Fixing floaters

Clement Mochet, Vryhof

October 30, 2017

Clement Mochet, of Vryhof, discusses ways to bring down the costs associated with floating wind projects.

WindFloat project, offshore Portugal. Image from Principle Power.

While costs in fixed offshore wind have more than halved in recent years to reach a zero-subsidy bid by developers (See panel on page 40), floating offshore wind still has some way to go.

The opportunities are attractive. Enabling offshore wind development in >40m water depth brings into play more steeply shelved coastlines, otherwise not viable for fixed turbines. According to Bloomberg New Energy Finance (BNEF), there could be as much as 237MW of floating offshore wind capacity installed by 2020, albeit mostly demonstrators and pre-commercial wind farms.

The French government is supporting four 25MW arrays for installation in the Mediterranean and Atlantic starting in 2020. However, that the government had to provide a feed-in tariff of $282/MWh (€240/MWh) for 20 years shows how much incentives are still required. Floating wind still needs to cut development costs significantly if it is to move into the mainstream as a viable energy source.

Where can changes be made to reduce costs? Initially, there will be a period of concept rationalization, from the 30+ concepts cited by the Carbon Trust in 2015, to probably three or four floater technologies that will move towards full commercial development, allowing the industry to focus on cost improvement, rather than technical feasibility.

Innovations in other areas, such as higher voltage cables and more cost-effective means of delivering electricity back to shore, will also help reduce capex.

One key of area of cost savings and technology advances will be anchoring and mooring – technology to tether the floating structure to the seabed while withstanding extreme offshore environments. Such technologies comprise up to 20% of the total costs of a floating wind offshore project and in complex deployments and extreme water depths, could be substantially higher.

The challenges facing mooring and anchoring

Stevtensioner on the Wikinger wind farm.
The STEVTENSIONER in action on the WindFloat project, the first offshore wind turbine in Portugal

It is likely that by the time the first floating wind farms become a commercial reality, they will be hosting 10MW+ turbines (8MW turbines have already been deployed on fixed structures), if not 12-15MW, with ca.200m rotor diameters.

The coupled loads generated by a turbine of this size on the floater could be huge - potentially up to 20,000kN per mooring line – with the units engineered for a lifespan of 25-30 years.

A large part of the cost of the mooring package is the installation costs, at about 50%.
Vryhof’s STEVTENSIONER system, used in the oil and gas industry for more than 20 years, would save costs by allowing two opposing anchors to be cross-tensioned simultaneously in constrained areas. A repeated heaving up and slacking of the system in a yo-yo action builds up the load in the mooring chain until the required tension is achieved.

The reduction in the required bollard pull capacity, the use of winches and the ability to use anchors with shorter leads enables operators to use much smaller and cheaper vessels in mooring installations, saving cost.

This system has been used on the WindFloat project. This is a tripod semisub, developed by Principle Power, installed as a pilot in 2011, 5km off the coast of Aguçadoura, Portugal, with a 2MW turbine. WindFloat was the first offshore wind turbine in Portugal, and the first to be installed without any heavy lift vessels or piling equipment at sea.

All final assembly, installation and pre-commissioning of the turbine and substructure took place on land in a controlled environment and the complete system was then wet-towed offshore using simple tug vessels.

As well as supplying the full mooring system solution, including procurement, production follow-up, certification, integration and delivery, Vryhof’s cross-tensioning system helped keep installation costs within budget.

Optimizing mooring spreads

However, the offshore industry is not yet used to dealing with high volumes of mooring lines. Getting there implies a significant scale change for all the components of the lines, moving from a project-orientated approach to mass production.

Mass production is considered as one of areas with the highest potential to help reduce costs. Early stage, confidential studies are currently running in cooperation with wind farm developers, floater designers and the mooring industry to assess the positive impacts to be expected.

Another means of reducing costs is through optimal mooring spreads that share anchoring points, rather than traditional single mooring points. The development of floating wind farms with a ‘networked’ mooring system will mean fewer anchoring points and a streamlined approach to geotechnical site investigations.

The quest for ongoing stability

Another important area is the importance of securing stability in challenging hard soils, such as complex gravelly soils, over-consolidated clays, limestone, or complex cemented soils of carbonite origin. As floating offshore wind expands further offshore, soil conditions are likely to become increasingly prevalent.

