Seeking new waters: the mooring challenge facing floating offshore wind
Floating offshore wind can draw on experiences of wave energy developers to help understand and overcome the challenges of operating and mooring in the mid-water depths from 40-80m.
Image; Hywind floating wind turbine being assembled in the Åmøy Fjord. Image credit: Lars Christopher - originally posted to Flickr as Hywind.
As deployment of offshore wind grows, competition for sites is seeing developers seeking to deploy their technology in deeper water, and more exposed sites forcing the jump from fixed to floating turbines. This change brings several engineering challenges and an easy step to managing these might seem at first to move just a little bit deeper – into the 40-80m zone for example. However, mooring in this ‘transitional zone’, between shallow and deep water, is in many regards the bigger challenge.
In this zone, the still relatively shallow depth gives mooring designers less space to incorporate catenary weight, synthetic lines and other elements which can help give the mooring compliance needed to avoid large load generating snatch events. The water depth also requires careful design thought to minimise touchdown motion for both the mooring lines and the electrical cabling, to avoid scour or soil build up at the anchor points, and to adequately allow design tolerance for tidal range and storm surges which can lead to significant changes in the water depth.
And that’s not all… another challenge comes from the wave profiles which at this depth are becoming increasingly influenced by the seabed interaction. This interaction creates conoidal, sometimes breaking waves and increases inertial loading. The implications of these effects on the structure being moored depends very much on specific responses and so general assumptions on loading and response cannot be made at these depths – successful designs may differ with water depths over this range. Meaning what works well at 40m depth may not be the best at 80m and vice-versa.
Image; OrcaFlex simulation of a two machine Pelamis farm with dynamic interconnection cabling and mooring systems. Image used with permission from Wave Energy Scotland.
While floating wind can learn much from the offshore oil & gas industry, their experience and approach leaves two big gaps. Firstly, the engineering and construction budgets are not compatible with renewables – a different approach is needed. Secondly, there is relatively little experience of addressing the mid-water range issues highlighted above. Floating wind developers can therefore also benefit greatly from the experience of the Quoceant team and their past work in the wave energy sector. The Pelamis project saw mooring and connection of six individual Pelamis wave energy converters including three deployed in an electrically interconnected mini-farm – a world first! The 1500te displacement structures were surface floating and moored with a compliant system in 40-60m water depth, designed to DNV codes and insured for operation in oceanic conditions. The approach taken on the project included incorporation of a rapid remotely operated connection and disconnection system to drive down the cost of installation and maintenance and allow for an iterative approach to testing. The design philosophy (and financial necessity) resulted in a different price point for the mooring system to that seen in the oil & gas industry.
The Quoceant team all draw on our pioneering background of developing the Pelamis technology as we approach challenges in the wider offshore sectors. Our combination of real-world experience and know-how enables us to offer a fresh approach to engineering challenges and drives our innovative outlook.
Rosalind Hart is a Senior Consultant Engineer at Quoceant and specialises in offshore engineering, and project and site development. She has a MEng in mechanical engineering and over 12 years’ experience working in the marine renewables sector. Prior to joining Quoceant, Rosalind worked at Pelamis Wave Power and was a Senior Engineer leading the mooring analysis and associated verification process for four full-scale Pelamis deployments.
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