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CHIKSAN® LNG MARINE LOADING ARMS ENHANCED FOR APPLICATION IN EXPOSED AREAS

BRAS DE CHARGEMENT MARINE GNL CHIKSAN® POUR APPLICATION SUR SITES EXPOSES

C.A.C. van der Valk J.J.M. Vreeburg

Shell Global Solutions International B.V.

R. Le Devehat J. C. Cartereau

FMC Technologies

ABSTRACT

Shell Global Solutions Int. and FMC Technologies jointly developed an offloading system using a Chiksan® Marine Loading Arm for loading of LNG in exposed areas. A 16-inch diameter Double Counterweighted Loading Arm (‘DCMA-S’ type) has been dynamically analyzed and tested utilizing a purpose-built dynamic analysis tool and test bench.

The project, which started in 1999, concentrated on the development of a semi-automated connecting mechanism to assist the operator in making a controlled connection between the Chiksan® hydraulic Quick Connect/Disconnect Coupler (QC/DC) and the Manifold Flange on the LNG Carrier (LNGC) under highly dynamic conditions.

The connecting operation consists of crawling the QC/DC to the receiving flange on the LNGC using a constant-tension guide-cable between the base-riser of the Loading Arm and the Manifold on the LNGC. The closer the two flanges come together, the smaller the relative motions.

Connecting operation tests have been successfully completed in 2002 for motions, which became available from scale 1:50 basin model tests on weathervaning barges with alongside moored LNGC.

The Marine Loading Arm enhanced with the newly developed connecting mechanism showed that reliable and safe connections can be made for relative motions between the mating flanges in the range of 4m. For comparison, the conventional Marine Loading Arm will not be connected when the relative motion is more than 0.5m depending on motion frequencies. This enhanced Marine Loading Arm offers the captain of the LNGC a much wider mooring envelope.

The new system therefore opens opportunities not only for Floating LNG Export and Re-gasification facilities, but also for more exposed onshore LNG plants and terminals, thus reducing the need for breakwaters and allowing consideration of different sites.

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RESUME

Shell Global Solutions Int. et FMC Technologies ont développé conjointement un Bras de Chargement Marine Chiksan® pour le transfert de GNL sur sites exposés. Un bras de diamètre 16 pouces à double contrepoids (type ‘DCMA-S’) a fait l’objet d’études et d’essais à l’aide d’outils d’analyse dynamique et d’un banc de test spécifiquement conçus pour ce projet.

Le projet, initié en 1999, a porté sur le développement d’un système semi-automatique pour assister la connexion et permettre un accouplement en douceur entre le coupleur hydraulique Chiksan® (QC/DC) et les brides de Manifolds des méthaniers, en présence d’importants mouvements générés par les vagues.

La procédure de connexion consiste à amener le coupleur hydraulique (QC/DC) jusqu’à la bride du méthanier en utilisant un câble de guidage à tension constante entre l’embase du bras de chargement et le Manifold du méthanier. Plus les deux brides se rapprochent, plus l’amplitude des mouvements relatifs diminue.

En 2002, les tests de connexion ont été réalisés avec succès dans des conditions dynamiques réelle, issues d’essais en bassin à l’échelle 1:50 sur des méthaniers amarrés à des barges flottantes en weathervaning, soumis à des conditions environnementales réalistes.

Le bras de chargement marine équipé de ce nouveau système a montré que des connexions fiables peuvent être effectuées en toute sécurité avec des mouvements relatifs entre les brides et les bras de chargement de l’ordre de 4 m. A titre de comparaison, les bras de chargement marine conventionnels ne permettent pas la connexion avec des mouvements de plus de 0.5m suivant la fréquence des mouvements. Ces nouveaux bras de chargement marine offrent aux capitaines de méthaniers des enveloppes d’amarrage bien plus larges.

Ce nouveau dispositif ouvre donc des opportunités non seulement pour des unités GNL flottantes de production ou de re-gazéification, mais aussi pour des terminaux GNL terrestres en zones exposées, évitant ainsi l’utilisation de brises lames onéreux et permettant de considérer de nouveaux sites.

INTRODUCTION Liquefaction of natural gas and re-gasification of LNG are more and more considered

as activities that could be done in an offshore environment. In the case of an LNG production facility, this may be an LNG Floating Production Storage and Offloading system (LNG FPSO, or FLNG), and in the case of an LNG receiving terminal this may be an LNG Floating Storage and Re-gasification Unit (LNG FSRU).

