GB1578220A - Offshore terminal - Google Patents

Description

(54) AN OFFSHORE TERMINAL (71) We, DAVID BROWN - VOSPER (OFFSHORE) LIMITED, a British Company of Graphic House, Castle Street, Portchester, Hampshire, also of Tyler Way, Swalecliffe, Whitestable, Kent, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an offshore terminal.

In the North Sea and other areas there are gas fields in which wells can be drilled to produce gas. Further in all offshore oil fields the oil is associated with gas. Currently associated gas from an oilfield is normally flared off on a production platform as there is not known economic method of transferring that gas from the oil production terminal to the shore.

It is an object of this invention to provide an offshore terminal which alleviates to some extent the stated problem.

According to this invention there is provided a terminal intended to be moored to the seabed and to receive gas, the terminal comprising gas liquefaction apparatus, a liquefied gas store and transfer apparatus for transferring liquefied natural gas from the store to a liquefied natural gas carrier.

If the gas to be received by the terminal is associated gas from an oilfield the terminal should include apparatus for cleaning associated gas.

The size of the store determines how often liquefied natural gas has to be transferred from the terminal to a carrier.

A method of collecting gas from offshore gas terminals is the subject of our copending patent application No. 13717/76.

A terminal in accordance with this invention may well be required to operate in waterdepths between 100 and 200 m and it has been proposed that the terminal should have a storage capacity for four days production.

Preferably the terminal comprises a flare for flaring off gas.

Preferably the terminal comprises a turntable which carries the transfer apparatus, said transfer apparatus being connected to the store through a swivel.

Preferably the turntable carries the flare which may be of the horizontal stack type.

Preferably the turntable carries a mooring line hawser.

The terminal is preferably buoyant and may be held in position by anchor chains.

An embodiment of this invention will now be described, by way of example only, with reference to the accompanying drawing (Fig. 1) and Fig. 2 of the drawings accompanying the Provisional Specification of which: - Figure 1 is a block diagram of gas liquefaction apparatus forming part of the terminal; and Figure 2 is a schematic view of the terminal which is of the catenary type.

Referring to Figure 2 the terminal comprises three basic units that is a liquefied natural gas (LNG) transfer turntable 1, accommodation and LNG process plant structure 2, a concrete buoy chamber 3 containing auxiliary machinery and LNG storage tank.

An LNG transfer system and a mooring line upcoiler are mounted on the turntable 1 which is mounted on bogies. An emergency flare stack 4 is mounted horizontally on the turntable 1, perpendicular to the line of a mooring hawser. A helideck 5 and navigational aids are mounted on the turntable 1. Since the terminal freeboard changes 6.4. m during loading, a transfer boom of the transfer system has to be fitted as low as possible.

A cryogenic swivel (not visible) is mounted on the turntable 1 to transfer the LNG to the transfer system from the LNG tank 7 within lower part of the chamber 3. This swivel is also used for the excess fuel gas supply to the flare stack 4. A slip ring system provides electrical power to the transfer system and helicopter facilities.

The structure 2 is of steel lower portion is denoted at 15. The accommodation provides living quarters and facilities for the operational crew. The accommodation area is separated from the LNG plant by a cofferdam into which inert gas and water can be injected. A double wall shaft containing LNG and flare gas lines to the turntable pass through the area.

The LNG process plant is divided into gas zones and gas free zones, the zones being separated by cofferdams. The liquefaction plant is grouped around a central double wall steel riser. All electrical and gas turbine machinery is separated from the process plant by a cofferdam with the rotating shaft passing through gas tight seals.

All gas zones are fitted with forced ventilation and inert gas injection systems. Cofferdams are provided with inert gas and water spray injection systems. The process plant control room is in a gas free zone adjacent to the equipment. Anchor chain tensioning winches are fitted in this area in gas free zones.

The prestressed concrete buoy chamber contains all the auxiliary machinery and the LNG storage tank. The auxiliary machinery space 8 is in upper portion of the chamber 3 and contains the electrical generating plant and switchgear. The fuel gas tank and emergency diesel tank are mounted on a separate deck. The fire fighting system and inert gas generator are mounted around the steel riser with the associated gas and LNG control valves.

The LNG tank is separated from the machinery space by a double cofferdam which is fitted with a forced ventilation system and an inert gas injection system. Concrete ballast 9 required in the largest terminal is positioned in the base of the buoy chamber.

Gas riser pipes pass down the outside of the buoy structure in a steel housing 10 from the top of the concrete structure to a hose manifold 11 on the underside of the buoy.

The terminal is anchored to the seabed by eight catenary chains 12 piled into the seabed. The chains 12 pass through guides 13 fitted to the buoy chamber 3. The anchor chains are tensioneby winches mounted on the LNG process plant deck.

These winches are operated continuously to allow for the variation in freeboard due to changes in the volume of LNG in the tank.

