Examples of the Emergency Situations with Liquefied Gas Carriers

Work on gas carriers (LNG or LPG) demand excellent understanding of safety rules, good concentration and qualification. This article shows examples, where mistakes led to incidents, which can be named emergency situations.

Case Study No. 1 – Overfill of Cargo Tanks and Subsequent Freezing of a Relief Valve

A semi-pressurised ethylene carrier was loading 2 500 m³ at a river berth. The Chief Officer was in charge of the watch and by his estimation loading of cargo should finish in 2,5 hours.

The ethylene carrier was fitted with a gas blower that was able to transfer large amounts of gas ashore while loading. Removal of gas from the tanks in volume caused a flash cooling effect.

A gas blower is like a vapour compressor.

The Chief Officer heard a noise from the gas blower and, as he investigated it, liquid ethylene erupted from the mast riser. At this point he stopped the blower and closed the tank filling valve. He did not activate the ESD but instead went to advise the jetty operator verbally.

Liquid ethylene again erupted from the mast riser and spread onto the vessel’s deck. The fire hoses that had been rigged and charged either side of the ship’s manifold were used to flush the liquid ethylene overboard, causing further evaporation. The cargo tank relief valves had failed to re-seat.

Emergency stations were called and, at this point, the fire hoses were directed on to the frozen relief valve in an attempt to warm it up and allow it to subsequently re-sit.

The amount of gas that the ship was venting was getting worse when the Captain gave the order to go “dead ship”, i. e. the generators were to be shut-down. For safety, all crew were disembarked, with only two officers remaining on board. A further 2 hours elapsed before the relief valve re-seated.

Case Study No. 2 – Tank Overfill and Mast Fire

A ship was loading propane in calm conditions at a sheltered jetty. The final tank was topping-off but, due to lack of attention from the OOW it overfilled. This caused the relief valve to lift and liquid to be vented from the mast riser.

The liquid that escaped from the mast riser was rapidly vapourising and when it flowed over the ship’s side it was ignited by the engine of a boat moored alongside.

A number of similar incidents, where escaping LPG vapour has been drawn into the air inlet of a petrol or diesel engine and resulting in the engine over-speeding, have occurred. In this instance, the engine increased speed before finally disintegrating. The vapour was ignited and flashed back to the mast riser.

The mast riser was fitted with a flame screen that prevented the flames returning to the tanks. Until the relief valve sat, fire continued to rage at the top of the mast riser as it was being fed with vapour from the Types, Layouts and Designs of the Liquefied Gas Carriers (LNG/LPG)cargo tanks.

Case Study No. 3 – Collision

In thick fog, a collision occurred between a semi-refrigerated gas carrier of approximately 2 000 m³, with a full cargo of butadiene, and a container ship. The container ship pierced the hull of the butadiene tanker between the two horizontal cylindrical cargo tanks. The depth of the contact was sufficient to indent the aft end of the forward cargo tank and carry away some deck cargo piping.

The prompt activation of the gas carrier’s ESD reduced the amount of gas released to atmosphere to a minimal amount. Flooding of the hold space exceeded the capacity of the pump system and the decision was made to abandon ship. The engine room was abandoned with the main engine still running and the vessel sailed in circles, completely unmanned, until she eventually heeled over to an angle where the funnel was partly immersed and the machinery stopped of its own accord.

Although the vessel’s cargo tanks were full of butadiene at a Specific Gravity of, 0,57, the buoyancy was sufficient to keep the vessel afloat.

Following an investigation of the ship the decision to salvage the vessel and its cargo was made. A semi-refrigerated vessel of similar capacity was arranged to lighter the partly submerged ship and, with the aid of flexible hoses, transfer of the cargo was started using the compressors of the loading vessel to create over-pressure in the damaged tanks.

During the transfer, an emergency condition arose and the lightering vessel broke away from the hulk without disconnecting the hoses. The release of gas caused by the cargo hoses parting did not result in an explosion or fire.

The cargo transfer was then resumed and completed without further incident.

