AU2017341380A1

AU2017341380A1 - Method and system for side-by-side offloading of liquid natural gas

Info

Publication number: AU2017341380A1
Application number: AU2017341380A
Authority: AU
Inventors: Daniel Craddock; Sanne Eva Groot; Jason MCCONOCHIE; Finlay MCPHAIL
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 2016-10-13
Filing date: 2017-10-11
Publication date: 2019-04-11
Anticipated expiration: 2037-10-11
Also published as: BR112019007584A2; KR102443625B1; WO2018069365A1; AU2017341380B2; NL2017614A; KR20190059929A; NL2017614B1; MY195850A

Abstract

System for supporting side-by-side transferring of hydrocarbons between a weathervaning vessel and a carrier vessel, the weathervaning vessel comprising thrusters. The system being arranged to: a) obtain metocean forecast data, the metocean forecast data comprising at least wind speed, wind direction and at least one wave parameter for a forecast period, b) obtain a carrier vessel identifier associated with the LNG carrier vessel, c) obtain or compute for a plurality of headings of the weathervaning vessel, a plurality of vessel responses for different time slots within the forecast period, based on the metocean forecast data and the carrier vessel identifier, d) compare all vessel responses to predetermined safety limits and generate safety indicators for the plurality of time slots of the plurality of headings, and e) output the safety indicators.

Description

TECHNICAL FIELD

The present invention relates to a method and system for supporting side-by-side transferring of hydrocarbons, e.g. liquid natural gas, between a weathervaning vessel and a carrier vessel, the weathervaning vessel comprising thrusters. The weathervaning vessel may be a turret moored floating liquid natural gas producing vessel (FLNG vessel). The carrier vessel may be any kind of carrier vessel suitable for transporting the hydrocarbons. In case of liquid natural gas, the carrier vessel is a LNG carrier vessel comprising LNG tanks suitable for carrying cryogenic LNG, e.g. membrane type LNG tanks or MOSS type LNG tanks.

BACKGROUND

Various concepts of offloading LNG are known, including side-by-side off-loading and tandem off-loading.

Side-by-side offloading has the benefit to use the existing fleet of LNG carriers since, in side-by-side, a near-standard berth, mooring line and tendering arrangement, and importantly, existing loading/return line flanges can be used all without modifications.

In contrast, tandem offloading of LNG requires carriers to be specifically modified for LNG loading from the bow of the carrier including modifications to the mooring system and offloading and return hoses. Tandem offloading is possible with the use of dynamically positioned carriers obviating the need for a bow mooring system but greatly increasing the cost of the LNG carriers.

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- 2 However, for LNG carrier vessels, side-by-side mooring and offloading of liquid natural gas from a weathervaning LNG producing vessel is not currently practiced.

The methods of an LNG carrier vessel to approach and berth to a weathervaning LNG producing vessel are dependent on (among other things) the metocean conditions, the heading of the weathervaning LNG producing vessel, the expected behaviour of the weathervaning LNG producing vessel during the operation,tug capacity, number of tugs, motions/behaviour of the LNG carrier vessel, etc.. During the operation it is possible that the weathervaning LNG producing vessel heading is changing due to the external environmental forces applied by the wind, waves and currents (metocean conditions). The situation of the weathervaning LNG producing vessel heading constantly changing is not suitable for approach and berthing of LNG carrier vessels in side-by-side arrangement. If the weathervaning LNG producing vessel was to change heading unexpectedly in response to, say changing wind, the LNG carrier vessel is unlikely to be able to respond quickly enough to avoid a collision with the weathervaning LNG producing vessel.

Reference is made to Effect on heading control on LNG offloading, by Arjan Voogt, Proceedings of the nineteenth (2009) International Offshore and Polar Engineering Conference, ISBN 978-1-880653-53-1. Further reference is made to A high capacity floating LNG desing, by Barend Pek, Harry van der Velde. Further reference is made to OFFLOADING

OPERABILITY WITH HEADING CONTROL OF SIDE-BY-SIDE

MOORED FLNG AND LNGC, by Hyun Joe Kim et al, Proceedings of the ASME 2012 31st International Conference on Ocean,

Offshore and Arctic Engineering, OMAE2012, July 1-6, 2012,

Rio de Janeiro, Brazil.

