Designing infrastructure for an evolving seabed

Offshore infrastructure is an essential element of our energy system and our communication networks. This infrastructure rests on a seabed that evolves with time – due to environmental processes, and due to the action of the infrastructure itself.

The design of cables, pipelines, foundations and anchoring systems is made complex by the evolution in soil properties caused by the whole-life loading history – from installation, through in-service loads, to the end-of-life solution. In addition, ocean-seabed-infrastructure interaction can cause sediment transport and scour, altering the seabed geometry and causing changes in embedment and support.

This Geotechnique lecture tackles the geotechnical challenge of designing infrastructure for an evolving seabed, examining ‘whole-life’ changes in seabed strength and geometry.

Changes in soft soil strength arise from both monotonic dead loads and cyclic live loads. Examples for pipelines, foundations and anchors will be shown, drawn from centrifuge model tests, numerical simulations and field data.

Significant changes in strength by a factor of 2-3 are typically evident. These effects matter most at shallow depths, close to the seafloor. New types of element test and penetrometer will be shown, which target this near-surface strength.

Meanwhile, in both coarse-grained and fine-grained soils, excitation from the ocean or the infrastructure can lead to sediment transport, scour and subsidence. These effects are often, but not always, detrimental to design. Examples of this fluid-structure-seabed interaction are described, from the laboratory and the field.

The lecture will close by showing how these whole-life effects can be harnessed through innovative technologies, such as:

• New types of robotic penetrometer test that simulate whole life effects
• Foundations that deliberately slide across the seafloor
• Anchoring systems that rely on whole-life changes in strength

Modern numerical techniques, such as finite element limit analysis, now provide definitive solutions for the stability and bearing capacity of structures, if the geometry and strength of the surrounding soil are known. The examples in this lecture highlight the remaining uncertainties for offshore infrastructure, which are the changing soil properties and geometry.