2011X63 Explosion on Tanker Mersey
Explosion on Tanker Esso Mersey - report No. Mars10 offrep
Events
The vessel, loaded with naptha and premium grade petrol, arrived at Fawley during the afternoon of 3rd September 1991 and commenced safety checks prior to discharging. The Esso Vessel Inspection Officer boarded and, after consultation with the Chief Officer, the ship/shore safety check list was completed satisfactorily. The "EEC Tanker Check List" was also approved. A cargo surveyor took samples, ullages etc. and agreed with the refinery the cargo discharge plan previously prepared by the Chief Officer (C/O) and approved by the Master in accordance with Esso's established procedures.
At 2000, when the C/O handed over to the 8/12 Second Officer (2/O), all checks had been completed but no valves had been opened. The standard Company "Enclosed Space Entry Permit" system had been put into operation at 1630, this required a hydrocarbon gas test to be taken at least every 4 hours in the pump room. The discharge plan was to use 1 & 2 cargo pumps to discharge the naptha via manifold 2, and number 3 pump to discharge petrol through manifold 3.
When the valves were opened to No 3 pump, the pump separation chamber was noticed to be leaking, the line was shut down and the C/O called. A further gas test was carried out before the C/O entered the pump room, and it was decided that, because a leak had occurred, additional testing over and above the permit requirements should be carried out in the pump room before entry and that the cargo control room should also be tested at intervals.
A revised discharge plan was approved and implemented using No 4 pump instead of No 3, and discharge started at 2048. At 2400 the watch was taken over by the 12/4 2/O with another able seaman. During the next 4 hours two visits were made to the pump room: the small leak from No 3 pump separator chamber was confirmed and an additional small leak from No 4 pump top mechanical seal noted. The pump room entry permit was again revalidated at 0030 and an acceptably low hydrocarbon gas reading was obtained in the pump room. Two further tests during the watch also gave acceptable readings. The cargo pump temperatures were monitored during the watch and gave no cause for alarm.
The C/O, together with a new able seaman, took over the watch at 0400 and a decision was made to alter the discharge sequence to give a slight port list in order to assist stripping the cargo tanks. At 0430 the pump room gas reading was less than 1% of the lower explosive limit (LEL). At this time it was decided to further assist stripping by increasing the vessel's stern trim. This required opening valves on deck and the C/O went with the A.B., who was new to the vessel, to show him the correct valves.
On return to the poop deck at approximately 0500, the C/O immediately smelt a strong concentration of petroleum vapour just outside the cargo control room. He told the A.B. that something was not right and to be ready to go forward and close the manifold valves. A gas reading just outside the cargo control room now showed 20% LEL. A different meter was used to check inside the control room, this also showed 20% LEL.
Deciding to shut down all cargo operations, the C/O called to the A.B. to close the manifold valves, and as he was turning towards the stop button of No 4 pump, there was a rattle, an orange flash, a very loud bang and he found himself on the control room deck.
The 12/4 2/O went immediately towards the cargo control room. On the way he met the C/O, who was badly burned and in a state of shock, led him to the nearest cabin for cold water treatment, and then went to the pump room. He found debris all over the deck and thick black smoke coming out of the cargo control room doors, he reported this to the Master, who had been wakened by the explosion and gone to the bridge. The Master, who was by this time on the bridge, contacted Southampton VTS and the Esso Marine Control on the VHF whilst the 2/O went back to the forward deck to secure the cargo system. He found the missing A.B. slumped over the flying bridge rail and apparently dead. He shut all 3 manifold entry block valves using the remote controls on the flying bridge before going on to the main deck to manually shut No 2 and 3 manifold valves. He then informed the Master of the situation via the hand-held radio situated by the gangway and said that he was going to shut all valves on deck, he also advised the shore personnel that the discharge had stopped. All cargo operations on other berths were stopped and under the Esso Emergency Plan the incident was classified as "Category 2" - a situation that required back-up facilities beyond the scope of Esso site personnel. VTS suspended traffic movements past the berth until further notice.
