FIELD OF THE INVENTION

The present invention relates to a system and a method of transferring hydrocarbon liquids between ships. In particular, the present invention relates to a system and a method of transferring oil between tankers using a single-point mooring buoy.

BACKGROUND TO THE INVENTION

The offshore oil and gas industry often uses floating production storage and offloading (FPSO) units to produce and store hydrocarbons. The FPSO may, for example, receive hydrocarbons from a subsea platform, process them and store the resulting oil. The FPSO unit will then typically transfer the oil to a tanker that transports the oil ashore or to a transhipment point for export.

The following process describes use of a dynamic positioning (DP) tanker to offload the oil. A DP tanker is used to receive the oil from the FPSO unit. Typically this will be a Large Range tanker or Suezmax tanker that has a size of 120,000- 200,000 DWT (tons deadweight). A DP tanker is equipped with systems that enable the tanker to automatically maintain its position and heading using its own propellers and thrusters. These DP tankers are, therefore, specialist vessels and are significantly more expensive than conventional oil tankers to build, charter and operate.

Once the DP tanker is loaded with oil, the DP tanker may then move to a location where the oil can be transferred to a conventional (non-DP) tanker using ship-to- ship transfer methods. The conventional tanker may be another Suezmax tanker or a VLCC (very large crude carrier). In these operations, the two tankers are secured alongside each other and continue to remain alongside during the transfer. By transferring the oil to a conventional tanker the more expensive DP tanker is freed to be able to return to the FPSO.

Such ship-to-ship transfers are, however, not possible in adverse weather conditions, such as when swells of greater than 3m are experienced for example. As adverse conditions such as these are frequently encountered in the seas and oceans near oil production facilities and in which tanker-to-tanker oil transfer is desired, it is frequently not possible to use this method, or the delay due to the adverse weather causes disruption to the export schedule.

In some cases it is, therefore, necessary for the DP tanker to transport the oil significant distances to a suitable harbour or location at which the oil can be offloaded or transferred. This means that the DP tanker is in use for a significant period of time transporting the oil to the harbour and returning to the FPSO unit. The time taken can be around 14 days at a cost of around USD $50,000 per day and about USD $800,000 of fuel in total. Furthermore, additional DP tanker vessels are required to service the FPSO during this time period. These DP tankers may cost around USD $120M each.

A solution that is being considered, to reduce the time that the DP tanker is away from the FPSO, is the provision of suitable ports or terminals in the desired locations closer to the FPSO or other production facility. The build cost of these is, however, in the region of USD $2 billion each.

There is, therefore, a need for a more reliable and most cost effective system that allows oil, or other hydrocarbon liquids, to be offloaded from an FPSO or other offshore hydrocarbon production facility and that minimises the period of time for which a DP tanker is away from the production facility.

SUMMARY OF THE INVENTION

The present invention provides, in a first aspect, a method for ship-to-ship transfer of hydrocarbon liquids comprising: mooring a first tanker to a single-point mooring buoy, the buoy having a first port and a second port and a fluid conduit connecting the first and second ports; connecting a first hose string between the first port of the buoy and the first tanker;

positioning a second tanker near the buoy, the second tanker having a dynamic positioning system;

connecting a second hose string between the second port of the buoy and the second tanker; and

pumping a hydrocarbon liquid between the first and second tankers, via the first hose, the fluid conduit of the buoy, and the second hose.

In a second aspect the present invention provides a system for ship-to-ship transfer of hydrocarbon liquids comprising:

a single-point mooring buoy, the buoy having a first port and a second port and a fluid conduit connecting the first and second ports;

a first tanker moored to the buoy;

a first hose string connected between the first tanker and the first port of the buoy;

a second tanker holding position near the buoy; and

a second hose string connected between the second tanker and the second port of the buoy,

wherein the first hose string, the fluid conduit and the second hose string are configured to allow hydrocarbon liquid to be pumped between the first and second tankers.

The system and method of the invention enables ship-to-ship transfer of hydrocarbon liquids in conditions in which a direct connection between two tankers may not be possible. The hydrocarbon liquid will typically be oil, but may be another hydrocarbon or a product derived from hydrocarbon processing. The hydrocarbon liquid may be a petrochemical product.

