Geosynthetica’s GeoAmericas series continues with a look at the utilization of geomembranes in oil and gas operations. This article was originally published in the GeoAmericas 2016 proceedings (published by Geosynthetica) under the title of “Fortified Geomembranes and Dike Expansion Keep Gas Flowing in the Northeast United States.” The topic is timely, not only for the exemplary geosynthetics work but because the oil and gas market in North America has experienced some volatility that has raised concerns for the economic health of small operators and the potential for post-operation environmental issues. In the article, Stephen Valero, P.E. and Brian Frasier show how geosynthetics are used to optimize oil and gas work while enhancing near and long-term environmental protections.
1. BACKGROUND AND INTRODUCTION
The owner of an underground gas storage facility in the northeast US was faced with several problems related to their surface brine storage reservoir. The facility is designed to store gas in underground salt caverns and uses saturated brine to equalize/stabilize the caverns as gas is transferred in and out. The brine reservoir is critical to operation of the facility as it safely stores brine at the surface until it is needed in the caverns.
The nearly 60 million liter (500,000 barrel) capacity brine reservoir had been in continuous operation for more than 30 years. It was fitted with a single, reinforced polyethylene geomembrane liner that had reached the end of its design life. In addition, the capacity of the pond was no longer adequate due to natural growth of the caverns and rain/snow water dilution of the brine in the pond was exacerbating this situation. Dilution of the brine concentration is not desirable as it leads to increased growth rate of the underground salt caverns. Cavern growth rates and total size are closely regulated by government and if left unchecked could lead to premature closure of the facility.
Given these concerns, the owner contacted the authors seeking a design-build proposal to reline the reservoir and fit it with a floating cover system to preclude future dilution of the brine. To reduce risk, the owner required a modern liner system with leak detection capability. Construction schedule was also an important concern since the brine reservoir could only be offline for the summer months when heating gas demand is low and the caverns could be used to store saturated brine from the reservoir. In addition, the owner had determined that approximately 14.3 to 17.0 million liters (120,000 to 140,000 barrels) of liquid in the reservoir was non-saturated brine. It was their desire to permanently remove that liquid from the reservoir/cavern system and temporarily store it on site until it could be treated and discharged. Finally, the owner wanted to expand the total capacity of the brine reservoir by over 50 percent, if possible. It had been determined that this added storage volume would allow full usage of the cavern capacity for the remaining life of the facility. Therefore, expansion was highly desirable from a commercial standpoint but would not be implemented unless it could be accomplished during the planned summer outage.
After considering the situation, the authors’ design team developed a comprehensive solution that would provide temporary, safe, onsite storage of diluted brine, replace the existing single liner with a durable double liner and leak detection system, dramatically expand the pond’s capacity, preclude future dilution of the stored brine and provide other operational enhancements. Each component of the solution relied heavily on the unique capabilities of geosynthetics. The final plans included the following components: geomembrane lined, high-volume, temporary Aboveground Storage Tanks (ASTs); a vertical dike expansion using Mechanically Stabilized Earth (MSE) technology; a modern, double liner system with leak detection; and, a highly fortified floating geomembrane cover system. Through careful planning and a design/build approach, this solution was successfully implemented during the summer of 2014 allowing the facility to get back on-line and keep the gas flowing to the northeast US for the winter of 2014-2015. The following sections describe how geosynthetics coupled with significant design and construction experience with these products were critical to success of several parts of this challenging project.
2. TEMPORARY ONSITE DILUTED BRINE STORAGE
Several options were considered for on-site storage of the diluted brine that could not be pumped into the underground caverns during construction. It was determined that the most expedient and cost effective alternative was to use large (51.2 meter by 3.4 meter high, 5.1 million liter capacity), circular corrugated steel tank rings lined with factory fabricated geomembrane liners to create a battery of four onsite ASTs having a total storage capacity of 20.5 million liters (172,000 barrels). Similar systems are commonly used to temporally store liquids and brine associated with hydraulic fracturing and recovery operations in oil and gas wells. The selected ASTs had several advantages over in-ground ponds, smaller mobile tank systems and single, larger modular above ground systems including: cost effectiveness, minimal regulatory/permitting requirements, minimal ground/area disturbance, simple and rapid construction, easy visual leak detection, structurally efficient circular shape requiring no external bracing and redundancy/excess capacity.
