Innovative Suction Anchor Design and Installation

Abstract

Abstract Continued demand for cost effective methods of installing mooring systems for both FPSO's and MODU's has created a special market for marine contractors. The market pertains to development of innovative methods for installing mooring systems for a variety of water depths and soil conditions. Currently, suction anchors are being utilized mostly in permanent moorings due to the cost of installation, installation techniques and the availability of suitable installation vessels. Installation method development for suction anchors has generally been adapted to installation by crane-type installation vessels. Further development of installation methods will increase the pool of available installation vessels to include standard anchor handlers because of suitable deck area and the existing anchor handling winch. Methods are now being developed to utilize anchor handlers to both install and recover suction anchors by application of standard anchor handling techniques. This will allow suction anchors to be used more frequently in both temporary and permanent mooring systems. This paper will present the innovative design and installation method of the Schiehallion FPSO suction anchors. The paper will address the impact on suction anchor design caused by the short time span from contract award to anchor installation. This short time span demanded focus on simplicity in anchor design, fabrication, transport logistics and installation. Other aspects we will address that impacted anchor design include the large variations in soil conditions at each anchor location. Finally, we discuss the anchor design choices made and their impact on installation method and the use of a very simple anchor launch method. Introduction The mooring system for the Schiehallion FPSO, BP's offshore development West of Shetland. is located in 400 meters of water and was installed in the summer of 1997. The mooring system was initially designed with suction anchors for holding power. However, after additional soil data became available in the late spring of 1996, it became uncertain whether the previously designed suction anchors could be penetrated with suction. The- mooring system designer consequently changed the anchor design from suction to hammer driven pile anchors. A proposal was submitted wherein AkerlMaersk offered a solution with suction anchors designed for the new soil conditions. The proposal also included an innovative suction anchor launch method using standard AHV's, providing BP with a cost-effective solution to the hammered piles. Development of the launch method for the Schiehallion suction anchors was based on a pre-existing launch procedure developed by Aker Marine, Inc. for Shell. (I) In November 1996 BP awarded a contract to Aker Marine for the design and fabrication of 14 suction anchors. The mooring installation portion of the project was to be executed in 1997 by Aker/Maersk, a joint venture company. The weather West of Shetland is generally very harsh for most of the year and not very suitable for installation activities. In order to benefit from the calm weather AkerlMaersk elected to start the installation on July 1st. The late summer installation start date ret1ected the availability of installation vessels most suitable for the project. Maersk vessels were engaged in other projects, and consequently the vessels were not available to start the project before this date. The outcome of selecting the late start date resulted in having to complete the anchor design by early February, anchor construction by June, and finally transporting the anchors to the shore base by July 1st. This required a very aggressive planning and engineering sch