Structural Platform Solution For Seismic Arctic Environments-Sakhalin II Offshore Facilities (2005)

Abstract This paper describes the development of a technical solution for the two offshore platforms of the Sakhalin II project, in order to cope with the special characteristics of the arctic and seismic environments. The emphasis is on describing the design challenges and how they were addressed during the Front-End-Engineering-Definition leading to the concepts currently going through detailed design. Transportation and installation aspects, in such a remote environment with a limited weather window, form an integral part of concept development and needed to be addressed early on. The structural solutions for the two offshore platforms comprise a four-legged concrete gravity based structure (GBS), supporting an integrated deck that includes a seismic isolation system between topsides and GBS. In addition to providing seismic isolation, this system also effectively isolates the topsides from wave, sea ice and thermal contraction loadings and hence simplifies the design considerably. The structural concepts, which at the time of writing are at the final stages of detailed design, provide effective solutions taking due consideration of constructability, transportation, installation and operation for 30 years. Introduction This paper provides a description of the design challenges and how they were overcome during the Front-End-Engineering-Definition (FEED) work for the Lunskoye A and Piltun-Astokhskoye B platforms for the Sakhalin II development. There is currently little track record of feasible platform solutions in the new frontier environment of the seas around Sakhalin Island. Thus, the focus of the paper will be the structural and naval architectural design issues that were faced due to the remote location of the Sakhalin II fields, in combination with a harsh arctic environment and the significant seismicity of the region. This paper describes the conceptual solution that addressed the specific design drivers that are unique to this region of the world and these will be presented and critiqued in the paper. At the time of writing this paper the detail of the design has advanced, however, the design drivers and principal solutions remain unchanged from the FEED. The technical challenges and the engineering solutions developed are addressed with reference to the following: Sea Transportation, Installation, Arctic Design (material and temperature issues), Sea Ice influence (topside to substructure interactions), Seismic Design (topside to substructure interactions including seismic isolation methods considered), Wave Impact and Blast (highlighting differences encountered in arctic conditions). Readers may find the design experience presented here beneficial for future developments in this region of the world. It is intended that this paper will initiate the population of a database of structural platform solutions. The paper highlights the unique combination of design drivers and presents structural design solutions that could be referenced for future field developments in this region of the world. At the date of publication of this paper, there is only one existing platform facility on location in the Sakhalin region. This is the Piltun-Astokhskoye A platform, a caisson based oil production platform with seasonal tanker offloading.