Arrangement for a tank for transport of environmentally harmful liquids

("Vacuum Safety System")

TECHNICAL FIELD

The present invention relates to an arrangement for a tank for transport of environmentally harmful liquids in vessels, pipes being connected to the tank for loading or unloading of the liquid as well as for inert gas treat- ent of remaining air, which pipes are provided with valve members which make possible screening off of the tank space from the surrounding atmosphere, and which vacuum pump members are arranged for reducing the inter¬ nal pressure of the tank, until this pressure falls below the surrounding atmospheric pressure.

PRIOR ART

Transport of oil and other environmentally harmful liquids by means of vessels always involves certain risks of spillage as a result of collisions, running aground or explosions. The development of technical aids for redu¬ cing the risks is going on constantly, but the human fac¬ tor after all lies behind a large proportion of the acci¬ dents. It has furthermore been proposed that all oil tan¬ kers be provided with a double bottom in order to reduce the spillage in the event of running aground. This mea¬ sure is, however, relatively costly, in the case of both new construction and reconstruction. Moreover, the double bottom never affords a guarantee against major oil spil¬ lages in the event of a serious running aground accident. Furthermore, the risk of explosions increases in tanker vessels with a double bottom.

TECHNICAL PROBLEM

One aim of the present invention is therefore to bring about an arrangement which makes it possible to reduce considerably the environmentally harmful consequences of a running aground accident and which provides effective preventive protection against this damage.

SOLUTION

This aim is achieved according to the invention in that the tank is designed with an expansion space, to which the pipes for inert gas treatment as well as the vacuum pump members are connected, in that the expansion space communicates with the remaining part of the tank via a relatively narrow passage, in that the width of the ex¬ pansion space in the transverse direction is considerably smaller than the corresponding width of the remaining part of the tank, and in that the expansion space is pro¬ vided with internal longitudinal and transverse bulk¬ heads.

If a leak should now occur in the tank below the water- line of the vessel, in the first instance the surrounding water will be pressed into the tank until the gas pres¬ sure in the latter is equalised, so that it is in equilibrium with the surrounding pressure.

Expediently, the width of the expansion space in the transverse direction is considerably smaller than the corresponding width of the remaining part of the tank. As a result, the area of the free liquid surfaces can be reduced to a minimum, which is advantageous with regard to the stability of the vessel. If the invention is applied to a new ship, the latter can be classed in such a manner that full tanks will always be carried. This makes it possible to reduce or eliminate the internal reinforcements (antislosh baffles) in the tanks. The ste-

el weight saved can instead be used for reinforcements in webs, reinforced decks and bulkheads. Depending upon the construction of the vessel, the loading capacity can then be increased while maintaining the dead weight.

If the vacuum pump members are connected to a control centre, which is arranged to start the vacuum pump mem¬ bers automatically when certain conditions are met, the advantage is achieved that echo-sounders, radar and/or other navigation aids can be used in order to activate the vacuum pump members automatically when conditions which can lead to running aground are detected.

DESCRIPTION OF THE FIGURES

An exemplary embodiment of the invention will now be described in greater detail with reference to the attached drawings, in which

Fig. 1 is a perspective view of a tanker which is equipped according to the invention.

Fig. 2 is a transverse section through the tanker in Fig. 1, shown directly after damage to the bottom,

Fig. 3 is a partially sectioned side view of an expan¬ sion space, and

Fig. 4 is a cross-section along the line IV-IV in Fig. 3.

PREFERRED EMBODIMENTS

The cargo space of the vessel shown schematically in Figs. 1 and 2 is divided into a number of central and wing tanks 11 via a number of transverse and longitudinal bulkheads 10 (see Fig. 2) . The tanks 11 are provided in a conventional manner with conventional vertical trunks (not shown) with pumps for loading and unloading the tanks as well as for handling the ballast water.

Each tank is, according to the invention, connected to an expansion space 12 via a pipe 13 of a relatively small cross-section. The connecting pipe 13 extends between the bottom of the expansion space and the top of the tank and can be shut off by means of a valve 14. As can be seen from Fig. 1, the expansion spaces 12 are oblong and arranged centrally on the tank-top 15, so that they run along the cargo space of the ship. It can be seen from Figs. 3 and 4 that the expansion spaces are provided with internal longitudinal and transverse bulkheads 12a and 12b respectively.

The expansion spaces 12 are connected via pipe connec¬ tions 16 to a conventional system for inert gas treatment of the gas in the expansion spaces 12. Further pipe con- nections 17 connect the expansion spaces to a pump system for generation of vacuum pressure. The pipe connections 16 and 17 can also be shut off by means of valves (not shown in the drawings) , so that the spaces 12 can be screened off hermetically from the surrounding atmos- phere.

The vessel is loaded in a conventional manner while inert gas is supplied simultaneously. Subsequently, the oil is allowed to rise in the pipes 13 until the expansion spaces 12 are approximately half full. In this con- nection, the temperature conditions which will prevail during the voyage of the vessel are taken into considera¬ tion. That is to say that allowance is made for the oil being subject to contraction if the voyage takes place from warmer to colder latitudes or to expansion in the opposite case.

During the voyage, the inert gas treatment is maintained in a conventional manner and a conventional condensing plant can be utilised in order to take care of volatile gases and maintain the fire safety on the vessel.

When the vessel approaches a coastal region with narrow channels and difficult bed conditions, the inert gas system is shut off, all tank openings are closed and vacuum-pressure setting of the gas proportion of the ex¬ pansion spaces 12 is started. An expedient low pressure is variable in the range 0 to 5 metres column of water. Since the tank 11 forms a communicating container with the expansion space 12 via the pipe 13, a low pressure also prevails in the tank 11, in parity with the low pressure in the expansion space 12.

This state is maintained until the vessel has reached its place of anchorage or has passed the risk region, where¬ upon the gas proportion of the expansion spaces can be ventilated until normal atmospheric pressure again prevails.

If the vessel should meet with damage to the bottom in the shell plating 18, such as is shown at the arrow 19 in Fig. 2, the surrounding sea water 20 will flow in through the hole in the shell plating 18, until pressure equalisation has taken place. In this connection, the sea water pushes the oil away into the tank, equalisation taking place to equilibrium between the relative pressure of the oil and of the sea water. Oil is then moved from the tank 11 to the expansion space 12.

Since the oil has a considerably lower density than the sea water, there is no risk that oil 21 will force out through the hole in the shell plating, provided that the

vessel does not make such headway through the water that a turbulent flow is formed in the tank.

It can be expedient to control the pump members for main¬ taining low pressure in the expansion spaces from a cen- tre which is associated with the navigation system and radar of the vessel. Automatic control devices can then control the transition from normal operation to operation with low pressure, for example in the event of a change in course outside a channel.

Check valves can be arranged in the pipe connections 17, so that the low pressure is maintained even if the power supply of the vessel should be put out of operation.

The vacuum pumps are expediently positioned in the engine room of the vessel, and doubled in order to reduce the risk of breakdowns.

The invention is not limited to the exemplary embodiment described above, but various alternatives are conceivable within the scope of the subsequent claims. For example, the expansion spaces 12 can be designed otherwise than shown and for example be welded directly to the tank-top 15. The invention can be applied both to existing tankers and to new ships, in which connection considerable gains can be made in the case of a newly built optimised vessel in which the cargo capacity can be increased while ain- taining the dead weight.