Ibrahim Al Awadhi
Mechanical and Civil Engineering Department Manager
Adnoc Gas Processing
ADNOC Gas Processing
11:45 - 12:15
Thursday, 19 September 2019
T2.8 Cryogenic LPG Storage Tanks – Integrity Threats & Mitigation
Objectives / Scope:
LPG storage facility in an ADNOC Gas Processing in plant in UAE is 35 years old and contains two propane tanks and two pentane tanks each of 60m diameter and 24m height surrounded by massive conical embankments. Cryogenic storage temperature (-450C) 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 massive conical 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.
Methods, Procedures, Process:
Tank heating system consist of heating tapes installed in 1” GI conduits concealed in soil embankment/concrete base slab. 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 failure risk of 300mm thick tank bottom slab that was not designed for frost heave load.
Results, Observations, Conclusions:
Frost heave is caused by Frost susceptible soil, Water presence and Freezing temperature; role of each form the basis for this study. Soil samples collected from 5 Nos. 30m deep boreholes were tested and confirmed near tanks were tested and confirmed for frost-vulnerability. Permanent piezometers installed in boreholes upto 4m below tank base continuously monitored ground water level and temperature. Borescope/ videoprobe investigation provided heating tapes condition in embedded conduits.
Based on all gathered information comprehensive simulations 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 degrees C (41% duty-cycle), soil shall not freeze until 6 adjacent heaters fail; while in reality there are only 2 adjacent heaters failure. It was also inferred that with 100% duty-cycle, upto 10 adjacent heaters could 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.
Novel/ Additive Information:
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 industry in managing and operating similar ageing storage facilities in safe manner.