Ensuring safety and efficiency through compatibility and risk mitigation

Generally, there is a certain distance between the LNG supply market and the demand market, and LNG carriers connect the global LNG supply market and the demand market. On the demand market  side, large LNG carriers can use ship-to-ship (STS) transfer to increase downstream cargo distribution channels through LNG floating  storage and regasification units  (FSRU),  small and medium-sized LNG carriers and LNG bunkering ships .

In the process of STS transfer, the larger LNG carrier usually becomes the mother ship (or unloading ship) and the smaller ship becomes the sub-ship (or receiving ship).

The LNG STS transfer solution involves the technologies such as transfer compatibility assessment, site risk assessment, transfer process, risk assessment, and contingency plans.

Evaluation of LNG Ship-to-Ship Transfer Compatibility

When  STS  transfer  operations  are planned, the compatibility of the mother ship and sub-ships should be ensured from the point of design and equipment, and the compatibility shall be evaluated to confirm the ship’s suitability for planned transfer operations. Besides, any factors that may require special attention shall be identified .

Compatibility evaluation of STS transfer operation includes but is not limited to:

1. Ship characteristics required by STS transfer plan;
2. Minimum  operating  speed  and corresponding rotational speed (RPM);
3. Layout , size and quantity of manifolds used;
4. Number of available hose connections;
5. The minimum and maximum estimated height of cargo manifold from waterline, and the freeboard difference during cargo transfer;
6. Whether the hose lifting equipment is in good condition and has appropriate safe working load (SWL) and working range;
7. The side hose bracket shall be sufficient to prevent damage to the hose due to friction;
8. Cargo handling equipment, types of cargo pumps and any restrictions necessary to ensure the safe transfer of cargo;
9. Maximum permissible pumping/ receiving rates for all ships involved in the design;
10. Mooring arrangement, including the mooring line type and the requirements of closed cable guide;
11. Relevant requirements for displacement, draft, freeboard and parallel hull length and fender on arrival and departure of ships;
12. In terms of the horizontal and vertical maps of hazardous areas and gas safety areas, the height change of cargo decks of all participating ships and the overlap of flammable areas of each ship shall be taken into account;
13. Information about any obstructions that obviously affect the airflow, may cause eddy current and affect the shape and concentration of any gas in the vent. When two ships are alongside, the discharge of the smaller ship will cause danger to the upper deck and operating area of the larger ship;
14. The danger of smoke and spark caused by wind direction;
15. Personnel transfer scheme (if applicable);
16. Emergency shutdown device (ESD) and communication system;
17. Emergency plans and procedures;
18. Cargo transfer and ballast plan, including estimated duration;
19. Cargo vapor handling capacity of each ship, and cargo vapor handling capacity and compatibility when cargo vapor recovery/equalization is used;
20. Temperature, pressure and density of cargo in two ships.

In STS transfer operation, ships with total length difference less than 10% are usually considered to have similar ship lengths. Additional measures shall be taken to reduce the risk to an acceptable level for STS transfer of ships with similar ship lengths:

1. The optimal mooring arrangement shall be determined.  As the head line becomes a horizontal line, it is necessary to arrange the additional lines at the fore and aft of the ship for compensation;
2. The best fastening  scheme of the fender shall be determined to ensure that the fender will not be damaged due to the lack of fairlead/fairleader;
3. The front and rear positions of the ship shall be adjusted so that the bridge wings of the two ships are offset and staggered. In this process,  all the main fenders are ensured in parallel with the hull of the two ships during the whole operation, and the hose length shall be sufficient to adapt to the manifold offset caused by the movement of the two ships;
4. In the mooring and transfer operation, additional headlines shall be added to resist the impact force acting on the exposed bow to prevent the bow from deviating;
5. A larger-diameter fender is adopted to increase the distance between two ships;
6. After considering the guidance and effectiveness of mooring lines configuration, the environmental factor limitation can be reduced .