Each tank was designed with a primary liner, secondary liner and protective ground pad. Important factors went into specification of the geomembrane materials used in the ASTs. Most importantly, it was required that the liners be fabricated using flexible, durable geomembrane capable of high tensile elongation and fatigue cycles without puncture or tear. The selected geomembrane also had to be highly resistant to degradation under exposure to UV light and brine for the required temporary storage period. High strength/low elongation such as scrim reinforced or High Density Polyethylene (HDPE) geomembranes were not considered acceptable for this application as they tend to bridge over voids, tank corners and ground debris leading to stress concentration, punctures, tearing and leakage.
In addition, scrim reinforced liners have relatively thin surface laminations and coatings, rendering them more susceptible to installation related abrasion resulting in leakage. Tank construction was to take place in the normally wet, spring conditions of the area so it was also important that the liners be factory fabricated to eliminate the need for field welding, rolled, folded and packaged such that they could be stored and handled without damage and installed from the center of the system without flipping panels or extensive dragging along the ground. The selected product was a highly flexible grade 30 mil thick, Linear Low Density Polyethylene (LLPDE) geomembrane developed specifically for use in this type of AST.
Construction of the temporary AST battery began in May of 2014. The foundation soil in the area to receive the AST battery was evaluated and determined to be acceptable by the project geotechnical engineer. The area was already relatively flat and level such that only minimal grading was required beyond removal of vegetation/topsoil and creation of internal drainage sumps. Construction of the entire tank battery and transfer of about 15 million liters of excess diluted brine from the primary reservoir to the tanks took about three weeks. Figures 1 and 2 illustrate the tanks under construction and in service, respectively. The entire operation went as planned and the tanks performed well without any leakage or other operational issues.
3. DIKE EXPANSION ENABLES BRINE RESERVOIR CAPACITY EXPANSION
Options considered to expand the brine reservoir capacity included: Excavating (lowering) the floor, steepening the inside slope of the containment dike, increasing the dike height, expanding the overall footprint and various combinations of these alternatives. Expanding the overall footprint was ruled out almost immediately as it would require extensive permit revisions and approvals coupled with excessive construction time. Excavating the floor was also eliminated as a possibility due to the shallow ground water table beneath the structure. After the Geotechnical Engineer confirmed that the existing dike foundation could support anticipated loads, the design team moved forward with an approach that combined steepening of the internal slope and increasing the height of the existing dike.
It was estimated that the dike height had to be raised by approximately 1.4 to 1.6 m depending on the final interior slope grading to achieve the required capacity expansion. The design team determined an MSE wall near the top of the existing dike outside slope would provide the required structural support and could be rapidly constructed relative to other viable wall options (sheet pile and cast-in-place concrete). The geogrid reinforced, wrapped face MSE wall had several additional advantages including: lower cost (about 1/3 the cost of other options); allowed for balancing of cut/fill as material removed during interior dike steepening could be used for reinforced fill; facilitated a 3m wide access road around the entire perimeter and could be vegetated to blend with the surroundings. As illustrated in the typical dike expansion cross section (Figure 3), a non-structural concrete ring wall was also added at the top of regraded inside dike slope to facilitate later anchorage of the liner and cover components.
The total final design dimensions of the MSE wall was approximately 630 m long by 2.75 m high for a total face area of about 1,730 square meters. Construction of the dike expansion began in mid-June 2014 and despite several periods of poor weather, all components shown in Figure 4 were substantially complete within about 60 days. The MSE design proved simple to construct (Figure 4) at very high production rates (nearly 100 square meters of face completed per day) and is performing well.
4. BRINE RESERVOIR LINER AND COVER SYSTEM
The upgraded brine reservoir liner system was designed to exceed regulatory requirements and to provide both leak detection and secondary containment capability. The new floating cover system was designed to prevent rainwater and snowmelt from diluting the stored brine. Specifications required a 25 year design life for the liner and cover system. Therefore, it was critical that all components be highly resistant to degradation in contact with concentrated brine through temperatures ranging from below freezing to nearly 50 degrees Celsius. Furthermore, the cover system would be constantly exposed to sunlight, weather and fluctuating liquid levels meaning it had to be highly ultra-violet (UV) and fatigue resistant while maintaining flexibility.
