[0001] The present invention relates to an offshore structure and a mooring arrangement.
[0002] Such an offshore structure may be used to produce hydrocarbon fluids from a subsea
or underwater well and to process these fluids into a transportable form. In some
cases, e.g. when a pipe line is not technically and/or commercially feasible, the
processed hydrocarbon fluids need to be transferred into a tanker ship for transportation
away from the offshore structure to a receiving site. In case of natural gas, for
instance, the gas may be first liquefied to produce LNG in which form it can be transported
by an LNG tanker, generally referred to as an LNG carrier (LNGC).
[0003] Alternatively, such an offshore structure may be used to receive hydrocarbon fluids
that have been transported by a carrier ship from elsewhere, and optionally store
and/or process the received hydrocarbon fluids.
[0004] Liquefied natural gas ("LNG") is produced when natural gas is cooled to a cold, colourless
liquid at -160°C (-256°F). Storage of LNG requires much less volume for the same amount
of natural gas. A number of storage tanks have been developed to store LNG in cryogenic
form. In order to use LNG as a power source, the LNG is converted to its gaseous state
using a re-vaporisation process. The re-vapoured LNG can then be distributed through
pipelines to various end users.
[0005] One advantage of LNG is that LNG may be transported by ship to markets further than
would be practical with pipelines. The ability of produce LNG offshore and to bring
it onto a transport carrier allows for exploitation of gas reservoirs that would otherwise
be too far removed from consumers of natural gas. Importing LNG by ships has led to
the establishment of LNG storage and re-vaporisation facilities at on-shore locations
that are close to shipping lanes. There is also a desire in the industry to provide
offshore capabilities for receiving LNG and LPG producs from a transport carrier.
As an example,
WO 2006/052896 discloses a floating LNG storage and regasification unit ("FSRU").
[0006] Both types of offshore structures described above may advantageously employ a mooring
arrangement in order to enable mooring of carrier ships to the offshore structure
during loading and/or offloading of the hydrocarbon fluid(s).
[0007] In
WO2006/101395 a mooring arrangement is provided on an offshore structure, for mooring a tanker
vessel alongside of the offshore structure. The mooring arrangement uses a quick release
coupling hook. The mooring lines are rather short because of the small distance of
the side-by-side mooring geometry compared to mooring on-shore on a jetty. A relatively
short nylon rope can be used in view of an extendable shock absorber that is provided
on the hook in the form of a hydraulic cylinder and an extendable arm.
[0008] In a first aspect, the present invention provides an offshore structure comprising
an outer hull having longitudinal sides and at least one fairlead located adjacent
a longitudinal side, and a hook, connected to the structure and located inboard relative
to the fairlead and being displaced along the hull longitudinally from the fairlead,
such that an angle of displacement (α) between a tangent to the outer hull at the
centre of the fairlead and the hook, is less than or equal to 45°.
[0009] In a second aspect, the present invention provides a mooring arrangement comprising
at least one mooring line having a pennant at an end thereof, and an offshore structure
according to the first aspect of the invention, wherein the at least one mooring line
is configured to pass inboard of the outer hull through the fairlead and is attachable
to the hook such that the angle of displacement of the at least one mooring line between
the tangent to the hull at the centre of the fairlead and the hook is less than or
equal to 45°.
[0010] The present invention and its advantages will now be further illustrated by means
of examples and with reference to the drawing wherein:
Figure 1 shows a schematic plan view of an offshore structure in which the positioning
of the fairlead and hook relative to the longitudinal sides of the structure are depicted;
Figure 2 shows a schematic plan view of a section of a mooring deck of the offshore
structure of Figure 1;
Figure 3 shows a cross-sectional representation of an offshore structure according
to an embodiment of the invention in which the mooring deck is located within the
outer hull and is bounded by the hull;
Figure 4 shows a schematic representation of a mooring arrangement according to an
embodiment of the invention;
Figure 5a shows a schematic representation of a mooring arrangement with mooring points
A to P in operation mooring a LNGC;
Figure 5b shows a schematic representation of the mooring layout of Figure 5a in operation
mooring a further LNGC;
Figure 6a shows a schematic representation of the mooring layout of Figure 5a in operation
mooring a LPGC;
Figure 6b shows a schematic representation of the mooring layout of Figure 5a in operation
mooring a further LPGC; and
Figure 7 shows a schematic representation of a mooring arrangement of the invention
for an LNGC mooring position relative to an offshore structure in the form of an FLNG.
[0011] Although the invention will be described in terms of specific embodiments, it will
be understood that various elements of the specific embodiments of the invention will
be applicable to all embodiments disclosed herein.
[0012] An improved mooring arrangement for mooring a transport carrier to an offshore structure,
which offshore structure is typically deployed in a body of water, is proposed. The
mooring arrangement is particularly suitable for application on an offshore liquefied
natural gas production unit and/or an offshore liquefied petroleum gas production
unit. The offshore structure may be a floating structure, preferably moored in a weathervaning
arrangement, which facilitates the mooring procedure of the transport carrier to the
offshore structure.
