GB2399320A - Semi-submersible jetty for transferring LNG from a production vessel to a transport vessel - Google Patents

Abstract

An 'L' shaped semi-submersible jetty is provided for use in the transfer of LNG from a production vessel to a transport vessel. In use the jetty is connected to the production vessel by means of a ball and hydraulic clamp coupling 3, which allows no relative motion between the jetty and the production vessel in a longitudinal, transverse or vertical direction, but does allow rolling motion of the jetty. Once the jetty is connected to the production vessel the transport vessel is moored alongside the jetty and LNG is transferred through pipes running along the jetty.

Description

Page Nol of 5 2399320 Semi Submersible Floating Jetty for the Safe

Transfer of LNG between a Floating Production, or Storage Barge, to a Transportation Vessel at an Exposed Offshore Location.

Malcolm Newell 146 Nyetimber Lane, Aldwick, West Sussex, P()21 31IX, United Kingdom Contents Page No The design requirements for a Proposed Floating Offshore LNG Jetty I The Floating, Semi Submersible LNG Jelly 2

Conclusions 5

The design requirements for a Proposed Floating Offshore LNG Jetty The jetty described here is to be used in conjunction with a Tandem Mooring arrangement similar to those used on many Offshore Oil l,oilding systems. In this application the 1,NG Shuttle vessel approaches close enough to pick up single Mooring Hawser which is attached to the Barge at one end, and then the outboard end of this hawser is attached to a BOW Chock on the Shuttle. By slowly running astern, the Shuttle is able to take up this mooring and apply a low tension force to the stern of the Barge. This a) keeps the Shuttle at a fixed and well defined distance from the Barge and also, b) slightly dampens any surge or sway motions of the Barge about its Turret Mooring, reducing the footprint swept out by the loading manifold on the Shuttle and simplifyhg the problem of aligning any Offtake system which is linking the two vessels.

The Ship:- relatively low cost solution to the LNG Offtake Shuttle requirement will be to take a standard LNG vessel (probably having a Moss or other self supporting cargo tank arrangement and powered by a highly reliable steam turbine based propulsion plant) and adapt it for Tandem Mooring. This involves fitting a suitable bow fairlead, mooring chock and a winch capable of handling both the mooring hawser itself and its attendant chaffing chain, floatation buoys, messenger rope etc. The Shuttle so produced is already fitted with mid- ships mounted loading and discharge facilities to international standards and is compatible with most anticipated discharge terminals throughout the world. It would benefit from the addition of forward facing berthing radar as well as an automatic relative heading control and hawser tension monitoring equipment linked to systems on the Barge. The cargo manifolds are already mounted at a longitudinal position on the shuttle where heave and pitch motions are at their minimum but, being at the ship side' are subjected to its maximum roll movements.

The Barge:- is fitted with a Tandem Mooring Hawser mounted, along with its stowage winch, on the centre line of its all (Poop) deck. The adoption of Tandem Mooring virtually eliminates any requirement for sophisticated D.P. and associated power control systems on the Shuttle. By using this mooring system it is possible that the LNG Shuttle Tankers can be conventional steam turbine propelled vessels rather than sophisticated Dynamically Positioned units. it_

Page No2 of 5 The Jetty:- With the two vessels tandem moored in line ahead, pointing into the weather and with the hawser tension stable, we are ready to arrange for Cargo Transfer by fitting:- (Refer to Fig 9).

a) A floating platform? carrying a set of loading arms, which is attached (by units powered from the jetty) to the Shuttle side at its mid-ships position and which is clamped to and movhig with the Shuttle as it rolls.

b) A second floating platform attached to the stern of the Barge at the C/L End coupled to the vertical transom and moving with the Barge in heave.

c) Between items a) and b) must be fitted a rigid but resilient pipe-rack structure capable of safely carrying a normal complement of insulated l, NG and vapour pipelines (plus associated valves and fittings) and which holds them in a dry accessible position well above the sea surface.

d) A second set of loading arms is fitted on a small platform rigidly attached to the stern of the Barge and cantilevered out near item h).

These four basic ingredients form the design basis of the proposed Floating Semi Submersible LNG transfer Jetty which is described next. I he forward end of the light weight, low inertia floating structure is directly coupled to the Barge at its C/L, directly below the Tandem Mooring Hawser attachment and carries platform b!. The coupling is resiliently mounted on the Baige from which it is hydraulically operated. It is colmected to the lloathig platform structure by an articulated ball joint which allows the platform to move freely in roll but restrains all relative longitudinal and heave movement between the Barge and Jetty at this point.

