Samenvatting

The increasing demand on renewable energy in the past decade resulted in the development of more windmill parks in the north-sea. The north-sea is an ideal sea for the development of offshore structures such as windmill parks due to its morphological benefits of being a shallow sea with soft soils consist of sand and clay. The generated energy from the windmills is collected in the transformer. The collected energy is being transferred from the transformer to the mainland through export cables. The cables that transfer the energy towards land are buried to protect them from other activities in the sea such as commercial fishing. The process of burring the cables in the seabed is called trenching. The trenching equipment use pressured water jets to create trenches for export cables that connect offshore windmill parks with the mainland. Boskalis B.V. is an international service provider in the field of dredging, maritime infrastructure and related services such as subsea, heavy transport, hoisting and installation. Their current trenching equipment is developed for soils consisting mainly out of fine-grained sand. However, the soils of the north-sea also contain sections of clay. This means that the equipment must go multiple times of the same area to get the required depth. This takes up valuable time.

The goal of this research was to improve the current trenching equipment by using alternative jet nozzle configurations. This research has been divided in a theoretical and practical part. In the theoretical part a literature study is performed to gain more knowledge about the jet nozzles and the effects of jet trenching on cohesive soils such as clay and loam. The practical part of this research has been conducted in the hydro-lab located at Boskalis in Papendrecht. These tests are conducted on clay samples where numerus tests have been done to see the effects of the different jet nozzle configurations. During the testing phase, the focus was on three variables that changed. Throughout the testing three different nozzle sizes has been used to see their effectiveness on the clay. Also, the distance between the nozzle and the plough was also changing variable. In this way we could see what happened if the water jet sprays inside, on the edge or outside the area of influence created by the plough. Finally, the pressure of the water jet varies. That way the water jet would always cut through the entire block of clay to get rid of the excess water more effectively.
Research shows that different nozzle setups can make a difference in the trench making capability. The 5mm nozzle shows to have a bigger impact on the trenching process. When checking the distance between the nozzle and plough, the smallest distance appears to have the largest effect on the force reduction. Because this nozzle is inside the area of influence, it actively removes the clay being pushed forward by the plough. The middle distance appears to have mixed results and more tests needs to be done. However, the results show that the active nozzle is on the edge of the area of influence. The plastic zone in front of the plough cannot properly develop making more resistance in the clay pushing forward.
In order to have a better conformation on how to improve the trenching capabilities in cohesive soils, the recommendation is that more test must be conducted. to have more reliable results and a more accurate conclusion. At the end possible alternatives for future researches within the field of water jetting in cohesive soils are included.