Access systems: The access challenge

Access systems The access challenge

Cost reduction is the number one priority for the offshore wind industry and operations and maintenance (O&M), representing about a quarter of overall costs, is a significant opportunity for delivering savings. As wind farms get bigger and are built further from shore, in harsher met ocean conditions, the challenge of undertaking O&M will become even greater.

To illustrate this challenge, if today’s infield crew transfer vessels were to be used at the Dogger Bank offshore wind farm, the weather window would only allow transfer of technicians from these vessels to the turbines during about 200 days a year, constraining the potential for O&M activities, which in turn would impact turbine availability and yields.

TAS initial trials, courtesy of BMT Nigel GeeTo drive down costs and to ensure that far shore wind farms are economic, new O&M strategies must be developed and new technologies commercialised.

This is where the Carbon Trust Offshore Wind Accelerator (OWA) is making significant progress. The OWA is a joint industry project initiative which aims to commercialise new technologies to reduce the cost of offshore wind in the near term. It is one-third funded by the UK’s Department of Energy and Climate Change (DECC), and two-thirds by the industry.

The OWA consists of nine energy companies, namely DONG Energy, E.ON, Mainstream Renewable Power, RWE Innogy, ScottishPower Renewable, SSE Renewables, Statkraft, Statoil and Vattenfall, who have licences to develop over three-quarters (36GW) of the UK’s capacity. Technical experts from these companies meet regulatory to solve common challenges, from foundation design to wake effects.

There are five research areas in the OWA, one of them, Access Systems, is specifically focused on solving the access challenge for O&M.

O&M strategy

Nearly all of today’s offshore wind farms use port-based strategies for maintenance. Technicians transit from port to the wind farm every day, and in the wind farm they transfer from the vessel to the turbine by stepping across onto boat landing ladders on the turbine foundations.

There are two problems with this approach for far shore wind farms.

Firstly, the further from shore you go, the longer the transit time. As a result technicians spend a significantly larger portion of their time on board the vessels rather than working in the wind farm. The longer the transit time, the bigger the weather window is required to safely complete the operations, which means technicians will spend longer in port waiting for the right weather.

Secondly, stepping across onto boat landings is safe in certain sea states, but further from shore, severe conditions are more likely to be encountered, which will constrain the window for transfers.

The solution to these problems is to base the technicians offshore in the wind farm using offshore bases, floatels or mother ships, and to develop more robust transfer systems.

New vessels and transfer systems

In 2010, the OWA ran an international Access System competition. The goal was to identify new transfer systems, infield crew transfer vessels, and launch and recovery systems concepts that could be developed for the offshore wind industry. The competition attracted over 450 entries from 30 countries. Thirteen concepts were selected by the OWA technical experts to receive technical and financial support from the OWA and marine specialists DNV Kema to develop, de-risk and demonstrate the concepts.

The OWA experts were delighted at the innovations proposed by the entrants, with ideas for technology transfer not only from other marine industries, but also from unexpected places such as the automotive sector.

Concepts from the competition are now at prototype testing phase, or have reached the market. The leading vessels make trade-offs between speed, manoeuvrability, stability, and fuel efficiency to give offshore wind operators better tools for O&M activities. It is likely that operators will include several different vessel types in their fleets to ensure that the right tool is available for the job.

Vessel designs

The best vessels include the following designs:

• The UMOE Mandal Wave Craft is a surface effect ship. Travelling on a cushion of air, the vessel is extremely fast, which means that it can reach turbines in short weather windows. When it reaches the turbine, the air cushion can be used to help dampen out the vessel motions to create a very stable platform for transfers. With its origins in the Norwegian navy, Wave Craft has been successfully tank tested and the next step is to build a full-scale prototype.

• TranSPAR is a radical vessel design inspired by spar buoys employed in the oil and gas sector. Although the vessel is small, it has the stability of a much larger vessel, with minimal motions in heavy seas. With relatively low construction and operating costs, it is envisaged that a fleet of TranSPARs would be deployed in a wind farm.

