Efficient capacity analysis of offshore suction caissons and monopiles

Overview

In offshore engineering, embedded steel foundations with relatively small length-to-diameter ratios (L/D = 0.5 to 5) are commonly used to support fixed structures and to provide anchorage points for floating platforms such as FPSOs. These foundations are typically open-ended and thin-walled (D/t = 50 to 150). They can either be driven into the seabed, or, in the case of suction buckets and suction piles, installed using self-weight assisted by ‘suction’ (differential pressure induced by pumping).

The ultimate bearing/holding capacity of such foundations can be estimated using various calculation methods.

This presentation will review some of the main available approaches, including:

• mechanism-based analytical methods (notably the influential paper by Murff and Hamilton, 1993)
• 3D finite element analysis (FEA) using elastic-plastic soil models
• 3D finite element limit analysis (FELA) using rigid-plastic soil models 

For each approach the underlying theory and assumptions will be outlined, and the resulting advantages and limitations will be discussed.

A selection of illustrative examples will be used, ranging from fundamental benchmark problems (e.g. pure lateral displacement of an isolated monopile) to complex design scenarios (e.g. combined vertical, horizontal, moment, and torsional loading acting on a three- or four-legged jacket structure supported by suction buckets). Some of these examples will explore the impact of diverse factors such as tensile gapping, spatial variation of strength properties, group effects, and the detrimental effect of scour on capacity.

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Programme