LNG load transfer system

Details LNG load transfer system LNG load transfer system

2000-08-28

2002-08-20

Capossela, Ronald (Department: 3744)

Refrigeration

Storage of solidified or liquified gas

Liquified gas transferred as liquid

C062S050700, C062S053100, C062S240000

Reexamination Certificate

active

06434948

ABSTRACT:

INTRODUCTION
This invention relates to a system combining two vessels for handling liquid natural gas and other natural petroleum products, with one vessel being a floating production, storage and offloading lying at a petroleum producing seabed arrangement, ant with the other vessel being an LNG tank vessel or an ordinary tanker.
STATEMENT OF PROBLEM
In connection with and during offshore production of floating gas (LNG, NGL and LPG) usually stabilized oil and/or condensate is produced. Delivery of LNG is usually tied to long-term contracts, while gas condensate contains light or “volatile” petroleum components which may be delivered at a spot marked to the refinery which at any time might need such light petroleum components for the refining process. The vessels devices for the export system from such a combined offshore oil/gas conversion FPSO must satisfy both these products, export demands in a way so that two different tanker vessels like LNG tank vessels and conventional tank vessels shall be able to moorage to the FPSO-vessel and be connected with their respective transfer systems, whereof the first is cryogenic for LNG transfer, and the second may be arranged for transfer of petroleum components being fluid at higher temperatures.
DEFINITIONS
Liquid Natural Gas “LNG” is a liquid, fluid methane, with boiling point between −165° C. and −163° C.
Gas condensate is the part of the gas from a producing well which is separated into liquid phase, consisting of light petroleum components.
The term “cryogenic” describes in this relation a thermally isolated system arranged for handling a gas being cooled down to its liquid phase. The liquid here is LNG. The isolation usually consists of vacuum combined with ordinary isolating material.
By “LNG pipe” pipes being cryogenic are meant, and may comprise pipes with several parallel channels or concentric channels, provided with isolation and possible return channels arranged outside of the main channel. LNG pipes may be stiff or flexible as defined below.
By “rigid LNG pipes” pipes are meant which are no more flexible than the occurring change of shape due to pressure or temperature expansion during use.
With the term “flexible LNG pipe” an LNG pipe is meant being arranged for and which repeatedly may be bent to a smallest radius of bending of e.g. 3 metres. Such flexible LNG pipes are usually provided with corrugated walls of austenitic steel.
An LNG tank vessel is a tank vessel with cryogenic tanks, usually spherically shaped, arranged for transporting LNG.
“STP” and “STL” are submerged moorage- or production loading buoys.
LNG
Methane arrives in gas phase from a gas producing petroleum well and must be condensed in a condensing plant to be brought into liquid phase as LNG. LNG has little volume with respect to the methane gas, and may be handled under low pressure. All heat energy supplied to LNG may lead to boiling and thus loss of methane gas if the gas is not reliquefied. LNG must thus be handled cryogenically during storage and transport, i.e. that tanks, pipes, swivels and valves must be thermally isolated. During LNG production at sea this liquid gas must be transferred to an LNG tank vessel which brings the LNG load ashore to separate tank devices arranged for receiving LNG.
Gas condensate.
Gas condensate consists of other lighter hydrocarbon fractions which must be stored in ordinary tanks separate from the cryogenically stored LNG. Normally the gas condensate must be transported at ordinary tank vessel and may not be transported at LNG tank vessels. Gas condensate may be transferred via e.g. floating loading hoses or STL systems to tank vessels or other export systems. A floating loading hose arranged for being stored or mooraged along the FPSO vessel while not in use for transfer of liquid load, e.g. condensate, is described in Navion's patent application NO 19980431, filed 30.01.1998.
Known Production/export Systems
FIG. 12
illustrates a known solution for LNG production and export which implies storage of a very large volume of LNG on board the FPSO vessel's LNG tanks, and fast offloading to a tank vessel. The large storage volume contributes to the FPSO vessel's displacement and implies generally high construction, operating and maintenance costs. The large storage volume demands a volume which could rather have been utilized for other production processes or storage of other petroleum fluids. LNG tank vessels are already available, and they are less specialized than FPSO vessels, and may thus represent a more economic solution for intermittent storage during production of LNG at the field.
Known LNG Transfer Systems
Due to the known moorage device's limitations, and in order to keep safe separation, preferably 100-150 metres between the FPSO vessel and the tank vessel, known LNG pipes must be long. Existing solutions for LNG transfer to LNG tank vessels imply use of stiff transfer pipes typically 16″ (40 cm) inner diameter, and relatively fast transfer to a LNG tank vessel. The known systems for LNG transfer with 16″ pipe implies e.g. loading of 135000 m
3
of LNG during about 12 hours, i.e. 10000 m
3
/hour. Such long transfer pipes arranged in a crane boom are heavy, stiff and difficult to handle and are often damaged by connection or disconnection, or break during transfer. Moorage and load transfer under demanding operating conditions is difficult to perform due to the mechanical loads such a transfer pipe would exerted to.
A mooring system comprising hawsers from the FPSO vessel's stern to the LNG vessel's bow in cooperation with about 40 to 50 tonnes constant force aftwards from the tank vessel propulsion engine in order to keep a very close, still tensioned mooring between the FPSO vessel and a tank vessel, is described in Navion's patent application NO 998 0579 filed 10.02.1998, of which this application claims priority. A support vessel which would otherwise be present anyway for handling of loading hoses, moorings etc., may replace the aftward force from the tank vessel's propulsion engine. The combination of a close and still tensioned mooring position facilitates transfer of ordinary liquid load, preferably gas condensate, through the floating loading hose to a midship manifold or a bow manifold on an ordinary tank vessel, but also launches the possibility of LNG load transfer via a flexible LNG pipe extended between the stern of the FPSO vessel and the bow of an LNG tanker. This flexible LNG pipe may either hang freely and dry between the vessels, be held e.g. on sheaves by means of a support wire extended between the vessels, or running via the sea.
A need therefore exists of a system being able to handle a continuous production of both a large volume of LNG and a smaller proportion of gas condensate, and which may export these two products with each their different and special demands for storage, handling and load transfer, preferably to two different kinds of vessels, by means of two different load transfer systems.
The system which here is being applied patent for, represents a unitary solution for several of the above mentioned problems of technical, operational and logistic nature.
Reference to the Set of claims; Definition of the Invention
The Invention According to claim
1
The solution to the above mentioned problems consists of a system for production, storage and export of liquid natural gas (LNG) from an FPSO vessel with an LNG liquefaction plant, with the novel trait by the invention is the combination of the following points:
an LNG buffer tank on the FPSO vessel, with buffer storage capacity for temporary storage of the continuously produced LNG during the absence of an LNG tank vessel,
a mooring device arranged for short separation moorage between the stern of the FPSO vessel and the bow of an LNG tank vessel,
a cryogenic transfer device arranged between the FPSO vessels stern and an LNG tank vessel's bow, comprising a flexible LNG pipe and arranged for consecutive transfer of produced LNG, and
at least one or more LNG

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