CHINA SHIP SCIENTIFIC RSEA.RCH CENTER
The Joint ,China/Caxiada Development of Two
Mobilè Offshore Production Systems
for the South
Peter R. Gibb,
Ewig Wong Chen
BOX 116, W.UXI, JIANGSU
2.0 MARGINAL FIELDS IN BEIBUWAN
3.0 GUAM Floating Frodút1OEi System
3.1 Design Objectives -3.2 System Description
33 System Characteristics
.3.4 Advantages oï the GUAM System
4.0 RISER MOORED TANKER (R MT) SYSTEM
4.1 Systém Description.
4.2 System Operation
4.3 System Behaviour
4.4 Advantages. of the Riser Moored Tanker System.
50 ECONOMIC COMPARISON OF MARGINAL FIELD DEVELOPMENT
OFFSHORE CHINA BY PLATFORM OR FLOATING SYSTEM
FIGURE 1 GUAM LAYOUT
FiGURE 2 GUAM SYSTEM EXCURSIONMOORiNGFORcELAW FiGURE 3a EXCURSION FOR GUAM
FIGURE 3b HORIZONTAL MOORING LOAD FOR GUAM
FIGURE 4 RISER MRÈD TANKER. - LAYOUT OF SYSTEMCOMPONENTS FIGURE 5 LOWER RISER PACKAGE
FIGURE 6 RISER CONNECTOR
FIGURE 7 MULTI-PASS SWIVEL
FIGURE 8 MOORING AFD UNMOORING OF RISER MOORED TANKER
FIGURE 9 FPSO VS PLATFORM IN THE SOUTH CHINA SEA
LIST: OF TABLES
The China Ship Scientific Research C ntre of Wuxi, China, and the CanOceanDivision of Novacorp
International Consulting Ltd. of Canada, have individually developed a mooring
system to be used
for floating production systems. The two organizations havedecided to cooperate with each ot!er
to complete the design of their systems and to perform the necessary analyses and model testing.
Both the systems are low cóst, highly mobile and recoverable for reusefor several small of fshoré
olifields over, their lifetime. Their designs are based on field-proven principles and possess excellent
dynamic characteristics.The CSSRC's Gravity-based Underwater Articulated Mooring (GUAM)
System Is more suitable for shallOwer water depths andCanOcean's Riser Moored Tanker (RMT)
lystem is easily deplqyed to a wide range of water depths.. In vièw of the foregoing, they are
excellent chOices for floating production systems used to produce marginal offshore oilfields which otherwise would not be economically developed by conventional production methods.
A current example of such marginal olifields are those discovered in the Beibuwan area of South
china Sea for which both the systems described, are considered suitable.
2.0 MarginaI Fields in Beibuwan
Alter séveral years of exploration, a number of oil bearing Structures have been discovered in the Beibuwan area of the South Chiná Sea. One of the larger structures, the Wei 10-3 f leid, is now in the development stage. It is the second offshore oilfield to be developed after Chengbei field in the Bohai.
However, exploration data indicated that the geology of the reservoirs in Beibuwan are complex and
the structures are highly faulted.
Besides the main reservoir, there are large numbers of small
reservoirs. The difficuLty to
size and recoverable reserve has an
unfavourable impact on the economic evaluation of the field as the estimate tends to be overly
conservativé in an attempt to reduce risk. As a resült, some marginal fields will notbe developed. The environmental conditions in Beibuwan are favourable: shallOw water, f lat seabed, sandy soil, môderate weather and sea states. However, typhoons during summer could cause severe damage to offshore structures. This factor cannot be overlooked in any field development in that area.
lt is obvious that a production system based on fixed ôffshore platforms has many advantages.
However, its consturction and instàllation periods are longer and the capital intieìtment is higher ¡n
omparison with a floating production system (FPS).
Moreover, it is not mobile and re=usable.
Hence, the high risk in capital investment rnders it unsuitable for developing ,thsmall margina!