[0013] Figure 1 depicts an embodiment of an offshore structure incorporating an embodiment
of the invention. The offshore structure, in some embodiments, may be a floating liquefied
natural gas production unit ("FLNG"). In other embodiments, the offshore structure
is a liquefied petroleum gas production unit ("FLPG"). Often, the offshore structure
produces both liquefied natural gas (LNG) as well as liquefied petroleum gases (LPG)
and/or other condensate(s). For the purpose of this specification, any floating unit
producing LNG and/or LPG will be referred to as an FLNG/FLPG structure. The invention
may also be incorporated in an offshore structure capable of receiving of, and optionally
storing and/or processing of, LNG or LPG or other hydrocarbon fluids from a transport
carrier. An example of such a unit for receiving, storing and regasification of LNG
is a floating storage regasification unit ("FRSU").
[0014] The offshore structure is intended to allow transport carriers, such as for instance
LNG and/or LPG carriers (LNGC and/or LPGC), to berth directly alongside the structure
and unload and/or load hydrocarbon fluids such as LNG and/or LPG. Such a LNGC or LPGC
may be moored alongside the starboard side of the offshore structure, optionally with
Yokohama floating fenders protecting each of the (steel) hulls from contact with one
another. Mooring of an LNGC or a LPGC with the offshore structure may be accomplished
using mooring lines. The mooring lines are deployed from the LNGC/LPGC to the offshore
structure in order that the carrier and the offshore structuer be moored alongside
one another for transfer of the hydrocarbon fluids.
[0015] In the present example, the offshore structure 1 will be referred to as FLNG/FLPG
structure 1 although the basic principles of the invention are not limited to this
type of offshore structure. The FLNG/FLPG structure 1 has an outer hull having two
longitudinal sides 2. Bow and stern sections join the longitudinal sides 2 to from
the outer perimeter of the outer hull. Although not necessary for the invention, the
outer hull, when viewed in cross-section from above, is in this particular embodiment
a substantially obloid structure, with two parallel longitudinal sides 2, and curved
bow 12 and stern 13 sections. In alternative embodiments, the stern and/or bow sections
may for instance be a straight hull section between the longitudinal sides of the
structure.
[0016] The FLNG/FLPG structure 1 may be moored at a mooring point, for instance in the form
of a turret 17, at the bow 12. Preferably, the FLNG/FLPT structure 1 is capable of
weathervaning around the mooring point.
[0017] At least one fairlead 3 is positioned adjacent a longitudinal side 2. A hook 4 is
connected to the structure 1 and located inboard relative to the fairlead 3 such that
the angle of displacement α between a tangent to the outer hull 5 at the centre of
the fairlead 3 and the hook 4 is less than or equal to 45°.
[0018] Selecting the angle of displacement to be less than or equal to 45°, allows for using
longer mooring lines than is the case in a conventional alongsides mooring arrangement,
whilst the mooring arrangement consumes considerably less lateral depth between the
fairlead and the hook. Herewith it is possible to avoid needing a special hook with
shock absorbing properties, or at least to reduce the shock absorbing capacity, because
the mooring line can be long enough to absorb shocks in the conventional way.
[0019] The saving in lateral space between the fairlead and the hook may translate in reducing
the inboard space required for the mooring deck within the limits of the outer hull
of the structure, which is particurly welcome for an FLNG/FLPG structure or an FSRU
structure since the deck space is needed for top sides hydrocarbon processing equipment.
Alternatively, the space saving may be translated in reducing the need for outboard
space (i.e. protruding out of the perimeter of the hull), which is considered to enhance
the safety during approach operations of a transport carrier to the offshore structure.
[0020] Further still, by providing for a greater line length inboard of the fairlead over
conventional mooring arrangements, the offshore mooring arrangement of the invention
emulates the stability of an on-shore mooring arrangement within a more limited special
environment. An angle of displacement less than 45° allows for a longer pennant than
in prior known systems, thereby allowing the offshore mooring arrangement to more
closely resemble an onshore mooring arrangement in which a more secure mooring is
provided due to the longer pennant length applicable to the mooring arrangement.
[0021] When referred to herein, the "pennant" is the tail end of a mooring line or rope.
The tail end of the mooring line or rope is the end which is attachable to the hook
on the offshore structure or a hook on a liquefied natural gas carrier and/or a liquefied
petroleum gas carrier.
[0022] Preferably, the distance from the centre of the fairlead to the hook is in the range
of from 10 to 22 metres. This exceeds the distance of conventional arrangements, which
rely on an 11 metre pennant length and therefore typically do not employ hooks in
excess of 10 m away from the fairlead. The additional available length in excess of
10 m is advantageous for shock absorbing as in offshore alongside mooring the length
that need to be brided by the mooring lines from the offshore structure to the transport
carrier is relatively small. An inboard length of about 15 m is typically recommended,
i.e. the distance from the centre of the fairlead to the hook is preferably about
15 m.
[0023] The angle of displacement α is preferably less than or equal to 25°. Herewith it
is possible to maintain approximately 15 m of mooing line length between the fairlead
and the hook while consuming less than 7 m of lateral mooring deck depth. This is
considered an acceptable lateral space for a mooring deck on an FLSO/FLPG.
