At a SIGTTO Regional Forum in early 2004, one Member raised a question regarding safety issues associated with the mixing/co-mingling of refrigerated Butane and refrigerated Propane on board a vessel while loading at their Jetty. It would appear that this was being carried out without the shore staff being aware that the operation was taking place at all.

Very little guidance has been published about this operation; there is only a short warning note in the ICS Tanker Safety Guide (Liquefied Gas) which mentions the hazard of "flash gas" and the potential for "roll over". It is apparent that LPG traders require mixed LPG’s for commercial reasons and are instructing ships to carry out the mixing, either during loading, on passage or during discharge, often without any appreciation of the hazards involved.

At the 49th SIGTTO GPC meeting in San Francisco in April 2004 it was agreed that a working group be set up to look into this issue and produce guidelines for the industry. The working group met in August 2004 and this document was produced as a result of the meeting.

Propane (C_{3) and Butane (C4) are the two cargoes most commonly carried by LPG tankers. Their physical properties are shown in table 1.}

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Table 1 – PHYSICAL PROPERTIES OF n-BUTANE, i-BUTANE AND PROPANE

When Petroleum Gases are produced, the Butane and Propane streams are usually separated. The degree of the separation depends on the process and the intended use of the products.

The specification most commonly observed for the commercial grades is for the Propane and Butane content to be "95%" volume respectively. This was defined by the Gas Processors Association in the 1930’s and requires Commercial Propane to contain at least 95% Propane and Propene, with the balance made up from Ethane and Butanes.

Likewise, Commercial Butane contains at least 95% Butane isomers and butenes (97% in some specifications), with the balance usually Propane and Pentane; Butane does not usually contain any Ethane.

When used as fuel gas, this composition range is perfectly satisfactory, though there may be limits on constituents such as un-saturates, volatile Sulphur compounds and Ammonia as well.

In temperate climates and countries where the ambient temperature at night falls towards zero, Butane has a serious limitation as a fuel gas during the winter months. The ambient temperatures can drop to below -5°C when the Butane product effectively has no vapour pressure. In large plants, heated vaporisers are used to maintain vapour pressure within storage vessels. However, butane cylinders become useless in sub-zero temperature conditions as vapour cannot be generated for user consumption. So in these ambient conditions, some Propane may be added to the Butane to maintain a positive vapour pressure.

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The mixtures specified for these reasons may be 70/30 volume % C_{4/C3 or 60/40 volume % C4/C3, depending on the local conditions.}

In practice, the burners of LPG-fired units can usually handle either Propane or Butane with little adjustment. Therefore the two fuels can be interchanged or mixed according to the prices of the products – which can fluctuate relative to each other. LPG mixtures in any ratio may be requested (e.g. 50/50 volume % or 70/30 volume % C3/C4), dictated by the market prices for each product.

Buyers have also requested co-mingling of LPG prior to arrival in the territorial waters of the receiving country for tax purposes. India is one country where this occurs.

At present, it appears to be an increasingly common practice for charterers, or others with commercial interests, to request the owners/operators of LPG vessels (or floating storage vessels) of any size or type to co-mingle LPG either whilst loading, on passage or during discharge. The majority of vessels conducting co-mingling have, generally, been fully pressurised or semi-pressurised LPG carriers although an increasing number of fully-refrigerated vessels are being asked to conduct this operation. The biggest concerns, at present, relate to fully refrigerated LPG carriers which do not have the pressure capabilities of fully-pressurised and semi-pressurised LPG carriers.

The instructions to co-mingle LPG cargoes appear to be issued without taking into account whether those on board have any previous experience of such an operation and often no operational guidance is given.

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.

A co-mingling operation will often involve much slower loading rates than normal, and the fact that charterers, or others with commercial

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interests, have been known to express surprise at the necessity for these slow loading rates indicates a lack of background knowledge regarding safety concerns inherent in the practice.

It is known that vessels have been co-mingling, whilst loading, without the terminal being informed. An unusually slow loading rate may indicate that a co-mingling operation is taking place, although there have been instances of vessels trying to conceal this fact by misleadingly attributing the slow loading rate to other factors.

If a fully pressurised LPG ship is asked to mix Butane and Propane cargoes, both at ambient temperature – there is no fundamental problem from the "process" standpoint. The systems can withstand the highest pressure (i.e. Propane) and the mix will not generate additional pressure as both components are at the same temperature.

The question of mixing is much more complex if a fully refrigerated vessel co-mingles Butane and Propane which have both been refrigerated to atmospheric pressure, where the vessel’s tanks are only able to withstand an over-pressure of, typically, 250 mBar(g).

