Hielke Brugts
Mireille Soeters
Max Krekel
Bluewater Energy
Services B.V.
The FPSOMuninwas modified, mobilized and put into operation on the West River field, offshore China, to continue production while the field’s permanently moored FPSO was in dry dock for essential maintenance. The project was unique in that theMuninrelies solely on its dynamic positioning system for station keeping during production, while also offloading to conventional non-DP off-take tankers.
In October 2004, the FPSO producing the West River field offshore China was scheduled for dry dock for its 5-year special survey and extensive maintenance, repair and life extension works. These activities were scheduled to last 160 up to 190 days. To minimize production loss during this period, the field’s operator searched the market for a temporary production facility capable to replace the FPSO. The main criteria for the replacement facilities were:
•Easy hook-up to the existing mooring and riser system in order to minimize installation time and associated ‘non-production’ time
•A disconnectable mooring system, because of hurricane/typhoon risk in the area
• A production capacity of approximately 70,000 b/d oil
• Storage capacity and offloading capability to ensure minimal production interruption
• Offloading to conventional off-take tankers.
At the time, Bluewater was producing the Lufeng field in the South China Sea with its FPSOMunin on behalf of Statoil Oriental Inc. (SOI). Contractually, the Munin was to remain on Lufeng until February 2004 and Bluewater proposed the vessel as temporary replacement for the FPSO on the West River field upon completion of that contract. Due to the high oil price and the higher than expected production on Lufeng, however, Bluewater and SOI agreed to produce on the Lufeng field as long as possible.
External loading structure, FPSO buoyancy tank and Munin FPSO.
To maximize production time, the project team was challenged to minimize the yard time of theMunin. Initially, a period of three weeks was planned in which all modifications, repair, and maintenance work would have to be completed.
This was stretched to 11 weeks for more extensive repair and lifetime extension program, which would allow theMuninto sail directly from the West River field back to the Lufeng field without any further yard visit.
The West River field lies in the Pearl River Mouth Basin approximately 120 km southeast of Hong Kong in the South China Sea. The production facilities consist of two wellhead platforms connected via two 10-in pipelines and risers to a buoyant turret mooring (BTM) system that can be pulled-in and secured to the turret of the West River field’s permanent FPSO. The BTM is fitted with eight anchors legs, and supports the two risers.
Concept development
The initial concept relied on mooring theMuninto the original West River BTM by means of a hawser in sea states up to Hs = 4 m. In higher conditions, theMuninwould revert to its DP mode, with the hawser ready to disconnect in adverse weather. Fluids would be transferred between BTM and FPSO by two flexible hoses, one hose dedicated to each wellhead platform. Serious concerns on the technical feasibility of this concept and costs arose, and this path was subsequently abandoned.
The project team then considered an architecture that minimized the modifications to the BTM and allowed it to float at its design draft when disconnected from the FPSO. The most appropriate configuration was to leave the buoy in its normal submerged position and not use it for station keeping of the FPSO but to use the DP capabilities of theMuninfor this.
Using the FPSOMunin’s DP system for station keeping had several more advantages. The modifications required to the BTM would be minimized, as would the investment in field specific equipment. Further, the field installation activities would become easier and quicker to complete.
Apart from the mooring arrangement, the project team would have to design an external loading structure for reception of the well fluids. Normally these enter the vessel in the submerged turret production (STP) room, where the submerged buoy is connected to the existing product piping. Modifications in this area would prevent remobilization to Lufeng field.
Propulsion, power generation
TheMuninis propelled by a single propeller at the stern, electric driven by two motors on a single shaft. The total maximum continuous rating is 12 MW at 103 rpm. The main propeller and high lift rudder are tied into the DP control system. Additionally, the dynamic positioning system is equipped with two tunnel thrusters in the bow (2,000 kW each) and two azimuth thrusters at the stern (3,000 kW each). All thrusters have controllable pitch propellers and operate at a fixed rpm.
DP capability
DP capability plots were calculated in line with and were based on the characteristics of the DP systems and the local environmental conditions to establish the operational envelope of theMunin.
To define the capability of theMuninto operate on the West River field under the governing weather conditions, a number of conditions were simulated:
• FPSO in full and ballast draft
• Thrusters failures
•Power failures
•FPSO offloading with offtake tanker moored to the FPSO
•FPSO offloading with offtake tanker moored to the FPSO & failures
However, increasing wind and wave conditions result in higher FPSO motions and excursions. The latter were not accounted for in the DP simulations and therefore sufficient “spare” thrust must remain available to compensate for these. Excursions up to 10 m were estimated, based on the experience of masters and DP operators. Thus the overall operating limits for safe production and offloading were reduced to Hs = 5.5 ~ 6.5 m.
An outrigger structure was installed on the original BTM to support the two new flexible risers, connecting the existing BTM with theMunin. The outrigger comprises a tubular space frame and incorporates two spool pieces to connect the West River risers on one side and the flexible FPSO risers on the other. The two flexible FPSO risers are connected to a buoyancy tank that carries the weight of the flexible risers and the outrigger structure.
On the FPSO side, the flexible risers are connected to a second buoyancy tank, referred to as the FPSO buoyancy tank, which keeps the risers afloat when disconnected from the FPSO. The FPSO buoyancy tank consists of two riser pull-in tubes, a single central lifting lug on top, and a connection system to the FPSO. A floating pick-up rope assembly is attached to the main lifting point on top of the tank and is used to lift the tank and risers up to the FPSO for hook-up to the topsides systems. The two flexible risers are connected to the riser hang off plates on top the riser pull-in pipes.
Installation
Bluewater subcontracted the DP2 diving and installation vesselFu Lai,which equipped the FPSO with saturation and air diving spreads, a work-class ROV, a 32-ton capacity A-frame, and a 50-ton revolving crane. The FPSO buoyancy tank was mounted on a hydraulically-operated elevator external to the hull on the starboard side. This was used to transport and deploy the FPSO buoyancy tank.
Production from the West River reservoir arrives at theMunin via two wellhead platforms, located 5 and 10 km away. The well fluids form a waxy gel upon cooling, and heat tracing is therefore provided on the topsides to avoid the wax formation during non-flowing conditions. The wax mitigation strategy for the subsea pipeline is to flush the line with high water cut production fluids (90% water / 10% crude), and finally with seawater prior to a planned shutdown.
TheMunin has remained connected to the flowlines throughout the contract period.
Oil production peaked at 80,000 b/d for prolonged periods. During deployment on West River, it was found that the actual DP position excursions were tighter than estimated during the design. The weather during the operation varied with wind speeds from 15 ~ 40 knots, and corresponding sea states from H 1.5 ~ 4.5 m. The actual deviation from the target set point was normally well within 1.5 m. Only when rapid heading changes were required, did the position excursions increase; a maximum offset of 5 m was encountered during such conditions.
The successful modification, mobilization and operation of theMuninfor a short contract period on the West River field again demonstrates the flexibility of the leased FPSO concept, both technically and commercially.
One of the main advantages of FPSOs lies in its ability for re-use. This is further enhanced by using DP as the station-keeping system. This allows operations that are otherwise uneconomical, such as extended well testing and early production systems, and producing in typhoon/hurricane areas (disconnectable FPSO).•
Acknowledgements
The authors wish to thank Kongsberg, Global Maritime, Marin, Hallin Marine Systems Ltd., and Vecto Aibel for their contribution to the innovative aspects of the project.
