An Alternative Approach to Scour Monitoring Around Offshore Wind Turbines.
Philip Bishop, Lucy Maclennan
Fugro, Portsmouth, Hampshire, UK
Changes in seabed level around a vertical structure, otherwise known as scour, can present big problems across the offshore sector. Scour pits can be caused by tidal currents moving around foundations, and can impact the integrity of offshore structures by gradually washing away the supporting sediments. The result of this can, and has, led to the unnecessary premature decommissioning of expensive offshore assets.
The extents and effects of scour is of particular importance around the nearshore environment, where the tidal effects are amplified due to the relatively shallow waters involved. This brings the issue into the spot light for all manner of renewables developments, but particularly the offshore wind sector, where the infrastructure is in place on such a large scale.
Round 1 and 2 wind farms have begun experiencing the effects of scour, leading to newer wind farm developers increasingly taking notice of this potential threat. Some sites have been known to witness up to 20 m of sediment being eroded away from the foundations, and this can lead to early decommissioning as turbines may only be piled 30 – 40 m into the seabed.
Bathymetry surveys, traditionally used for monitoring scour, can miss short-term scour pits generated by extreme conditions. In-situ scour monitors can be installed on WTGs, and when correlated with other oceanographic measurements, provide invaluable information on the integrity of offshore assets.
Scour monitors are secured to foundations below LAT, ideally with one device placed on each principle tidal axis; ensuring the most extreme effects are monitored. The devices use four acoustic beams, configured at four angles from the vertical and can be programmed to capture data at user-specific sampling rates. A depth sounding is calculated from the reflection of each beam off the seabed and can produce a time-series of seabed elevation.
Data can be transmitted in real-time via a cable and displayed on a dedicated website, or remain logged on the instrument for post-processing. Measurements of local tide, waves and currents can be collected concurrently to evaluate how environmental conditions affect the generation of scour and which conditions pose the greatest threat.
Results from individual wind farms are highly site-specific as scour is predominantly driven by factors such as seabed morphology, sediment type and local environmental conditions as well as the age of the wind farm. We would expect a high correlation between extreme weather conditions and short-term scour generation. Longer term scour pits are usually caused by a combination of site specific factors, and require further investigation due to the potential for impact on the structural integrity.
Scour monitors offer a simple and cost effective solution that require a minimal amount of maintenance following initial installation. Data can highlight which turbines are most at risk, ensuring plans can be made well in advance to secure their long-term operational future.
Compared to traditional bathymetry surveys, the scour monitors provide data on a 24/7 basis, irrespective of weather and this allows for the assessment of both short and long-term scour pits. Continuous monitoring allows offshore operators to decide on appropriate protective measures, such as rock dumping, to mitigate against further scouring and damage to the structure.
In addition to providing site monitoring, the scour data could also be used for numerical modelling initiatives; contributing to future wind farm development and foundation engineering design.
This presentation will give developers and site managers an overview of the potential risks of scour development and highlight a comparatively inexpensive approach to monitoring seabed changes, and the potential for mitigating future problems.