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State-of-the-Art Design Standard specifically developed and applicable for Offshore Wind Turbine Structures The DNV services includes the Manufacturing, Installation and In-service Phase and the requirements are given with focus on the structural integrity of the installed structure Join our committees By Tove Feld, DNV Global Wind Energy The wind energy industry requires cost-effective design for offshore wind turbine structures in order to make the projects economically viable. A year ago none of the existing standards were truly applicable to offshore wind projects with a different loading scheme compared to the traditional oil and gas offshore structures, e.g. going from wave dominated to wind dominated. As a member of Offshore Center Denmark, you may also participate in the work of the four permanent working groups within the centre. The four committees each represent the interest of a number of members, who themselves have defined the groups The committees are: • Mobile & stationary offshore installations • Offshore Wind Turbines • HSEQ - Health, Safety, Environment & Quality • Decommissioning of offshore structures 18 Offshore Center Denmark Newsletter ON/OFF 3 - August 2004 This let to the development of a new state-of-the-art offshore standard ‘Design of Offshore Wind Turbines Structures’, I DNV-OS-J101. (First edition issued June 1st 2004), specific developed and applicable for offshore wind turbine structures. The new II standard is based on DNV´s experience from participation in more than 24 offshore wind projects and general rule development from III maritime and offshore industries for decades, resulting in a rational set of state-of-the-art rules allowing fit-for-purpose design. IV DNV Project Certificate The challenge has been to establish a sound basis for development of projects both technically and economically that would be V recognized by all parties involved in offshore wind projects: owners, manufacturers, finance, insurance and authorities. Consequently the newly issued DNV-OS-J101 is VI a cost effective design standard, living up to safety levels required by authorities and other parties involved in offshore wind projects. In addition the standard account for the fact Certificate Maintenance that the structures are unmanned and the risk for pollution of the environment is limited, opposite the existing offshore standards. The new design standard has been chosen and is currently being used as Design standard on a number of Offshore Wind Projects around the world. The standard and the project certification serves the purpose of creating the trust and confidence between the parties that is needed for accelerating the development of offshore wind farms. ■ Design Basis Design Manufacturing Installation Commissioning In-Service Statement of Compliance Statement of Compliance Statement of Compliance Statement of Compliance Statement of Compliance Schematics over the DNV Project Certification Scheme Each group has appointed a chairman. To apply for membership of a committee please forward your request to Offshore Center Denmark, who will relay the request to the chairman.
Materials in offshore wind turbine wings by Birgit Kjærside Storm, Aalborg University Esbjerg The materials used in wind turbines wings must fulfil many requirements, as the wind turbines build today, are very large, and hence the wings get great tip space. At the same time the wind turbines are placed within environments, where also large chemical and wearing requirements to the materials are extreme. The key requirements set to the wing materials, are suitable rigidity and strength in bending out, and at the same time the materials must possess wearing qualities with regards to sand, hard rain, hail as well as salt and also be chemical resistant to water, bird droppings, insect’s blood and salt. Many other chemical influences to the wings are possible, but these mentioned are the most common. A high strength/weight scale is also required from the wing material. To fulfil the requirements and ensuring at the same time that the weight of the wing is as low as possible, almost solely composites are used for manufacturing of wind turbine wings. A composite is a piece of material that consists of a strengthening element as well as a matrix material which can “glue” the strength giving parts together. In wind turbine wings preferably fibre glass are used as strengthening element, but in the future many wings will be made with carbon fibres instead of or together with fibre glass. The carbon fibres are more rigid than the fibre glass. They have a higher strength/weight scale than fibre glass, but they are more costly than fibre glass. The matrix material is the material that surrounds the fibre material.The matrix material must be consistent with the fibres, as the fibres have to be moistening all over the surface in order to let the strength be transferred from the fibres to the matrices. In order to obtain a sufficient fastening between fibres and matrix materials, the fibres have to be coated so that moistening and fastening can be carried out. This coating is called a sizing. All wind turbine wings have a coating either as a gel coat or as painting on the surface. The coating is made in order to protect the wing material itself (the composite) against the wear and chemical attacks. The matrix material in wings is either hardening plastic polyester or a type of epoxy. Both non-saturated polyester and epoxy belong to the group hardening plastic materials. Being hardening plastic means that the materials during the hardening will cross bind. During the cross binding process strong primary bindings between the molecule chains are formed, and thereby a good strength and rigidity within the materials is obtained. By cross binding the chemical resistance of the materials is increased, as chemical substances cannot pass through a cross binding but have to find another route, in order to get in connection with an active group which can be influenced by the chemical substance. Epoxy and polyester each have advantages and disadvantages. They each have their own hardening method but the hardening course can be changed on the materials by inserting other chemical active groups in the materials during the polymerization process. Epoxy is used a lot in the so-called prepreg (pre-treated fibre glass in which the epoxy is found as pre-polymerized) which is put into a mould before it is hardened under thermal influence. By adding heat the hardening process is started, as a hardener is put in active condition and therefore can act with the epoxy group over which the cross binding can take place. The most commonly used epoxy type is DGEBA which cross binds with an amine. During heating the epoxy resin receives fluid by means of which it can moisten the fibres. Polyesters are mostly used as resines to be dissolved in the styrene. The styrene is used as dissolving material for the non-saturated polyester, but during the hardening process the styrene is activated by means of a peroxide and is used as cross binder in the non-saturated polyester. The hardening process can here be started at room temperature, as it is started by an initiator. The starting of the process can also happen in other ways, but an initiator (MEKP) is the most used method. The moistening of the fibre glasses is essential to get a good strength transfer from the fibre glasses to the matrix. The fibre glass is treated with different coupling agents depending of the type of resin (epoxy or polyester) to be used as matrices. To obtain good moistening the resin must have a suitable viscosity. If the viscosity of the resin is too high (the resin is too thick), the resin cannot float sufficiently easily over the fibres and therefore can not moisten the fibres. If on the other hand, if the resin has too low a viscosity (too thin) it will run off the fibres and therefore the moistening will be too low. Both epoxy and polyester are able to absorb water. This quality is unfavourable as adding water produces a risk of destroying the composite itself. Water can also reduce the binding between fibres and matrix materials. When wind turbines are placed offshore, they will be under almost constant influence from water and salt. Salt is a hygroscopic material and salt deposits on the surface of the wings are able to absorb water. Hence the requirements to the chemical resistance of the wing materials will be increased offshore. The wind turbine wing material of the future will probably still be composite materials with hardening plastic matrix material even though the hardening plastic can not directly be re-used. Hardening plastic possesses great strength and good chemical resistance and at the same time the moulding process can be done in as large dimensions as required. The fibre material will probably still partly be glass but carbon fibres will replace some of the glass. The matrix material will probably still be polyester and epoxy materials but new resin types such as polyimides and other strong and chemical very resistant plastic types can be envisaged. Offshore Center Denmark Newsletter ON/OFF 3 - August 2004 19
Page 1 and 2: Offshore Center Danmark Newsletter
Page 3 and 4: A Logistic Challenge By Poul Skjær
Page 5 and 6: Additionally the economic part of a
Page 7 and 8: Foundation design considerations By
Page 9 and 10: A2SEA A/S pioneers in offshore wind
Page 11 and 12: Numerical Simulation of hydrodynami
Page 13 and 14: Loads on wind turbine, tower and fo
Page 15 and 16: Member questionnaire Offshore Cente
Page 17: accustom themselves to be together

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