The length of jetty / pipe support structure between the Barge and the Shuttle Mid-ships position is supported on small water plane columns tuned in heave to match the Barge motion. The Jetty columns are tied together by a light weight lattice cmsiruction which enables the Boating structure to form a strong, fight beam between the Barge and the Shuttle Mid-ships where it connects to a deep narrow pontoon containing machinery as well as ballast tanks and which supports the mid-ships loading arms as well as the location and clamping devices for attachment to the Shuttle side.

A brief outline of the operation of the floating jetty as well as a tcw more details of some of its features now follows.

The Floating, Semi Submersible LNG Jetty À LNG Transfer is by Stainless Steel Pipes with LNG Loading Arms at the Barge to Jetty & Jetty to Ship connections.

À Pipe-work expansion and flexure is taken up by proven Omega Bends and LNG grade Swivels À All Mooring and Transfer system equipment is based on well proven and readily available Marine products.

À All pipe-work & fittings associated with LNG transfer are located well above sea level and easily maintained.

À All pipe-work utilizes conventional "Dry and Sealed" marine LNG system Insulation.

À Jetty systems arc easily maintained "On Sitc". Production Bargc facilities can be easily utilized in order to perform more stringent Jetty maintenance. Or, jetty can be easily removed to Shore Base or Shipyard for major work.

À When on site, all Jetty systems arc powered and supplied, via umbilical cables, from the Bargc (Fig 9, Item 1).

À System operation, monitoring and control as well as E.S.D and other Emergency functions are easily integrated with the Bargc Systems and D.C. S. so that the Bargc Control Room can operate and Monitor all Jetty functions.

À Jetty has its own independent Emergency and Base l,oad Generator which provides essential safety back-up for Power, Control & Emergency Systems and for periods when Jetty is dc-coupled or when under tow (Fig 9, Item 2) .

À Jetty connection to the Barge is via a "Ball Joint" on the jetty which is connected to the Barge within a hydraulically operated "Tow Coupling" mounted at the Barge C/L and located well above sea level on the Transom of the Barge (Fig 9, Items).

À Direct coupling of the Barge and floating Jetty minimizes the excursion envelope which the Barge end LNG transfer system has to cope with and minimizes the angular motion in both the vertical and horizontal planes.

À The LNC Omake Vessel (LNG.O.V) is independently Tandem Moored from the stern of the Barge with the mooring hawser passing through a Fairlead mounted vertically above the Jetty Ball Joint connection (Fig 9, Item 4).

À Mooring equipment tor the LNC.O.V is mounted on the Barge Stern at the C/L in a safe location at main deck level.

Page No3 of 5 À No mooring loads *tom the LNG.O.V are transferred to the Jetty structure, which is lightweight and low inertia.

À Restraining forces transferred between the Jetty & the Barge (due to relative Heave and Roil motions) are low as a result of the lightweight Jetty construction in vicinity of the Barge.

À Any Jetty (and LNG O.V) Fishtail" motion relative to the stern of the Barge takes place about a vertical axis through the Hall joint and Tandem Mooring Fairlead, which results in very small relative longitudinal movements between the Jetty and the LNG O.V.

À The upper flange ofthe Jetty beam is tommed by the longitudinal walk way and pipe rack support structure.

À Power and Control is transferred between the Barge and the Jetty via Umbilical cables suspended from retractable Umbilical Crane booms on the [large. This allows the l)mbilical cables to be quickly detached (e.g. Or Jetty detachment as part of ESD or for Tow off).

À All Jetty and LNG O.V mooring and un-mooring operations take place in "Safe Zones" on both Barge and Jetty.

À LNG Transfer from the Barge and Vapour return from the Jetty, are via loading arms slightly offset from the CiL moorings with all related power and controls mounted on the Barge.

À Hazardous Areas associated with LNC transfer are minimized in size and number and are clearly definable. They do not encroach on permanently manned areas or on Mooring and Power transfer systems.

À At a position on the Jetty which coincides with the Mid-ship's Loading Manifold on the 1,NG O.V there is a deep, narrow Ballast Tank and Motor Room on which are mounted the LNG Transter Arms and Fire Fighting equipment (Fig 9, Item S).

