Loading and Offloading of LNG in open seas.

By Wim van Wijngaarden, Jean-Pierre Queau, and Luc Pescio, Single Buoy Moorings, Monaco

Introduction

During the early 80’s the attractiveness of placing a natural gas liquefaction plant on a barge, moored ator close to an offshore field was identified. Such barges can either be part of an oil development in whichit liquefies associated gas or a stand-alone field development of remote green gas fields. In the associatedgas case, the Floating LNG system makes remote oil developments feasible on environmental grounds byturning the problem of associated gas disposal into a moneymaking opportunity. In the green gas fieldcase, the Floating LNG is an economical means to develop remote offshore gas fields.

In the wake of the already successful Floating Production Storage and Offloading Systems (FPSO’s),which have proven their suitability to develop both small and large oil fields, the industry is nowconsidering LNG FPSO’s. As with the known FPSO’s, the key to attractiveness of Floating LNG lies inthe integration of well control, processing, storage and offloading on a barge positioned in the vicinity ofthe field. It replaces the offshore platform and pipeline, as well as the jetty and the onshore storage andprocess plant. This provides a significant opportunity to reduce cost and project time. The topsides andthe barge can be built and commissioned in a yard under controlled conditions using highly experiencedstaff. During operations, the barge operates offshore unhindered by local conditions. Additionaladvantages are that after depletion of a reservoir, the barge may be moved to a different field, and that atthe end of its service life, the barge can be economically abandoned.

The idea of Floating LNG is not new. However, recently the industry has returned its focus to LNGFPSO’s. The main reasons for this renewed interest in Floating LNG are:- The extensive experience and the confidence gained with oil FPSO’s- The technology advances made over the past twenty years, and- Liquefied Petroleum Gas (LPG) FPSO’s with relatively complex topsides, storage and offloading

at -45oC.

The LNG technology itself has become mature, and process plants and carriers have proven to be capableto operate in excess of their initial 20 years design life while achieving an excellent safety record. Ingeneral, extensive operational experience has been gained on both FPSO operations and all aspects ofLNG production, shipping and marketing. Today, all major oil and gas production companies arestudying LNG FPSO’s.

Technically, Floating LNG is similar to other FPSO’s. It consists of a basic hull with accommodation, amooring system, storage tanks, LNG processing, and offloading facilities. After pre-conditioning, theincoming gas is liquefied and the LNG is stored at atmospheric pressure in thermally insulated tanksonboard the FPSO. When sufficient LNG has accumulated, it is offloaded to a LNG carrier and shipped tothe customer. The produced condensates are stabilized and stored onboard to await offloading usingstandard offloading methods and tankers. The main technical challenges are the marinisation of theprocess plant, the offloading of LNG at cryogenic temperatures (–162oC), and the topsides integration andinstallation.

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Mooring and Offloading in Open Sea – A Historical Perspective

Single Buoy Moorings (SBM), a member of the IHC Caland Group, has been in the business of the design,supply and operation of Single Point Mooring (SPM) systems and FPSO systems for over 40 years. SBM hasalso been heavily involved in the design, engineering and supply of key components (swivels) for the transferof high-pressure gas for gas injection, gas lift and for transport to/from offshore platforms. One of the firstproducts supplied was the mooring and gas transfer system on the ARDJUNA SAKTI concrete barge forLPG storage in 1975. In addition to SBM’s involvement in many other projects, SBM owns and operates aLPG Floating Storage and Offloading System (FSO) offshore Congo. In total, SBM owns 15 operationalFSO’s and FPSO’s and is deeply involved in the offloading to shuttle tankers on the units SBM operates.

Some of the early FSO’s or FPSO’s used:1) A rigid arm between a standard type Catenary Anchor Leg Mooring System with flexible lines or with

a Flexible Pipe type of Fluid transfer system2) A tension leg type of mooring system so that rigid piping with only short flexible pipe jumpers couldbe used.3) Soft Yoke Mooring Tower with rigid piping, in shallow waters only.

Most of the recent FSO’s and FPSO’s have been fitted with turret mooring systems. As high-pressureflexible pipes are now well proven in the industry, a move towards turret type mooring systems, whichalso provides anchor leg redundancy, has generally occurred except for very shallow waters, whichusually precludes the use of catenary mooring systems.

