Removal of a dumped rock cover with a low pressure jet

Removal of a dumped rock cover with a low pressure jet

Schoen, J.J.

Van Rhee, C. (mentor)
Talmon, A.M. (mentor)
Nobel, A.J. (mentor)
Verheij, H.J. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

2014-10-22

Pipelines and cables on the sea bottom are usually protected by a cover of dumped rock. In case of maintenance or removal works, this cover has to be removed in order to expose the pipeline. This removal can be performed by means of a mass flow excavation process, created with a submerged jet flow. However, the exact erosion processes of a gravel bed with a low pressure water jet are not yet fully understood; as a result, the production cannot be accurately predicted. The consisting theory about jetting in gravel is very limited. Most jet operations are performed in sand or clay, while the majority of the theory about erosion of gravel is based on relatively low energetic situations, such as rivers. Therefore, a literature study based on different aspects of hydraulic engineering is performed. The aim is to combine the theory of the various specialisations into one study, containing all relevant information. A preliminary test series with a scale of 1:30 is performed in order to get insight in the erosion processes of a vertical jet, horizontally moving in the same direction as the pipeline. This method is commonly used with jetting operations in sand, and forms therefore a logical starting point for the experiments. However, tests shows that this working method results in a low production and seems therefore ineffective for the removal of a dumped rock cover. Three test series focussing on alternative working methods are subsequently performed to determine the most feasible removal method. The trail direction of the nozzle, the jet angle relative to the bottom and the design of the nozzle are varied in these tests. These tests show that, depending on the flow and trail velocity, a trailing jet can behave as either a deflecting or a penetrating jet. Both jet behaviours have a different working method that is effective. The most feasible working method turns out to be a horizontal, penetrating, jet with a high hydraulic power, slowly moving along the pipeline. An erosion model is created in order to model the penetrating behaviour of a jet. It is found that the generally used pick-up function of Van Rijn is not applicable to a situation with a jet flow eroding grains with a large diameter. The pick-up function of Fernandez Luque results in more accurate outcomes of the model, although the modelling of the settling of the stones does still not correspond with the observed processes. A dimensionless erosion parameter Epen is defined as a function of the initial flow velocity, stand-off distance, jet diameter and trail velocity. This parameter can be used to predict the penetrating behaviour of a trailing jet.

jets
mass flow
dumped rock
erosion

http://resolver.tudelft.nl/uuid:b4102780-99e4-48f5-bc7e-b89cbc390989

2016-10-22

Student theses

master thesis

(c) 2014 Schoen, J.J.