Calculating and testing the stability of an air-cushioned floating suction pile foundation

The Tri Suction Pile Caisson (TSPC) is being developed as an innovative alternative to existing wind turbine foundations. It consists of three open bottomed suction piles connected to a central monopile column. By placing it on the sea floor and depressurizing the suction piles, the TSPC is penetrated into the seabed, effectively creating a permanent foundation for a large offshore wind turbine. One of the large novelties in the concept is the secondary functionality of the suction piles as floaters of the foundation during wet-tow transport. Due to the air volume in the suction piles, the TSPC foundation has sufficient buoyancy to stay afloat on its own. Hence it can be towed to site in a relatively straightforward manner with offshore tugs, by-passing the need of a dry-tow mode or transport using offshore going transport barges. Moreover, the on-site installation requires a much lower crane capacity than what otherwise would be required. The TSPC combines the benefits of the suction pile (no noise, nor vibrations during installation, rapid and accurate installation and decommissioning of the entire structure) with much lower fabrication costs than a jacket.
In the TSPC research project (2022-2024), MARIN has investigated the hydrostatic and hydrodynamic properties of the concept design, in order to determine the workability offshore and the methodology of transportation and installation for a floater which obtains as much as 97% of its total buoyancy from the enclosed air pockets. This was firstly done with numerical modelling in MARIN's time-domain simulation software aNySIM/XMF, incorporating models for non-linear hydrostatics and dynamic tanks. Secondly, wave basin model tests with a scale 1:40 model were performed for limited environmental conditions. Finally, the simulation model was calibrated against the basin model tests and a workability assessment was performed to determine the TSPC workability offshore at a North Sea site. During model testing, to achieve a long term stable trim, a passive volume control by means of a stand pipe was added to the TSPC concept.
This paper will discuss the key findings of the project, including, the aNySIM/XMF time domain simulations, the results of the wave basin model tests and the workability assessment.