Experimental Modeling of the Unburial Behaviour of Pipelines

Abstract

Abstract This paper is concerned with the interaction of a buried pipeline and the surrounding soil during various unburial processes. This follows a three year research program at Oxford University investigating mechanisms of pipeline unburial including upheaval buckling and pipeline floatation. The results include 2D plane strain uplift tests where a section of pipe is pulled upwards through soil, with the resulting forces and displacements being measured. A loose fine uniform sand is the focus of the study as this is often found in the North Sea and also reflects the soil conditions after pipe installation such as by jet trenching. The soil was tested in a dry state to benchmark drained capacities and in a saturated state to explore rate dependent responses. Particle image velocimetry was used to identify failure mechanisms at different depths as the pipe is pulled from the soil. A second theme of larger scale experiments, using a slender length of pipe, was also carried out to identify 3D effects that might not be captured in the conventional 2D tests. One set of experiments was carried out to determine the axial buckling load of a pinned and buried strut. The tests explored responses in dry sand and saturated sand showing that buckling loads are a function of soil cover depth. Further work was carried out in a specially designed pipeline testing tank (8m long) to explore buckling and floatation responses of a long slender pipe. The experiments were instrumented so that pipe displacements along the length were acquired as well as loads applied to one end of the pipeline. Summary conclusions from the work so far are presented. Introduction Submarine pipelines are typically used to transport hot crude oil or gas from satellite production fields to a central offshore production platform. These pipelines are commonly laid in trenches that are subsequently backfilled, so that they are protected from damage and to gain additional thermal insulation from the surrounding soil. Trenching typically involves processes that can be categorised as;jetting: the soil (usually a sand) is fluidized allowing the pipe to sink into the resulting sand-water dispersion or,ploughing: a trench is mechanically ploughed into the soil (both sands and clays) into which the pipe is laid and the spoil is mechanically filled back over the pipe. In both cases the remoulded strength of the soil above and around the pipe is significantly lower than that of the in-situ material. The oil and gas that is pumped through the pipe is generally 50 to 150 degrees Celsius hotter than the ambient temperature and causes the pipe to undergo thermal expansion. The fluid is also at a high pressure which induces additional strains in the pipe. The embedment depth of the pipe is designed so as to restrain the hot pipe sufficiently against expansion or lateral movement. However, in locations of imperfections or insufficient embedment, the thermal expansion can be relieved by the occurrence of a buckling event (Boer et al., 1986). As the backfill material in the trench is significantly weaker than the in-situ material, the pipe will tend to move upwards to the seabed surface forming a confined buckle. The resulting ‘kink’ or sharply bent section of pipe is vulnerable to low cycle high strain fatigue as the pipeline heats and cools and thus expands and contracts respectively, during the following operational cycles (Nielsen et al., 1990). If the pipeline protrudes from the seabed it also loses its original protection against trawl gear and drag anchors (Cathie et al., 1996). Since remedial actions are very costly the temperature induced buckling is a well known area of concern for offshore pipeline designers and is usually called upheaval buckling.