Spacer Design and Fabrication for Sakhalin II Project

Abstract Sakhalin Energy Investment Company has refurbished and redeployed the Molikpaq, an arctic offshore drilling caisson originally designed for use in the Canadian Beaufort Sea. The original structure has been deepened by adding a new pontoon base, named the "Spacer", and converted into a drilling and production unit. It is now placed 16 km off the coast of Sakhalin Island as the first production unit in the Russian subarctic. This paper describes the design of the 15,000 tonne Spacer unit, its fabrication in Russia and the mating of the Spacer to the 35,000 tonne modified Molikpaq structure at a Korean shipyard. The paper also describes the interrelationships among the participants involved in this fast track project; the challenges of designing for a new frontier area with severe wave, seismic and ice loadings; and the challenges for the Russians and the Owners of building a new type of structure in a yard designed for building submarines. Project Overview The Piltun-Astokhskoye (PA) Field, located in the Russian Far East in the Sea of Okhotsk, was discovered by Sakhalinmorneftegas (SMNG) in 1985. Subsequently, SMNG drilled 13 additional parametric wells on the structure. Members of the consortium now called Sakhalin Energy Investment Company Ltd. (Sakhalin Energy) were awarded the rights to further explore the PA Field in March 1992. In 1995, the Upstream Operator, Marathon Upstream Sakhalin Services Ltd., with the aid of several engineering contractors, evaluated the use of the "Molikpaq" (or platform PA-A) as a way to bring early production to the PA Field. The Molikpaq, a massive, octagonal, Mobile Arctic Drilling caisson, was constructed in the early 1980s for Gulf Canada Resources Inc. and used as a drilling platform at several locations in the Canadian Beaufort Sea (ref: 1, 2). Although the Molikpaq was constructed specifically as a mobile offshore drilling unit, the engineering studies indicated that it could be modified to support additional wells and process facilities and become a full production platform operating safely in the challenging wave, ice, and seismic conditions characteristic of the Sakhalin region. Figure 1: Modified Molikpaq Platform at the PA Field (Available in full paper) The Sea of Okhotsk presents enormous challenges for offshore production. Ice-covered for half the year, it generates severe waves during the open-water season and is one of the world's most seismically active regions. The challenge was to develop a platform that could accommodate the environmental forces and yet be brought on stream on a fast track basis. To accommodate the 30 m water depths and harsh wave climate, the Molikpaq had to be deepened. After assessing a number of alternatives, the solution chosen was to design a pontoon-type steel base, ultimately named the "Spacer", which added 15 m to the platform depth.