Guidelines for Automatic Cargo Tank Overfill Protection Aboard Gas Carriers

Overfill Protection of cargo tanks on gas carriers is a critical safety aspect in the maritime transportation of liquefied gases. The potential consequences of overfilling, ranging from structural damage to environmental pollution and significant safety hazards, necessitate robust and reliable systems to prevent such incidents.

This article will delve into the various technologies, regulatory requirements, and operational procedures that constitute effective Overfill Protection for these specialized vessels, highlighting the importance of each layer of defense in ensuring the safe and efficient carriage of gas cargoes.

Purpose of Guidelines

Currently, liquefied gas carriers interfacing with gas terminals have a range of protective measures to prevent overfill of cargo tanks and collectors.

Progressive development of the IMO Codes has led to a co-existence between vessels equipped to pre-1976 standards with those complying with Resolution MSC5(48) of the IMO “International Code for the construction and equipment of ships carrying liquefied gases in bulk”.

The IMO requirements have been further extended by the SIGTTO initiative in developing ship/shore emergency shutdown (ESD) systems. The most sophisticated of these provides for Ship to Shore Access Guidelines for Gas Terminal and Vessel Operatorsship to shore and shore to ship linkage within a single ship/shore interface. Such schemes may employ pneumatic, electrical or optical signalling methods to create the links. They are capable of being manually or automatically initiated responding to adverse conditions by shutting down cargo transfer and isolating cargo storage in a surge-free fashion. This is the first phase. Should the event become sufficiently serious that ship and shore must distance themselves, then phase two will “dry break” manifold ship-to-shore connections allowing the vessel to depart without serious spillage.

The rather broad spectrum of overfill and other protective capability has prompted a review of, and a recommendation for, a minimum set of requirements.

IMO requirements currently in force

As a consequence of the developments mentioned above, terminal operators must accommodate ships with varying degrees of protection. Ships built before 1976 require cargo tanks to have High level alarms activated by independent sensors. Either by means of this alarm sensor or another independent sensor, programmed isolation of the subject cargo tank takes place. The sequenced isolation in response to the threat of overfill shall avoid surge pressure within the cargo transfer system.

Pressure vessels or vessels within which Maximum Allowable Relief Valve Settings (MARVS) can never be achieved require none of the foregoing. For ships built between 1976 and 1980 the above applies, except that pressure vessels of greater than 200 m³ are no longer exempt from the alarm/shutdown requirements.

Ships built between 1980 and 1986, in addition, offer the alternative of using emergency valves, required by the IMO Code (Resolution A328(IX) as a means of overfill protection. This results in a stoppage of cargo transfer and isolation of ship from shore. If this option is taken, Use of Cargo Pumps on Liquefied Gas Carrierscargo pump and compressor shutdown shall also accompany the programmed valve closure.

Finally, ships built after 1986 have all the options and requirements relating to 1980-86 ships but with removal of the alternative to use a single sensor for both alarm and triggering of the isolation process. IMO Resolution MSC5(48).

Prime causes of overfill in cargo tanks

Cargo liquid may enter cargo tanks via discharge/loading lines, reliquefaction return lines (if fitted) and spray lines. Ingress through these routes are regarded as “prime sources” since a single error or mistaken act produces a risk of overfill conditions. Cargo flowing into a tank from either the vent line or vapour line must be regarded a secondary event consequent upon an initial tank overfill problem. Prevention of the former obviously removes the latter.

Typical examples of inadvertent tank overfill are as follows:

• During the early stages of a discharge sequence cargo being pumped ashore is discharged to a full tank which the discharge plan had intended to isolate. Due to incorrect closure of isolating valves, cargo passes into a full tank either through loading lines, spray lines or reliquefaction lines.
• During the early stages of a discharge sequence in which more than one cargo tank is being discharged simultaneously into a common manifold, the pump in one cargo tank trips and cargo from the other tanks flows into this tank bypassing the non-return valve until it overflows.
• During loading maloperation of the loading valves can lead to overfill. In the case of refrigerated LPG vessels loading without vapour returns, flow from the reliquefaction plant could be misdirected into an already full tank. This latter situation can also occur on passage.

Recommendations for cargo tank overfill protection

Experience has shown overfill tends to occur during the discharge sequence more often than at any other time. Liquid transfer is sustained entirely by the ship’s cargo pumps, and hazard analysis has shown that overfill protection is significantly improved if the tripping of cargo pumps is added to the basic IGC Code requirements.

Read also: Non-Standard and Emergency Operations on Liquefied Natural Gas Carriers

It is therefore recommended that all cargo pumps are stopped if the risk of tank overfill reaches a defined degree of severity.

Exceptions to recommendations

• Those vessels which already exceed basic IMO requirements and are fitted with cargo pump trips initiated by independent level sensors.
• All vessels fitted with an ESD system activated by high liquid levels in cargo tanks.
• Those vessels built after 1980 which use IMO-defined emergency shutdown systems to stop cargo transfer and, therefore, by definition cause the cargo pumps and compressors to shutdown.
• Shipborne pressure vessels of cargo capacity less than 200 m³ and tanks within which MARVS can never be exceeded.

