Skirted foundations are used in offshore applications to resist the large horizontal and moment loads that are characteristic of the ocean environment. The ultimate capacity of a skirted foundation subjected to vertical-horizontal-moment (VHM) loading combinations can be described using failure envelopes in VHM load space. This represents a far more versatile design approach than classical bearing capacity solutions and is able to accommodate complex loading scenarios and similarly complex stress conditions in the soil. Typically, VHM envelopes are constructed using deterministic plastic limit analyses or numerical methods. However, soil is a natural material and as such is inherently variable. The value of soil parameters fluctuates through the ground, as evident in field or laboratory test results. In this study, the ultimate capacity of a skirted foundation in spatially variable undrained clay under VHM loading is investigated. Spatial variation is taken into account using a random field representation of undrained shear strength and the increase of strength with depth typical of a normally consolidated marine clay is considered. The random field is coupled with a finite element method to assess the ultimate limit state. The failure mechanism and ultimate capacity are consequently direct results of the spatial variation of undrained shear strength. Monte Carlo simulation is used to produce probabilistic failure envelopes, based on the probability that the true combination of VHM loads causing failure will fall inside on or inside that envelope. Hence, this method allows designers to select a failure envelope that is associated with an acceptable level of risk.