Maritime University of Szczecin
Akademia Morska w Szczecinie
2011, 25(97) pp. 13–20 2011, 25(97) s. 13–20
Modelling of seakeeping ability of FPSO vessels
Modelowanie właściwości morskich statków FPSO
Tomasz Cepowski
Maritime University of Szczecin, Faculty of Navigation, Institut of Marine Navigation Akademia Morska w Szczecinie, Wydział Nawigacyjny, Instytut Nawigacji Morskiej 70-500 Szczecin, ul. Wały Chrobrego 1–2, e-mail: t.cepowski@am.szczecin.pl
Key words: FPSO, offshore, design, preliminary stage of design, seakeeping ability, green water loading,
slamming, heaving
Abstract
Problems concerning preliminary design of FPSO vessels are presented in view of their seakeeping ability. The article analyzes the presently applied approach in which seakeeping quality of FPSO vessels is taken into consideration and discusses possibilities of using this approach at the preliminary stage of design. Besides, the current approach used for predicting such phenomena as heaving, slamming and green water loading is discussed.
Słowa kluczowe: FPSO, offshore, projektowanie, wstępny etap projektowania, właściwości morskie,
zale-wanie pokładu, sleming, nurzania
Abstrakt
W artykule przedstawiono problematykę projektowania wstępnego statków FPSO, biorąc pod uwagę właści-wości morskie tych statków. W artykule przeanalizowano aktualnie stosowane podejście, dotyczące uwzględniania właściwości morskich statków FPSO i możliwości wykorzystania tego podejścia na wstępnym etapie projektowania. W artykule przedstawiono aktualnie stosowane podejście do prognozowania m.in. nu-rzań, slemingu i zalewania pokładu statków FPSO.
FPSO vessels
As possibilities of crude oil production on land are shrinking, new technologies of increased oil extraction from the sea bottom continue to be developed. Originally, crude oil was sought and exploited from fields found in the continental shelf. At present the exploration of oil deposits takes place in increasingly deeper waters.
The demand for crude oil produced at deeper and deeper seabed goes in line with continually developed oil production technology. Due to the fact that pipeline installation is difficult to build and maintain in deep and ultra deep waters, that is why as early as in the 1970s first ideas were considered to build ship-type multifunctional installations for the production, storage and transshipment of oil. These functions have been performed by FPSO type vessels (Floating Production, Storage and
Offloading Unit). The functions they perform include production, pre-purification, storage and transshipment of crude oil and gas extracted from submarine deposits.
Since their origin, both TLP and SPAR plat-forms and FPSO units have evolved to satisfy in-creasingly stricter requirements, concerning e.g.: depths of mooring,
hydrometeorological conditions prevailing at the production site,
number of connected oil production wells and chemical injection modules for maximizing the production.
At first, FPSO units were not important for crude oil production. They generally served as tem-porary systems of oil production until other tech-nologies involving stationary techtech-nologies were implemented, such as TLP and SPAR platforms or
semi-submersibles). In time, however, advantages of FPSO units were appreciated. These include: easy transfer to an oil field,
short time from design approval to putting into operation,
lower initial costs if a conventional ship is con-verted into an FPSO unit,
higher resistance to environmental conditions, possibility of combined collaboration with other
vessels / units.
One major advantage is that FPSO vessels can produce, pre-process and store crude oil or gas in-dependent of land-based transport infrastructure. Consequently, the mumber of FPSO vessels and their share in the global offshore fleet has been systematically increasing [1].
Seakeeping ability of FPSO vessels
FPSO vessels have to be operated in any envi-ronmental conditions. They cannot avoid a storm zone or be towed for repairs. Therefore, these ships are expected to satisfy high requirements concern-ing, inter alia, their seakeeping ability, i.e. to resist weather conditions as much as possible. The proper behaviour of the FPSO vessel in heavy weather conditions affects [2]:
comfort and safety of the personnel, efficiency of production systems operation, efficiency of transshipment to a shuttle tanker, helicopter use.
The above factors are mainly affected by ship motions and their secondary effects, i.e.:
relative motions, accelerations, sea sickness, shipping of water, slamming.
Of the various phenomena, the most essential for FPSO vessels are:
pitching and heaving, slamming,
shipping of water (green water loading).