The 8/12 2/O mustered the crew aft for a personnel check, obtained a hand held radio and was instructed by the Master to lower the port lifeboat to the embarkation deck and to activate the bridge front water wall. At 0525 the Master instructed the engine room to flood the pump room with CO2 but not to operate the multispray at that time. The Fire and Ambulance services arrived at approx. 0527 and took the C/O to hospital. At 0555 the Fire Brigade injected foam into the pump room as a precautionary fire measure as well as to contain any gas emission.
Containment and Recovery Operation
High gas readings were still showing at 0800. About an hour later firemen and the 2/O were able to confirm that three leaks were present in the pump room, two of sea water from the ballast system and one of cargo from the bottom of No 4 pump. The cargo valves still open to No 4 pump were the suction valve, deck discharge valve and No 4 centre bulkhead valve. Due to concern over the state of the hydraulic system, these valves were closed progressively during the day using a portable hydraulic jack. Whilst the valve closing was going on, it was decided to shut the vessel down due to the danger from the high gas levels still in the pump room. At 1000 the ship's power system was shut down and all sources of electrical power isolated, including the batteries.
The cargo and gas containment operations continued over the next two days with the Fire services remaining in attendance. At 0305 on 6th September the vessel was cleared of personnel and gas freeing of the pump room started. At 0630 on the 7th the Fire services considered the situation safe and left the site. Inerting of the cargo commenced at 0210 on the 8th and discharge resumed at 0640 that day.
The Chief Officer died in hospital from his injuries on 7th September.
Cause of Explosion
The conclusions reached after the preliminary examination were that:
1. The fire damage in the pump room at plate level was consistent with the combustion of a pocket of rich gas (at the upper explosive limit) at, or above, head height in the vicinity of No 4 cargo pump and the propagation of a flame front away from this area.
2. The damage in the upper parts of the pump room and in the cargo control room was consistent with the passage of unburned product mixture followed by the flame front and combustion products into the cargo control room via the epoxy partition below the control console and mimic panels before finally venting out forward through the control room doors.
Report of Failure Sequence
The detailed exam-ination of No 4 cargo pump suggests a probable sequence of events from pump failure leading to release of product and eventual explosion:
The lower impeller 1st stage outer locknut came loose and backed off. This allowed the locknut holding the 1st stage impeller to free sufficiently to allow the 1st stage impeller to move on the shaft and allow free movement of the interstage sleeve between the impellers. The cause of the initial slackness in the impeller outer locking nut was not positively identified. However, it was noted that the locking grub screws were absent and that the products of corrosion were evident in the 1st stage impeller lock nut grub screw holes. The debris found in the threaded holes was such that it was not possible to say whether grub screws had been refitted after a 1989 overhaul and the material had corroded away, or simply that the screws had not been replaced.
The resultant mechanical looseness in the rotating unit made it less stiff and brought the critical running speed of the rotor close to the actual running speed of the machine. This looseness caused imbalance in the rotor and increased the vibration levels. Discoloration of the faces of the locknuts indicated that they had been loose for some time.
One of the top bearing pedestal dowels was not fitted; this was likely to mean that the pump shaft was not running through the centre of the casing. This being the case, it caused instability through the centre bush and affected wear ring clearances, thus further increasing vibrations.
The lower wear ring on the 2nd stage impeller most likely came into contact with the casing wear ring, heated, expanded and started to move off the impeller. This caused further rubbing and heat until it finally came off and rested in the intermediate piece. This put the rotor further out of balance.
The high vibrations in the machine were sufficient to loosen the bolts in the cardan shaft guard and the top bearing pedestal. It would appear that these bolts came out over a lengthy period of time with the last remaining bolt being the second from the starboard side.
