Multiline anchors for floating offshore wind towers

Roughly 60% of potentially exploitable offshore wind power is located beyond the range of water depths suitable for fixed foundations, where floating offshore wind towers (FOWTs) moored to the seabed are required. Anchors comprise a critical component of the mooring system. A variety of anchor types are potentially suitable for this purpose, but all have limitations in regards to the types of seabed soils in which they may be deployed and the type of mooring systems (catenary, taut) for which they are suitable. Additionally, foundations for offshore wind make up a large portion of project cost; therefore, minimizing the costs of fabrication, transport and installation of anchors is a key aspect of overall project feasibility. In contrast to offshore oil-gas installations, offshore wind towers are deployed in arrays, which offer the possibility of reducing project costs by attaching more than one mooring line to a single anchor. In addition to direct cost savings, the multiline anchor concept permits a reduced scale of costly offshore geotechnical site investigations. This paper first examines different anchor types that are potentially suitable as anchors for FOWTs, largely within the context of their traditional usage in securing a single mooring line to the seabed. Then, the potential for adapting these anchors to multiline systems is assessed. Anchor types examined include: driven piles, dynamic piles, suction caissons, drag embedded anchors, vertically loaded anchors, pile driven plate anchors (PDPA), dynamically embedded plate anchors (DEPLA), and suction embedded plate anchors (SEPLA). Performance considerations for each anchor include: Soil profile constraints, vertical load capacity, horizontal load capacity, precision of positioning, installation cost, efficiency, performance under sustained loading, potential loss of embedment, as well as other anchor specific considerations.