AU2018232894A1 - A Method for Offshore LNG Transfer from a FLNG Facility to a LNG Carrier - Google Patents

Abstract

A method of offshore LNG transfer between a floating LNG vessel (FLNG) 10 and a LNG carrier 12. The method involves a first step of tandem mooring of the LNG carrier 12 to the FLNG10 by a mooring hawser 16. A semi-submersible pontoon 18 s ("SSP 18") is then positioned near the FLNG 10. A tug 24 is then connected with a stern 26 of the LNG carrier 12. LNG is transferred from the FLNG 10 via the SSP 18 to the LNG carrier 12. This is done using hoses 20 which are always kept above the water plane, and therefore do not need the capability to float on the water which comes with substantial associated complexity and cost. CD r14 rn rn r14 bLo r'4 r-4 ( o IZI, CD rn r-4 T-i IZI, C-4 r14 r14 blo C-4 0 00 r14 r-4 rn blo iz A - C-4 ( o r-I rn

Description

2018232894 17 Sep 2018

A Method for Offshore LNG Transfer from a FLNG Facility to a LNG Carrier Technical Field

A method is disclosed for offshore LNG transfer from a floating LNG (FLNG) facility/vessel to a LNG carrier.

Background Art

A floating LNG vessel (hereinafter referred to in general as “FLNG”) is a vessel moored offshore for the purposes of producing LNG. The FLNG has storage tanks for temporarily storing produced LNG. The stored LNG is transferred from time to time to LNG (offtake) carriers and subsequently shipped to market.

io With a conventional barge or ship shape FLNG the present methodology for offtake is side-by-side mooring of the FLNG and LNG carrier subject to suitable weather and sea conditions. The side-by-side relationship is required to enable the transfer of LNG from an amidships manifold of the FLNG to an amidships manifold of the LNG carrier by hoses or hard arms. FLNG side-by-side offtake in open waters can be quite restrictive with respect to sea conditions and vessel movement. It has inherent risks with berthing and mooring operations which can require at least two tugs which themselves have restricted capability in open sea situations. The risk of damage in berthing and mooring operations with a FLNG is higher than similar operations at a sheltered terminal.

The concept for FLNG production and offtake offshore is essentially the same as that presently undertaken for oil production and offtake by Floating Production, Storage and Offtake tankers (FPSO’s). However the practice is for the offtake oil tanker to moor astern of the FPSO by head line(s) in tandem and the oil transferred from the FPSO by floating oil hoses from the stern of the FPSO to the amidships manifold on the oil tanker. Tandem mooring reduces the risks in berthing and improves the weather envelope for offtake.

As with oil tankers, the loading manifold on a standard design LNG carrier is amidships. Although tandem mooring for LNG offtake is feasible there are inherent difficulties with floating LNG hoses. Due to the cryogenic nature of LNG, floating 30 hoses have problems with respect to handling, use and storage for LNG transfer.

An alternative is to provide a tandem moored LNG offtake carrier with a bow manifold and gantry offtake arrangement to load over the bow. A further refinement would be to include dynamically positioning (DP) into the LNG offtake carrier so that it can maintain position relative to the FLNG without mooring. While this is feasible, 35 the cost and more importantly, the restriction from a commercial market perspective of being unable to use standard LNG carriers, would seriously impact the likely approval for investing in a project.

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2018232894 17 Sep 2018

In addition to ship or barge shaped FLNG facilities, a circular designed facility has been proposed. The advantage is in the area or response to sea conditions negating the need for the FLNG facility to incorporate an expensive swivel system. Due to the FLNG moorings, sea dynamics and its circular design, this facility does 5 not facilitate side by side operations. DP LNG offtakes are feasible however the downside of cost and market restrictions also apply.

As mentioned above, a key requirement to ensuring a commercially viable project in today’s LNG market is to use standard LNG carriers. The market has moved in the last 10 years from generally fixed long term project destinations with dedicated seller io shipping to a mix including buyer providing shipping, global trading and shorter term deals. This all drives the need for utilising standard LNG ships which do not have DP or bow loading.

There exists a need for an alternate and flexible method of transferring LNG from a FLNG to a standard LNG carrier without the need to berth alongside an FLNG facility.

SUMMARY OF THE DISCLOSURE

In a first aspect there is disclosed a method of offshore LNG transfer between a floating LNG vessel (FLNG) and a carrier comprising: tandem mooring of the carrier to the FLNG;

positioning a semi-submersible vessel (SSP) near the FLNG; and transferring LNG from the FLNG to a standard LNG carrier via the SSP.

In a second aspect there is provided a method of offshore LNG transfer between a floating LNG vessel (FLNG) and a LNG carrier comprising:

tandem mooring the LNG carrier to the FLNG;

positioning a semi-submersible pontoon (SSP) alongside LNG carrier; connecting a tug to a stern of the LNG carrier while moored to the FLNG; and transferring LNG from the FLNG to the LNG carrier via the SSP.

