The Weekly Reflektion Week 04 / 2020
In our Weekly Reflektion for week 35/2018 we presented a near miss at the Sullom Voe terminal in Shetland and used the phrase ‘There but for the Grace of God go I’. The phrase was allegedly a statement by the English Reformer John Bradford in the sixteenth century made while watching a group of prisoners being led to their execution. The phrase is also appropriate when considering an explosion in 2006 on the Rough platform in the UK Southern North Sea.
Are you aware of safety flashes from past incidents that required immediate action? Were you made aware of them at the time? Were the actions followed up and documented? Are you sure the incident would not repeat itself on your facility?
On the morning of 16th February 2006 a senior production technician was on his way to the process module on the central processing platform on the Rough facility in the UK sector of the Southern North Sea. A scaffolder stopped him to request a signature on a work permit and the two spent 10 minutes discussing the planned work. At about the same time two instrument technicians working on a pressure switch on a separator on the central processing platform decided to remove the switch and were on their way back to the workshops on the accommodation platform. Just at that moment there was an explosion on the central processing platform followed by a fire that engulfed the platform. Two people received burn injuries however these were not serious. The production technician and the two instrument technicians should have been working near the location of the initial leak. They got lucky on that day. There but for the grace of god.
The cause of the explosion was a catastrophic failure of a shell and tube heat exchanger where seawater was used to cool high pressure (HP) hydrocarbon gas. The tube sheet had been manufactured from carbon steel with an explosively bonded, 13mm thick titanium cladding plate on the sea water side to provide corrosion protection. A key feature of this design is that it allows the HP hydrocarbon gas, condensates and water to come into contact with the steel tube sheet/titanium interface, thus creating the potential for in-service degradation. The water in the process stream led to galvanic corrosion of the steel at the bond interface. The hydrogen released from the corrosion process reacted with the titanium and resulted in the progressive formation of brittle titanium hydrides at the bond interface. The failure of the bond resulted in rapid pressurisation of the bond interface, complete separation of the titanium cladding plate from the steel tube sheet, followed by over pressurisation of the shell and catastrophic failure of the cooler. Shell and tube heat exchangers are normally protected from overpressure on the shell side by a bursting disc, however the disc is normally dimensioned for a single tube failure and not a failure of the whole tube sheet. The internal leak exceeded the dimensioning case for the shell side overpressure protection.
A safety flash was issued by the Health and Safety Executive in the UK and this called for identification of heat exchangers manufactured to this or similar design and a risk assessment of the continued use of the heat exchangers involving the manufacturer and third-party verifiers.
There was no mention of incorporating the learning into the appropriate standards for the selection and design of heat exchangers and no requirement for the operators to consider the incident in future designs.
Safety flashes are important in the communication of incidents and their causes and to suggest actions to prevent recurrence. Unless however these actions are made permanent by inclusion in relevant standards for design, construction and operation or are incorporated into the management system and project execution system for the operators then the learning may be lost.
Are there safety flashes that are relevant for your facility that you may have missed or never received? What are you going to do about this?
How on earth can we be expected to follow up incidents that we have never heard of?
HAZOPs are normally carried out on existing facilities every five years or in the event of a major modification. Next time the HAZOP is planned, include a review of safety flashes for process systems and equipment. Be proactive, be curious and don’t make your people become dependent on luck to get home after work.