Post-trenching with a trailing suction hopper dredger

It is common practice to protect subsea pipelines by embedding them into the soil. Trenches can be made before or after the pipelines have been laid. In the latter case, the excavation process is called post-trenching. The essence of post-trenching, as handled in this thesis, is erosion of sand by a water-jet. The literature study focused on the processes of jets and erosion. A lot of research has been done in the field of water jets and useful information is widely available. Nevertheless the available information on the subject of impinging jets is rather limited and the validity remains questionable. Water jets used for post-trenching create high flow velocities for which the traditional erosion equations are not valid. Therefore use is made of a special set of equations for high speed erosion. With the information provided by the literature study a description of jetting in sand was made. The known processes were arranged resulting in a set of equations. Following the rules for scaling the set of equations was converted into a properly scaled model. Preliminary model tests were conducted to observe the jet-process and narrow down the possible jet angles. These preliminary tests were followed by scale model tests to determine the erosion depth for different nozzle angles, flow velocities etcetera. A numerical model was developed to simulate jet-induced erosion. Since the erosion equations, making use of the average flow velocity, could not model the erosion behaviour of a jet, a turbulence term was introduced. The results of the simulations were compared with the model tests. Though the numerical erosion model showed promising results, it could not be validated due to a lack of data. The most important conclusions are that soil can be eroded to the desired depth, a data-set has been created and much insight is gained with respect to the post-trenching process. Last but not least, a numerical model was made that can prove to be useful after better validation.Hydraulic EngineeringHydraulic EngineeringCivil Engineering and Geoscience