ABSTRACT

Various foundation types for offshore wind energy converters (OWEC) are currently discussed and used. Shallow offshore foundations can rarely be placed on the seafloor, as weak soils usually need to be excavated to place the foundation structure on more stable ground, thus resulting in anthropogenic submarine pits. Steep but stable slopes of the pit meet both economic and ecologic aims as they minimise material movement and sediment disturbance. Natural submarine slopes of sandy soils are usually less steep in reality than in theory. According to Terzaghi (1957) the angle (β) between slope and the horizontal ground surface of cohesionless soil is at most equal to the critical state friction angle (φcrit). However, hydrodynamic forces act on the slopes and natural submarine slopes are already shaped by perpetual loads of waves, tide and mass movements. Artificial slopes of foundation pits do not appear and behave as natural submarine slopes, or as theoretically predicted. Physical simulations of different scales were used to analyse the stability of artificial submarine slopes with sandy soil of the North Sea. The laboratory tests focused on gravitational forces and impacts from the excavation processes. In addition, numerical simulations of wave-induced bottom pressure supported considerations of suggested submarine pit slope angles. Based on these slope angles, in situ tests will be performed and both dredging process and resulting foundation pits will be surveyed.

1. Introduction

Submarine foundation pits are usually necessary for gravity based foundations to place the structure on stable ground. The required depth of projected pits in the German North Sea varies from 2m to 7m, and the excavation of temporary submarine foundation pits results in huge amounts of dredged material. From economic and ecologic points of view, this amount should be reduced to the only what is most necessary.

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