Case Study No. 4 – Slippage Alongside the Berth

A gas carrier was loading propane at a jetty
where there are strong tides and currents of
up to 4 knots. The Chief Officer observed that a spring line forward was caught under a mooring dolphin.

As he slackened the spring line to clear it the ship was moved along the jetty by the current, causing the cargo hoses to rupture and result in a loss of cargo.

Case Study No, 5 – Inappropriate Use of Cargo Hose During Discharge and its Subsequent Rupture

A ship was discharging a full cargo of fully refrigerated anhydrous ammonia, at -33 °C, via hard arms. When it was alongside, the jetty staff realised that the ship’s manifold was out of the hard arm envelope and so a direct connection was not possible.

Having no hoses available on the jetty, it was decided to lash the end of the hard arm to the ship rails and then connect the ship’s manifold to the connecting flange of the hard arm with a ship’s hose.

After discharge was started and the maximum cargo discharge rate achieved, the hose burst.

The spilled ammonia caused severe burns to two crew members. Another five crew members were treated in hospital for minor burns and symptoms of asphyxia.

Case Study No. 6 – Loss of Hold Pressure

A 35 000 m³ Semi-Pressurised LPG Carrier was on passage to the Black Sea to load NH₃.

The carrier had 8 individual tanks, each with an individual hold space. The hold spaces were all under dry air at a slight pressure of 40-50 m/bar.

In the evening, the vessel was in the Marmara Sea where the outside temperature was +15 °C. The following morning it was in the Black Sea, where the outside temperature was -5 °C (a drop of 20° overnight).

Each hold space was fitted with a dedicated “top-up” dry air compressor and this was set to automatically top-up the holds with dry air if required.

While each hold space was fitted with a positive pressure alarm, there was no alarm for vacuum occurrence.

In the morning all the hold spaces were shown to be at least -100 m/bar in vacuum and the top-up compressor was not able to keep up with the contraction experienced by the cargo tanks that was caused by the dramatic drop in the outside air temperature.

The action taken on the vessel was to vent each hold space, and then crack open the by-pass valve until each hold space was at ambient pressure and so could be topped up.

Case Study No. 7 – Lightning Strike While Gas Freeing

The vessel concerned was a 30 000 m³, fully refrigerated gas carrier.

The vessel was completing a grade change from LPG to NH₃ and was under commercial pressure to be ready to load the following morning. On board, this caused a sense of urgency to complete the grade change.

That evening, while the vessel was still on passage to the load port, a “far off” thunderstorm was sighted in the distance, but the vessel thought it “safe enough” to continue venting.

The vent mast was struck by lightning and, although no fire or explosion occurred (perhaps due to the NH₃ that was being vented), the upper half of the mast was stripped of all paint.

Case Study No. 8 – Increased Loading Time on a Semi-Pressurised Ship

A vessel was nominated to load 29 000 m³ of propane at a loading temperature of -20 °C. It was to load to 98 % capacity.

The vessel loaded to the capacity of the refrigeration plant and, as she was to load to 98 % full, the propane needed to be fully refrigerated to -42,3 °C, on board.

Read also: Basic Information about Liquefied Natural Gas Bunkering Operations

This is believed to be an example of where the charterer did not fully comprehend the implications of loading a “warm” cargo in a 98 % lift, which can only be achieved when the propane is fully refrigerated.

Loading the cargo, for which the charterer had allowed 2 days, took 10 days. This was at a considerable financial loss and was unlikely to have been factored in to the commercial arrangement made.

Case Study No. 9 – Vapour Release Caused by the Enthusiastic Actions of an Over-Familiar Inspector

A fully refrigerated LPG Carrier of 52 000 m³ arrived to discharge a full cargo of propane and butane, and berthed starboard side alongside the jetty.

The vessel had completed the ship/shore safety check-list and the OOW was assisting in setting the lines for discharge. Between the ship’s port and starboard manifolds there was a manual valve on the liquid line, but no manual valve on the vapour line.

The starboard manifold was being prepared for discharge and the blanks had already been removed from the liquid and vapour lines with the OOW, some crew members and shore staff in attendance.

While the ship was preparing for discharge the maritime authority of the ship’s flag state was conducting an inspection. One of the maritime inspectors had previously sailed on the ship and was excited to be back on board.