The turret mooring systems are composed of a turret column rotatably held by the internal or externalvessel’s structure via a roller bearing arrangement. The vessel bound components can thereforeweathervane around the turret, which is fixed in orientation with respect to the seabed. The FPSO adopts,at any time, the direction of least resistance against waves, wind and current. The turret column is securedto the seabed by catenary anchor legs. Fluid swivel joints and stacked toroidal swivels allow transfer offluids across the rotating interface while the FPSO is weathervaning.

The catenary anchor legs must provide sufficient restoring capability to maintain the vessel in the fieldwithin a limited radius from its rest position in order to ensure the feasibility of the underwater fluidtransfer system.

The response of the moored vessel to the action of waves, wind and current can be split into threecomponents:1) A mean offset due to wind, current and mean wave drift force2) Slow varying oscillations around the mean offset induced by the second order wave drift forces withperiods associated with the wave groups occurring in irregular waves. These second order forces arerelatively small but their frequencies correspond to the range of natural frequencies of horizontal motionsof the moored vessel3) Wave frequency component caused by the first order wave-induced vessel motions with periods in therange of 5 to 20 seconds. The resulting velocities and accelerations give rise to significant dynamic forcesin the mooring system.

Turret concepts come in a number of different design layouts but all use a number of catenary or in thedeeper waters (semi) taught mooring lines. The family of turrets includes those designed to permanentlymoor the vessel in 100-year storm conditions, such as:1) Simple external turrets for West African conditions,2) Clamped type bow turrets, etc

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3) Internal turrets for the more harsh environments offshore West of Shetlands and in the Central NorthSea,

4) Internal turrets for a large number of fluid lines. The ESPADARTE FPSO has a total capacity of 46risers in the turret.

For the shallow waters, Soft Yoke mooring systems arefrequently used due to the fact that the catenary anchorlegs cannot provide sufficient restoring capability. TheseSoft Yokes can either be connected directly to a jacketstructure fixed to the seabed, as in the case of SBM’sFPSO VI offshore Nigeria (Figure 1) and the recentlyShell-ordered tower Soft Yoke for the EA field offshoreNigeria, or to a floating buoy system. Several of thesemooring systems are in operation. The Soft Yoke systemconsists of a rigid mooring yoke, connected at the jacketend by a yaw, roll and pitch articulation, and at the vesselend by two articulated mooring legs to the mooringstructure mounted on the bow or stern of the vessel.

The mooring yoke is a structure with the shape of an “A”, the base of which consist of two cylindricaltanks filled with ballast and suspended from two mooring legs hanging verticallyfrom the vessel when thesystem is in equilibrium. When the vessel moves out of its equilibrium position due to wind, wave orcurrent action, the two vessel mooring legs lift the weights, thus creating a restoring force.

The deployment of FPSO’s in certain tropical storm areas or in fields where winter ice may form hasrequired the development of disconnectable mooring systems. These systems will enable the FPSO vesselto sail away from the site without external assistance leaving part of the mooring system behind. Severalof these FPSO systems are now in operation. Disconnectable systems have the added advantage that theFPSO can be sailed to a shipyard for upgrade or routine inspection.

For the mooring of Floating LNG vessels, a turret-type mooring will be the most likely option, given thesize and mass of the FLNG vessel. However, the mooring of the LNG carriers to the FLNG vessel or toanother loading or offloading point in open sea is a new phenomenon, yet to be proven by the industry.SBM has taken up this challenge, and has started the development of several products to support thisoperation in relatively harsh environments. The SBM mooring and LNG offloading system is based on atandem mooring principle using a Soft Yoke to accommodate the relative motions between the FLNG andcarrier or between the carrier and a fixed SPM.

Open Sea LNG Offloading – Challenge

The main challenge in open sea LNG offloading, either between a fixed structure and a vessel, or betweentwo floating vessels, is the fluid transfer method, which is subject to significant relative motions.