For the application of FLNG (export terminal) or FSRU (receiving terminal), LNG needs to be transferred to and from the Shuttle Carriers while both vessels are moving. There are basically two transfer options: side-by-side, where the shuttle tanker is moored alongside the floating barge, and stern-to-bow (tandem), where the shuttle tanker is moored at the stern.

The desired goal is to transfer LNG using ‘standard’ Shuttle Carriers and ‘standard’ offloading technologies, and only move away from that objective if necessary to meet unique project constraints/limitations. In the absence of field-proven stern-to-bow

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offloading systems for LNG, the preferred offloading method for Shell Global Solutions is side-by-side using ‘standard’ Chiksan® Marine Loading Arms with constant motion swivel joints. However, even though there is considerable experience with these Loading Arms on jetty terminals, a fair amount of development work is needed to deal with the dynamic conditions in offshore exposed areas.

In order to obtain a better insight into the design specifications for Loading Arms installed on weathervaning (turret-moored) barges with an alongside moored Shuttle Carrier comprehensive motion-response analyses have been carried out. At the request of Shell Global Solutions, MARIN (Wageningen, The Netherlands) performed both numerical analyses and scale 1:50 basin model tests for various field-specific metocean conditions.

Simultaneously, Shell Global Solutions and FMC Technology jointly initiated a development program with the objective to enhance a ‘standard’ Chiksan® Marine Loading Arm for application at offshore exposed locations.

The main requirements for the loading arms are:

• The structure shall be able to withstand inertia forces, since it will be subjected to motions.

• Being-connected, the constant motion swivel joints and structural bearings shall withstand considerable rotations.

• Connecting operations shall be safe and reliable under relative movements between the hydraulic Quick Connect/Disconnect Coupler (QC/DC) and the Manifold Flange on the Shuttle Carrier.

• The hydraulic system shall be able to allow for high velocities and accelerations in the system.

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The first issue can relatively easily be dealt with in the design, while the constant motion swivel joints and structural bearings have already field proven designs at relatively exposed locations, notably Bruneï for ten years and Oman for five years, Oman having the larger motions. Therefore, the development has focused on solving the latter two issues, which are more complicated. To avoid damage to flanges or other components a more automated mating of the QC/DC and the Manifold Flange of the LNGC is required.

The project concentrated on the development of a semi-automated connecting mechanism to assist the operator in making a smooth coupling between the QC/DC and the Manifold Flange of the LNGC. The connecting operation consists of crawling the QC/DC to the Manifold Flange using a constant-tension guide-cable stretched between the base-riser of the Loading Arm and the Manifold. The closer the two flanges come together, the smaller the relative motions.

The development program has been completed with extensive testing of the operability of a 16-inch diameter Double Counterweighted Loading Arm (‘DCMA-S’ type) equipped with the newly developed connecting mechanism. Important objectives of the tests were to study the amount of oscillation in the system due to inertia effects during connecting and disconnecting under dynamic conditions. For this purpose, a dynamic test bench, representing the Manifold Flange on the Shuttle Carrier, has been designed and constructed.

Figure 1. Connecting operation testing of enhanced

Chiksan Marine Loading Arm

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In parallel to the preparation of the test campaign, a numerical model has been developed to enable dynamic analysis of the enhanced Marine Loading Arm. The design tool has been validated and calibrated using the findings of the tests.

Finally, a parameter study has been performed using the newly developed numerical analysis tool. The results provide insight into the connecting and disconnecting performance of this type of Marine Loading Arms with different dimensions and weights.

This paper presents the results of all facets of the development program with emphasis on the full-scale testing of the connecting and disconnecting operations, which have been successfully completed in 2002.

TRACK RECORD OF SIDE-BY-SIDE OFFLOADING WITH LOADING ARMS Since the seventies, experience has already been obtained in offloading from floating

units using Loading Arms for LPG and crude oil.

FMC installed the following Marine Loading Arms on floating units:

• 1970 An LPG Marine Loading Arm on a turret-moored FSU in Ardjuna field, Indonesia (see Figure below).

• 1978 Two crude oil Marine Loading Arms on FSU (type RCMA 16" x 110') in Abu Dhabi.

• 1979 Two LNG Marine Loading Arms on LNGC (type P 378 jumpers of 8" x 13.5m) on Das Island in Abu Dhabi.

• 1986 Four crude oil Marine Loading Arms on FSU (type RCMA 12" x 110’) in Colombia.

• 1986 Two crude oil Marine Loading Arms on FSU (type RCMA 16" x 85') in Bouri, Libya.