The factors that increase slamming and shipping of water on deck are as follows:
significant vertical motions due to vessel length and unfavourable wave angle,
continual changes in FPSO vessel’s draft and trim,
heavy weather conditions.
Data and research on modelling seakeeping qualities of FPSO vessels are limited. Those
pre-sented in [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21] cover a restricted group of watercraft and are concerned with the influence of the shape of selected FPSO hull parts on a given phenomenon.
Classification societies recommend that sea-keeping ability of FPSO vessels should be deter-mined through model tests [2, 22, 23, 24]. In case such tests are not possible, it is suggested that sim-plified calculations of vessel seakeeping ability be used.
Heaving, rolling and pitching of FPSO vessels
Excessive ship motions can cause vertical move-ment and accelerations, which mainly affect loads on the production and mooring systems, production facilities and cause sea-sickness of the crew and production personnel.
Classification society rules and regulations do not enable predicting the amplitudes of heaving, pitching and rolling of FPSO units at the prelimi-nary design stage on the basis of specific wave parameters prevailing in a given area. For instance, the work [22] indicates formulas for calculations of selected ship motions in order to determine internal and additional dynamic forces resulting from liquid movement in tanks. These regulations include sim-plified formulas for calculating only:
amplitudes and natural pitching periods:
L c k B PMO 4 / 1 1 10 (1) i B p k c d T 2 (2) where – pitching amplitude,
Tp – natural pitching period,
cB – block coefficient,
L – vessel length,
di – mean draft at a given loading condition,
PMO, k1, k2 – coefficients of, amplitudes and
natural rolling periods:
1000 35 di d f B RMO R c LBd k C k C (3) GM k k T r r 4 (4) where: – amplitude of rolling,
Tr – natural rolling period,
B – vessel breadth,
df – vessel draft,
GM – initial transverse metacentric height, kr – radius of rolling inertia
CR, Cdi, RMO, k, k4, kd – coefficients.
At the preliminary design stage general informa-tion on FPSO vessel behaviour in a given area is obtained by calculating natural rolling periods and comparing them with wave periods prevailing in the area [22, 25, 26]. Vessel’s natural heaving, pitching and rolling periods can be calculated from these relations: w z gA A T 33 π 2 (5) L yy GM g A k T 55 2 π 2 (6) GM g A k T xx 44 2 π 2 (7) where:
Tz – natural heaving period,
T – natural pitching period,
T – natural rolling period,
Aw – surface area of waterplane,
A33 – added mass of water for heaving,
A44 – moment of intertia of added mass for
rolling,
A55 – moment of intertia of added mass for
pitching,
GM – initial transverse metacentric height, GML – longitudinal metacentric height,
– volume displacement,
– water density,
g – gravitational acceleration,
kxx – radius of gyration relative to the x axis,
kyy – radius of gyration relative to the y axis.
Slamming
The impact of bow slamming on FPSO vessels may cause:
damage to the forward part of the hull, damage to the production facility installed
ahead of the FPSO bow,
increase in the intensity of green water load-ing on deck.
Damage to the bow structure may in extreme situations lead to necessary dry docking repairs. Damage to production installation, in turn, may cause a break in the production process and neces-sity of various repairs. This may constitute a great problem as disconnection of an FPSO vessel or one
of its subsystems from the production or mooring installations is a technically complex and costly operation.
Damage to the bow of the FPSO Schiehallion due to slamming that took place on 9 November 1998 drew oil companies and scientists attention to the issue of slamming [19, 27].
Studies on slamming impact on FPSO vessels are discussed in [2, 6, 17, 19, 20, 28, 29] and are related with the effect of the bow hull shape on slamming loads for a specific vessel type. Design methods presented in those studies come down to the reduction of slamming effect by modifications made in the bow shape of the hull. It follows from those studies that the impact of slamming on struc-tural load strongly depends on the shape of vessel bow, which in FPSOs is generally considered in one of two configurations (Fig. 10):
fully rounded bow, sharp bow.
