A containment liner integrity conundrum
PWSRCAC commissioned study into ways of testing the condition of the tank secondary containment at the Valdez Marine Terminal
Following the discovery a few years ago of cracks in an area of the impervious liner of the secondary containment system for the oil storage tanks in the Valdez Marine Terminal, the Prince William Sound Citizens Advisory Council continues to worry about the condition of the liner at other locations in the terminalís east tank farm. During the councilís board meeting on Jan. 25 the board reviewed the results of a study it had commissioned into possible non-destructive ways of testing the liner. The study has suggested a couple of possible methods that might be used. The board agreed to send a letter to Alyeska Pipeline Service Co., the terminal operator, asking the company to consider a pilot project for evaluating the effectiveness of the techniques.
Scott Hicks, Alyeskaís Valdez Marine Terminal manager, commented to the board that Alyeska had previously commissioned a similar study into testing methods and had run into problems with the practicalities of the testing, given the size of the containment area involved. During excavations conducted in conjunction with work on the industrial wastewater system, most of the liner was found to be undamaged - it is still in flexible condition and very functional, Hicks said.
Austin Love, PWSRCAC project manager for terminal operations, commented that the frequency of liner damage found in the excavations that had been carried out in the east tank farm suggests that similar damage exists in regions of the liner that have yet to be inspected. Love also commented that Alyeska had put considerable effort into repairing areas of the liner that had been found to be damaged.
Oil storage tanksThe marine terminal accepts oil from the trans-Alaska oil pipeline and loads the oil into tankers in Port Valdez. The oil tanks provide buffer storage, to enable the efficient loading of oil into the tankers. The oil containment system around the tanks, designed to hold any oil that might leak from the tanks, was installed in 1977 and involves a thick impervious membrane composed of catalytically blown asphalt, or CBA. The membranes are covered by several feet of gravel and, in total, cover a huge area.
The integrity of the liner matters, not just because of the protection it provides to the environment, but also because of the economics of oil spill contingency arrangements at the marine terminal. State and federal regulations spell out the permeability requirements for the liner. And the state allows Alyeska to reduce the oil volume specified in the terminalís spill response requirements by more than half, on the assumption that the tank secondary containment meets the regulatory standards and would, therefore, be effective. Any increase to the response requirement as a consequence of the secondary containment being inadequate would require a corresponding increase to the equipment and people that would need to be available to respond to an oil spill.
Damage found during refurbishmentAs previously reported in Petroleum News, the issue relating to the condition of the liners came to light in the course of carrying out a successful project to refurbish the system of pipe work used to drain rainwater and snow melt from the secondary containment enclosures at one of the storage tanks. Renovating the drainage system involved removing some of the gravel from the containment system, thus exposing some sections of the liner underneath. This uncovering of parts of the liner revealed a network of cracks permeating one section of the liner, and some holes in the liner. Alyeska remedied the problem by installing a sheet of modern impermeable material over the damaged area.
No explanation for the cracking has been forthcoming, and no evidence of similar cracking is evident elsewhere at the terminal. Alyeska concluded that something must have happened to damage the membrane, such as a diesel oil fuel spill, in the area of the damage.
Need for non-destructive testingThe widespread digging up of the gravel cover of the 54-acre containment area, to check for further membrane damage, would be a massive exercise that would involve a significant risk of damage to the membrane as a consequence of the digging. So PWSRCAC commissioned Geosyntec Consultants to investigate possible ways of conducting non-destructive testing.
Jay Griffin, a project engineer with Geosyntec, described to the board what the study had found.
The consultants had considered the use of ground penetrating radar. Electromagnetic surveying is also somewhat similar to ground penetrating radar and can be used to detect objects in the subsurface. But neither of these techniques would be practical for testing the containment membrane because of the widespread existence of metal features around the tanks, and because the technologies would not provide sufficient spatial resolution for detecting small defects, Griffin said.
Instead, the consultants found two techniques that may work: tracer gas testing and electric leak detection, a method used for testing liner systems in the mining industry.
Tracer gas testing would involve the injection of some form of gas or smoke under the liner, and then testing for the injected material escaping through leaks. Griffin suggested that this technique could maybe be used in small isolated areas of the tank farm, perhaps around where something penetrates the membrane. Electric leak detection, on the other hand, relies on the fact that the membrane is an electrical insulator - an electric current is applied to the cover material and then voltage readings are taken on the ground above the membrane, with a voltage anomaly indicating a leak.