After successfully using an all steel Pre-Installed Mooring System (PIMS) to double Transocean Mariana's mooring capability in the 1990s, Shell improved the PIM system's performance by using polyester ropes in the system. The upgraded mooring system allowed Transocean DW Nautilus to successfully drill several wells in water depth greater than 9000 ft.
This paper summarizes the key learning based on the Shell's experience in the Pre-installed Mooring System since 1997, and on the performance results of the Second Generation PIMS using polyester ropes since 2001. The findings based on the experience and test results of mooring hardware after Hurricane Lili in 2002 and Hurricane Ivan in 2004 provide a solid basis for us to improve our design, handling and installation of future polyester mooring systems for both mobile and permanent floating platforms.
In the mid 1990s, Shell (US) faced challenges to drill some of Company's prospects in water depth nearly twice as deep as the existing water depth rating of rigs on contract. Another challenge in exploring deepwater prospects is to virtually eliminate the risk of drive-off, drift-off or emergency disconnect of a DP rig without significant increase of capex or opex. Extending the water depth capability of a moored MODU on Shell's contract would address both challenges.
A cost-effective upgrade to a rig's maximum water depth rating requires upgrading both drilling and stationkeeping capabilities within the platform's variable deck load capacity. In terms of stationkeeping, Shell with support from contractors developed an effective Pre-Installed Mooring (PIM) System to supplement a MODU's on-board mooring system in the mid 1990s. The first generation PIM system typically includes suction-pile-anchor/subsea-connector/wire/buoys. The top end of Pre-Installed Mooring lines would be connected to rig wires upon a MODU's arrival at a well site. This solution was successfully applied to enable Transocean Marianas to drill many wells, such as NaKika and Coulomb fields, in water depth from 5000ft to 8000ft, even though the rig had marginal variable deck load capacity to drill in 4500ft of water. The Pre-Installed Mooring concept typically utilizes two sets of mooring lines so that the second set of mooring lines could be preset at the next well site. By pre-investing in two sets of PIM lines, a MODU could be quickly put under tow to the next well site by disconnecting and buoying-off PIM lines rather than wait for the entire mooring system to be retrieved. Since another set of PIM lines have been preset and buoyed off waiting to be hooked-up at the new site, a MODU could typically spud a well within 24 hours upon arrival at the next site.
