Advancing the art of pipeline padding

 recent study conducted by Battelle for the INGAA Foundation and the U.S. Department of Transportation assessed enhancements in safety and productivity that could be realized with new and emerging padding and backfill practices.

The study examined a number of recently developed construction practices, and identified and evaluated opportunities for cost reduction, improved pipe protection, and decreased construction-related pipe/pipe coating damage.

In general, the study noted that increased demand for fuels and other hydrocarbons has begun to deplete transmission pipeline supply basins located near traditional markets, requiring longer pipelines to increasingly remote supply basins located typically in challenging geographic and climatic conditions. Shifting population centers and related markets such as distributed power generation have likewise driven this process.

These trends have prompted evolution of new or emerging construction practices, as well as developments in thinner and stronger line pipe, and new coatings. Coupled with developments in construction equipment and practices to deal with these challenges, such improvements could add value through increased safety and reliability due to reduced extent and/or severity of construction-related coating damage that necessitate repairs.

As part of the study, data were gathered subject to the practical constraints imposed by use of construction equipment, and the contractor’s permission to enter or approach the ditch for “hands-on” measurements and close-up observation.

Measurements and observations

On-site measurements and observations made in reference to the many spreads visited lead to the following general conclusions:

• The performance of bedding and padding machines is dictated by the properties of the soil and soil mechanics, in addition to design and operating features that might be unique to a given machine.

• Differences in performance characteristics between integral in-line loader and screen machines and independently loaded stacked vertical-screen machines preclude a onesize-fits-all basis for machine selection. Economics and many other factors dictate overall productivity on the spread and constrain decisions. For example, where many tie-ins are involved because of routing or topography, the tie-in welds appear to control, and this is but one such factor.

• Coating damage resistance is a major factor in the viability of bedding and padding procedures, as tough durable coatings were resistant to damage under loading conditions characteristic of bedding and padding.

• The relative significance of bedding and padding practices designed to avoid damage tends to be of limited value where adequate damage-resistant coatings were used. Performance-based specifications for coatings and construction practices appear viable in this context, with validation needed to make them widely accepted.

• Some areas have been identified for future development, the focus of which is combination crusher/padding machines and the evolution of damage-resistant coatings. Work underway in Europe is particularly significant in this context.

As noted, data were gathered subject to the practical constraints imposed by use of construction equipment and the contractor’s permission to enter or approach the ditch. Accordingly, safety and other conditions dictated by the contractor were complied with. On-site measurements and observations made in reference to the five sites considered lead to the following general conclusions:

• The significance of bedding and padding practices designed to avoid damage is limited in practice where damage-resistant coatings were used. Performance-based specifications for coatings and construction practices seem viable in this context, with validation needed to make them as effective as possible, in balance with longer-term concerns related to coating quality.

• The potential for cost reduction due to improved bedding and padding procedures cannot be assessed independent of the coating used, because as just noted the significance of bedding and padding practices designed to avoid damage is limited in practice where damage-resistant coatings were used. This means that selection of bedding and padding practices depend on the coating system used as well as the conditions known or anticipated along the construction routing.

• The potential for cost reduction reflect not only the coatings performance during construction but also on the coatings survivability in-service. Long-term durability and reliability means reduced maintenance associated with avoided metal loss, as well as reduced costs and problems associated with other threats like possible SCC at coating failure sites or service induced damage such as dents and/or gouges. It follows that an up-front investment in quality coating brings the potential for immediate value through enhanced resistance to construction-related damage almost independent of the bedding and padding practice, plus longer-term cost-reduction and increased safety related to avoided corrosion, possible SCC, and mechanical damage.

• The behavior of the fill material is also a critical bedding and padding parameter, as forward flow of the fill material and the natural selectivity for coarser material to roll down the slope created by the angle of repose lead to flow of fill under and around the pipe-string. Forward flow once the pipe-string is enveloped is controlled by soil mechanics through the angle of repose for the soil, which depends on the nature of the soil, including the size and to a lesser extent the shape of the fill material. Any bedding and padding machine that provides for these aspects should reasonably supply compacted fill that flows around the pipeline, with the little chance for coating damage. Industry personnel associated with companies that sell or operate such equipment note this inherent property of the fill material is a key to the bedding and padding success.

• The presence of select material under the pipe and the exclusion of larger rock from the immediate vicinity of the pipeline should lead to a reduction in rock dents and other such mechanical damage during construction. While not directly quantified in the project as such results follow from post-construction ILI and other such data, anecdotal information gathered from contractors indicates such benefits are common when the investment and effort is made to quantify such factors.

Recommendations

Data concerning coating damage resistance are limited in reference to contact conditions and what size/depth chip or scrape is a short- or long-term threat. A formal evaluation of coating damage resistance under parametric contact conditions could be considered to quantify this in reference to contact bedding and padding or related construction practices are viable versus problematic as a function of coating type and thickness, and factors affecting their properties such as temperature, thereby maximizing productivity without posing a practically significant short- or long-term threat or burden on maintenance.

Because tough durable coatings were observed to resist damage, damage resistance becomes a major factor in the viability of bedding and padding procedures. As the viability of coatings becomes better characterized, consideration should be given to performance-based specifications for coatings and construction practices, with validation drawn from the results of the above noted parametric coating evaluation. The role and effectiveness of CP should be a consideration in developing and validating any one-sizefits-all construction specification involving performance-based coating and bedding and padding practices.