In the absence of the Ship’s Officers (who were preparing the ship for discharge) the inspectors were working their way through the inspection. One of the items on the inspectors list was to check the closing time of the remote actuated valves, and the inspector felt it acceptable to open the remote operated vapour manifold valve on the port side. However, the inspector inadvertently opened the starboard side vapour valve (that did not have a manual valve) and, with the vapour valves on all the tanks open and the tank pressures of 180 m/bar; this opening caused a release of “wet” LPG vapour over those in the vicinity.

Case Study No. 10 – Gassing-up from the Deck Storage Tank

A ship that had 3 tanks was gassing-up with LPG vapour from the deck tank. The following, which is a too frequent occurrence, provides an example of an outcome that demonstrates no comprehension of the properties of liquefied gases by those involved.

The practice on board was to put only one tank under LPG vapour from the deck tank.

As a cost-saving exercise, it was the established practice to take only the minimal amount of LPG necessary to gas up a single cargo tank from the deck tank on each occasion. This would leave the gassing-up and cool-down of the other two cargo tanks until after the vessel had loaded an amount of cargo to facilitate a full vessel cool down at the load port.

This practice, while saving a few tonnes of liquid in the deck tank, results in a longer cool-down period at the load port while the other two cargo tanks are gassed-up and cooled down.

What frequently happens is that, when only a minimal amount of vapour has been introduced to the cargo tank at ambient temperature, when the compressor is started in order to cool-down the tank it rapidly goes into vacuum.

Note:

The total cargo quantity to gas up and cool-down a tank to a given temperature ready for loading is:

DT is calculated using the formula in Cargo Total Weight Calculation of Liquefied Gas on the LNG and LPG Carriers“Cargo Calculations on Modern LNG and LPG Tankers”:

where:

• P = requested tank pressure in barg (i. e. 1,030);
• T = requested tank temperature in °C (i. e. -35).

The pressure to achieve in the tank before cooling down will be:

which, neglecting the tank shrinkage factor, will simplify to:

hence:

So for instance if the vapour temperature to get before loading is -35 °C and the gassing-up is done at 20 °C, the tank must be pressurised to the follwing pressure to keep 30 mbarg (1,030 bar) positive pressure after the cooling-down:

Case Study No. 11 – Tank Overfill on Commencement of Discharge

A 30000 m³ gas carrier, that had 3 cargo tanks, was on passage to discharge a full cargo of ammonia.

About 7 days from the discharge port, the bilge alarm failed when tested. The alarm card from the No. 2 tank hi-hi level level alarm was removed as a substitute for the bilge alarm card. This was not replaced.

The day before arrival at the discharge port the cargo compressors were set up to return condensate to the No. 2 tank, to a level where the hi-level alarm initiated.

It is not clear what actions were taken after the hi-level alarm was activated, but it was later discovered that the No. 2 hi-level alarm was still in “alarm” when the ship commenced discharge. Discharge commenced from No. 1 tank and minutes later liquid ammonia belched out from the forward mast riser.

It was discovered that the two ball loading valves on No. 2 tank were leaking, so when discharge started from No. 1 tank cargo was entering No. 2, where the hi-level alarm was currently in alarm and from where the hi-hi level alarm had been removed. Because of this, when the No.2 tank filled to overflow there was no alarm indication at all.

The authorities would not allow the vessel to continue discharge in that country and a very unhappy charterer had to find an alternative discharge port for this cargo of ammonia.

Case Study No. 12 – Rupture of Cargo Heater

A 15 000 m³ gas carrier was discharging a full cargo of ammonia into pressurised storage using the ship’s booster pumps and cargo heater.

Discharge was about halfway through when a large “thud” was heard from the deck area, in the locality of the cargo heaten and a zero reading showed on the manifold pressure gauge.

Sometime later, it was discovered that the shore had inadvertently closed a valve against the ship, while the ship continued to discharge, and that the weakest point between the booster pumps and the closed valve was the shell tubing in the cargo heater.

