In the LNG industry chain, the LNG supply market is in the upstream, the end point is usually natural gas liquefaction plant, and the LNG demand market is in the downstream, with the LNG receiving terminal as the starting point. 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 .

Risk Assessment of LNG Ship-to- Ship Transfer Operation Site

A risk assessment shall be carried out each time a new transfer site is selected, and the results of the risk assessment shall be incorporated into the specific operating procedures for establishing the transfer site, including the implementation of appropriate safeguards to ensure that the identified risks are effectively managed .

The risk assessment shall consider the impacts and possibilities associated with the identified hazards specific to the transfer site, and shall also include the assessment of residual risks after appropriate safeguards, controls or mitigation measures have been taken. Factors to be  considered in risk assessment include, but are not limited to:

(1) Requirements of local laws and regulations;

(2) Open water or sheltered water for operation under the normal environmental conditions, including the appropriate marine meteorological analysis;

(3) Whether berthing and unberthing operations are carried out when the ships are sailing, anchoring or alongside;

(4) Whether transfer operation is carried out when two ships are sailing, anchoring or alongside;

(5) Traffic density in the vicinity of the transfer site, including the existence of other STS transfer operations;

(6) Gas leakage and diffusion trajectory and potential environmental impact;

(7)  Whether there are any other resources and availability of spill response requirements at the transfer site;

(8) Availability and capability of on-site ship support;

(9) Completeness of operations and resources for any support provided by local interested parties on site;

(10) A security threat present on the transfer site;

(11) Operating environment restrictions, including standards for suspension of operations;

(12) Navigation risks near the transfer site.

When considering the applicability of the new transfer site, anchoring or mooring analysis shall be performed to determine the operating environment parameters of transfer operation.

Figure 1 Flow Chart of LNG STS Transfer Operation

Operation Flow of LNG Ship-to- Ship Transfer

The main process of LNG STS transfer operation includes pre-operation preparation stage, operation stage and post-operation stage, as shown in Figure 1.

Risk Assessment of LNG Ship-to- Ship Transfer Operation

Risk assessment shall be conducted prior to STS transfer operations and shall contain sufficient information to ensure a good understanding of the operations and effective control. Risk assessment shall cover the physical and operational hazards and their management means, as well as the suitability of equipment .

Risk a ssessment is an important part before STS transfer operation plan, and at least the following factors shall be considered:

(1) Ship compatibility , including mooring arrangements;

(2) Suitability of specific working place;

(3) The characteristics of the transfer cargo;

(4) Training, experience and qualifications of personnel;

(5) Proper ship preparation for operation and adequate control measures during operation;

(6) Adequacy of navigation procedures;

(7) Enough personnel to control and execute the transfer operation;

(8) Adequate communication between the ships and/or the persons in charge;

(9)  Influence of the difference of freeboard during transfer;

(10) Equipment, including fenders and transfer hoses;

(11) Expected environmental conditions;

(12) Emergency plans and procedures .

For LNG STS transfer operations, some high-risk scenarios usually need to be considered. There are cases of causal factors that may lead to high risks, and describes the events or activities that may help trigger specific risks. Understanding the causal factors will help to develop the appropriate measures against obstacles and the risk prevention measures to minimizing the occurrence of related risks, as well as the risk mitigation measures .

Contingency Plan for LNG Ship-to- Ship Transfer Operation

Contingency plans shall be made for LNG STS transfer operation to deal with the accident risks and various potential consequences. Risk mitigation measures and contingency plans shall cover all possible emergencies and provide comprehensive and integrated responses.

The contingency plan shall also be relevant to the operation site and have the available support resources in and around the transfer area considered. Where appropriate, the contingency plan shall be combined with similar plans or requirements formulated by local competent authorities.  The contingency plan shall at least cover the following contents:

(1)Procedures for issuing alarms;

(2)Stop operation during emergency;

(3)Notification procedure;

(4)Emergency workstation and preparation to start emergency procedures;

(5)Personnel assembly at the mooring point;

(6)Emergency disassembly of cargo transfer equipment;

(7)Main engines preparation for manoeuvring;

(8)Unberthing .

Common emergencies of LNG STS transfer operation include the emergency during berthing operation, gas accumulation on deck, accidental cargo leakage, activation of emergency disconnection system, hose treatment  after  release of emergency disconnection device, activation of emergency disconnection device of paralyzed ship, inspection and test of transfer system, emergency cut-off system in connected state, compatibility, connection and use of emergency cut-off system, test of emergency break-off system and emergency cut-off system, etc .

Prospect of Application Scenario of LNG Ship-to-Ship Transfer Technology

STS transfer mode provides great convenience for downstream cargo distribution of LNG carriers and improves the speed of LNG trade circulation. At present, this solution has attracted the attention of clean energy suppliers and shipping enterprises.  First, in the countries  and regions with weak infrastructure and slow construction speed, the technical solution of liquefied natural gas floating storage and regasification unit (FSRU) is often adopted for natural gas supply, and FSRU usually needs to supply cargo from LNG carriers by STS transfer. Second, as some LNG receiving terminals are located in the limited waters of waterway water depth (such as Yangtze River and Pearl River inland waters), large LNG carriers cannot enter the port to unload cargo, STS transfer is usually used to unload cargo to small LNG carriers, and small LNG carriers that are not limited by waterway water depth unload cargo, which is adopted by Dongguan LNG receiving terminal of JOVO Group. Third, the distance between the polar Yamal LNG project and the LNG receiving terminal now includes two parts: the ice waters and the open waters. The open water performance of large ice-class LNG carriers invested in this project is poor in open waters, and thus the STS transfer is generally used to unload cargo to the ordinary large LNG carriers in open waters near ice areas, and the ordinary large LNG carriers sail into ports to unload cargo. Finally, since the second half of 2021, LNG power ships have developed rapidly, the port LNG fuel filling market has flourished, and the LNG bunkering ships can receive cargo at LNG receiving terminals near the port by reverse transportation, or directly receive cargo from large LNG carriers by STS transfer.

China Classification Society (CCS) assists CNOOC and JOVO Group to carry out LNG STS transfer operation at home and abroad, and provides full technical support, which is unanimously recognized by customers and competent authorities .