Self-Burial Of Laterally Loaded Offshore Pipelines In Weak Sediments

Abstract

ABSTRACT A laboratory study was initiated to determine the extent to which pipelines resting on the ocean bottom would self-bury when subjected to cyclic wave and current forces. Experiments were conducted in a large sediment-filled tank at approximately one-quarter scale, applying controlled lateral loads or displacement to a pipe test section with a servo system. Preliminary results provide a means for estimating horizontal forces required to move embedded pipes, and estimating rates of self-burial. INTRODUCTION Pipelines resting on or partially embedded in the sea floor are subjected to cyclic wave and current forces which can result in eventual burial below the mudline. To enable this problem to be addressed quantitatively, a laboratory study was initiated to determine how much laterally loaded pipelines would self-bury in soft cohesive sediments. The objective was essentially to characterize the behavior of a transversely loaded pipeline in soft clay, and the effects of vertical forces other than pipe self-weight were not considered at this stage. A program of model testing using scaled horizontal forces and displacements was conducted in order to observe pipeline behavior. Both load- and displacement controlled tests were performed on pipes installed at various initial burial depths. The effective specific gravity of the pipe could be varied by adding or subtracting dead weights. Force-displacement data throughout a test were digitally recorded and subsequently plotted. Vertical displacement of the pipe was measured with a dial gauge, and checked by external survey levels and manual embedment measurements. SOIL CHARACTERIZATION The soil used was a green-grey calcium bentonite mixture provided in barrels at an undrained shear strength of approximately 8 kPa (150 psf). To obtain the desired field undrained shear strength of approximately 1 kPa (20 psf), it was mixed with distilled water in a large mixer, then placed into large tanks and mixed again by hand to ensure uniformity. The final undrained shear strength was determined using a motorized miniature vane rotated at a standard rate (0.0143 radians per second). A water content versus undrained shear strength curve was also developed for this soil, so that spot measurements of water content could be used to check shear strength. Grain size analysis of the soil indicated that the soil consisted of a very small fraction (5%) of fine sand (> 0.074 mm) and a large fraction (53%) of clay (<0.002 mm). The liquid limit of the soil was 101, and the plastic limit was 39, giving a plasticity index of 62%. The soil was classified as a highly plastic clay, CR. DESCRIPTION OF EQUIPMENT Figure 1 shows the overall layout of the experiments. Tests were conducted in a steel tank, which had been fitted with a false bottom to reduce the total soil volume, so that its internal dimensions were 1.8 m (6 ft) square with a soil depth of 0.6 m (2 ft). The soil surface could be levelled with a removable scraper that ran along two side rails.