Experimental Evaluation of Dynamic Rock Scour Protection in Morphodynamic Environments for Offshore Wind Jackets

Abstract

Bottom-fixed offshore wind turbines are generally built on continental-shelf sections that are morphodynamically active due to their shallow depths and severe wave and current conditions. Such sites are commonly protected against scour to prevent the loss of structural stability. Scour protection can be designed using static or dynamic solutions. Designing dynamic protection requires experimental validation, especially for singular or unconventional structures. This article presents an experimental method for the laboratory analysis of scour protection for jacket foundations placed at morphodynamically active sites. The test campaign was conducted within the project East Anglia ONE (UK) as part of the asset owner studies and aimed to evaluate operation and maintenance (O&M) aspects, independent of the contractor’s original design assessments. The physical experiments explored morphodynamic changes on the sea bottom and their importance to scour protection, as well as the importance of the history of the wave loads to the deformation of the rock scour protection. This was explored by repeating different cumulative tests, including a succession of randomly ordered sea states (Return Period (RP) 1-10-20-50 years). The experimental results show that the deformation of the rock sour protection was the greatest when the most energetic sea states occurred at the beginning of the experimental test campaign. The maximum deformation was at 5D50 when the first test was also the most energetic, while it was at 3D50 when not included as the first test, yielding a 40% reduction in the scour protection deformation.

Scour Protection Measures for Offshore Wind Turbines: A Systematic Literature Review on Recent Developments

“…From the falling apron point of view, a low dependency of scour protection on hydrodynamic load history was found by the authors. In addition, the maximum deformation of the scour protection was found to be greatly affected by the arrival order of storms [105]. Another form of hard scour protection (shown in Figure 18b) with four layers for four-pile jacket foundations was proposed in [106] and found by tests to be effective in preventing scouring.…”

“…When compared with monopiles, only a handful of studies focused on scour protections for jacket foundations. Sarmiento et al [105] mentioned that a kind of rock protection (shown in Figure 18a) has been applied on at least one of the foundations of the wind farm East Anglia ONE (UK). The authors analyzed the falling apron effect and the stability of this rock scour protection for a three-pile jacket foundation.…”

Methodology to assess scour development around complex structures

“…Physical experiments allow to properly reproduce the interaction between flow, structure and soil, therefore the main objective of the physical experiments is the final estimation/validation of the equilibrium scour depth along the pile clusters, as well as the potential assessment of the global scour around the foundation. The experimental methodology was developed at IHCantabria facilities (CCOB, equivalent to the basin tests described in [15]). Considering the main characteristics of the basin (CCOB) and the size of the platforms, the scale of the test can vary from 1/35 to 1/45.…”