Optimising Vibrojet® Performance for Offshore Installation

Using 2D Lab Tests and Particle Recognition Software

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Abstract

Offshore wind energy is a pivotal element in the renewable energy transition and achieving net-zero emissions by 2050. Most offshore wind turbines require the installation of a sturdy foundation called a monopile. The current method of installing monopiles offshore is known as piling, which involves driving large steel cylinders into the seabed with a hydraulic hammer. One of the major problems with this method is that it generates high levels of noise, which travel through the water and are harmful to marine life. Therefore, several European governments have introduced laws that regulate the amount of noise that may be emitted.
Multiple solutions have been developed to reduce the amount of noise that is generated but they are costly or negatively impact operational speed. Therefore, GBM Works, a startup in the offshore wind industry, is developing an innovative method for installing monopiles known as the Vibrojet®. The Vibrojet® combines both a Vibrohammer on top of the monopile with a jet at the bottom. The jet aims to fluidise the sand inside the monopile to reduce the friction on the inner shaft. This not only reduces the amount of noise emitted but also increases the installation speed of the monopile and may reduce the required pile dimensions.
This research aims to optimise the Vibrojet® performance during offshore installation by reducing shaft friction as much as possible while minimising jet flow. This approach maximises the installation speed of the monopile while reducing the capacity requirements for all parts of the Vibrojet® system. When installing a monopile while jetting, a soil skeleton forms (soil plug) in the middle of the pile while all sand particles near the inner shaft gets fluidised. The displacement of the surface of the soil plug has been assessed in this research to estimate the plug shape and optimise the performance of the Vibrojet®.
At Deltares a test setup of a soil container was constructed to imitated a 2D version of the inside of a monopile during Vibrojet® installation. The front of the container was made of see-though Perspex to enable analysis of the processes inside the pile. Inside the container holding 1,500 kg of sand, 0.3\% of the particles were ultraviolet (UV) coated to enable tracking. Visual software was written to track these particles with a camera, allowing for the measurement of flow velocities and visualise the surface of the plug shape.
It was discovered that the equation for plug surface displacement overestimated the displacement observed in the laboratory tests. By analysing the results from various installation settings, discrepancies in the equation were identified. The following improvements were recommended to enhance the accuracy of the equation:
• Addition of a seepage force term to the equation
• Addition of separate term for sedimentation effect
• Factor for the return flow of particles
• Influence of the slope angle on flow erosion
A relationship was discovered indicating that the optimal flow rate for Vibrojet® installations can be determined by the total volume of sand that needs to be fluidised. A model was proposed to optimise the performance of the Vibrojet® by assessing the displacement of the plug and utilising this relationship to calculate the connection between the flow rate and the volume of the fluidised zone.

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