Increasing numbers of subsea structures related to wells and pipelines are being placed on the seabed as part of typical subsea or tie-back developments. Given the proliferation of these structures and the marginal cost of offshore developments, controlling installation and fabrication costs for subsea structures can be key to project viability. Skirted mudmats are often the most cost-effective foundation type, and particular additional design focuses on optimising their cost by minimising foundation weight and installation time.

Subsea foundations must be designed to withstand all applied loads during their design life (e.g. during set-down, tie-in, hydrotest, operation etc.) with suitable reliability. Using skirts, peripheral or internal, to improve the sliding resistance is an efficient solution provided the self-weight of the subsea structure on set-down is sufficiently large to ensure installation of the skirts (even for the strongest likely seabed conditions), but can lead to significant cost increases if additional ballast is required to ensure this.

The paper examines how foundation skirt geometries can be optimised in order to provide sufficient foundation in-place capacity whilst minimising the amount of self-weight required for their installation. Parametric studies are presented that show how the sliding capacity of individual skirts is affected by the weight of the structure, and also the spacing and position within the foundation plan.

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