Development of OPASS Code for Dynamic Simulation Mooring Lines in Contact with Seabed

Abstract

The mooring system of a floating platform keeps the structure in the desired sea location and can also contribute to the system stability. It consists of several cables attached to the platform in a point called fairlead, and with the lower ends anchored to the seabed. A new code for the dynamic simulation of mooring lines on wind turbines based on Finite Elements is presented. The code, called OPASS (Offshore Platform Anchoring System Simulator), includes effects as inertia, added mass, hydrodynamic drag, structural damping and contact and friction with seabed. The equations of motion are integrated in time using the Runge-Kutta-Nystr ¿om scheme. It has been loosely coupled with the FAST code for the simulation of offshore floating wind turbine dynamics. OPASS has been verified with semi-empirical expressions and with other codes as 3DFloat and SIMO-RIFLEX. Simulations prescribing a harmonic horizontal motion with 5m amplitude at the fairlead with different periods were performed with the objective of evaluating the importance of dynamic effects. In these simulations, dynamic effects on the lines are more important at moderate line tensions and can increase the tension up to 60% in comparison with the static tension. For taut lines dynamic effects are smaller and the quasi-static model could be an acceptable approximation. A comparison with FAST quasi-static mooring lines model has been also performed for a spar-buoy floating platform. The dynamic model predicts an important increase (30%) in the tension peaks of the cable with respect to the quasi-static approach and the consideration of dynamic effects can also have influence on the global motions of the platform. In particular, the quasi-static model can underestimate the surge damping.