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- 3 These references show that some progress has been made toward understanding impact of turret mooring system and consequent weathervaning on the LNG carrier vessel approach to berth and on the offloading both separately (berth manoeuvring studies and offloading operability studies in various levels of detail) and together (Voogt).

Other works have suggested that applying thrusters during the approach and berthing phase can be advantageous (A marine view on offshore LNG, by Dekker and Doom, OTC 20593, 2010 offshore Technology Conference held in Houston, Texas, USA, 3-6 May 2010) .

The thrusters may be located on the stern of the weathervaning LNG producing vessel. For instance, 3 thrusters may be provided which can be used to affect the heading of the weathervaning LNG producing vessel and also stabilise the heading during the approach and berthing phases to within a few degrees .

In addition, during the approach and berthing phase there are a number of operations which must be conducted that are sensitive to the metocean conditions such as pilot transfer onto the LNG carrier vessel, transfer of equipment and tug operations. Conditions must be such that these operations can be conducted safely within the operating limits.

As the LNG carrier vessel begins to connect mooring lines to the weathervaning LNG producing vessel, other limits begin to become important such as for mooring line tensions and fender loads .

Once the LNG carrier vessel is berthed alongside the weathervaning LNG producing vessel, the operation continues to make fast all mooring lines followed by connection of the offloading arms to the LNG carrier vessel offloading flanges. Offloading then commences and continues for about 2 to 40 hours depending on the LNG flow rates and the capacity of the

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- 4 LNG carrier vessel. Criteria that are important for the safe offloading of LNG include the metocean conditions, motions of the LNG carrier vessel, the relative motions of the weathervaning LNG producing vessel and the LNG carrier vessel as well as forces induced in the connected LNG loading arms, the mooring lines tensions, the fender compression and the resulting internal pressures. Criteria for sloshing in the LNG tanks of the LNG carrier vessel as well as the weathervaning LNG producing vessel also are relevant. For each of the criteria, limits are defined on the basis of structural capacity (e.g of the LNG loading arms, mooring lines, fenders), for the avoidance of contact between the vessels, and personnel comfort levels of motions with the vessels in close proximity. The entire operation must be planned in such a way to avoid exceeding these complex limits .

Furthermore, as the loading operation progresses the LNG carrier becomes more heavily loaded; as a consequence the motion of the LNG carrier changes. With more LNG on board, the LNG carrier motions change and consequently the forces in mooring lines, fenders and within the LNG loading arms change; all responses are affected to varying degrees. For all criteria, the responses must be maintained with their respective limits continuously throughout the offloading operation .

To maintain safe operations, it must be determined prior to any offloading operation whether safety limits will or will not be exceeded for the planned operation period.

Finding out during the operation that limits are exceeded is not a planned operating mode even though systems and procedures will be in place to safely terminate the operation .

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- 5 However, there is no system or method available to provide operational support to the decision making process to decide on if and when to go ahead with the side-by-side offloading, the preferred heading through to completion of the operation upon LNG carrier sail-away.

It is an object to provide a system for supporting sideby-side offloading, in particular a system for supporting planning side-by-side offloading.

According to an aspect there is provided a system for supporting side-by-side transferring of hydrocarbons between a weathervaning vessel and a carrier vessel, the weathervaning vessel comprising thrusters, the system being arranged to:

a) obtain metocean forecast data, the metocean forecast data comprising at least wind speed, wind direction and at least one wave parameter for a forecast period,

b) obtain a carrier vessel identifier associated with the LNG carrier vessel,

c) obtain or compute for a plurality of headings of the weathervaning vessel, a plurality of vessel responses for different time slots within the forecast period, based on the metocean forecast data and the carrier vessel identifier,

d) compare all vessel responses to predetermined safety limits and generate safety indicators for the plurality of time slots of the plurality of headings, and

e) output the safety indicators.

The weathervaning vessel may be a turret moored vessel.

