Suction caisson foundations offer a compelling alternative to piled foundations since (1) they have a lower environmental impact during installation with reduced noise emissions, (2) installation can be faster and (3) the foundations can be more easily removed at the end of their lifetime. During installation, suction is applied inside the caisson with pumps, effectively pushing it into the seabed. For extraction, the pumps can apply an overpressure, pushing the foundation back out of the ground.
Since most of the offshore wind turbines are installed using monopile foundations, recent innovation projects such as the Blue Pilot and the HyPE-ST projects have investigated the use of hydraulic overpressure for decommissioning these foundations.
This article introduces an application and it's implementation to calculate the required suction for installation and overpressure for extraction.
The calculation method uses a one-dimensional finite element limit analysis approach. An overview of the method is presented here, but full details can be found in Beuckelaers et al. 2021. The vertical equilibrium conditions inside the caisson/pile are:
where σ'_i is the vertical effective stress inside of the pile, p_i is the pressure inside of the pile, τ_i is the internal shaft friction and γ'_s is the effective unit weight of the soil (symbols shown in the figure below). The maximum shear stresses that can be developed inside the pile are:
The equations above can be solved using a linear optimisation program where the pressure inside the pile is maximised while maintaining equilibrium and staying within the allowable stress ranges for the shear friction. The maximum pressure then corresponds to the break-out pressure for hydraulic pile extraction.
The Beuckelaers et al. 2021 paper initially presented the method for hydraulic extraction, however, it can also directly be applied to suction installation by updating the objective function to minimise the pressure to failure instead of maximising it (using a -1 multiplier in the objective function). The resulting approach is in line with the mechanism based method for installation design in the Suction Caisson Design Guidelines Report by the OWA, but allows for extension to multi-layered system since we use a finite element based approach.
The suction installation / hydraulic extraction application is freely accessible using this link.
The application allows users to define multiple soil layers and experiment with pile/caisson geometries. Calculations only take a fraction of a second, which makes the method ideal for sensitivity analyses at early stage of a project or for detailed design stage where many different designs and soil layering profiles need to be analysed.
The figure below shows the sample output of an analysis, where you can see that the break-out pressure is a bit over 10 bar. The graphs present (from left to right):
The vertical effective stresses on the inside and outside of the pile
The inside and outside shear stresses along the pile wall
The vertical stresses of the pile
The water pressure distribution on the inside of the pile
An overview of the soil types (red: sand, blue: clay)
In conclusion, the application presents a useful approach to calculate the required suction for caisson installation or overpressure for extraction. The method is in line with the mechanism-based approach in the OWA suction caisson guidelines, but also has the capability to handle layered soil conditions, making it much more useful for offshore installation studies. The fact that the application runs very quickly (in a fraction of a second) makes it ideal for preliminary stage studies where sensitivity analyses need to be performed, or at detailed design stage when many different designs and layering profiles need to be analysed.