The Development and Testing of Polyester Moorings for Ultradeep Drilling Operations
Authors: Dove Peter , Weisinger Don , Abbassian Fereidoun , Hooker John 
Published: October 2008

Polyester taut-leg mooring systems (TLMs) promise superior stationkeeping and reduced cost (compared to conventional systems using wire and chain) for drilling and production applications in deep water. The Brazilians have successfully proven use of polyester moorings anchored with Suction Embedment Anchors (SEAs) or Vertically Loaded Anchors (VLAs), yet several technical and cost issues are evident with the approaches taken to date.

GOM acceptance of this important technology, and possibly other areas of the world, requires demonstration and recognition of existing, improved components and installation techniques by both regulatory authorities and deepwater industry players. In late 1998, a JIP was sponsored by (now BP Amoco), BHP, Chevron, Mobil, Aker Marine Contractors and Marlow Ropes attempting to achieve these objectives. The intent was to prove a new anchor design (Suction Embedded PLate Anchor -- SEPLA) and torque matched polyester rope (suitable for connection to conventional sixstrand wire). Two preset moorings using these components were to be used as part of an eight leg preset mooring attached to the semisubmersible Ocean America. While several objectives and lessons were established in this initial test, use of the polyester components and a SEPLA was not realized due to unrelated equipment failures; however, important lessons about polyester handling and deployment equipment, SEPLA installation and recovery, and preset mooring benefits were achieved.

Further development was undertaken by Aker Marine Contractors, Inc. (AMC) and Marlow, culminating in a second component test with the semisubmersible Ocean Victory to be moored in 4,500 ft of water late in 1999. Vastar joined the JIP for the second test by providing the rig and an installation vessel.

This paper describes the development and testing processes, installation and equipment techniques, test results and lessons learned and makes an assessment of how the technology can be used for future drilling and production applications. The work will provide a critical benchmark for industry acceptance of the technology.

Introduction

Interest by the offshore oil and gas industry in deep water exploration and production intensified during the 90s, with the sale of many deep water leases in the Gulf of Mexico and other areas. While several new generation Dynamically Positioned (DP) drill ships, primarily designed for exploration drilling, were committed to late in the decade, the need for cost effective and efficient stationkeeping for floaters using moorings, especially where development drilling and long term production were anticipated, became evident.

Taut Leg Moorings (TLMs), defined as mooring systems where a vessel's restoring force is achieved from a combination of catenary weight and mooring line elasticity, provide efficient and cost effective station keeping in deep water, especially where polyester rope is used. Recent studies comparing steel and polyester TLMs for semisubmersible and Spar-based Floating Production Units (FPDUs) have shown substantial reductions in maximum operating line tensions, vertical mooring load, offsets and costs with polyester TLMs (Reference 1).

Late in 1997 the U.S. Society of Petroleum Engineers (SPE) organized a conference at a venue in Brazil.