Assessing the stability of piled tripod foundations for offshore wind turbines under cyclic loading

As offshore wind projects move further offshore into deeper waters, tripod and jacket structures are becoming the most economic method of supporting wind turbines. These support structures are subjected to cyclic loading arising from wind, waves and currents. Traditional offshore oil & gas tripods / jacket structures generally support large deadloads and the magnitude of the cyclic loading is low in comparison to these permanent loads. The wind turbine support structures are much lighter than traditional jackets meaning the cyclic loading is a significant proportion of the total loads. The cyclic loads are transferred to the foundation where they could cause degradation of the subsoil. The response of the piles to long term cyclic lateral and axial loads is complex and there are no generally accepted methodologies to predict the effect of cyclic loading on the foundation resistance. This paper describes the available methodologies to account for the potential effects of cyclic loading on the foundation resistance of tripod piles driven in dense sand. A rational design approach is outlined based on available field- or model scale results and past experience in the offshore oil & gas industry. The design approach shows how the effects of cyclic axial and lateral loading on pile resistance can be accounted for. The limitations of the design approach are reviewed as well as the inherent conservatism. Due to the relatively small database of pile tests, conservatism is inevitable. Further research into the long-term behavior of tripod piles is clearly warranted.As offshore wind projects move further offshore into deeper waters, tripod and jacket structures are becoming the most economic method of supporting wind turbines. These support structures are subjected to cyclic loading arising from wind, waves and currents. Traditional offshore oil & gas tripods / jacket structures generally support large deadloads and the magnitude of the cyclic loading is low in comparison to these permanent loads. The wind turbine support structures are much lighter than traditional jackets meaning the cyclic loading is a significant proportion of the total loads. The cyclic loads are transferred to the foundation where they could cause degradation of the subsoil. The response of the piles to long term cyclic lateral and axial loads is complex and there are no generally accepted methodologies to predict the effect of cyclic loading on the foundation resistance. This paper describes the available methodologies to account for the potential effects of cyclic loading on the foundation resistance of tripod piles driven in dense sand. A rational design approach is outlined based on available field- or model scale results and past experience in the offshore oil & gas industry. The design approach shows how the effects of cyclic axial and lateral loading on pile resistance can be accounted for. The limitations of the design approach are reviewed as well as the inherent conservatism. Due to the relatively small database of pile tests, conservatism is inevitable. Further research into the long-term behavior of tripod piles is clearly warranted.P