Tri-Suction Pile Caisson - Analysis of Soil-Structure Interaction

Abstract

Offshore wind turbines are getting larger and are being installed in ever deeper waters. This requires increasingly stronger yet affordable foundations. The Tri-Suction Pile Caisson (TSPC) is a Wind Turbine Generator (WTG) foundation concept patented by SPT Offshore that requires a smaller amount of structural steel or concrete than a conventional monopile. It can be towed from port to location, and can, thus, be installed by smaller crane vessels. Additionally, it does not require piledriving or noise mitigation measures. Despite its promising potential, unlike its competitors and proven technologies, i.e., the monopile and the Suction Bucket Jacket (SBJ), little is known about the in-place behaviour of the TSPC in sandy or clayey soil types.
This study aims to further existing knowledge regarding the interaction between the three clustered suction piles and the surrounding soil and, subsequently, contribute towards making the TSPC an economically feasible option. To this end, a parametric analysis was conducted using PLAXIS 3D, a Finite Element Modelling (FEM) software, to assess the effect of suction pile centre-to-centre distance, load combination and soil type on the TSPC behaviour under monotonic static loading.
To validate computational results against realistic values, the calculation of the environmental loads acting on the TSPC and the soil constitutive model parameters calibration were made using metocean and geotechnical data from the Aberdeen Offshore Wind Farm (AOWF).
In total, five centre-to-centre distances were considered ranging from 1.2 to 2.0 times the diameter of the suction buckets. Two governing Ultimate Limit State (ULS) load combinations were identified, corresponding to WTG rated wind speed and 50-year storm conditions. Finally, two soil types were addressed, medium-loose sand, modelled by the Hardening Soil Model (HSM) and soft-stiff clay, represented by the NGI-ADP model. The contribution of the above parameters to the mobilization of failure mechanisms, and the evolution of foundation stiffness, was investigated.
Based on this work, it is concluded that the TSPC behaves very similarly to a mono-caisson for the most clustered geometrical configurations, especially in clay, where the formation of an inner soil plug is evident. For larger centre-to-centre distances in sandy soils, the TSPC approaches the behaviour of the SBJ, with the mobilization of axial caisson pair capacity being the main resistance factor against overturning moment loading. With the current knowledge, it is not straightforward to determine which geometrical configuration is the most optimal in terms of structural steel efficiency and installability in sand and clay. Thus, the challenge of the TSPC design optimization remains.
The results of this work, however, can be used to outline the optimization process by providing starting points for normalised centre-to-centre distances in sand and clay, combined with further research on the quantification and normalisation of connection beam costs and rigidity. The investigation of different embedment ratios, accounting for installability, would also contribute to that direction. Finally, the assessment of TSPC behaviour under cyclic and dynamic load is encouraged since it can shed light on a potential strategic advantage against conventional WTG foundations.