On the. other hand, FPS has many advantages for developing such marginal fields. Its short lead time results in early productionand early return on investment. Furthermore, the low capital cost and
high mobility feaure result¡n lower ¡nlvestment risk. Therefore, FPS is obviously the best
development concept for Beibuwan oilfields.
Ïo establish the economic viability of a marginal oilfield, it is necessary to reduce the capitál and operating costs. This can be aciieved by:
o simplifymg the system
o rationalizing the structure
o improving the system's operating condItions, e.g. reducing the mooriñg load
o simplifying the operations, increasing productivity
o reducing maintenance by minmizing component failure.
In' addition, the system should be mobile and re-usable because the life of FPS could be 10 to 20 years, whilst a small ailfield would be depleted in 3 to 5 years. The capital Cost of such a system could, therèfore, be amortized over several oilfields with each field bearing only a part of the total Cost.
Thesystem operation must be simpie so that the production/storage vessel could be easily mçored
ar unmoored. In this way, during typhoons, the vessel can qúickly disconnect and leave. This will Increase the system safety, simplify the system design ánd achieve, the objective of low capital cost. This is obviously important to olifield development in Beibuwan.
An economic analysis comparing a floating production system with a platform development. undèr China's taxation structure is discussed ¡n Section 5.0.
Design environmental conditions in Beibuwan are as föll9ws:
Water depth . 20 -50 meters
SOil condition sandy and. clayey layer
Seabed topography flat
Survival cönd it ions (100 year return) Maximum wind speed
3.sec. gust : '72 rn/sec
1 minute mean : 60 rn/sec
Maximum wave height approx. £9 mèters
Surface current 4 knots
operating conditions: -.
Maximum 'wind speed (1 minute mean) : 40 knots
Significant wave height 3.6 meters
Surface current 2 knots
3.0 GUAM Floating Productioh Sstem
The GUAM (Gravity-;based Underwäter Articulated Mooring) system is developed by China Ship Scientific Research Center (CSSRC). lt is a new FPSoncept, patticúlary sùitáblê. for use in shallow waters
3.1 Design Objectives
o Safety and reliability, minimum use of unproven components.
o Good dynamic characteristics, low mooring load and high operatiOnal efficiency
o Simplified system structure.
o Ease of mooriñg and unmooring, increasing productivity and minimizing possible damage due to unmooring during a typhoon.
o Good mobility and reusage, increasing system utilization.
o Ease of towing and installation, increasing self-installation abillty, time and cost savings.
The above objectives are aimed at reducing capital cost and increasing operational efficiency.
32 System bescription
The system layout is shown in Figure 1. It cônslsts of the following: gravity-based mooring tower,
turntable, underwater- articuláted mooring arm, vertical mooring arms, ballast bcarn, quick disconnect joints, universal joint, crude transfer line and swivel, production and storage tanker
CSSRCJUMPER TO MANIFOLD
1VEAN LEA LEVEL
23000 BASI 44000 ROTATING TOWER auicx CONNECTOR UNIVERSAL JOINT
UNDERWATER MOORING ARM
Fig. i GUAM Layout
o Gravity-based mooring tower:
The gravity base is of rectangular shape with a "cross" inside the rectangle. lt is made of box construction with a flat square base resting on the seabed. The base serves as a ballast chamber. When deballasted, it also provides sufficient buoyancy so that the tower can be floated. This facilitates towing and installation, and provides mobility. When a reservoir is depleted, the system can be deballasted and moved to another held to start production again. The central 'cross' of the base supports a mooring tower.
o Mooring arm and mooring characteristic:
The storage tanker is moored to thé mooring tower via the vertical and underwater
mooring arms. The underwater mooring arm is connected to the mooring tower by the universal joint and bearing. Thus, the tanker cap weathervane around the mooring tower and take up position of minimum mooring forces.
The underwater ballast beam contains solid baIIas. and water. The ballast provides the
surge restoring force and restricts the tanker to surge within the allowable limit. By
varying the ballast weight, the mooring characteristic can be altered so as to reduce the low frequency surge and mooring load.
