Performance of perforation drilling in easing spudcan-footprint interaction issues in moderate clays

© Copyright 2016 by the International Society of Offshore and Polar Engineers (ISOPE).This paper reports a measure for easing spudcan-footprint interaction issues, with the efficiency of perforation drilling tested through model tests carried out at 1g. The soil conditions tested simulate a moderate seabed strength profile close to the mudline, with undrained shear strength of 25 kPa. The most critical reinstallation location of 1D (D = spudcan diameter) was investigated. An existing footprint depth of 066D was considered, with the evidence from a half-spudcan test. A 'shower head' apparatus was employed to perform perforation drilling operation on an optimized pattern Three spudcan tests were conducted on un-perforated footprint, perforated footprint and level ground. The spudcan was rigidly connected to a leg instrumented by three sets of bending strain gauges and one set of axial strain gauges to record the vertical load applied and the bending moment generated during testing. Free lateral displacement of the leg was allowed during the installation The removal of soil (through perforation) inside the spudcan perimeter, with an area of 9% perforated, reduced the induced maximum moment and horizontal sliding distance by 39% and 23~52% respectively. This was supported by the observed failure mechanisms: catastrophic sliding failure turned to a near vertical penetration.