Experience from Full-Scale Suction Caisson Trial Installations at the Seagreen Offshore Wind Farm-Paper

Paper

Pdf Summary

The paper reports a full-scale suction caisson trial campaign for the Seagreen Phase 1 offshore wind farm in Scotland. Conducted in 2019, it comprised 30 installations at 20 locations: 25 monotonic tests, three cyclic tests and two set-down tests. The trials assessed installation feasibility across dense sands, strong clays and layered soils, and calibrated predictions of installation resistance.<br /><br />Installations succeeded at 19 locations. The sole geotechnical refusal occurred where surficial cobbles and gravel prevented a seal from forming, highlighting the importance of identifying high-strength, permeable surface layers—conditions that CPT data alone may not reliably reveal. No significant seabed disturbance was observed during set-down, although excessive caisson tilt was associated with soil gapping. Tilt in the single-caisson test setup was considered less representative of jacket installation, where multiple caissons can be used for levelling.<br /><br />Back-analysis produced site-specific installation resistance factors that improved predictions using the DNVGL method. These factors were lower than standard recommendations but generally matched observed suction pressures reasonably well; the standard method tended to overpredict resistance in sands, potentially because it does not account for beneficial water-flow effects.<br /><br />Pressure cycling was an effective way to reduce resistance in clay, especially two-way cycling and the first cycle. It was less effective in sand. Post-retrieval CPTs indicated soil-plug loosening in sand, potentially creating a weakened footprint that could affect nearby foundations; no significant changes were observed in clay.<br /><br />Overall, the campaign reduced installation risk and helped identify areas unsuitable for suction caissons while expanding the areas considered feasible. It also supported design optimisation and potential cost savings.

Keywords

suction caisson

offshore wind farm

installation trials

geotechnical refusal

installation resistance

dense sands

strong clays

soil gapping

pressure cycling

DNVGL method