The vertical mooring arm are connected to the tanker via the quick connectors. This
facilitates quick disconnection and hook-up of the tanker. The long lengths of the
vertical mooring arms permit larger tanker excursion and greater flexibility enabling it to absorb higher impact energy and to reduce the peak mooring load.
Figure 2 shows the system excursion and mooring force characteristics with 100 tonnes of ballast. The curves are relatively flat at low excursion or low mooring stiffness. At
large excursion, the curves are steep and show good motion-restraining characteristic. Underwater mooring arm is adopted here so that the mooring point is low. This reduces
the overturning moment at the gravity base and the bending moment at the bottom of the mooring tower produced-by the mooring load.
During typhoons, the tanker is disconnected and the ballast beam can be baltasted to tue sea bottom. This allows the system to safÏy survive the typhoons.
CSSRC'o 300 - MOORING FORCE (ton) 25o 200 10 100 50
-VERTICAL MOORING ARM FORCE
liNDEN WATER MOORING .RM FORCE
KONIZONTAL MOORING FORCE
10 15 X(vn( 20
Crude transfer systerÑ
The product swivel is the most crucial component of the crude transfer system lt LS located above water ¡n order to facilitate maintenance 3urnper hose is USed to connect the f lowline to the riser insider the gravity base The remaining f lowlines are hard pipes except at the articulation'poiñts where jumper hoses are used.
The turntable and deck on top of the mooring tower, is equipped
with .a small ce,navigational aids, firefightingand lifesaving equipment.
3.3 System Characteristics,
The CSSRC in-house computer software package ¡s used to perform the frequency
domain and time domain motion analyses of the tanker and the mooring system. The ÇSSRC computer program
based on 3-dimensional motion response. Figure 3 shows the time domain
simulation results of the eccursion and horizontal mooring load of the GUAM system under operating conditions.
A 30,000 DWT tanker was used in the simulation. The curves show that the mooring loads change gradually with no high peaks.
3.4 Advantages of the GUAM System
o Self -iloating to facilitate towing, relocation and installation.
o Mobile and reusable.
o Rigid underwater mooring arm prevents the tanker from cçiliding with the macring
Sóf t moor ¡ng-charaCteriStic5 mooring stiffness can bè altered,' low mooring load.
Moring load produces small overturningmoment and bending moment.
'After the tanker is disconnected, the rigid mooring arm can either stay afloat or sink to the seab?d depending upon environmental conditions The system is easy to operat' nd suitable for severe environmentalconditions.
Quick-connector makes operatiod and quick disconnect easy.
Fig. 3a Excursion for
Fig. 3b Hogizontal
Mooring Load for GUAM
4.0 RiSer Moored Tanker (RMT) System
The riser moored tanker system ¡s being developed by CanOcean as a very mobile floating
production system suitable for shallow or deep water and, where ñecessary, very high sea states. lt
is a mooring system based on the field proven single anch& leg principle where the horizontal
component of riser tension ¡s used to restrain the tanker.
The main feature is the use of a slim
deployable riser, which can be connected and disconnected at the seabedrapidly and easily, and
recovered to the tanker. Heave motions of the tanker are accommodated by a uniqueweight-type motion compensation system. R 11, pitch, sway and surge motions are taken care of by articulations
at the ends of the riser.
The result is a floating production system that ¡s very versatile and adaptable to virtually any field
production scenario. Once a tanker size has been selected ànd a maximum sea state chosen, the
riser moored tanker can be moved from one field to another by simply releasing the subsea
connector, moving the tanker to the new site, redeploying the riser to the length requiredby the
neW water depth, and reconnecting the riser connector at the seabed. The system is particularly
suited to fields where minimal subsea e4uipmtt is desired and all the wells are flowed directly and Individually up the riser to the tanker. Because of the sysiem's versatility and its ability to be used on resérvoirs requiring more complex reservoir management, the capital cost can be spread over the
llfàtime of mañy fields. 4.1 System Description
The system layout of the riser moored tanker ¡s shown in Figure 4.lt consists of the following?
riser base, lower riser package, riser, gimballed riser mast, rocking beäm and counterbalance
o The riser base ¡s the anchóring point for the riser at the seabed and the cöllection point for all the flowlines. The base can be either a gravity-type or piled depending on the soil
o The löwer riser package serves as the connection point between the floating production system and the. seabed equipment. lt provides the guidance and locking system f cyr
attaching the riser to the seabed base. A universal joint is also part of the package.
