A LNG ship-to-ship transfer is when cargo or fuel is moved directly from one ship to another while they are both at sea. It is often done to transfer large quantities of goods more efficiently than traditional port methods.
During an LNG ship-to-ship transfer, two LNG carriers come alongside each other. Hoses or arms are connected between the vessels to transfer the liquefied natural gas from one ship to the other. The process is carefully controlled to ensure safety and avoid any spills or accidents.
LPG and petrochemical gases
Ship-to-ship transfer is a very common operation in the oil industry. Over the years, a number of Very Large Crude Carriers (VLCC) or Ultra Large Crude Carriers (ULCC) have systematically discharged all or part of their cargo into smaller ships which, in turn, discharged at ports too small to accommodate the larger ships.
This has also been a common practice in the LPG trade and, to a lesser extent, in the petrochemical gases trade. The procedure has been used at ports too small to accommodate the large ships, or when there was not enough storage available and the larger ship was used as additional storage.
In recent years, many large LPG ships have been anchored off the coasts of India and China, being used as storage vessels. Their cargo was discharged into a fleet of coastal vessels able to reach the shallow domestic ports to discharge in the small storages available or even directly into rail or road trucks or bottling plants. The large vessels were either anchored for long periods, receiving cargo from other ships or they sailed to a loading port, from time to time, to replenish their tanks. This had the added advantage of providing employment for a number of large older ships which, without this opportunity, might have found themselves surplus to requirements. The construction of new terminals, both in India and in China, will reduce greatly the demand for such ships.
The same practice has been used in South America and occasionally in Europe, for instance when a terminal was being serviced.
A recent trend has been the appearance of floating storage units attached to production platforms for oil and LPG. These units are either ships modified for this task or purpose-built units. They receive the warm gas directly from the production platform and cool it, for storage in their tanks. Ships come alongside to collect the Ports, Terminals and Jetties – Role and Significancegas for transport to the port of delivery. Several such units are already in service and it is probable that more will follow.
A very comprehensive guide book «Ship-to-Ship Transfer Guide (Liquefied Gases)» is compiled by ICS, OCIMF and SIGTTO. This guide gives all necessary recommendations to proceed with a safe ship-to-ship transfer operation.
Without going into too many details, one of the main factors for the success of such an operation is the selection of a safe area, well protected from the weather, with enough space for the two ships to come side by side with plenty of room to manoeuvre. It is also important to choose a period of good weather with low wind speed, reduced wave height and minimum tidal current. The local weather forecasts must be followed with great care. Other factors to consider are the number and quality of fenders available, which should be suitable for the size of ships involved, and the quality of the transfer hoses. The rest of the recommendations are not very different from those listed for a transfer operation in a terminal:
- good preparation;
- detailed exchange of information;
- careful supervision during the operation;
- and suitable contingency plans in case of emergency.
The critical element, particularly when the transfer involves refrigerated propane or ammonia, is the cargo hoses linking the two ships. Hoses can handle the product at ambient temperature, even at very high pressure, without difficulty. However, the cryogenic element introduced by the low temperature of the transferred product creates a different situation. When combined with the relative movements of the two floating ships, it makes the hoses much more fragile and careful selection and good maintenance are essential if long service is to be expected. The reliability of cargo hoses used for transferring the refrigerated products is very important as, even at the temperature of propane or ammonia, (approximately -45 °C or -33 °C), a large amount of cold cargo coming into contact with a ship’s structure, as the result of a burst hose, can have serious damaging effects on the integrity of the structure. The majority of Ship-to-ship LNG transfer operationsship-to-ship transfers has, so far, involved pressurised cargoes. It is only recently, and particularly with the appearance of the production storage units, that the transfer of refrigerated cargo is becoming more common and the suppliers of hoses have had to work on improving their products.
Although an extra element of risk is added in a ship-to-ship transfer operation by the fact that the two ships are not within the protected environment of a safe port, the operation must be reasonably safe as, to my knowledge, there is no record of a serious accident happening in these conditions.
LNG
Commercial or routine ship-to-ship transfer between LNG ships has never been tried. On several occasions, emergency ship-to-ship transfer of an LNG cargo has been carried out, following an accident or a mechanical problem incapacitating one ship. According to available records, this has been done only four times since the start of LNG shipping, the last time being 15 years ago. In each case, the necessary equipment had to be provided from far away and ample time for extensive preparation was available. An area of very protected waters and the availability of plenty of expertise on site was arranged in time to carry out the operation in the most favourable conditions. All four operations were successfully completed.
Since then, all LNG ship operators have arranged, either individually or within a pool, to have all the necessary material available for such an operation. This includes mainly cargo hoses, fenders, a prefabricated bridge for linking the two ships, and emergency cabling to supply the damaged ship with power, if need be.
There has not, until now, been any need for commercial LNG ship-to-ship transfers to be considered because all LNG projects were fully integrated, with all elements of the chain being available at the same time. The ports were tailored for the ships involved in the project and the storage facilities designed according to requirements.
In recent years, however, some projects have been suggested, which would have involved ship-to-ship transfer, but they did not materialise. Some serious new proposals are under consideration to provide floating liquefaction and storage units on site for the development of small isolated wells or floating storage and vaporisation units to supply LNG to places where the construction of a storage and vaporisation plant would be either difficult, not considered economical or simply delayed. None of these proposals have yet materialised either. But very serious studies have been conducted and it is probable that, eventually, such a project will be successfully implemented.
Ship-to-ship transfer between LNG ships is not as straight forward as for LPG ships. LNG ships are much larger, although this should not be a serious problem as large oil tankers have been able to find suitable locations. The most serious problem is the potential risk of the accidental release of a large amount of LNG in case of cargo hose damage. In view of the very low temperature of LNG, this would have a devastating effect on the ships’ structure. Some known accidental cargo spillages, which occurred during normal cargo transfer in port, even though never very large, have resulted in some frightening damage to the ships involved: extensively cracked decks, even cracked hulls. In the present state of cargo hose technology, and despite the hose suppliers’ claim, there is no available hose able to sustain long-term service when handling LNG. Some hoses are adequate for one emergency transfer but their long-term reliability is very doubtful. The other problem of cargo hoses is that, due to the very low temperature of LNG, the hoses are very quickly covered with a thick coat of ice. This makes them loose their flexibility and become much less able to accommodate the relative movements of the two floating ships.
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In presenting the report of a Working Group on LPG ship-to-ship transfer, SIGTTO recommends that, until there is a serious improvement in the cryogenic hose technology, only rigid arms must be considered for commercial operations.
In preparation for some floating liquefaction and storage units to be installed on the open sea, where the relative movements of the unit and of the ship would preclude any side by side transfer, much work has been done by a group of oil producers and loading arm suppliers on the possibility of having ships connected in a tandem configuration. The cargo would be transferred through a very long cantilevered arm terminated by an articulated pantograph. The receiving ship would maintain its position behind the floating storage unit through dynamic positioning and would receive the pantograph which would automatically connect to its receiving pipe. This system is claimed to be able to accommodate relative vertical and lateral displacements of several metres. It is, however, still under development.
Commercial LNG ship-to-ship transfer, if envisaged for a specific project, would require extensive planning and preparation. Substantial modifications of the ships to be used would have to be designed and put in place. Extensive surveys of the selected area, statistical research into the prevalent weather and sea conditions and accurate modelling of the relative ship movements would all be necessary. If the operation was planned in the vicinity of an existing port, navigation assistance, in the form of pilotage and tugs, would have to be considered. Finally the authorisation of the local authorities would have to be obtained.
