MEGATHYST - A Compact Dynamically Positioned Semisub for Deep Waters

Abstract MSV Amethyst is a compact, fully Dynamically Positioned semisub, designed for workover on subsea wells in the North Sea, supplied in 1987. In 1996 a new rig, based on the Amethyst, was designed with drilling capacities for deep water applications (1200 – 1500 m). The capacities of this new design should match with those of a large drilling vessel. In order to be able to design such a drilling vessel, close cooperation with the drilling equipment suppliers and the drilling operator was necessary to get a cost effective compact light weight semi submersible. This paper describes the process of upgrading the design of the workover unit into a deep water drilling vessel. This involves:– The increase of the payload from 1700 to 4000 tomes.– Design of the powerplant / DP system according IMO DP Class 3.– Reduction of the amount of steel used to build the vessel.– Design of the box girder derrick to reduce the amount of steel and thus weight of the derrick.– Vertical storage of risers in the aft columns of the vessel to lower the centre of gravity.– Integration of the drilling equipment in order to reduce the building time of the vessel in the yard, the weight due to the reduction of the amount of steel needed and as a result costs.– Dual derrick design for simultaneous operations like preparing casing or riser while drilling. Introduction In the beginning of 1980's five dynamically positioned diving support vessels were built by our yard for different owners (Fig 1). During the design and construction of these vessels the design team was asked to investigate the possibility of carrying-out workover through a moonpool in the aft deck. A demand for workover on the various subsea wellheads, being installed in the North Sea, was expected (Fig 2). After studying motion-study characteristics the amount of expected downtime was not acceptable to the field operator. The starting-point of carrying-out workover from a monohull was quickly abandoned and, also based on the experience of diving support vessels, a semi-submersible structure was adapted. The market approach was using a dynamic positioned vessel (DP) for workover in the summer and in the winter, as a diving support vessel. The diving work would consist of hyper baric welding, installing templates and spool pieces etc. The vessel, supplied in 1987, started working in the Northsea in 1988 and was named M.S.V. Amethyst (Fig 3). Due to the market situation in 1987 / 1988 the workover equipment was not installed. The vessel was used as a diving support-installation vessel in the North Sea until the beginning of 1995. In 1995 the owners / operators of the vessel were contracted to carry-out workover in Brazil. After completing the workover equipment the vessel left for Brazil in the middle of 1996 (Fig 4). MEGATHYST design concept In 1996 the design team of the Amethyst was asked to develop a new design based on the Amethyst. However the operators demanded better performances for drilling in deeper water.