Advances in de-risking installation of bucket foundations
Per Magnus Sparrevik
Norwegian Geotechnical Institute, Oslo, Norway
Caisson or "bucket" foundations have been in use for many decades by the offshore oil and gas industry for anchoring and fixed platform foundations. Recently the concept has been implemented for OWT foundations. The main benefits with bucket foundations are possible timesavings during installation, scalability to larger water depths and larger turbines with use of less steel in total. In addition, the offshore installation is silent with respect to noise under water.
The upside down "bucket" initially penetrates by self-weight such that a soil seal is formed between the entrapped water inside the bucket and the outside sea. The entrapped water is then evacuated from the bucket using a submerged pump and the skirts are driven to the penetration depth that is required for sufficient holding capacity and stiffness during operation of the turbine. The suction pressure and driving force which is generated when the entrapped water is pumped out from the bucket is a function of the encountered penetration resistance, the soil permeability, and the weight of the foundation. The pumping outlet is closed after completed installation, which significantly improves the in place performance of the foundation.
Penetration of the buckets can be sensitive to the soil conditions. Therefore, proper knowledge about the limiting conditions and failures modes is important in order to avoid problems during installation and reduce the risk. Challenging seabed conditions and limited water depths have brought substantial new knowledge about installation feasibility, allowing for bucket installation also in "difficult" seabed conditions and further optimisation of the design.
Dependent on the soil conditions, different geotechnical processes occur during suction penetration. In permeable sands, the pressure difference generates a seepage around the skirt tip and reduced effective stresses such that the penetration resistance diminish. In impermeable clay or silt, no seepage occurs and the suction pressure does not affect the penetration resistance. In layered soils a combination of these processes are present.
The following challenges apply for suction penetration in difficult soil conditions:
Field experience has verified that the following mitigation methods can ensure successful suction penetration to the required depth in challenging soil conditions:
If leakage occurs when suction penetration is started:
If the penetration resistance must be reduced:
Leakage around the skirt tip is mitigated by further penetration. Cycling is most efficient in clay, pumping the bucket up and down reduces the penetration resistance more than on-off pumping. Water injection is relevant if the skirt tip penetrates into dense sand and seepage is blocked by overlaying clay. The water injected at the skirt tip increases the pore pressure and the end bearing capacity becomes negligible. Jetting must be avoided as this may reduce the in-place bearing capacity of the foundation.
With a sound design and installation methodology, bucket foundations can be installed in most of the soil conditions relevant for offshore wind farms. At locations with competent soils near the surface, the shallow bucket foundation is an economic alternative to pile foundations.
To ensure successful utilization of the bucket foundation concept within the offshore wind industry, proper understanding of the limitations and failures modes for installation is important. Bucket penetration is straightforward for many seabed conditions but for some types of soils, it can be more complicated. Other limits such as cavitation at sites with shallow water depth may require more advanced mitigation methods to enable suction driving of the bucket foundation to the required penetration depth. As for pile driving, experience from field installations is important input in order to specify relevant soil investigations, improve installation predictions and methods. This presentation explains the principles and mechanisms involved during suction penetration in challenging soil conditions. The relevant failure modes and possible mitigation methods are also discussed.