TWI Industrial Member Report Summary 996/2011
By M Ali
The offshore wind energy sector has been a rapidly growing industry in the United Kingdom, Europe and the USA within the renewable energy sector for the past few decades. Offshore wind turbines (OWT) have gained popularity due to their high energy efficiency and limited social and environmental implications compared to onshore wind turbines. In order to achieve target contributions (ie 20% by 2020 in EU) set for renewable energy as a substitute for other sources of energy, offshore wind farms all over the world are not only being extended, but also the designs of individual wind turbines are being adapted to deliver higher energy per unit. The associated support structures of offshore wind turbines therefore need to be robust to withstand additional loading imposed as a result of design modifications for higher energy output, ie increase in the size of rotor blade, generator and auxiliary components. This has led to stringent requirements for the integrity of the support structures, ie monopiles and transition pieces. There are few standards which provide guidelines for design and manufacture of offshore wind turbine support structures (OWTSS); these guidelines originate from the standards prepared for other offshore foundation structures/platforms used in the oil and gas industry.
This report, prepared in a TWI exploratory project, presents a general survey of standards used for design and manufacture of offshore wind turbines with a specific focus on the requirements set for the selection of steel materials used in the fabrication of the foundation and other support structures. The selection criteria for steel properties (ie strength and toughness) recommended in various standards/guidelines has been explored with particular emphasis on the structural integrity of the welded components of the support structures. Using a simple fracture mechanics model, an engineering critical assessment (ECA) was carried out to estimate the minimum toughness requirements, which were compared with those recommended in the standards. The ECA model incorporated a hypothetical flaw and a conservative assumption of the applied stress level.
- Review existing standards/guidelines addressing OWT with a particular focus on the materials used for fabrication of support structures.
- Review and compare the recommendations on materials property selection criteria (toughness) given in standards and compare those with the results obtained by using a simple fracture mechanics based assessment.