Figure 5 shows the basic principles behind the lower riser package.
lt uses a separate
guide cone frame to lote the riser onto the baseo
This allows the three functions of location, orientatIon and stab-in to be performed
independently of each othere The geometry of the cone provides positive location to be made independently of significant riser angles, loads or mâtions.
Release at high ris'r
CanOceaflSTORAOF EUUIPMENT MULl-PASS SWIVEL RISER LIVING OUARTI%S -(EXISTING N ROCKING BFAM'
WATER TREATUG POtJER GENERATION.
WORICI4UP. C(flTßOL ItOOM 0W OECK
LOWER RISEN PACRAGE RISER BASE
,'_)_-. -GBOUNU ILARE ; PROCESS EQUIPMENT S.
OFF LOADING BOOM..
Fig. 4 Riser Moored Tanker - Layout of System
angles is possible because the cone angle Is not steep-sidéd..The features are similar to those used for a well workover lower riser package designed and built by CanOcean n the past.
o The riser is a compösite type where the outer case takes the structural load ánd houses
all the individual production pipes. The riser is made up of several sections which are joined together by a high performance connector(Figure 6). lt is designed for high loads, good fatigue life and easy make-up. The riser connectOr is presently undergoing an
extended fatigue iife test. With metal seals lt has already demonstrated Its ability t
withstand the maximum bending and tension loads with 15,000 psi in the production tubes.
o The riser mast is gimbailed at the end of the rocking motion compensation beam. Its
fwction is to provide the upper termination of the riser and the mans of connecting and lowering the riser. lt serves a similar function as the derrick rn a drill rig except it is much simpler because It has only a few tasks to perform.
o The multipass swivel is located at the top of the riser in e. ris mast. lt allows me tanker to weathervane around the riser without crossing of f lowlì . This component is
probábly the most critical at present for any weathervanin f loa 'ig production system, particularly where high pressures are involved
or wre sptiisticated reservoir
management is needed. CanOcean has been developtig a swivel
to meet this
requirement and Is shown In FIgure
The heave motion coinpensation and riser tensioning is provided by a rocking beam with a. counter balance Weight at the, opposite end to the riser. The weight ¡s in the form of a
tank that can be filled with a liquid such as water,.or, [f corpactness is required, drilling mud. This allows the riserension to be varied easily. Weigit-type motion compensation systems are inherently simple but they are usually not used because of the uneven load response d',e to tanker acceleratiors. lhls acceleration s magnified in a counterweight system. To overcome this problem, a rocker arrangement is used which changes the
lever arm of the weight. When the weight Is at its highest anc the accel'rations are
negative, the moment arm is at its greatest. Conversely, when the weight is at Its lowest with high positive acceleration, the arm IS at its shortest. To trwnit horizontal load to the tánker, a rack and jear arrangement ¡s used With the same pitch circle as the rocker.
Fig. 6 Riser Connector
4.2 System Operation
The riser moored tanker can be configured to suit many diflerent operating scenarios. The most important being the number of disconnections that are expected to be made At one extreme the system could be considered permanent where, after it is ¡nstalléd, it is not removed Until the field is
depleted, On the other hand, if necessary, it could leàve the field when the tanker is fil and deliver the crude oil to market instead of using shuttle tankers. In this case, the syitem would have Itsown Propulsion and maneuvering capability. Running and retrieying the riser is straight forward and çan be automated as required. The design of the riser connector makes the automation of the riser attachment and running rèlatively easy.
For attaching the riser to the seabed, the riseris connected together in the riser mast and lowèred to the seabed (Figure Sa). As it nears the seabed it can be positioned relative to the base by moving the riser mast (Figure Sb).
