OFD in operation
The number of offshore installations for expanding the use of renewable energy and the construction of coastal infrastructure has grown rapidly in recent years. As a result, demand for economical and environmentally friendly installation technologies for offshore foundations has increased. These are needed for wind turbines, substations, tidal power plants and bridges as well as for harbor infrastructure such as quay walls, breakwaters and jetties.
The Herrenknecht OFD technology uses a full-face drilling concept and a down drive behind the cutterhead to overcome the limits of conventional methods. This applies in particular to difficult ground conditions unsuitable for pile driving and where established pile driving methods would cause critical noise emissions. As seawater is used to transport the excavated soil to the surface through a slurry circuit, a separation plant on the surface is not required.
Depending on the installation method, the vertical advance force is provided e.g. by a lowering system or by using the force of gravity. The lowering of the foundation pile or the casing occurs simultaneously with the advance of theHerrenknecht OFD machine. The lowering speed is controlled by the drilling operator. Hydraulic cylinders measure the downforce on the cutting wheel.
When the final depth is reached, the overcutters are folded in and the locking system is disengaged. The machine is attached to the crane on the surface, smoothly lifted out of the foundation pile or casing and placed into the seafastening system on the platform structure.
Installation of foundation piles
The OFD machine can be used to install a foundation pile either directly in the seabed or in a two-stage process initially using a temporary casing. In the first case, the foundation pile must have a constant diameter through which the OFD machine can be recovered. In the second case, a temporary casing is used for the drilling process. The final foundation pile, e.g. a (conical) monopile, is inserted into the casing after the Herrenknecht OFD machine has reached the target depth and has been recovered and placed in a seafastening system on board the platform structure.
Direct installation of a foundation pile (exemplary)
When a non-conical foundation pile needs to be installed in the seabed, the Herrenknecht OFD technology allows direct installation. This means the Herrenknecht OFD machine is installed directly in the final foundation pile, which can then be lowered to the target depth in parallel with the drilling process.
In a first step, the foundation pile is lowered down to the seabed and calibrated in its vertical position. A pile gripping unit and/or crane on the platform structure is necessary to hold the pile in its vertical position and carry out the lowering process in tandem with the advance of the OFD drilling machine down to the target depth. After completion of the drilling process described above (steps 2 and 3), the extendable overcutters are folded in and the OFD machine is recovered. In a final step, the overcut has to be grouted with cement and connected with the surrounding geology (step 4).
OFD Offshore Foundation Drilling by Herrenknecht is a mechanized solution that increases the feasibility of offshore projects requiring deep foundations. New project sites will be explored in future, in particular where soil conditions, water depths and currents have been limiting parameters in the past. Environmental considerations and associated regulatory limits will continue to drive the development and use of mechanized construction methods for offshore foundations, nearshore and offshore, for both renewable energy and infrastructure projects.
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