[0024] On the other hand, the angle of displacement α is preferably at least 4°. Maintaining
at least 4° ensures that a mooring line, passing through the fairlead to the associated
hook approximately 15 m away from the fairlead, is displaced sufficiently inboard
of the longitudinal side of the structure to enable safe operation of the hook by
an operator who may thus be positioned away from the longitudinal side of the structure
during the mooring operation. Moreover, the stress on the mooring line is within operating
limits by way of ensuring that the mooring line passing through the fairlead does
not turn sharply towards the longitudinal side as would be the case with an angle
of displacement less than 4°.
[0025] The hook and the fairlead may as shown in Figure 1 may be provided onto a mooring
deck. Figure 2 shows a schematic plan view of an example of such a mooring deck 7
being positioned within the perimeter of the outer hull 2 of the FLNG/FLPG 1. The
fairlead 3 is located on the mooring deck at the longitudinal side of the outer hull
2, and the associated hook 4 is located on the mooring deck, inboard of the longitudinal
side of the outer hull 2 and offset from the position of the fairlead 3 longitudinally
of the hull of the FLNG/FLPG. An operator, heaving a mooring line deployed from a
LNGC/LPGC to the FLNG/FLPG onto the mooring deck for attachment to the hook 4, will
be positioned away from the longitudinal side of the outer hull 2 and is, therefore,
protected from the elements and from falling overboard from the FLNG/FLPG.
[0026] As generally depicted in Figure 2, the inboard lateral displacement of the hook 4
from its associated fairlead 3 is in the range of 1 to 5 metres to permit safe access
to the hook set inboard of the outer hull 2 of the FLNG/FLPG. In a preferred FLNG/FLPG
of the invention, the inboard lateral displacement of the hook from its associated
fairlead is 3 metres. The lateral width of the mooring deck 7 itself may be within
a range of from 1 to 7, preferably of from 1 to 6 meters.
[0027] In some embodiments, as generally depicted in Figure 3, the mooring deck 7 is bounded
by the outer hull 8 on two sides 8a, 8b and is open at the longitudinal side of the
outer hull 8c. The outer hull 8c may have a guard means 9, which may be a guard rail,
a rope, a steel panel wall, or combinations thereof, at the open longitudinal side
thereof. The operator is, thereby, further protected from both the elements and from
the risk of a man overboard scenario.
[0028] The FNLG/FLPG depicted in Figure 3, further shows the mooring deck 7 with a hook
4 located thereon and a fairlead 3 located at the longitudinal side of the outer hull
8. The mooring deck 7 in this embodiment is contained within the side ballast tanks
10 of the FLNG/FLPG, therefore the cargo carrying capacity of the FLNG/FLPG is not
diminished or comprised by the presence of the mooring deck 7 within the bounds of
the outer hull 8. Further, the mooring deck being provided in a recess in the outer
hull line provides additional safety for an operator heaving a mooring line onto a
hook than would be the case with an exposed decking arrangement.
[0029] By way of providing a mooring deck within the outer hull of the FLNG/FLPG, and integral
with the structure, the FLNG/FLPG has a clean hull line and no deck protrusions therefrom.
Thus, the risk of a protrusion to the outer hull being broken off due to collision
with a LNGC/LPGC is reduced or even eliminated.
[0030] The invention may be implemented on an FLNG/FLPG that is provided with one or more
LNG/LPG storage tanks, preferably insulated tanks to hold LNG or LPGs in a cryogenic
state. In some embodiments, the offshore structure has a length that is at least equal
to a length required to provide sufficient berthing alongside the offshore structure,
for a transport carrier, e.g. a tanker, having capacity of greater than about 200,000
cubic metres.
[0031] Mooring lines may lead from the carrier to fairleads 3 and associated mooring hooks
4 on the FLNG/FLPG as depicted generally in Figure 2. The hook may be a quick release
hook operable to receive the mooring line from the FLNG/FLPG side fairlead. The quick
release hook may incorporate a powered capstan to heave a mooring line from an LNGC/LPGC
to the FLNG/FLPG structure. In some embodiments, the hook will be a twin set quick
release hook with an associated powered capstan set.
[0032] In embodiments where all of the mooring lines are passed by the LNGC/LPGC to the
FLNG/FLPG, the remote operation of the quick release hooks facilitates release of
the carrier vessel from the offshore structure in a single operation in the event
of an emergency.
[0033] The hook may comprise a mooring line monitoring load cell. The load cell will be
operable to transmit data pertaining to the load applied by the mooring line to the
hook to the FLNG/FLPG control room, thereby providing a real time indication and recording
of the mooring loads being applied to each and all of the hook sets. Remote release
of the mooring hooks may be provided from the control room. Release of the mooring
line from the hook under tension may be provided.
[0034] Mooring line load forces should preferably be kept below about 55% of the Minimum
Breaking Load. Increasing mooring line length by leading lines through fairleads 3
on the FLNG/FLPG to remote Quick Release Hooks (QRH) may cause chafing. In some embodiments,
mooring line flexibility may be in a nylon tail pennant. A mooring line may lead directly
from the carrier, through a fairlead 3 to an associated hook 4 as depicted in Figure
2. Mooring lines may be designed to comply with OCIMF guidelines.
[0035] In some embodiments, mooring line flexibility is in the tail pennant. Exemplary materials
suitable for use in a tail pennant are polyamide and PET, for example. It is generally
appreciated that longer tail pennant lengths reduce line loading and increase fatigue
life. The use of more elastic tail materials may increase the line life of the main
mooring line due to decreased line loads.