Whereas in benign conditions the LNG offloading method may be a “marinised” version of the well-known jetty offloading arms in a side-by-side mooring arrangement, this method is not suitable inweather conditions where the resulting sea states cause excessive relative motions between the twofacilities.Even relatively moderate sea states (Hs ~ 1.5 m) may be prohibitive for side-by-side offloading, if thenatural roll periods of the storage barge and the LNG carrier differ, which is practically always the case.Both vessels will react differently to wind, waves and current, due to differences in wind profile, hull-

Figure 1 – Soft Yoke tanker mooring

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shape and mass. The carrier motions will thus differ from those of the storage barge, thus regularlycausing out of phase motions of both vessels.Once the carrier is moored to the storage barge, the relative motions will be dampened. However, theberthing of the LNG carrier will be done in the same way as for oil tankers and would require tugassistance.

SBM has a long-standing experience with open sea side-by-side offloading between crude oil tankers(Aquila) and LPG tankers (Nkossa II). Based on this experience SBM has adopted a conservative viewwith respect to the feasibility of side-by-side offloading. Although in those operations there is nosignificant downtime to “bad” weather, this is mainly due to the fact that the weather windows are largeenough to delay offloading when necessary.Also, a few spread-moored FSO’s in West Africa, which have used side-by-side offloading in the past incombination with low offloading frequencies, are being converted to a CALM offloading system now thatthe offloading frequencies are being increased to higher production rates. This is another indication thatside-by-side offloading in open sea, even in benign climates, may result in excessive downtime.

When considering offshore LNG production and offloading, one must realise that the offloadingfrequencies will be high, ranging typically from 1 to even 4 offloads per week. In such operatingconditions, “bad” weather downtime cannot be accepted, as this would quickly result in the need to shutdown the LNG liquefaction plant: a costly event.

Therefore SBM believes that unless full confidence for side-by-side offloading has been obtained on theberthing and relative motion behaviour between the two vessels, based on extensive hydrodynamicstudies, model tests and berthing simulations, which demonstrates that “bad” weather downtime fallswithin acceptable operating criteria, tandem offloading is the preferred and safer solution.

In a tandem offloading arrangement, the relative motions between the vessels are much less critical.Furthermore, the berthing and mooring is much safer. Tug assistance may be required during very calmweather, when wind, wave, and the current forces are such that the LNG carrier drifts towards theoffloading vessel or fixed tower. Also during LNG offloading the relatively large distance between thevessels provides a safer operating condition.

When deciding for the tandem mooring arrangement, there is another challenge: how to perform the fluidtransfer. SBM has opted for a rigid structural type mooring arrangement, in combination with hard-pipedfluid transfer lines, with swivels to allow for the relative motions.

Soft Yoke Mooring and Offloading (SYMO) Concept

Prior to the development of the SYMO Arm Concept, SBM has developed a Pipe-in-Pipe LNG loading arm,of which the concept was presented at Gastech 2000. The design was started during 1999 and the first phaseengineering study was completed early 2001, ending with an “Approval in Principle” from DNV on theconcept. The starting point of the study was based on earlier work performed on cryogenic swivel designsand tests performed during the mid 1980’s. During the first phase study the Pipe-in-Pipe concept feasibilitywas proven.

To obtain a high availability for LNG offloading in harsh environment areas, SBM designed the Pipe-in-Pipe LNG Loading Arm (Figure 2) for tandem offloading up to 5 - 5.5 m significant wave height. Thearm is supported from a cantilevered rotating structure on the back of a FPSO and is based on theSBMwell known Soft Yoke principle. The shuttle vessel can be passively moored using the gravity restorationof the weights located in the horizontal part of the loading arm. In addition to its mooring function thegravity restoring system also controls the overrun (incursion) case and thus avoids a possible collision

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between the LNG carrier bow and the stern of theLNG FPSO when connected. The SYMO could alsobe used in combination with a DP (DynamicPositioning) assisted LNG carrier using both the LNGarm mooring function and the shuttle thruster system.When connected the arm provides for a continuousflow path of LNG to and vapour from LNG FPSOvessel and the shuttle carrier.

In 2001, several companies stimulated SBM todevelop a tandem LNG offloading system, based onthe same Soft Yoke principle, but with multiple fluid-transfer paths. This concept is called the SYMO Armconcept. Recently the engineering, including a model test, for the first phase of the concept was finalized.

