Full Scale Field Trial of Taut Leg Mooring using Fiber Rope and Suction Anchor attached to a Semi-Submersible Drilling Unit
Authors: Ølberg Stein , Guttormsen Tom , Molland Gunvald , Andersen Jarle 
Published: October 2008

1 Abstract

This paper describes a full scale field trial of taut leg mooring performed by Saga Petroleum ASA in August/September 1996 offshore Norway, using suction anchors and two types of fiber ropes attached to a semi-submercible drilling unit. The purpose of the field trial was to qualify taut leg mooring as an effective mooring system for floating drilling units.

The subject drilling unit was conventionally moored by 8 chain/wire lines. The field trial was based on installing two additional mooring lines consisting of one polyethylene(Dyneema) and one polyester fiber rope in a taut leg mode using suction anchors. The operation was performed in 200 m water depth.

The two large suction anchors were installed and removed from a standard AHTS vessel by use of a purpose built launching frame. This operation is normally performed by large, heavy lift vessels or crane barges.

The tension and elongation of two fiber ropes were continuously monitored and stored by a data acquisition unit at site during the trial. Additional laboratory tests of the fiber ropes were carried out before and after the field trial.

Saga Petroleum's field trial has demonstrated that taut leg mooring using fiber ropes and suction anchors is a viable and effective alternative to conventionally moored and dynamically positioned drilling units, especially in congested seafloor areas and in deep water.

2 Introduction

About 5 – 6 years ago the industry in Norway started to look into exploration drilling in deep waters (800 – 1500 m water depth). This was initiated in preparation for exploring the deep prospects offshore Mid-Norway (Mere and Vering Basins). Saga Petroleum has been working with a simplified rig concept for this application, called "The EfEx Rig" (Effective Exploration). In autumn 1995, taut leg mooring using suction anchors and fiber ropes was raised as an alternative mooring system for this rig concept.

Taut leg mooring using suction anchors and fiber ropes soon developed into a project of its own. As conventional installation of suction anchors so far has been done with expensive heavy lift vessels or barges, it was imperative to develop a more cost effective installation method. In a cooperation between a small engineering company, named Structural Engineering, and Saga Petroleum, a method of deploying suction anchors from a standard anchor handling (AHTS) vessel was developed. This was accomplished by designing a special launching frame.

Saga Petroleum found that in order to qualify this deployment of suction anchors and to evaluate the behaviour of fiber ropes in a taut leg mode, a full scale field trial would be essential. The semi-submercible drilling unit, Deepsea Bergen, was scheduled to drill a well for Saga Petroleum at the Tordis Field in The North Sea, summer 1996. Even though the water depth was only 200 m, this offered a good opportunity for afield trial as Deepsea Bergen was only to be moored by 8 out of possible 10 lines.