Response-Based Metocean Criteria for the Optimization of Floating Production Facility for Marginal Oil Field at Java Sea

7/24/2019 Response-Based Metocean Criteria for the Optimization of Floating Production Facility for Marginal Oil Field at Java

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ICSOT Indonesia, November 4th

5th

2015, Surabaya, Indonesia

2015: The Royal Institution of Naval Architects

RESPONSE-BASED METOCEAN CRITERIA FOR THE OPTIMIZATION OF FLOATING

PRODUCTION FACILITY FOR MARGINAL OIL FIELD AT JAVA SEA

W D Aryawan andG M Ahadyanti, Institut Teknologi Sepuluh Nopember, Indonesia

SUMMARY

Indonesian oil patch is full of marginal oil fields which if developed will help to boost the economy of the continent

tremendously. Such marginal field is uneconomic to be developed using current technologies based on the existingproduction facilities. This research proposed a methodology to optimize FSO principal dimension based on motion

criteria subject to defined platform and metocean at a specified location. Parametric study involving 72 main dimensions

variation had been carried out for various L/B and B/D ratios. Such main dimension variations were derived based on

the existing FSO operated in Java Sea. Numerical simulations have been conducted using Hydrostar to investigate

motion responses in both regular and irregular waves. The results obtained from the investigation indicated that heave,

roll and pitch motions were influenced by L/B and B/D ratios. It is concluded that the design methodology to suit

specific metocean conditions is an effective approach to minimise motion behaviours.

NOMENCLATURE

Oscillation frequency (rad/s)

p Peak frequency (rad/s)

M Inertia matrix of the body

MA Added mass (kg)

B Damping components

K Stiffness matrix

F Excitation load amplitude (N)X Motion amplitude

S Sea spectrum

A Normalizing factor

Hs Significant wave height (m)

Peakedness parameter

Shape parameter Wave propagation direction (degree)

Tp Peak period (s)

Lwl Load waterline length (m)

Bmld Moulded breadth (m)

Hmld Moulded depth (m)

L Length (m)B Breadth (m)

D Depth (m)

T Draught (m)

Displ Displacement (Ton)

1. INTRODUCTION

Global oil prices have fallen sharply over the past year,

leading to significant revenue shortfalls in many oil

exporting nations, and forcing them to decommission

rigs and sharply cut investments in exploration andproduction. Consequently, Indonesia as one of oil-

producing countries will face economic challenges. In

addition, Indonesian oil patch is full of marginal oil and

gas fields which if developed will help to boost the

economy of the continent tremendously. It is therefore

necessary to optimize such marginal oil field facilities inorder to reduce capital and operating costs.

FSO may be used as a floating production facility for oil

marginal field. In the life cycle of a floating facility, it is

in the development design stage that there is the greatest

potential to impact the earning power. It is well knownthat the strategy to optimize such design stage is using

parametric study with respect to hull sizing. In this study,

hull sizing was performed involving motion criteria

subject to defined platform and metocean at a specified

location, i.e. Java Sea.

2. RESEARCH METHODOLOGY

In this study, seakeeping analysis has been carried out

using Bureau Veritas software, Hydrostar. HydroStar is

a fluid dynamics software based on potential flow theory,

in which the seakeeping problem is solved by 3-D

diffraction-radiation theory based on motion equation atsix degrees of freedom which can be written as follows

[1]:

[( ( (] (Motion behaviors of the floating structures have been

evaluated in two wave conditions, i.e regular waves and

irregular waves within wave frequency range between

0,182-1,462 rad/s.

At irregular wave condition, spectra used in this study

refers to JONSWAP and the formula can be written as:

( )

)

( )

Spectral analysis was carried out with Hs=4.6m

calculated based on Java Seas wave distribution for a

100-year return period [2].

3. PRELIMINARY DESIGN AND INITIAL

SIMULATION

3.1 3D HULLFORM MODELLING

The initial hullform used in this study is FSO Cinta

Natomas that has been operating in Java Sea. The


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hullform was modeled using Hydrostar with the scale of

1:1. The main size of the vessel is shown in the Table 1below:

Table 1. Main Dimension of FSO

Lwl (m) 178.8

Bmld (m) 40.64

Hmld (m) 25.09T (m) 15

Displ (Ton) 105577

3.2 VALIDATION

Before analyzing the results of the FSO seakeeping

simulation by numerical methods, validation should bemade to determine whether the simulation is correct. The

validation process was done by comparing the simulation

results obtained with the experiments that have been

done in previous studies [3]. A typical validation result is

shown through Roll RAO as can be seen in Figure 1.

