Scour development around a jacket structure in combined waves and current conditions compared to monopile foundations

M Welzel,A Schendel,A Hildebrandt,T Schlurmann

doi:10.1016/j.coastaleng.2019.103515

Abstract

This paper presents the results of an experimental study on the scour development of a hydraulic-transparent offshore foundation exposed to combined waves and current. Irregular waves propagating perpendicular to a current were simulated in a wave-current basin. The physical model tests were conducted in a length scale of 1:30 while measurements of the scour development over time were achieved by echo sounding devices placed at several locations at the upstream and downstream side of the jacket structure. Insights were gained on the scour development and time scale of the scouring process around a complex jacket structure for different wave-current conditions. The results were presented with respect to the Keulegan-Carpenter KC number and the relative wave-current velocity. Wave conditions were adjusted so that KC numbers between 6.7 and 23.4 could be tested in a systematic wave-current test program with tests reaching from wave dominated conditions up to current dominated conditions. Measured scour depths were critically assessed by an extrapolation to expected equilibrium scour depths. With respect to the current flow direction, the experiments showed generally larger scour depths at the upstream side and lower scour depths on the downstream side for each pile of the jacket structure. The development of global scour around the structure intensified with increasing relative wave-current velocity. As a result, a practical formulation is proposed for the reliable prediction of local scour depths around a jacket foundation in combined wave-current conditions. Finally, dimensionless time scales and observed as well as predicted scour depths are compared to values for the scour development around monopiles.

Highlights

Introduction and motivationIn the search for sustainable renewable energy forms, offshore wind parks within coastal areas are expanding to meet rising energy demands
For the scour development around a jacket structure, it is reasonable to assume that its main mechanisms are related to the local flow acceleration around the piles, which in turn lead to a mobilization and to a transport of sediment downstream
It can be presumed that structural elements like near bed diagonal braces cause additional vortex shedding and streamline contraction compared to a single pile, potentially increasing the sediment mobilization around the jacket structure

Summary

Introduction

Introduction and motivationIn the search for sustainable renewable energy forms, offshore wind parks within coastal areas are expanding to meet rising energy demands. In contrast to onshore installations, the emergence of scour around foundations needs to be taken into account in the design process of offshore wind structures. Due to a lack of knowledge about scour around complex, hydrodynamic transparent jacket-type foundations in particular, offshore wind energy converters (OWEC) are often designed following an approach for monopiles (Bolle et al, 2012). This can lead to a conservative and uneconomic design (Stahlmann and Schlurmann, 2010) or may lead to an uncertain prediction of local and global scour depth (Rudolph et al, 2004)

Methods

Results

Conclusion