ABSTRACT

Pile foundation is widely used to support offshore wind turbines in Donghai Bridge Wind Farm, China. The scour problem around the pile group is found severe and efficient scour protection is a concern of offshore wind farm here. A new scour countermeasure using solidified slurry have been proposed recently. But there is a lack of understanding on the flow and diffusion of solidified slurry pumped into the developed scour holes for scour repair as the fluidized material solidifies gradually. In this study, a multiphase flow numerical study was carried out on the pumping operation by using computational fluid dynamics (CFD) method. The flow and diffusion characteristic, and the residual rate of solidified slurry around the monopile foundation and pile group foundation are systematically investigated. It is found that the location downstream of the foundation is optimal for pumping operation. Although the flow characteristic of slurry around the two foundation types is largely different, the comparison of final residual rate indicates that pile group can be simplified as a monopile to conduct a rapid assessment for the engineering pumping location design in scour repair.

INTRODUCTION

The rapid development of renewable energy resulted in the increase of offshore wind farms (Strach-Sonsalla and Muskulus, 2016; Wu et al., 2019). High-rise pile cap foundation is widely used to support offshore wind turbines in Donghai Bridge Wind Farm, China (Chang et al., 2014; Chen et al., 2018). The foundation consists of a high-rise pile cap platform and the lower pile group, which is composed by eight inclined supporting piles distributed evenly around the cap. This structure has many advantages over other conventional supporting systems (Chen et al., 2016), such as manageable risk, high stiffness, economical cost etc. However, the local scour around the pile group is found severe after a long-time operation and has been recognized as a general issue. Scour pits develop around pile group in waves and currents due to the complex hydrodynamic condition, and thus reduce the bearing capacity and stability of the foundations when the scour depth reach to some extent (Qi et al., 2019; Sumer and Fredsøe, 2001; Zhang et al., 2017b). This may lead to serious failure and collapse of the offshore structure (Smith, 1976; Wardhana and Hadipriono, 2003). Thus, efficient scour protection is a concern of offshore wind farm here.

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