Figure 1. Roll RAO

The straight line shows the simulation results obtained

with the software Hydrostar (H) while the simulation

results obtained in previous studies (S) indicated by a

small circle on the increase in frequency. As can be seen

in the graphs above that the difference between the

seakeeping simulation results using software Hydrostar

are not significant with seakeeping simulation results

obtained in previous studies. The difference between the

results are below 5%, this indicates that the simulation

results in this study are valid.

3.3 PARAMETRIC DESIGN

One of the main factors that influence the size and

arrangements on the FSO is the oil storage capacity. This

factor is directly related to displacement vessels, which

in this study will be used as constraints in determiningthe main dimension ratio of the alternative hull forms.

Based on the review that has been conducted previously

[4], new build FPSO has an L/B ratio between 4 to 6 and

the B/D ratio between 1.5 to 2.

In this study, 72 ships have been analyzed for their

seakeeping ability, where 60 main dimensions variedbased on the L/B ratio and 12 main dimensions varied

based on the B/D ratio. Variation of the ratios of the FSO

main dimensions in this study are shown in Table 2

below.

Table 2. Ratio Variations of FSO Main DimensionNo. L (m) B (m) D (m) T (m) L/B B/D

1 153.75 37.5 25 18.9 4.1 1.5

2 157.5 37.5 25 18.45 4.2 1.5

3 161.25 37.5 25 18.03 4.3 1.5

4 165 37.5 25 17.62 4.4 1.5

5 168.75 37.5 25 17.22 4.5 1.5

6 172.5 37.5 25 16.85 4.6 1.5

7 176.25 37.5 25 16.49 4.7 1.5

8 180 37.5 25 16.15 4.8 1.5

9 183.75 37.5 25 15.82 4.9 1.5

10 187.5 37.5 25 15.5 5 1.5

11 191.25 37.5 25 15.2 5.1 1.5

12 195 37.5 25 14.91 5.2 1.513 198.75 37.5 25 14.62 5.3 1.5

14 202.5 37.5 25 14.35 5.4 1.5

15 206.25 37.5 25 14.09 5.5 1.5

16 210 37.5 25 13.84 5.6 1.5

17 213.75 37.5 25 13.6 5.7 1.5

18 217.5 37.5 25 13.36 5.8 1.5

19 221.25 37.5 25 13.14 5.9 1.5

20 225 37.5 25 12.92 6 1.5

21 164 40 25 16.62 4.1 1.6

22 168 40 25 16.22 4.2 1.6

23 172 40 25 15.84 4.3 1.6

24 176 40 25 15.48 4.4 1.6

25 180 40 25 15.14 4.5 1.6

26 184 40 25 14.81 4.6 1.6

27 188 40 25 14.49 4.7 1.6

28 192 40 25 14.19 4.8 1.6

29 196 40 25 13.90 4.9 1.6

30 200 40 25 13.62 5 1.6

31 204 40 25 13.36 5.1 1.6

32 208 40 25 13.10 5.2 1.6

33 212 40 25 12.85 5.3 1.6

34 216 40 25 12.62 5.4 1.6

35 220 40 25 12.39 5.5 1.6

36 224 40 25 12.16 5.6 1.6

37 228 40 25 11.95 5.7 1.6

38 232 40 25 11.75 5.8 1.6

39 236 40 25 11.55 5.9 1.640 240 40 25 11.35 6 1.6

41 205 50 25 10.63 4.1 2

42 210 50 25 10.38 4.2 2

43 215 50 25 10.14 4.3 2

44 220 50 25 9.91 4.4 2

45 225 50 25 9.69 4.5 2

46 230 50 25 9.48 4.6 2

47 235 50 25 9.28 4.7 2

48 240 50 25 9.08 4.8 2

49 245 50 25 8.90 4.9 2

50 250 50 25 8.72 5 2

51 255 50 25 8.55 5.1 2

52 260 50 25 8.38 5.2 2

53 265 50 25 8.23 5.3 254 270 50 25 8.07 5.4 2

55 275 50 25 7.93 5.5 2


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56 280 50 25 7.79 5.6 2

57 285 50 25 7.65 5.7 2

58 290 50 25 7.52 5.8 2

59 295 50 25 7.39 5.9 2

60 300 50 25 7.27 6 2

61 150 37.5 25 19.38 4 1.5

62 160 40 25 17.03 4 1.6

63 170 42.5 25 15.09 4 1.7

64 180 45 25 13.46 4 1.8

65 190 47.5 25 12.08 4 1.9

66 200 50 25 10.9 4 2

67 225 37.5 25 12.92 6 1.5

68 240 40 25 11.35 6 1.6

69 255 42.5 25 10.06 6 1.7

70 270 45 25 8.97 6 1.8

71 285 47.5 25 8.05 6 1.9

72 300 50 25 7.27 6 2

4. RESULTS AND DISCUSSION

4.1 THE EFFECTS OF MAIN DIMENSIONRATIOS MODIFICATION ON THE

SEAKEEPING

To determine the optimum design point, it should be

examined whether there was an effect of modification ofmain dimension ratios on their seakeeping. Seakeeping

