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Meeting the challenges facing small-diameter combination inspections.

The inspection of small-diameter pipelines has always presented unique allenges to the pipeline industry. perience and technology advancements have allowed for not only the capability to inspect these lines successfully but to do so utilizing inline inspection (ILI) combination tools.

Combination ILI tools which collect multiple data sets in a single run have been around for several years. These tools are frequently used by pipeline operators who are under regulatory requirements to assess their pipelines for both metal loss and geometry and wish to do so in a single tool run. These tools have also made identifying metal loss and geometric defect interaction much easier. With a single integrated report there is no longer a need to manually align two data sets.

Most ILI vendors have focused their efforts on large-diameter combination tools as there are significant challenges facing small-diameter combination tool designs. The tight tolerances present in small-diameter pipelines make it difficult to fit the required components into a small-diameter tool. This is magnified by the need to maintain good tool collapsibility, short tool length, and the ability to traverse tight bends.

Benefits Of Combination Technology

A great deal of the cost associated with running an ILl tool comes from the support of the inspection rather than the cost of the inspection itself. These costs include lost production due to slowing or stopping of flow rates, personnel to support launching, receiving and tracking, as well as the costs associated with the disposal of hazardous materials produced by the inspection.

Anytime multiple data sets can be collected in a single inspection as opposed to two or three, support costs are considerably reduced. Costs are also reduced for the ILl vendor as there is no need to mobilize multiple tools to the location or to manually align common features from two data sets. This translates into a lower overall inspection cost when compared to performing multiple single technology inspections.

Being able to associate metal loss features, which directly interact with geometric features (dents, gouges, etc.), is critical in obtaining an in-depth picture of a pipeline's integrity. Many regulations state that metal loss interacting with a geometry feature constitutes an immediate repair condition. These features pose a significant threat to pipeline integrity.

Small Diameter Considerations For Combination Technology

The challenge with designing a small-diameter combination tool is the ability to fit all of the required elements into a small enough package to traverse the many tight tolerances found in small-diameter lines. Maintaining tool collapsibility with the ability to pass tight bends, while keeping the tool length, weight and overall drag to a minimum, are the main considerations when designing a small-diameter combination tool. Each of these issues must be addressed in order to achieve a successful multiple-technology inspection.

Simply adding a geometry measurement module onto an existing small-diameter MFL tool often requires additional proving such as a standalone geometry inspection to ensure there aren't any restrictions that will pose a threat to the magnetic measurement portion of the tool.

Traditional small-diameter MFL tools are designed to be able to pass restrictions of 10% or less, while the borepassing capability of standalone geometry tools at 20-25% provided a good method of passage clearance for the larger MFL tool.

By employing an MFL measurement unit that is 20-25% collapsible, the prerun proving requirements are reduced to the running of a gauging plate, similar to what has been traditionally used prior to geometry inspections. While this is relevant for all combination inspections, it is more prevalent in small-diameter where a 10% restriction in a 4-inch pipeline can be a change of 9.7mm (.38-inch) where having 25% collapsibility would have passed that same restriction with 14.6mm (.6-inch) to spare.

The need to identify tight bends often present in small-diameter pipelines is typically left up to a pre MFL inspection geometry run. This verifies there are no bends present in the line that are tighter than what could be traversed by the MFL tool. By adding bend passing capability down as low as 1.5D on small-diameter combination tools, less pre-run proving needs to be done. With high collapsibility and a small minimum bend radii most of the functions of the pre MFL geometry run can be eliminated.

To solve the issues of collapsibility and bend passing capability the tool is designed with short-jointed sections forming the inspection train. While this typically works on larger diameter lines as their launching and receiving facilities have been designed for many years with intelligent pigging in mind, many small-diameter lines only have facilities to accommodate utility pigging.

While modifying inadequate facilities is an option, the costs can often reach three times the cost of the inspection being performed. Efficient utilization of space within the tool and miniaturization of the onboard systems can often shorten an inspection tool as much as 30-40%, allowing it to be launched and received in facilities that would normally be too small for other inspection tools. Keeping the length of the tool down also reduces its weight which helps to reduce speed excursions in gas pipelines.

Consistent speed in low-pressure, small-diameter gas pipelines is critical to data quality and tool reliability. If a tool's speed cannot be sufficiently controlled in a gas line, it can become necessary to shut down the line and propel the tool using liquids. This can quickly become expensive.

Suspending the inspection train on rollers to reduce drag as it moves down the pipeline greatly reduces the amount of differential pressure required to propel the tool. This allows the negotiation of tight restrictions while avoiding large speed excursions seen on tools that employ ride cups to support the tool.

Combination inspection tools help pipeline operators meet their regulatory requirements by running a single tool to collect the necessary geometry and metal loss data. Although small-diameter combination tools require special considerations in both their designs and procedures for inspection, high-resolution data can be obtained in a single pass.

With recent advances in the small-diameter combination tool technology, high-resolution data can easily be attained while cutting operational costs associated with multiple tool runs.

Chris Maier, Onstream

Pipeline Inspection Ltd.
COPYRIGHT 2007 Oildom Publishing Company of Texas, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
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Title Annotation:2007 P&GJ and PPSA Pigging Section
Comment:Meeting the challenges facing small-diameter combination inspections.(2007 P&GJ and PPSA Pigging Section)
Author:Maier, Chris
Publication:Pipeline & Gas Journal
Geographic Code:1USA
Date:Aug 1, 2007
Words:1017
Previous Article:2007 P&GJ and PPSA pigging section: buyer's guide.
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