GB2295372A

GB2295372A - Fluid Transfer Between Ships

Info

Publication number: GB2295372A
Application number: GB9523966A
Authority: GB
Inventors: K Re Breivik; Olav Wiik; Arne Smedal
Original assignee: Den Norske Stats Oljeselskap AS
Current assignee: Equinor ASA
Priority date: 1994-11-24
Filing date: 1995-11-23
Publication date: 1996-05-29
Anticipated expiration: 2015-11-23
Also published as: NO944504D0; CA2578769A1; NO300536B1; GB9523966D0; GB2295372B; CA2163726A1; NO944504L; IE950891A1

Abstract

A method and a drift anchor system (7) for fluid transfer between two ships (1, 5) in the open sea. At least one of the ships is at its stern provided with a loading hose (2), a reel or similar for said hose, and a mooring hawser (4). The drift anchor system (7) is arranged forward on the ship and is designed for providing breaked aft driftage. The system comprises a bottom weight (8, Fig 2) having rounded shape, a drag chain (9), an anchor wire (10), a winch (11) on the deck and a vertical or inclined chain shaft (12) from the deck and down to the ship bottom. The method incorporates activating of the drift anchor system (7), manoeuvring of the second ship (5) towards the first ship (1), mooring of the ships to each other by means of the mooring hawser (4) and activating of a bow loading system (6) on the second ship to establish fluid transfer by means of said loading hose (2). <IMAGE>

Description

Method and system for fluid transport between ship a 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 fluid or load transfer, a better employment of the fleet will i.a. be possible.

A good load transfer flezibility will particularly be valuable for a flee in which the individual ships have different types of loading/discharging equipment; an board.

Until now known methods to transfer fluid between tankers in the open sea have turned out problematical during eo- called marginal weather conditions (by significant wave heights over 6 feet t2 it)). The reason is as follows:If two ships to be interconnected for load transfer are noting 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 recornendable to make the connection and carry out the subsequent load transfer between the ships, since the risk for collision would be too great. If, on the other hand, the ship machinery is deactivated, the shipe 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.

Different 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 for heavier, thereby leading to more serious bonsequences in the event of a possible direct aontact between the ships.

On this background the present invention is proposed, said invention 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 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 sohematioally the drift anchor system in activated condition.

Referring to Fig. 1 there is shown a first ship 1 having a loading hose 2 at its stern and 4 reel 3 or a similar storing device for the loading hose, which aleo 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 first ahip 1. The second ship is preferably a buoy loading ship having excellent nianoeuvring properties due to a system for dynamic positioning (DB) for the control of trueters forward and aft and turnable mein propellers.

When the DP system is activated the mooring hawser 4 has the function of an extra safety mechanism between the ehips. A typical distance between the ships may be 80 - 100 m during 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 n rounded shape, without anchor hooks and secured to the lower free end of a drag chain 9, an anchor wire 10 having a oonsiderable length, for example 1000 - 2000 m and being seoured to the upper end of said drag chain, a winch 11 mounted on deck and adapted for reeling of the anchor wire, and a 'rtrtical - or inclined chain shaft extending from the ship deck and down to its bottom.

When the drift anchor system is inactivated, the winch 11 is keeping the anchor wire 10 reeled, while the drag chain 9 is hanging vertically and extended within 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 case the winch is relieved. In both situ scions the bottom weight 8 will be drawn with a certain force 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, snd the shaft mouth 14 will be correspondingly complementary shaped.

During use the first ship forward machinery is stopped, the drag chain 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 turag 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 elowly being dragged forwards. It is of' import'anoa that the bottom weight is not designed as an ancor - 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 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 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/ahain equipment along the eea 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 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 aan be kept relatively small due to that the wind facing area will be far less than if the ship should be allowed 4 free driftage, since it then probably would end up in an azimuth perpendicular to the wave and wind direction. A such uncontrolled driftage therefore would be considerably greater and could lead to a driftage sped of for example 4 - 5 knots when the wind strangth 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 riak even during great wind speeds and at wave heights (given as significant wave height S.) up to around 5 - 6 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 hip stops its forward machinery and activates its drift anchor 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). Alter natively and to save time the ship may first be turned against the wind, thereafter the forward machinery is reversed so that the ship by own means starts its aft driftage, then the machinery is switched on and the drift anchor system is activated.

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 mesning: 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 mooring/connection to an offshore buoy installation.

Thereafter the ships are connected together, and the second ship may keep an approximately constant distance to the first ship by means of its DP system.

o) The bow loading system 6 is activated and made to establish a fluid transport connection between the ship. by means of the loading hose 2, and the fluid transfer may start.

d) At the end of the loading operation the ships are disconneoted, 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, even by marginal weather conditions.

Claims

Claims:

1. A method for fluid transport between two ships in the open sea, at least one of the ships having a loading hose at its stern, a reel or a corresponding storage device for the loading hose, a mooring hawser, and a drift anchor system forward to allow breaked aft driftage due to the prevailing weather forces, the second ship having a bow loading system for fluid transfer to or from the first ship, the method comprising:- stopping the forward machinery of the first ship; activating the drift anchor system of the first ship until its driftage is becoming stable; manoeuvring the second ship towards the stern of the first ship and to a position ahead of it, in relation to the aft driftage of the first ship, the bow of the second ship being faced against the stern of the first ship; mooring the second ship to the first ship by means of the mooring hawser in the stern of the first ship; and activating of the bow loading system on board the second ship and establishing a fluid transport connection between the ships by means of the loading hose.

2. A method as claimed in Claim 2, which includes stopping the forward machinery of the first ship and activating of its drift anchor system after turning the ship towards the wind, and thereafter over a period, letting its machinery work in reverse so that the ship by own means begins its after movement in the driftage direction.

3. A method as claimed in Claim 1 or Claim 2, in which the machinery of the second ship for forward movement or positioning is kept in operation, even after the ship is manoeuvred into position approximately parallel to or aligned with the first ship, with its bow in the direction of the stern of the first ship and connected to it by means of the mooring hawser, in order to ensure that the distance between the ships does not become too small, but is preferably kept constant and at a value corresponding approximately to a tight mooring hawser.

4. A method for fluid transport substantially as herein specifically described with reference to and as shown in the accompanying drawings.

5. A drift anchor system to allow fluid transport between two ships in open sea by means of a method as claimed in any preceding Claim, comprising:- a bottom weight having a rounded shape and without anchor hooks; a drag chain secured to the bottom weight at its lower free end; an anchor wire having a considerable length and being secured to the upper end of the drag chain; a winch on the deck for reeling the anchor wire; and a vertical or inclined chain shaft from the deck of the first shop down to its bottom.

6. A drift anchor system as claimed in Claim 5, in which the bottom weight at least at its upper portion is conical or spherical, and the shaft mouth of the chain shaft in the ship bottom is shaped in a complementary fashion to the conical or spherical form of the bottom weight.

7. A drift anchor system constructed and arranged substantially as herein specifically described with reference to and as shown in the accompanying drawings.