Offshore wind farms are rapidly developing in Western Europe as an alternative means of clean energy. The foundation of wind-turbines in the shallow water of the Northern Sea is almost exclusively (over 87%) performed by short rigid monopiles, as an efficient and relatively cheap solution. Scour formation, meaning the soil removal around the pile due to the actions of the waves and currents, is a potential hazard for the functionality and the integrity of the structures supporting the turbines. This thesis investigates the effect of scour formation in the soil-pile capacity and examines the possible contribution of the scour protection layers in the stiffness of the system both under static and cyclic loading. The methodology followed included 18 main static and cyclic centrifuge experiments and 9 basic cyclic numerical analyses. The physical modelling included an aluminum pile, equipped with strain gauges in order to calculate bending moments and ultimately derive the “p-y curves”, which was embedded in a dense dry Geba sand. Numerical analyses have been conducted in PLAXIS 3D, with a symmetrical fully drained model, in prototype scale, with the same set of materials (Geba sand, aluminum pile).
The results have shown that scour formation can diminish the lateral soil capacity in the ultimate limit state (ULS). Scour depth is the most critical characteristic of the scour hole geometry, but the scour width at a specific depth can also make the difference between structure’s integrity and failure. Therefore, the term “local” scour is deemed insufficient, as it cannot be described by a unique geometry. A valid design against scour is recommended to include a series of analyses with combinations of scour depth and width in realistic ranges. Experimental derived “p-y curves” have been compared with the ones proposed by the API method, concluding that the API overestimated both the initial response and the ultimate capacity of the soil reaction. It is proposed to update the existing regulations for the rigid piles and add rotational springs to simulate the effects of the considerable shear developing in the tip of rigid monopiles. The effect of the scour width in the “p-y curves” was observed to be limited in the shallower depth of the pile, as going deeper resulted almost to the same soil response regardless of the type of scour. The cyclic centrifuge experiments focused on the load type, investigating different scenarios of “one-way” and “two-way” load patterns. It was shown that the “one-way” case is more favorable in terms of accumulating deformations compared to the “two-way” case which experienced higher residual displacements, as long as the same maximum load was applied. This was attributed to the smaller dissipation of energy and hence destruction of the soil structure by the “one-way” loading. However, when the maximum load applied in a “two-way” experiment is considerably smaller than the equivalent of the “one-way” test, smaller deformations observed in the “two-way” test, implying the significance of the maximum load. The last section of this thesis, the numerical analyses, focused on the scour protection effect in the mechanical properties of the soil-pile system when subjected to cyclic loading. It was shown that a typical protection layer can highly increase the stiffness of the system and hence decrease the accumulated displacements. The length of the protection layer is not crucial, as change in its magnitude does not alter considerably the reduction of the accumulated displacements, in contrast with the thickness which has a larger impact on the stiffness of the system. It is concluded that scour protection layers can considerably increase the soil resistance around the monopile, allowing for smaller embedment length, so their contribution in the soil-pile stiffness should be taken into account for a more economic design.

Wind turbines are nowadays the main means for producing renewable energy in Northern and Western Europe, and therefore research is conducted to secure their efficiency. One of the critical aspects of the windfarms is the foundation type, as it can define the functionality and the cost of each turbine. Monopiles have prevailed as the most efficient foundation, since 75% of the existing wind farms are founded in this way. However, despite their dominance, there are still mechanisms that affect their stability during their lifetime. Scour formation, which is the erosion of the soil around a monopile, is a crucial one that is the topic of this project. More specifically, the effect of the scour formation (both depth and type) in the stiffness and the lateral capacity in sandy soils has been investigated, followed by analyses about the efficiency of the scour protection layers. The methodology that was followed included numerical simulations, which have been performed in the finite element code of PLAXIS 3D. More specifically, 26 simulations have been conducted, in which the vertical load, the scour depth and type and the scour protection length were the parameters that have been investigated. The conclusions drawn could be divided into four categories, the vertical load, the scour depth and type and the scour formation effect in the stiffness and the lateral capacity. It was shown that the increase in the vertical load had a positive influence in the lateral capacity of the soil-monopile system. However, it could be characterized negligible, as the lateral capacity increase was less than 5%. In the next set of results, the scour depth impact in the stiffness and lateral capacity of the soil was investigated. It was observed that for the same type of scouring, the increase of the depth of the scour hole significantly reduced the soil resistance. It is noted though that scour up to 1.0D could be managed except for the global scour case. On the other hand, for larger depths the situation was becoming more critical and in depth of about 2.0D, the loss of the capacity was too great leading to a no-return state, for all scouring types. Then, the type of scouring impact was investigated, as local scour (narrow and wide) or global can occur. It was shown that the narrow type of scour was the most favorable case, which could approach the no scour case for small scour depth, while the global scour case was the most critical, as even in small scour depths could lead to dramatic reduction of the capacity. The main point is that the narrow case and the wide one can be manageable in depths up to 1.5D under certain conditions, while the global type seems to become a no-return case for much smaller erosion depths. The last part included the scour protection effect which was similar to the vertical load effect, as it had a positive influence in the lateral capacity of the soil-monopile system, but in a range of 5 to 15%. Therefore, it could be ignored in the design phase, unless a thicker protection layer is used. The graphs of the evolution of soil pressures and pile deflection clearly indicated that the monopile presents a rigid response, as the pile was rotated in all cases from a certain rotation point. The rigid behavior of the pile states that the API method is outdated as it is based on slender piles and it cannot capture the rotation point and the distribution of soil pressures that occur due to the rigidity of the monopile. The final output of this current report is a design recommendation, depicted in two normalized charts that correlate load and stiffness at failure displacements with the scour depth and type.