Shear strength of saturated sand-steel interfaces: Geotechnical issues found at landfall operations

Abstract

Landfall operations are conducted for connecting an offshore pipeline with process facilities on the shore. During a landfall operation an offshore pipeline is pulled by means of a steel wire rope on the shore with a velocity of 7 cm/s. During this process the steel wire rope interacts with the soil located on the seabed and on the shore. Allseas Engineering BV has a long experience on landfall operations and it was occasionally noticed that the steel wire ropes were buried beneath sand dunes which resulted from sedimentation and wave actions. In these instances excessive pulling forces were required to mobilise the steel wire ropes. A prototype experimental setup was developed for simulating the pulling process in medium scale. Physical modelling involved the pulling of a steel element through saturated sand with a relative density of 80%. The aim of this apparatus was to examine the shear failure mechanisms that develop in saturated sand while a steel pipe, with a significantly rough surface, is pulled through it. The main focus points of this study were the peak pulling force of the steel pipe and the change of pore pressures around the steel pipe's circumference during the pulling process. The latter two were examined with respect to six burial depths (0 - 0.31 m) and three pulling velocities (1, 4 and 7 cm/s). It was observed that at every test a momentary decrease of pore pressures was taking place around the steel pipe during the pulling process. A peak was always recorded at the same time as the peak pulling force and this was attributed to the tendency of sand particles to dilate around the steel pipe. Undrained loading conditions were caused by the high pulling velocities and low permeability of the soil. Therefore, dilation was restrained by the pore water and consequently tensile pore pressures developed which increased the shear strength of the soil, momentarily. Burying the steel pipe at different depths influenced, as it was expected, the peak pulling force due to the increase of the vertical effective stresses. In addition, the peak decrease of pore pressures was found to increase in magnitude while the burial depth ranged from 0 to 0.09 m and this was an unexpected event as the tendency of the soil to dilate was expected to be restrained. The magnitude of peak pore pressure decrease was also found to reduce at burial depths ranging from 0.09 m to 0.31 m due to the increase of vertical effective stresses that restrained the soil's tendency to dilation. The effect of pulling velocity to the peak pulling force and the peak pore pressure reduction values was also examined. The latter were found to increase linearly with the increase of pulling velocity, at each burial depth that was tested. Also, the peak pulling forces were found to increase linearly while comparing tests at the same burial depth, conducted with different pulling velocities. In addition, the increase of pulling velocity caused a linear increase on the stiffness index of the test specimens. The scientific significance of the results of the current study can assist to performing landfall operations in a more efficient way. It is recommended that during a landfall operation the initiation of the pulling of the steel wire rope should take place at the lowest possible rate. As a result, the maximum pulling force will be minimised and the pulling velocity can be increased gradually once the steel wire rope is mobilised. Moreover, the findings of this study can be useful for the (un-)installation of (offshore) piles, sheet pile walls, soil nails and dredging operations on saturated sands.Civil Engineering and GeosciencesGeoscience & EngineeringGeo-Engineerin