Author(s): B. Mutlu Sumer
Linked Author(s):
Keywords: Scour; Floating offshore wind farms; Subsea structures; Drag embedment anchors
Abstract: Offshore wind energy is one of the most important renewable energy resources. Offshore wind farms (OWFs), bottom-fixed or floating, are used to harvest offshore wind energy. Floating OWFs, with water depths of O(20-30) m or larger, are the energy production sites where wind turbines are installed on floating platforms. Floating platforms are held in place with a system of mooring lines (likely chains) and anchors. There are mainly three types of anchors, namely, conventional drag embedment anchors (DEAs), suction caisson anchors, and gravity anchors, all involving dimensions of O(10) m. For semi-submersible and spar-type floating offshore wind turbines, DEAs are the optimum choice, given their high holding capacity against lateral loads. DEAs and the associated subsea structures, namely, a tensioner (or subsea chain adjuster), clump weights and chains, may be subject to unfavorable effects of scouring and liquefaction. The purpose of this presentation is to review scouring issues for foundations of floating offshore wind turbines, the effect expected to be a potential threat for the stability and integrity of floating offshore platforms. Scouring is not an issue for DEAs themselves because they are completely buried in the seabed. However, it is an important issue for other subsea structures associated with DEAs, such as tensioners, clump weights, and chains. When a tensioner is placed on the seabed and exposed to the flow, scour and eventually undermining develop at the tensioner. As a result of the combined effect of scour and undermining, the tensioner sinks in the soil. This sinking process stops when the tensioner sinks to such levels that the tensioner is protected against the scour. In this scouring and sinking sequence, the structure may experience a tilting (and even toppling) towards the upstream direction in currents, due to the relatively larger scour depths at the upstream side of the structure, largely caused by the strong presence of the so-called horseshoe vortex. In waves, however, the scour hole is more symmetric with respect to the offshore-onshore direction. Therefore, the aforementioned tilting and toppling are practically nonexistent. Engineering projects essentially call for prediction of time series of scour and sinking depths under given metocean conditions. An extensive review of the subject will be given in the presentation, including recent research with special reference to the physical processes associated with scour in combination with tilting and toppling.
Year: 2024