The associated gas passes through a a flexible hose 14 from a seabed manifold on the subsea pipeline to the hose manifold 11 on the underside of the buoy. A second hose is fitted for standby purposes. There is sufficient slack in the hoses to allow for the draught variation of the terminal.

The associated gas has to be cleaned of moisture, carbon dioxide and heavy hydrocarbons (C.3-C ). The trace quantities of water are removed from the gas by a two bed molecular sieve dehydrator (not shown). The carbon dioxide is removed in a three bed molecular sieve (not shown) which also contains an activated carbon bed for the removal of the heavy hydrocarbons. The absorption systems are operated on a sequential basis, each bed going through a cycle of absorption, regeneration and cooling. The treated gas is then filtered to remove solid pick-up. The regenerated gas, being rich in carbon dioxide and heavy hydrocarbons, is stored as fuel gas.

Liquefaction of the treated gas is achieved by subcooling the gas through a series of six core heat exchangers 21 to 26 (Fig. 1) at a pressure of 5.0 bar. The pressure is then reduced to the storage pressure of 1.1 bar. During the pressure reduction process, some flash gases are produced. This gas is compressed by a boil off compressor and added to the fuel gas tank. Should the boil off compressor be unserviceable, the flash gases are flared off.

The boil off from the tank is no more than 0.5% per day based on a full tank.

The boil off gas is either re-compressed and passed to the fuel gas tank or flared off.

A nitrogen expansion cycle, using a two-stage axial flow compressor 27 with fresh water cooled inter-coolers 28 and 29 is used to liquefy the gas after cleaning.

The nitrogen is compressed to a pressure of 10.7 bar. The inter-coolers 28 and 29 are used to keep the temperature of the gas below 27"C. Turbo expanders 30 to 35 are associated with the exchangers 21 to 26. The compressed nitrogen passes into the heat exchanger circuit with a portion of the nitrogen diverting through each stage of the turbo expander system. The refrigeration cycle is shown in Figure 1.

Refrigerant make-up is supplied from nitrogen bottles.

The main refrigerant compressor 27 is situated in its own compartment. The gas turbine compressor drive, comprising a fully marinised gas generator, supplying a single stage power turbine, is situated in a compartment separated from the compressor by a cofferdam. The drive shaft passes through gas tight seals. All areas containing gas plant are force-ventilated by electrically driven blowers providing at least 30 changes of air per hour.

Electrical power is provided by two generating sets with gas turbine prime movers, one unit being on standby. The gas turbines normally operate on fuel gas but, in emergency conditions, diesel fuel could be used. An inert gas generator is provided to produce a mixture of nitrogen and carbon dioxide for purging the gas tanks and lines. In addition, inert gas is used in the LNG storage tank insulation space. Sea water cools the fresh water of the inter-coolers 28 and 29 via a heat exchanger. The fresh water cooling system is a closed loop system with a small fresh water make-up tank. A fuel gas storage tank is provided for the gas turbines. The storage tank provides a collecting point for boil off gas, flash gas and regenerated gas.

The LNG storage tank is based on the Pittsburgh - Des Moines Steel Company/ Gaz Transport free standing tank. Three sizes of tank are considered, 4883 m3, 6348 m3 and 9766 m3 for the terminal sizes.

The tank is designed to be built of aluminium alloy. A typical alloy is the Alcoa 5083 aluminium alloy, containing magnesium as the principle alloying constituent. This material maintains its physical properties to temperature lower than --180"C. The tank consists of a hemispherical top, cylindrical centre portion and a conical bas. There its a central column in the tank through which pass the LNG discharge pipes to the transfer system. The tank is unstiffened and is supported on its conical base. The insulation for the top, sides and central column consists of a layer of fibrous glass 200 mm thick. This allows for tank expansion. Outside the glass is a layer of perlite insulant. The base of the tank sits on three layers of PVC foam insulant incorporaing an aluminium alloy drip tray.The whole assembly is encased in a thin non load carrying steel shell. The tank is separated from the concrete structure by a dead space filled with inert gas.

Two submerged centrifugal pumps, one working and one standby, are provided in the base of the storage tanks for transferring LNG to the LNG carrier transfer system. The pump outputs for the three terminal sizes will be 660 m3/h, 900 m3/h and 1400 m3/h respectively. The pumps discharge, via the LNG pipework, within the central column, to the cryogenic product distribution unit mounted coaxially with the turntable structure.

The LNG transfer system is designed to allow loading of an LNG carrier in sea conditions of up to 5 m. significant wave height. The system will allow for a change of terminal freeboard of 6.4 m between the loaded and unloaded conditions. The transfer system consists of the lattice structure boom 6 supported on the turntable structure immediately above the mooring line position. The transfer hose 40 hangs from the end of the boom. The mooring line is indicated at 41. The elevation of the boom can be controlled from the turntable.

The LNG and return vapour pass into the boom 6 through a cryogenic product distribution unit. A system of interlocked shut 40 will allow for the motion of the LNG carrier relative to the terminal. A manifold cryogenic coupler is fitted to the free end of the hose 40. An auomatic fail safe system is fitted to effect rapid shut off and disengagement of the transfer system in an emergency.

