Samenvatting

When a pipeline is laid on the seabed it can be exposed to hazards of ship anchors, large fishing gear or ice keels. To prevent the pipeline of being damaged, the pipeline can be trenched. Trenching is a technique where the pipeline will be buried in the seabed to a certain depth. This can either be done before or after the pipe lay process.

This graduation project ‘Design of trencher main structure’ contains the design and development of the complete structure of the new trencher. This structure has to meet the requirements for both trenching depths and needs to be strong enough to hold different equipment.

A modular structure is developed which can be adapted for both scopes. Pipe profiles of DIN Ø273*6.3-mm are used for the complete structure. The structure exists of a main frame, skid frames and skids. These components are connected to each other with the use of welded slip-on flanges. The main frame exists out of two frames for the small structure and three frames for the large structure when a middle frame is added. The increased length is needed to install two jet swords. The second jet sword will keep the soil fluidized for a longer time, giving the pipeline more time and distance to lower into the trench.

The small structure is supported by two skids and two skid frames, and the large structure with four skids and four skid frames. For the large structure the skids frames are wider, which increases the width of the structure. This is needed to overcome the width of the trench which can collapse during trenching. For the inner pipe profiles of the skid frames a profile of DIN Ø2191.1*8-mm is used.

An Inventor model is made to see the interaction between the structures and the installed equipment. The complete structure is modelled including the jet swords, guidance frames and all the flange connections. By this, a parts list for both structures is made. The list is used as a weight-input for the FEM analyses and calculations.

Different load cases have been made to determine the loads which the structures are subjected to. The worst case scenario is when the structure is deployed by the crane underwater and the heave compensation will fail. This causes the structure to accelerate with the same motions as the tip of the vessels crane of which the trencher is deployed with. Besides the extra acceleration, water drag will occur. This will increase the loads on the structure.

FEM analyses have been made for both structures above and under water. For the small structure an extra weight of four tonnes is added to simulate the weight of submerged pumps in case they will be used. For the large structure a weight of eight tonnes is added, since there are two jet swords. The results show that the maximum stresses will stay below the allowable stress in all the load cases. For the longer skid frames an extra stiffener is added to keep the stresses acceptable.

Hand calculations have been made to check the FEM results. They confirm that the stresses will be acceptable and show that the bending stress in the large skids frames will be too high without stiffener. This confirms the need for a stiffener in the intern connections of the large skid frames.