WO1980002825A1

WO1980002825A1 - A method of reducing the escape of oil from oil tankers which have gone aground

Info

Publication number: WO1980002825A1
Application number: PCT/SE1980/000164
Authority: WO
Inventors: B Stenstroem
Original assignee: Salen Technologies Ab; B Stenstroem
Priority date: 1979-06-11
Filing date: 1980-06-10
Publication date: 1980-12-24
Also published as: SE7905090L; SE432397B

Abstract

In a method of eliminating or reducing the escape of oil from a damaged oil tank (1) of an oil tanker which has gone aground, there is first created an underpressure in the gas space of the damaged tank. While maintaining this underpressure there is then applied a liquid lock. This lock is created by means of a trunk (10), which may be portable and which is accommodated in a pressure vessel (9). Subsequent to evacuating the pressure vessel via a working hatch (8) the trunk is then lowered into the gas space of the tank. The trunk is lowered beneath the surface of the oil, whereafter the pressure vessel is removed, the liquid column in the tank forming a lock for the underpressure in the tank. There is then lowered into the tank a portable submersible pump - or a hose (17) which is caused to operate as an air-lift pump by feeding finely divided air to the bottom thereof. Above deck the air is passed to a deaerator (18), while the oil is passed to an empty or partially filled undamaged tank (2; 2') through a hose (7; 19).

Description

A method of reducing the escape of oil from oil tankers which have gone aground

Technical Field The discharge of oil from tankers normally occurs for two reasons. Either oil is discharged as an operative measure, or is spilled as the result of an accident. Since the coming into force of international laws making it an offence to purposely discharge oil at sea, contamination resulting from this source has become less and less, causing contamination resulting from oil spilled from, e.g. vessels which have run aground to be more and more prominent.

Accidental oil spillages are normally caused by a ^'vessel running aground or colliding with another vessel. To a lesser extent oil may be discharged unintentionally as a result of explosions or fire on board.

A ship which runs aground can be placed in two categories, namely a first category in which the ship does not founder, but remains afloat, with oil being discharged as a result of the hydrostatic overpressure of the load at the region of the damage suffered by the ship, this category being the most usual, and a second category in which the ship suffers serious damage and sooner or later breaks up, with the major part of the load being discharged into the sea.

Background Art

Previous solutions have proposed the provision of double-hull structures, at least in the vicinity of the ship's hull, to prevent oil from being spilled in the event of the ship running aground. This solution, however, is expensive and difficult to put into effect. Moreover the measures proposed cannot be carried out on existing ships without great difficulty.

OMPI Λ. WIPO » The Swedish Patent Specification No. 72 11486-1 proposes another solution, based on creating an under¬ pressure by moving a volume of liquid from a damaged tank to an undamaged tank which is initially empty or only partially filled. This requires the ship's pumping and conduit systems to be in working order, however, which is not always the case when a ship has gone aground.

Disclosure of Invention

The present invention is based on the concept of creating an underpressure in the gas space or free- surface area of a damaged tank, and the object of the the invention is to provide simple ways and means which can be readily put into practice on board^" ship and which are effective in eliminating or reducing oil spills from ships which have run aground and are placed in the aforesaid first category. Such groundings usually occur in protected waters and fairways, and consequently represent a serious threat to the environment.

In such cases the only oil able to escape is that which exerts a hydrostatic overpressure greater than the pressure of the surrounding water at the ship's bottom. In the case of large tankers this overpressure is of the order of 2 metres water column, and of the order of 1 metre water column in the case of smaller vessels.

There is no possibility of avoiding this escape in single-hull tankers built along traditional lines, with traditional equipment on board. The present invention provides a method of preventing or reducing the spill of oil by a combination of substantially the following measures:

- sealing of all tank hatches and other openings in the tank deck. The normal "breathing valves" of the

OM Λ. I tanks are sealed by placing over them special protective hoods;

- creating a restricted underpressure in the gas space of a damaged tank between the surface of the oil load in the tanks and the deck;

- transferring in a later stage part of the load to other, underdamaged tanks which are not completely full. In this respect there is used in accordance with the invention a special trunk which forms a liquid lock, permitting the arrival and departure of oil without losing the aforementioned underpressure.

The method according to the invention can be put into effect at low cost, while the devices and apparatus used are simple and highly reliable in operation.

Conveniently, the underpressure is created in the tanks by means of ejectors, preferably portable projectors, driven by air under pressure from the ship's normal working-air systems, which shall be in operation when the ship approaches the coastline.