In some embodiments the hydrocarbon liquid is pumped from the first tanker to the second tanker. In these embodiments the hydrocarbon liquid is pumped from the first tanker, through the first hose string, through the fluid conduit, through the second hose string and to the second tanker. In other embodiments the hydrocarbon liquid is pumped from the second tanker to the first tanker. In these embodiments the hydrocarbon liquid is pumped from the second tanker, through the second hose string, through the fluid conduit, through the first hose string and to the first tanker.

Preferably the first hose string is a floating hose string and the second hose string is a floating hose string. Ideally when not in use the hose strings are preferably tied back to the buoy in a bight. The first and second hose strings may be tied back to the buoy in a bight during manoeuvring of the tankers and then untied once the tankers have departed. In preferred embodiments one or both of the first hose string and the second hose string includes a marine breakaway coupling. The first hose string is preferably connected to an emergency disconnect coupling provided on the first tanker. The second hose string is preferably connected to an emergency disconnect coupling provided on the second tanker.

The first tanker is preferably moored to the buoy by a hawser. The second tanker is positioned a safe distance from the first tanker and the buoy, and maintains position using its dynamic positioning system. The second tanker is preferably positioned between 50m and 600m away from the first tanker.

The first hose string is preferably connected to the midships manifold of the first tanker. In some embodiments the second hose string is connected to the second tanker at the bow of the second tanker. In other embodiments the second hose string is connected to the midships manifold of the second tanker.

After hydrocarbon product has been pumped between the first and second tankers, the method preferably includes flushing the second hose string with sea water. The method may include flushing the first hose string with sea water.

After transfer of hydrocarbon liquid between the tankers the method comprises disconnecting the second hose string from the second tanker and tying the second hose string back to the buoy in a bight. The method preferably further comprises disconnecting the first hose string from the first tanker and tying the first hose string back to the buoy in a bight.

The system for ship-to-ship transfer of hydrocarbon liquids preferably further comprises a first support craft equipped to handle the first hose string for connection to and disconnection from the first tanker. The system may additionally comprise a second support craft equipped to handle the second hose string for connection to and disconnection from the second tanker.

Preferably the buoy comprises a surge protection system including a surge tank connected to the fluid conduit. A pressure relief valve between the fluid conduit and the surge tank permits hydrocarbon liquid to enter the surge tank from the fluid conduit when the pressure in the fluid conduit exceeds a predetermined level. A telemetry based monitoring system determines the level in the surge tank and includes an alarm function.

In preferred embodiments the buoy comprises a telemetry system arranged to transmit data to and receive data from at least one of the first and second tankers.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a block diagram illustrating a system for ship-to-ship transfer of hydrocarbon products according to the present invention;

Figure 2 illustrates a system for ship-to-ship transfer of hydrocarbon products via a single point mooring buoy according to an embodiment of the present invention; Figure 3 illustrates a system for ship-to-ship transfer of hydrocarbon products via a single point mooring buoy according to another embodiment of the present invention;

Figure 4 illustrates a system for ship-to-ship transfer of hydrocarbon products via a single point mooring buoy according to a further embodiment of the present invention;

Figure 5 shows an end view and a side view of a midships connection of a hose to a tanker forming part of a system for ship-to-ship transfer of hydrocarbon products according to an embodiment of the present invention;

Figure 6 is a further view of the midships connection of Figure 5 showing an end of the hose connected to a midships manifold of the tanker;

Figure 7 shows a side view of a bow connection of a hose to a tanker forming part of a system for ship-to-ship transfer of hydrocarbon products according to an embodiment of the present invention; and

Figure 8 is a further view of the bow connection of Figure 7 showing a bow loading system and an end of the hose connected to the tanker via a bow slot.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a method of transferring hydrocarbon liquids between two tankers using a single-point mooring (SPM) buoy. The method will typically be used to transfer crude oil, but may be used to transfer other hydrocarbon liquids including refined hydrocarbon products, liquefied natural gas and liquefied petroleum gas. Accordingly, it will be understood that in the following description, references to oil and crude oil may be more generally considered to be references to any hydrocarbon liquid. It is envisaged that this system will be used to transfer oil from a tanker equipped with a dynamic positioning system (DP tanker) to a conventional tanker, which does not include a dynamic positioning system. This releases the DP tanker to return to an offshore oil production or processing facility, while the conventional tanker can travel longer distances to a local onshore processing plant or onwards to another destination. This is advantageous as DP tankers are significantly more expensive than conventional tankers, both in terms of build cost and daily running costs.