Level settings

Acceptable tank filling limits vary under the IMO Code. A limit of 98 % of tank volume covers all vessels, however exceptions are permitted. Individual governments (Administrations) may allow filling to greater volumes for ships entitled to fly their flag.

LNGCs employing the Moss Rosenberg cargo tank system can fill to 99,5 %. Some LNG membrane cargo tank systems are permitted to fill to 98,5 %. These concessions are allowed where tank geometry gives certain advantages.

The alarm and shutdown sequence

The sequencing of automatic protective devices has to be arranged to avoid surge pressures in the cargo lines. Surge protection has been a requirement, at all stages, of the development of the IMO Code. SIGTTO has published methods which show how surge may be contained.

Although a full ESD is not called for in these Guidelines, they do ultimately require Liquefied Natural Gas Plant and Regasification Terminal Operationscargo transfer streams to be stopped, making SIGTTO‘s “Guidelines for the Alleviation of Excessive Surge Pressures on ESD” entirely relevant.

Restarting cargo pumps after activation of High level sensors

The Guidelines will protect against all ship-to-shore cargo transfer overfill events and will protect against overfill resulting from any ship internal cargo movements involving cargo pumps.

Should an event progress to the activation of High High level sensors whereby manifold valves are closed and all cargo pumps are both stopped and inhibited from further use, it is probable that cargo pumps must be used to rectify the overfilled condition. Under these conditions, circuitry inhibiting the use of cargo pumps has to be overridden. Means of overriding the system shall be provided only to the Master or Chief Cargo Officer.

Recovery from incidents of this nature must be accompanied by careful, cautious consideration of action to be taken. When the recovery plan is implemented, ship’s staff must be vigilant in performing their allotted tasks properly and shall keep the terminal operators informed of their activities.

It should be noted that the sequencing of overfill protection measures avoids depriving the operator of the use of his cargo pumps until there is no other option.

Testing of sensors and alarms

Independent sensors and tank level system alarms should be provided with adequate means to enable regular testing. Their function shall be demonstrated without the need to bring tank liquid levels to the alarm condition.

Unwanted alarms

During the final stages of loading, during the loaded passage and at the early stages of discharge, cargo levels may exist which, although operationally normal, will activate the Early Warning and High level sensors. Under such circumstances a means must be found to safely inhibit alarms affected in this fashion. Once the conditions are removed, then the relevant alarms must resume their true protective role.

Relief valve venting systems

Piping systems and tank structures are protected by relief valves. No part of the ship’s cargo system can be damaged by excessive pressure build-up resulting from isolation of cargo containment and line isolation. This means relief valves open to eject gas vapour and/or liquid to a vent header connected to a mast riser, or a cargo tank, or, in the case of cargo or spray pumps, the suction side of the protected pump.

Returns to cargo tanks or pump suction can be regarded as remaining within the cargo containment system. Any liquid or vapour ejected to the vent header will have escaped from the Cargo containment system of gas vesselcargo containment system.

The earliest IMO Codes required provision to be made for detection and disposal of any escaped liquids:

Relief valves discharging liquid cargo from the cargo piping system should discharge into the cargo tanks, or alternatively, they may discharge to the cargo vent mast and within six years (1982) means should be provided to detect and dispose of any liquid cargo which may flow into the vent system. Relief valves on cargo pumps should discharge to the pump suction.

All subsequent amendments maintain these requirements. Liquid may escape to the vent piping only from Independent Cargo Tankscargo tanks or line pressure relief valves.

Escape of liquid from cargo tanks may either be from pressure relief valves or tank venting systems that have either been maloperated or have malfunctioned. In either case a tank must first have been overfilled.

So long as the source of ejected liquid is the tanks themselves and those tanks have been protected as these Guidelines propose, then both tank isolation and cargo transfer shutdown with cargo pump inhibition must have failed before liquid may get into the vent system. This risk must be regarded as negligible.

There yet remains cargo line pressure relief valve operation as a source of gas liquids ejected into the vent header. Should liquid gas carriers be dedicated to a single product, all such escaped liquids may be returned to cargo tanks directly or via liquid trapping devices.

Multi-product vessels, however, will be unable to return liquids ejected from main cargo line relief systems to cargo tanks. Such liquid will pass, via the vent header, to collectors usually installed at the base of the mast riser. Quantities of liquid passing to the vent header from this source will be very small being restricted to the volume of those line lengths which can be isolated and yet contain cargo. Relief valves will only open when overpressure occurs due to warming of the trapped cargo. Pressures can be contained by releasing very small quantities.

The risk of significant liquid escape from this source is considered to be negligible. IMO Code requirements in connection with collectors need not be exceeded.

It will be interesting: LNG System Features and Controls

Means of detecting liquids should be provided at each collector. Such detection should take the form of an alarm activated by a liquid level. Since no level should exist within the collector, the lowest value which can be reliably registered should activate the alarm. An audio-visual response to the alarm condition should be incorporated into any centralised cargo control centre.

Alternatively, the audio-visual alarm may be initiated by a low temperature detector. Collectors not permanently connected into the Piping System of pressure vessels on gas tankerspiping system shall also have alarm arrangements of a similar type. Level settings shall be appropriate to the duty the collector system has to perform.

Author

Olga Nesvetailova

Freelancer

Literature

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