[0036] A mooring line length of at least about 15 metres between the fairlead 3 and the
QRH may ensure the nylon pennant and joining shackle are clear of the ship's fairlead
and not subjected to chafing. In an embodiment, the minimum safe working load of each
mooring hook may be more than the minimum-breaking load of the strongest mooring line
anticipated. In some embodiments, the operational mooring line may not exceed the
greater of 2.5 times the winch brake holding capacity or 2500 KN. The extreme mooring
load may not exceed the greater of 2.5 times the minimum breaking load line or 3125
KN. The capstan barrel may be at a suitable height to permit safe handling of messenger
lines. The QRH-assembly may be electrically isolated from the platform decks. The
insulation may provide an electrical resistance of at least about 1 mega-Ohm.
[0037] The QRHs may be positioned on the FLNG/FLPG. The mooring lines may lead from the
carrier to the fairleads 3 and the QRHs on the FLNG/FLPG. Decks may have rounded edges
in front of the mooring hooks to prevent chafing of the mooring lines. In some embodiments
the at least one fairlead is connected to the structure. The at least one fairlead
may be connected to the structure adjacent a longitudinal side.
[0038] The fairlead may be an open fairlead having an open top. However, a closed fairlead
may be preferred, e.g. of the so-called Panama type, in order to avoid the mooring
line to be lifted out of the fairlead. This may happen when the relative free boards
between the offshore structure and the carrier vary from case to case. whereby a mooring
line may be passed through the opening in the fairlead. Particularly, a Panama design,
closed fairlead may be fitted at the longitudinal side of the FLNG/FLPG to accept
the LNGC/LPGC mooring lines and to direct the mooring lines inboard of the longitudinal
side to a quick release mooring hook situate on the mooring deck of the FLNG/FLPG.
The specification of the fairlead is preferably consistent with the safe working load
(SWL) of the quick release hook set. Exemplary SWLs are 125t, SWL, considering the
minimum breaking strain (MBL) of the LNGC/LPGC mooring lines.
[0039] In some embodiments, a single fairlead may exclusively service one single mooring
line and hook.
[0040] Fairleads may be mounted in abutment with, and faired to the FLNG/FLPG longitudinal
side. The fairlead will preferably have no protuberances or sharp edges on either
the inboard or outboard sides in order to avoid additional wearing on the mooring
line as it passes through the fairlead.
[0041] A Panama design closed fairlead will typically have a base portion attachable to
the offshore structure. The fairlead will be in the form of a ring-shape extending
upwards from the base portion and having an annular hole in the centre thereof. The
annular hole is generally oval. The base portion is typically 1300mm in length and
560mm in width. From the base portion to the centre of the annular hole is typically
505 mm with the fairlead being 930mm in height. The annular hole is typically 450mm
in height and 600 mm in length, with a radius of approximately 225 mm. The internal
radius of the fairlead will be as large as practicable to reduce stress on the mooring
line.
[0042] The fairlead may be lined with a friction reducing agent or material. The friction
reducing agent may be a Nylacast
™ protective insert for reduction of chafing of a synthetic mooring line pennant. The
Nylacast
™ material is a synthetic material incorporating a lubricant that will minimise chafing
damage to the LNGC/LPGC mooring pennants in the fairlead.
[0043] Monitoring systems may be provided on the offshore structure, to detect the carrier
speed of approach; mooring line loads through strain gauges on QRHs; and/or pressure
monitoring system in air block fenders. Data from the monitoring systems may be centrally
collected and displayed in a control room.
[0044] An embodiment of an FLNG/FLPG incorporating the invention is depicted in Figure 4.
The FLNG/FLPG 1 comprises a plurality of fairleads 3 and associated hooks 4. In the
depicted embodiment the fairleads 3 are spaced along the outer hull 8 of the FLNG/FLPG
and are configured so as to provide mooring points for a liquefied natural gas carrier
11 and/or a liquefied petroleum gas carrier 11. For ease of reference, the fairleads
may be numbered from bow to stern using letters from the alfabeth, in the present
case ranging from A to P. A mooring deck may be defined along the starboard side of
the hull (assuming the FLNG/FLPG is moored at its bow) to accommodate side fairleads
3 and mooring hooks 4 generally configured as shown in Figure 4. The mooring arrangement
of the FLNG/FLPG may typically comprise mooring lines deployed from the LNGC/LPGC.
Typical mooring lines may include headlines, sternlines, breast and spring lines.
[0045] The FLNG/FLPG may comprise a loading/offloading connection for transferring the hydrocarbon
fluid(s). The connection may comprise a manifold of one or more transfer arms. Such
manifolds
per se are known, and typically an LNG transfer manifold comprises three arms of which the
middle one is a vapour arm. An LPG transfer manifold may also comprise one or more
transfer arms, but typically a vapour transfer line is combined on an other arm such
that no separate vapour arm is provided. The tranfer arms may be Chiksan unloading
arms available from FMC Energy Systems. Typical LNG/LPG transfer equipment may include
power packs, controls, piping and piping manifolds, protection for the piping from
mechanical damage, ship/shore access gangway with an operation cubicle, gas detection,
fire detection, telecommunications capabilities, space for maintenance, Emergency
Release Systems (ERS), Quick Connect / Disconnect Couplers (QCDC), monitoring systems,
and/or drainage systems.