• 1997 Three crude oil Marine Loading Arms on FSO (type 16” x 93’) in Gulf of Mexico (see Figure 2 below).

Figure 2. Above: Side-by-side loading of LPG

in the early seventies Right : Side-by-side loading of

crude oil today

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Through the years, the design of the QC/DC and the procedure for the connecting operation has been improved to assist the operator in making controlled and safe connections under dynamic conditions. However, in particular for the less benign areas, there is no doubt that a proper connecting operation with the conventional Loading Arm strongly depends on the ability/experience of the operator. The operator has to estimate the maximum motion amplitudes and to choose a favourable moment for making the connection.

In general, connections will only be made when the relative motion between the flanges is less than 0.5 m and the relative velocity is less than 0.5 m/s. This motion envelope however is too small for the FLNG and FSRU applications currently anticipated. The intention is to further enhance the Loading Arm design such that smooth connecting operations are possible for relative flange movements in the order of 4 to 5 m at relative velocities of about 1 m/s. A controlled connection of the flanges, however, can only be made if the relative motions are minimal at the moment of mating. Therefore, a method has been developed, which significantly reduces the relative motions during approaching of the flanges.

NEW CONNECTING MECHANISM AND PROCEDURE As part of the project, many different ideas have been studied, but the use of the

currently reported constant-tension steel guide-cable between the Loading Arm and the Manifold Flange has been found to be the best solution.

Figure 3. New connecting mechanism added to the Chiksan Marine Loading Arm

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The newly developed connecting procedure consists of three steps:

1. Connect the cable to the Manifold on the Shuttle Carrier. For safety reasons, this is the only operation carried out by the operator in the vicinity of the manifold area.

2. Put constant-tension on the cable.

3. Crawl the QC/DC to the Manifold across the cable.

Final positioning of the flanges is achieved automatically by engagement of male and female alignment-cones.

The new connecting procedure requires a completely new maneuvering system. In the conventional system the Loading Arm is moved to the Manifold Flange using three different hydraulic cylinders: (1) the outboard arm drive cylinder, (2) the inboard arm drive cylinder, and (3) the slewing drive cylinder. At the time of the connection, the hydraulic system is locked, and just after the connection, the Loading Arm is put into the freewheel mode to allow the Loading Arm ‘to follow’ the manifold motions of the LNGC.

Figure 4. View on acquisition winch

(crawler) mounted at the side of the ‘Style 80’ ∗)

Figure 5. View on ‘Style 80’ connected to the Manifold Flange

In the new system, the Loading Arm is maneuvered to the Manifold via the

hydraulically activated crawler, which is an integral part of the ‘Style 80’. The hydraulic system of the Loading Arm itself is in the freewheel mode, allowing the Loading Arm to

∗) Product name of the assembly of constant motion swivels, HQCDC and Emergency Release System (ERS), see Figure above

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follow the motions of the tensioned guide-cable, avoiding any risk of interference with the adjacent loading arms already connected.

The new connecting mechanism consists of the following components:

• A steel guide-cable between the base-riser of the Loading Arm and the ‘Style 80’.

• A constant-tension winch with hydraulic motor at the base-riser to provide the constant-tension to the guide-cable. A hydraulic power unit provides a constant hydraulic pressure to the motor. The system is field proven as a mooring system and it has a roll on/roll off speed of 1.4 m/s.

• An acquisition winch with hydraulic gear motor (crawler), attached to the ‘Style 80’, for crawling the ‘Style 80’ to the Manifold Flange across the cable, with a nominal speed of 0.3 m/s.

• A female alignment-cone attached to the ‘Style 80’ for aligning of the QC/DC with the Manifold Flange, which is provided with a male alignment-cone.

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In addition, a Chiksan® hydraulic QC/DC with integrated seal protection system is being used to avoid potential damage to the seal during contact of the flanges. FMC has developed and field proven this component separate from the current project, but it is an essential part of the new connecting mechanism.

Figure 6. Chiksan® hydraulic QC/DC with integrated seal protection system

Furthermore, to assist alignment in cases of considerable offset in longitudinal (surge)

direction between Loading Arm and Manifold due to berth inaccuracies, a rotation system may be added to the ‘Style 80’. This system is already in operation on Chiksan LNG arms. It was incorporated in the new connecting mechanism assembly, but it was found to be redundant in combination with the guide-cable arrangement for the offset conditions considered in the tests.

