Volume-Based Assessment of Erosion Patterns around a Hydrodynamic Transparent Offshore Structure

Abstract

The present article presents results of a laboratory study on the assessment of erosion patterns around a hydrodynamic transparent offshore foundation exposed to combined waves and currents. The model tests were conducted under irregular, long-crested waves in a scale of 1:30 in a wave-current basin. A terrestrial 3D laser scanner was used to acquire data of the sediment surface around the foundation structure. Tests have been conducted systematically varying from wave- to current-dominated conditions. Different volume analyzing methods are introduced, which can be related for any offshore or coastal structure to disclose physical processes in complex erosion patterns. Empirical formulations are proposed for the quantification of spatially eroded sediment volumes and scour depths in the near-field and vicinity of the structure. Findings from the present study agree well with in-situ data stemming from the field. Contrasting spatial erosion development between experimental and in-situ data determines a stable maximum of erosion intensity at a distance of 1.25 A, 1.25 times the structure’s footprint A, as well as a global scour extent of 2.1–2.7 A within the present study and about 2.7–2.8 A from the field. By this means, a structure-induced environmental footprint as a measure for erosion of sediment affecting marine habitat is quantified.

Influence of Structural Elements on the Spatial Sediment Displacement around a Jacket-Type Offshore Foundation

“…The present study follows two earlier studies of Welzel et al [24,34], presenting in-depth analyses previously not addressed. While Welzel et al [24] concentrated on the local scour depth, measured temporally, introducing a prediction approach to calculate scour depths around jacket structures, Welzel et al [34] proposed a methodology to assess spatial erosion processes around offshore structures, applied on erosion patterns around a complex jacket structure. This novel analyzing method is enhanced in the present study and utilized for erosion, as well as deposition, patterns.…”

“…They proposed an equation predicting the normalized erosion volume of the scour around a pile in steady currents and combined waves and currents scenarios together with the rough scour shape (flat or steep slope angle). The idea to analyze spatial erosion processes with the actual information on the normalized relocated sediment volume was pursued and introduced in Welzel et al [34]. The present study follows two earlier studies of Welzel et al [24,34], presenting in-depth analyses previously not addressed.…”

“…The idea to analyze spatial erosion processes with the actual information on the normalized relocated sediment volume was pursued and introduced in Welzel et al [34]. The present study follows two earlier studies of Welzel et al [24,34], presenting in-depth analyses previously not addressed. While Welzel et al [24] concentrated on the local scour depth, measured temporally, introducing a prediction approach to calculate scour depths around jacket structures, Welzel et al [34] proposed a methodology to assess spatial erosion processes around offshore structures, applied on erosion patterns around a complex jacket structure.…”

“…To the authors' knowledge, most studies on scour around offshore foundations still focus on the erosion process due to a pointwise scour depth analysis. However, spatial measurement techniques such as photogrammetry [25][26][27], laser probe bottom profiler [28][29][30][31], multi-beam echo sounder [30] or terrestrial, as well as underwater, laser scanner [32][33][34] are more commonly used to assess scour around offshore foundations. Although these techniques are partly applied, very few studies concentrate on a systematic assessment of spatial erosion and deposition patterns by means of volume-based analyzing methods.…”

“…Additionally, scour development over time and equilibrium scour depth were analyzed, point-wisely, with an echo sounder system. Due to an adjustable distance of the lowest nodes to the seabed, the influence of the distance of the structure nodes was investigated and compared for the same hydrodynamic conditions as analyzed in Welzel et al [24,34]. Based on the above identified lack of knowledge, and using the methodological approach presented so far, the specific objectives of this paper include the following:…”