In one embodiment of either aspect transferring LNG comprises maintaining at least one LNG transfer hose which extends from the FLNG to the SSP above a surface of a body of water in which the FLNG and the LNG carrier are located and enabling a flow of LNG from the FLNG to the SSP.

In one embodiment of the first aspect the method comprises connecting a tug to a stern of the LNG carrier while moored to the FLNG.

In one embodiment of either aspect the method comprises mooring the SSP to the LNG carrier.

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2018232894 17 Sep 2018

In one embodiment of either aspect the method comprises mooring the SSP to a side of the carrier to facilitate amidships LNG connection.

In one embodiment of either aspect the method comprises making a flexible mooring 5 between the SSP in the carrier.

In one embodiment of either aspect the method comprises moving the SSP to an offshore mooring when not required for transferring LNG to a carrier.

io In one embodiment t of either aspect he method comprises using the tug to move the SSP to and from the offshore mooring.

BRIEF DESCRIPTION OF THE DRAWINGS

Notwithstanding any other forms which may fall within the scope of the method as 15 set forth in the Summary, specific embodiments will now be described, by way of example only, with reference to becoming drawings in which:

Figure 1 is a schematic plan view of an arrangement and of equipment and vessels to facilitate an embodiment of the disclosed method of offshore LNG transfer between a circular hull FLNG and a LNG carrier;

Figure 2 is a view of the arrangement shown in Figure 1 from a first angle;

Figure 3 is a view of the arrangement shown in Figure 1 from a second angle which is orthogonal to the first angle; and

Figure 4 is a plan view illustrating application of the embodiment of the disclosed method in relation to a rectangular hull FLNG and a LNG carrier

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Figures 1 to 3 show a plan and two mutually orthogonal side views of an arrangement of equipment and vessels which facilitate a first embodiment of the disclosed method for the offshore transfer of LNG between a floating LNG vessel 10 (hereinafter referred to in general as “FLNG 10”) and an LNG carrier 12.

In this first embodiment the FLNG 10 has a circular hull type as for example manufactured by the company Sevan Marine ASA. However as explained later, embodiments of the disclosed method and system are not limited to use with a FLNG having a circular hull.

The first embodiment of the method entails the steps of:

(a) tandem mooring of the LNG carrier 12 to the FLNG10 for example by an appropriate mooring hawser 16;

10661094_1 (GHMatters) P106132.AU

2018232894 17 Sep 2018 (b) positioning a semi-submersible pontoon 18 (hereafter referred to in general as “SSP 18”) near the FLNG 10; and (c) transferring LNG from the FLNG 10 via the SSP 18 to the LNG carrier 12.

One of the ideas of this embodiment is to enable the transfer of LNG from the FLNG

10 to the LNG carrier 12 by hoses 20 which are maintained above the sea. This may be done for example by using a gantry or other form of hose handling facility 38 on the FLNG 10 with flexible hoses. The flexible hoses can be connected to flexible and /or hard lines 22 on the SSP 18.

This use of the SSP 18 in the method provides significant commercial and io operational benefits.

First and foremost it provides a cost effective way to facilitate a tandem mooring arrangement negating the need for side by side operations with high damage risk and limiting operational envelopes. Tandem mooring requires the use of one tug whereas side by side mooring with standard LNG carriers would require at least two 15 tugs offshore.

Second it negates the need for floating cryogenic hoses with the following advantages.

• The reliability, handling and maintenance of floating hoses is a recognised issue that is difficult to mitigate in an offshore environment.

· Reduced capital expenditure because floating hoses are more expensive than non-floating hoses • Reduced capital expenditure where the length of floating hose required facilitating the LNG transfer from the FLNG to the LNG carrier manifold is not insignificant with high per metre costs.

· Floating hoses are subject to damage during handling.

• The SSP 18 facilitates the use of multiple smaller hoses and pipes instead of larger diameter floating hoses.

• With the hoses and transfer pipework on the SSP 18, methods of cooling and purging can be designed to minimise time and effort.

In this embodiment in order to minimise the risk of weather vanning of the LNG carrier 12 and to otherwise hold the LNG carrier 12 at a substantially fixed juxtaposition to the FLNG 10 the act of mooring the LNG carrier 12 also includes connecting a tug 24 or other support vessel to a stern 26 of the LNG carrier 12.

The SSP 18 is in the general form of a small vertical structure such as a framework 35 28 with one or more decks 30 and a submersed hull 32 which provides substantial stability. When the SSP 18 is not in use transferring LNG to a LNG carrier 12 it is moored at another location such as by a buoy. The SSP can use moorings to make fast to the side of the LNG carrier or utilise vacuum mooring technology that is being increasingly used in the marine environment. An example of such a mooring system

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2018232894 17 Sep 2018 is the vacuum based automated mooring system by Cavotec SA marketed under the brand name MoorMaster™.