evaluation results are represented in the form of

Response Amplitude Operator (RAO) in which the wave

propagation direction ( = 90o) subject to heave and roll

as well as the direction of wave propagation coming from

the direction of the bow ( = 180) for pitch. Theseangles of wave direction have been selected to

demonstrate the extreme value of the amplitude of thesethree motions.

Figure 2. Heave RAO based on the increase of L/B

Figure 3. Pitch RAO based on the increase of L/B

Figure 4. Roll RAO based on the increase of L/B

From the figures above, we can see that the increase in

the ratio L/B, there is a decrease in the maximum

response of heave, pitch, and roll motions. Interestingthings that can be seen on all three graphs, along with

increasing price ratio L/B, there is a tendency of heaves

natural frequency to shift gradually towards the right.

While on the pitch chart, along with increasing price

ratio L/B, there is a tendency to shift its natural

frequency gradually to the left. Furthermore, along withthe increasing ratio L/B, the maximum response for

heave and pitch motions are also decreasing gradually.

Figure 5. Heave RAO based on the increase of B/D


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Figure 6. Pitch RAO based on the increase of B/D

Figure 7. Roll RAO based on the increase of B/D

It can be seen in the heave graph, along with increasingB/D ratio, there is a tendency its natural frequency

shifted gradually to the right. Whereas viewing from

pitch motion, along with increasing B/D ratio, there is a

tendency to shift its natural frequency gradually to the

left. Furthermore, along with the increasing ratio B/D,the maximum response for heave, pitch, and roll motions

are also decreasing gradually.

From the analysis above, when viewed from the heave

and pitch motions, based on the increase of L/B ratio,

the most optimum FSO is FSO no.60, whereas based onthe increase of B/D ratio, the most optimum FSO is FSO

no.72. Based on the increase of L/B ratio, FSO no.60 hasthe greatest L/B ratio, while based on the increase of

B/D ratio, FSO no.72 has the greatest L/B ratio. From

this analysis it can then be concluded that the greater L/B

and B/D ratio result in the minimum response of heave

and pitch motions. Because FSO no.60 and FSO no.72

are the same FSO, in order to ease reference, hereinafter

the FSO will be referred as FSO no.72.

However, when viewed from the roll motion, based on

the increase of L/B ratio, the most optimum FSO is FSO

no.34 while based on the increase of B/D ratio, the most

optimum FSO is FSO no.72. One thing to remember is

that the results in the analysis above are characteristic ofmotions based solely on modeling results in regular

waves. In designing the offshore structure, designers also

have to look at the wave period of the waters in which

the offshore structure will operate [5]. This can be doneby performing spectral analysis which will be explained

further in the next section.

4.2 SPECTRAL ANALYSIS OF SURVIVAL

CONDITION (100-YEAR RETURN PERIOD)

Based on the modeling results in regular waves, in terms

of heave and pitch motions based on the increase in the

ratio L/B and B/D, the most optimum FSO is FSO no.72.

Meanwhile, in terms of roll motion, based on the increase

of L/B ratio, the most optimum FSO is FSO no.34. As

for the roll motion based on the increase of B/D ratio, the

most optimum FSO is FSO no.72. Therefore, it is

necessary to carry out spectral analysis of both FSOs in

order to compare their responses in irreguler waves.

The survival condition for the FSO has been selected at a

100-year return period because the unit shall withstandthe site metocean conditions covering the installation,

operating and the most severe conditions. Figure 8 is roll

spectral response curve of FSO no.34 for 100-year return

period calculated according to the increase of peak

period from Tp=2s up to 8.5s with a variety of headingangles ranging from =0

o up to =330

o. Based on the

computation result, the maximum roll response occured

in wave direction angle of 270oat 0.301 m

2/(rad/s).

Figure 8. FSO no.34 Roll Response Spectra (100-Year

Return Period)

Figure 9 shows roll spectral response curve of FSO no.72

for 100-year return period calculated according to the

increase of peak period from Tp=2s up to 8.5s with a

variety of heading angles ranging from =0o up to

=330o. Based on the computation result, the maximum

roll response occured in wave direction angle of 270oat

1.985 m2/(rad/s). This maximum response is much larger

(6.6 times larger) when compared to FSO no.34

maximum response.