The wind farm’s electrical substation could potentially be an offshore accommodation base for technicians. After a comprehensive tank testing programme, the next step for this Canadian design is to build a full-scale prototype.

• Nauti-Craft is a twinned-hulled vessel with suspension. Designed by the team responsible for the suspension on the McLaren MP4-12 and Paris Dakar-winning Mitsubishis, Nauti-Craft is an excellent example of technology transfer. Originally conceived as a four-hulled vessel, extensive prototype testing has led to today’s catamaran design. The suspension allows the vessel to maintain a constant ride height during transfers, and provides greater comfort for technicians during transits, reducing the risk of sea sickness which can prevent technicians from working offshore. A 8m prototype is under construction which will prove the design for the offshore wind market.

• Fjellstrand WindServer is a trimaran vessel that planes like a hydrofoil during transit, and which has tremendous stability when station keeping. Following design development and tank testing, Fjellstrand received an order to build six vessels in May 2012. This will be the first vessel design from the OWA Access Competition finalists to reach the market.

• North Sea Logistics’s Pivoting Deck Vessel includes a long section of deck that pivots from near the stern of the vessel to provide a very stable platform when thrusting into the boat landing. This feature allows heavy equipment to be lifted from the vessel using the turbine’s crane in harsh sea conditions. Following tank testing, the next step for the Pivoting Deck Vessel is to build a full-scale prototype.

Access systems

The competition also attracted a number of ideas to eliminate the dangers when crossing from the vessel to the boat landing. The best transfer systems include the following concepts.
won the Safety Innovations Award at the 27th Offshore Achievement Awards organised by the Society of Petroleum Engineers (SPE), Aberdeen Section.

• Otso’s Autobrow is another lightweight compensated gangway. The system with its simple but robust compensation system has been successfully factory tested, and the next step is to trial the prototype at sea.

• Momac MOTS is a robotic arm that uses visual references to maintain constant height next to the boat landing. Looking like a robot out of The Terminator, the MOTS design is based on technology developed for the robot arms that build cars in assembly lines. The MOTS system has recently been fitted to an infield crew transfer vessel to be deployed at a wind farm in the Baltic.

Launch and recovery systems

Launch and recovery systems will be important enabler to allow mother ships to be adopted to deploy daughter craft infield crew transfer vessels.

• The Divex LARS system is one of the leading designs from the competition. Using cradling technologies used on dive vessels for RIBs, the LARS system can be retrofitted to offshore service vessels to lift daughter craft out of

the water. The design is currently undergoing tank testing, and should allow safe and reliable deployment without the need for bespoke mother ship designs.

Sea trials

As the vessels and transfer systems start to be deployed, offshore wind developers face the challenge of selecting the best access systems for their projects. To date, the performance of access systems is typically approximated as the significant wave height (Hs) in which transfers are possible. However, this is a crude metric, as the limit will depend on wave direction and wave period as well as the wave height. Developers also need to factor in fuel costs, time to deploy, and crew comfort into their decision.

For this reason, the OWA has developed a set of sea trial procedures to provide a more rigorous assessment of performance. The output of the trials will be a series of performance plots known as ‘P-plots’. The ambition is that the industry will adopt P-Plots, quoting an ‘as-designed’ metrics when the concepts is designed and tank tested, and ‘as-tested’ metrics when the concepts are first deployed at sea. This will provide greater transparency about performance, and encourage designers to develop ever more capable designs.

The OWA members will be conducting a series of sea trials over the coming months to establish P-plots for a number of vessels and transfer systems, and the procedures will be published later in 2013 so that they can be adopted by the industry.

Big challenge, big opportunity

Offshore wind has the potential to deliver a large proportion of Europe’s electricity. This renewable, low carbon opportunity also has the potential to create thousands of green jobs. For the industry to be a success it must deliver cost reduction and Governments must provide stable incentive regimes.

By working together in OWA, industry has been able to identify new innovations to reduce costs and to accelerate technology development. This has the potential to transform O&M and solve the access challenge. New technologies are not just on their way, they are getting into the water and starting to be used in commercial projects.

Phil de Villiers, Head of Offshore Wind at Carbon Trust