Numerical Investigation on the Effect of Scour Formation & Scour Protection on the Stiffness & Lateral Capacity of Monopiles

Abstract

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.