Note: Descriptions are shown in the official language in which they were submitted.
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Method and system for fluid transport between ships
There are several reasons to transfer fluid such as oil or gas between two
tankers in the open sea, and by having available good and flexible solutions for a such
5 fluid or load transfer, a better employment of the fleet will i.a. be possible. A good
load transfer flexibility will particularly be valuable for a fleet in which the individual
ships have different types of loading/discharging equipment on board.
Until now known methods to transfer fluid between tankers in the open
sea have turned out problematical during so-called marginal weather conditions (by
10 significant wave heights over 6 feet [2 m]). The reason is as follows: If two ships
to be interconnected for load transfer are moving by their own forward machinery to
keep a minimum steering speed (the machinery in position "forward, slow"), the
forward or rearward speed still may be at least approx. 5 knots, and therefore it will
not be recommendable to make the connection and carry out the subsequent load
15 transfer between the ships, since the risk for collision would be too great. If, on the
other hand, the ship machinery is deactivated, the ships could easily break adrift in
an uncontrolled manner and end up being laterally exposed against waves and wind,
and even then it will most often be problematic to arrange a load transfer from one
ship to another.
Differing ways of transporting fluid, such as by fuel supply to military
vessels neither can be used advantageously for tankers, because the dimensions of
ships and tankers and their loading hoses are far heavier, thereby leading to more
serious consequences in the event of a possible direct contact between the ships.
On this background the present invention is proposed, said invention
25 being directed towards a method for fluid transport between ships, particularly loading
of hydrocarbons (HC) between tankers in the open sea and allowing the loading to be
effectuated securely and effectively. The invention further comprises a drift anchor
system to allow fluid transport between two ships in the open sea. In the following
paragraphs the invention will be described more closely with reference to the
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accompanying drawings, wherein:
Fig. 1 demonstrates a connection between two tankers in order to carry
out fluid transport according to the invention, Fig. 2 illustrates a drift anchor system
on one of the ships, and Fig. 3 shows schematically the drift anchor system in
5 activated condition.
Referring to Fig. 1 there is shown a first ship 1 having a loading hose 2
at its stern and a reel 3 or a similar storing device for the loading hose, which also
may be stored in a horizontal support. At the stern there is also provided a mooring
hawser 4. Further is illustrated the bow portion of a second ship 5, moored to the
10 first ship 1. The second ship is preferably a buoy loading ship having excellent
manoeuvring properties due to a system for dynamic positioning (DB) for the control
of trusters forward and aft and turnable main propellers. When the DP system is
activated the mooring hawser 4 has the function of an extra safety mechanism
between the ships. A typical distance between the ships may be 80 - 100 m during1 5 mutual mooring.
The free end of the loading hose 2 from the first ship is connected to a
bow loading system 6 of traditional type on the other, second ship 5.
At its bow portion the first ship 1 is provided with a drift anchor system
7 (Fig. 2) comprising: A bottom weight having a rounded shape, without anchor
20 hooks and secured to the lower free end of a drag chain 9, an anchor wire 10 having
a considerable length, for example 1000 - 2000 m and being secured to the upper end
of said drag chain, a winch 11 mounted on deck and adapted for reeling of the anchor
wire, and a vertical or inclined chain shaft extending from the ship deck and down to
its bottom.
When the drift anchor system is inactivated, the winch 1 1 is keeping the
anchor wire 10 reeled, while the drag chain 9 is hanging vertically and extendedwithin the chain shaft 12. The winch pull on the wire may be slightly greater than the
combined weight of bottom weight 8 and drag chain, or the upper end of said chain
may be secured under tension by means of a stopper 13 on the deck, in which case30 the winch is relieved. In both situations the bottom weight 8 will be drawn with a
certain force
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against the shaft mouth 14 in the ship bottom in order to close the shaft. For this
reason the bottom weight may be conical or spherical at least at its upper portion, and
the shaft mouth 14 will be correspondingly complementary shaped.
During use the first ship forward machinery is stopped, the drag chain
5 with bottom weight connected is lowered down to the sea floor, and the anchor wire
is reeled out gradually when the ship breaks adrift and turns into the direction of the
wind and sea current. The turning will partly be under control due to the friction of
the drift anchor system. Fig. 3 illustrates this situation, and it is obvious that also a
portion of the anchor wire will be positioned along the sea floor and slowly being
10 dragged forwards. It is of importance that the bottom weight is not designed as an
anchor but instead is rounded to be drawn alongside the sea bottom under resistance,
together with the drag chain. The drag resistance acting upon a ship having a such
drift anchor system will depend upon the weight and length of the drag chain and its
connected anchor wire. If a particularly great drag resistance should be wanted, for
15 example if a ship serving as a barge vessel has a considerable tonnage, a coarse drag
chain and a corresponding large and heavy bottom weight 8 may be used. The drag
resistance may also be varied within certain limits by adjusting the length of the
portion of the anchor wire being dragged along the sea floor.
Preferably the drift anchor system of the ship is provided with two sets
20 of wire/chain equipment, as illustrated in Fig. 3. Thereby is given the possibility to
adjust the ship heading or deviation direction to a certain extent by arranging a
different length of the wire/chain equipment along the sea floor on the steerboard and
port side of the ship, respectively.
By letting the ship heading in this way be under control of the drift
25 anchor system, the angular or azimuth direction of the ship during its aft driftage will
approximately become in line against the prevailing weather forces. In this way the
wind influence can be kept relatively small due to that the wind facing area will be far
less than if the ship should be allowed a free driftage, since it then probably would
end up in an azimuth perpendicular to the wave and wind direction. A such
30 uncontrolled driftage therefore would be considerably greater and
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could lead to a driftage speed of for example 4- 5 knots when the wind strength is,
say 50 knots. By the method and drift anchor system of the invention is assumed
that a barge operation may be executed under minimum risk even during great windspeeds and at wave heights (given as significant wave height Hs) up to around 5 - 6
5 m.
A loading operation for fluid transport between two ships in open sea,
for example the transfer of oil/gas from a first to a second tanker will follow these
steps:
a) The first ship stops its forward machinery and activates its drift anchor
10 system, whereafter it is allowed to break adrift and turn into a position with its bow
towards the prevailing weather forces (waves, wind and current). Alternatively and
to save time the ship may first be turned against the wind, thereafter the forward
machinery is reversed so that the ship by now means starts its aft driftage, then the
machinery is switched on and the drift anchor system is activated.
15 b) When the aft driftage (rearward movement) of the first ship 1 has
become stable, the second ship 5 is manoeuvred with its bow against the stern of the
first ship and is positioned ahead of the bow (the expression "ahead of" here
meaning: in relation to the direction of movement of the first ship). The manoeuvring
may for example be effectuated in the same way as is common by
20 mooring/connection to an offshore buoy installation. Thereafter the ships areconnected together, and the second ship may keep an approximately constant
distance to the first ship by means of its DP system.
c) The bow loading system 6 is activated and made to establish a fluid
transport connection between the ships by means of the loading hose 2, and the fluid
25 transfer may start.
d) At the end of the loading operation the ships are disconnected, the first
ship is heaving in the loading hose and the mooring hawser and winching the anchor
wire until the bottom weight is lifted to close the shaft mouth, thereafter the ships
may navigate to their destination sites.
Carried out in this way, hydrocarbon fluid may be transferred faster and
more secure from one ship to another out at sea, compared to known methods, evenby marginal weather conditions.
