This paper outlines a proposal for a slim drillship optimised for re-entry into existing subsea wells. The ship is designed to perform intervention operations using wireline and coil tubing equipment, through a high pressure riser string between the ship and the subsea wellhead, for access to the pressurised well. The slim drillship is also equipped with machinery and tools enabling it to drill new hole sections inside existing wells, for example long horizontal extensions, using slim jointed pipe that enters the well inside its production tubing. A key feature of a slim drillship is its high pressure (HP) riser, giving intervention tools access to the well containing hydrocarbons, eliminating the need to kill the well first with drilling mud. This feature does also enable the ship to use Manage Pressure Drilling (MPD) methods.
The drillship is initially designed for 7500 ft of water depth as well as well depth below the drill floor of 22500 ft. This is believed to be a practical depth limit for some known subsea fields in the North Sea area, Mexican Gulf, Brazilian shelf and areas offshore West Africa, and future areas in South Asia. It is possible to configure the ship for deeper wells.
This paper focus on the drill floor layout and open deck areas for storage of tubulars and equipment. An open mast for hoisting is proposed in order to enable a large drill floor for third party equipment. The mast is telescopic so the ship can lower the crown block and pass through the Straits of Bosporus and the Panama Canal, enabling the ship to move fast between arctic locations and tropical waters without passing south of South America. A novel vertical pipehandling method is used, with cylindrical rotating setbacks at a lower elevation than the drillfloor, and offline stand building on this lower deck. The use of an open mast layout as opposed to an enclosed derrick gives access to the well centre for intervention equipment, and enables rapid turnover between various tools going into the well.