Structural performance and dynamic response of semi-submersible offshore platforms using a fully coupled FE-SPH approach

An explicit Finite Element (FE) software package with an embedded and fully coupled Smoothed Particle Hydrodynamics (SPH) solver is used to investigate the dynamic response of a floating (moored) offshore platform subject to strong wave action. The water is modelled using SPH while the platform and anchor cables are modelled using FE. The main body of the platform is modelled as a rigid body with appropriate mass, centre of gravity and moments of inertia. Coupling between the SPH and FE is automatically handled by contact algorithms available in the solver1. To represent the effect of the mooring system on the dynamic response of the platform, the anchor cables are modelled using flexible 1- dimensional (1D) finite elements. The stable time step for these 1D elements is much smaller than for the SPH particles and including the flexible anchor cables in the model would ordinarily cause excessive simulation times. However, a technique known as multi-model coupling is used to partition the analysis model into 2 separate models, each of which advances at its own stable time step2. This technique is shown to result in a total simulation time that is comparable to that obtained when running the case without anchor cables. The use of periodic boundary conditions applied to the SPH particles allowed the computational domain to be limited to 4 wavelengths in the direction of wave travel. The waves are generated by applying the moving floor technique3. The dynamic response of the platform is presented for the case when the platform is operating at a depth of 310 m under the influence of waves with a wavelength of 365 m and amplitude of 7.8 m. This represents the survival condition for a reference semi-submersible platform. An extension of this approach considering the stresses induced in structural members of the platforms is also demonstrated by replacing the rigid representation of the cranes with flexible finite elements