Abstract

In 2004, a supply ship struck West Venture, a semi-submersible drilling rig operating in the North Sea. This paper describes the accident and the resulting damage, considers the operational factors that led to the collision and compares the damage with current guidance.

Introduction

West Venture is a 5th generation semi-submersible drilling unit with dual Ramrig, operated by Smedvig Offshore as. In 2004, it was operating in the Troll Field of the North Sea. In the middle of the night of 7th March, a supply ship of 5000 tonnes displacement collided with the rig. The wind was calm, the significant wave height was only 1.3m, and although it was dark, there was no reduction in visibility due to weather conditions.

The supply boat entered the safety zone around West Venture for a scheduled delivery, but was not able to manoeuvre normally due to operational problems with the autopilot. The ship struck the fore port column head on at a speed of 7.3 knots. It took some time to stop its engine, and the supply ship actually pushed the drilling rig more than 6 metres before the incident was over.

The damage to the drilling rig was moderate. The column shell was indented 10-20 cm and punctured in several places. There was some minor damage to a fairlead, which had been struck by the bulbous bow. The supply ship was more severely damaged, with an indentation almost 3m deep in the bow.

In the following we compare the extent of the actual damage against current guidance. We also describe in more detail the operational background for the accident.

Operational considerations

The supply ship was on a scheduled trip with cargo to the drilling rig. Its autopilot was engaged and was commanded to steer a true heading that would pass 700m away from West Venture. Unfortunately, wind and current caused sideways drift such that the resulting track was directly towards the fore port column of the drilling rig.

Normal procedure when approaching an offshore installation is to disengage the autopilot when entering the 500m safety zone. The box on the checklist was ticked to confirm that this action had been carried out, but for some reason that will not be discussed further here, the autopilot was still engaged as the supply ship approached the drilling rig.

When the officer in command tried to steer the ship to bring it alongside the drilling rig, he quickly noticed that it was not responding to his manoeuvring commands. The autopilot would not permit control wheel steering (CWS) nor would it allow manual override while it was engaged.

As the situation became more critical, the officer tried everything to prevent the accident, except the one thing that could have saved his day and that was to disengage the autopilot.

He tried to turn the thrusters 180 degrees and add full power for backing up. The system rejected the steering command but accepted the power command.

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