This paper details experience gained from full-scale suction caisson trial installations at the planned Seagreen offshore wind farm, which is located in the Firth of Forth, offshore Scotland. Issues encountered during the trial campaign are discussed and key conclusions provided. A total of 30 trial installations were undertaken across 20 locations; these comprised 25 monotonic tests, 3 cyclic tests and 2 set down tests. Trial installations were conducted in very dense sands, overconsolidated clays and complex layered soils representative of the wide range of ground conditions present at the site. The main objectives were to prove the feasibility of suction caisson installation across the site and calibrate future predictions of installation resistance. Successful trial installations were conducted at 19 of the 20 locations. Observations confirmed installation feasibility in various strata, including high strength clays and layered profiles, without indication of piping or plug failure. The single installation refusal highlighted that the presence of high resistance and high permeability surficial layers was a critical consideration for suction caisson installation. With this experience, isolated areas of the site were deemed unsuitable for suction caissons and mitigations planned accordingly, thereby reducing foundation installation risk. Quantitative back-analysis of the trial installation results enabled improved site-specific predictions of foundation installation resistance, thereby further de-risking suction caisson foundation design and identifying areas for potential design optimisation. The results of the back-analysis also meant more areas of the site were considered feasible for suction caisson installation. Performance of the trial installation campaign therefore provided notable benefits to de-risking the project and to lowering overall foundation cost.
Lewis Jones and Andrew Harding
Keywords: Suction caisson, foundation installation, offshore wind
Publisher: DFI