Pipeline Trenching Using Plows: Performance and Geotechnical Hazards

Abstract Based principally on experience with prediction and subsequent back-analysis of the performance of pipeline plows, this paper presents an overview of plow performance and the various geotechnical hazards that are particularly relevant for pipeline trenching. Performance models relating plow tow force, share depth and speed are presented theoretically and calibrated with data from actual plowing experience. The major geohazards for pipeline trenching are then discussed with the aid of case histories covering soft soils, cemented soils, fine sands and trench instability. Introduction Pipeline plows are effective tools for trenching pipelines provided the rates of progress can be assessed reliably and hazards identified before trenching is started. The purpose of this paper is to provide guidance for predicting plow performance and mitigating the risks inherent in all trenching operations. Only the performance of pipeline plows is addressed in this paper, based on the first authors experience of many trenching projects. A schematic of a typical heavy plow is shown in Figure 1. The basic characteristics include a share which self-corrects its own depth to follow the skid settings (long-beam plow principle), a V-shaped share and trench (typically about 35 degrees to the horizontal for stability in sand) with a shorter foreshare ahead of the main share, a weight on the order of 120t and a tow force capacity of about 250t steady pull. Clearly, performance will differ according to the individual design and operational procedures. Features such as water injection may make a considerable difference to performance in certain soils. The performance results presented in this paper are based on more that 30 sites in which trenching was performed and results analyzed by the first author for Northern Ocean Services between 1990 - 1994. The plow weighed about 120t in water. Water injection was not used on any project for which data is presented here. Plow performance models - theory Cohesive soils. A plow is in contact with the soil at three points - the two skids which run on the seabed and the share. Resistance to forward movement derives from these three points. Plows move quickly enough for undrained shearing to take place at these contact points. This results in relatively simple equations for modelling the resistance to plowing. The plowing resistance, or tow force (F) required to advance a plow in cohesive soils (clays) can be written as: F = Fw + Cc Su D2 (1 + Cd V) [1] Where Fw is the adhesion of the underside of the skids and share to the soil during plowing, Cc is a coefficient similar to a bearing capacity factor, Su is the soil undrained shear strength, D is the trench depth, V is the plow speed and Cd is a coefficient relating the strength of the soil at normal shear strain testing rates (Su) to the strength of the soil at plowing rates of strain. Two comments may be made regarding Equation [1]. Firstly, it is quite rare (except when trenching in soft clay areas) for both the skids and share to be in the same material.