The application of semi-analytical diffraction formulas to predict second-order dynamic response of a TLP floating wind turbine in monochromatic waves

Floating offshore wind Tension-Leg-Platform (TLP) concepts benefit from a reduced hull tonnage and limit response in vertical modes of motion through the use of pre-tensioned mooring lines to resist aero-hydrodynamic forces. However, this design choice requires an accurate prediction of extreme loadings in the mooring cables such as those generated from slack-line events. Literature shows that slacking events are better predicted when including higher-order hydrodynamic loading as these can generate high-frequency resonant responses in TLPs referred to as springing and ringing for second-order and third-order sum-frequency responses respectively. Therefore, understanding the amplitudes and phases of the various harmonics of the forces on TLP platform’s hulls can be an important aspect of their design. This paper compares two common engineering methods for computing second-order loads, the strip theory and second-order potential flow approaches to be applied for a TLP type Floating Offshore Wind Turbine (FOWT). Whilst the strip theory approach is computationally efficient for second-order force calculation on cylinders, its accuracy decreases with high radius to wavelength ratio. On the other hand, commercial BEM solvers compute second-order forces with a much higher degree of accuracy but they suffer from high computational time. Therefore, a third method is proposed in this paper, aiming at combining better accuracy than with strip theory while keeping computational time low, by using semi-analytical equations in the form proposed by Huang & Eatock Taylor [1]. The harmonics of the force signal generated by unidirectional monochromatic waves on a truncated vertical cylinder in finite depth are compared with BEM and strip theory models to understand the difference of force amplitudes and phases between the various models and their exact domain of application. The comparison is then extended by applying the same method on the transition piece of an academic TLP while modelling the remainder of the structure with strip theory. The harmonics of motions and line tensions are then compared for the three models with particular attention given to springing response. The results suggest the viability of applying semi-analytical potential flow solution as an alternative to the traditional strip theory methods for the estimation of springing response in TLP FOWTs in early design optimisation loops. However, the paper highlights the issues of their use on complex structures and proposes further work to determine their full potential.