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This paper elaborates on (main) conclusions from pile advanced monitoring data, regarding the quality of the monitoring data under the specific circumstances, the modelling and validation of pile driving resistance in subject extreme soil conditions, and the effectiveness of pile design features such as internal- and external driving shoe. Results are expected to be of added value to near-future offshore wind farm developments in areas having soil conditions similar to this area of the North Sea. Statoil on behalf of partners Masdar and Statkraft are developing a large offshore wind farm (OWF) in the central part of the North Sea. The OWF consists of 67 wind turbines and one transformer substation platform. The wind turbines have been installed on monopiles foundations, whereas the substation has been founded on suction piles. Total area of the site comprises around 50 square km. The monopile foundations dimensions are typically 6.3m diameter at the pile top and 7.0m, 7.2m and 7.4m diameter at the embedded section of the pile, length varies between 53m and 72m. Mass of largest monopile is around 1000 metric ton. Seaway Heavy Lifting has been main contractor for installation of the monopiles. The monopiles have been installed by means of hydraulic impact hammers with rated energy up to 3000 kJ. The hammers were equipped with a 6.5m diameter pile sleeve as to suit the top of the monopile. Total weight of the hammer assembly was around 580 metric ton. During design phase as well as during preparation of the offshore campaign, it became clear that the soil conditions would have a major impact on the project. This was due to presence of a wide variety of soil types and strength that existed within the OWF site. Site investigation had revealed presence of very weak chalk layers with potential risk of dropfalls during installation, but also high strength clay layers and rock-type soils were encountered with potential risk of pile driving refusal conditions. To mitigate the risks related to installation of the monopiles, a number of measures were taken. These included good regular engineering and evaluation of field experience from nearby installed OWF, but also a pile driving test-program on application of internal- and external driving shoe to reduce the pile driving soil resistance was included. It was decided to use advanced pile monitoring during pile driving operation for a selection of monopiles for confirmation of engineering design. Monitoring was performed by subcontractor GeoDrive. Due to limitations originating from pile design and from offshore operations, the monitoring sensors had to be placed on the inside of the monopile. This is not common practice since experience has revealed some uncertainty regarding reliability of the monitoring data under these circumstances, but was considered feasible in this case. |