Figure 7. Hydraulically activated rotation system added to ‘Style 80’

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The LNG Shuttle Carrier is provided with an assembly of a spool piece, an alignment cone, and a guide-cable connecting and release system. The guide-cable is automatically locked when the operator aboard the LNG carrier pulls the guide-cable through the locking system. He uses the messenger line connected to the end of the cable. To release the cable, the operator uses a manual handle.

SAFETY The newly developed connecting procedure has less potential for human error. For

connecting the conventional Loading Arm the operator has to control separately (1) the slewing of the Loading Arm, (2) the rotation of the inboard arm, and (3) the rotation of the outboard arm, while watching the centring of the QC/DC around the Manifold Flange of the LNGC. In the absence of a hydraulically activated rotation system, he also may have to rotate manually the ‘Style 80’ swivel joint assembly to ensure that the flange of the QC/DC is parallel to the Manifold Flange.

With the newly developed system, the operator needs to give only one command to activate the crawler in order to move the Loading Arm to the mating position. The ‘Style 80’ aligns and centres itself on the Manifold Flange. Potential misalignment of the flanges due to considerable offset in longitudinal (surge) direction will be corrected by controlling the hydraulically activated rotation system.

ADAPTED EMERGENCY RELEASE PROCEDURE After connection of the Loading Arm with the Manifold, the steel guide-cable

remains connected, because it is needed again for controlled disconnection of the Loading Arm. During the LNG loading period there is a low constant-tension in the guide-cable to avoid slack conditions.

Figure 8. Guide-cable release after emergency disconnection of Loading Arm

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Following the emergency disconnection the loading arm is automatically retracted without any risk of interference with the manifold and surrounding area of the LNG carrier.

The continuous presence of the guide-cable connection between the Loading Arm base and the Shuttle Carriers’ Manifold requires a special emergency release procedure. In case of an ESD, caused e.g. by drifting away of the LNG carrier, the guide-cable stays connected to the Manifold on the carrier, but it fully rolls-off from the constant-tension winch at the riser base of the Loading Arm. An ESD procedure has also been developed for the condition that the actual connecting operation is ongoing.

PURPOSE-BUILT TEST FACILITY In order to test the operability of the Loading Arm equipped with the newly

developed connecting mechanism, a dynamic test bench, representing the Manifold Flange on the Shuttle LNG Carrier, has been designed and constructed.

The dynamic test bench basically consists of a Manifold Flange with support structure, which is able to produce harmonic (sinusoidal) as well as irregular motions. Motions can be applied simultaneously in a two directions, namely in heave and sway direction, with maximum velocities of 0.85 and 1.15 m/s, respectively. The maximum envelope for the heave motion is 4 m, while the maximum envelope for the sway motion is 5 m. Note that the relative vessel motions to be applied in the heave direction are a summation of the heave and the vertical component of the roll motion.

Figure 9. Dynamic test facility showing the envelope for heave and sway motions

The relative vessel motions to be simulated in the sway direction are a summation of

the sway and the horizontal component of the roll motion.

The test facility has not been designed for simultaneous application of heave, sway and surge motions, since surge motions have long periods, and will therefore have a limited effect on the overall dynamic behaviour. In order to investigate the effect of

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different offset positions (in surge direction) on the dynamic behaviour of the Loading Arm, the test bench can be installed at different locations relative to the Loading Arm position, with a maximum offset of 5.3 m or the equivalent of a total drift of 10.6m in a fore and aft direction.

The test facility has been set-up such that the Loading Arm is fixed to the ground, while the test bench (Manifold Flange) produces relative motions. The effect of the simplifications have been analysed using the newly developed numerical analysis tool discussed further on in this paper.

MOTION SPECIFICATIONS The last couple of years Shell have extensively evaluated the motions of side-by-side

moored floating units, of which the production unit is a weathervaning barge with an external turret, moored to the seabed with a 9-leg mooring system. The Shuttle Carrier is moored to the barge with 16 pre-tensioned mooring lines, while there are 4 cylindrical waterline fenders and 5 back-up block fenders close to deck level in between the units.

Figure 10. Experimental evaluation of motions between side-by-side moored floating units performed in offshore basin of MARIN

Extensive scale 1:50 basin model tests have been carried out for various field-specific

conditions of wind, current and waves. Wind seas and swells have been considered as individual components, each with its own wave height and direction. For this purpose the ultra-modern capabilities of the new offshore basin of MARIN have been utilised.

Motions at the location of the Loading Arms/Manifolds were measured for a large number of irregular seas, each with duration of 3-hours. The 3-hours conditions are ‘storm’ conditions in relatively benign offshore environments and are ‘at the limit’ from a perspective of marine operations such as berthing, mooring, and being-moored. The design philosophy for the enhanced Chiksan Loading Arm is that it should be able to be

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and to stay connected for any condition the captain of the LNGC considers safe for being moored.