This system has particular advantages in offshore locations which frequently experience adverse weather conditions, and where there is a reliance on tankers equipped with dynamic positioning to safely transfer oil for export, or transportation away from the offshore location or oil field.

The following description describes an embodiment of the invention in which a DP tanker is transferring hydrocarbon liquid to a convention tanker via a single-point mooring buoy.

The single-point mooring buoy 10 is located in the vicinity of a floating production storage and offloading (FPSO) unit or other offshore oil production facility, such as an oil well or oil platform. Preferably the buoy 10 is located less than one day DP vessel transit time from the FPSO unit or other facility. The buoy may, for example, be located between 5 miles and 700 miles away from the FPSO unit or other facility. More preferably the buoy is located about 50 miles away from the FPSO unit or other facility.

The SPM buoy 10 comprises a main body 12 and a plurality of anchor chains or mooring chains 13 that anchor the buoy 10 to the seabed. In the system of the present invention the SPM buoy 10 is not connected to a pipeline end manifold or subsea piping, and therefore does not include a flexible riser. The SPM buoy 10 is configured to enable direct ship-to-ship transfer of goods and comprises a fluid conduit 14 terminating at a first end at a first hose connector or first port 16 and at a second end at a second hose connector or second port 18. The fluid conduit 14, therefore, directly connects the first port 16 and the second port 18.

In the illustrated embodiment the SPM buoy 10 further comprises a surge protection system including an accumulator vessel or surge tank 20. A pipe 22 extends from the fluid conduit 14 to the surge tank 20. A pressure relief valve 24 is disposed in the pipe 22 and is configured to permit fluid flow along the pipe 22 when the pressure in the fluid conduit 14 exceeds a pre-set value. If the pressure in the fluid conduit 14 rises above the pre-set value due to, for example, a valve on one of the tankers closing suddenly, the pressure relief valve 24 will open and permit oil to flow from the fluid conduit 14 into the surge tank 20. The SPM buoy 10 may include a pump arranged to pump hydrocarbon liquid from the surge tank 20. This allows the surge tank 20 to be emptied and the hydrocarbon liquid to be transferred to a tanker 2, 4.

The SPM buoy 10 further comprises a telemetry system configured to gather information and transmit data to and from the tankers 2, 4. The data preferably includes weather conditions, sea conditions such as wave current, and location data. The tankers 2, 4 will receive the telemetry data from the buoy 10 and will use the data, in particular, to assist during mooring. The data will be used by the DP tanker 4 to hold position relative to the buoy 10 and the other tanker, as described further below. The DP tanker 4 will, therefore, be equipped with the necessary software to receive and process the telemetry data from the buoy 10, as well as from the other tanker. Furthermore, during ship-to-ship transfer of hydrocarbon liquid, the data will include pressure, flow rates and other data related to the transfer of the liquid. The telemetry system may be configured to provide an emergency shutdown system. The emergency shutdown system may be configured to trigger emergency disconnect couplings on the tankers and to monitor conditions, such as for surge protection.

The SPM buoy telemetry system may include means to detect if there is an oil spill around the buoy. Such a system may, for example, be a system developed by the modification of advanced instrument technology such as from https://www.nortekgroup.com A first hose or hose string 26 is connected at a first end 28 to the first hose connector (first port) 16 of the buoy 10. A second hose or hose string 30 is connected at a first end 32 to the second hose connector (second port) 18 of the buoy 10. Each of the first and second hose strings 26, 30 is preferably between 200m and 400m in length, and more preferably about 300m in length. The second hose string 30 may be longer than the first hose string 26. In the illustrated embodiment each of the hose strings 26, 30 comprises a floating hose. Each of the hose strings 26, 30 also includes a marine breakaway coupling (MBC) 34. The MBC 34 is configured to automatically shut off flow along the hose string 26, 30 in the event of an adverse tensile load on the hose string 26, 30.

