The influence of incorporating a forecutter on the performance of offshore pipeline ploughs

Abstract

Installation of offshore pipelines in the seabed can be efficiently achieved using pipeline ploughs. Increased efficiency may be achievable through incorporating a smaller forecutter in advance of the main plough share. Currently guidance is limited and conflicting as to the advantages or disadvantages of incorporating a forecutter. To investigate the effect of forecutter inclusion model tests were undertaken at 1/50th scale under laboratory conditions in sand beds prepared at different relative densities in both dry and saturated conditions. Dry sand tests were used to determine the effect of the forecutter on the static or passive components of plough tow force. The currently adopted passive pressure coefficient (Cs) did not appear to vary with relative density to the same degree as previously suggested and the forecutter increased the magnitude of the passive or static resistance to ploughing. Saturated tests were used to determine the effects of the forecutter on the rate dependant component of ploughing resistance and allow verification of a dimensionless form of rate effect representation. The forecutter acts to reduce the rate effect component of plough tow force in both fine sand (low permeability) and to a lesser extent in medium sand (higher permeability). In fine and silty sands, however, incorporating a forecutter would seem highly beneficial at all ploughing depths and soil densities but in medium sand (higher permeability) the benefits of incorporation are limited to an operating window at shallower trench depths and lower relative density.