[0046] Assuming mooring on a turret at the bow, the loading/offloading manifold may be suitably
located for safety reasons between the offshores structure's mid and aft in order
to be removed from as far aft as possible from the process equipement and the turret.
Ideally, however, the manifold is also further removed from any crew's quarters on
the offshore structure than is the case for a typicall transport carrier.
[0047] For the purpose of illustrating the invention, it is assumed that the transfer manifold
comprises both an LNG transfer manifold as well as an LPG manifold. In the examples
below, it is assumed that the LNG vapour arm is situated on the FLNG/FLPG at 185 metres
from the stern of the structure, with the LPG connection being located some 10 m aft
of the LNG vapour arm. Thus, LNGC/LPGC manifold offsets (relative to midships point
of the carrier) of up to 25 metres are accommodated. The position of the fairleads
3, referenced to the LNG/LPG vapour arm location, may be configured so as to be capable
of receiving mooring lines from an LNGCs ranging from 75,000 cubic metres to 217,000
cubic metres capacity, and from LPGCs ranging from 74,000 cubic metres to 84,000 cubic
metres capacity.
[0048] With reference to Figures 5a, 5b, 6a and 6b, in certain embodiments of the invention,
the locations (A to P) of the mooring points, represented by fairleads 3, relative
to the LNG vapour arm 6 are in accordance with
Table 1.
Table 1 |
Lead |
Distance to LNG vapour arm |
Number of leads/hooks |
Direction of lead to hook |
Distance from lead to hook |
A |
175.0 |
Double |
Forward |
15m |
B |
155.0 |
Double |
Forward |
15m |
C |
140.0 |
Double |
Forward |
15m |
D |
120.0 |
Double |
Forward |
15m |
E |
100.0 |
Double |
Forward |
15m |
F |
70.0 |
Double |
Towards vapour |
15m |
G |
40.0 |
Double |
Towards vapour |
15m |
H |
20.0 |
Double |
Towards vapour |
15m |
I |
-30.0 |
Double |
Towards vapour |
15m |
J |
-55.0 |
Double |
Towards vapour |
15m |
K |
-80.0 |
Double |
Towards vapour |
15m |
L |
-110.0 |
Double |
Towards vapour |
15m |
M |
-125.0 |
Double |
Towards vapour |
15m |
N |
-145.0 |
Double |
Inboard |
15m |
O |
155.0 |
Triple |
Inboard |
15m |
P |
-175.0 |
Triple |
Inboard |
15m |
[0049] A positive value for the distance between the mooring point (A to P) and the LNG
vapour manifold in Table 1 indicates a mooring lead location forward of the LNG vapour
arm (A to H), and a negative value indicates a mooring lead location aft of the LNG
vapour arm (I to P).
[0050] In the embodiment of Table 1, each lead location accommodates multiple fairleads.
Lead locations A to N each accommodate two fairleads and lead locations O and P each
accommodate three fairleads. The direction of lead to hook is indicated, being the
direction the mooring line will be led when passing through the fairlead to the associated
hook set. The aft three sets of leads N, O and P, situated within the accommodation/service
area of the FLNG/FLPG, will be led inboard on the FLNG/FLPG to the associated hook
set. In the embodiment described in Table 1, the fairlead to associated hook distance
is 15 metres. This distance enables the total LNGC/LPGC mooring line length deployed
to be of sufficient length, in combination with the mooring line nylon/polyester pennant,
to absorb peak mooring loads in sea states up to 3 m sea height whilst moored.
[0051] In some embodiments an 11 m tail pennant length will be sufficient, however, in certain
embodiments a 22 m tail pennant is preferred.
[0052] In embodiments of the invention wherein the lead to hook distance is 15 metres and
the pennant length is 22 metres, the 15 metre lead to hook distance allows the 22
metre mooring pennant to be located in the FLNG/FLPG side lead thereby avoiding conflicting
damage of alternate LNGC/LPGC synthetic HMPE and wire mooring lines placed in the
same leads. In addition, the 15 metre lead to hook configuration will provide for
a greater mooring line length (LNGC/LPGC winch to FLNG/FLPG hook) than would typically
be the case. In a conventional shore terminal berth, a mooring line length of 35 to
50 metres is typical, which mooring line length typically is not available in a conventional
offshore side-by-side mooring configuration.
[0053] In order to maximise the length of mooring line inboard of the fairlead, the distance
from fairlead to associated hook is preferably as large as possible.
[0054] As can be seen in Figures 5a, 5b, 6a, 6b, the mooring arrangement allows for alongside
berthing of a variety of carrier types and sized while being able to align the carrier's
loading/offloading manifold with the corresponding manifold on the FLNG/FLPG structure,
even where the carrier's manifold is not located in the centre of the carrier.
[0055] Figures 5a and b depict an FLNG mooring layouts with mooring points A to P, showing
that the FLNG is capable of alongside berthing of 290 metre long LNGCs 11 having their
loading/offloading manifolds at different positions relative to the carrier's centre.
