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

Abstract

This research advances the understanding of jacket-type platform induced local and global erosion and deposition processes for combined wave–current conditions. To this end, a laboratory study was carried out comparing the equilibrium scour depth for two structural designs that are differentiated in the geometrical distance of the structure’s lowest node to the seabed. Measurements of local scour depths over time have been conducted with echo sounding transducers. An empirical approach is proposed to predict the final scour depths as a function of the node distance to the seabed. Additionally, 3D laser scans have been performed to obtain the digital elevation model of the surrounding sediment bed. Novel methodologies were developed to describe and easily compare the relative volume change of the sediment bed per surface area due to structure–seabed interaction, enabling spatial analyses of highly complex erosion and deposition patterns. The seabed sediment mobility around the structure is found to be highly sensitive to a change in node distance. The decrease of the node distance results in a higher erosion depth of sediment underneath the structure of up to 26%, especially for current-dominated conditions, as well as an increased deposition of sediment downstream of the structure over a distance of up to 6.5 times the footprint length. The results of this study highlight the requirement to consider the interaction of the structure with the surrounding seabed within the design process of offshore structures, to mitigate potential impacts on the marine environment stemming from the extensive sediment displacement and increased sediment mobility.

Oyster Reef Surfaces in the Central Wadden Sea: Intra-Reef Classification and Comprehensive Statistical Description

“…In the fields of hydraulic engineering and fluvial hydraulics, the analysis of bed topographies utilizing high-resolution digital elevation models (DEMs) and spatial statistics has become more and more popular in the past decades, as this approach offers an opportunity to link surface characteristics with hydraulic roughness (Powell, 2014). Apart from some exceptions (Krämer and Winter, 2016;Welzel et al, 2019Welzel et al, , 2020 this approach has been used less in the field of coastal or ocean engineering. Conventionally, bed roughness due to small-scale roughness elements on plane beds, such as sand-grains or gravel, has been linked to a characteristic surface descriptor (e.g., grain size d 50 or d 84 ) or to the equivalent sand roughness, i.e., the roughness used in the work of Nikuradse (1933).…”

GWK+: Erweiterung des Großen Wellenkanals – Analyse des Einlaufbereichs

“…Zur effizienten Weiterentwicklung maritimer Technologien ist es daher essenziell, dass die Vorhersage und Abschätzung struktureller Belastungen sowie die Analyse anthropogen verursachter Umwelteinflüsse wie z. B. Sedimentumlagerungsprozesse um Offshore‐Strukturen [3–5] durch maritime EE verbessert werden. Die kombinierte Belastung aus der komplexen Überlagerung von Wellen und Strömung ist als realistische hydrodynamische Randbedingung maßgebend für die Bemessung von Offshore‐Strukturen und die Vorhersage möglicher Umwelteinflüsse.…”

Experimental Study of Local Scour around Tripod Foundation in Combined Collinear Waves-Current Conditions

“…For the foundation with complex shapes, such as tripod and jacket, the scour process was more complicated than the case of monopile due to the blockage effects induced by structural elements [27][28][29], so a unique scour morphology around foundation with complex shapes would be expected compared with a single pile. Thus, the scour results from the single pile can't be applied directly for foundation with complex shapes.…”

“…Thus, the scour results from the single pile can't be applied directly for foundation with complex shapes. Welzel et al [28,29] carried out a series of wave flume tests to study the scour evolution around jacket foundation, the results implying the streamline contraction and flow acceleration adjacent to the structural elements, resulting in the shear stresses on the seabed amplified and consequently the greater scour depth. The tripod foundation consists of three piles, one main column and structural elements between piles and the main column.…”

“…The tripod foundation consists of three piles, one main column and structural elements between piles and the main column. According to the experimental results of Welzel et al [28,29], the structural elements exerted a significant effect on the scour evolution and scour morphology, so it was vital to investigate the scour development and scour topography around tripod in combined waves and current. Yuan et al [27] conducted several groups of scour experiments to study the scour development around tripod in steady current, and the test results instruct that all the maximum scour depths occurred at the downstream tripod's pile.…”