Wax removal using pipeline pigs

Abstract

The deposition of paraffin wax solids in pipelines and risers represents a continuing challenge to flow assurance in offshore installations. Wax deposits reduce product throughput, requiring increased energy expenditure to re-establish flow levels. In severe cases, wax deposits can completely block a pipeline. Preventative solutions to the problem such as pipeline insulation, active heating of pipes or chemical dosing with wax inhibitors are not always economically viable, so mechanical removal using a device known as a 'pig' remains an economical solution to the problem of wax removal. A pig is a cylindrical tool that is driven through the pipe by the flow of product, scraping deposits from the pipe wall as it travels. Despite the importance of pipeline pigging to the oil and gas industry, the effectiveness of pigs in removing wax is poorly understood and it is this problem that is addressed by this thesis. One of the first necessities in undertaking this work has been to define the mechanical properties of wax deposits. This has required critical analysis of published material on the subject of wax deposition along with practical experimentation to create representative models of wax deposits that require mechanical removal from pipelines. Previously, studies of wax removal using pigs have assumed the mechanics of the process to be adequately represented by uniaxial compression or simple shear load models. In this work wax removal is analysed using the orthogonal cutting model. This provides a more accurate description of the process as it includes the effect of material after yielding (the chip) on the net wax removal force. Experiments were designed to allow testing of the validity of the orthogonal cutting theory to the pigging process under a variety of conditions. An original contribution from this work is through experimental and theoretical results that are given context through comparison with established metal cutting theory. Through experimentation a specific cutting energy is obtained for wax removal. The results of the wax cutting experiments have identified particular differences between wax cutting and metal cutting regarding the homogeneity of chip formation. These observations have important implications in predicting wax removal forces using mechanical removal tools. Although the affect of removed wax chips on pigging forces has been neglected in theory, it is well known in practice. The fluid used to drive cleaning pigs is often used to produce a jet radiating centrally from the front of the pig intended to blast wax chips away from the pig body, avoiding formation of a 'plug' of wax ahead of the pig. In this study a novel variation of this process in the form of an annular bypass jet is experimentally studied. A semi-empirical model of wax removal using an annular bypass jet has been developed and empirical constants obtained to allow prediction of removal rates for different waxes under various conditions. The new model introduced here allows balancing of pig velocity with wax removal velocity so that a non-contacting wax removal system is obtainable. The bypass-jetting model has been validated using a full-scale trial of the process by industrial sponsors.