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At the bottom of the deep blue sea - Part 1

Published by
World Pipelines,

Ask any pipeline inspection engineer about what it’s like to inspect deepwater pipelines. More often than not, they respond with a knowing grin, and mutter: “It’s a tough job, but somebody’s got to do it.” It’s tough because a pipeline might lie more than 2000 m (6500 ft) subsea, in the hurricane-prone Gulf of Mexico, offshore West Africa or the frigid North Sea. And while remote, harsh environments pose their own challenges, inspection is frequently further complicated by the design of the pipeline itself.

Given the varying sizes and dimensions that a single section of a deepwater pipeline may exhibit, inspection is not typically possible with an off-the-shelf solution. Instead, customised inline inspection (ILI) solutions are necessary.Pipeline inspection and integrity assessment specialist NDT Global developed its first high-pressure tool for small diameter deepwater pipelines a decade ago. Since then, NDT Global has drawn upon its collective knowledge to extend its range of inspection options, including a line of ultrasonic technology (UT) tools that it has used to successfully inspect deepwater pipelines around the world.

Challenges for intelligent pigging

While there is no doubt that co-operation at the design stage of pipeline construction can allow many inspection problems to be avoided, it must be accepted that, in most cases, this co-operation does not take place. Many deepwater pipelines have not been designed with inspection in mind.

NDT Global has developed a range of strategies to successfully address these complications and simplify the inspection process.

With regard to the problem of thick pipe walls, when inspecting a line with a wall of up to 50 mm (1.96 in.), magnetic flux leakage (MFL) tools are not suitable because the magnets they employ are not strong enough to fully magnetise the pipe wall. As a solution, UT provides quantitative, not qualitative – hence, absolute – measurements and requires no calibration.

UT is also useful for inspecting pipe that features internal CRA cladding, which can pose inspection problems for other ILI methods. With CRA cladded pipe made of metallurgical-bonded plates with a seam weld, the internal surface is typically quite smooth, thus posing no limitations with regard to UT measurements.

Inspection managers must also address the challenges posed by the special pressure requirements of deepwater pipelines. Due to water depth and operating conditions, a deepwater pipeline must be able to withstand static pressure and pump pressure of up to 500 bar (7250 psi). This means that inspection tools must be designed and rated accordingly to operate in this environment, for example, by using high-strength materials. NDT Global offers a range of such tools to cope with these unique pressure and temperature challenges, and a solid track record of experience.

When designing ILI tools, non-constant internal pipe diameter is often the greatest challenge. There are a number of reasons for this lack of constancy. For example, there might be different wall thicknesses for risers, flowlines, topsides piping and subsea structures. Or, manufacturing tolerances on seamless pipe can cause variations in thickness, as can dual-diameter design (e.g. 6 in. risers and 8 in. flowlines). NDT Global developed special multi-diameter tools and proved them in a variety of environments, so that the problems posed by non-constant internal pipe diameter can be efficiently addressed.

Unique solutions

While each of the problems discussed can complicate the ILI task, it is the combination of these pigging challenges that makes inspection of deepwater pipeline systems especially difficult.

When an inspection manager reviews the specific challenges posed by a pipeline, he or she frequently finds that there is simply no ILI tool that will fulfil all requirements, and small-scale modification of existing tools is usually not sufficient. In these instances, the expertise and flexibility of NDT Global becomes paramount, as it is generally possible to design a bespoke solution for the particular pipeline.

Once a bespoke solution is completed, NDT Global conducts test-runs with the ILI tools in a pipeline mock-up in order to make sure that it will work in the particular deepwater context. Given its history of experience in inspecting subsea pipelines, NDT Global knows that testing viability in a test yard is preferable to carrying out this critical activity when the pipeline is located on the seabed.

Edited from published article by Stephanie Roker

To read the full version of this article, please download a copy of the July 2016 issue of World Pipelines.

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