The number of sub sea wells in the world has increased greatly in the last few years. It is estimated that there are approximately 750 sub sea wells in the North Sea with many more than this number world wide. All the indications are that the number of sub sea wells will continue to increase as operators exploit this proven technology in ever deeper water depths. The costeffectiveness of sub sea completions with floating producti on facilities combined with the environmental advantages of not installing large fixed platforms means that the number of subsea developments will continue togrow.
Coiled tubing as a means of well intervention has matured considerably over the last five years with coiled tubing performing operations that previously would have been the preserve of workover rigs or hydraulic workover units. These two developments lead to the inescapable conclusion that a light subsea well intervention vessel equipped with coiled tubing as it's primary means of intervention could offer a cost effective method of working over sub seawells.
Sub-sea well workover is an essential part of field maintenance. It can be divided into two categories;
Heavy workover where there is a requirement to deploy a marine drilling riser to retrieve the down hole completion string, necessitating the use of the full capabilities of a drilling rig,
Light intervention that can be performed through the existing completion using coiled tubing or wire line requiring a light riser or a sub seariser.
Light well intervention is currently carried out by mobilising portable equipment onto semi-submersible drill rigs. These rigs may be diverted from infield drilling, or spot hired for the task. Well intervention therefore tends to be performed only when deteriorating well performance has become critical and it can no longer be avoided.
With the growing inventory of sub-sea wells, the industry has recognised the need for a more cost effective, efficient and comprehensive ability to intervene in, and maintain, sub-sea wells. Previous industry studies have concluded that a semi-submersible would be too expensive, and that the motion characteristics of known monohulls would necessitate shut in and disconnection of the riser in relatively modest sea states. Thus both options were seen to be uncompetitive, and the decision to design a new ship was taken.
The Light Well Intervention Ship must have a large operating window, and the target of 95% up-time in the North Sea was set. This equates to winds of Beaufort 7 to 8 and sea state 7.
The ship must have a high transit speed so as to maximise the time available at site, minimise mobilisation costs and reduce turn-round times. The minimum acceptable speed was set at 15 knots.
The ship must have low operating costs. This would be achieved by:
designing for low maintenance and infrequent dry-docking,
designing for low fuel consumption on D.P.
designing systems for low manning,
providing a large bunker capacity,
designing systems with sufficient flexibility and capacity that the ship would be capable of autonomous operation.
The ship must be able to provide all standard well service solutions and these must be available at all times, thus providing operational flexibility and the ability to respond to the client's requirements at the well site and deal with contingencies as they occur. In addition it must be able to operate world wide, so the motion characteristics must suit oceanic as well as North Sea conditions, and the riser must be designed for use in these conditions. A target of water depths between 75m and 600m was set, covering the North Sea and West of Shetland. This was then extended to 2000m to cover the Gulf of Mexico, Brazil and West Africa.