A large number of tubes in the cargo heater had ruptured and the thudding noise was caused by the reaction of the ammonia with the sea water when the cargo heater imploded.

Discharge was suspended for 12 hours to carry out repairs.

Case Study No. 13 – High Dew Point While Purging and Subsequent Damage

A 78 000 m³ gas carrier with 4 tanks left Singapore, bound for the Arabian Gulf, to load a full cargo of propane for discharge in Brazil.

The cargo tanks remained under air on passage and there was a debate on board about how much IG to introduce to the tank. The Chief Engineer wished to inert the tank and then pressurise it to 200 mbar.

The Chief Engineer’s request prevailed and the tanks were purged with inert gas before the vessel passed the Quoins, some 24 hours before the load-port. What the ship staff were unaware of at this time, or did not fully appreciate, was that the silicone drying towers were not functioning properly and so the inert gas entering the tanks was moist and not at the desired dew point of -50 °C.

The vessel loaded 2 000 m³ of propane to No. 2 tank and then proceeded to anchor to cool-down. It returned alongside to load the balance of the cargo.

As loading to all tanks commenced, the cargo pump shafts were “spun” to check for free movement and to confirm they were not freezing. At this point, reports of the pumps having frozen on all of the tanks were received. Extra crew were posted to the tank domes to continue turning the pump shafts and soon the entire stock of methanol that was on board had been injected into the cargo pump deep wells.

The ship completed loading, but now proceeded to Dubai to receive 15×200 ltr drums of methanol, which were injected/ poured down the tanks. For the first 3 days on passage, the crew were posted on the tank domes around the clock to try and ensure free movement of the cargo pumps. The ship managed to complete its discharge but left the Brazilian discharge port with 3 inoperative cargo pumps in 3 different tanks.

Case Study No. 14 – Discharge on a River Berth with Strong Tidal Flows

A 30 000 m³ LPG carrier was discharging on a river where the tidal flow past the vessel was 4 to 5 knots.

The OOW failed to pay attention to the change of tide on his watch, although he had been earlier advised to ensure that sternlines, forward spring lines and breast lines forward and aft were secure at the 9 pm change of tide. The vessel slid 10 metres down the jetty, causing the hard arm to disconnect.

The vessel was not able to re-connect the hard arm until slack water, at 4 am the following morning, at which time the ship was able to pull herself back along the jetty.

Case Study No. 15 – Lightning Strikes LNG Carrier

An LNG carrier was hit by lightning at the UK’s LNG-receiving terminal at the Isle of Grain, causing a brief mast-riser fire.

TradeWinds understands the 129 767-cbm 1979 built vessel had finished discharging its cargo and was waiting for a pilot escort when, on the 29^th May, the lightning struck. A Flammability, Explosion and other Hazards of Liquefied Gasbrief fire, that lasted around 50 seconds, broke out before the ship’s in-built nitrogen system extinguished the flames.

As a result of tank pressures being exceeded, the vessel had been venting gas for around a minute before the lightning strike. Experts say this should not have been happening and a thorough investigation was carried out.

Lightning strikes on LNG carriers are not uncommon. The Society of International Gas Tanker and Terminal Operators (Sigtto) cites incidents at Trinidad and Das Island, Abu Dhabi, and reports in 2004 mentioned a lightning strike causing a fire and minor damage on a 1969 71 500-cbm LNG Carrier.

Case Study No. 16 – LPG Mixing Alert

In 2005, numerous warnings about the dangers of “co-mingling” propane and butane cargoes on board liquefied petroleum gas carriers were issued.

SIGTTO warned that the practice could have “serious consequences” such as the release of cargo vapour.

Co-mingling of propane and butane has been conducted on board vessels for years, but the practice has recently become more common.

Charterer’s instructions to co-mingle LPG cargoes often appear to be issued without taking into account crew member’s previous experience of such an operation, and rarely is support or operational guidance given.

The guidance is mainly aimed at owners and operators of fully refrigerated LPG vessels that do not have the pressure capabilities of fully-pressurised and semi-pressurised vessels.