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Figure 9. FSO no.72 Roll Response Spectra (100-Year

Return Period)

After observing the spectrum response, the next step is to

observe the value of stochastic period and frequency as

well as the specific amplitude of both FSOs roll

motions. The motion qualities in irregular waves can be

analyzed by plotting changes in the motion intensity as a

function of the significant wave height increment as

shown in Figure 10.

It can be seen in the figure that at all the significant wave

height, FSO no.34 has a roll amplitude intensity lower

than the FSO no.72. At low significant wave height,

Hs=0.2m, the roll significant amplitude of the two do not

differ siginificantly. However, the greater the value of

the significant wave height results in the greater value ofthe roll significant amplitude of FSO no.72. At

significant wave height 3m, the roll significant amplitude

of FSO no.72 reached 4.67m. This means the roll

significant amplitude of FSO no.72 is approximately 8

times larger than the roll significant amplitude of FSOno.34.

Figure 10. The Increase of Significant Roll Amplitude

as a Function of Significant Wave Height Increment

From the analysis above it indicates that for 100-year-

return period, FSO no.34 has better roll motion quality

when compared to FSO no.72 at Java Sea.

5. CONCLUSIONS

After performing numerical simulation of initial hull

form and alternative hull forms as well as conducting

analysis of seakeeping and spectral analysis, it can be

drawn into these following conclusions:1. Main dimension ratios modification affects in FSO

seakeeping. Along with the increase of L/B and B/D

ratios, there is a decrease in maximum reponse

amplitude of heave and pitch motions.

2. Along with the increase of L/B ratio, there is atendency heave natural frequency shifted gradually tothe right. Along with the increase of L/B ratio, there

is a tendency pitch natural frequency shifted

gradually to the left.

3. Along with the increase of B/D ratio, there is a

tendency heave and roll natural frequency shifted

gradually to the right. Along with the increase of B/D

ratio, there is a tendency pitch natural frequency

shifted gradually to the left.

4. Based on the modeling results in regular waves, in

terms of heave and pitch motions based on the

increase in the ratio L/B and B/D, the most optimum

FSO is FSO no.72.5. Based on the modeling results in regular waves, in

terms of roll motion based on the increase of L/B

ratio, the most optimum FSO is FSO no.34. As for

the roll motion based on the increase of B/D ratio, the

most optimum FSO is FSO no.72.6. Based on the response spectrum analysis on irregular

waves, taking into account roll motion, FSO no.34 is

the most optimum hull because it produces minimum

roll amplitude.

6. ACKNOWLEDGEMENTS

The authors would like to thank the Bureau VeritasOffshore Business Regional Manager, Mr. Kuan Yeh

Sheng for his help in providing the software Hydrostar as

well as Bureau Veritas Executive Engineers, Mr. Binbin

Li and Mr. Pan Qi for giving references and software

guidance during this study.

7. REFERENCES

1. JOURNEE, J. M. J., MASSIE, W. W, OffshoreHydrodynamic First Edition,Delft University of

Technology,2001.

2. FUGRO, Design Criteria and Fatigue

Assessment Metocean Data for OffshorePlatform at OffshoreNorth Java Sea, 2014.

3. AHADYANTI, G. M., Studi Optimasi HullForm FSO Di Laut Jawa, Institut Teknologi

Sepuluh Nopember Surabaya, 2015.

4. MACGREGOR, J. R., and SMITH, S. N.,

Some techno-economic considerations in the

design of North Sea production monohulls,

1994.5. DJATMIKO, E. B., Perilaku dan Operabilitas

Bangunan Laut Di Atas Gelombang Acak,ITS

Press, 2012.


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9. AUTHORS BIOGRAPHY

Wasis Dwi Aryawan holds the current position of

lecturer at Institut Teknologi Sepuluh Nopember

Surabaya. He is responsible for teaching Ship Design

within Department of Naval Architecture and

Shipbuilding Engineering. He also serves industrialservices for: oil and gas companies, shipping companies,shipyards, and other marine related industries.

Gita Marina Ahadyanti has graduated with a Master

Degree at Institut Teknologi Sepuluh Nopember

Surabaya. Having graduated with a BS degree in Naval

Architecture and Shipbuilding Engineering from the

same institute, her previous experiences include

Technical Investigation of Mooring Hawser Break at

FSO Cinta Natomas and Technology Selection

Feasibility Study for Pertamina Hulu Energi Offshore

North West Java (PHE ONWJ) Marginal Field.

Documents

Response-Based Metocean Criteria for the Optimization of Floating Production Facility for Marginal Oil Field at Java Sea