Mudmats are used in the offshore oil and gas industry to support subsea infrastructure for pipeline terminations and well manifolds. Expansion and contraction of connected pipelines and jumpers due to changing thermal and pressure conditions impose fully three-dimensional loading on the foundations, dominated by horizontal, moment and torsional loading rather than high vertical loads. The mudmat foundations are rectangular, and include shallow skirts in order to increase capacity, particularly for sliding. Offshore design guidelines for shallow foundations tend to excessive conservatism; optimisation of mudmat capacity under general loading has thus become critical in order to keep foundation footprints within the limits of current installation vessels. The paper presents an alternative design method, based on failure envelopes derived from an extensive programme of three-dimensional finite-element analyses, focusing on the sliding and rotational capacity of the foundation. Starting from expressions that quantify the uniaxial capacity under each of the six degrees of freedom, failure envelope shapes for different biaxial combinations are developed. Ultimately, the allowable capacity under the six-degree-of-freedom loading is expressed in terms of a two-dimensional failure envelope for the resultant horizontal and moment loading, after due allowance for the vertical and torsional components of load.

A generalised framework is presented for predicting the consolidated undrained capacity of rectangular mat foundations on normally consolidated soft clay under combined loading in six degrees of freedom as a function of relative preload and degree of consolidation. Consolidated undrained response is investigated by coupled small-strain finite-element analysis using the modified Cam Clay plasticity constitutive model. Increases in the load-carrying capacity of a foundation under combined loading in six degrees of freedom following vertical preload with subsequent consolidation are demonstrated and quantified. The results are presented as failure envelopes in multi-directional load space and are shown to expand proportionally as a function of degree of consolidation for a given relative preload. A methodology and a set of expressions are provided to predict the shape and size of failure envelopes for rectangular mat foundations for any degree of preloading and consolidation.

The effect of soil–foundation interface condition on the undrained capacity of rectangular mudmat foundations under loading in six degrees of freedom is investigated. Undrained failure envelopes for mudmats with zero-tension interface have been derived from finite-element analyses, and compared with the solutions from traditional methods and established for an unlimited-tension interface condition. The zero-tension interface has minimal effect on failure envelopes in the absence of moment, but significantly reduces the load-carrying capacity of mudmats under all other load paths that include moment. The traditional method for predicting the capacity of shallow foundations under multi-directional loading generally predicts lower capacity under any combined loading condition in comparison with the finite-element results for mudmats with zero-tension interface. Algebraic expressions are proposed to describe failure envelopes for mudmats with zero-tension interface. The proposed expressions can be implemented in an automated calculation tool to enable essentially instantaneous generation of failure envelopes and optimisation of a foundation design as a function of foundation dimension or material factor.