Drilling Solutions for Offshore Wind Foundations and Export Cable Landfalls

Abstract

The development of renewable energies is the focus for future energy supply. Large offshore wind farms are to be constructed and connected to a high-capacity trans-mission grid. Due to the public environmental awareness, smart installation methods are required for fast and safe construction of offshore foundations and the eco-friendly landfall installation for the export cables, with minimal impact on surroundings. The installation of offshore foundations requires drilled installation methods, espe-cially in non-drivable soil conditions and medium to higher water depths. In addition to limitations in certain ground conditions, conventional pile-driving is reaching its limits with increasing turbine sizes. Furthermore, pile-driving has a deep environ-mental impact. Noise emissions can either require complex and expensive noise mitigation systems or cause lasting damage to flora and fauna. In the past, offshore drilling rigs have only been a non- performance driven backup option for pile-driving installations. Today, the Offshore Foundation Drilling technology provides a com- pletely drilled installation of offshore foundations, e.g. for monopile structures, pin-piles or floating anchor piles. High drilling performances can be achieved, that were previously only known in the horizontal drilling industry. Lower forces on the foun-dation pile during installation allow a more economical static pile design. Acoustic emissions can be largely avoided which finally can also lead to longer and more flexible time frames for construction. In 2022, the first equipment worldwide has been used as a standard installation method for XXL monopiles for the St. Nazaire wind farm in France, installing a total of 73 monopiles by drilling and grouting with a drilling diameter of 7,700 mm, in wa-ter depths of 15 to 25 m in rocky seabed. The landfall sections of export cables coming from offshore wind farms are usually located in sensitive coastal areas where the operation of heavy construction equip-ment is not possible due to environmental protection requirements. Also, when dyke structures have to be crossed, this sets further increased demands on the safety as-pects of respective trenchless installation methods. Even if HDD presents a common technology for the installation of casings for subsequent pull-in of the export cables, the alternatives with remote- controlled microtunnelling systems such as Pipe Jacking or Direct Pipe need to be carefully investigated. In any case, the drilling works can be carried out from onshore to offshore to reduce marine construction costs, whereas the protective pipe can be inserted from the seaward side in a second step. This paper will give an overview of the drilled approach for offshore founda-tions and the application range of trenchless methods for export cable landfalls.