For both the liner and cover system, the design team selected a specific fortified polyolefin geomembrane (Enviro Liner® 6000HD) that had demonstrated high flexibility under a wide range of temperatures as well providing exceptional UV and chemical degradation resistance. A fortified geomembrane is defined as a product heavily treated with stabilizers providing enhanced heat, UV stability and chemical resistance (Schiers, J 2009). The fortified material chosen was initially developed in the early 2000’s and was part of a longer term UV weathering study on geomembranes published in 2009 (Mills, Martin & Sati, 2009). The referenced study included a 30,000 hour accelerated UV weathering test with results outlined in Figure 5 below. This study showed that the useful life of polyolefin geomembrane materials could be significantly extended when properly stabilized. As a result of this study a fortified polyolefin material was developed specifically for longer term exposed geomembrane and floating cover applications and made available with a 25 year weathering warranty.
In addition, the geomembrane chosen had been subjected to high temperature brine immersion testing proving the materials proprietary anti-oxidant stabilization package had performed extremely well (Layfield, 2012). Results of antioxidant stabilizer evaluation during this program are presented in Figure 6.
After selection of the geomembrane material, the liner and cover system structural details were developed. As shown in Figure 7, the liner system consisted of a 1.52 mm (60 mil), primary liner over a 5 mm (200 mil), HDPE geonet leak detection/conveyance layer over another 1.52 mm (60 mil).
The floating cover system was designed in a “double Y” defined sump configuration (Figure 8) consisting of a 1.27 mm (50 mil) geomembrane fitted with prefabricated floats and weights arranged to keep the cover tight and properly positioned while allowing full range of brine level fluctuation in the pond beneath (Figure 9). The 1.27 mm (50 mil) thickness for the floating cover material was chosen based on prior experience confirming this thickness to be an optimum tensile strength to weight ratio while still providing good material flexibility required in the define sump section. Small float actuated pumps placed in the intersections of the weighted areas (sumps) were included to evacuate rain water from the cover as required. All pump lines were also heat traced to avoid freezing in winter conditions.
The highly skilled construction crew was able to install the liner and cover systems in parallel to save time and reduce the risk of flooding/subgrade damage due to rain (Figure 10). Despite a the large total surface area (24,000 square meters +/-) and challenging weather, installation of the liner and cover systems began in mid-August 2014 and was substantially complete by late September, 2014. Following construction, the primary and secondary liners were ultra-sonically tested for defects using a proprietary technology developed by the construction team. Several small pinholes were effectively located and repaired before the reservoir was placed back into service in October of 2014.
Figure 11 illustrates the completed cover system before the pond was placed back into service (empty) while Figure 12 shows the system at near full capacity. The entire system continues to perform as designed, with no major issues since being placed back into service.
The containment project profiled in this paper demonstrates how an experienced design-build team leveraged newer generation geosynthetics to dramatically optimize performance and improve value to the owner. If not for geosynthetics, the owner of the facility described in this paper would NOT: have been able to complete the described upgrades in a single summer outage; been able to justify the return on investment of capacity expansion; have no leak detection/secondary containment capability; and have no cover system to preclude future dilution of stored brine. This highlights the important role that modern geosynthetics can play in construction projects.
The authors would like to acknowledge the confidential facility owner for permission to publish this case history. In addition, we would like to thank Ross Hartsock, Project Manager with Layfield USA Corporation who was instrumental in providing photographs and construction documentation related to the project.
ABOUT THE AUTHORS
Stephen N. Valero, P.E., is an engineer and business leader with a deep background in geosynthetics, including consulting and manufacturing roles. Brian Fraser is with Layfield, a diverse geosynthetics manufacturing and installation company.
Layfield Environmental Containment (2012). EnvirolinerHD Technical Data and Specifications, Layfield Environmental Containment, Edmonton, Alberta, Canada.
Schiers, J. (2009). A Guide to Polymeric Geomembranes: A Practical Approach (Wiley Series in Polymer Science), 2009Edition. Chemical Resistance of Geomembranes, p 341
Mills, A. Martin, D. Sati, R. (2009) Long-Term Weathering Stability and Warranty Implications for Thin Film Geomembranes, Geosynthetics ’09, IFAI 1801 County Road B W Roseville, MN, USA