3 Post-trenching Technologies
This is the most common method of pipeline trenching though little work has been done to improve this technology. Basically it uses the compressed air or water to cut or liquefy the seabed soils around and in front of the pipeline by the surface pumping equipment and passes through the nozzle at the end of horse to create high pressure intensity. Excavated soils are removed by air lifts, water eductors or submersible pumps.
But there are some limitations of this method in deep water due to the adverse effects of quasi-static and dynamic responses of the umbilical horses. Because of this long and bulky horse in deep water, it is extremely hard to control the trenching location which might cause damage to the pipe, sled or horses. Typical jet trenching operation and jet barge are illustrated in figure 8 and figure 9 respectively.
Conventional jetting has used compressed air but it was ineffective in removing soil in deep water due to the high hydrostatic pressure. High hydrostatic pressure reduces the air volume and thus provides small lifting force on bottom. So, in modern jetting machines, water jet employs for cutting.
In order to stabilize the jetting machine, skids that slide on the sea bed or buoyancy tanks on a machine that rolls along the pipeline can be used. These methods are further referred as a jet sled and a jet machine respectively and discussed later in this chapter.
Encountered soils along the pipeline play a crucial role when determining the trenching speed and jetting pressure. In cohesive soils, trenching speed is a function of ratio of jetting pressure to undrained soil strength. High speed trenching is used for larger ratios and vice versa. Normally, this trenching speed is within few meters per minute.
In the cohesionless soils are generally easy to cut and requires quite low jetting pressures. But it has some serious limitation because of the cohesionless nature. In order to achieve trench slope stability and prevent the infill behind the trenching machine, a shallow trench slope should be made. Further, excavated soils should be deposited far enough away so that they do not infill the trench before the pipeline installation and use the air lifts at 8-40 m depths and the water eductors at depth lesser than 8m.In very shallow depth(<4m),a small pontoon barge can be used to remove this excavated soils. In deep water, high trenching speed and low number of passes should be required.
3.1.1 Jet Sleds
This is an early version of jetting machine and slides along the seabed on skids. This skid width should be enough to make the required trench width and generally wider skids use for cohesionless soils to make a shallow wider slopes. This machine is relatively high weight and required a large pull force. In order to transport and handle a large barge with a large crane is required so that has enough deck space where the large pumps can operate to give more power for cutting. Therefore, jet...