Suction Caisson Foundation System Design

Project Brief

To assess the stability of a suction caisson foundation system for an offshore meteorological mast gathering data in advance of a full wind farm development offshore Hong Kong SAR, China.

 

Design Methodology

The design of the suction caisson foundation system for storm loading was performed by Arup using Oasys Safe. This was achieved through the ability of Oasys Safe to model the cyclic behaviour of the soil (at an elemental level), entered as data strings representing the stress-strain response softened for an equivalent number of cycles (of the maximum wave force) representing the full storm’s effect. This element level response curve allows the boundary value problem to be solved, such that the stability and displacement of the suction caisson supported substructure under the environmental loads is computed.

Laboratory Centrifuge Testing

Laboratory centrifuge testing of a single suction caisson was undertaken. The cyclic loads computed to represent the design storm were applied to the suction can, and the resulting displacements in the centrifuge tests were recorded. These tests were performed with idealised soils to represent the true site soils. A suite of lab tests were also undertaken to characterise the idealised soils used and their cyclic response.

Back analysis

Arup used Oasys Safe to back analyse the centrifuge testing performed using the storm loading functionality available within Oasys Safe. The stress-strain behaviour of the soil (derived from the lab testing discussed above) was entered into the package. This analysis was performed using the axisymmetric model shown below. The Oasys Safe results exhibited good agreement with the centrifuge test results, providing confidence to the approaches adopted.

Analysis and design

The end result of this back analysis was confidence in the approaches to the assessment of soil lab testing data and the numerical soil model for prediction of stability and displacement of the foundation system. These interpretation approaches were then extended to lab tests results on the in-situ soils, for input to Oasys Safe. These models were used for the structural and geotechnical design of the meteorological station, checking the settlement, stability and dynamic aspects.

Conclusions

Oasys Safe was a good choice for this analysis as it has a storm loading module for the analysis of stability and displacement under design storm loads. The results from analysis gave credible predictions of the behaviour of the constructed meteorological station and the subsequent design of the wind turbine generator foundations.

“The use of Oasys Safe allowed successful back-analyses of trial and model tests to be performed and extend the results of these calculations to the in situ caisson design”  - Arup lead engineer