When. the lower riser package lands on. the seabed base, the weight in the end of the motion
compensating beam i reduced so that the beam starts to compensate for heave of the vessel. After
the riser is lock& cnto the basé, the counterbalance weight is increased to the maximum riser
operating tensiofl th is required. The tanker can then be allowed to drift back under the influence of the environrner.tal forces until these forces are balanced by the horizontal component of rider tension.
When the tanker nc to be released it is brought to an approximäte position over the seabed base
either by is owi prpu!sion or by tug, and the counterbalance weight redUced. When the weight is
less than the lower riser package weight bUt still enough to tension the riser! the subsea connector is released. The counterbalance weight is now increased until the lower riser package starts to lift off the seabed base. At Ihis point the. tanker can leave the area (Figure Sc) and recover the riser.
4.3 System Behaviour
The most importas-it consideration for a floatliig prOduction is
to be ableto survive the
environmentalforces and have motions that are acceptable for the Production eqUipment to fUnction and allow continuous produciôn. There are three main mooring considerations for the riser moored tanken
Vessel seakeeping and motions due' to the environment.
Slow wave drift oscillations.
. Pirectwnal stability.
Fig. B Mooring and Unmooring of Riser
Fig. 8bFig. Sc .
Seakeeping Is primarily controlled by the tanker size and shape. The mooring system has little effect on the bàsjc motiOn characteristics. Therefore, apart from storage capacity, the maximum seastate requirement will determine the minimum tanker size.
Slow rnötion wave drit is primarily dependent i the stiffness of th mooring system which, n this
case, is determined by the riser tension The higher the mooring stiffness, the smaller the
excursions, but the higher the mooring loads The slow drift oscillation case is a more dominant load case thañ the dirçct environmental mooring loads.
To provide weathervaning and thus reduce tanker motiôñS and loads the, mooring point must be
forward of midShips. To prevent fishtailing instabilities the mopring point is well forward of the
4.4 Advantages of the Riser Moored Tanker System
The RMT system shares many of the advantages of the GUAM system. It is very mobile, self-installing, reusable on other fields, can have its mooring stiffnèss changed easily and follows proven principles.
-It is also very easily redeployed to a wide range of water depths and the design can accommodate the worst sea states, i.e. depending on the basic tanker seàkeeping ability. These high capabilities are possible not only becauseof the configuration bút because Of new technology equipment that is
currently being tested. . .
5.0. Economic Comparison of. Marginal. Field DeeIopment Offshore. China by. Platform or Floath
An economic analysis has been done to define the minimum field size. which may be consideréd to be developed by means of a floating production storage and off loading (PPSa) systém and to compare such an FPSO system development, with a platfor.m development. The analysis is focused on, and attempts to qûantify, the apparent ecOnomic advantage to be gained by utilizihg uth ¡n FPS() system on more than one field. This type of development is Particularly relevant to the Chinese Continental Shelf as the majority of discoveries to date have been considered to be marginal in nature.
The systems to be compared haye been chosen as being representative of the type of development
- which Would be considered for sucha marginal field. The field parameters are ontlined in Table 1.
These are based orn
MARGINAL FIELD PARAMETERS
1985 $ x i6 .
Exploration Costs $ -
-Capital Costs (Estimated) 47.2 54.2 81.8
Annual operating Costs (Estimated) 7.4 7.7 7.7
Transportation Costs mcl. . md. mcl.
Removal Costs 2.0 2.0 4.0
Salvage Còsts Value . 5.0 10.2 4.9
Oil Price 26.0/bbl 26.0/bbl 26.Ó/bbl
Corporate Tax Rate 50%
Commercial Tax Rate. 5%
Inflation Rate . Costs 6%
Inflation Rte - Oil Price Constant through 1987, then increase at
raté of inflation
-Discount Rate. for Net Present Value 10% greater than inflation, or 16%
Recoverable Reserves (Base Case) 6.3 MMbbl
Number of Wells 3
Peak Pröduction 4500 bId for 1 year
Decline Rate 20% exponential
Operating Time 90%
Water Depth loo m
In the plätlorm case:'
A 4-pile drilling and prOduction platform with criIiing supported by a drilling tendér. A singleoffloading CALM-type buoy backed up by a small platform storage tank to allow. tanker changeover
- Time to first oil from decision point - 22 monthso In the FPSO systerncase:
A minimum tripod weilhead platform with drilling supported by a drilling tender.
