Assessment of Pile Length Criteria for Monopile Foundations

For the geotechnical monopile design of offshore wind turbines, the required embedded monopile length is commonly derived by so-called pile length criteria. As no clear procedure is specified by the current offshore guidelines, different criteria are used in design practice. Four often used pile length criteria are investigated by means of a comparative study on monopiles in homogenous sand. Herein, the influences of pile diameter and stiffness, load level and applied p-y approach on the outcome of the individual pile length criteria are discussed on basis of the head rotation due to characteristic ULS loads and the results of different ULS and SLS design proofs. Based on the identified inadequacies of the criteria, it is concluded that the consideration of a maximum pile head rotation is more suitable to reflect the pile stability due to cyclic load effects than existing criteria. However, the disregard of any pile length criterion is not in line with the DNVGL guideline and the EA-Pfähle recommendations and thus may induce problems in the certification and approval process.