GhubrahIncidentInves.. - authority for electricity regulation, oman

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Report Number: R 8991 Issue: 1.1 2 FINDINGS On arrival at site, Senior Sogex personnel (including the site General Manager) were interviewed to determine the circumstances and the probable root cause of the incident. Work permits, work orders, procedures, evidence of competencies and health and safety documents were requested and received. The author received full and open co-operation from Sogex personnel throughout the investigation. On the 25/03/06, a Limitation-of-Access (LOA) Permit-To-Work (PTW), LOA No.10795/L was issued to carry out routine maintenance on the external surfaces of the Phase 3 Hypochlorite Storage Tanks, C and D (See Appendix A). The permit was issued for (1) cleaning, (2) surface preparation and (3) painting. The cylinders were to be left in service and the permit clearly states that no isolations were to be carried out and that the civil department (conducting the painting operation) were to take all safety precautions. In this respect this specific PTW/LOA covered activity of a ‘cold’ work nature, although it is observed that the relevant section of the permit designed to indicate ‘hot’ or ‘cold’ status was left unspecified. On inspection of the job site on cylinder ‘D’, the civil department reported that the cylinder jacket was badly corroded and hence a work order (Reference No. LO 42/04/06) was raised on 05/04/06 to the Mechanical Department, to replace the cladding (hood) on cylinders A, B, and D (See Appendix B). The work order No. LO 42/04/06 references PTW/LOA No. 10795/L. Evidence was requested regarding the competency and authorisation of the permit issuer and work order issuer. Sogex personnel produced relevant CVs and authorisation memos stating that the persons concerned had been assessed and found competent to sign permits (see Appendix C). The work to replace the cladding involved the removal of the corroded cladding and the fabrication and installation of replacement cladding. To replace the cladding it is necessary to use drills for the riveting process. It appears that the mechanical fitters were left to complete the work with no periodic site inspections by the management and no safety departmental presence. Work on the ‘C’ Hypochlorite cylinder had been performed under a separate internal work order (LO 37/03/06, not included in this report), without incident. However, during the process of installing the newly fabricated hooding of the ‘D’ cylinder, an explosion occurred resulting in a ‘fire ball’ that fatally injured the two Sogex maintenance fitters involved. They reportedly were able to climb down from the top of the cylinder, a difficult feat, to seek assistance. Once the incident had been discovered, the Hypochlorite plant was shut down and the two employees were immediately transferred to hospital where they were placed in intensive care, but subsequently passed away. Page 5 / 11
Report Number: R 8991 Issue: 1.1 3 PROCESS Hypochlorite is used during the desalination process to control bacterial growth within the system. Hydrogen is a by-product of the process. The Hypochlorite treatment process separates the majority of the hydrogen in separators prior to being pumped into the hypochlorite storage cylinder. Each cylinder is allowed to vent to atmosphere, and to assist the ventilation process, three fans, two running and one in standby are used, common to the four cylinders. Although the majority of the hydrogen is separated, prior to storage, there still remains a ‘trace’ level. The ‘D’ hypochlorite cylinder was reportedly 80% full (liquid) allowing the remaining head space to fill with hydrogen separated from the hypochlorite before venting to atmosphere. Note, the cylinders vent to atmosphere and are therefore not classed as pressurised vessels. 4 PROBABLE CAUSE 4.1 Inadequate Permit to Work considerations Permit PTW / LOA No. 10795/L does not state whether it is a hot or cold permit to work, neither box is ticked. However, the wording on the permit indicates that it was considered to be a ‘cold’ PTW covering the cylinder painting activity only. Under a ‘cold’ permit to work the painters contacted mechanical maintenance to assess and / or replace the hooding on the four hypochlorite cylinders. Work order No. LO 42/04/06 was issued and progressed for Tanks A, B and D with reference to LOA No. 10795/L with no Risk Assessment evident. Repair maintenance to Cylinder ‘C’ was completed without incident under a separate Work Order LO 37/03/06 (not included). During the maintenance activities on the ‘D’ cylinder the two maintenance fitters were using an electric drill in proximity to the cylinder vent, where the probability of hydrogen gas being present should have been considered to be the norm, and therefore a significant risk to health and safety given the work involving a potential source of ignition. 4.2 Probable source of ignition The electric drill being used at the time of the incident was of a standard ‘DIY’ type with the original plug removed and ‘taped’ to extension cord with a 13 amp, standard 3-pin domestic plug. The plug was inserted into an extension lead drum (still coiled (not good practice)) with an industrial plug fitted. An electric drill will always issue sparks from the brushes and in all probability a spark from the drill brushes (or at least, some similar form of ignition) ignited the hydrogen gas venting from the hypochlorite tank which caused the hydrogen within the cylinder to explode (gas was under the constraints of the cylinder) and hence a ‘fire ball’ issued from the vent causing the fatal injuries to the two maintenance fitters. 4.3 Related Observations The explosion was of sufficient force to lift the entire cylinder c. 100mm from its anchoring bolts and snap a 200mm drainage pipe with pipe wall thickness of c. 12mm. Page 6 / 11
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GhubrahIncidentInves.. - authority for electricity regulation, oman

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