If the refrigerated cargoes are mixed in these circumstances, the loading temperature of the cargo is of the order of – 1°C ~ –5°C for Butane and –42°C for Propane. The temperature of the resultant mixture is somewhere between the two, which means that the Propane is raised in temperature above its boiling point. The mixing operation therefore has the potential to create a large amount of "flash gas", which would need to be controlled by the ship’s reliquefaction plant, otherwise there could be uncontrolled venting from the cargo tank relief valves. Clearly, the rate of mixing needs to be monitored and controlled carefully to be well within the capacity of the reliquefaction plant.

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Mixing fully refrigerated Butane with fully refrigerated Propane generates flash gas, and there is a risk that if the mixing is conducted too fast and/or the pressure tendency is not being monitored scrupulously enough, the ship’s cargo tank relief valves will lift – causing a hazardous, uncontrolled release of a LPG vapour cloud on the jetty and around the vessel. This is most likely to occur during the loading operation when the vessel’s tank pressures are typically higher than they normally are at any other stage of the voyage.

During the loading operation all of the vessel’s compressors will normally be running at maximum capacity. If the vessel was to experience a loss of compressor capability due to any power problems or even a complete loss of power during the co-mingling operation there would be no contingency for disposal of the vapour generated unless a vapour return line was fitted. The presence of a vapour return line would allow the vapour to be sent to the shore thus potentially preventing the relief valves from lifting.

A real concern would be to ensure that the final volume of any tank did not exceed more than 98% full. Co-mingling

Co-mingling whilst at sea involves the hazards as described in 1.5.1 with the addition of further hazards.

In order for the vessel to co-mingle, at least one tank would have been only part-filled at the loading port. This has potential stability and stress issues for the vessel upon departure due to one tank being "slack". At least two tanks will be slack once the operation commences which means the vessel’s effective centre of gravity is raised through "free surface effect" and the vessel’s stability correspondingly reduced. In addition the part-filled tank may affect the vessel’s shear forces and bending moments as may the change in distribution as the co-mingling operation takes place. This could "over stress" the vessel during the course of the operation and

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it would be necessary for the vessel’s crew to carry out sufficient intermediate loading evaluations to ensure that stability, longitudinal strength and all other operational limits are not exceeded during co-mingling. When a vessel is in a seaway the dynamic stresses are greater which could exacerbate the stability and stress issues. Most LPG carriers are today designed to operate with two or more slack tanks, however the effect of "sloshing" on the tank internal structure should probably still be considered.

Due to the vessel’s motion in a seaway, it is usually difficult to accurately calculate the cargo tank levels and therefore the quantities. The risk of over filling, while co-mingling at sea, is therefore increased due to this uncertainty. Indeed, the movement of the vessel could be such that the inaccuracy of the readings prohibits the continuation of the operation. Other items of the ship’s equipment may also not be designed for use at sea, such as cargo pumps and float level gauges. Damage to such equipment could be sustained when being used at sea due to, for example, excessive strain on bearings when the vessel is rolling or pitching or deflection of deep-well pump shafts if the vessel is stressed in any way.

When co-mingling is carried out during discharge, this is conducted by mixing at the manifold. This means that the cargo tanks are not subjected to the risk of being over-pressurised. Some of the hazards described above are therefore avoided, although this does not mean that the rate of co-mingling would not still need to be carefully monitored and the discharge rate adjusted accordingly in close consultation with the shore.

When mixing during discharge, both of the products would be pressurised by the cargo pumps, so they would be at a pressure above saturation, and therefore the "flash gas" would be reduced naturally. If the shore tank were not refrigerated, then heating of the C3, to the temperature required for the mix, would reduce the amount of flash gas. A terminal’s tank farm typically has a refrigeration capacity much larger than a ship’s plant, and if a vapour return to the ship were provided, the combined capacity of both plants could deal with the flash gas. That said, shore plants have minimum safe operating temperatures and these could be exceeded,

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perhaps if a fully refrigerated Propane cargo were not warmed to an acceptable temperature prior to a co-mingling operation at discharge.

The co-mingling operation can also cause "apparent" losses, as the density of the mixture will not be a mathematical average of the densities of the components – because the molecular composition is different. Calculation of cargo densities in such circumstances is discussed in Appendices 3 & 4 of the SIGTTO publication

Furthermore the temperature of the mixture may not be equal throughout. Non-equal temperature distributions can lead to problems in correcting for cargo tank shrinkage, float immersion and the temperature correction to be applied when quantifying the cargo.

Co-mingling on board fully pressurized vessels is not a major safety issue as there is no temperature gradient and the vessels are designed to be able to carry the cargo at ambient temperatures well within the safety valve settings. On board semi-refrigerated vessels it is also not considered to be a major safety issue although the relief valve settings can vary considerably in range from vessel to vessel.

If the vessel is able to contain the Propane at Butane temperatures then there is no major safety issue as there is no risk of pressure rise leading to uncontrolled release of vapour. This is the reason why co-mingling on board fully refrigerated vessels causes the most concern as these vessels are not designed to carry Propane at Butane temperatures.