The SYMO concept differs from the Pipe-in-Pipe concept in two ways:- The SYMO allows simultaneous transfer of different fluids (gas or liquid).- In the Pipe-in-Pipe concept, the mooring function and the fluid transfer function are integrated,

whereas in the SYMO Arm concept both functions are separate. This follows from the fact thatthere are multiple fluid transfer lines.

Typical SYMO Applications

The SYMO Arm consists of a Soft Yoke mooring arm, which provides the connection between an FPSOor fixed tower and the LNG carrier bow. In case of the FPSO, a fixed support structure is placed at itsstern from which the yoke is hung down. In case of a fixed tower, the yoke is hung down from arevolving support boom structure required for freeweathervaning of the LNG carrier around the tower. Inthis case the concept is called the Soft Yoke Tower Mooring and Offloading (SYTMO) concept.

The Soft Yoke (Tower) Mooring and Offloading Arm can be used for a variety of applications, such as:- LNG unloading to a permanently moored Floating Storage and Re-gasification Unit (FSRU).- LNG unloading to a fixed jacket supported tower, with a topsides regasification plant. The LNG

is regasified on the tower, and gas is sent to a pipeline distribution network.- LNG unloading to a fixed jacket supported tower, but without a regasification plant on the tower.

Here the LNG is sent to a shore-based import terminal via asubsea cryogenic pipeline (Figure 3).- LNG loading from a fixed jacket supported tower. The LNG is supplied to the tower via a

cryogenic subsea pipeline from the shore-based export terminal.- LNG offloading from a LNG FPSO (or FLNG) to a LNG carrier. In this case the tower is

replaced by a fixed structure on the FLNG (Figure 4).

Figure 2 – Pipe-in-pipe LNG offloading system

Figure 3 – Fixed SPM tower LNG(off)loading

Figure 4 – FLNG (off)loading

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The SY(T)MO can be designed for operation is relatively harsh environments, such as frequentlyoccurring in the Atlantic Basin near the USA east coast, the Gulf of Mexico, or in the Pacific near theUSA west coast.

The SY(T)MO is also designed to achieve high LNG carrier offloading availability in sea states ofapprox. 3.0 m (Hs), or when already connected, to remain moored in sea states up to 5.5 m significant. i.e.similar to the Pipe-in-Pipe system.

Loading Arm Design

Both the Soft Yoke Mooring and Offloading (SYMO) and the Soft Yoke Tower Mooring and Offloading(SYTMO) were detailed but for clarity only the SYTMO has been further described.

Description

The SYTMO yoke consists of 2 vertical members, which serve as flexible hangers for the horizontal A-frame of the yoke. The vertical members can rotate freely around their articulation joints at the top andbottom. This allows the A-frame to swing relatively freely around its equilibrium position, thus allowingfor the surge, sway and yaw motions of the LNG carrier. Its natural position (when hanging in the airdisconnected) is at an angle ofaround 45° with the horizontal plane, with the connector-end of the A-frame upward. This is caused by the counterweight at the opposite side of the 2 legs of the A-frame.Figure 5 shows an outline of the yoke configuration.

At the connector-end of the horizontal A-frame, a roller bearing allowsthe connector to rotate around the horizontal longitudinal axis, thusallowing relative roll of the LNG carrier compared to the yoke itself.Finally, an articulation joint at the connector-end of the yoke alsoallows the LNG carrier to pitch.

Because of the softness of the connection, via the various articulationson the yoke, the arms of the yoke not only compensate for roll, yaw orpitch, but can shorten or lengthen the distance between the two mainbodies, move up or down, or swing from left to right. Thesedisplacements can therefore also compensate for the heave, sway andsurge motions of the LNG carrier.

The structural design of the yoke, such as the main dimensions and theweight of the counterweight, is such that the maximum expected

mooring loads under the specified design conditions are safely absorbed, while keeping the LNG carrierin a reasonable operating envelope around the equilibrium position.

The yoke structure supports a number of LNG (or other) fluidtransfer lines (Figure 6). These fluid transfer lines convey thefluids from one facility to the other (in either direction). A typicalLNG offloading facility will consist of 4 identical loading lines, ofwhich two will be used for LNG transfer, one for vapour return,and the fourth one serves as a spare line, either for LNG or forvapour return. A typical size is 16”, which allows a LNGoffloading rate of 10,000 m3/hr.

