The cyclic characteristic of offshore environmental load may cause the accumulation of foundation settlement and excess pore pressure in the subsoil. This may lead to conditions that jeopardize the structural stability and operability of the wind turbine. This paper discusses the analysis steps of the responses of non-cohesive subsoil under cyclic loads for a gravity base foundation system. These steps include the simplification of the irregular load, prognosis of the cyclic settlement accumulation using the simplified High-Cycle Accumulation Model (Niemunis, 2003), and the pore pressure development.
Two of the main design criteria for an offshore wind turbine foundation are the structural stability and operability during its designated service life. These include the stability of subsoil and the settlement of the foundation. In the common design practice for civil structures, the structural stability is proven for several preselected singular static loads. This approach is in general accepted for structures which only experience monotonic loading condition. However, it is still questionable whether this approach is applicable for structures which constantly experience dynamically changing loading condition. In reality, the loading history and sequence play a significant role in the subsoil response and the overall structural stability, particularly due to the influence of pore fluid and permeability of the subsoil. With the current computing resources and knowledge, it is still difficult, if not impossible, to directly analyze the offshore structure response for the whole load sequences during the service live. The number of load cycles can easily exceed 100 Million for an offshore wind turbine platform in the North Sea with a 20 years service-life. Even after reducing the number of load cycles, e.g. by neglecting cyclic loads with maximal load level less than 20 percent of the static foundation resistance, we still need to deal with over 100.000 load cycles.
