Maintaining pipeline integrity sometimes impacts other types of infrastructure. That was the case when a new natural gas pipeline near Seattle started to experience erosion issues related to a faulty draining channel. Milliken Infrastructure Solutions was called in to solve the problem to ensure pipeline safety and reliability.
In late 2014, a new gas line lateral installed on a steep 2:1 slope faced severe erosion issues due to the effects of heavy Pacific Northwest rainfall. Within weeks of installing a conventional rip-rap drainage channel, excess runoff water bypassing the faulty channel began eroding the soil near the gas line and potentially destabilizing the bedding material around the pipe.
Below grade oil and gas transmission lines are regulated by federal, state, county and city entities to ensure safety to the public and environment. These regulations are primarily focused on protecting the pipe from accidental damage and/or disturbances from above grade activities or weather related issues.
Some of the standard requirements for protecting the pipe include:
• Maintaining minimum burial depth
• Geotechnically stabilizing soils adjacent to the pipe
• Maintaining the easement right of way
Along with a regional technical manager from Milliken Infrastructure Solutions (MIS), erosion control distributor ACF West reviewed the situation and developed a comprehensive plan to correct the problem. The design goal was to convey the water down the slope and eliminate the erosion caused by the shear forces from the water flow (more than 14 lbs/ft2). The group provided recommendations to repair the erosion without disturbing the existing pipe, and create a low-maintenance, environmentally friendly channel that could handle runoff water from continuous storms. Per Federal Energy Regulatory Commission (FERC) requirements, the ground surface also had to remain at the same elevation as preconstruction.
Rip-rap typically requires maintenance when used in drainage channels, riverbanks, slopes, etc. While the solution usually creates an acceptable drainage structure, it degrades easily on slopes greater than 3:1. Rip-rap also moves over time — creating instability issues and excessive vegetation growth that can lead to flow restrictions, hydraulic capacity reductions and more. These were the major causes of the problem at
Possible repair solutions included:
• Poured concrete: This solution caused concerns for the asset owner, since FERC codes require the slope, with its 80-ft elevation change, be returned to its normal state after construction. The steep grade made direct cement truck access practically impossible; a large pumper truck would not have had a direct laydown area to set up.
• Turf Reinforcement Mats: TRMs require constant maintenance to ensure vegetation is healthy and meets expected hydraulic performance values. This solution was immediately eliminated because vegetation could not be established in the drainage channel to meet the acceptable shear values prior to the next storm (which could have occurred within days or weeks after repairs completed). There were also concerns about wildlife entanglement and mowing issues that affect TRMs when they are not soil filled.
• Articulate Concrete Blocks: Potential issues transporting ACBs up the steep slope without causing a major disturbance around the pipe — not to mention the time and expense to remove and dispose of soil off site — made this option unviable.
• Pipe: A pipe would need to be set on the ground since excavation would impact the current gas line. This would make it difficult for sheet flow from the slope to enter the pipe, and for debris such as leaves, blown trash and sediment to be removed.
• Gabions: Soil piping underneath the gabions, as well as impacts to the area and the pipe due to hauling materials up the slope, caused major concerns about this repair method.
• Geosynthetic Concrete Composite Mats: GCCMs offered quick installation with limited disturbances to the area, advanced hydraulic performance capabilities and natural blending with the surrounding vegetation.
Ultimately, Milliken’s Concrete Cloth GCCM was selected for the project. The material is frequently used for these types of applications in addition to slope protection, secondary berm protection, armoring for utilities, geosynthetics liner protection, and more.
MIS evaluated the channel width, depth and several dynamic external forces on the steep slope — including water movement and other live loads — to determine how to best apply the GCCM, and what kind of anchoring system would be needed to hold it in place. Less than two weeks after the initial assessment, the owner approved the repair solution, and ACF West supplied approximately 30 portable batch rolls.
The contractor carried the batch rolls to the top of the slope by hand and positioned the GCCM in place over a span of two days. The ability to hydrate the material in the field using an overflow catchment pipe was another advantage that allowed the contractor to reduce time and resources needed for installation. The area was re-vegetated to enhance aesthetics and return the slope to its natural condition, per FERC standards. Throughout the process, MIS made recommendations to the contractor to accelerate construction and meet budget goals, while minimizing disturbances to the area.
The repair met all FERC requirements and project criteria. Long-term maintenance requirements for the slope as a result of Concrete Cloth GCCM installation were reduced to virtually nothing, versus other active systems susceptible to degradation and displacement.
For the pipe itself, installing a GCCM solution ensured proper burial depth of the pipe along the drainage ditch and prevented runoff precipitation from saturating the slope. At the same time, the adjacent properties and natural runoff water from the surrounding areas remained unaffected. This functional longevity was an important benefit to the asset owner, who will not need to request access to the right-of-way from FERC for additional repairs for the lifespan of the product.
Tags: ACF West, Concrete Cloth, FERC, July 2016 Print Issue, Milliken Infrastructure Solutions
John Hepfinger is global market manager for Milliken Infrastructure Solutions (MIS).