Figure 5 – Soft Yokegeometry

Figure 6 – Fluid transfer system

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The fluid transfer lines are hard-piped steel lines, each fitted with 8 cryogenic swivels including the yawswivel on the shuttle carrier but excluding the cryogenic toroidal swivel located on the tower for theweathervaning function. These swivels allow the piping to follow all required yoke movements duringconnection, in the connected state, after disconnection, and in stored position. The swivel alignment issuch that the rotation points align with those of the yoke articulation joints. Thus no mooring loads aretransferred from the yoke to the piping, except from some minor mass inertia forces caused by thedynamic behaviour of the yoke.

At the connector, quick connect/disconnect coupling (QC/DC) valves are provided, which are closed priorto breaking the connection. Separate powered emergency release couplings (PERC’s), which are normallyinstalled on jetty loading arms, are not installed on this system, as they are not required: instantaneousdrifting of the carrier will not occur due to the robust mooring system.

The connector is a special design (Figure 7), which combines thestructural mooring function with the fluid transfer function. Theconnection principle is similar to that of conventional LNG offloadingarms (e.g. spring-loaded clamps or hydraulically operated collets), withthe difference being that the actual mechanical connection is made onthe structural mooring part, instead of on the fluid transfer lines.

The connector is designed to enable connecting in the specified designsea states, without causing damage to the fluid piping or ship deckequipment. The yoke-end as well as the carrier-end of the connector isfitted with a ring fender. This fender has two purposes: to protectagainst damage during the connection procedure, and to enable fixingof the yoke-part of the connector to the carrier-part of the connectorbefore the actual connection is made. Hydraulic jacks on the LNGcarrier are used to fix the yoke-part of the connector.

The yoke is provided with a winched mooring line, which runs along pulleys on the yoke. This line isused during the connection procedure to lower the A-frame of the yoke and bring it in position just abovethe LNG-part of the connector for fixing by the hydraulic jacks.

Operation

Mooring

The approaching LNG carrier picks up the messenger line with the help of a small workboat. Themessenger line is used to pick up a temporary mooring hawser. The hawser is used to keep the LNGcarrierduring berthing at a rather well defined distance from the fixed tower. Tug assistance might berequired to keep the LNG carrier at a “fixed” distance from the tower during this operation but this isdepended on local environmental conditions. However, as soon as connected the SYTMO avoids anycollision between the LNG carrier and the tower due to the gravity restoring function in the Soft Yoke.Therefore, as soon as the LNG carrier is connected the SYTMO assures a safe distance between towerand the LNG carrier.

After the hawser is connected the revolving yoke support boom on the fixed tower is rotated above theLNG carrier bow, by means of a driving mechanism. The mooring line is lowered from the winch on thetower via the yoke, and is connected to the mooring line on the carrier. Then the winch is put on its brake,and the winch on the LNG carrier is used to pull taut the line, and to bring down the A-frame of the yoke.

Figure 7 - Connector

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Once the yoke-part of the connector is approx. 0.5 m above the LNG carrier-part, hydraulic jacks withclamps grab the ring fender, and fix the upper part of the connector in position.Then the yoke-part of the connector is fixed relative to the LNG carrier-part of the connector, and thefluid transfer lines are lined up for connection, by rotation of the LNG carrier-part of the connector intothe right position. The LNG carrier-part of the connector is then pushed upward against the yoke-partusing the hydraulic jacking system, and the spring-loaded clamps connect both ends. Finally the fluidtransfer line connection is made by springs pushing the lower connector part upward against the upperpart. The ship-part of the connector is kept slightly retracted, to avoid damaging the pipe connectorsduring the connection procedure. The hawser is disconnected or slackened once the connection iscompleted.

The system is then ready for LNG transfer between the two facilities after pipe integrity testing has beencarried out.

Loading or Offloading

The actual (off)loading takes place in the same manner as applied for onshore jetty offloading arms. Thefluid transfer piping design is such that no low pockets are present. This allows quick drainage of thepiping once the offloading is complete. Shut-off valves at either end of the loading system are provided,allowing a complete shut-in of the loading arms. At the top of the loading arms, a purge connection isprovided to assist drainage, and to perform purging if required.

