Installation of Suction Buckets

Description:

Suction buckets are a relatively new type of foundation in the offshore wind energy sector, which can be designed as monopod and multipod foundations.  Suction buckets are first installed by placing them on the ground and penetrating them under their own weight. Subsequently, an underdrain is created inside the bucket by pumping out the water trapped in the bucket. The driving forces are the structure's own weight and the pressure difference acting on the bucket lid. The penetration resistance results from the outer and inner shell friction along the skirt and the peak pressure under the base of the bucket.

In a non-cohesive subsoil, the underpressure leads to an upward flow of water into the bucket, which significantly reduces the internal casing friction and the peak pressure under the foot. In this case, the negative pressure has a double benefit: Increasing the driving forces and reducing the penetration resistance.

However, negative pressure cannot and must not be generated at any desired level; its level is primarily limited by the following aspects:

• The pressure must not drop so low as to cause cavitation. In practice, the maximum possible negative pressure results from the sum of atmospheric and hydrostatic pressure at the current height of the bucket lid.
• The negative pressure must not lead to buckling of the bucket. The buckling pressure must be analysed taking into account the current embedment depth and can be increased by design measures such as stiffening, greater wall thickness or cross-section optimisation. This can also have an effect on the penetration resistance.
• In cohesive soil, excessive negative pressure can lead to soil material being broken out or even the entire soil plug being sucked in. In the case of non-cohesive soil, it can lead to the formation of erosion channels and ultimately hydraulic ground failure. Uncontrolled penetration and tilting are then the result. Irrespective of this, the non-cohesive soil is loosened and lifted anyway by the upward flow forces.

The negative pressure that leads to a hydraulic fracture when exceeded is referred to as the critical negative pressure Δukrit and can be determined using flow network calculations. It is usually assumed for the sake of simplicity that a stationary flow condition prevails due to the comparatively low penetration velocity. Time-dependent effects are therefore neglected. To avoid the problems mentioned above, a permissible negative pressure Δu_zul must be defined for each installation depending on the penetration depth. Whether a certain penetration depth can be achieved can be determined by comparing Δu_zul with the required negative pressure Δu_erf. 

Δu_erf can be calculated using CPT-based calculation methods or methods based on the effective stresses. These analytical methods work in a similar way to methods for calculating the load-bearing capacity of a pile foundation, but also take into account the influence of the surrounding flow on the penetration resistance. For the sake of simplicity, it is assumed that the penetration resistances can be reduced as a linear function of the ratio Δu/Δu_krit. It is also assumed that the penetration speed has no influence on the achievable penetration depth.

Whether the assumptions and simplifications mentioned here are sufficiently accurate and realistic for the prediction of the penetration process is to be checked with the aid of transient, hydraulically-mechanically coupled FEM calculations using the FE programme Abaqus.