The ability to predict offshore pipeline plough performance (e.g. tow forces and progress rates) in a variety of ground conditions is an important part of project planning. This may be estimated based upon experience from nearby locations or similar ground conditions, but existing empirical plough force prediction models are available and recent updates have been made. With the increasing use of digital data, it is hoped that greater reliance can be placed on these models and this process can be automated and integrated into Geographic Information System type applications. This paper uses real offshore plough data from three trenching projects in sand to compare plough performance prediction models.
To allow appropriate planning for offshore pipeline ploughing installation campaigns it is necessary to reliably predict tow forces and advance rates prior to undertaking works, or at the tendering stage. This is not only important for planning, but also in selecting appropriate tow vessel support and deciding the approach to ploughing i.e. can the ploughing be undertaken in a single pass whilst minimising tow forces and maintaining adequate progress rates, or are further passes required. A multi-pass approach may be adopted where although the design trench depth could be achieved in a single pass, trenching is undertaken in two or more passes with an initial shallow trench depth followed by trenching to the final design depth (Machin, 1995). A multipassing approach may seem counterintuitive because of the associated additional plough handling time and the requirement of pulling the plough across the seabed more than once, but this can be more efficient and save time when compared to a single pass.
The potential benefits of a multipass approach are demonstrated by Eq. 1 (Cathie and Wintgens 2001) where the passive pressure component of tow force (F) is shown to increase with depth (D) cubed and there is a rate dependent term or potential for tow forces to increase with increasing plough velocity (v). This latter term is dependent again on the depth of the plough share but also on the soil permeability and its potential to dilate (relative density, or unit weight γ and depth of ploughing) during shearing, resulting in low pore pressures and increased effective stress (Lauder et al., 2012).