Frost Heave in Cryogenic LPG Storage Facilities – Impact and Mitigation

Abstract

LPG storage facility in a GASCO plant is 35 years old and contains two propane tanks and two pentane tanks each of 60m diameter and 24m height surrounded by massive conical earth embankments. Cryogenic storage temperature (-45°C) of the products freeze the soil below tanks causing frost heave, a phenomenon of soil expansion due to capillary intake of ground water in freezing zone to form ice. To avoid potential risk of frost heave causing additional loads on tank bottom slab, original design provides heating systems at bottom and sides. Its partial failure and inaccessibility through embankments led to serious concerns on tanks safety/ integrity. This paper discusses several investigative studies performed on progressive heating element failures, subsoil frost heave prospects and mitigation measures to assure the integrity of tanks during its residual service life. Tank heating system consists of heating tapes in 1" GI conduits concealed in embankment/concrete base. In each tank, 64 Nos. of conduits exist at 900mm c/c; about 17% of them failed and 45% are in blocked conduits, but functioning. Aged inoperative tapes could not be replaced due to corroded conduits, snapping and access problems. Based on the status of heaters failure/ performance & their current duty-cycle, soil/ ground water temperature below tanks, thermal analysis was conducted and appropriate "duty-cycle" implemented - to avoid the failure risk of 300mm thick tank bottom slab which was not designed for frost heave load. Frost heave is caused by Frost susceptible soil, Water presence and Freezing temperature; role of each forms the basis for studies. Soil samples collected from 5 Nos. 30m deep boreholes were tested and confirmed for frost-vulnerability. Permanent piezometers installed in boreholes upto 4m below tank base continuously monitor ground water level and temperature. Borescope/videoprobe investigation provided heating tapes and conduits condition. Based on gathered information, comprehensive simulation for different heater failure scenarios were performed using ground-freezing analysis models. Results of the study showed that by adjusting the heater duty-cycles to maintain cut in/out temperatures at +5/+10°C (41% duty-cycle), soil shall not freeze until 6 adjacent heater fail; while in reality there are only 2 adjacent heaters failures. It was also inferred that with 100% duty-cycle, upto 10 adjacent heaters can fail before freezing begins. Continuous maintenance activities namely de-choking/ nitrogen purging of conduits, monitoring soil conditions and implementation of recommendations from sound engineering analyses resulted in maintaining safety and integrity of ageing storage tanks. The multi-perspective studies about frost heave effect on cryogenic tanks in the wake of progressive failure of heating elements are exclusive in nature. The findings and mitigation measures will provide ample guidance and knowledge to the industry in managing and operating similar ageing storage facilities in a safe manner.