The weathervaning vessel may be a liquid natural gas producing vessel or a LNG regasification vessel. In both cases, the carrier vessel may be a LNG carrier vessel and the transferring of hydrocarbons is to be understood as transferring liquid natural gas from the liquid natural gas

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- 6 producing vessel to the LNG carrier vessel or from the LNG carrier vessel to the LNG regasification vessel.

The carrier vessel may be a LNG carrier vessel comprising LNG tanks suitable for carrying cryogenic LNG, e.g. membrane type LNG tanks or MOSS type LNG tanks.

The hydrocarbons may be (pressurized) liquid natural gas (LNG) or liquefied petroleum gas (LPG).

The safety indicators can be used by a decision making tool or an operator to plan side-by-side mooring and offloading and decide when to commence side-by-side mooring and which heading of the weathervaning vessel will be chosen.

The predetermined off-loading time is typically in the range of 2 - 40 hours.

The forecast period is typically in the range of 1 - 10 days. This permits possible offloading windows to be assessed by the system and the determination of the heading of the weathervaning vessel up to the end of the weather forecast data.

The metocean forecast data may be updated at regular intervals (for example each 3, 6 or 12 hours) and include wind, waves and water current parameters. These parameters may be used to calculate the forces on the vessels both in the weathervaning vessel alone case as well as the side-byside (in residence) case and throughout the entire forecast period.

According to an embodiment, a) is performed repeatedly every so many hours, e.g. every 3 hours.

The metocean conditions control the natural weathervaning heading and in the case of thruster usage, determine the amount of thruster power required to sustain a particular heading. Thereby all headings can be scrutinised to determine the operability, permitting the system to define

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- 7 the optimal heading, manage thruster usage and maximise the offloading operability.

According to an embodiment the metocean forecast data comprise one or more:

- wave parameters, such as wave height, wave direction spectrum, (full) wave frequency, or parameterised wave spectra using significant wave height, wave period, wave direction and wave spreading,

- wind parameters, such as wind speed, wind direction, variations in wind speed and/or direction, maximum wind speed,

- water current parameters, such as current speed and current direction at one or more depths relevant for the weathervaning vessel and the carrier vessel.

The metocean forecast data is used to obtain or compute vessel responses.

In b) a carrier vessel identifier is obtained which may be used by the system in step c) to obtain or compute the vessel responses. As different carrier vessels have different dimensions, shapes etc. and therefore respond differently to metocean conditions, the carrier vessel identifier is obtained each time a carrier vessel approaches the weathervaning vessel to inform the system about the type of carrier vessel that is approaching the weathervaning vessel.

If the system is programmed for a specific weathervaning vessel, no weathervaning vessel identifier needs to be obtained. If the system is used for different weathervaning vessels, b) may also comprise obtaining a weathervaning vessel identifier.

According to an embodiment obtaining or computing vessel responses in c) comprises:

- obtaining predetermined vessel responses from a vessel response database, or

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- 8 - obtaining vessel responses by performing simulation calculations .

The vessel responses comprise vessel responses for the weathervaning vessel as well as for the carrier vessel.

The vessel response database may comprise a collection of predetermined (computed or measured) vessel responses for the weathervaning vessel alone case as well as the side-by-side (in residence) case for the carrier vessel for different metocean conditions. The vessel response database may further comprise vessel responses for different loading conditions of the carrier vessel, as it will be understood that the behaviour of a carrier vessel depends on the loading conditions (e.g. empty, full).

Using the metocean forecast data and the carrier vessel identifier as entry for the vessel response database, the relevant vessels responses may be obtained. It will be understood that the vessel response database may only comprise a limited amount of different metocean conditions that will not always exactly match the metocean forecast data. In that case, the system is arranged to identify the metocean conditions present in the vessel response database that are a close match with the metocean forecast data. This may be done for each combination of heading and time slot.

The metocean conditions may be defined from a site specific database, but could also be made generic so as to be applicable at any site.

Vessel responses may be comprised in the vessel response database for a range of pre-selected combinations of weathervaning vessels and carrier vessels, incorporating the effect of different loading conditions of the carrier vessel and, if required, the weathervaning vessel.

