Numerical Simulation of Inline Forces on a Bottom-Mounted Circular Cylinder Under the Action of a Specific Freak Wave

Yuqi Wang,Zhanbiao Zhang,Fuyou Xu

doi:10.3389/fmars.2020.585240

Abstract

Freak waves pose a great threat to the tension-leg platforms (TLPs) and monopile foundations of offshore wind turbines (OWTs), which necessitates comprehensive investigations on the characteristics of freak waves and the wave actions on those offshore renewable energy structures with circular cylinder. The recorded freak wave series “New Year Wave” (NYW) was numerically simulated using the Computational Fluid Dynamics methods. The compensation measure was adopted to effectively improve simulation accuracy. Under the action of the NYW, the inline forces and secondary load cycle (SLC) on a vertical-mounted cylinder, as the classic form for the TLPs and foundation of OWTs, were fully addressed. The simulation results were compared with the empirical formulations and experimental data to reveal the differences and the possible causes. The development of SLC was found to be closely related to the downstream vortex and return flow, which induces the reduction of the wall pressure and thus the inline force. The maximum inline forces vary with the cylinder position relative to the wave peak, and the simulation results reveal that the linear inline forces calculated by Morison formulation may be less than 65% of the total wave forces.

Highlights

With the utilization of the renewable marine energy and resources, the offshore structures, including the tension-leg platforms (TLPs) and offshore wind turbines (OWTs) have rapidly developed over the past few decades
The maximum inline forces vary with the cylinder position relative to the wave peak, and the simulation results reveal that the linear inline forces calculated by Morison formulation may be less than 65% of the total wave forces
Some major conclusions are drawn as follows: (1) For the simulation of the extreme freak wave, e.g., “New Year Wave” (NYW),” the simulation accuracy can be improved by the compensation for the difference between the target wave series and initial numerical results

Summary

INTRODUCTION

With the utilization of the renewable marine energy and resources, the offshore structures, including the tension-leg platforms (TLPs) and offshore wind turbines (OWTs) have rapidly developed over the past few decades. Chaplin et al (1997) and Bredmose and Jacobsen (2010) experimentally studied the inline forces on the cylinder structures under the action of the freak wave, and found that the maximum wave forces are larger than the results calculated by Morison formulation. As mentioned by Bachynski and Moan (2014), none of the aforementioned theoretical approaches could capture the secondary load cycle (SLC) that was observed in experimental and numerical studies, and Grue et al (1993) believed that SLC makes a crucial contribution to higher-harmonic component of inline force, which may cause the ringing vibrations of the marine structures. The non-linear inline forces acting on a bottom-mounted vertical cylinder (widely used for the support structures for TLPs and monopile foundations of the OWTs) and the characteristics of the SLC are analyzed.

NUMERICAL METHODOLOGY

CONCLUDING REMARKS

Findings

DATA AVAILABILITY STATEMENT