Transforming Floating Wind Turbine Anchoring with Oilfield Technology of Casing Drilling

Abstract

Abstract The oilfield technology of casing drilling in the form of Riserless Casing Drilling is being adapted to provide a robust and cost-effective anchoring system for offshore wind turbine and other floating offshore structures. As the world transitions from the global dependence on hydrocarbons for energy to sustainable, renewable electrical generation, hundreds of offshore wind generation stations will need to be installed. The development of offshore floating electricity generation has been struggling with its economic feasibility. One of the key issues is that increasing costs, due to the water depth and adverse subsea soil conditions, has been challenging project economics. Floating wind generated electricity has a key role to play in the transition from fossil fuels to renewable energy. However, to be competitive and for its market share to dramatically increase, floating wind electricity generation needs to reduce its development costs to levels at or below that of competing sources of electrical generation (NREL, 26 Feb 2020). Anticipated cost reductions will come from many project elements including increased capacity wind turbines, reducing the number of turbines required per project. However, these larger turbines increase the anchoring and mooring system requirements increasing the project costs. The current anchoring systems of higher capacity, which include suction anchors, hydraulically installed pile anchors and drilled-grouted anchors, have limited anchoring capacities. Increased anchoring requirements for larger turbines would require more anchors. More anchoring and mooring systems increase cost and increases the environmental impact. In addition, the current anchoring systems are not suitable for many subsea soil conditions, particularly hard formations, nor can they be installed in inclement conditions placing additional constraints on floating wind generation. Subsea Anchoring System can provide very high load (pullout) anchoring capacities, better suited for all soil conditions, and can be installed in far rougher sea conditions. One advantage is that fewer anchors would be required, compared to current anchoring systems, for the high anchoring capacity requirements of tension leg platforms (TLP). This anchoring technology is a pile or casing drilled in to predetermined depth using casing drilling technology. It is a one trip system of drilling the hole with the casing to then immediately cement upon reaching anchoring depth. This system provides anchoring capacities determined by the drilled depth, diameter of anchoring casing and competent casing-to-cement-to-formation contact to provide anchoring capacities far exceeding existing anchoring. This operation would be done with existing mobile oilfield drilling units. The installation costs are more costly than the current high capacity anchoring technologies however since less anchors as required per floating wind turbine than current practices, including safety factor redundancies, the total anchoring and mooring cost would be reduced.