À Two electric thrusters with variable speed, f xed pitch propellers are fitted in Tunnels through the Ballast Tanks.

À The side of the Ballast Tank, Motor room facing the l.NG O.V is fitted with a pair of fender i attachment units.

À It is intended that "Vacuum Attachment" units are used (hydraulically tensioned mooring lines could also be used).

À Cantilevered out from the bottom of the Ballast Tank structure and under the LNG O.V Hull arc two "Location Arms" each with a pair of fender / vacuum attachment units mounted on their top sides (Fig 9, Item 6).

À In operation, the Jetty Unit can be raised and lowered between LNG O.V Berthing draft and LNG Loading draft by de-ballasting and ballasting the Ballast Tank, Motor room structure.

À Jetty length and Moorings arc arranged so that the de-ballasting of the Ballast Tank, Motor room structure raises the Loading Arms to the ideal height and alignment for connection to the LNG O.V's mid-ship's loading manifold.

À Prior to arrival of the LNG O.V, the Barge end LNG Transfer Arms are disconnected from the Jetty and lifted cicar.

À The Jetty I hrusters swing the Jetty clear (pivoting about the ball connection) of the Barge Tandem Mooring C/L (Figs 4 and 5).

À The Jetty Ballast T ank, Motor room structure is also ballasted down to the Berthing draft.

À Ideally, Thrusters on the stern old the Production Barge are used to hold the Heading of the Barge and Jetty throughout the LNG O.V's Berthing operation. However, the use of such thrusters is not essential.

À The LNG O.V approaches and connects the Tandem-mooring Hawser (Figs 6 to 8).

À Once the Chaffing Chains are secured in the mooring Bits, the LNG O.V drops back to give a tight Mooring line and it takes up and maintains the same heading as the Production Barge.

À The Jetty Thrusters are used to propel the Jetty back to a position alongside and in contact with the LNCi O.V.

À Initially the jetty and LNG O.V are held together by counter thrust (between Jetty and Ship lateral thrusters).

À The Barge LNG Transfer arms are re-connected to the Jetty pipe-work at the forward end.

À The Jetty Baila.st Tank, Motor room structure is de-ballasted to bring the lower locating arms (and Fender Attachment units) into contact with the bottom ofthe LNG O.V's [lull.

À The astern thrust being applied by the l.N(i O.V is adjusted against the Mooring Hawser elasticity to centrc the LNG Loading Arms on to the mid-ships Loading Manifold.

À Vacuum is applied to the Fender Attachment units and the Ballast Tank, Motor room structure is fully de-ballasted to the loading condition. The Jetty end is now locked against the LNG O.V and moves with the Roll of the Ship.

This results in the virtual absence of motion between the Mid-ships Jetty Loading Arms and the LNG O.V.

À Counter thrust between Jetty and Ship lateral thrusters can now be stopped.

À The Loading Arms are connected up using the Jetty's own hydraulic systems and standard cool-down and loading procedures are followed to allow the loading operation to commence (Fig 8.

À the LNG O.V's astern thrust is maintained throughout the Loading operation and the longitudinal alignment of the l.oading Arrns / Manifold positions are monitored throughout.

À T hroughout the loading operation, control, monitoring & alarm functions are directly available in the Barge CCR.

Page No4 of 5 À Back-up and emergency power is available from the Jetty Base Load (Generator and Switchboard.

À T he Fire Fighting systems at the LNG O.V's Manifolds are augmented by the Monitors mounted at the fore and aft extremes of the Jetty Ballast 'l'ank, Motor room structure.

Advantages from the use of the Jetty, TO THE BARGE: À It Minimizes and Simplifies the Mooring and 1,NG Transfer Facilities which must be mounted ON the Barge.

À It minimizes the footprints required on the Barge for Mooring and LNG Transfer.

À It removes the Marine Mooring and LNG transfer equipment from the vicinity of the Barge Production Facilities.

À It provides easily and clearly defined interfaces between the Barge and the Off-take early in the Project schedule.

À Integration ol'the Mooring and Offtake systems is simplified & impact on the Barge layout & construction reduced.

À There are only minor loads transferred into the Barge structure.

À There are no major structures to be mounted on and integrated with the Barge structure.