Disconnecting

Once the (off)loading is complete, the lines are drained from the top of the loading arm structure, thequick connect/disconnect coupling valves are closed, and the connector clamps opened. The A-frame ofthe yoke then automatically moves to its equilibrium free-hanging position. The LNG carrier can directlysail away in reverse thrust.

In case of a required emergence disconnection (typical ESD-2 situation), first the quickconnect/disconnect valves are closed before the mooring clamps can be released. In such a situation thedisconnection will be made with the loading arm fluid lines still filled with LNG. The LNG carrier canimmediately sail away.

The need for such a rapid disconnection is not as critical as for jetty offloading systems, since themooring is much more secure with the yoke structure being designed to withstand the sea state conditionsunder which it can be offloaded. Even a sudden deterioration of the weather leaves ample time to(partially) complete an offload allowing the carrier to safely disconnect and sail away. Offloading mayeven be terminated before the LNG carrier is entirely empty, provided that only full or empty tanksremain before actually disconnecting (to avoid a partial loading situation during sailing). With this type ofmooring system, sufficient time is available to safely complete such operations.

Technology Development Status

Soft Yoke Mooring

The mooring behaviour of the SYMO has been tested in model basin test in Marin earlier this year. Thefollowing typical environmental conditions were used in those tests, amongst others:

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Table 1: Model Basin Test ParametersTest No. Significant

wave height(Hs) m

Period(Ts) s

Windm/s

Currentm/s

Directionality

1 5.5+ Swell

14 15 0.3 Wind, waves @ 270°Current @ 180°

The tests were based on tandem offloading from a large body FLNG vessel to a standard LNG carrier of135,000 m3 in relatively deep water (> 100 m). These conditions are more severe than loading oroffloading from a fixed tower to a carrier, since in the test both floating bodies have different motioncharacteristics, which cause them to move out of phase regularly. These tests demonstrated that theSYMO could actually operate in the conditions stated above.

Connector

The connector is a special design developed by SBM. Real-time simulations have been performed tosimulate the connection and disconnection procedures, including a correct representation of distributedmass, suitable damping in articulations etc. The main results from these calculations are the dynamicbehaviour and loads. Model testing is planned to confirm these analytical simulations.

Swivels

Although standard industry-type LNG pipe swivels may be used for the SY(T)MO application, SBM isdeveloping their own cryogenic pipe swivel. The size of this swivel will be 16”. The swivel fabricationhas started, and cryogenic dynamic testing will start in due time. The swivel will be tested for a period ofapprox. 6 months, thus simulating 5-year operational life.

In addition, SBM will also fabricate a cryogenic toroidal swivel of 16” located on the SYTMO towerstructure. The design of this swivel has started, and fabrication and testing will start early 2003. Thisswivel will have a 16” flow path.

Conclusion

The SY(T)MO Arm Concept currently developed by SBM for LNG Tandem Offloading between twovessels or for LNG Import/Export through a SPM Terminal, is based on the integration of two wellknown technologies – the Soft Yoke principle for the mooring, and the hard-piped loading arms withswivels for the fluid transfer.

Thanks to the Soft-Yoke mooring principle, the LNG will be offloaded using an extremely safe operation:- No risk of unexpected breakaway from the LNG carrier with all associated risks of damage to the

loading arms or ship manifold.- High offloading availability even in relatively harsh environmental conditions. LNG offloading is

feasible in sea states with a significant wave height up to 5.5 m. The connection threshold is asignificant wave height of 3 m.

- Limited DP or tug assistance. DP assistance may be required to help aligning the LNG carrierwith the LNG FPSO during the berthing operation. Tug assistance may be required only duringvery calm conditions, when the environment does not give a well-defined load direction to thecarrier.

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Based on the above, the SY(T)MO Arm Concept will therefore be the enabling technology to allowoffshore LNG transfer operations, such as for:- Floating offshore liquefaction vessels (FLNG)- Floating storage and regasification units (FSRU)- Single Point Mooring (SPM) export/import terminals, combined with a subsea cryogenic pipeline

from/to shore.