Ultra deepwater floating production facility for Makassar Strait
Gde Pradnyana
Jafar Korloo
The West Seno TLP brings extremely valuable experience to the application of TLP concept based on a drilling tender assist scenario in the southeast Asia region. Indonesia began its first deepwater installation and operation in early 2003 by development of Unocal’s West Seno field based on a wellhead TLP with a drilling tender assist vessel and a floating production barge. The West Seno TLP is installed in 3,350 ft of water based on a wellhead TLP concept with a drilling tender assist vessel and a floating production barge moored approximately 200 m from the TLP.
This is the first wellhead TLP with a drilling tender assist installed in deepwater having a large number of top tensioned risers.
The West Seno TLP is a conventional TLP with four columns and four pontoons which is moored by 8 tendons. Total displacement at mean design draft is about 23,000 mt. It supports 28 dry tree production risers and a tender drilling rig with a minimal floating production unit (FPU).
The production fluids are transferred to the FPU by flexible flowlines.
The TLP went into operation in early 2003 with the drilling campaign. The drilling tender vessel was moored to the TLP for approximately two years without any unplanned disconnections. During the drilling operations several severe storms have passed through the site. The overall system has been performing in accordance with expectations. From time to time some field adjustments have been made to the pretensions to accommodate ease of operations.
West Seno TLP is unique in terms of having more steel area due to its large number of risers from the surface to seabed than the hull itself. Considering the strong persistent surface and submerged currents, the design of the TLP and in particular the risers were quite challenging. Extensive amount of analytical work and model tests were carried out during FEED and detail engineering. Substantial effort was concentrated around VIV and design of the risers and tendons.
The risers are an integral part of the TLP, restoring stiffness. The 28 risers were incrementally installed; addition of each riser resulted in an increase in the transitional stiffness as well as an increase in the overall current load. In extreme current events, the benefit of increase in stiffness is undermined by higher current load.
Consideration of current loads can be of prime importance when selecting a hull form or type. For example, under extreme conditions, 75% of the total environmental load on the West Seno TLP is from the current.
During the FEED, a series of detailed model tests were carried out with the following objectives:
• Verify the design
• Capture any phenomenon that might have not been realized during analytical work
• Study and validate the drilling tender assist concept
• Provide a comprehensive data base to calibrate and verify analytical tools and procedures to be used during detail design.
To monitor the performance of the TLP and the coupled drilling tender vessel/TLP, the West Seno TLP has been instrumented. The following measurements are made:
• Tendon tensions (total of 8)
• Riser tensions (28 risers)
• TLP surge and sway motions
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Volume 66 Issue 2
February 2006
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