Design of a constant diameter retractable Jet-ring with inside-out jetting combined with a Vibro-hammer

Abstract

Reducing noise pollution during offshore monopile installation is crucial for marine life. Current installation methods create too much noise or do not have the certainty of reaching the required depths. By using a Vibrohammer combined with a Jet-ring this problem could be solved. A Jet-ring is a tool to reduce the sand induced inside shaft friction by fluidizing the inside of the monopile during installation. To make this concept a viable option, the Jet-ring should be retrievable as a reduction in cost but also in materials. The aim of this thesis is to design a retrievable Jet-ring for a variable-diameter monopile without changing the diameter of the Jet-ring. For this goal to succeed the focus will be on the possibility of using inside-out jetting. This design would be an addition to the Vibro-jet® of GBM Works, which is not retrievable yet. Keeping the diameter of the Jet-ring constant keeps the design simple, reducing the chance of failure inside the monopile where the Jet-ring has to withstand harsh conditions under a lot of water and sand.

After creating a complete design of the retrievable Jet-ring, an experiment was designed to prove the concept. The designed concept is a Jet-ring which is small enough to fit through all sections of the monopile. In order to reduce the shaft friction from a distance, the nozzles are pointed outwards. This way the nozzles first have to erode a layer of sand before reaching the monopile wall. For comparison, 4 different water inlet concepts were tested, including the designed Jet-ring. All additional concepts were chosen to confirm and compare certain
aspects of the Jet-ring. The test setup included several cameras, a flow and a pressure meter. The concepts were placed at the bottom of the tank with a sand bed above it during the experiment. A repeatable experiment was created by keeping the relative density equal which was done by keeping the starting bed height constant combined with an equal weight of sand at every experiment. At the top of the sand bed, a bolt was placed which would fall through when the sand was fluidized. After execution, the height of the sand bed was measured
when the bolt fell through. In addition to the bed height, the duration until fluidization was measured. For all concepts, three different flow rates were executed minimally three times per flow rate to reduce outliers.

The processed results can be divided into two categories: visual and numerical results. The visual results contained the flow patterns and other mechanisms helping to fluidize the sand bed. By looking at the flow patterns, explanations were found for the numerical results. The numerical results are the bed height and duration until fluidization. Looking at the results, the importance of having pressure behind the flow was shown. Comparing the base case, with a homogeneous water inlet, to the other concepts jetting with ten nozzles showed that fewer nozzles with a higher pressure resulted in faster fluidization. Furthermore, the importance of prolonging the erosion mechanism is shown. A concept directly jetting into the side of the tank lost a lot of energy in the
flow resulting in almost double the fluidization time. While the energy dissipation against the wall of the tank is substantial, energy dissipation due to erosion seems relatively small. This was concluded by comparing a concept that jetted from the centre of the tank and a concept that jetted from the side. The last conclusion makes the designed tool using inside-out jetting a viable option for the retrievable Jet-ring.

While the results are promising, some remarks should be made on the experiment and the design. While congestion by sand could occur during the experiments the results still give a relevant view on the capability of the concept to reduce the inside shaft friction of the monopile because the same mechanisms such as erosion and fluidization are used to penetrate the sand during the congestion as during monopile installation. Furthermore, because of the smaller diameter of the tank, the complete inside was fluidized. This could be not the case during monopile installation as sand could accumulate at the centre of the monopile. However, this possibly accumulating sand is not sure to influence the shaft friction as the sand against the monopile wall that induces the shaft friction is fluidized or weakened. These additional studies could strengthen and prove the concept of inside-out jetting further in the future.