Evaluation of second and third-order numerical wave-loading models for floating offshore wind TLPs

With the large-scale development in the last decades of fixed offshore wind across Europe and the ever-more present threat of climate change dominating national and global agenda, the exploitation of wind power in deep-water using floating wind turbines is gathering a significant amount of interest. Compared to other types of floating wind platforms, Tension-leg-platforms (TLPs) are less compliant systems resisting dynamic forces through their pre-tensioned tendons. Whilst this reduces the weight of the platform hull, understanding extreme loading cycles in the mooring system becomes an important design issue. Recent research has shown that including sum-frequency second-order and third-order loads is important to capture extreme events such as slack-line events on large and stiff floating systems. This article exposes a review of some of the existing numerical methods available to engineers to calculate second and third-order forces, including both potential (semi-analytical and BEM) and strip-theory approaches (Rainey and FNV). It compares their application in monochromatic waves on a truncated cylinder and a full TLP platform by evaluating forces amplitudes against CFD calculation. The results show that large fully submerged elements of the TLP floater contribute significantly to the third-order forces and are not well represented by existing engineering methods.