Fig. 1. Typical hull shapes of newly built FPSO vessels, one with a round bow (upper hull), the other with a sharp bow (lower hull) [3]
Rys. 1. Typowe kształty kadłubów nowobudowanych statków FPSO z częścią dziobową zaokrągloną (górny rysunek) i ostrą (dolny rysunek) [3]
The fully rounded bow, typical of FPSO new-buildings, provides:
maximum displacement of this part of the ves-sel,
minimum weight of the hull structure.
However, this shape of hull structure, due to flatness of the bottom, is significantly liable to loads caused by slamming. Besides, waves hitting the flat bottom deform upwards, thus increasing the intensity of deck flooding.
On the other hand, the sharp shape of the hull may minimize the wave impact and reduce loads on the mooring system. However, in sea areas where wind, current and waves act at certain angles
rela-tive to each other, the effect of slamming on the hull structure is still essential. Besides, the sharp bow shape:
is more expensive due to design solutions, limits the volume of tanks and the surface area
of the deck,
makes the location of the production facilities in the forward part more complicated (due to the smaller deck area).
The work [21] gives design guidelines referring to the prediction of pressure from slamming. The following formula was worked out through empiri-cal experiments and computer simulation:
S FW G Z V DAF CE g p 1 ea S2 (8) where p – design pressure, C – coefficient,E(Sea) – coefficient accounting for the sea state,
F(W), G(Z) – coefficients accounting for the
bow shape,
VS – conventional slam velocity,
DAF – dynamic amplification factor,
account-ing for dynamic phenomena, deter-mined from experiments.
Classification society requirements for slam-ming refer exclusively to the control of pressure due to waves impact. The American Bureau of Shipping in [22] recommends that in cases where experimental data or accurate calculations are not available, design pressures from slamming should be calculated by this simplified formula:
Vi i
f i si kk v M n E P 2 ln 0 (9) wherePsi – conventional pressure from slamming at
section i, k – coefficient, 0 * 2 . 2 d b ki ,
b* – half breadth of the flat bottom part,
d0 – 1/10 of draft of i-th section for ballast
condition (ballasted holds),
L v0 0.29 , Ri i Vi BM M
Bi – coefficient depending on i-th section, B vm i Ri c L A M 1.391 , Ri i Vi Ri Vi i M d M v M M n 5730 exp 02 2
di – draft of i-th section,
Ai, vm – coefficients,
L f Ef 1 1
1 – resonance frequency from vertical
vibra-tions of ship under ballast,
f1 – coefficient.
Green water loading
Green water loading (shipping of water) of an FPSO vessel adversely affects the personnel safety and work effectiveness of personnel and facilities on deck.
In order to reduce the intensity of water flowing onto the deck or the effects of green water loading the following solutions are proposed [2, 17, 30]: • design of deck structure and equipment resistant
to loads caused by green water;
• increasing the local height of freeborad along the vessel to reduce the intensity of shipping of water;
• optimization of bow structure shape aimed at reducing the amount of green water on deck, and consequently, reducing related loads;
• optimization of deck structures and equipment: additional protections on deck,
raised forecastle or bulwark,
raised equipment or pipeline installations on deck,
additional protection of production facilities on deck.
On board FPSO vessels where green water heav-ily floods the deck operational actions are taken to mitigate the intensity or effects of green water loads. These actions include [2, 17, 30]:
control of vessel draft, trim control,
indication of dangerous zones on deck due to green water,
determination of green water loads in the dan-gerous zones,
determination of limit values of wave parame-ters (significant wave height, period), which, if exceeded, will result in increased intensity of the shipping of water,
control of personnel presence in spaces where green water is shipped.
Despite developments in the modelling of green water loading, it turns out that such methods some-times are not effective. The results of research per-formed on FPSO vessels operated in the UKCS oil field (UK Continental Shelf), presented in [2, 7, 8, 10, 11] have shown that:
nearly half of FPSO vessels operated in the UKCS field may be subject to shipping of water; with a vessel fully loaded, the magnitude of green water loads is mainly dependent on short-er wave pshort-eriod (12 s);
freeboard of the existing FPSO vessels is gener-ally too low.
The intensity of green water loading is first of all influenced by wave parameters. Model-based investigation and observations [7, 12] lead to a conclusion that the wave period particularly affects the phenomenon of shipping of water. Figure 2 presents the relation between the wave period and freeboard exceedence, while figure 3 shows a com-parison of wave parameters at which green water is shipped, assuming design and real conditions. It follows from these charts that dangerous incidents took place when waves were lower and shorter than those specified in the design [7].