There are two possible methods for moving the SSP 18 from one of the mooring locations to the transfer location at which the LNG carrier 12 is located.

A first method which may be used when the SSP is a “dumb” barge with no SSP propulsion is to use the tug 24 to tow the SSP 18 between a mooring location and the transfer location.

A second method is to construct the SSP 18 with its own power and propulsion system which can be controlled remotely or manually by one or more people on the io SSP 18; or remotely from the FLNG 10.

With the dumb barge concept for the SSP 18, the LNG carrier 12 and the SSP 18 (under tow by the tug) would do a side to side underway connection. The LNG carrier 12, with the SSP 18 fast alongside, would do a standard tandem moor approach with the tug now fast to the stern of the LNG carrier, and make fast to the 15 FLNG.

The FLNG 10 is fitted with suitable mooring points for the LNG carrier 12 and, as previously described, an adjacent gantry 38 for carriage of the flexible (soft) LNG hose connection to the SSP.

Once the LNG carrier 12 with the SSP 18 alongside is moored to the FLNG 10, the 20 LNG hoses 20 can be connected to the SSP 18 LNG piping (hoses). The SSP 18 is also connected with a mooring hawser 17 to the FLNG 10. This is left slack but is a safety line as well as facilitating departures of the LNG carrier 12.

The LNG connection between the SSP 18 and the LNG carrier 12 would be similar to the soft hose connection now used extensively in LNG ship to ship operations.

In one embodiment the gantry 38 can be formed with jib 42 (Fig 3) that is telescopically extendable to reach a location in substantial vertical alignment with the SSP 18. In this way the LNG hose or hoses 20 can be paid out and lowered to the SSP 18. The hose 20 may then be connected to the SSP 18 pipework 22.

In an alternate embodiment with the jib 42 is not extendable a feed line (not shown), 30 which is attached to the hose and can be projected, for example by use of a compressed air gun, to the SSP 18. The feed line is then hauled in bringing with it the hose for connection to pipework of the SSP 18.

There are many other ways to manage the mooring and connection of the LNG carrier 12 and SSP 18 in this tandem formation.

It will be recognised that for all variations, the hose 20 is always maintained above the sea and does not, nor need, to float. The hose 20 is in a catenary formation allowing for a safe movement envelope while transferring product. An emergency

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2018232894 17 Sep 2018 disconnect systems (not shown) which is common in the LNG industry would be incorporated in embodiments of the disclosed system and method to facilitate disconnection of the hose 20 form the SSP 18.

The pipework 22 on the SSP 18 to which the hose 20 is connected is plumbed through, or otherwise run, to a tower 46 on the SSP framework 28 which facilitates a soft hose connection 48 to the LNG carrier 12.

Once a transfer of LNG is complete the system is purged from the LNG carrier manifold and the hoses 48 disconnected. The SSP 18 remains connected to the

FLNG 10 and disconnected from the LNG carrier 12 with the tug made fast to the io stern end of the SSP 18 to hold the SSP 18 clear of the LNG carrier 12 which is disconnected from the FLNG 10 and goes astern to clear the FLNG 10 and SSP 18.

The SSP lines and connection can be cleared of LNG and once the hoses 20 are safely disconnected, the SSP 18 can be manoeuvred by the tug to its offshore mooring.

The disconnection and clearing of the LNG carrier 12 and SSP 18 can be achieved by a number of methods; the above is a suggestion to describe its feasibility. A powered SSP would facilitate a different methodology also.

Figure 4 illustrates an embodiment of the disclosed method applied to an FLNG 10a having a normal ship shape hull rather than the circular hull of the FLNG 10.

Claims (7)

1. 2018232894 17 Sep 2018

   Claims

   1. A method of offshore LNG transfer between a floating LNG vessel (FLNG) and a LNG carrier comprising:

   tandem mooring the LNG carrier to the FLNG;

   5 positioning a semi-submersible pontoon (SSP) alongside LNG carrier;

   connecting a tug to a stern of the LNG carrier while moored to the FLNG; and transferring LNG from the FLNG to the LNG carrier via the SSP.
2. 2. The method according to claim 1 wherein transferring LNG comprises maintaining at least one LNG transfer hose which extends from the FLNG to

   10 the SSP above a surface of a body of water in which the FLNG and the LNG carrier are located and enabling a flow of LNG from the FLNG to the SSP.
3. 3. The method according to any one of preceding claims comprising mooring the SSP to the LNG carrier.
4. 4. The method according to claim 3 comprising mooring the SSP to a side of the

   15 LNG carrier to facilitate amidships LNG connection.
5. 5. The method according to claim 3 or 4 comprising making a flexible mooring between the SSP in the LNG carrier.
6. 6. The method according to any one of the preceding claims comprising moving the SSP to an offshore mooring when not required for transferring LNG to a

   20 carrier.
7. 7. The method according to claim 6 comprising using the tug to move the SSP between the LNG carrier and the offshore mooring.