Investigation of the critical specific gravity for cables and pipelines during various backfilling and installation processes

Abstract

Embedment of pipeline and cables in the seabed is achieved to protect them from issues such as loading by currents, damage from fishing trawler vessels and to provide thermal insulation. Depending on the type of soil and on financial and operational constraints, the embedment of pipelines/cables can be achieved using either ploughing or jet trenching with remotely operated vehicles.

In this thesis the issues arising while embedding pipelines and cables using jet trenching and backfill ploughing were investigated by means of scaled physical model tests. The purpose of the investigation was to optimise the specific gravity (SG) of pipelines and cables to minimise flotation or movement during installation. The issues under considerations were the flotation of pipelines/cables in high moisture content fine grained materials due to jet trenching and the uplift of pipelines/cables due to backfilling in granular materials at different speeds.

The effect of embedment depth and trench width on the soil resistance to the flotation of pipelines/cables were studied. Two types of test, free flotation and pull-out tests were used for validation at two different model scales. To summarise the results in a unified framework a relationship between undrained shear strength and liquidity index was calibrated using two fine grained materials at very high moisture contents. While the existing literature lacks a criterion to define flotation, a standard framework to quantify the mobilised resistance as a function of the movement of the pipeline/cable was developed to represent uplift displacement due to flotation. This was required as no abrupt flotation event actually occurred. The pipeline typically moved and then stopped as greater soil resistance was mobilised. This allows customisation of the flotation criterion to different industry needs (i.e. varying the allowable pipeline or cable displacement). Design charts and tools are provided to estimate undrained shear strength at high moisture content and resistance provided by soils at different embedment depth and trench width. These can be used to determine the lowest safe SG of pipelines/cables which will not exceed the maximum allowable movement.

The effects of backfilling velocity on soil and pipeline or cable movement and the forces exerted by the soil on the pipeline/cable were also studied in separate testing. Two underwater plain strain model tests, at 7.5th and 50th scale, were employed to verify the scalability of the results. The effect of backfill ploughing in granular material was investigated using accelerometers and pore pressure transducers to quantify the magnitude and duration of forces on the model pipes. The soil flow behaviour and mechanisms were monitored via high speed video recording. The aim of the tests was to provide characterisation of the backfilling mechanism and quantify the forces acting on the pipe, whilst investigating their velocity dependency. The type of backfilling mechanism and the force acting on the pipe were found to be dependent on the backfilling velocity and scalable via the Froude number (Fr) calculated using plough velocity and pipe diameter. The changing point was found to be at Fr=0.1. The pore pressure measurements confirmed the dependency on the ploughing velocity of both the backfilling mechanism and the forces acting on the embedded product.

Results from the model backfill ploughing tests were interpreted and used to calibrate an empirical correlation of upward forces as a function of backfilling velocity. The results of the pore pressure instrumentation were used to predict the duration of the uplift force action. These correlations allow the use of the model test results to determine the mechanical properties of pipelines/cables required to achieve correct embedment depth during installation to minimise movement during installation. Estimation of the most influential mechanical properties of pipelines/cables being installed is provided via simple structural analysis implementing Euler beam equation to represent the pipepline/cable being backfilled.

Date of Award	2020
Original language	English
Awarding Institution	University of Dundee
Supervisor	Michael Brown (Supervisor), Andrew Brennan (Supervisor), Toby Powell (Supervisor) & Howard W. Chandler (Supervisor)