In Figure 5a the manifold 16 is at the carrier's centre c, but in Figure 5b the manifold
16 is some 15 metres ahead of the carrier's centre c. In both cases, a good alignment
between the carrier's manifold 16 and the target represented by the LNG vapour arm
6 is achievable.
[0056] Trials using Optimoor analysis tool have shown that even of the LNG manifold on a
290 metre long LNGC is displaced by 24 metre ahead of the carrier's centre line, the
carrier can be safely berthed alongside the mooring arrangement. In the latter case,
mooring point P is employed. The mooring arrangement is also capable of receiving
carriers having their LNG manifold aft of the carrier's centre line, but this situation
may be rare in view of the industries desire to remove the manifold as far as possible
from the crew's quarters.
[0057] In the embodiments depicted in Figures 6a and 6b, an offshore structure 1 is shown
with a mooring layout having mooring points A to P, capable of alongside mooring of
LPGCs having lengths of 203 metres and 214 metres, respectively. The carrier's LPG
loading/unloading manifolds 16' are in the shown cases relatively close to the carrier's
centre lines c, but the target manifold 6' on the offshore structure 1, being the
LPG connection, is some 10 m aft of the LNG vapour arm 6. Again, good alignment is
achievable.
[0058] Thus, an offshore structure 1 accommodating the mooring arrangement as proposed is
adapted to accommodate LNGCs and/or LPGCs of widely differing sizes and designs, including
manifold offsets.
[0059] In certain embodiments an FLNG/FLPG accommodating the invention may comprise at least
three spring fairlead and associated hook sets. Thus, the mooring layout of the FLNG/FLPG
accommodates spring deployment from an LNGC/LPGC. Where weather conditions require,
an LNGC/LPGC may deploy two springs from the main deck and one spring from the focsle
deck, with a consequential requirement for the corresponding spring lead and hook
sets on the FLNG/FLPG.
[0060] Figure 7 shows another example of an FLNG/FLPG 1 being anchored to turret 17 at its
bow 12. A mooring deck 7 is provided on the starboard side of the FLNG/FLPG's outer
hull. As is generally depicted in Figure 7, the mooring deck will run from the stern
of the FLNG/FLPG 1 forward for approximately 380 metres and at 14.5 metres above the
operating waterline. In some embodiments, the FLNG/FLPG 1 may have a mooring deck
having a width in the range of 1 to 6 metres to permit safe access between the back
of the mooring hook sets and the fore and aft bulkhead, to permit sufficient space
to gather the mooring messenger when an operator is heaving mooring lines 20 to the
FLNG/FLPG, e.g. with a powered capstan. In an embodiment, the FLNG/FLPG may have a
mooring deck with a width of 4 metres. As is further illustrated in Figure 7, a number
of fenders 22 are positioned between the FLNG/FLPG and the LNGC/LPGC to prevent damage
occurring during the berthing and mooring of the carrier 11 alongside the offshore
structure 1.
[0061] An offshore structure, e.g. in the case of an FLNG/FLPG, may accommodate LNG/LPG
storage tanks and allow LNG/LPG vaporization equipment and/or other process equipment
and utilities such as liquefaction equipment, gas treating equipment (e.g. acid gas
removal equipment, dehydration equipment, mercury removal equipment and the like),
gas intake separators and slug catchers, condensate stabilization equipement, etc.,
to be positioned on an upper surface of the FLNG/FLPG, and safely enable LNGCs/LPGCs
to berth directly alongside the FLNG/FLPG. It is also envisaged to provide equipment
for carbon (carbondioxide) capture and sequestration equipment to handle CO
2 removed from post combustion flue gases and/or from the hydrocarbon feed.
[0062] In the case of an FSRU, the offshore structure may accommodate revaporization, heating
value control, and metering equipment and other options such as described in for instance
WO 2006/052896, the disclosure of which is incorporated herein by reference.
[0063] An external turret system 17 may be a preferred option for anchoring the offshore
structure in a typical water depth of greater than 30 metres. An external turret may
be preferable to a Yoke Mooring System but may be dependent on water depth and may
require a complete riser design as part of the concept selection. A double hump riser
configuration may be a feasible arrangement.
[0064] A further mooring system of an FLNG/FLPG of the invention may be a weathervaning
arrangement to obtain a sufficiently high connecting threshold for the berthing operations
of an LNGC/LPGC. A further mooring system and high pressure gas export line may be
located at the forward end of an FLNG/FLPG of the invention. After selection of the
location of an FLNG/FLPG of the invention, an assessment should be made of the technical
feasibility of a further mooring system comprising, for example, an external turret
system, an internal turret system, a Yoke Mooring System (YMS), and combinations thereof.
An example YMS comprises, for example: a jacket (the jacket may comprise a four legged
tubular structure that may be fixed to the seabed via one or more, generally four,
piles, driven through the corner tubulars), a mooring head (a mooring head may be
located on top of the jacket and may be free to rotate; the mooring head may support
the pipe work and equipment, including the swivel stack), a yoke (a yoke may be a
tubular triangular frame that may be connected to the mooring head via a roll and
pitch articulation; permanent ballast tanks may be a part of the yoke structure to
provide the required pretension in the mooring legs), mooring legs (the mooring legs
may comprise tubular steel members connected to the adjacent structure via uni-joints;
an axial thrust bearing may also be included to provide rotational freedom; the mooring
legs with the yoke weight suspended underneath may provide the pendulum mechanism
of the mooring system), a mooring structure on the FLNG/FLPG (a mooring structure
on the FLNG/FLPG may comprise a tubular frame mounted onto the bow of the FLNG/FLPG;
the structure may overhang the bow of the vessel to provide clearance for the yoke;
lifting means may be provided for handling of one or more jumper hoses), gas transfer
may be performed via one or more, generally two, 16" flexible jumper hoses that may
provide a 2x 100% capacity.
