Marine pipelines are widely used for transporting hydrocarbon material. However, they can be damaged by marine geo-hazards such as seabed liquefaction, as they may sink, float or be dragged by the moving soil. One of the triggering mechanism of seabed liquefaction is the increase of seabed inclination as a result of the souring process or human construction activities. Experiments are carried out to simulate seabed liquefaction and to study the drag forces on a shallowly buried gas pipe at a centrifugal acceleration field of 50g. A tilting mechanism is applied to trigger sample liquefaction. A fluidization system equipped at the bottom of the strongbox is designed to prepare fully saturated, loose and uniform samples. Viscous fluid made of Hydroxypropyl Methylcellulose powder is used as the pore fluid. A hollow model pipe is embedded in the sand layer shallowly with a specific embedment ratio. The pipe fixities are made to be adjustable for adjusting pipe locations horizontally and vertically. Strain gauges attached on fixities are used to monitor the loads exerted on the pipe. The effect of the presence of pipes on the sand layer instability is presented. Furthermore, the drag forces acting on the pipe at a specific embedment ratio is discussed.

As soil behaviour is stress-dependent, a centrifuge model with tilting sample box was developed to generate liquefaction flow slides at higher confining stresses. The design and experimental set-up were based on the large liquefaction tank (de Jager et al., 2017). Fluidisation was used as sample preparation technique to produce a saturated, loose and uniform sand bed. The performance of the fluidisation system was evaluated by experimental investigation of the sample by considering the relative density, uniformity, degree of saturation and the influence of viscous pore fluid. The reproducibility of the initial sample was considered acceptable. A series of centrifuge experiments was conducted where the fluidised sand bed was accelerated to varying gravity levels and inclined to a slope with constant tilting rate. In most tests the soil response was characterised by a rapid liquefaction flow slide and a sudden increase in pore pressures. The moment of failure was consistently influenced by a variation in fluid viscosity and tilting rate, regardless of the gravity level; these effects indicated that instability was caused by the restricted seepage rate during loading. It is believed that the liquefaction potential is governed by the extremely loose and highly contractive top layer, which yields a sudden loss of strength under limited drainage conditions. The pore pressure measurements, which showed no excess pore pressures building up prior to failure, can lead to a misunderstanding of the failure mechanism and false assumption of fully drained conditions. Monitoring the pore pressures is therefore not suitable to predict liquefaction flow slides in submarine slopes. Mitigation of liquefaction should be focussed on densification of the looser part of the sandy slope, which is usually the top layer.