The 3-hours time-series produced in the basin include a complete record of displacements, velocities and accelerations at the locations of the Loading Arm and the Manifold as well as between those locations. The relative motions served as input to the Loading Arm tests.

TESTING OF ENHANCED LOADING ARM An irregular hydrodynamic motion of 3-hours duration shows alternation of ‘quiet’

and ‘heavy’ moments in such a period. This is clearly visualized by the moving Manifold Flange. The actual connecting operation have been tested during quiet as well as heavy moments, and thus providing an extensive insight into the connecting capabilities of the Loading Arm. Test were performed for a variety of different 3-hours time series for different geographical areas and different barge particulars.

In addition to testing the connecting and disconnecting operations, the Loading Arm performance was tested for the ‘being-connected’ condition. This part provided insight into the ability of this type of Loading Arm to withstand the most severe motions in a particular 3-hours storm condition.

More specifically, the tests provided insight into the following aspects:

• Dynamic response of the complete offloading system (damping properties).

• Functionality of newly added equipment, mechanisms and control systems.

• Logicality of methods and procedures.

• Operational and ergonomic aspects (operator handlings, communication/co-operation between operators).

The main measure for the assessment of the acceptability of the connecting process is

the range of the oscillations of the ‘Style 80’ relative to the Manifold at various stages of approach. The test demonstrated that the damping in the system caused by flow resistance in the hydraulic system and friction in the constant motion swivel joints and support bearings was sufficient to avoid undesirable oscillations in the Loading Arm.

A related measure for the assessment of the acceptability of the connecting process is the range of the load fluctuations in the guide-cable between ‘Style 80’ and Manifold, relative to the applied ‘constant’ tension.

NEWLY DEVELOPED NUMERICAL DESIGN TOOL The tests performed, though extensive, are rather specific. They have been carried out

for a particular Loading Arm type, for particular positions of the Loading Arm relative to the Manifold, and for particular motion characteristics. Furthermore, two main simplifications have been made: (1) motions have been applied in two rather than three degrees of freedom, and (2) the Loading Arm itself has not been subjected to motions.

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Figure 11. Numerical model of the Chiksan Marine Loading Arm (red arrows represent tension in of guide-cable)

In order to obtain a better insight into the effects of different parameters on the

dynamic response of the Loading Arm, and as aid into the design, a numerical tool has been developed. It is based on the use of the ‘ADAMS’ software.

The parameter that has an important effect on the dynamic behaviour of the system is the weight (distribution) of the Loading Arm. A Loading Arm subjected to motions might need additional structural reinforcement to cope with inertia effects; it is therefore heavier, and consequently, it will show a different dynamic behaviour. If the same Loading Arm has to be connected to a Manifold at a different position (e.g. due to a difference in LNGC freeboard), its dynamic behaviour will be different due to a difference in weight distribution.

The Loading Arm is modeled as an articulated system, consisting of solid bodies (cylinders, cones, blocks, etc.) connected by swivel joints/pivot points.

Values for the geometry, mass, and inertia terms define the mechanical properties of the solid bodies. Small, but heavy components of the Loading Arm are represented by ‘point masses’. Friction data are provided for the swivel joints/pivot points. Damping in the hydraulic system is modeled as an additional value for the friction in the swivel joints/pivot points.

The numerical tool is not only able to simulate the ‘being-connected’ condition, but also the actual guide-cable-guided approach of the Loading Arm to the Manifold. Furthermore, it has been made suitable for reading the same irregular motion time-series as used in the tests, allowing the tests to be simulated in detail. The tool has been calibrated by adjusting the values for the friction in the swivel joints/pivot points (which

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include the damping in the hydraulic system). The main calibration parameters are the oscillations of the ‘Style 80’ and the load fluctuations in the guide-cable.

CONCLUSIONS Operation tests of the Chiksan Marine Loading Arm, enhanced with the newly

developed connecting mechanism, showed that reliable and safe connections and disconnections can be made for relative motions between the mating flanges in the range of 4m and with relative velocities in the order of 1 m/s.

The semi-automated connecting operation has less potential for human error than the connecting procedure of the conventional Loading Arm.

The new system therefore opens opportunities not only for Floating LNG Export and Re-gasification facilities, but also for more exposed onshore LNG plants and terminals, thus reducing the need for breakwaters and allowing consideration of different sites.