The final bolt in the top bearing pedestal worked its way out allowing the pedestal to swing about the remaining dowel. This movement then shattered the seal secondary containment carbon and severely chipped the primary seal faces on both the top and bottom seal causing major seal leakage.
The edges of the 16 bolts clamping the cardan shaft Hardy Spicer swivel joint to the pump drive coupling, protruded over the edge of the coupling periphery. Following the release of the cardan shaft guard restraining bolts and the pedestal retaining bolts, the pump would have been vibrating severely. At this time, movement of the assemblies would have been such that the coupling bolts and nuts would come into contact with the cardan shaft guard. Evidence of the impact was clearly visible inside the guard and is considered to be the ignition source.
Pump maintenance
The mechanical condition of this cargo pump prior to the incident gives rise to concern as some of the defects noted in the examination have their origin in quality control of contractor overhauls as well as the standard of maintenance and supervision on board the vessel.
In 1981 an incident involving the failure of a cargo pump and fire raised the question of maintenance on the frequently utilised pumps. The Esso report of that incident noted that the then current maintenance system based on elapsed time overhauls and defect rectification should be reviewed and concluded that "a need exists for an effective planned maintenance system to be introduced on board".
That report was acted upon with the result that modifications were carried out to the rotating element of the pumps, the type of mechanical seal used was changed and a planned maintenance system introduced. The records on the Esso Mersey showed that routine work required under the Planned Maintenance system had been carried out at the frequency specified.
Structural Integrity
At the time the ship was built there were no statutory requirements for the segregation of the cargo control room and the cargo pump room. For regulation purposes, this area was defined as a "Dangerous Space" and therefore had to comply with the various special requirements laid down by the Classification Society and their Rules for these designated areas. All the electrical equipment in these areas also had to comply with the regulations for "Dangerous Spaces". All those requirements were met in the combined cargo pump and control room installed on the Esso Mersey when built in 1972. The requirement for these spaces to be separated is contained in the 1984 Fire Protection Regulations but is not retrospective and would therefore not apply.
A large number of small diameter hydraulic pipes led from the underside of the cargo control console. A gas tight seal around these was made by the use of poured resin panels, this was not a mandatory requirement but was a reasonable and practical measure to provide a better environment for the operators and to reduce their exposure to a dangerous atmosphere. After the explosion an examination of the properties of the resin panels was undertaken by Esso. They found that the material was flammable, had low thermal conductivity and had poor impact or significant loading characteristics.
Safety Procedures
The standard of safety training within Esso ensured that the emergency procedures specified both in the Safety Manual and those developed for the Esso Mersey were all put into effect without delay or confusion. Following the explosion the ship's staff immediately and effectively put into operation their emergency procedures. Both Southampton VTS and Esso Marine Control were informed by the Master whilst an assessment of the damage and the cargo state was carried out by the 2/O. All cargo tanks were isolated, the procedure for "Fire in the Pump room" carried out and all non-essential crew mustered aft. Esso Emergency Control Centre responded and classified the incident as "Category 2" thus involving Hampshire Fire Brigade.
An incident room was established with the Marine Manager assuming overall control. Close co-operation between the Fire Brigade, Esso Authorities and ship's staff over the next four days ensured that a hazardous containment and retrieval operation was brought to a safe and successful conclusion. On the day of the explosion and subsequently, members of the ship's staff and relieving crew members made a number of entries to the pump room wearing SCBA sets to make an assessment of the damage and to carry out valve closures. These personnel, all volunteers, carried out their duties in a known dangerous environment and their actions are to be commended.
The accident highlights the reliance which is placed on hand-held radios, particularly in an emergency. The vessel normally had eight such radios with two fixed base stations. The Muster List specified that the Command Centre on board would normally be manned by three persons, on this occasion two of those would have been the C/O and the AB who was killed in the explosion. This left the Master alone on the Bridge. The way in which the emergency procedures were handled on this occasion is testimony that the Master successfully coped with his responsibilities single handed, for which he is to be commended.