Predictability of Scour at Large Piles due to Waves and Currents

Abstract

Installing a structure offshore interferes with the existing flow conditions? The wave and current induced velocities are locally heightened and therewith sediment is lifted and washed away. A so-called scour hole, a deepened area around the structure, is formed. In the design of an offshore wind turbine the formation of a scour hole needs to be considered or scour protection must be provided. Design rules dictate a design scour depth of one to two times the diameter of the pile. The piles of offshore wind turbines are at least 3.5 meter wide, so that the predicted scour depth differs in a size of 3.5 meter or more. The average scour depth is much smaller. Most of the knowledge about scouring at piles is referred to relative slender piles. If water particles move far compared to the width of the pile the flow separates and vortices are formed. These vortices are known as the main causes of scour. At large piles however, the flow does not separate. Instead the wave field is transformed by reflection and diffraction. Therewith the relative scour depth around large piles is much smaller than at small piles. Basing on this understanding the predictability of scour around monopile-founded wind turbines is analysed. Three flow regimes are characterized and defined in dependency of the Keulegan-Carpenter number. This is explained in Section 2.1 and applied to offshore wind turbines in Section 5.2. Evaluating scour the flow characteristics, the sediment transport and the stability in relation to the foundation need to be considered. An insight into the most important aspects of scour development is given in Chapter 2. Scouring around slender piles has been widely investigated. Important determinations related to offshore installed piles are summarized in Chapter 3. Scouring around large piles is not understood in detail. Based on the review of insights in flow characteristics, empirical and analytical approaches scour around large piles is discussed in Chapter 4. Under consideration of uncertainties, which are explained in Section 5.2, the knowledge of flow regimes and maximal scour depths are applied to offshore wind turbines and the design of scour protections in Section 5.3 and 5.4. The application to offshore wind turbines shows that under moderate wave conditions the depth of the scour hole around a pile is less than thirty percent of its diameter. However, under extreme wave conditions, which are expected less than once a year, the scour hole can increase up to a limit of 1 time of the diameter. According to current design standards, even scour depth of 1.5 and 2 times of the diameter may occur. This effect a more than five times too large considered impact of scouring in the fatigue analysis of offshore wind turbines. Investigation of scouring processes leads to a better insight and with this knowledge one can reduce the uncertainties and thereby decrease the "best practice" safety factors.