Subsea Tracked Trencher Mobility

Abstract

The deployability, regarding mobility, of Boskalis trenchers, is currently determined by practical knowledge. The lack of theoretical knowledge means that risk assessments in the tender phase are less accurate. This reduced accuracy has two negative consequences. First, the trencher may fail. Secondly, the safety margins can be too large. Both consequences result in additional duration and costs. This research considers two ways of failure. The first one is slipping; as a result of this, the track does not have enough traction, which causes the tracks to turn while the trencher does not move forward. The slipping can happen in two ways. The first way is due to soil failure with the grousers fully penetrated. This results in the trencher digging itself in. The other way is that the grousers are not penetrated and the trenchers slips due to the tracksoil interaction. In order to prevent the trencher from slipping, horizontal stability between the soil and tracks should be obtained. The second way of failure is that the trencher sinks into the soil. Vertical stability between the applied pressure and the bearing capacity should be achieved to prevent this kind of failure. In this research a model has been developed that can approximate the operating range of different trenchers. With the help of these operating ranges, the model can make risk assessments in an early phase of the project. There will be little knowledge about the soilspecific parameters and the exact external conditions in this phase. Should there be a risk in the area of mobility along with the project, the model is also able to provide a more indepth analysis. In addition, the model aims to apply to different types and sizes of trenchers. The research answers the following question: ”Which different operational environments and soil conditions have a critical influence on the deployability of a subsea tracked trencher regarding the vertical stability and horizontal stability of the soil¬track interaction system? The model considers the trencher’s mobility in terms of vertical and horizontal stability, where the axes rotate with the slope. Meyerhof’s effective area method approximates the vertical stability. This method is used to take into account the eccentric loads. A traction calculation model approaches horizontal stability. Based on the penetration depth of the individual elements of a track, a shear mode is determined. The available traction force per element can be calculated using these shear modes using the Mohrcoulomb shearing theory. With a case study for the CBT2400, the research question is answered. In this case study, we first look at the different external processes that influence mobility. In addition, the various soil parameters and trencher dimensions are examined. Finally, the results of this study will be compared with two reallife projects. The results show that the mobility of the trencher is better in granular soils than in cohesive soils. In cohesive soils, the trencher’s mobility depends on the external factors that change in the driving direction, like pitch slopes and currents that have a frontal impact. In cohesive soils, the sensitivity is a vital soil parameter besides the undrained shear strength. Since the sensitivity is not included in the soil research, it is recommended to do it in future projects. In granular soils, external factors from all angles must be taken into account, like pitch and roll angles and currents from all directions. With granular soils, especially the relative density and internal friction angle are essential; in addition, the grain size must be taken into account. The grain size influences the permeability of the soil. The permeability, together with the relative density, determine if contractancy and dilatancy will take place in the soil. It could be interesting to change the grouser and track dimensions to improve the trencher’s mobility. However, this depends on the circumstances in which the trencher would have to operate. The model was validated using two cases involving the Borssele and Moray East projects. An important conclusion is that the model can be used to estimate the potential risk areas reasonably. However, it is recommended to test the model in predefined circumstances and soil parameters.