Session: 09-02-03 Wind Energy: Moorings I

Submission Number: 156904

Evaluation of Mooring Tension Reduction Ratio of Load Reduction Device Using HydroQus 

To be competitive in the floating offshore wind market, it is essential to be economical. To this end, efforts are being made to reduce the capital expenditure on mooring systems. Reducing the size of the mooring chain or the length of the mooring line through mooring load reducing devices (LRDs) has been proposed. These LRDs are characterized by nonlinear stiffness with three phases. This study aims to verify that their proposed 3-phase nonlinear stiffness is realized in practice and to evaluate the mooring load reduction.

This study is based on a FOWT consisting of IEA 15 MW wind turbine, VolturnUS semi-submersible floater, three catenary moorings with 185 mm R3 studless mooring chain. The wind turbine, tower, and floater are modeled as rigid elements. Beam elements were placed along the centerline of the floater and tower. This is to account for drag force due to current and wind. The catenary lines were modeled by placing beam elements and joint elements along the layout calculated by the elastic catenary equation. For LRD, we chose products with relatively large amounts of information. Before applying LRD, it is necessary to determine the level of mooring load experienced by the target FOWT. To do this, an integrated load analysis (ILA) must be performed. The ILA was performed with HydroQus, developed by the authors. For the ILA with HydroQus, frequency response analysis was performed on the floater to derive the hydrodynamic coefficients. To define the environmental conditions required for ILA, we analyzed the environmental conditions in Ulsan, South Korea. The ILA was performed for DLC 6.1, which is the ultimate limit state. The environmental loads were wind speed, wave height, wave period and current with a 50 year return period. The reason for the selection of the DLC was to obtain the mooring peak loads required for LRD design and performance verification. From the ILA results, the specifications of the LRD could be determined.

The determined LRDs were verified to reproduce the stiffness curves presented in the reference data. We modeled the LRD as a cylinder-shaped buoyant body with two arms. To derive the stiffness curves, we constrained the 6 degrees of freedom of one arm and moved the other arm. The direction of translation of the arm was the same as the direction of the mooring load. HydroQus was used for the validation simulations, which accounted for hydrostatic pressure and drag forces acting on the buoyant body of the LRD. Through validation, we found that the stiffness curves differ from those presented in the reference. The difference is likely due to the drag force acting on the buoyant body. To reproduce the stiffness curves presented in the reference data, it was necessary to increase the rotational speed of the LRD exponentially during the simulation. The fact that the stiffness curve changes with the speed of motion means that the performance of the LRD can vary depending on the sea state.

As mentioned earlier, the purpose of this study is to evaluate the mooring load reduction rate. ILA was performed for the case with the directly modeled LRD and for the case where the springs placed at the location of the LRD were defined with the nonlinear stiffness presented in the reference data. A comparison and analysis of the mooring load reduction in these two cases will be presented at the conference.

Presenting Author: Dong Ho Yoon Inha University

Presenting Author Biography: Dongho yoon is a doctor program in the Department of Naval Architecture and Ocean Engineering at INHA University, South Korea

He has been studying FSI of floating offshore wind turbines.

Today he will present Evaluation of Mooring Tension Reduction Ratio of Load Reduction Device using HydroQus.