IE950891A1

IE950891A1 - Method and system for fluid transport between ships

Info

Publication number: IE950891A1
Application number: IE950891A
Authority: IE
Inventors: Kare Breivik; Olav Wiik; Arne Smedal
Original assignee: Norske Stats Oljeselskap
Priority date: 1994-11-24
Filing date: 1995-11-23
Publication date: 1996-05-29
Also published as: GB9523966D0; GB2295372A; NO944504L; NO944504D0; CA2578769A1; GB2295372B; NO300536B1; CA2163726A1

Abstract

A method and a drift anchor system (7) for fluid transport between two ships (1, 2) 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) 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). <Fig. 1>

Description

Method ι md flu:d transport between ships There are several reasons to transfer fluid such as oil s 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.e. 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 is to x; PC z c H o LU a. co co V) CM J? UJ —I M> CC Q >9 Z < co CM to Z rl- o ..H o o UJ z ω tankers in the open sea have turned out problematical during socalled marginal weather conditions (by 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 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.

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 end their loading hoses are far heavier, thereby leading to more serious consequences in the event of a possible direct contact between :he ships.

On this background the present invention ie proposed, ι laid 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 : n vent ion will be desaribed more closely with reference to the, 85089 1 accompanying drawings, wherein: Fig. 1 demonstrates a connection between two tankers in order to aarry out fluid transport according to the invention, Fig. 2 illustrates a drift anchor system on one of the ships, end s Fig. 3 shows schematically 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 a reel 3 or a similar storing device for the loading hose, whioh also may be stored in io 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 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 is 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 during mutual mooring. so 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 2i a rounded shape, without anchor 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 end adapted for reeling of the anchor wire, and a vertical or 3o inclined chain shaft extending from the ship deok and down to its bottom.

When the drift anchor system is inactivated, the winch 11 is keeping the anchor wire 10 reeled, while the dreg 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 ohain, or the upper end of said ohain may be secured under tension by means of a stopper 13 on the deck, in which case the winch is relieved. In both situations the bottom weight 8 will be drawn with a certain force «5089 η 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. s 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 turning will partly be under control due to io 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 dragged forwards. It is of importance that the bottom weight is not designed as an anohor but instead is rounded to be drawn is 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 dreg chain and its connected anohor wire. If a particularly great drag resistance should be wanted, for example if a ship serving as a jo 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 wlre/ohaln equipment, as illustrated in Fig. 3. Thereby ie given the possibility to adjust the ship heading or deviation direction to a certain extent by arranging a different length of the wire/chaln equipment along the sea so floor on the steerboard and port side of the ship, respectively.

By letting the ship heeding in this way be under control of the drift anchor system, the angular or azimuth direction of the ehip 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 lt 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 speed of for example 4-5 knots when the wind strength ie, say 50 knots. By the method and drift anahor system of the invention Is assumed that a barge operation may be executed under minimum risk even during great wind speeds s and at wave heights (given as significant wave height H,) up to around 5 - 6 m.

A loading operation for fluid transport between two ships in open sea, for example the transfer of oil/ges from a first to a second tanker will follow these steps: io a) The first ship stops its forward machinery end activates its drift anchor system, whereafter it is allowed to break adrift end 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 is 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 2o its bow againat the stem 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 moorlng/connectlon to an offshore buoy installation, as Thereafter the ships ere connected together, and the second ehip may keep on 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 se of the loading hose 2, and the fluid transfer may start. d) At the end of the loading operation the ehips 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 olose the shaft mouth, thereafter the ships as 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

1. A method for fluid transport between two shipe in β the open sea, et least one of said ships (1, 2) having at its/thair stern a loading hose (2), a real (3) or a corresponding storing device for said loading hose, and a mooring hawser (4), and forward a drift anchor system (7) to allow breaked aft driftage due to the prevailing weather forces, and said at least xo one other or second ship (S) having a bow loading system (6) for fluid transfer to or from said first ship, characterized in: stopping of the forward machinery of the first ship (1), activating of the drift anchor system (7) of said first is ship (1) until its driftage is becoming stable, manoeuvring of said other ship (5) towards the stern of said first ship and to a position ahead of it, in relation to the aft driftage of said first ship, the bow of said second ship being faced against the stem of said first ship, mooring of said other ship (5) to said first ship (1) by means of the mooring hawser (4) in the stern of the first ship, and activating of the bow loading system (6) on board the other ship (5) and establishing of a fluid transport connection as between the ships by means of said loading hose (2).

2. Method according to claim 1, characterised in stopping of the forward machinery of said first ship (1) and activating of its drift anchor system after turning the ship towards the wind and thereafter in a period letting its machinery so work in reverse so that the ship by own means is starting its aft movement in the driftage direction.

3. Method according to claim 1, characterized in that the machinery of the other ship (5) for forward movement or positioning - even after said ship is being manoeuvred in ss position approximately parallel to or aligned with the firet ship (1), with its bow in the direction of the stem of the first ship end connected to it by meane of the mooring hawser (4) - is kept in operation to make sure that the distance between the ships is not becoming too small, but preferably being kept constant and 850891 by a value corresponding to approximately a tight mooring hawser.

4. Drift anchor system (7) to allow fluid transport between two ships (1, 2) in open sea and according to the method of one of the preceding claims, characterised in: e a bottom weight (8) having rounded shape end without anchor hooks, a drag chain (9) secured to the bottom weight (8) at lte lower free end, an anchor wire (10) having a considerable length and io being secured to the upper end of said drag chain (9), a winch (11) on the deck for reeling of said anchor wire (10), and a vertical or inclined chain ehaft (12) from the deck of the first ship (1) and down to its bottom. is

5. Drift anchor system according to previous claim 4, characterised in that said bottom weight (8) at least at its upper portion is conical or spherical, and that the shaft mouth (14) of said chain shaft (12) in the ship bottom is shaped complementary to the aonloal or spherical form of said bottom » weight (8).