Concerns have been raised by SIGTTO, about employing the practice on semi-pressurised vessels as “a mismanaged co-mingling operation can have serious consequences and, although fortunately very rare, there have been instances of vessels cargo tank relief valves lifting whilst alongside due to excessive tank pressures caused by the co-mingling operation. The lifting of relief valves may lead to an unacceptable release of large clouds of heavier than air cargo vapour, which has serious consequences.”

SIGTTO says traders are instructing ships to carry out the mixing during loading, on passage, or when discharging, often without any appreciation of the hazards involved.

Case Study No. 17 – Loss of VCM from a Semi-Pressurised LPG Carrier

A semi-pressurised gas carrier of 3 500 m³ was preparing to discharge a part cargo of Vinyl Chloride Monomer (VCM).

As he took cargo samples, the cargo surveyor asked the Chief Officer to run a cargo pump in each tank. The Chief Officer was unaware of the need for sampling but he proceeded without making any safety or operational precautions.

The Chief Officer opened the valves on the aft tank, which allowed recirculation of the cargo in that tank. He then started the aft tank cargo pump using local controls sited on the tank top.

The cargo surveyor began filling his sample cylinder from the designated tank sampling point. After a few minutes, the cargo alarm klaxon sounded on deck. The Chief Officer walked around the tank dome and, using a local control, stopped the klaxon from sounding. He assumed the alarm indicated that the cargo pump had tripped but he could not be certain without going to the CCR. A few moments laten the klaxon sounded again. The Chief Officer then noticed a large cloud of white vapour advancing down the deck towards him. He quickly ran aft, taking hold of the cargo surveyor and pulling him with him, hitting the Emergency Shut-Down (ESD) button as he ran past it. They managed to reach the shelter provided by the accommodation before the cloud overtook them.

A little under 600 kilograms of liquid and vapour VCM had erupted from the vessel’s forward cargo tank mast riser after the forward tank had become over-pressurised. A cargo valve had been left open on the forward tank, which resulted in cargo being transferred inadvertently from the aft cargo tank to the forward cargo tank during the sampling process.

The vessel’s crew, the port agency representative, the cargo surveyor, two Terminal Operations for LNG or LPG Carrier after Arriving in Portterminal staff who had been connecting the manifold, two people on the berth and a number of members of the public were exposed to VCM as the resulting gas cloud drifted downwind.

Case Study No. 18 – Explosion in a Gas Compressor

A 2 223 gt liquefied gas carrier had loaded initially 320 tonnes of propane, followed by 640 tonnes of butane. During loading, the vessel’s cargo cooling system was in operation. After loading and while on passage, the Chief Officer began a planned leak test on the condenser of the starboard cargo cooling unit. He had earlier drained the condenser of liquid butane, which was the last parcel of cargo loaded. The intended test method was to pressurise the condenser with air at 15 bar using the system’s compressor, and measure any drop in pressure. To remove butane from the system, he closed the gas outlet from the condenser, opened the inlet of the compressor to atmosphere, and ran the compressor to pump air into the condenser to a pressure of 3 bar. This was enough to allow butane to condense and be drained. The procedure was repeated several times. Judging the unit to be gas free, the Chief Officer ran the compressor and gradually raised the condenser’s pressure to 15 bar.

He then stopped the compressor, and began to close the inlet valve to the condenser. Just as he did so, there was a powerful explosion followed by a fire in the compressor room on deck. Seeing these events from the wheelhouse, the Captain immediately activated the deck sprinkler system. The mate and chief engineer began to tackle the fire using hoses. The remainder of the crew was mustered to assist them, and the Coastguard was alerted.

Comments

1. By compressing air to 15 bar, a temperature above the auto-ignition temperature of butane was generated. With traces of butane still in the system such as that remaining in the compressor’s lubricating oil, an explosive mixture was generated in the compressor.
2. This procedure did not comply with the owner’s instructions for leak testing, which specified that propane should be the medium used to pressurise the condenser, not air.
3. This explosion resulted from incorrect procedures being followed, and could have caused serious injury or loss of life. Changes to well-established system operating procedures should always be very carefully considered where the consequences of a system failure could be serious.

