the Average Period

(Tav) variation of 6.09 seconds, 5.60 seconds, 4.82

seconds and 3.70 seconds. This seawaters condition

is categorized as moderate waters or sea state

number 4.

ITTCϛ

(ω) =















exp



(1)

where:

ω = wave frequency (rad/s); A = 172.75

ave

; B =

691

ave

The wave spectrum (Sω) generated from Eq. 1

significantly depends on the value of the wave

frequency. As a result of the influence of floating

body speed and the angle of attack of the wave, the

frequency of the incident wave (ωw) will change to

the encountering wave frequency (ωe). This

encountering wave is used to make the encountering

wave spectra (Se). To calculate the encountering

wave frequency, Eq. 2 might be used.

= ω



















cos1

(2)

where:

ωe = encountering waves frequency (rad/s)

ωw = wave frequency (rad/s)

V = ship speed (m/s)

g = gravity acceleration (9.81 m/s2)

The offshore conditions in this study refer to the

conditions (Sea State Code) that have been

determined by WMO (World Meteorological

Organization) on the review of three variations of

sea conditions with different parameters covering

the highest third of wave height (significant wave

height), wave period, and wind speed (Sustained

Wind Speed). Variations in offshore conditions are

small waves (Slight), moderate waves (Moderate),

and large waves (Rough), see Table 2.

North Offshore Natuna-Indonesia is identified as

a sea waters with medium wave height. Based on

data from the Climatology and Geophysics

Meteorology Agency, North Offshore Natuna-

Indonesia has an average wave height of 1.0 m - 2.0

m with a maximum wave height of 2 m - 3 m. The

average wind speed in the Java Sea is 15 knots - 20

knots.

Sea Keeping Performance Investigation of Cylindrical Floating Production Storage and Ofﬂoading (FPSO) on the Indonesian Offshore

Environment

155

Table 2: World Meteorological Organization Sea State

Code.

Sea

State

Code

Sustained

Wave

Height

1/3

) (m)

Sustained

Wind

Height

(knots )

Wave

Period

(s)

Description

Range Range

3 0,5 – 1,25 11 – 16 7,5 Slight water

4 1,25 – 2,5 17 – 21 8,8 Moderate

water

5 2,5 - 4 22 – 27 9,7 Rough

water

3 RESULTS AND DISCUSSIONS

Evaluation of the performance of a floating body

depends on the environment condition. This

environment is being expressed in the form of a

wave spectrum. Based on the FPSO environment

condition, namely in the North Natuna Offshore

Indonesia, the ITTC wave spectrum will be adopted

for this research, by regarding that recently the

offshore environment are considered more

dangerous than 20 years ago. In the sea keeping

analysis, it is consisted of three categories of

problems that are included:

1. Estimation of environmental conditions

which is applied to the FPSO model

2. Prediction of the response characteristics of

the FPSO model

3. Specification of criteria that is used to assess

the behavior of FPSO motion.

The performance assessment also can be done by

comparing with the other FPSO. The comparison of

the two FPSO designs or reviewing a design against

the predetermined acceptance criteria depends on

accurate information from the three categories.

In this study, the evaluation of motion was

carried out to get varied movements from the bottom

design configuration. Comparison between the two

designs of the FPSO Sevan Piranema with several

types of bottom variations is done to find out the

advantages and disadvantages of the behavior of one

another. This is needed in providing a bottom design

recommendation that is better for use on cylinder

FPSO. The acceptance criteria for floating body

performance are determined in NORDFORSK 1987

and the operating criteria are adopted from the

operability criteria for the Essar Wildcat.

0.0 0.3 0.6 0.9 1.2 1.5

0.00

0.15

0.30

0.45

Pitch Motion RAO

Encounter Frequency (Rad/S)

Disc-Shape

Original

0.0 0.3 0.6 0.9 1.2 1.5

0.0

0.5

1.0

1.5

Heave Motion RAO

Encounter Frequency (Rad/S)

Disc-Shape

Original

Figure 3: The FPSO Response Amplitude Operator: Heave

Motion (Top); Pitch Motion (Bottom).

Motion analysis is calculated using

hydrodynamic diffraction method. The results of the

analysis of the ship motion are divided into several

parts, namely:

1. Response Amplitude Operator (RAO) from

heave and pitch motion

2. Wave spectra of the North Natuna Offshore

Indonesia

3. Vertical acceleration of the two FPSO design

The results of this motion analysis that are

represented in the form of Response Amplitude

Operator graphics for heave and pitch can be seen in

Fig. 3.

It can be observed that the motion characteristics

of the two cylinder FPSO model have a fairly small

rotational motion (pitch). The Disc Shape Skirt

design has a larger rotational motion compared with

the original FPSO model. This is caused by

differences in geometry configuration, and bottom

dimension size, that gives a difference in the cross-

sectional area of the structure that might be

influenced by sea waves, which in turn gives

different characteristics of motion. In heave motion

RAO, it can be seen that Original Model has the

higher maximum value of 3.84 m/m, at the

encounter frequency of 0.31 rad/s. In pitch motion

RAO, the higher maximum value is obtained in the

ISOCEEN 2018 - 6th International Seminar on Ocean and Coastal Engineering, Environmental and Natural Disaster Management

156

Disc Shape Model with the magnitude of RAO is

0.41 deg./m, at the encounter frequency of 0.44

rad/s.