The vessel response database comprises a look-up table wherein forecast metocean conditions are matched to the

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- 9 closest condition in the pre-calculated database, and then responses are taken from that pre-calculated table entry.

The vessel response database may comprise wave-frequency induced vessel responses. Vessel responses within the same frequency range as the waves are computed directly using transfer functions that convert the forecast wave spectrum into forecast motions directly. This is the so-called frequency domain approach using response amplitude operators (RAO's).

The weathervaning vessel may be much larger than the carrier vessel.For instance, the weathervaning vessel may have a length overall (maximum length of the ship between the forwardmost and aftermost parts of the ship) twice as big as the length overall of the LNG carrier. In such a situation, the weathervaning vessel and the carrier vessel are expected to react differently to the metocean conditions.

Also, in particular in the case of a weathervaning LNG producing vessel, it may be assumed that the loading condition of the weathervaning vessel has little effect on the behaviour of the weathervaning vessel in response to the metocean conditions, as the weight of the cargo is relatively small compared to the overall weight of the weathervaning vessel.

According to an embodiment the vessel responses comprise one or more of the following parameters for a side-by-side moored situation:

- motions of the weathervaning LNG producing vessel,

- motions of the LNG carrier vessel,

- relative motions and relative positions of the weathervaning vessel and the carrier vessel,

- loads, forces or tensions in mooring lines between the weathervaning vessel and the carrier vessel,

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- 10 - loads, forces or tensions in fenders positioned in between the weathervaning vessel and the carrier vessel,

- loads, forces or tensions in loading arms for transferring hydrocarbons between the weathervaning vessel and the carrier vessel.

All or a subset of these parameters may be stored in the vessel response database or may be computed on the fly.

With respect to the loading arms, it is noted that the relative motions and relative position/orientation of the loading arms. These limits limits can be calculated during design based on the structural limitations of the components, the loading arms and the system.

According to an embodiment the system comprises or has access to a limit database comprising the predetermined safety limits .

The limit database comprises all sorts of safety limits, including safety limits relating to the different vessel responses, for instance maximum allowable movements of the weathervaning vessel, maximum allowable movements of the carrier vessel, including heave, pitch, roll motions, maximum allowable relative movements and relative positions of the weathervaning vessel and the carrier vessel, maximum allowable loads, forces or tensions in mooring lines between the weathervaning vessel and the carrier vessel, maximum allowable loads, forces or tensions in fenders positioned in between the weathervaning vessel and the carrier vessel and maximum allowable loads, forces or tensions in loading arms for transferring hydrocarbons from the weathervaning vessel to the carrier vessel.

According to an embodiment in e) the safety indicators are outputted by generating a visual representation of the safety indicators in a two dimensional coordinate system wherein

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- 11 - one coordinate of the safety indicator is determined by the heading associated to the safety indicator, and

- the other coordinate of the safety indicator is determined by the time slot associated to the safety indicator.

According to an embodiment the two dimensional coordinate system is a polar coordinate system having radial and angular coordinates, wherein

- the angular coordinate of the safety indicator is determined by the heading associated to the safety indicator, and

- the radial coordinate of the safety indicator is determined by the time slot associated to the safety indicator.

The visual representation can be outputted via any suitable user interface. The visual representation can be plotted on a display, printed on a piece of paper, outputted as a digital file (e.g. pdf) or outputted by any other suitable means.

The method may further comprise permanently storing the outcome of the steps a) - e) for later use as evidence of the information based upon which the decision to start the transferring, the selected time to start the transferring and the decided heading under which the transferring took place and the selected heading was taken. The permanent storing may be done in a memory unit comprised by the system and/or in a remote memory unit (not comprised by the system).

According to an embodiment the safety indicator is visually represented by a colour indicating whether or not the combination of heading and time slot meets the safety limits .

A first colour (e.g. green) may be used to indicate the safety limits are not exceeded, while a second colour (e.g. red) may be used to indicate that the safety limits are exceeded.

