Research design analysis of a floating oil and gas production platform for the Russian Far East offshore for a medium sea depth (of 120-160 m) is presented. The base task is how to provide the platform stability against wave load as well, as to seismic loads. The well-known "Spar" conception is assumed as prototype for the platform substructure design There are considered the "Spar" design defects (as results from operational experience):

  1. substructure large angular inclinations due to wave influence;

  2. impermissible set-down and offset under wind, current and ice influence;

  3. probable instability of anchors and all mooring system owing to seabed seismic accelerations. To avoid the "Spar" defects a new conception of platform substructure for a medium sea depth (100–200 m) is presented. In this purpose, a number of heavy pendant chains are arranged near the floating leg bottom. The slacked chains lie partially at seabed and provide additional vertical stability as well, as horizontal and angular stability. Therefore the platform motions are reduced and leg bottom transference is kept in allowed limits. Seismic accelerations are not dangerous for the slacked chains. Computer simulation for a design pattern had confirmed the mentioned results. The proposed variants are considered in reference to Russian shipbuilding yards technology possibilities.

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