Hydrocarbons

The sequence of events on 16th April 2001 unfolded rapidly. Prior to the incident, the SGP unit was reported to be operating normally with no recent process upsets. Unbeknownst to the operators, the critical elbow in pipe P4363 was severely weakened. At approximately 14:20 hrs, the elbow failed catastrophically due to the internal pressure exceeding the reduced strength of the pipe wall. This resulted in a high-pressure release of LPG, modeled as a full-bore rupture directed downwards at 45 degrees, with an initial flow rate of 133 kg/s. A large, dense vapour cloud quickly formed, expanding rapidly; modeling suggested a maximum flammable cloud volume of 10,000-15,000 cubic meters, reaching approximately 175m in length and 80m in width within seconds. Between 20 and 30 seconds after the initial release, the cloud encountered an ignition source, identified by the HSE investigation as likely being a fired furnace located 65m away. This triggered a massive VCE, with calculated overpressures reaching up to 0.5 bar within congested areas of the plant. The resulting blast wave caused immediate and severe damage across the SGP and adjacent areas, shattering windows and damaging structures both on and off site. A large fire erupted instantaneously, engulfing the SGP. The intense heat from this fire impinged on other nearby pipework and equipment. Approximately 10-15 minutes after the initial explosion (around 14:35-14:50 hrs), this heat exposure caused secondary failures (domino effects) in other pressurised systems due to material weakening. These subsequent releases fed the fire further, resulting in additional fireballs and increasing the overall intensity and scale of the blaze. The site emergency response team, supported by public fire services (Humberside Fire Brigade) and firefighters from the adjacent Lindsey Oil Refinery, responded promptly. The fire was brought under control by approximately 15:30-15:40 hrs (within about 70 minutes of the explosion). Efforts focused on isolating fuel sources by manually closing valves where possible. The fire was largely extinguished by 17:10 hrs and officially declared “struck out” by the fire brigade at 21:01 hrs.

Failure Mechanism Deep Dive (Erosion-Corrosion): The failure mechanism was definitively identified as erosion-corrosion occurring internally at the failed elbow. The carbon steel pipework (P4363) normally relies on a protective passive layer of iron sulphide (FeS) forming on its internal surface in the presence of corrosive species like H2S within the process stream. This layer significantly slows down the rate of general corrosion. However, the introduction of the continuous water wash, injected without a dispersion quill just upstream of the 90-degree elbow, fundamentally altered the internal environment. The jet of water physically impinged on the inner wall of the elbow, particularly at the outer radius where flow direction changes and turbulence is highest. This mechanical action (erosion) continuously removed the protective FeS scale layer. Once the protective scale was removed, the underlying bare carbon steel was directly exposed to the corrosive components (H2S, ammonia, chlorides) in the high-temperature, high-pressure gas stream. This led to rapid localized corrosion. The synergistic combination of mechanical erosion removing the protective layer and subsequent chemical corrosion of the exposed metal resulted in a dramatically accelerated rate of wall thinning specifically at the point of impingement. Over time, this localized metal loss reduced the pipe wall thickness from its original 7-8mm to less than 1mm, with a minimum recorded thickness of just 0.3mm. At this point, the pipe could no longer contain the internal operating pressure, leading to the sudden and catastrophic rupture.Â

Analysis of Management System Failures: The detailed analysis points to systemic failures rather than isolated errors. The MoC process failed critically when the water wash was introduced and later changed to continuous operation. It did not adequately identify or assess the significant erosion-corrosion hazard created by the modification, focusing instead on the intended operational benefit (fouling reduction). Using an existing vent point without proper engineering design (no quill, poor location) compounded the risk introduced by the change. The pipework inspection program was ineffective in detecting this specific failure mode. It likely relied on generic corrosion allowances or inspection intervals not suited for the localized, accelerated degradation occurring post-modification. The inspection strategy did not adequately target the high-risk area created by the water wash impingement near the elbow. Corrosion management practices failed to recognize or address the specific threat of erosion-corrosion in this context. Even if general corrosion was monitored, the localized mechanism driven by the water wash was overlooked or underestimated. Reports suggest that specific alerts about potential problems at injection points were not sufficiently acted upon. Communication breakdowns contributed significantly. The lack of effective information sharing about the change to continuous water injection prevented other groups (like Inspection or Engineering) from reassessing the risks or adjusting integrity management plans accordingly. Essential knowledge, potentially available from industry experience regarding this failure mechanism , was not effectively applied.Â

Mitigating Factors and Potential Severity: The most significant mitigating factor was the timing of the incident on a public holiday during shift change preparations, which drastically reduced the number of personnel exposed outdoors. Weather conditions (7.2 m/s northerly wind) influenced cloud dispersion, but modeling indicated that alternative wind scenarios were unlikely to have caused ignition at other potential sources or resulted in a significantly worse outcome, such as impacting the nearby HF Alkylation unit. Despite the low casualty count, the physical destruction clearly demonstrates the event’s massive energy release and catastrophic potential. The HSE explicitly stated the incident could have been far worse, drawing comparisons to the scale of the 1974 Flixborough disaster.