À On going maintenance of the Mooring and Of Stake systems have little impact on the Production Barge Operation.

À Integration of the required power, control, alarm and monitoring systems is shnplified.

À No new Gas Hazardous Zones are created and those around the Offlake system are minimized.

À There is no requirement for the Offtake LNG Vessel to lie alongside the Production Barge.

À Tandem Berthing is Sai'er, Simpler and a more Standard operation than Alongside Berthing against a moored Barge.

À The Offtake LNG vessel has no need to closely approach the Barge. The possibility of contact is much reduced.

À The Jetty is virtually an independent Project. Once the Barge / Jetty interfaces are defined, changes to the details of the Jetty or Barge have no impact on the design, construction, installation and commissioning schedules.

À Jetty installation and commissioning is very mpid and has virtually no impact on the Barge progress.

À It is possible for the Jetty to be easily removed from site to Shore if major repairs or maintenance become necessary.

À The construction ofthe Jetty is very simple and can be undertaken by most small to medium sized Shipyards.

À It is possible for a complete Back-up Jetty unit to be held in reserve and introduced very rapidly so that Production and Offlake operations are not disrupted in the change over.

À In extreme emergency it is possible to incorporate Barge / Jetty detachment as part of the ESD function.

Advantages from the use of the Jetty, TO THE LNG Omake Vessel: À Tandem Berthing is less Labour intensive and exposes fewer persons to Mooring Hazards.

À Tandem Berthing can be performed in more extreme sea & wind states, with-out Tugs or Line handling vessels.

À There is much less likelihood of Ship 1\ Barge impact due lo the presence of the Floating Jelly structure.

À ESD and un-berthing operations in an emergency are achieved more quickly, more simply and more safely.

À Offlake Vessel & Barge Hazardous Areas i Safety Zones remain independent. There is minimal operational conflict.

À No modifications required to standard cargo tank and accommodation ventilation systems on 1,NG O.V.

À I lazards to LNG O.V from Barge Production Facilities are much reduced.

À Shrapnel & Blast pressure Hazard to Moss type Cargo Tanks on LNG O.V (from Barge explosion) is much reduced.

À Modif cations to LNG O.V are limited to the addition of Bow Mooring equipment located in non hazardous area.

À There are few changes required to Standard Cargo Loading and Discharge connection and Monitoring systems.

À Ships data transfer, monitoring, control, alarm and EST) functions easily integrated with the Barge D.C.S system.

À The Tandem Mooring operation is identical with that of conventional Oil FPSO (Training & Experience available).

À The presence of the Jetty has virtually no impact on the Berthing operation performed by LNG O.V.

À Pitching of Bow of LNG O.V at extreme of weather window has only minimal impact on LNG transfer connections at the Mid-ships manifold. Weather window is improved if Jetty is used rather than a Bow Loading system.

À No Mooring loads are transferred to the LNG O.V's Hull via structures (or near fluids) at cryogenic temperatures.

À Berthing of LNG O.V involves the use of Thruster assisted Joystick control with electronic safety features, rather than fully sophisticated Dynamic Positioning system.

À When Moored to the Production Barge, there is no requirement for the use of D.P system.

À While moored, propulsion loss or black-our on LNG O.V ol'fers no immediate hazard to Production Barge or Ship.

À Machinery system on LNG O.V can be simpler, Cost and Complexity of LNG O.V is much reduced.

7/3/ 3 Page No5 of 5

Conclusions

The l.NC, 1,oading Jerry becomes an irdependeii Project so that its build site can be completely separate from the Barge construction site and Design or Construction delays on Jetty have no impact on Barge or Shipping Schedule (and vice versa). Tow out and I look up of Jetty and LNG transfer system is much reduced in both time and cost to Project and the Jetty can be completely pre-commissioned prior to ISt connection at Production site. The Jetty can be removed and towed to satiety ir; ar ticipation of "Hurricane" or Storm conditions and Jetty modifications, repairs and maintenance can easily be achieved either on or offthe Production site.

The Jetty design is easily scaled up or down to suit any sized Project or LNG Vessel Capacity. It can be built for (Lightweight) Benign conditions or in (Robust) Exposed location versions. It is also suitable for the transfer of Crude Oils and any other fluid Cargo and could be used for the transfer of any Pumped Slurry-on any scale. It can be re- designed to allow the handling of free flowing granular solids, making it suitable for the loading of e.g. Gas Hydrates or Manganese Nodules from a deep sea dredger.