Fig. 2. Effect of wave period on freeboard exceedence [7] Rys. 2. Wpływ okresu fali na przekroczenie wolnej burty [7]
The report [12] brings the results of model tests on FPSO vessel with a displacement D = 105 877 t. The tests confirmed that the intensity of shipping of water was greater in one-year waves (shorter period) with these parameters:
significant wave height Hs = 10 m,
characteristic period T1 = 9.76 s (peak period Tp = 11.7),
than in a hundred-year wave with these parameters: significant wave height Hs = 13 m,
characteristic period T1 = 14.76 s (peak period Tp = 17.7).
In both cases the wave spectrum JONSWAP was assumed, with the coefficient = 3.3.
Fig. 3. Wave parameters at which green water is shipped in design and actual conditions [7]
Rys. 3. Parametry falowania, przy których występuje zalewa-nie pokładu w warunkach projektowych i rzeczywistych [7]
Taking into account design parameters of an FPSO vessel, we can state that the intensity of shipping of water is affected by:
freeboard height, shape of the bow hull, vessel length.
The study [3] presents physical aspects of green water loading and develops semi-empirical methods of predicting this phenomenon, taking into consid-eration the shape of bow hull. Based on the results, the work [31] analyzes green water loading in view of practical FPSO design issues, i.e.:
design of the bow part of the hull,
design of structures aimed at reducing the effects of green water on deck.
That study proposes, inter alia, a method for calculations of:
water height on deck:
h a
H H (10)
where:
H – water height on deck,
aH – coefficient depending on the assumed
place on deck and the shape of above water part of the bow,
h – height of freeboard exceedence;
pressure of green water on deck:
2 h a
p p (11)
where:
p – pressure of water on deck, 0 1 2 3 4 5 6 7 8 9 10 11 8 9 10 11 12 13 14 15 16 17 F re eb oa rd e xc ee de nc e [m ]
Peak wave period [s]
w av e pe rio d ca usin g m ax im um re tu rn m ax im um d esig n pe rio d of 5 0/1 00 y ea r w av e 0 2 4 6 8 10 12 14 16 18 20 8 9 10 11 12 13 14 15 16 17 18 19 S ig ni fi can t w av e h ei gh t [ m ]
wave period (peak) [s] wartości występujące wartości projektowe actual values design values
ap – coefficient depending on the underwater
and above water shape of the bow,
h – height of freeboard exceedence;
horizontal load of the deck structure due to green water loading:
2 h a F F (12) where: F – transverse load,
aF – coefficient depending on the underwater
and above water shape of the bow,
h – height of freeboard exceedence.
Table 1 shows the values of coefficients aH, ap,
aF calculated from model tests for the design of DP
FPSO with a displacement D = 215 000 t, and these particulars:
length between perpendiculars Lpp = 260 m,
breadth B = 46 m, lateral height H = 28 m, draft d = 20,5 m.
Table 1. Values of the coefficients for the equations (10–12), depending on the forecastle deck shape (Fig. 4) [31]
Tabela 1. Wartości współczynników do równań (10–12) w zależności od kształtu pokładu dziobówki (rys. 4) [31]
Flat deck Truncated deck
aH 0.56 0.10
ap 2.02 0.28
aF 93.4 15.6
Fig. 4. FPSO unit with a truncated deck (left) and flat deck (right) [31]
Rys. 4. FPSO ze ściętym pokładem (lewa strona) i płaskim pokładem (prawa strona) [31]
It follows from the study [31] that the problem of taking into account the effect of green water loading and deck structures may be mainly solved
by model-based tests. This is due to numerous non-linearities, which first of all result from [3]:
effect of the weight of green water flooding the foredeck,
effect of above water hull (particularly when high waves occur causing high amplitude mo-tions),
inaccuracies resulting from a linear model used for the calculations of relative motions at great significant wave heights.
However, the key quantity allowing to calculate the values of the above quantities is the height of freeboard exceedence. In the study [31] that height is calculated by using this algorithm:
calculation of relative motions in waves by linear numerical methods for the most adverse wave parameters,
taking into account the non-linearity between the wave height and the freeboard exceedence height, based on data obtained from measure-ments.