Each of the tankers 2, 4 preferably includes an emergency disconnect coupling, emergency shutoff and disconnect coupling or emergency release coupling (ERC) 36 connected to a manifold. When each of the hose strings 26, 30 is connected to the respective tanker 2, 4, the ERC is, therefore, disposed at a second end 38, 40 of the hose string 26, 30. The ERC will form part of an emergency disconnect system which also includes associated software or control systems. The ERC 36 is designed, in an emergency situation, or when triggered by the telemetry system of the SPM buoy 10, to shut off flow through the coupling 36 and separate. The ERC 36 therefore causes the hose string 26, 30 to separate from the tanker 2, 4. A shut off valve mechanism within the ERC 36 blocks flow such that when the coupling 36 separates both parts of the coupling 36 are blocked or shut off such that there is no or minimal loss of oil from the hose string 26, 30.

When not in use, each of the hose strings 26, 30 is tied back to the buoy 10 in a bight. The hose strings 26, 30 therefore remain floating on the surface of the sea around the buoy 10.

A mooring hawser 42 is connected to the SPM buoy 10 at its first end. The hawser 42 will typically be about 60m in length. A further pickup line or rope is preferably attached to the hawser 42. The pickup line is preferably about 200m in length. Two support craft or support vessels, such as tugboats, (not shown) are available to assist in connecting the hawser 42 and the hose strings 26, 30 to the tankers 2, 4, as described further below. The support craft are also configured to assist in maintaining the position of one or both of the tankers 2, 4 if required. Each of the support craft will also be equipped with pollution and safety equipment, such as one or more of booms, oil spill recovery equipment (skimmers), storage tanks for recovered liquid, detergent spray, spray arms and firefighting equipment.

In a first stage of the method of ship-to-ship transfer of hydrocarbon liquids a first, conventional tanker 2 approaches the SPM buoy 10. A first one of the support craft extends the mooring hawser 42. The first tanker 2 is moored to the buoy 10 by the hawser 42.

The first hose string 26 is then extended and presented to the tanker 2 by the support craft. The second end 38 of the first hose string 26 is connected to the first tanker 2 at its midships manifold 44. The first support craft remains near the first tanker 2, to assist in manoeuvring the first tanker 2 if necessary.

As the first tanker 2 is connected to the SPM buoy 10 by the hawser 42 it will be appreciated that the first tanker 2 is free to weathervane around the buoy 10.

A DP tanker 4 then approaches the SPM buoy 10 and maintains position at a pre set location relative to the SPM buoy 10 and the first tanker 2. The DP tanker 4 is preferably positioned between 50m and 600m from both the first tanker 2 and the SPM buoy 10. In some situations the DP tanker 4 will maintain position off the starboard quarter of the first tanker 2. Telemetry received by the DP tanker 4 from the SPM buoy 10 and/or the first tanker will assist in maintaining the correct position, even when the first tanker 2 is weather-vaning.

The second hose string 30 is then extended and presented to the DP tanker 4 by the second support craft. The second end 40 of the second hose string 30 is connected to the DP tanker 4 either at its midships manifold 46 or through its bow slot 48. Typically the bow slot of a tanker is configured for loading; however, the bow slot of the DP tanker may be configured or adapted to permit discharge of oil from the DP tanker at required flow rates.

Figure 2 illustrates an arrangement in which the second hose string 30 is connected to the midships manifold 46 of the DP tanker 4. Figures 3 and 4 illustrate arrangements in which the second hose string 30 is connected through the bow slot 48 of the DP tanker 4.

Once both hose strings 26, 30 are connected, the DP tanker 4 pumps oil through the second hose string 30, through the fluid conduit 14 of the buoy 10, and through the first hose string 26 to the first tanker 2.