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- 12 According to an embodiment c) further comprises obtaining or computing expected thruster parameters for the different time slots of the plurality of headings of the weathervaning vessel.

The expected thruster parameters may be outputted as part of step e). The thruster parameters may for instance be a number ranging from 0 - 100% indicating the required thruster usage as part of the maximum thruster capacity. The number may be shown as number, graph or colour in the two dimensional coordinate system described above or in a separate visual representation.

For instance, the system is arranged to determine which headings are achievable with or without the use of thrusters and determine which headings are not achievable even with the use of thrusters. Obtaining or computing a plurality of vessel responses for different time slots within the forecast period, based on the metocean forecast data and the carrier vessel identifier, may only be done for the achievable headings of the weathervaning vessel, optionally taking into account a safety margin of for example 10 degrees into account.

According to an embodiment the system is further arranged to compute and output berthing and departure safety indicators for the different time slots of the plurality of headings of the weathervaning LNG producing vessel, the berthing and departure safety indicator being:

- positive in case the carrier vessel is forced towards the weathervaning vessel,

- negative in case the carrier vessel is forced away from the weathervaning vessel, and

- neutral in case none of the above applies.

According to an embodiment the system is a computer system.

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- 13 The term computer system is used here to for instance refer to a system comprising a processor and a memory unit, the memory unit comprising instruction lines readable and executable by the processor to provide the computer system with the required functionality as explained in this text.

The computer system comprises an input/output unit to output information, e.g. to a user, and receive user feedback and/or instructions. The input/output unit further enables the computer system to communicate with remote systems and remote computer systems.

The input/output unit, the processor and the memory unit may all be arranged to communicate with each other.

SHORT DESCRIPTION OF THE FIGURES

The invention will be further illustrated hereinafter, using examples and with reference to the drawing in which;

Fig. 1 schematically shows a system according to an embodiment,

Fig. 2 schematically shows visual representation of the safety indicators according to an embodiment.

In these figures, same reference numbers will be used to refer to same or similar parts.

DETAILED DESCRIPTION

There is provided a system and method for supporting side-by-side transferring of hydrocarbons between a weathervaning vessel and a carrier vessel, the weathervaning vessel comprising thrusters.

According to an embodiment the weathervaning vessel is turret moored weathervaning LNG producing vessel which permits the heading of the weathervaning LNG producing vessel to change in response to the external environmental forces (metocean conditions). This weathervaning motion may be

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- 14 referred to as heading into the weather which generally means forces and motions of the weathervaning LNG producing vessel are lower. According to this embodiment the carrier vessel is a LNG carrier vessel.

However, it is not always the case that the motions of the weathervaning LNG producing vessel will be lower when weather-vaning. For example, the weathervaning LNG producing vessel may weather-vane into the wind and at the same time, swell waves approaching from the side (or so-called beam) with long periods may cause higher roll motions than if the weathervaning LNG producing vessel was to head into the swell waves .

In addition to the weather-vaning mode of operation, the weathervaning LNG producing vessel heading can be affected by the thrusters, e.g. stern thrusters. Stern-thrusters located on the keel of the vessel on the aft end, opposite to the turret, produce thrust that can rotate the heading of the weathervaning LNG producing vessel such that the heading is deflected away from the natural weather-vaning heading. Thrusters are provisioned to facilitate operations during offloading and can be set to automatically maintain a near constant heading of the weathervaning LNG producing vessel.

By adjusting the heading of the weathervaning LNG producing vessel for offloading, vessel responses will be affected. Furthermore, it is possible to select a heading which can provide the minimum possible vessel responses in order that the likelihood of meeting the offloading criteria limits is increased.

