Ship-shaped units set to intervene in North Sea

Subsea oil wells need regular maintenance to operate efficiently. In the past, divers carried out much of this work. More recently the task has been performed from semi-submersible platforms. Attention has now turned to the potential use of ship-shaped well intervention vessels, which are self-propelled and able to carry out other tasks.

Einar Tyssen, head of the marine and process systems (offshore technology) division at DNV says the main impetus behind the development is the desire to reduce the cost of maintaining subsea wells. Large numbers of subsea wells on the Norwegian Continental Shelf need well intervention in the coming years, he explains.

DNV was part of a recent Joint Industry Project (JIP), into well intervention ships. This concluded that operations such as subsea wirelining could be performed by a ship-shaped vessel as safely and as efficiently as a semi-submersible. The study determined that well intervention carried out by a dynamically positioned ship-shaped vessel would cost significantly less than the cost involved in mobilising a rig. A ship-shaped vessel would also have a higher transit speed and the ability to carry out other tasks such as ROV operations and general supply duties.

DNV has developed two types of new class notation for well intervention vessels: for light well intervention (which requires DP2), and for more ?heavy duty? tasks such as direct contact with a live wellhead (which needs DP3).

Two newbuild vessels are now on order which are expected to be built according to DNV?s new class. One is an MT6014 design for newly-formed company, Wellships, based in Aberdeen, Scotland. The other is a UT737L design, which is derived from the well-known family of Rolls-Royce Marine UT designs.

Wellships scope

Wellships, which was established specifically to own and operate well intervention ships, put pen to paper in June of this year and signed a contract with Aker Brattvaag for the construction of its MT6014 well intervention vessel. Delivery is scheduled for early 2004. The company raised £70 million ($109 million), through debt and equity, to build the vessel, making Wellships one of Scotland?s largest corporate start-ups in recent years.

At 101m overall, with a beam of 22m, and 6.5m draught, the MT6014 design has accommodation for 80, and installed power amounting to 11,584kW. Transit speed is 12 knots. The builder will fit an Integrated Topside Package (ITP) to the ship to allow the deployment, control and recovery of the subsea and well intervention equipment.

The scope of services that Wellships intends to offer from the vessel include: subsea mechanical services and inspection, repair and maintenance; downhole services (including well performance enhancement and data acquisition); and decommissioning services.

The vessel?s operating parameters are:

l Riser-less well intervention operations to depths of 1,500m

l 2,500m maximum for all other subsea intervention operations

l Well intervention operations in up to Sea State 6

l Non-well intervention operations in up to Sea State 7.

DNV classes the ship as a ship-shaped well intervention vessel type 1.

Island Frontier

Island Offshore is behind the second ship-shaped well intervention unit ordered. The

Island Frontier

, as the ship is to be named, will also be classed as a Mobile Offshore Unit under Norwegian Maritime Directorate regulations.

Nils Peter Pilskog, sales manager for ship technology at Rolls-Royce Marine explains that apart from well intervention, the vessel will undertake a variety of offshore support tasks including subsea installation, ROV operations, and diving support and supply work.

The UT737L?s closest ?relation? in the UT ?stable? is Island Offshore?s UT737 Normand Flower, itself an enlarged ? and greatly altered ? UT745 design (see following article).

Due for delivery from Norway?s S?viknes shipyard in 2003,

Island Frontier

is a DP3 class vessel with full redundancy and logically planned subdivision.

The hull is 106m long overall and has a beam of 21m. A deadweight of about 4,600t is available on a draught of 6.2m, of which about 3,400t can be taken on deck.

Tank testing of the design indicates that it will have extremely low motions, helped by the roll reduction system using four passive tanks. A pumped anti-heeling system is fitted, independent of the roll-reduction tanks, to counteract the heeling effect of the 100t long-outreach offshore crane which is installed on the starboard side of the working deck.