For offshore wind turbines in deeper waters, a jacket sub-structure supported by axially loaded piles is thought to be the most suitable solution. The design method recommended by API and two CPT-based design methods are compared for two uniform sand profiles. The analysis shows great contrast in the predictions of bearing capacities calculated by means of the three methods for piles loaded in both tension and compression. This implies that further analysis of the bearing capacity of axially loaded piles in sand should be conducted.
The increased focus on renewable energy sources has resulted in the development of large offshore wind farms in the North Sea. As the interest for offshore wind farms increases, locations with deeper water are included as possible sites for the wind farms. At water depths greater than approximately 25 m, current technology implies that the most suitable foundation solution for the wind turbine is a jacket substructure supported by axially loaded piles. The recommended design method for the axial capacity of the offshore driven piles in cohesionless soil is the so-called β-method, suggested by API (2000), in which the effective overburden pressure at the depth in question is used. However, comparisons between the capacity calculated using the API β-method and full-scale measurements show that the β-method under-predicts the capacity of short piles (length less than 20 m) in dense sand, over-estimates the capacity of long piles in loose sand, and gives a shaft capacity less conservative for piles in tension than for piles in compression (Lehane et al., 2005a; Schneider et al., 2005). The API method is developed for predictions of pile capacities for piles used in the oil and gas industry. For this type of construction, the selfweight is so great that the piles are loaded in compression at all times.
