The Prelude’s power problems began on December 2, 2021, when a fire broke out in a room housing a backup battery system. Shell’s investigation was unable to determine the exact nature of the failure. The company therefore plans to address all possible causes it has identified.
The fire triggered an emergency shutdown of the Prelude, after which there were several days of unsuccessful attempts to fully restore power.
To be allowed to restart production, Shell must convince NOPSEMA that it can safely recover power and essential services after a power failure and that it can ensure the safety of the Prelude crew.
A NOPSEMA spokeswoman said it was reviewing Shell’s plan to determine if it was satisfactory.
“Ventilation and air conditioning, potable water and sewage systems were damaged during the outage, making the facility increasingly uninhabitable,” Shell’s report noted, leading to reduce the number of crews by more than half from 293 to 137.
During power outages, the crew had to haul diesel canisters up many stairs in the humid heat to fill a fuel tank for an emergency generator. Two crew members involved in the effort, but not carrying fuel, required medical treatment for heat stress.
Shell’s investigation found that manual filling could have been avoided, but the operation of an automatic fueling system had not been documented in emergency procedures.
A total of seven crew members fell ill from the heat and four required treatment.
Shell’s survey revealed hot living quarters with limited airflow, the need to relay messages on stairs due to failed communication systems, and lack of sleep due to long hours and to the constant emergency calls contributed to the heat stress of the crew.
When Prelude closed her processing equipment was emptied of oil and gas to make the ship safer. The oil was burned atop a giant flare designed to draw intense heat away from the ship and crew.
The process, called purging, should happen in a certain order, but electrical problems caused the final stage to start two minutes early.
Shell discovered that an untimely purge could cause excessive vibration leading to uncontrolled gas leakage and excessive heat which would make it more difficult for the crew to traverse open areas of the ship.
NOPSEMA’s initial investigation in December found that there could have been a “catastrophic failure” due to structural steel overcooling near the Prelude’s LNG tanks during the power outage.
Shell’s report said steel could have fractured had the power outage continued, but a gas leak was “not credible” due to the design of the tanks.
The NOPSEMA spokeswoman said the regulator had identified the risks associated with LNG in the tanks and the potential impacts on the integrity of the facility.
“Shell responded to this finding in its report and NOPSEMA is reviewing this information,” she said.
The December incident came at the end of the first year the unique and complex Prelude had achieved significant production since arriving in Australian waters in 2017.
The vessel accounted for the bulk of Shell’s 3% increase in gas production in 2021 and a 6% jump in liquids production, according to Shell’s 2021 annual report released on Thursday.
At full capacity, the Prelude can produce 1.3 million tons of light oil called condensate and 400,000 tons of LNG per year, as well as 3.6 million tons of LNG.
The Prelude has a poor record of flaring, or burning, excessive gas. This practice, partly due to frequent shutdowns, produces significant amounts of carbon emissions.
Shell has excluded Prelude emissions from its calculation of the emissions intensity of its global production in 2021.
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