Case Study No. 19 – Sloshing Damage on Membrane LNG Carrier

In 2006, a 2OO3 built 138 000 m³ LNG Carrier fitted with the GTT No. 96 membrane tanks entered dry-dock for routine repairs.

The ship had sustained damage to the insulation boxes located on the longitudinal bulkheads between 6-8 metres from the tank bottom behind the membrane in No. 2, No. 3 and No. 4 cargo tanks in the area above the hopper, however the membrane lining of the tanks was not breached.

GTT and observers have speculated that the damage to the plywood boxes behind the cargo tanks membranes was caused by sloshing, during intermediate tank level filling giving opportunity for greater liquid motion to occur.

Case Study No. 20 – An Unexpected Ammonia Incident

A small semi-pressurised gas carrier was alongside a berth discharging her cargo of LPG. It was daylight hours and the situation was well under control, with ship staff both on deck and in the Cargo Control Room (CCR).

The berth was in a large port that handled a range of different liquid and solid cargoes.

Suddenly, the ship’s personnel in the LPG noticed that their colleagues out on deck were in some form of distress, coughing, writhing around and choking but for no immediately apparent reason.

The crew on deck failed to answer attempts at radio contact and so the CCR staff ran out to investigate. They too began coughing and choking, with uncontrollable weeping at the eyes and rapid loss of breathing capability.

An emergency condition had now arisen on the ship. It required prompt breathing apparatus rescue and fresh air resuscitation under shore-side medical supervision.

It was found that the cause of the incident was another gas carrier that was berthed upwind at another jetty while loading anhydrous ammonia. There had been a cargo spill on this other ship and a light wind had carried the vapours toward the semi-pressurised gas carrier while those on board the ammonia ship had been busy attending to their own incident.

This incident also serves as a reminder that the harmful effect of anhydrous ammonia on the body is a rapid caustic burn to all humid or moist parts of the body such as the eyes, throat, lungs, under-arms, genital areas and, in hot humid perspiration circumstances, all exposed skin areas.

Case Study No. 21 – Cargo Incident When Loading Ammonia

A 30 000 m³ fully refrigerated liquefied gas carrier was loading ammonia alongside a dedicated ammonia jetty. The port was in the tropics, it was a fine clear day and the ship and crew had loaded at this port for a considerable number of consecutive cargoes.

Halfway through loading the cargo, the jetty informed the ship that they needed to shut- down and cease loading for a brief period so the ship staff closed all ship valves, as did the jetty staff ashore. The shore-to-ship loading arm was fitted with a QCDC quick connect/disconnect coupling that was designed to operate if a ship failed in its mooring management and drifted out of the loading envelope.

At the time of close down, the loading arm was covered with a layer of ice (ammonia at minus 33 °C).

The shore stoppage was longer than anticipated and the ice on the loading arm slowly melted. Suddenly, the coupling flanges parted, just as a member of the ship’s crew walked under the loading arm.

The QCDC had been incorrectly set and a quantity of liquid ammonia sprayed onto the crew member, who subsequently died from the ammonia reaction against his throat, lungs, eyes and skin (the moist areas of the body). The vapour cloud generated from the liquid spill held back the “would-be” rescuers. The expansion of liquid to vapour was substantial and there were further victims who suffered toxic, corrosive and cold burns.

The following practices were incorporated after this incident:

• Prohibition of personnel near the cargo manifolds, both on ship and on shore, with barrier tape used to indicate those areas;
• Path ways are marked to ensure no attempt at passage past the working manifold;
• Any person gaining entry to the manifold area must be properly dressed, in the appropriate PPE and must have a valid purpose for being in the area.

Case Study No. 22 – Ethylene Incident

A semi-pressurised/refrigerated ethylene ship was carrying its cargo of ethylene at minus 102 °C in transit in a river channel. It was under pilotage and approaching the discharge berth.

A container ship on a reciprocal course coming down-river collided with the ethylene ship.

The collision resulted in the container ship piercing one of the ethylene tanks. This caused a substantial gas leakage that resulted in river water entering the ship’s hull. This, combined with the angle of impact, meant that both ships were literally frozen together for some time.