Hard soil conditions, however, have traditionally caused significant operational and cost challenges for anchors, with highly developed, multiple and expensive anchoring arrangements required to provide the necessary stability.

There are options entering the market. Vryhof, for example, has recently introduced a new anchor, STEVSHARK REX, based on its geotechnical and modeling expertise and the principles of soil mechanics. The drag embedment type anchor has a holding power of up to 20% more than previous anchors and can be used in all hard soil conditions.

Following testing in the North Sea and the United Arab Emirates, the new anchor recently completed extensive testing in Australian waters in close collaboration with Woodside Energy. The 18-ton anchor with 7.2-tonne ballast was tested in 100m water depth at four different locations, each with their own ‘extreme’ soil characteristics based around cemented soils of carbonite origin. All the tests were successful with the anchor’s geometry and the protruding fluke tips effectively penetrating the hardest rocks.

With future floating wind farms likely to be in relatively complex geological settings where standard anchors may not work as effectively, such anchoring developments are likely to be crucial in the future.

The perfect blend

While mooring and anchoring is only an enabler to some of the exciting new developments in the floating wind power industry today, the focus on innovation and pushing the boundaries will ensure that it plays a crucial role in the viability of marine renewable energy for many years to come.

About the Author
Clément Mochet is commercial director for Vryhof. Before to joining Vryhof, Mochet was Sales Director and Export Manager at Le Béon Manufacturing. Mochet holds an MSc in mechanical engineering from The National Institute of Applied Sciences (INSA) of Lyon, France, and from the Tampere University of Technology (TUT) in Finland. 

Wind blows, costs fall

Offshore wind has been making headlines for the scale of cost reduction in the industry. Recent contracts for new offshore wind farms in Europe have been agreed pegging the cost of energy from them at as low as US$76.27/MWh (£57.50/MWh)(Hornsea 2 and Moray Offshore).

This is via UK Government issued contracts for difference (CFD), a support mechanism providing project owners a guaranteed price (strike price) for electricity.

In comparison, Westwood Global Energy says the first CFD round saw strike prices averaging $155.20/MWh (£117/MWh). Meanwhile, the CFD award to the Hinkley Point C nuclear project in the UK at $122.70/MWh (£92.50/MWh). Westwood points out that wholesale prices in the UK have ranged between $42.45-66.33/MHW (£32-50/MWh) over the last five years.

Scale, competition, efficiency and reduced costs from the oil and gas sector have helped offshore wind drop its costs, says Westwood Global Energy’s Head of Research, Global Oilfield Services, Steve Robertson.

“The UK has established itself at the forefront of offshore wind activity with 5.1GW installed to date,” he says. A further 17.2GW of capacity planned for installation over the 2018-2026 period at a cost of $84.55 billion (€72 billion). Including projects at the concept or speculative stages, this could rise to 19.8GW, with capital expenditure over 2018-2026 reaching $91 billion.

“The offshore wind sector has benefited from the increased scale of projects, both in terms of overall capacity and capacity per turbine,” Robertson says. “Westwood data indicates that, until recently, turbines of 3-4MW capacity were the norm for most installed projects. Now, projects at the planning phase are typically evaluating turbines of 7-10MW.

Greenpeace, which welcomed the cost reduction, said that by the mid-2020s, turbine capacities are set to reach 15MW. “Furthermore, the sector has seen intense competition and efficiency gains as major engineering and construction firms are lured by the attraction of what are now multi-billion dollar megaprojects,” Robertson adds.

Vryhof’s Clement Mochet says the economies of scale achieved with huge projects (the He Dreiht offshore wind farm from Germany’s EnBW for instance aims at 900MW) and the synergies with other projects nearby have allowed for capex and operations costs optimization.

“A cyclical downturn in the oil and gas industry has helped by putting downward pressure on offshore construction and support assets, a number of which serve both the oil and gas and wind sectors,” he adds. Shorter cycle times on offshore wind farms, of two to three years for a build out, also minimizes the risk of cost overrun and sees new technologies adopted more quickly.

“It is not yet clear the extent to which these awards are indicative of a new price paradigm, or if the industry can consistently deliver projects at these levels, but it is a remarkable milestone in the progression of offshore wind to becoming a commercially viable power-generation proposition in its own right,” Robertson adds.