Mooring is fundamental to all wave energy technologies. While its primary purpose is station keeping, mooring systems can provide much more depending on the characteristics of the wave energy converter (WEC). The right mooring design can enhance control, increase power absorption, provide greater machine survivability, and make operations and maintenance far more straightforward.
At present, the wave energy sector is some way from being cost competitive with established energy generation. This is unsurprising given the early stage of many of the devices under development and the challenges associated with harnessing such a harsh resource. As the industry progresses there remains a clear need to demonstrate the path to cost competitiveness. One area which can make a significant inroad is the mooring and foundations. In this blog, we take a quick look at the range of moorings underdevelopment, some of the challenges and the exciting opportunities which exist to innovate and ultimately push down the cost of energy.
Just as there are a range of wave energy technologies under development so too there is wide variety of mooring and foundation types. Selection is driven by the specific requirements of the technology (if for example, the mooring is also the point of reaction for power take-off) and the site environment (such as water depth and seabed conditions). Common mooring types under development include; compliant systems such as catenaries, taut systems sometimes including an element of elasticity, active mooring systems capable of altering heading, submergence or response, coupled moorings designed to integrate the WEC with existing infrastructure such as break-waters or offshore wind, and fixed where the WEC is built rigidly to the seabed or shoreline.
The Pelamis used a compliant mooring system as the primary mooring and included an active rear mooring to give control over the yaw heading of the machine.
Some of the challenges for mooring designers working with wave technology differ from those seen in conventional offshore applications. These difference include:
Cost: much of the existing mooring infrastructure technology has been developed for offshore oil and gas use but renewables must operate at a different cost level.
Sites: Wave energy developers seek exposed sites, often in remote locations. This can make installation challenging and require design of components for a higher load range with greater consideration of fatigue.
Water depth: wave energy is often deployed in shallow to medium water depths where loading and access can be more problematic and where there is a general lack of experience to draw on.
Fortunately, the open and early technological stage of wave energy offers many avenues for innovations and reductions in cost of energy. These include;
Infrastructure sharing - Wave energy is a denser resource than wind, this allows for devices to be sited closer together in an array. This offers the opportunity for cost saving by sharing infrastructure such as anchoring points. Further there is opportunity to reduce cost by sharing of installation vessels and specialist tooling designed for reuse across projects and on a range of wave energy designs. Sharing mooring lines with neighbouring devices is also an option.
Power Optimisation – the primary purpose of the mooring system is to keep the WEC on station however, for many classes of WEC, it also has a significant impact on the response of the machine either directly where the mooring lines are part of the PTO drivetrain, or through heading or attitude control to best match wave conditions, or as a more passive response tuning or damping. Innovation in the mooring system may therefore have direct and very material impacts of the absorption efficiency of the WEC.
Rapid mooring connection/disconnection systems - For many offshore WEC designs it is possible to incorporate quick connection and disconnection systems allowing for machines to be towed to shelter. This enables an offsite maintenance strategy which can have significant benefits to both availability and OPEX costs.
Development of specialist installation systems - in a similar way to the specialised vessels, equipment and processes that can be seen in each part of the marine and energy sector, there is a major opportunity to reduce cost by creation and sharing of installation vessels and specialist installation tooling and processes designed for reuse across projects and on a range of WEC designs. With the rough locations intended for WEC applications, more weather tolerant processes are likely to have a particularly strong impact.
Function sharing & optimising redundancy – at present main and back-up moorings and electrical connections are generally provided as separate functions. Offshore codes and standards provide advice on necessary levels of redundancy and back-up, often not well matched to the lower consequences of failure for a WEC compared to other marine installations. Within this there is the possibility to combine some of these functions to reduce complexity and cost.
Technology innovations - new materials, load monitoring technology, modular anchors, rock bolts... the list is vast.
This is not an exhaustive list, and there are many more, but it illustrates that the scope and reach of mooring and foundation technology can deliver well beyond the obvious impacts to CAPEX to have potential impacts on nearly all aspects of cost of energy for a wave power project. It’s time to get excited about moorings!
Rosalind Hart is a Senior Consultant Engineer at Quoceant and specialises in offshore engineering and site development for the marine energy sector. She is a chartered mechanical engineer with the IMechE and has over 12 years’ experience working in the sector. Prior to joining Quoceant, Rosalind worked at Pelamis Wave Power and was a Senior Engineer taking a leading role in the mooring analysis and associated verification process for four
full-scale Pelamis deployments.
Get in touch; Rosalind.Hart@quoceant.com
Richard Yemm is a Director at Quoceant, he specialises in offshore technology innovation and is a renowned expert in marine energy. He has a PhD in mechanical engineering and has worked in the marine renewable sector for over 20 years. Prior to Quoceant, Richard was the inventor of the Pelamis wave energy converter and founded and led Pelamis Wave Power. He was a key driver in the mooring design and machine installation concepts.
Get in touch; Richard.Yemm@quoceant.com