Piles for offshore wind turbines

Abstract

The paper considers the current state of the art for estimating the pull-out capacity of driven open-ended piles used to support wind turbine foundations founded on sand. The latest edition of the American Petroleum Institute guidelines for pile design includes a conventional earth pressure approach and four alternative cone penetration test (CPT) methods for estimating pile shaft resistance in sand. A database of open-ended pile tests was used to assess the predictive reliability of the design approaches. While the earth pressure approach was unreliable, exhibiting bias with pile slenderness and sand relative density, the CPT methods were shown to provide improved and relatively consistent estimates of pile capacity. However, the tension loads experienced by wind turbine foundations are significantly higher than those applied to piles in the database. When the CPT methods were used to estimate the pile length required to support a 5 MW turbine installed in typical offshore soil conditions, the CPT methods provided a wide range of predicted pile lengths. The reasons for this divergence are discussed and an alternative framework for considering driven pile shaft resistance is put forward.