Pipeline and cable plough stability is an important issue that must be considered when planning trenching operations in soft clays. Of a plough has insufficient bearing capacity, the result can be over deep trenching, high out-of-straightness, a poor quality trench and higher than expected tow forces. Excessive sinkage can also render pipeline or cable loading impossible. Plough stability is mainly affected by the plough characteristics, the applied forces and the undrained shear strength of the soil. When operating correctly, the skids and/or share are in state of continuous sliding failure and do not cause a bearing failure of the soil. This paper discusses the mechanics of trenching in soft soil, and presents examples of how the applied loads and load distribution on the skids and share can be calculated and compared to the failure enveloper to determine satisfactory ploughing conditions. A basic framework for rational selection of factors of safety is discussed.
Pipeline and cable plough stability is an important issue that must be considered when planning subsea trenching operations in soft clays. If a plough has insufficient bearing capacity the result can be over deep trenching, high out-of-straightness, a poor quality trench and higher than expected tow forces. Furthermore, upon plough deployment, excessive sinkage may endanger a pipeline or render the product loading impossible. Although this paper concentrates principally upon pipeline ploughs, the concepts can also be applied to cable plough (Section 6).
A pipeline or cable plough typically operates using the ‘long beam’ plough principle. This means that the cutting depth of the plough share, which excavates and displaces the soil to form a trench, is controlled by the height of skids which run ahead of the share. The plough share and body can freely rotate relative to the skids, which are designed to slide along the natural seabed without sinkage. The principal components and configuration of a typical pipeline plough are illustrated in Figure 1. A typical cable plough operates in a similar manner.
In order to retain proper control of the plough it is important that the skids do not experience significant sinkage. Excessive sinkage of the share must be avoided to obtain a high quality trench. The plough is considered ‘stable’ if both the skids and the share operate as intended, either at seabed level for the skids or at a controlled dept below seabed for the share.
Plough stability is mainly affected by the plough characteristics (submerged weight, skid or share dimensions), the applied forces on the skids and share, and the bearing capacity of the soil under undrained conditions. In order to ensure successful operation of plough (i.e. stability) it is necessary to consider the two main operating scenarios:
plough deployment on the seabed and pipeline loading; and
These two operating scenarios are discussed further. Illustrating the difference between them.
In order to deploy the plough over a pipeline, the share is opened and the plough is lowered over the pipeline.