As presented here, the Jetty can handle LNG and Gas flowing in either direction i.e. onto as well as off the Barge. This makes the Jetty suitable for use in LNG import and Re-Gasification Projects e.g. for the import of LNG into an Industrialised Country such as the USA. 11 is suggested that 1,NG from the 1,NG Ol't:take Vessel arriving from the distant Producing Field is transferred (using the Jetty) to a Turret moored LNG reception and storage vessel which itself is equipped to re-gasify the LNG and pass the warm gas into the adjacent shore Gas distribution system via a short sub- sea pipeline. This use is particularly attractive as it allows the simple introduction of new LNG reception facilities in locations where shore reception and storage of LNG and the location of re-gasitication facilities are difficult or prohibitively expensive. The Moored re- gasification Vessel can be located well offshore in a safe and preferably sheltered position allowing the shipping operation to take place in many places where shore facilities would otherwise be banned. This type of application will allow LNG import to take place quickly and cheaply in most parts ofthe World, especially where the Construction of the necessary shore tanks and other facilities would be difficult (or slow) to introduce.

Claims (1)

1. Semi Submersible Floating Jetty for the Safe Transfer of LNG between a

   Floating Production, or Storage Barge, to a Transportation Vessel at an Exposed Offshore Location.

   Claims: Claim 1) The Jetty is shaped like a capital "L" with the height (corresponding to the length of jetty-see Fig 9) being roughly / the length of the Offlake vessel + 60 meters. The base is roughly equal to the Beam of the Offlake vessel plus 10 meters.

   The Jetty Ball mooring attachment is located on a forward extension to the jetty fitted half way along the base of the "L" and locates with the hydraulically operated Claw type coupling which is mounted on the stern of the Barge at the C/L and at an elevation of at least 10 meters above the loaded waterline. When connected to the Barge, the coupling allows no relative motion in a Longitudinal, Transverse or Vertical direction but offers no restriction to Roil motions. The dimensions and geometry of the Barge to Offlake vessel Tandem mooring and the dimensions and geometry of the Jetty are such as to ensure that longitudinal movement between the Offlake vessel and Jetty Pontoon are minimised.

   Claim 2) concerns the methods of achieving the "Clamping" Forces between the Jetty Pontoon and the Offlake Vessel and is the main claim but is dependent upon the Jetty shape and the use of the forward attachment method described in Claim 1 above. The "L" shaped Jetty is fitted with the narrow machinery and ballast pontoon as described in the main "Description" and located at the top of the "L" which corresponds with the mid-ship loading position of the LNG Offtake vessel. Claim 2 covers the form of the clamping attachment made between the Jetty and the Offtake vessel. These clamping forces will be applied by the following means: a) Initially the Jetty and Ship are forced together by counter running the Tunnel Thrusters fitted to both vessels.

   b) When the Rubber Fenders mounted on the Jetty are in contact with the Offtake vessel and slightly compressed, a low pressure "Plenum Chamber" is formed from which the Jetty Thrusters extract the water and sea water pressure on the two hulls will hold the vessels together.

   c) The Moulded Rubber Vacuum attachment Fenders are pressed firmly into contact with the Offlake vessel Hull and sea water pumps in the Jetty Pontoon evacuate the Attachment units.

   d) Conventional 'Soft Rope Mooring Tails" stored on the Jetty Pontoon are secured to the mooring bits on the Offlake vessel and tensioned by hydraulic Rams located on the Jetty Pontoon.

   e) In Benign weather conditions the Main Thrusters on both the Jetty and Offlake Vessel are shut down following step d).

   fl "Shearing" of the Vacuum attachment is prevented by the application of slight positive buoyancy to the Jetty Pontoon which forces the lower "Location Arms (Fig 9, Item 6) with their additional Vacuum attachment Fenders into contact with the Offlake vessel bottom which also self aligns the LNG Omake loading Arms mounted on the Jetty Pontoon.

   9) The LNG Vessel and Jetty Pontoon remain clamped securely together and move together with virtually no relative motion so that the LNG Transfer Arms can remain connected to the loading manifold throughout the transhipment operation.

   Malcolm Newell pimp,-__ 7/03/2003