Scour hole collapse at an offshore wind farm foundation

Couldrey, A. and Whitehouse, R.J.S. and Harris, J. (2021) Scour hole collapse at an offshore wind farm foundation. In: 4th Virtual Geoscience Conference (VGC 2021), 29 September to 1 October 2021, Online.

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At an offshore wind farm foundation, where the seabed sediments are mobile, the process of tidally-induced scour removes sediment from around a foundation’s base, creating a steep-sided scour pit. Scour pit slopes are known to periodically destabilise and collapse, with results similar to slope failures observed in terrestrial environments. Until recently these elusive events had not been observed in detail (HARRIS et al., 2019). This is because, unlike their terrestrial counterparts, there are processes that can transport material back up and out of the scour pit, restoring the slope. Capturing these events using discrete surveying techniques is near impossible because the events cannot be predicted and operate on short timescales. Little is known about the prevalence of these events and the driving causes. Here, the authors present an example of one of the rare cases where an event was captured using multibeam bathymetry surveying. The area of seafloor surrounding the base of an offshore wind farm foundation was surveyed using multibeam bathymetry sonar. The same area was resurveyed the next day. Over this 24 hour period the scour hole at the base of the foundation collapsed, raising the seabed level by as much as 3.3 m, almost half the scour depth (the area in red in Fig. 1). The edge of the scour pit extended >10 m upstream (the area in blue in Fig. 1). The volume of material deposited within the scour pit approximated 400 m3 and was near equal to the volume of material eroded from upstream, i.e. the process was a closed system and little material had yet scoured from the pit following the collapse. From these two time-steps the frequency and longevity of these events could not be determined. To answer this an echosounder transducer, capable of measuring the level at four fixed points on the seabed, was affixed to a foundation (Fig. 2). The echosounder levels show a reversing rather than monotonic progression with time. The scour collapse events occurred regularly, with a frequency of approximately once every 10 days. The largest collapse events happened during the spring tides. These events typically took place over less than an hour and the scour depth recovered over a period of days. The weight of the material infilling the scour pit will provide overburden and temporarily stiffen the foundation (MAYALL et al., 2019). The stiffness of the foundation has implications for the longevity of the foundation, with less stiff foundations more quickly reaching the end of their life. The prevalence of these events means that they should be considered during the foundation design process. The occurrence of these events has implications for the survey design and interpretation. If an infilling event occurred just before surveying, reducing the scour depth, then it could be (incorrectly) assumed that there is minimal scour, altering the decision made about mitigation (e.g. scour protection). Where this process is known to occur the interpretation of periodic surveys should be performed with this in mind.

Item Type: Conference or Workshop Item (Paper)
Uncontrolled Keywords: Multibeam bathymetry, Subsea, Monitoring, Geomorphology, Infrastructure, Renewables
Subjects: Maritime > General
Divisions: Maritime
Depositing User: Helen Stevenson
Date Deposited: 16 Sep 2021 08:15
Last Modified: 16 Sep 2021 08:15

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