0.0 0.3 0.6 0.9 1.2 1.5

0.0

0.3

0.6

0.9

S

Wave encounter frequency (Rad/S)

1.5m

2.0m

2.5m

3.0m

Figure 4: The wave spectrum of the North Natuna

Offshore Indonesia.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0.00

0.01

0.02

0.03

0.04

0.05

Pitch Response Spectrum

Wave encounter frequency (Rad/S)

Disc-Shape

Original

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030

Heave Response Spectrum

Wave encounter frequency (Rad/S)

Disc-Shape

Original

Figure 5: The FPSO Motion Response Spectrum: Heave

Motion (Top); Pitch Motion (Bottom).

The spectral peak period distribution in the North

Natuna Offshore Indonesia has a range between 3.70

s - 6.09 s and the range of significant wave height

(Hs) is between 1.5 m - 3 m. Each peak period

consists of certain variations (Hs). Fig. 4 shows the

wave energy spectrum of the North Natuna Offshore

Indonesia in each period using the ITTC formula.

Response spectra were obtained after multiplying

the wave spectra with RAO. In this response spectra

computation, it is only conducted on the fluctuation

of heave and pitch motion modes that are conformed

to the operating criteria requirements. The response

spectra of the heave and pitch motion of the two

FPSO is shown in Fig. 5.

Table 3: Heave and Pitch Motion of the two FPSO.

Motion

Parameters

Significant

Wave

Height

Original

Cylindrical

FPSO

Disc-

Shaped

Skirt FPSO

Heave

Amplitude

3.0 m 0.023856 0.011821

2.5 m 0.013845 0.007178

2.0 m 0.005046 0.003330

1.5 m 0.000638 0.000647

Heave

Velocity

3.0 m 0.015973 0.008199

2.5 m 0.009775 0.005449

2.0 m 0.003996 0.002821

1.5 m 0.000596 0.000630

Heave

Acceleration

3.0 m 0.011035 0.006039

2.5 m 0.007131 0.004315

2.0 m 0.003249 0.002458

1.5 m 0.000564 0.000624

Pitch

Amplitude

3.0 m 0.094705 0.112535

2.5 m 0.059676 0.075440

2.0 m 0.025725 0.040202

1.5 m 0.004886 0.016221

Pitch Velocity

3.0 m 0.066637 0.084585

2.5 m 0.044356 0.061391

2.0 m 0.021307 0.038427

1.5 m 0.005086 0.018422

Pitch

Acceleration

3.0 m 0.048786 0.069121

2.5 m 0.034267 0.054329

2.0 m 0.018336 0.039073

1.5 m 0.005449 0.021085

The area under the response spectra curve for

each motion needs to be obtained, since it would be

used to find the statistical value of each movement

as the next procedure. Furthermore, significant

amplitude values and average amplitude can be

obtained for each FPSO design. The calculation

results can be seen in Table 3. The value of the

amplitude of the average movement response in each

movement is correlated with the operating criteria,

to obtain operability in each period and significant

wave height.

From the analysis results, it is show that the

Disc-Shape Cylindrical FPSO model has a different

motion acceleration compare with the original

model. The Disc-Shape model has a 45.27% lower

heave motion acceleration compare with the original

model. However the pitch acceleration of Disc-

Shape model is 41.68% higher than the Original

Model. It is indicated that the Disc Shape model is

an effective damper for the translation acceleration,

Sea Keeping Performance Investigation of Cylindrical Floating Production Storage and Ofﬂoading (FPSO) on the Indonesian Offshore

Environment

157

otherwise the original model have a better rotational

damper. It might be explained that the original

model have the flap skirt that is able to reduce the

rotational motion.

According to the acceptance criteria for the

operational requirement of the two FPSO, it can be

seen that the motion amplitude of both FPSO

variation models, for heave and pitch, up to the

highest significant wave height, are comply with the

NORDFORSK 1987 and the Essar Wildcat criteria,

see Table 4 and Table 5. Although in the Disc Shape

FPSO model have larger maximum RAO than

original model, the Disc Shape model generally

shows better heave motion response than the original

model, because of the wave spectrum have a small

value at the range of encounter frequency of 0.5-0.8

rad/s.

Table 4: NORDFORSK 1987 operability criteria.

Description RMS Vertical Acceleration

Light Manual Work 0.20 g

Heavy Manual Work 0.15 g

Intellectual Work 0.10 g

Transit Passengers 0.05 g

Cruise Liner 0.02 g

Table 5: Essar Wildcat operability criteria.

Operation Heave

Pitch/Roll

Single

Amplitude

Operability

0,2 g

(m/s

)

Land BOP on Wellhead 2.4 m 2.5 deg

Running BOP 4.6 m 2.5 deg

Running Casing 4.6 m 2.5 deg

Disconnect Riser 5.5 m 2.5 deg

Drilling or Tripping 4.6 m 2.5 deg

Hang-off 2.2 m 2.5 deg

Cementing 2.2 m 2.5 deg

Crane Operation 5.5 m 3 deg

End of Self Propelled Transit - 3 deg

Helicopter 5.5 m -

4 CONCLUSIONS

From the overall analysis results, it is obtained that

the heave and pitch motion characteristics of the two

FPSO design on the highest significant wave height

is comply with the operability criteria. It is indicated

that all of the design is reliable to be adopted for the

offshore exploration activities at the North Natuna

offshore Indonesia.

The Disc Shape design have larger heave motion

RAO than original design, however the Disc Shape

shows better heave motion response. It might be

explained that the maximum RAO of the two FPSO

design is occurred on the frequency which the wave

spectrum have very small magnitude. Since the Disc

Shape has shown larger RAO, the application of the

Disc Design should be thoroughly assessed by

considering the experimental studies. In the case of

pitch motion, the original design shows better

motion response characteristics than the Disc-Shape

design. It is indicated that the flap of the skirt was

effectively able to reduce the pitch acceleration of

the FPSO

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