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Damage development in scour protections around offshore monopiles
De Vos, L.; De Rouck, J.; Troch, P. (2007). Damage development in scour protections around offshore monopiles, in: McKee Smith, J. (Ed.) Coastal Engineering 2006: Proceedings of the 30th International Conference, San Diego, California, USA, 3-8 September 2006. Vol. 3. pp. 2391-2403. dx.doi.org/10.1142/9789812709554_0202
In: McKee Smith, J. (Ed.) (2007). Coastal Engineering 2006: Proceedings of the 30th International Conference, San Diego, California, USA, 3-8 September 2006. Vol. 3. World Scientific: New Jersey. ISBN 978-981-270-991-2. 2205-3240 pp., more

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Keyword
    Marine

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Abstract
    Some of the best locations for wind-farms are in more or less shallow coastal waters where wind is unobstructed by land. However, when a monopile is placed offshore and no countermeasures are taken, the sea bottom is subject to local scour due to waves and/or currents. Often, the expected scour depth is not acceptable for the structure and scour protection is necessary. A conservative way to design a scour protection (e.g. rip rap) is to allow no or only little displacement of individual rock stones. When the design criterion is no movement and the amplification factor due to the presence of the pile is known, the Shields criterion can be used to calculate the necessary individual stone size. A more economical solution may be building a protection with smaller elements, allowing movement of individual stones without failure of the protection. Under these latter conditions it is important to formulate a clear damage definition, to define an acceptable damage criterion and to account for damage development over time as important elements in the design of scour protections. This paper describes an experimental study on a scour protection of loose rocks around a monopile foundation. For different stone sizes, damage initiation and damage development during a storm is investigated, taking into account the influence of a combined wave and current action. A suggestion for a damage definition for a scour protection around a monopile foundation is made. The test results show that on average, movement of the stones is initiated for a bed shear stress equal to approximately half of the critical bed shear stress (Shields). Damage development over time exhibits an exponential trend. The results further show the importance of investigating the combined action of waves and currents, as damage is highly increased when wave action is superimposed on a steady current (e.g. tidal current).

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