Dynamic Analysis of Offshore Spar Platforms

Abstract

Offshore spar platform is a compliant offshore floating structure used for deep water applications for drilling, production, processing, storage and offloading of ocean deposits. The offshore spar platform is modelled as a rigid body with six degrees-of-freedom (DOFs), connected to the sea bed by multicomponent catenary cables which consist of a mooring line, a clump weight, and an anchor line attached to the fairlcads. The response-dependent stiffness matrix consists of three parts: (i) the restoring hydrostatic force, and the stiffness due to cables. (ii) nonlinear horizontal springs, and (iii) nonlinear vertical springs. A unidirectional regular wave model is used for computing the incident wave kinematics by Airy's wave theory and  hydrodynamic force by Morison's equation. The response analysis is performed in timc domain using the iterative incremental Newmark's method. Numerical studies have been conducted for sea state conditions with and without coupling of DOFs.