Stability of offshore systems

Abstract

Stability analysis is a major aspect of the design of offshore platforms. It focuses on averting any failures associated with excessive heel or trim, capsizing, catastrophic loss of buoyancy, or even sinking. In this chapter, we cover the buoyancy and stability of offshore platforms from both viewpoints of theoretical and regulatory issues. Intact and damaged stability of floating structures are discussed and essential formulations of the associated problems are introduced. In doing so, the stability of a platform (its ability to be at or to recover equilibrium at acceptable heel and/or trim angles) is assessed by comparison of the righting moment to overturning moments from external loads. Methods of calculating the righting moment that varies with the heel angle are discussed. At small heel and trim angles, the metacentric height and displacement may be used directly to calculate the righting arm. Overturning moments that account for all static loads, including environmental, operational, and live loads are listed as well. Assumptions are needed to calculate the applied loads, and stability analysis does not usually include the upsetting dynamic forces. To address these issues, regulators set criteria, with embedded safety factors, and they require sufficient reserve stability beyond the equilibrium of the platform. These and the risks associated with flooding are addressed through hull subdivision and damaged analysis in this chapter.