IOGP high potential event report

Incident description:

A well was opened under natural flow with initial diesel and inhibited water circulated in the annulus and wellbore. The flare pilot was ignited, surface facilities lined up to bitutainer no. 3 with the separator bypassed and an operator positioned at the sampling point checking to determine when the diesel had been circulated out. Immediately the operator observed crude at the sampling point, flow was then directed to bitutainer no. 2 via the separator (NG-V-100). Bitutainers are provided in the process area as holding tanks for the de-gassed crude before transfer into the 5000 bbls Crude Storage Tanks (CST). One hour into the start-up operation, an explosion and flash fire event occurred in bitutainer no. 2. The explosion overpressure blew open the bitutainer’s manway cover and ejected the manway’s gasket which landed 20 meters away. The bitutainer tank sustained minimal deformity on its roof. There were no injuries to site personnel or environmental spill. Immediately after the explosion, the crew manually activated the emergency shutdown (ESD) system and went to muster as per the emergency response plan (ERP). The crew waited for approximately one hour and then returned to the process area, closed the wellhead valves, de-pressurised the separator and isolated valves associated with it.

What Went Wrong:

Tools and Equipment - Accumulation of static charges inside the tank during filling of the bitutainer without an adequate path for dissipation to ground. The inner tank is coated to 280 µm thickness compared with NFPA 77 recommended thickness of less than 50 µm for tanks that store low conductivity fluids. This very thick coating created a barrier to safe dissipation of accumulated static charges across the inner tank to the external earthing on the outer skin. Tools and Equipment - The manway cover was not closed tight hence a gap was created that allowed air ingress into the bitutainer, leading to the formation of a flammable air-vapour mixture in the tank. The turbine flow meter installed upstream of the bitutainer consistently gave inaccurate volumetric readings of crude compared to actual dip measurements. Due to the ineffectiveness of the flow meter, the crew resorted to visual checks of crude level in the bitutainer, an action that required regular opening of the manway. A sight glass level indicator is not installed on the bitutainer. Engineering design - Inadequate technical data and/or information on physical properties of Ngamia crude specifically electrical conductivity and the associated hazard of static electricity. This technical data was not available at the time when the EOPS Basis of Design was issued and during both HAZID and HAZOP studies. Without sufficient information and understanding of the crude’s physical properties a vital opportunity was missed that would have ensured sound basis for the effective management of static electricity in relation to the low conductivity of Ngamia crude oil.

Corrective Actions:

To minimise the potential for static discharge, follow the guidelines in NFPA 77. This requires that the coating thickness of tanks used to store low conductivity fluids to be less than 50 µm and that the flow velocity of fluids discharging into a storage tank which can contain flammable atmosphere be no more than 1 m/s until the fill pipe is submerged in the fluid. Detailed technical data and information should be made available to ensure proper understanding of the physical properties and behaviour of materials and to enable conduct of suitable and sufficient hazard identification and risk analysis.