Offshore static and dynamic pile load tests results in low density chalk grade A1/A2 to optimize the foundation design of jacket piles

Iberdrola is developing the 350 MW Wikinger offshore wind farm in the German Baltic Sea where ground conditions are dominated by Deposits of the Upper Quaternary series and Postglacial Deposits overlying Cretaceous Upper Chalk (low density Grade A1/A2) of Maastrichtian age and at some locations with Paleogene Limestone (high to very high density grade A1/A2). The current pile design methods for open tubular steel piles highlighted significant design uncertainties that would lead to unnecessarily conservative pile dimensions for the seventy-four-legged jacket wind-turbine support structures. German legislation requires the execution of dynamic pile load tests during the first construction phase on production piles, immediately following pile installation. This practice offers no opportunity for pile design optimization. The pile tests can confirm the design assumptions if positive but risk costly delays in OWF commissioning if the dynamically ‘measured’ capacities fall below design requirements. Therefore, after careful consideration of costs and benefits, and with the clear advantages of eliminating ambiguity of the test results, it was decided that offshore tests at the Wikinger OWF offered the best return on investment regarding foundation design optimization, assurance and certification. To address these concerns, the project commissioned, in advance of final design, offshore dynamic and static pile load tests, that were completed 10 weeks after pile driving, on slightly downscaled piles with penetrations of up to 31m, in water depths of around 40m. This paper provides a short overview of the conducted offshore pile load tests on test piles in 40 m water depth in 2015, how results were applied in the detailed design in 2016 and validated during construction through dynamic pile tests on production piles in 2017.