Modelling sliding resistance of tolerably mobile subsea mudmats

Abstract

Subsea infrastructure for deep-water oil and gas developments is often supported by mudmat foundations. Traditionally, subsea mudmats are designed to resist the loads imposed by pipeline thermal expansion and contraction while remaining stationary. As subsea facilities have grown, the required size and weight of the mudmats challenge the handling capacity of installation vessels and raise costs. Tolerable mobility of a subsea mudmat can significantly relieve the applied loads, leading to reduced mudmat size and weight. In this paper, the cyclic shearing and reconsolidation response of fine-grained soil around a tolerably mobile mudmat is investigated through results of finite-element analysis using a critical state soil model. The mudmat was subjected to a simulated lifetime of operation, with many cycles of undrained sliding with intervening consolidation between cycles. The sliding resistance was shown to rise exponentially with cycles and reach the drained limit, accompanied by significant strength gain in the subsoil because of the intervening consolidation between movements. The degree of reconsolidation between slides affects the number of cycles required to mobilise the drained limit. The hardening response for periodic shearing with intervening consolidation is shown to scale from the hardening response for continuous undrained shearing by an amount depending on the degree of intervening consolidation during pipeline operation. Expressions for the rate of hardening of sliding resistance of a tolerably mobile mudmat foundation are proposed in this paper to assist design practice.