Dewatering a long pipeline requires special considerations regarding the design of the dewatering train components. Gels constitute a critical part of a dewatering train. This paper discusses the functional specifications that have been developed to select appropriate gels for a given application. These specifications pertain to thermal, chemical, and shear stabilities; contamination sensitivity resulting from dilution with other fluids; drying characteristics; solids content left in the pipeline upon drying; lubricity of sealing elements in the pigs; and fluid rheology to prevent fluid bypass. In addition to these properties, the gels selected should also be environmentally disposable. This paper also discusses the numerous criteria (biological degradation, ease of on-site quality control, etc.) developed for the gel selection process. Laboratory testing procedures applicable to long-term stability because of long pipeline lengths have also been discussed.
The developed gel systems have been successfully employed in the dewatering of two subsea pipelines of the Zeepipe system in the North Sea:
a 40-in. diameter, 505-mi. (814-km) long pipeline and
a 20-in. diameter, 155-mi. (250-km) long pipeline.
The performance of these gels has been excellent as verified through tests of fluid samples before and after the train travels.
As a result of this study, water-based and methanol-based gels have been developed that provide excellent performance. It has been demonstrated that it is possible to design and operate dewatering trains even for long pipelines with virtually no gas bypass.
The contributions of this paper are the functional specifications and the methodology for the development of appropriate gels for pipeline dewatering/drying applications. The laboratory-based simulations of the dilution and shearing of gels during the train travel will also be useful.
With the development of offshore gas fields in the North Sea, the use of subsea pipelines for gas transportation has increased substantially over the past decade.1 As the number of gas pipelines has increased, the need for large-diameter, longer pipelines has also increased, which has created technical challenges.2 One of the largest technical challenges is how to dewater a large subsea pipeline after a hydrotesting operation before the pipeline is commissioned for gas transportation. Dewatering is sometimes done with a dewatering train consisting of mechanical pigs and fluid compartments driven by nitrogen, air, or produced natural gas, wherever possible (Fig. 1).
The dewatering of long, large-diameter gas pipelines, using a methanol dewatering train propelled by gas, can be both a rapid and cost-effective method. However, the process should be considered a "one-shot" operation; if the operation fails, corrective action with the line filled with gas can be extremely time-consuming and expensive. The greatest risk of failure is the loss of pig sealing integrity, followed by the forward bypass of gas, which can result in hydrate formation that can partially or totally block the pipeline. Pig performance (better sealing and reduced wear) can be greatly improved by the use of appropriate gels.
