By Natalie Daly, Bertrand Breul, and Bernard Breul – The Chevron Willbridge Terminal is located in Portland, Oregon near the Willamette River. The facility stores refined petroleum products. Currently, the site features 11 light product tanks, 10 lube oil tanks of smaller diameter, and 3 waste tanks. The terminal has been in operation for more than 100 years, and the tanks are of varying sizes and fabrication styles: riveted, welded plates, etc. Products are received and shipped via truck, pipeline, and marine vessels.
To manage inventories and dispense product, a large number of pipes crisscross the terminal. Drain inlets, pumps, electrical panels, electrical conduits and other appurtenances are also located within the secondary containment zone. The terrain is essentially flat. Stormwater drainage consists of a series of drain inlets connected by pipes to an oil-water separator prior to connecting to the City of Portland sewer system. Terminal soils are mostly sand, which minimizes any ponding of stormwater around the tanks.
Over a six-year period, the site operators refurbished of the terminal, replacing a series of small and relatively old tanks with two new tanks.
The new tanks were designed in accordance with governing standards and the company’s only state-of-practice guidelines. Regarding the latter, it meant that the tanks needed to be constructed with concrete slabs on top. And, in replacing existing aboveground storage tanks (AST) with new ones, the engineering works needed to comply with the 2004 Stormwater Management requirements (City of Portland Department of Public Works).
GAS DEPOT SITE REGULATIONS
The Portland site required compliance with multiple tiers of regulations. Notable requirements included:
- Federal level: AST secondary containment systems in compliance with Clean Water Act (as amended by the Oil Pollution Act of 1990). These requirements are detailed in the Spill Prevention, Control, and Countermeasures (SPCC) and Facility Response Plan Regulations.
- State level: Oregon has adopted the International Fire Code that governs ASTs containing motor vehicle fuel. A permit from the state fire marshal is required for gasoline and diesel fuel tanks with a total storage capacity of more than 1,000 gallons.
- Local level: The City of Portland Stormwater Design Manual (2004) requires that for bulk fuel terminals, (a) secondary containment must equal 100% percent of the product’s larger container or 10% of the total volume stored, whichever is larger; and (b) an impervious floor within all containment areas. Floors shall be sealed to prevent spill from contaminating the groundwater.
The local requirements are in fact more stringent than the federal and state requirements, since they specify the installation of an “impervious floor.”
The site owners subsequently made their secondary containment selection based on a lining system optimization matrix.
SECONDARY CONTAINMENT SELECTION
The three main lining systems considered include:
- Flexible Geomembranes – Included resin-based geomembranes (HDPE, LLDPE, PVC, etc.), bituminous geomembranes, and Geosynthetic Clay Liner (GCL)
- Structural Liners – Included concrete, shotcrete, and asphalt concrete
- Soil-based liner or soil-treated liner – Includes amended soils (e.g., with bentonite, cement, etc.)
Each liner, of course, presented advantages and disadvantages, such as requiring cover or enabling long-term exposure, complexity of site work required, etc. Proposed costs varied from a few cents per square foot to a few dollars per square foot.
The selection criteria had to take into consideration how each system might impact construction and operation, performance, cost, and regulatory compliance.
The selection process also had to take into account many factors of the site design risk points.
- The new tanks were to be located in the middle of existing ASTs, so space was limited for materials and equipment; different activities would simultaneously occur (e.g., concrete, metal work, piping layout, electrical, etc.); and many existing pipes, electrical conducts, their support, and other appurtenances could not be disturbed.
- All penetrations into the ground (ducts, pipes) needed to be made watertight.
- Numerous appurtenances had to be included in the floor of the secondary containment system (electrical control boxes, pumps, walkways, concrete supports for pipes, etc.).
The flat topography also complicated re-grading options with respect to drainage.