- A riser moored FPSO system which cOntains al! crew accommodations and process equipment
- Time, to first oil, from decision point - 22 months.
The Chinese taxation strUcture was modelled with an oil price of $26.00/bbl through 1987, then escalated at the inflation rate of 6%. One case was run with' the recently waived royalty payments on fields producing less than I million tonnes annually.
All comparative results are básed on the original taxation structire includingthe 12.5% royalty.
This does not invaLidate' the comparisons. lt only affects the absolute values of the results.
At 100 m w.d. and' field sizes of 5 to 10 MMbbi reserves, there is very little economic difference in developing an òilfield by a platfOrm or by an FPSO system. This is displayed on Table. 2. The platform shows a slight edge in internal rate of return (IRR) and the FPSO system has an edge in net alter tax cash flow.
'the real difference occurs when the FPSO systém is utilizedon more than one field. ft was assumed that the FPSO ¡s returned to a shipyard for inspectloñ, maintenance and re-certification at 20% of' the original conversion and riser mooring system cOsts. Under the' above assumptions, the FPSO system shows a definite ecOnomic advantage. This is displayed on Table 2. Based on the consecutive development, of'. tWo 6.3 MMbbl fields, the FPSO system's internal rate of return is 14.7% vs 12.21% for the platform and more importantly to the operators, the FPSO system generates $70.4 mn vs $41.0 mn for the platform, in after tax cash f tow. This is an improvement of gieater than 70% in aftér tax cash flow.
- Financial Pax'ameters Platform Floating Production
z i Field
z i Field
Cumulative Net Income - $ 19.3 14.5
Cumulative After Tax Cash Flow 20.5 .26.7
Interrial Raie of Return (IRR) 12.2% 12.1%
Net Present Value (NPV) @ 16% ($3.9) ($49)
z 2 FIelds X 2 Fields
Curnúlative Net Income $ 38.6 58.1
Cumulative A.T; Cash Flow 41.0 70.4
IRR 12.2% 14.7%
The model which was run with the recently waived royalty payments, see Figure 9, highlights the r suiting dramätic improvement in profitability of marginal fields. In the case of the FPSO system developing two consecutive 6.3 MMbbl fields, the IRR improves from 14.7% to 19.9% and the curmitative after tax cash flow imprôves from $70.5 mn to$100.0 mn.
Each contemplated field development will in practice neèd to be r-cddled using specific field
parameters, in order to optimize the development method chosen. From the results of this analysis, it would appear that an FPSO system designed for utilization on more than one field wbuld be the first choice for investigation and the most probable choice development for fields of less than 10 MMbbl recoverable reserves.
The joint efforts of CSSRC and Canöcean In the design and development of the two floating
production systems signify the good will and technical cooperatiôn betweenChina and Canada. The systems developed are directly applicabl to the production of offshore oil from smaller fields. They are designed to meet the particular environmental requirements of the offshore regions of China, e.g.. the ability of. the production vessel to disconnect quickly in typhoon conditions. They are inexpensive and they can be easily relocated and reused in a number of oiltïeids, thereby spreading
the capital cost. An example of the Beibuwan fields is cited as a possible application for the
The authors wish to Thank the management of Çhina Ship Scientific Research Centre of China and Novacorp International Consulting Ltd of Canada for permission to publish this work and to the personnel of both organizations for their assistance and contributions toward compiling thispaper.
A.T. CASH FLOf (FPSO
A.T. CASH FLOW
A.T. CASH FLO!1 (IPSO 1)
RECOVERABLE RESERVES ÍFIELO- BARBELS MI MIWONS
Fig. 9 FP5d vs P(atform ¡n the South China Sea
RANGE Of ESTIMATE (±20% ON CAPITAL
FPS0(I) I-i-- IPLATFORM G--r-- FPSO(3c2)