However, the choice of this optimal heading accounting for the complex criteria and the forecast metocean conditions cannot be obtained by, for example, simply reviewing the forecast metocean conditions since the responses depend on complex factors. The complexity in the factors comes

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- 15 primarily from the metocean conditions impact on the motion responses of the vessels and the resulting responses of mooring lines loads and the like. Considering the above example where swell and wind comes from different directions, it can also be the case that ocean currents and other wave groups come from other directions - all affecting the natural heading of the vessel, but also the motions. Significant computational cost is required for the estimation of the responses such as mooring line tensions and the relative motions of the weathervaning LNG producing vessel and the LNG carrier vessel. As a result a system is described which permits fast operational calculations of the vessel responses and furthermore, the impact that the heading of the weathervaning LNG producing vessel can have on the vessel responses. The aim of the system is to provide maximum operability of the weathervaning LNG producing vessel berth for offloading operations. This is achieved by selecting a fixed or variable heading of the weathervaning LNG producing vessel on the basis of forecast metocean conditions as well as predicted vessel responses throughout the forecast window. Alternatively it may be that the maximum operability is achieved by allowing the weathervaning LNG producing vessel to continuously weather-vane throughout the offloading window. The system also makes this determination.

Furthermore, the system can allow selecting a suitable window optimised for the period when the LNG carrier vessel is due to arrive at site, either accelerating the arrival time or delaying the arrival time to match with forecast window in offloading operability, thus saving costs on LNG carrier fuel consumption during its' journey.

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- 16 Fig. 1 schematically depicts a system according to an embodiment. It will be understood that the system may be embodied by any kind of suitable computer system.

Fig. 1 shows a system 1, comprising a processor 2 and a memory unit 3. The memory unit 3 comprises instruction lines readable and executable by the processor 2 to provide the system with the required functionality.

The system comprises an input/output unit 4 which is arranged to communicate with other systems and may in particular be arranged to

a) obtain metocean forecast data from a metocean forecast providing service, here schematically represented with reference 20, and

b) obtain a carrier vessel identifier ID associated with the LNG carrier vessel 30.

The system 1 is further arranged to output information via input/output unit 4. The information may be outputted to an interface, shown here as a user interface embodied by a display 6.

The user interface may be comprised by the system 1 or may not be comprised by the system 1, for instance the user interface may be located remote from the system 1.

Based on the received metocean forecast data and the carrier vessel identifier ID, the system 1 is arranged to

c) obtain or compute for a plurality of headings of the weathervaning LNG producing vessel, a plurality of vessel responses for different time slots within the forecast period.

The plurality of headings may comprise all headings at predetermined intervals (e.g. at five degree intervals) or may be only those headings which are achievable with or without the use of thrusters ignoring those headings which are not achievable even with the use of thrusters. In

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- 17 addition, headings that are within 10 degrees of a non achievable heading may be ignored to create a safety margin.

According to an embodiment predetermined (precomputed or premeasured) vessel responses may be obtained from a vessel response database, which may be stored in memory unit 3. Alternatively the predetermined vessel responses are stored in a separate or remote memory unit (not shown) accessible by the system 1.

According to an alternative embodiment, the vessel responses may be obtained by performing simulation calculations using simulation calculation rules or software stored in the memory unit 3. Alternatively the simulations are performed in a separate or remote computer (not shown) accessible by the system 1.

The system 1 may comprise a limit database comprising predetermined safety limits. The limit database may be stored in the memory unit 3.

According to an alternative embodiment the system 1 may have access to a remotely stored limit database to which the system 1 has access via input/output unit 4

Next, the system is arranged to d) compare the vessel responses to the predetermined safety limits.

For instance, the vessel responses may comprise an expected maximum mooring line load to be generated for a certain heading during a certain time slot. For this particular mooring line a safety limit is available. The expected maximum mooring line load and safety limit are compared and in case the maximum mooring line load exceeds the safety limit a negative safety indicator is generated.

This is done for all vessel responses that are taking into account. If for a certain heading and time slot one of the vessel responses exceeds the corresponding safety limit a

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- 18 negative safety indicator is generated for that particular combination of time slot and heading.

Next, according to an embodiment, in step e) the safety indicators are outputted by generating a visual representation of the safety indicators. The visual representation is outputted via input/output unit 4 for instance to a display 6. The two dimensional coordinate system may be a coordinate system wherein

- one coordinate of the safety indicator is determined by the heading associated to the safety indicator, and

According to a preferred embodiment, the coordinate system is a polar coordinate system wherein

- the radial coordinate of the safety indicator is determined by the heading associated to the safety indicator, and

- the angular coordinate of the safety indicator is determined by the time slot associated to the safety indicator .

