Author(s): Liang Cheng; Xin Cheng; Lin Lu; Guoqiang Tang
Linked Author(s): Guoqiang Tang
Keywords: Scour protection; Monopile foundations; Offshore wind turbines; Riprap; Field experiences
Abstract: Scour protection measures are often installed around monopile foundations of offshore wind turbines installed on erodible seabeds in shallow waters to prevent the erosion of sediments around monopile foundations (e.g. Whitehouse et al. 2011; Sumer and Fredsoe 2002). Although scour protection measures are intended to protect wind turbines from the impact of environmental conditions, they may have negative impacts on the wind turbine systems if they are not adequately designed and installed. A number of incidents related to scour protection measures have occurred worldwide, leading to lengthy operational downtimes and significant maintenance costs. The talk will present lessons learnt from field survey data from Europe and China and potential solutions of the problems identified, based on existing knowledge and some preliminary results of physical model testing conducted purposely. Ripraps are one of the most commonly used scour protection measures for monopile wind turbines installed offshore (Whitehouse et al. 2011). Riprap scour protection is normally formed by placing rock particles around the pile to prevent the erosion of sediments. Ripraps can be installed either prior to or after the installation of monopiles, which are referred to as pre-installations or post-installations respectively hereafter. Field experiences suggested that pre-installations of ripraps appeared to work well in preventing excessive local scour around the wind turbines surveyed in a number of European offshore wind turbines (Whitehouse et al. 2011). On the other hands, significant secondary local scour was observed around wind turbines with post-installations of ripraps, based on the observations from wind turbines installed in European (Whitehouse et al. 2011) and Chinese waters. Subsequent remedial works through more rock dumping in some of the projects did not achieve the expected outcomes and even worsened the conditions. The excessive secondary scour around some of those wind turbines adopting post-installations has led to the exposure of buried cables and structural damages of power cables at locations near the wind turbines. An example of field survey results of excessive secondary scour after the post-installation of riprap scour protection in one of the offshore wind turbines is shown in Figure 1. The present talk provides interpretations of the occurrence of excessive secondary scour, based on the existing knowledge and the outcomes from purposely designed physical model tests. Finally methods for remedial rock dumping and cable protection are suggested.
Year: 2023