Offshore pile load tests in Chalk to support the design of jacket foundations

Abstract

Iberdrola developed the 350 MW Wikinger offshore windfarm (OWF) in the German Baltic Sea where ground conditions are mainly dominated by deposits of the Upper Quaternary series and post-glacial deposits overlying Cretaceous Upper Chalk (low-density, grade A1/A2) of Maastrichtian age and at some locations with Paleogene Limestone (high-density to very high density, grade A1/A2). Current pile design methods for open tubular steel piles in Chalk highlighted significant design uncertainties for open tubular piles that would lead to unnecessarily conservative pile dimensions of 70 m embedded length or longer for the 70 four-legged jacket wind turbine support structures and the six-legged offshore substation. For mainly axially loaded piles, German legislation requires the execution of dynamic pile load tests on production piles at a minimum of 10% of wind turbine generator locations immediately following pile installation. This practice offers no opportunity for pile design optimisation. If positive, the pile tests can confirm design assumptions, but risk costly delays in OWF commissioning if the dynamically ‘measured’ capacities fall below design requirements. After careful consideration of costs and benefits, and with the clear advantages of eliminating ambiguity of test results, it was decided that offshore tests at the Wikinger OWF offered the best return on investment regarding foundation design optimisation, assurance and certification. To address these concerns, in advance of final design, offshore dynamic and fully autonomous static pile load tests were commissioned. These were completed 10 weeks after pile driving on 50%-downscaled piles with penetrations of up to 31 m in water depths of around 40 m. This paper provides an overview of the test results and how they were applied in detailed design and validated during construction through dynamic pile tests. Lessons learned are shared and conclusions are drawn regarding on-site offshore pile test results and their transfer into the foundation design and planning process.