Fig. 2 schematically depicts a visual representation of the safety indicators according to an embodiment wherein a polar coordinate system is used.

Fig. 2 shows a visual representation wherein step c) is executed for 16 different headings and 4 time slots of 1 hour each. It will be understood that in practice, more headings, more time slots and shorter time slots may be used.

Based on the outcome of d) for each combination of time slot and heading a single shading is used to indicate that no safety limits are exceeded, crossed-shading is used to indicate that at least one safety limit is exceeded and no shading is used to indicate that the heading is not achievable even with the use of thrusters.

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In practice, instead of using different shadings, different colours may be used.

It will be understood that Fig. 2 is only one of many possible representations and only provided here by means of example .

The visual representation may further comprise an indication of thruster usage to allow an operator to select a preferred heading.

The person skilled in the art will understand that the present invention can be carried out in many various ways without departing from the scope of the appended claims.

Claims

1. System for supporting side-by-side transferring of hydrocarbons between a weathervaning vessel and a carrier vessel, the weathervaning vessel comprising thrusters, the system being arranged to:

b) obtain a carrier vessel identifier associated with the LNG carrier vessel,

e) output the safety indicators.

2. System according to claim 1, wherein the metocean forecast data comprise one or more:

- water current parameters, such as current speed and current direction at one or more depths relevant for the

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- 21 weathervaning LNG producing vessel and the LNG carrier vessel.

3. System according to any one of the preceding claims, wherein obtaining or computing vessel responses in c) comprises :

- obtaining predetermined vessel responses from a vessel response database, or

- obtaining vessel responses by performing simulation calculations .

4. System according to any one of the preceding claims, where the vessel responses comprise one or more of the following parameters for a side-by-side moored situations:

- motions of the weathervaning LNG producing vessel,

- motions of the LNG carrier vessel,

- loads, forces or tensions in fenders positioned in between the weathervaning vessel and the carrier vessel,

- loads, forces or tensions in loading arms for transferring hydrocarbons from the weathervaning vessel to the carrier vessel.

5. System according to any one of the preceding claims, wherein the system comprises or has access to a limit database comprising the predetermined safety limits.

6. System according to any one of the preceding claims, wherein in e) the safety indicators are outputted by

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- 22 generating a visual representation of the safety indicators in a two dimensional coordinate system wherein

7. System according to claim 6, wherein the two dimensional coordinate system is a polar coordinate system having radial and angular coordinates, wherein

8. System according to any one of the claims 6-7, wherein the safety indicator is visually represented by a colour indicating whether or not the combination of heading and time slot meet the safety limits.

9. System according to any one of the preceding claims, wherein c) further comprises obtaining or computing expected thruster parameters for the different time slots of the plurality of headings of the weathervaning vessel.

10. System according to any one of the preceding claims, wherein the system is further arranged to compute and output berthing and departure safety indicators for the different time slots of the plurality of headings of the weathervaning LNG producing vessel, the berthing and departure safety indicator being:

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- 23 - positive in case the carrier vessel is forced towards the weathervaning vessel,

- neutral in case none of the above applies.

11. System according to any one of the preceding claims, wherein the system is a computer system.

12. Method for supporting side-by-side transferring of hydrocarbons between a weathervaning vessel and a carrier vessel, the weathervaning vessel comprising thrusters, the method comprising:

a) obtaining metocean forecast data, the metocean forecast data comprising at least wind speed, wind direction and at least one wave parameter for a forecast period,

b) obtaining a carrier vessel identifier associated with the LNG carrier vessel,

c) obtaining or computing for a plurality of headings of the weathervaning vessel, a plurality of vessel responses for different time slots within the forecast period, based on the metocean forecast data and the carrier vessel identifier,

d) comparing all vessel responses to predetermined safety limits and generating safety indicators for the plurality of time slots of the plurality of headings, and

e) outputting the safety indicators.