JUNE 26, 2013 — Classification society DNV is responding to developments that see offshore wind farms move further offshore and into deeper waters. It has DNV  launched a significant revision to its rules for wind farm service vessels (WFSV). And its DNV KEMA unit has released a new standard for floating offshore wind turbine structures that, it says, will help ensure safety and reliability in floating wind turbines, and give the nascent floating-turbine sector the confidence to continue its development to commercial maturity.

The revisions to the WFSV rules aim to improve the safety of these specialized vessels as they undertake multiple roles ever further offshore. DNV was first to publish rules for WFSV in January 2011 to provide the wind farm industry with an improved construction standard that would be accepted by regulators in the North Sea Basin. These early tentative rules were very well received by industry and quickly became the industry standard for wind farm service vessels.

"However," says DNV, "given the regulatory complexity and wide operating profile of the vessels we found that improvements were required in order to realise our vision of enhancing safety. We therefore established a cross industry rule development forum which provided us with very constructive and comprehensive feedback in a number of key areas."

The new rule set, which comes into force July 1, incorporated this industry feedback as well as the experience DNV has gained from design approvals, newbuilding supervision projects and in-service inspections of classed vessels.

DNV says that its work on developing this new standard offers the following benefit for the industry:
1) Rule requirements for equipment certification have been clarified reducing the build cost
2) Key lessons from in-service inspections have been fed back into the rule development, for example added requirements to reduce cracking caused by the increased vibration in the aft end of the vessel when pushing up against turbines. This will reduce maintenance and repair cost for the owner.
3) It provides an enhanced safety standard which local authorities and domestic associations can use as a benchmark thereby contributing further towards improved safety for the industry.

Thomas Grafton a Senior Engineer with DNV commented: "The original tentative rules met the expectations of the industry broadly however we realized that we needed to improve the clarity and accessibility of our rules further. The improvements we have made support further innovation leading to both lower through life cost and improved safety for these vessels."

Mårten Schei-Nilsson, Approval Engineer with DNV, added: "With the development of the wind energy industry, especially in European waters, there is a requirement for the vessels to go further afield and to transfer between jurisdictions such as Denmark, Germany and the U.K. We have delivered a proven solution and will continue to support this industry as wind farms are constructed in other parts of the world and in increasingly challenging environments."

DNV KEMA standard for floating offshore wind turbine structures

DNV KEMA, the energy arm of DNV, says its new standard will help accelerate the development of a new generation of floating offshore wind turbines by establishing design requirements for the floating structure and related systems.

According to Johan Sandberg, head of renewable energy at DNV KEMA, Norway and project sponsor, the standard covers a broad range of issues, including safety philosophy and design principles; site conditions, loads and response; materials and corrosion protection; structural design; design of anchor foundations; floating stability; station keeping; control and mechanical systems; transport and installation; in-service inspection and cable design.

"As demand for wind energy increases, we predict offshore deployments will continue to move into deeper waters and, consequently, there's a need to establish design standards that will help ensure safety, reliability, and confidence in future wind turbines," Mr. Sandberg says. "To that end, the new standard, developed as a Joint Industry Project (JIP) with 10 participating companies, aims to spur progress in floating offshore wind through a framework for best practices and technical requirements, plus producing guidance for design, construction and in-service inspection."

Mr, Sandberg notes that many densely populated coastal areas around the world are not suitable for traditional bottom-fixed offshore wind turbines. In other areas, the shallow water coast is already developed or challenging seabed conditions makes bottom-fixed offshore wind unsuitable. Also, local communities have been known to oppose projects due to negative visual impacts.

"Recent successful deployments of full-scale prototype configurations have demonstrated that floating wind turbines can be a viable alternative and the market is taking notice. Several companies and research institutes worldwide are already engaged in developing research programs, pilot projects and even planning for commercial development of floating wind farms," Mr. Sanberg says.

Countries like Japan and the U.S. have also made offshore wind energy one focus of their energy policy. According to Mr. Sandberg a tricky point in the development of offshore wind around the coastal belts of these countries, like the majority of coastal belts around the world, is that water depths can range from dozens to hundreds of metres. This situation demands new technology so in both Japan and the U.S., ideas are turning to floating structures for wind turbines.

"It is now time to take the next step: standardization. A new standard can increase the confidence in the industry and hopefully attract new investors to this new renewable energy technology," says Mr. Sandberg. "The decades of expertise that DNV has amassed in the standardization of maritime offshore oil & gas, and onshore and offshore wind is invaluable for the development of standards for floating offshore wind structures."

The new standard for floating wind structures, devised under DNV KEMA's leadership through project manager Anne Lene Hopstad and technical specialist Knut Ronold supplements the developed DNV Guideline for Offshore Floating Wind Turbine Structures, and the existing standard DNV-OS-J101 Design of Offshore Wind Turbine Structures.

The 10 participants in the JIP study are Statoil, Nippon Steel & Sumitomo Metal Corporation, Sasebo Heavy Industries, STX Offshore & Shipbuilding, Navantia, Gamesa, Iberdrola, Alstom Wind, Glosten Associates and Principle Power.