Like in the case of slamming, classification so-ciety requirements concerning green water loading are restricted to the control of loads caused by wa-ter shipped on deck. In ABS regulations design pressures from green water loading can be calcu-lated by this simplified formula [22]:
bi B i RVM gi c kF L B A K p 1 4 / 1 (13) where
Pgi – conventional pressure from green water
loading uniformly distributed across the deck at i-th section,
L – vessel length, B – breadth,
Fbi – local freeboard height at i-th section,
K, k1, RVM, Ai – coefficients.
Summary
Formal requirements concerning designs of FPSO vessels basically refer to the issues of the structure and are stated in [2]:
• NORSOK standards (initiated by the Norwegian production industry), first of all in NORSOK Standard N-004 Design of Steel Structures; • ISO / WD 19904 standard: Offshore Structures –
Floating Systems;
Lloyds Register of Shipping (LR): Rules and Regulations for the Classification of a Float-ing Installation at a Fixed Location, July 1999 [24];
American Bureau of Shipping: Guide for Building and Classing Facilities on Offshore Installations, June 2000 [22];
Det Norske Veritas (DNV) Offshore 2000 Rules for Classification of Floating Produc-tion and Storage Units, OSS-102, January 2001 [23].
The above requirements are compared in the report [13]. In reference to seakeeping ability of a vessel classification regulations are very general and do not permit taking into account wave condi-tions. It follows from the report that, inter alia, formal design recommendations concerning green water loading and slamming for FPSOs are insuffi-cient.
Informal design guidance is mainly restricted to research on slamming and green water loading for a specific ship. Such research seeks relations be-tween wave parameters, hull shape parameters and e.g. freeboard height versus loads due to slamming or green water on deck. The research results do not account for the impact of general geometric para-meters of the hull on these phenomena, therefore they cannot be taken into account at the preliminary design stage.
To sum up, it can be stated that the continued research and formal design guidelines (included in classification rules and regulations):
can be used for modelling of hull shape and structure of an FPSO at that design stage when the hull dimensions and overall shape (underwa-ter and above) are defined,
are not applicable at the preliminary design stage,
to a small extent take into account operational conditions of the vessel (waves, motions para-meters),
do not allow to predict the probability or fre-quency of occurrence of a given phenomen de-pending on basic design parameters and waves prevailing in a given area in severe weather conditions.
There is particularly little or no information on guidelines concerning the calculations of the fre-quency of green water loading and excessive slam-ming.
The existing design guidelines only allow to cal-culate natural periods of heaving and pitching as a basis for estimating the seakeeping ability of an FPSO in a selected sea area.
References
1. SHIMAMURA Y.: FPSO/FSO: State of the art, Journal of Marine Science and Technology, wydanie 7, 2002. 2. Ukooa FPSO Design Guidance Notes for UKCS Service,
R M Offshore LTD., Project Reviews LTD, 2002. 3. BUCHNER B.: Green Water on Ship-type Offshore
Struc-tures, Grafisch Bedrijf Ponsen & Looijen bv, Wageningen, The Netherlands, 2002.
4. BUCHNER B.: The Impact of Green Water on FPSO Design. Offshore Technology Conference, Houston, Texas, 1995. 5. BUCHNER B.: The Influence of the Bow Shape of FPSOS
on Drift Forces and Green Water. Offshore Technology Conference, Houston, Texas, 1996.
6. BUCHNER B.,VOOGT A.J.: Wave Slamming on External Turrets of FPSOs. Proceedings of the ASME 28th
Interna-tional Conference on Ocean, Offshore and Arctic Engineer-ing, Honolulu, Hawaii, 2009.
7. BOMEL Limited: Analysis of green water susceptibility of FPSO/FSU’s on the UKCS, Offshore Technology Report 2001/005, 2001.
8. ERSDAL G,KVITRUD A.: Green Water on Norwegian Pro-duction Ships. Proceedings of the Tenth (2000) Interna-tional Offshore and Polar Engineering Conference Seattle, USA, May 28–June 2, 2000.