[0065] A further mooring system comprising a YMS may include a gas swivel to transfer send-out
gas from the weathervaning FLNG/FLPG to a fixed pipeline riser. An in-line swivel
may be expected to provide sufficient reliability (typical MTTF of 20 Years) but an
'N+l' arrangement of the fluid transfer system may be obtained through additional
toroidal swivel modules. The in-line swivel may be used for operation; the toroidal
module may provide the back-up. In case of failure, the in-line may be changed out
while the send-out gas may be routed through the toroidal swivel path.
[0066] In some embodiments, the height of the upper surface, on which mooring equipment,
for example, quick-release hooks (QRHs) are disposed, above the surface of the body
of water may be such that an angle of mooring lines extending from the mooring equipment
to the liquefied natural gas carrier coupled to the body is less than about 30 degrees.
[0067] The centerline of the unloading arms may be positioned to create a maximum degree
of protection for all types of common LNG/LPGC/LPGCs.
[0068] Although a three-unloading arm concept as mentioned above may be technically acceptable,
a four- unloading arm concept may have more redundancy. Redundancy may increase the
integrity and/or reliability level. The spare unloading arm may be used on a day-to-day
basis. This may safeguard the proper functioning of the equipment. The installation
of one or more spare unloading arms may increase the normal overall LNG/LPG loading
capacity. The design of the FLNG/FLPG may account for severe weather conditions.
[0069] Transfer of LNG/LPG between a LNGC/LPGC and a FLNG/FLPG may be based on traditional
hard arms, which are currently used at onshore terminals for ship-shore LNG/LPG transfers.
To enable safe and reliable connecting and disconnecting under seaway motions, for
floating-to-floating transfer, a guide-wire system may be utilized to guide the loading
arm to the ship manifold.
[0070] A suitable overall length of the offshore structure on which the invention is applied
may be any length that provides for storing and/or processing of the hydrocarbon fluids
such as LNG/LPG as described herein, and is generally at least about 100 metres, specifically
at least about 200 metres, more specifically at least about 300 metres, and generally
no more than about 1000 metres, specifically no more than about 750 metres, and more
specifically no more than about 500 metres.
[0071] A suitable breadth of the offshore structure may be any breadth that provides for
storing and/or processing of hydrocarbon fluids, such as LNG/LPG as described herein,
and is generally at least about 20 metres, specifically at least about 30 metres,
more specifically at least about 40 metres, and generally no more than about 300 metres,
specifically no more than about 200 metres, and more specifically no more than about
100 metres.
[0072] A suitable draft of the offshore structure may be any draft that provides for storing
and/or processing of hydrocarbon fluids, such as LNG/LPG as described herein, and
is generally at least about 5 metres, specifically at least about 7 metres, more specifically
at least about 10 metres, and generally no more than about 25 metres, preferably no
more than about 20 metres. In one embodiment, the draft may be about 17.6 m.
[0073] A suitable length:depth ratio of the offshore structure may be any length:depth ratio
that provides for storing and/or processing of LNG/LPG as described herein and is
generally at least about 5, specifically at least about 7, more specifically at least
about 10, and generally no more than about 20, specifically no more than about 18,
and more specifically no more than about 15.
[0074] An example further mooring system of the offshore structure may be a Yoke Mooring
System ("YMS"), because the water depth for an inshore location may be in a range
of from about 15 metres to about 30 metres and may not allow for the catenary of an
external turret system. Maximum sea states should be obtained to ensure that the further
mooring system utilized can meet such maximum sea states.
[0075] An example approach procedure of an LNGC/LPGC to an FLNG/FLPG of the invention may
include: about 12 hours before the estimated time of arrival ("ETA"), prevailing weather
conditions and status of both FLNG/FLPG and LNGC/LPGC are exchanged; preparations
are made, for example testing of LNG/LPG arms, mooring equipment, fenders and selecting
LNGC/LPGC approach, about 1 hour before ETA, the LNGC/LPGC will arrive at the agreed
entry point, at some 2 to 3 nautical miles from the FLNG/FLPG and with a forward speed,
typically 4 knots; berthing master will board and tugs are ready to be connected,
the LNGC/LPGC will head for a position off the starboard side of the FLNG/FLPG and
target to come to complete stop near parallel to the FLNG/FLPG, at approximately 100
m separation, the LNGC/LPGC will move side-ways, whilst monitoring the applied thruster/tug
forces, heading relative to the FLNG/FLPG and approach velocity; if control over the
LNGC/LPGC position and heading becomes difficult, the approach will have to be aborted,
and pneumatic equipment may be used from the FLNG/FLPG to shoot across messenger lines.
