The vast majority of offshore wind farms constructed to date are supported on monopile foundations. These monopiles consist of a large diameter (>4m) open-ended steel piles driven into the seabed to a specified penetration. While laterally loaded piles have been used for many years in the offshore oil and gas sector, they typically have diameters below 2m and a slenderness ratio (ratio of pile length to diameter) in excess of 20. In contrast, monopiles used in the offshore wind sector typically have slenderness ratios of 5 to 8. Design methods developed for relatively slender flexible piles are unlikely to provide accurate predictions of the response of more rigid monopiles to loading. This paper presents the results of a field test performed on an instrumented monopile installed at a dense sand research site in Blessington, Ireland. The pile, which had an external diameter of 340mm, was driven into the dense sand to a slenderness ratio of 6. It was also instrumented with 11 levels of strain gauges to capture the load transfer and bending moments along the shaft. The load test results show that conventional design procedures, such as the Det Norske Veritas (DNV) or the American Petroleum Institute (API) approaches, grossly underestimated the lateral capacity of the monopile. At the end of the paper, a 3D finite element analysis of the pile load test is presented.

1. Introduction

There has been a significant drive to develop offshore wind energy resources in the last 20 years. To date, most offshore wind turbines (>75%) have been constructed on monopile foundations in relatively shallow water (<30m). Monopiles are large openended steel tubes, with typical diameters (D) that are between 4m and 7m, and embedded lengths (L) typically less than 30m. This gives a resulting slenderness ratio (L/D) of between 5 and 7.

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