The Use of Gelly Pig Technology for Removal of Stuck Mechanical Pigs
Authors: Nesbitt D.P. 
Published: October 2008

As mechanical cleaning pigs have evolved since their first introduction, so has their tendency to bind during travel. Most pig hang-ups are of two types. Stopping of the pig can be due to mechanical deformities of the line and/or excess buildup of material in front of the pig, where the driving pressure may exceed the pressure rating of the line. Extreme wear of the pig components andlor accumulation of debris in front of the device, where the driving fluid is allowed to bypass, can also stop a mechanical pig. This paper outlines the innovative use of gelled fluids, "Gelly Pigs" for the removal of stuck mechanical pigs in pipelines due to bypass of fluids. A recent successful offshore application of this technology is described, including details of the types of mechanical pigs removed, as well as procedures used for their removal.

INTRODUCTION

Gelly Pigs were developed in the late 1970's to overcome limitations of mechanical pigs in cleaning and batching fluids in pipelines. Comprised of a gelled solvent and usually water-based, these gelled pigs are special formulations designed with specific rheological characteristics, exhibiting mechanical integrity while retaining the properties of a fluid. The characteristics of these gelled fluids provide a method of suspending debris in front of mechanical pigs, so as to help prevent debris buildup and bypass of the driving fluid. Gelled pigs also provide a method for more effectively separating two fluids over long distances, such as used to batch different products or more effectively remove condensate from a gas line. Continued developments of Gelly Pig technology include a gelled formulation for efficiently laying down a corrosion inhibitor, as well as gelled alcohols/solvents and methods for use in efficient dewatering of pipelines where water intrusion can be detrimental. Mechanical pigs are commonly used by the pipeline industry for batching various products and cleaning deposits from pipelines, and are available in a wide array of configurations. These pigs move by means of a differential pressure across their sealing surface; the efficiency of the sealing surface varies depending on the type of mechanical pig used. This driving pressure differential can be lost due to worn cupsldiscs andlor accumulated debris, often resulting in binding of the pig and bypass of the driving fluid. The consequences of a stuck mechanical pig in a pipeline can be enormous. If the mechanical pig can not be dislodged through continued andlor reverse pumping andlor the flow and pressure restriction caused by the device is too large, the entire pipeline must be taken out of service, and lost production costs mount while the pig is located and removed. If the line is located offshore, the magnitude of removing this stuck pig dramatically increases. It is due to these limitations of mechanical pigs that many of the Gelly Pig fo'rmulations were developed. When a mechanical pig is first stuck in a line, very often a second mechanical pig is run to try and reestablish the driving differential pressure. Sometimes this will work, but often the second pig becomes stuck due to the same circumstances as the first, and there is an added risk of plugging off the line completely.