Advances in subsea recovery techniques allowing tie-in to existing subsea pipelines, mean that multi-diameter pipelines are now common practice for many offshore applications. These multi-diameter systems not only present a challenge from a pigging aspect, but also from a pipeline gauging perspective. The traditional approach to gauging, with a conventional aluminium plate, will not provide sufficient information on the as-built condition of a multi-diameter pipeline. As such, gauging enhancements must be made to keep up with the industry requirements.
Following construction of a pipeline, there are several further processes that are carried out to ensure the ‘as-laid’ conditions are acceptable. Operators usually undertake flooding, cleaning, and gauging operations to clean the asset from post-construction debris and verify that the line has been laid without significant defects. Pipeline pigging and gauging therefore play a key role in the pre-commissioning activities of a new pipeline.
Multi-diameter case study
A multi-diameter project located in offshore Middle East required gauging as part of the pre-commissioning campaign. The pipeline had an internal diameter range of 719 mm (30 in.) to 535 mm (24 in.) which equates to a 26% change. A three pig pre-commissioning pig train would run from onshore to offshore (1700 m water depth) to flood, clean and gauge the pipeline. Within the range of diameters, four main sections of pipeline with significant lengths would need to be gauged.
Typically, pipeline gauging involves using a pipeline pig with an aluminium plate fitted (Figure 1). This plate is generally sized at 95 - 97% of the pipeline minimum ID (depending on the specification used for sizing). Upon recovery of the pig, the gauge plate physical appearance is inspected for any deformation. In this case, because of the multi-diameter aspect, the size of the aluminium plate would be so small (being based on the minimum ID of the 24 in. section) that it would provide no information on the as-built condition of the larger (30 in.) sections at all – therefore a smarter approach to gauging would be needed for this project.
A range of SMART gauge technologies were required for the project to provide usable as-built information on the as-laid condition of each of the four main pipe sections. Through development, design and in-house testing Propipe provided a range of systems to gauge the pipeline. Table 1 summarises the gauge systems used for each section of pipeline …
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