The horizontal gas flare 4 is mounted on the turntable 1 perpendicular to the line of mooring hawser. The gas is fed to the flare via the LNG cryogenic product distribution unit. A system of interlocked shut off valves and inert gas purging is fitted adjacent to the product distribution unit to ensure that the LNG does not become contaminated with flare off gas. Gas to be flared off is stored in the fuel gas tank, until LNG loading is completed. In an emergency, the LNG supply can be remotely shut off to allow gas to be flared off.

The basic fire protection will consist of water hydrants and hoses throughout the terminal, water spray and deluge systems, dry chemical systems, foam systems, installation of fire-resistant insulation on important and exposed structures and equipment, a training manual, monitoring systems for detecting combustible gases and fires, and control of ignition sources.

The waterspray systems for fire fighting, personnel protection and cooling of exposed structures and components will be installed according to the Classification Society Rules. In addition, the following areas will be protected by a waterspray system: Exposed structures in process area con taining or supporting storage vessels or equipment containing LNG or gases.

Boundaries of superstructures, deck houses and control rooms interfacing with the process area and the LNG stor age area.

Typical areas that will be protected by fixed dry chemical systems are the LNG storage area, the process area, the feed, LNG and refrigerant transfer area, and the impounding areas and drainage channels.

The waterspray and fixed dry chemical system will partly operate automatically on fire detection. The systems will also be capable of remote manual operatio;n.

Monitoring systems will be provided so that every portion of the area protected is under surveillance by detecting devices.

Manualy operated alarm boxes located in key areas throughout the facility, will be incorporated into the monitoring system.

Permanently installed systems of combustible gas detectors, and fire detectors with audible and visual alarms will be provided.

The systems will indicate the location of fires or abnormal combustible gas concentrations. The systems will be able to shut down automatically and safely the whole terminal or portions of the terminal in case of dangerous gas concentraions or fires.

The hawser is fitted with a load measuring device. The mooring pick-up arrangements are linked with the LNG transfer system to facilitate connection of transfer hose to the LNG carrier. When not in use, the mooring hawser is upcoiled by the upcoiler.

WHAT WE CLAIM IS: - 1. A terminal intended to be moored to the seabed and to receive gas, the terminal comprising gas liquefaction apparatus, a liquefied gas store and transfer apparatus for transferring liquefied natural gas from the store to a liquefied natural gas carrier.

2. A terminal as claimed in claim 1 which includes apparatus for cleaning associated gas.

3. A terminal as claimed in claim 1 or claim 2 which comprises a flare for flaring off gas.

4. A terminal as claimed in any preceding claim which comprises a turntable which carries the transfer apparatus, said transfer apparatus being connected to the store through a swivel.

5. A terminal as claimed in claim 4 as appendant to claim 3 wherein the turntable carries the flare which may be of the horizontal stack type.

6. A terminal as claimed in claim 4 or claim 5, wherein the turntable carries a mooring line hawser.

7. A terminal as claimed in any preceding claim which is buoyant.

8. A terminal as claimed in claim 7 which is held in position by anchor chains.

9. A terminal substantially as hereinbefore described with reference to the accompanying drawings and Figure 2 of the drawings accompanying the Provisional Specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. key areas throughout the facility, will be incorporated into the monitoring system. Permanently installed systems of combustible gas detectors, and fire detectors with audible and visual alarms will be provided. The systems will indicate the location of fires or abnormal combustible gas concentrations. The systems will be able to shut down automatically and safely the whole terminal or portions of the terminal in case of dangerous gas concentraions or fires. The hawser is fitted with a load measuring device. The mooring pick-up arrangements are linked with the LNG transfer system to facilitate connection of transfer hose to the LNG carrier. When not in use, the mooring hawser is upcoiled by the upcoiler. WHAT WE CLAIM IS: -

1. A terminal intended to be moored to the seabed and to receive gas, the terminal comprising gas liquefaction apparatus, a liquefied gas store and transfer apparatus for transferring liquefied natural gas from the store to a liquefied natural gas carrier.

2. A terminal as claimed in claim 1 which includes apparatus for cleaning associated gas.

3. A terminal as claimed in claim 1 or claim 2 which comprises a flare for flaring off gas.

4. A terminal as claimed in any preceding claim which comprises a turntable which carries the transfer apparatus, said transfer apparatus being connected to the store through a swivel.

5. A terminal as claimed in claim 4 as appendant to claim 3 wherein the turntable carries the flare which may be of the horizontal stack type.

6. A terminal as claimed in claim 4 or claim 5, wherein the turntable carries a mooring line hawser.

7. A terminal as claimed in any preceding claim which is buoyant.

8. A terminal as claimed in claim 7 which is held in position by anchor chains.

9. A terminal substantially as hereinbefore described with reference to the accompanying drawings and Figure 2 of the drawings accompanying the Provisional Specification.