Minimising the adverse effects of water jetting

This proposal aims to facilitate the installation of piled foundations or other tubular structures through the application of water jetting.

Water jetting involves firing high pressure water out of the base of a pile whilst simultaneously lowering or pushing the pile into the ground. Water jetting significantly reduces the installation loads, shortening the construction time and reducing any noise and vibration compared to conventional hammering of piles into the ground. These benefits have the potential to reduce the environmental disturbance of construction projects, especially where excessive noise is produced offshore (an increasing issue with new very large diameter monopiles causing disruption to marine animal life), as well as to reduce the total cost of construction (especially relevant to the offshore industry due to the large equipment costs). Proper application of water jetting may even allow very-large diameter offshore pile foundations to be installed to support larger wind turbines, increasing the amount of wind power available to the UK. As a result, a range of industries in the offshore construction sector (including oil and gas and renewable energy) wish to use water jetting in practice.

Practitioners have concerns over the adverse effects of water jetting, hence the motivation for this research. These concerns arise from the large volumes of water required during construction and the effects water jetting may have on the final stability of the installation. The fluid used in water jetting has the potential to disrupt the surrounding soil structure, thus reducing its strength and stiffness. In the extreme this can contribute to catastrophic structural failures. Therefore, the research work proposed here is essential to allow water jetting to be more comprehensively understood, and its effects minimised and predicted prior to construction. This ought to increase its usage and allow the significant benefits of the jetting process to be realised.

The first part of this project seeks to understand the soil response to water jetting, through small scale tests of transparent sand-like particles and a matched transparent fluid. These materials allow the fluid-particle interaction to be imaged in the centre of the model using a laser sheet and multiple cameras. From this, the jetting mechanism will be directly observed for the first time without disrupting the surrounding boundary conditions. This transparent modelling approach will allow the applicant to develop analytical relationships that govern the fluidisation and transportation of granular materials in high energy fluid flows in order to predict the performance of water jetting in the field.

The second part of this project will see the modelling of several water jetted installations in sand. These will be similar to the installations in the transparent material but will be for larger depths in order to undertake axial and lateral load testing. The results will be compared with jacked installations without jetting to show the effects of water jetting on the final structural properties. This is essential if jetting is to be more widely used in the field.

This project has been proposed in collaboration with three industry partners (NGI, Cathie Associates and Shell) to bring advances to the arena of water jetting in the offshore construction sector. This is primarily in relation to south North Sea construction projects required to ensure continued exploitation of oil and gas reserves, and in the construction of large offshore wind turbines required to reduce the UK's reliance on carbon intensive electricity generation, both adding value to the UK economy.

The results will be disseminated across the directly linked industrial partners to ratify findings and to ensure the work is of the highest standard before presenting it more widely via industry-led conferences to maximise the impact of the research. To capture the interest of the industry, an end-of-project workshop will produce a position paper to describe the state-of-the-art in water jetting and to outline future prospects of the technology. This will ensure the research brings benefits well beyond the life of this project.

Beneficiaries from this project will principally be in the following sectors:

1. Oil and gas

For continued exploitation of North Sea reserves (which can contribute £200Bn to the UK economy) new efficiencies and cost reductions must be realised for projects to be profitable. This prospect has seen continued investment in the North Sea (such as the Culzean Field recently). Safe jetting of conductor tubes reduces construction times and therefore the cost of offshore drilling operations. This work, in seeking to produce an effective prediction of jetting and a potential control to the operation, will reduce costs further by removing or reducing the need for field testing to produce empirical design guidelines tailored for the North Sea.

2. Offshore wind

The next generation of offshore wind turbines will need larger foundations to cope with larger turbines and deeper water, but practitioners foresee difficulties in installing the required very large diameter monopiles. Working with Cathie Associates, the potential of using water jetting to install monopoles will be investigated, reducing the cost of the foundations (which accounts for 20% of the cost of a wind farm) and contributing to the planned doubling of offshore wind turbines in the North Sea.