9. GRAFF W.J.: Introduction to offshore structures – Design, fabrication, installation. Houston: Gulf Publishing Com-pany, 1981.
10. HSE Report Analysis of Green Water Susceptibility of FPSO/FSUs on the UKCS OTO 2001:005.
11. HSE Research Project 3794, MARIN Review of HSE Greenwater Study, completion 2000.
12. HSE Research Project 3959, FPSO Response in long and short crested seas, 2002/018, 2002.
13. HSE Review of API RP 2FPS, OTO 2001-006.
14. Lloyds Register of Shipping: Ship-Type FPSO Hull Struc-tural Appraisal, OS/GN/99002, June 1999.
15. LUO Y.,BAUDIC S.,PORANSKI P.,WICHERS J.,STANSBERG C.T., ORMBERG H.: Deepstar Study on Predicting FPSO Responses – Model Tests VS Numerical Analysis. Off-shore Technology Conference 16585, Texas, 2004. 16. MAZAHERI S., MESBAHI E., DOWNIE M.J.: Seakeeping
Analysis of a Turret-Moored FPSO by Using Artificial Neural Networks. ASME 2003 22nd International
Confer-ence on Offshore Mechanics and Arctic Engineering (OMAE2003), June 8–13, 2003, Cancun, Mexico.
17. PAIK J.K., THAYAMBALLI A.K.: Ship-Shaped Offshore Instalations. Design, Bulding and Operation, Cambridge 2007.
18. PARKER G.: The FPSO design and construction guidance manual. Houston: Reserve Technology Institute, 1999. 19. XU L., BARLTROP N.: Wave slap loading on FPSO bows.
(Research Report, No. 324), Health and Safety Executive, UK, 2005.
20. XU L., BARLTROP N., OKAN B.: Bow impact loading on FPSOs 1 – Experimental investigation, Elsevier, 2008. 21. XU L., BARLTROP N.: Bow impact loading on FPSOs 2 –
Theoretical investigation, Ocean Engineering 35 (2008), 2008.
22. ALMEIDA J., VASCONCELOS A., NEVES C., OLIVEIRA E., GOULART M.,GOMES R.: The Preliminary Design of the FPSO Maracana. International Student Offshore Design Competition, Rio de Janeiro Federal University Team, Rio de Janeiro 2002.
23. Det Norske Veritas (DNV) Offshore 2000 Rules for Classi-fication of Floating Production and Storage Units, OSS-102, January 2001.
24. Lloyds Register of Shipping: Rules and Regulations for the Classification of a Floating Installation at a Fixed Location, July 1999.
25. BANDA E., BELTON R., FALEYE W.,HOLMES B.,OGAH N., SPENCER A.: Design of a Floating Production, Storage, and Offloading (FPSO) System and Oil Offtake System For Offshore West Africa. Ocean Engineering Program, Civil Engineering Department, Texas A&M University, 2003. 26. CHIPUK C,,CLARK M, HOFFMAN C.,PEAVY J.,SABET R.,
WILSON B.: Design of Floating Production Storage Off-loading Vessel for the Gulf of Mexico. Ocean Engineering Program, Texas 2003.
27. GORF P.,BARLTROP N.,OKAN B.,HODGSON T.,RAINEY R.: FPSO Bow Damage in Steep Waves, Rogue Waves 2000, SeaTechWeek 2000, Le Quartz, Brest, France 2000. 28. BUCHNER B.,VOOGT A.J.: Wave impacts due to steep
fronted waves. Rogue Waves 2004, Brest, France 2004.
29. WANG G., TANG S., SHIN Y.: A Direct Calculation Ap-proach for Designing a Ship-shaped FPSO’s Bow Against Wave Slamming Load, Proceedings of The Twelfth (2002) International Offshore and Polar Engineering Conference, Kitakyushu, Japan, May 26–31, 2002.
30. ERSDAL G.: Green water workshop, NPD 12.05.2000. 31. BUCHNER B.,GARCIA J.L.C.: Design Aspects of Green
Wa-ter Loading on FPSOs. The 22nd International Conference
on Offshore Mechanics and Arctic Engineering Cancun, Mexico 8–13 June, 2003.
Recenzent: drhab. Leszek Smolarek, prof. AM Akademia Morska w Gdyni
