Backfill Improvement Techniques - Optimising Pipeline Burial Solutions
Authors: Cowie M. , Finch M. 
Published: October 2008

Backfill is frequently required on small diameter, steel pipelines for thermal insulation as well as a measure to mitigate upheaval buckling. Current industry practice for mitigating upheaval buckling is to rock dump on susceptible sections of pipeline routes. This process can lead to excessive and unnecessary remedial costs due to conservatism in backfill parameters.

Methods of increasing the download on pipelines will reduce or even eliminate the amount of rock dump required. Improvement of backfill material and optimisation of current upheaval buckling models provide a number of options for increasing pipeline uplift resistance.

Coflexip Stena Offshore has undertaken extensive research into upheaval buckling of pipelines and has carried out associated studies into backfill improvement techniques. The research incorporated a full-scale laboratory-testing programme that assessed the uplift resistance behaviour of backfill soils and rock dump material. The backfill improvement study reviewed traditional land based ground improvement techniques and their potential transfer to subsea applications. Potential innovative solutions were identified.

Improvement of backfill material and optimisation of upheaval buckling models will result in cost-effective solutions for the mitigation of upheaval buckling.

Introduction

Small diameter rigid steel pipelines are frequently used as in-field tiebacks to transport high pressure and high temperature hydrocarbons.

Since the distances over which these tiebacks are placed are relatively short, in comparison to export pipelines, the problem of upheaval buckling can be a significant concern for pipeline designers, installation contractors and field operators alike. Measures to mitigate upheaval buckling typically consist of trenching and perhaps backfilling the pipelines. One purpose of backfill material is to provide download over the pipeline to resist the vertical forcesassociated with temperature and pressure induced upheaval buckling. In areas where the backfill cover is not of sufficient thickness to resist upheaval buckling, additional rock dump is typically placed over the pipeline. In cases of highly sensitive pipelines blanket rock dump is normally required. However, it should be noted that whilst trenching is often required to provide physical protection and perhaps also on-bottom stability, the additional of backfill and rock dump inevitably leads to increased cost.

In order to reduce such costs and to fully understand the failure mechanisms associated with upheaval buckling and cyclic ratcheting, the Coflexip Stena Offshore Group (CSO), has undertaken extensive research and development programmes.

This paper describes typical trenching and backfilling tools, summarises the findings of the detailed upheaval buckling research programmes and describes potential backfill improvement techniques, which have been investigated by CSO, that could be employed to improve the download provided to pipelines.

Trenching

Small diameter rigid pipelines are usually trenched by ploughs or ROV based jet-trenching tools. Each of these methods has benefits and restrictions with respect to reliably providing backfill cover for mitigating upheaval buckling. Ploughs Modern pipeline ploughs are capable of trenching rigid pipelines post-lay in all soil types and even rocks where they are weak or heavily fractured