Upon completion of the transfer of oil, the DP tanker 4 preferably flushes the second hose string 30 with sea water before the second hose string 30 is disconnected from the DP tanker 4. More preferably the DP tanker 4 flushes the second hose string 30, the fluid conduit 14 and the first hose string 26 with sea water. This means that the hose strings 26, 30 are not left full of oil when they are disconnected from the tankers 2, 4. This minimises the risk of an oil spillage during disconnect of the hose strings 26, 30 and when the hose strings 26, 30 are tied back to the buoy 10, for example if one or both of the hose strings 26, 30 is damaged due to a contact.

The second hose string 30 is disconnected from the DP tanker 4 and is tied back to the SPM buoy 10 in a bight. The DP tanker 4 is then free to depart and return to the oil field or FPSO for re-loading.

The first hose string 26 is disconnected from the first tanker 2 and is tied back to the buoy 10 in a bight. The hawser 42 is disconnected from the tanker 2 and the tanker 2 is then free to depart. The first tanker 2 may transport the oil to a local refinery or to another destination. In preferred embodiments the hose strings 26, 30 are tied back to the buoy 10 in a bight during manoeuvring of the tankers 2, 4. Once the tankers 2, 4 have departed the second ends 38, 40 of the hose strings 26, 30 may be released from the buoy 10 so that the hose strings 26, 30 extend from the buoy 10. The hose strings 26, 30 may therefore weathervane around the buoy 10.

In the above scenario, the first tanker 2 and the DP tanker 4 were of similar size or capacity such that a single DP tanker 4 transferred its cargo to the conventional tanker 2, which then departed. In these scenarios, the class of both the DP tanker 4 and the conventional tanker 2 may be Suezmax, having a size of 120,000 - 200,000 DWT (tons deadweight).

In other scenarios the conventional tanker 2 may be significantly larger than the DP tanker 4. In these scenarios a first DP tanker 4 may transfer its cargo to the conventional tanker 2 that is moored to the SPM buoy 10. After the first DP tanker 4 has departed the conventional tanker 2 may remain moored and await a second DP tanker 4. A second DP tanker 4 may then transfer its cargo to the same conventional tanker 2. In these scenarios the class of each of the DP tankers 4 may be Suezmax, having a size of 120,000 - 200,000 DWT (tons deadweight) and the class of the conventional tanker 2 may be Very Large Crude Carrier (VLCC) having a size of 200,000 - 320,000 DWT.

In further embodiments the first tanker may be a floating storage tanker. In these embodiments the floating storage tanker is moored to the buoy by the hawser, as described above. The second end of the first hose string is connected to the midships manifold of the floating storage tanker.

In use, the DP tanker pumps oil through the second hose string, through the fluid conduit of the buoy, and through the first hose string to the floating storage tanker. In some circumstances the oil (or other hydrocarbon) may then be stored for a period of time on the floating storage tanker for future transfer to an export tanker. In other circumstances, an export tanker may be moored to the floating storage tanker at the same time as the DP tanker is connected to the buoy by the second hose string. In these embodiments a third hose string is connected between the floating storage tanker and the export tanker. In use, the DP tanker preferably pumps oil through the second hose string, through the fluid conduit of the buoy, through the first hose string, through pipelines of the floating storage tanker, and directly onto the export tanker via the third hose string.

Although in the embodiment described above oil was transferred from the DP tanker 4 to the conventional tanker 2, in other embodiments the transfer of hydrocarbon liquid may be from the first tanker 2 to the second tanker 4. In particular the transfer of hydrocarbon liquid may be from the conventional tanker 2 to the DP tanker 4, rather than from the DP tanker 4 to the conventional tanker 2.

It will be appreciated that the system of the present invention is more flexible than the provision of ports in fixed locations, and may be used in numerous locations around the world. Furthermore, the use of a single-point mooring buoy for the transfer of hydrocarbon liquids may be safer than bringing a tanker into a port.

The present invention offers benefits and risk reduction by keeping oil tankers in deep water away from environmentally sensitive areas and therefore provides a safer, more reliable and more flexible system and method that allows oil, or other hydrocarbon liquid, to be transferred between a DP tanker and a conventional tanker and that may be utilised to minimise the period of time for which a DP tanker is away from an offshore production facility, such as an FPSO.

Other modifications and variations not explicitly disclosed above may also be contemplated without departing from the scope of the invention as defined in the appended claims.