INDUSTRY ARTICLE
AT SEA SEA-GOING VESSEL VERSUS WIND TURBINE
Wind turbines off the German coast generally represent obstacles in the traffic routes of ships. What if a large sea-going vessel, unable to manoeuvre, were to impact an offshore wind turbine?
COMPUTER MODELS AND SIMULATIONS The analyses used to determine and assess the potential risks of a collision for people, ships, turbines and the environment are currently based on computer models and simulations. However, these models do not adequately consider important aspects – such as routine landing operations by service boats. Do we have to alter the course of collision analysis?
GUARANTEEING SAFETY As the authority in charge of the approval of German offshore wind farms, the Maritime and Hydrographic Agency (Bundesamt für Seeschifffahrt und Hydrographie, BSH) must ensure that Germany can guarantee the safety of international maritime transport at all times. To do so, the distance between a wind farm and any shipping routes must be at least 4.5 kilometres.
In addition, the authority requires evidence of the collision-friendly design of wind-turbine foundations for each offshore wind project. Given this, as part of the approval process, the project owners must go through the worst-case scenario and simulate events, i.e. a ship hitting an offshore wind turbine or a transformer platform.
BREAKING NEW TECHNOLOGICAL GROUND AT HIGH SEA However, the required simulations and analyses confront the wind power industry with numerous challenges as they are largely based on pioneering work and there is no long-standing practical and operational experience to fall back on. So far, the offshore sector is still lacking an adequate range of clear and straightforward frameworks and standards. While some of the pilot projects that have been realised off the coast are delivering
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www.windenergynetwork.co.uk COLLISION RISK
initial findings that can be used in future projects, many aspects indicate that the methods established today are not fully suitable for identifying and specifically assessing all hazards and risks involved.
‘INTENTIONAL’ COLLISIONS The same also applies in particular to the recurrent ‘intentional’ collisions, when a moving service boat first makes contact with the fixed, but flexible offshore structure. The coincidence of only one boat- landing position and unfavourable wave direction generates the biggest risk of higher impact loads.
The ongoing landing operation of the service boat, where the pilot maintains forward thrust – even in high stern waves – to keep the ship in contact with the boat landing platform do not typically apply higher loads. Today’s methods of static analysis work with estimated figures for a single static equivalent load (= Ultimate Limit State). The results of these calculations cannot be absolutely exact and, even worse, cannot be used for structural stability of the wind- turbine foundation and the components of the boat landing platform in the long term (Fatigue Limit States). The question now is whether wind turbines are designed in such a manner that safe servicing and
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1a - Tanker FE model middle cargo holds 1b - Tanker FE model double hull and transv bulkhea 1c - Tanker FE model fore cargo hold
maintenance operations are ensured on a long-term basis, while the risks in the case of a collision are kept to a minimum.
At present, the BSH only requires the usual structural analysis plus a calculation of the consequences in the case of a collision. Both the calculation model and the analysis are based on a reference ship representative of the sea area in question and on the foundation structure of the
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