The Development Of The Garden Banks Block 388 Fpf Mooring System
Authors: Dove P.G.S. , Librino F. , Scovell D.C. 
Published: October 2008

This paper discusses work conducted during the design, procurement and installation of Enserch Exploration, Inc.'s Garden Banks 388 FPF mooring system. The design began with the execution of a trade-off study evaluating and comparing previously installed floating production moorings in the Gulf of Mexico, coupled with evaluation of new concepts and emphasis on cost effective solutions.

The design effort involved dynamic analysis and wind tunnel and model tank testing, all in accordance with the newly completed API document RP 2FPI. Inspection of various components from the Placid GC-29 FPS moorings (installed in 1987 and recovered in 1990) determined that sections of chain, jacketed spiral strand wire rope, submersible buoys and connectors could be reused with suitable refurbishment. The excellent condition of the rig's onboard winching system also resulted in the reuse of the windlasses, with specified upgrades. Because a suflcientamount of used wire was not available, a bare spiral strand wire rope construction was adopted, including zinc anodes in the new sections, rather than jacketed strand.

The lack of cost, effective installation vessels in the Gulf of Mexico at the time of the installation bid posed challenges to Enserch. Howeverj an innovative preset mooring installation scheme involving Heeremac's SSC V Balder on its own moorings was adopted.

Since the vessel was already in the Gulf of Mexico oncontract for other projects, a cost effective contract was negotiated.

The results of this effort led to considerable cost savings for Enserch, compared to conventional FPF mooring systems previously installed in the Gulf of Mexico.

Introduction

The GB-388 FPF mooring system design commenced in mid-1 993. Aker Omega, Inc. was contracted by Enserch to perform this function as part of a larger engineering contract that also included the semisubmersible vessel conversion engineering and design of the production facilities. A major objective was to procure the on- and off-vessel components to allow preset mooring installation and platform attachment to be completed by the end of 1994. The preset mooring installation was completed on schedule in November 1994.

Enserch insisted the system meet the requirements of accepted industry design standards. The American Petroleum Institute (API) had just issued a first draft of specific Recommended Practices for designing permanent mooring systems (see below). This was the first mooring system in the world to be designed and installed according o these practices. It should be noted that these standards are more stringent than previously designed permanent ooring systems. Additionally, the system had to meet therequirements of the American Bureau of Shipping (ABS), ho classed the rig and mooring system and was engaged to represent the Minerals Management Service as Certification Verification Agency (CVA). At the same time, it was critical for the system to be as cost competitive as possible, both from a design and installation viewpoint.

The rig selected to be the basic platform for the GB-388 FPF was the Glomar Biscay 1, an Ocean Victory class Semisubmersible MODU. The rig, which was renamed the Enserch Garden Banks, was originally built in the mid 70's and previously belonged to Global Marine.