Anchoring FOWTs in shallow to deep water

Roderick Ruinen, Technical Director, Mooreast Asia Pte Ltd
While the offshore oil and gas industry has been using floating platforms for more than 50 years, most of the proposed and installed offshore wind turbines are in shallow water with a fixed platform type foundation. Only recently has the industry seen floating offshore wind turbines been proposed and designed. Keeping a floating offshore wind turbine (FOWT) on location poses new challenges for the industry. The aim of this paper is to explore the different solutions that are available for anchoring a floating offshore wind turbine to the seabed. Using knowledge from the offshore oil and gas industry, different mooring systems will be discussed together with their technical characteristics and suitability for FOWTs. Suitability of an anchoring solution will depend on the encountered soil conditions and type of mooring system. In shallow water a catenary mooring is generally selected. This results in little or no vertical force components on the anchor. Experience from the oil and gas industry shows that the seabed can vary from very soft clay to very hard clay, very loose to very dense sand, cemented soils and mixtures of these. For this type of application drag embedment anchors are a solution as they can be utilised in a wide range of soil conditions. In deep water a (semi) taut mooring is typically used. This type of mooring system results in both vertical and horizontal forces on the anchor. Experience has shown that in deep water locations the seabed can generally be described as very soft to soft clay. For these conditions the dynamically installed anchor (torpedo pile) is a suitable anchoring point. Drag embedment anchors have been used in the offshore industry since the 1970s around the world and torpedo anchors since the 1990s. Applications are for both temporary and permanent mooring systems. The presentation will discuss the design, installation and recovery of the anchor types. This will be based on project examples. As FOWT will generally be deployed in large numbers in a farm, a large number of anchors will also be required. Methods for the fabrication of large numbers of anchors and the efficient installation will also be discussed. Torpedo pile is a field proven, cost-effective solution for offshore Deepwater mooring systems which has a long track record in Brazilian Oil e Gas projects. Presentation will discuss design and installation methods of the torpedo piles, and the efficient installation method that makes torpedo pile a competitive option for a FOWT project.