Dynamic response analysis of floating offshore wind turbine with different types of heave plates and mooring systems by using a fully nonlinear model

Abstract

A finite element model is developed for dynamic response prediction of floating offshore wind turbine systems considering coupling of wind turbine, floater and mooring system. The model employs Morison's equation with Srinivasan's model for hydrodynamic force and a non-hydrostatic model for restoring force. It is observed that for estimation of restoring force of a small floater, simple hydrostatic model underestimates the heave response after the resonance peak, while non-hydrostatic model shows good agreement with experiment. The developed model is used to discuss influence of heave plates and modeling of mooring system on floater response. Heave plates are found to influence heave response by shifting the resonance peak to longer period, while response after resonance is unaffected. The applicability of simplified linear modeling of mooring system is investigated using nonlinear model for Catenary and Tension Legged mooring. The linear model is found to provide good agreement with nonlinear model for Tension Leg mooring while it overestimates the surge response for Catenary mooring system. Floater response characteristics under different wave directions for the two types of mooring system are similar in all six modes but heave, pitch and roll amplitudes is negligible in tension leg due to high restraint. The reduced amplitude shall lead to reduction in wind turbine loads.

Influence of Floater Response on Wind Turbine Tower Moments in Floating Wind Turbine System

“…The basic properties of the wind turbine are outlined here (table 1) with respect to some modifications that are essential for input in the numerical program. As the used scheme [ 2] cannot consider pitch control, wind turbine was considered as stall regulated. The aerodynamic and geometric details of the rotor blades and tower provided in Jonkman [ 12] were used.…”

“…A finite element scheme [ 2,14] that can use beam, pre-stressed beam and truss type elements and considers coupled interaction between floater, wind turbine and mooring system was used. The scheme uses Morison equation [ 17] with Srinivasan's Model [ 6] for estimation of hydrodynamic force, non-hydrostatic model [ 14] for restoring force and penalty method [ 18] for contact of mooring with seabed.…”

“…Denmark has the highest national contribution from wind energy at 21%. [ 1,2] The widespread application of wind energy has some practical limitation. Rural areas with steady wind and large space for wind farms have low energy demand while urban areas with high energy demand lack the space and steady wind resource.…”

“…Owing to their large size, they are more stable but have to carry large wave current loads, while the wind turbines suffer wake effect due to close proximity to other wind turbines on the floater. Over the last decade several concepts [ 12,13,14,15,16] of single wind turbine floaters have been investigated, these systems would resolve issues involved in the multi-turbine system but they suffer from stronger wave induced loads. The wave loads influence the wind turbine through six degrees of freedom of the supporting floater system as shown in figure 1.…”

“…In this paper, a previously developed numerical tool [ 2,14] is used to investigate the effect of floater response modes on the wind turbine tower moments over the range of operational wind speed for wind turbine. The catenary and tension leg type mooring system are considered to account for the level of restraint to floater response.…”