It may be expected that mooring lines will be passed after touching the fenders.
[0076] Currently, the significant wave height limit (Hs) for berthing of an LNGC/LPGC alongside
an FLNG/FLPG may be considered to be in the range of from about 2.0 to about 2.5 metres,
and in the range of from about 2.5 to about 3.0 metres for being moored alongside
an FLNG/FLPG of the invention.
[0077] An example departure manoeuvre looks very much a mirror image of the example approach
process. At the start of the actual departure, the ESD link systems is disconnected,
with radio links maintaining the integrated systems needed for a safe departure. The
LNGC/LPGC prepares to start the departure manoeuvre. Then the mooring lines are disconnected,
which may be implemented one by one, depending on prevailing weather conditions and
final operating procedures.
[0078] An example departure manoeuvre may see the LNGC/LPGC moving the bow clear from the
FLNG/FLPG using tugs or carrier bow thruster in combination with wind/wave/current
conditions. When the hulls are clear of each other the LNGC/LPGC will use its main
propulsion system to move clear and tugs will disconnect.
[0079] An LNGC/LPGC may be moored in the furthest forward and aft positions. A mooring arrangement
of the present invention may be utilised together with breast lines, spring lines.
[0080] The description above has made apparent that an offshore structure with a mooring
arrangement as described, transport carriers of different sizes and designs can be
effectively moored in most sea states up to 2.5 or 3.0 m significant wave height.
[0081] Moreover, the mooring arrangement mimics an on-shore mooring arrangement; that is
to say, the moorings deployed from the transport carrier are in a similar plan arrangement
to that provided in an on-shore terminal.
[0082] While the invention is susceptible to various modifications and alternative forms,
specific embodiments thereof have been shown by way of example in the drawings and
have been described in detail. It should be understood that the drawings and detailed
description thereto are not intended to limit the invention to the particular form
disclosed, but on the contrary, the intention is to cover all modifications, equivalents
and alternatives falling within the spirit and scope of the invention as defined by
the appended claims.
1. An offshore structure (1) comprising an outer hull having longitudinal sides (2) and
at least one fairlead (3) located adjacent a longitudinal side (2), and a hook (4),
connected to the structure (1) and located inboard relative to the fairlead (3) and
being displaced along the hull longitudinally from the fairlead (3), such that an
angle of displacement (α) between a tangent to the outer hull (5) at the centre of
the fairlead and the hook (4), is less than or equal to 45°.
2. An offshore structure (1) according to claim 1, wherein the distance from the centre
of the fairlead (3) to the hook (4) is in the range from 10 to 22 metres, preferably
about 15 metres.
3. An offshore structure (1) according to any one of claims 1 to 3, wherein the angle
of displacement (α) is less than or equal to 25°.
4. An offshore structure (1) according to any one of claims 1 to 4, wherein the angle
of displacement (α) is at least 4°.
5. An offshore structure (1) according to any one of claims 1 to 4, wherein the inboard
displacement of the hook (4) from the centre of the fairlead (3) is in the range from
1 to 5 metres, preferably about 3 metres.
6. An offshore structure (1) according to any one of claims 1 to 5, wherein the hook
(4) and the fairlead (3) are located on a deck (7), which deck is within the perimeter
of the outer hull (8).
7. An offshore structure (1) according to claim 6, wherein the deck (7) is bounded by
the outer hull (8) on two sides (8a, 8b) and is open at the longitudinal side of the
outer hull (8c), and further wherein the outer hull has a guard means (9) at the open
longitudinal side (8c).
8. An offshore structure (1) according to claim 6 or claim 7, wherein the deck (7) width
is within a range of 1 to 6 metres.
9. An offshore structure (1) according to any one of claims 1 to 10, wherein said structure
is a floating liquefied natural gas production unit or a floating liquefied petroleum
gas production unit.
10. An offshore structure (1) according to claim 9, wherein the deck (7) is bounded by
the outer hull (8a, 8b) on two sides and is open at the longitudinal side of the outer
hull (8c), and further wherein the outer hull has a guard means (9) at the open longitudinal
side (8c) and the deck width is within a range of 1 to 6 metres and contained within
the side ballast tanks (10) of the floating liquefied natural gas production unit
or the floating liquefied petroleum gas production unit.
11. An offshore structure (1) according to any one of claims 1 to 10, wherein the fairlead
(3) is lined with a friction reducing agent or material.
12. An offshore structure (1) according to any one of claims 1 to 11, comprising a plurality
of fairleads (3) and associated hooks (4), the fairleads (3) being spaced along the
outer hull (8) and being configured so as to provide mooring points (A to P) for a
liquefied natural gas carrier (11) and/or a liquefied petroleum gas carrier (11) having
a capacity of between 75000 cubic metres and 217000 cubic metres.
13. A mooring arrangement comprising at least one mooring line (20) having a pennant at
an end thereof, and an offshore structure according to any one of claims 1 to 12,
wherein said at least one mooring line (20) is configured to pass inboard of the outer
hull through said fairlead and is attachable to the hook such that the angle of displacement
of the at least one mooring line between the tangent to the hull at the centre of
the fairlead and the hook is less than or equal to 45°.
14. A mooring arrangement according to claim 13, wherein the pennant is 22 metres in length.