Also, since the terminal is a revenue-generating site, on-going operations could not be disrupted. To ensure operational flexibility, the engineering plan took into account:
- Utilizing smaller equipment (pick-up trucks, forklifts) to minimize site traffic issues
- Selecting a liner system with an easy repair potential, such as through local contractors
- Ease of cleaning up spills on the lining system
- The potential to extend the liner into other portions of the terminal as part of a long-term improvement plan
LINER SYSTEM SELECTION MATRIX
The selection criteria matrix developed for the site was built around achieving goals in construction and operation, performance, cost, and regulatory compliance. The liner options and criteria were organized in the matrix with each column representing an option and each row a criterion. Criteria were scored from 1 to 4 (very good to poor). These grades were assigned based on qualitative and quantitative bases. For example, “construction costs” are quantifiable and the cheapest liner to purchase and install was given a “1,” whereas the most expensive was given a “4.” By contrast “operational flexibility” is a subjective criterion and the grade assigned to each liner option was done using subjective and qualitative data based on personal experience and discussion with the operator.
Because of this, such a selection matrix will vary from site to site.
BITUMINOUS GEOMEMBRANES: DESIGN & INSTALLATION
The new tanks were constructed over the footprints of older, smaller tanks that had been decommissioned and demolished. The containment zone is surrounded by a concrete wall.
While the concrete protection met older, lateral containment requirements per federal and state rules, the local code’s requirement for an impervious floor merited the inclusion of a geosynthetic solution. A 4mm-thick (160 mil) bituminous geomembrane from Axter Coletanche was selected.
A berm was established around the works and the areas within the berms was regarding to optimize drainage to existing drain inlets. The final slopes were in the order of only 0.5 to 1%, due to the flatness of the site. Settlement, however, is not expected, due to firm, gravelly sand undernearth. Furthermore, the very low Manning coefficient of the bituminous geomembrane facilitates stormwater flow toward the drain inlet.
Once work commenced, a specialty industrial facilities contractor demolished the old tanks and associated pipes and equipment, constructed the ring foundations for the new tanks (to install the standard “Chevron double bottom”), built the new tanks, and installed new pipes, valves, and related systems.
Because of space limitations and time constraints, the work proceeded from one tank to the next with different activities performed depending upon availability, terminal operations, and weather conditions.
The construction schedule with so many simultaneous tasks was complex. Liner installation, in fact, was not even identified as a single task for each tank to be performed in one continuous period at a given time. The geomembrane installation was considered a “catch-up” task that would start/stop when time and other activities allowed. Here, the selection of bituminous geomembrane was important.
The robust material is sturdy and has great puncture resistance, but from an installation standpoint it is also easy to cut and seam. It does not require a specialty installer, can remain exposed, and can be driven upon. For this project, it was an ideal geomembrane.
This permitted the contractor to keep working in and around the tanks following geomembrane placement. The required scheduling flexibility was ensured.
Workers from the contractor personnel were trained by the manufacturer to lay and seam the geosynthetic liner. Seaming a bituminous geomembrane does not require special equipment. It simply need a torch with flame (or no flame for security requirements), a trowel, and a weighted roller to press the seam. The trained crew was efficient, performing other activities when liner was not being installed.
During installation, the same quality assurance standards for other geomembranes (HDPE, LLDDP, PVC, etc.) were followed for the bituminous geomembranes. Panels were numbered and their locations recorded. The 8-inch overlap on the seams were welded together by melting the bitumen over the overlap and pressing the two edges together.
Integrity testing for this type of liner was different from other geomembranes. Continuity of the seam was monitored by CQA personnel. The integrity and water tightness were spot checked with either a vacuum box or an ultrasound sensor. The ultrasound sensor monitored the thickness and continuity of the weld across the 8-inch-wide seam. A 1-ft. segment was tested for every 50 ft. of the seam. Every seam was tested at least once regardless of its length.
Five years into facility service, the lining system centered around bituminous geomembranes is still performing as designed and without evidence of settlement or wrinkles.
Natalie Daly, Bertrand Breul, and Bernard Breul work for Axter Coletanche. For more information on bituminous geomembrane characteristics and applications, visit www.coletanche.com.
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A full version of this article was published in the GeoAmericas 2016 Proceedings. The April 2016 conference was hosted by the International Geosynthetics Society North American Chapter. Learn more about the event and acquire a copy of the three-volume digital proceedings at www.geoamericas2016.org.
