Skirt Penetration Resistance of the Utgard Template-Paper

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The Utgard subsea template, supported by four 7 m diameter suction caissons, was installed in layered North Sea soils containing sand and clay. Installation measurements were compared with predictions to assess methods for estimating skirt penetration resistance.<br /><br />Using the semi-empirical bearing-capacity method of Andersen et al. (2008), calculations based on best-estimate soil parameters generally agreed with measured self-weight penetration and underpressure requirements when sand was treated as drained. Assuming undrained sand generally overpredicted resistance. In some dense sand layers, however, even drained calculations overestimated the measured resistance. Water flow and resulting pore-pressure gradients may have reduced resistance, but this effect was inconsistent and depends on sand-layer extent, fines content, and density. The evidence is insufficient to rely on this reduction in design, particularly where clay layers bound the sand.<br /><br />Measurements also showed additional penetration after underpressure was reduced to near zero and then reapplied, indicating that cycling may achieve the required penetration with a lower maximum underpressure. High-capacity pumping enabled a penetration rate of about 0.5 m/hour, helping limit water-flow duration and likely preventing excessive soil heave.<br /><br />Back-calculated clay parameters corresponded to CPT-method factors higher than DNVGL’s most-probable recommendations for stiff North Sea clays, though alternative parameter combinations also produced reasonable agreement. Overall, the installation confirms that the bearing-capacity approach can predict penetration in layered soils, while highlighting uncertainty in dense-sand resistance and the need for careful assessment of drainage and soil variability.

Keywords

Utgard subsea template

suction caissons

North Sea soils

skirt penetration resistance

bearing-capacity method

Andersen et al. 2008

drained sand

undrained sand

pore-pressure gradients

installation measurements