Evaluation of Walk-to-Work Accessibility for a Floating Wind Turbine

There is a growing interest from offshore and energy industry towards floating wind turbines. Since these systems are exposed to harsh metocean conditions, accessibility for inspection and maintenance purposes becomes of primary importance for a safe and cost-effective long-term operation. It is here proposed a methodology for evaluating the accessibility of a floating wind platform, by means of walk-to-work vessels. Two access means are considered: a catamaran equipped with fender and an offshore supply vessels with a motion-compensated gangway. The system composed by the platform and the vessel is modelled as a constrained multi-body system in the frequency domain. This allows to calculate transfer functions for the vessel motions and the contact forces. For given a short-term sea state the maximum crest height of a response variable is calculated assuming that crests heights are Rayleigh distributed, and compared to some threshold to determine whether the platform is accessible or not. For the catamaran, access is possible when the fender does not slip against the landing structure mounted on the floating platform; for the supply vessel, when the relative motions at the gangway tip do not exceed the compensation limits of the hydraulic system. Accessibility is calculated for a sample semisubmersible wind turbine located off the coast of Portugal using hindcast data for the period 1980–2013. Findings indicate that the catamaran and the supply vessel can ensure access for 20% and 76% of the time respectively. However, results are strictly dependent on the vessel types and the walk-to-work constraints.