Since the gas space or free-surface area in a full tank normally constitutes 2% of the volume of said

3 tank and thus with a tank whose volume is 10,000 m reaches to 200 m , it is necessary to remove about 3 70 m of air in order to lower the pressure in the tank by 25%. This reduction in pressure can be accomplished within the space of 10 minutes with a

3 portable ejector drawing from 3 to 4 m of air per minute from the ship's compressor system, which are fully acceptable figures.

The ejector can be dimensioned so as not to produce an underpressure of such magnitude as to create problems with regard to the mechanical strength of the ship's deck. The ejector, which may be portable, is either removably connected to a protective hood which is placed over the breathing valve of the tank, or may be permanently connected to the system of pipes which, on certain ships, connects the gas spaces of the tanks and which is normally used to feed an inert gas to the tanks.

As before mentioned, the Swedish Patent Specification No. 72 11486-1 proposes that the afore¬ mentioned underpressure is created by moving liquid from a damaged tank to an undamaged tank, using the ship's pumps and existing pipe systems. It cannot safely be assumed, however, that the pumps of a ship which has run aground will still be serviceable. By rapidly creating an underpressure in a damaged tank, it is possible to prevent or reduce the initial release of oil through the damaged bottom of a ship. The subsequent remove of oil from the damaged tank to an undamaged tank requires the use of apparatus which, while enabling the transfer of oil, will not cause the underpressure to cease. In technics known hitherto submersible pumps have been lowered through hatches in the ship's deck. When using this procedure, however, it is not possible to prevent the initial outflow of oil; it is only possible to take such measures in conjunction with the salvaging of the vessel.

A number of exemplary embodiments of the invention will now be described with reference to the accompanying schematic drawings.

Brief Description of the Drawings Figure 1 is a longitudinal sectional view of a ship which has gone aground, in which ship certain arrangements have been made to reduce the leakage of oil from a damaged tank.

_ OMPI Figure 2 illustrates the various stages of mount¬ ing a liquid lock, in the form of a trunk, in the damaged tank of the ship shown in Figure 1.

Figure 3 illustrates principally the use of an air-lift pump for transferring oil from the damaged tank to an adjacent undamaged tank which is only partially filled.

Preferred embodiment of the Invention In Figure 1 there is shown part of an oil tanker which has gone aground, whereat the reference 1 identifies a damaged tank containing oil. The reference 2 identifies a further, undamaged tank which is only partially filled. The reference 3 identifies so-called breathing valves arranged in the roof of a respective tank 1 and 2, while the reference 4 identifies a sealing cap or hood placed over the breathing valve of the damaged tank 1. A portable, air-driven ejector

5 is shown connected to the sealing hood. Also shown in the Figure is a liquid lock in the form of a trunk

6 mounted in the deck opening of the tank. A portable bilge pump is connected to the partially filled tank 2 via a hose 7.

Figure 2 illustrates the various stages of installing a liquid lock in the form of a trunk, for enabling pumps or suction hoses to be lowered into the damaged tank while maintaining an underpressure in the gas space of said tank. In this respect the reference 8 identifies a hatch cover in the ship's deck, which in the illustrated embodiment corresponds to the roof of the damaged tank. An arrangement in the form of a portable trunk 10, which is accommodated in a pressure vessel 9, is placed over the hatch cover 8, whereafter the pressure vessel is evacuated by means of the ejector 11. The hatch cover 8 is then

OMPI removed by means of manipulators 12, and the trunk lowered down into the tank. The length of the trunk is such as to allow it to be lowered to a satisfactory distance beneath the liquid level of the tank. The trunk has a lower ring whose weight is sufficient to overcome the upward drift from the deplaced part of the trunk when it is lowered into the oil. When the pressure in the vessel 9 has been equalized and the vessel removed, the column of liquid in the trunk 10 forms a lock against the underpressure in the tank. The trunk preferably comprises a soft material and lies against the edges or combing of the hatch in a gas-tight fashion.

A hoisting line for the trunk 10 is referenced 13 in Figure 2b, while an observation window is referenced 14 in Figure 2c. In Figure 2c the hatch cover 8 is shov/n withdrawn. The trunk is shown in its operative position in Figure 2d.

The various part figures a-d of Figure 2 illustrate the following procedural steps:

Figure 2a, the hatch cover 8 is closed, with its nuts loosened;

Figure 2b, the pressure vessel 9 has been brought into position and evacuated; Figure 2c, the hatch cover 8 is removed and the trunk 10 lowered into the tank;

Figure 2d, the pressure in the vessel 9 has been equalized and the trunk 10 occupies its working position. A submersible pump of known construction can then be lowered into the oil, through the trunk 10. Alternatively, instead of a submersible pump there may be used a hose 17 adapted to operate as an air¬ lift pump by introducing finely divided air to the lower end 21 thereof, as shown in Figure 3. Above

WIPO deck, the air is passed to a deaerator 18 and the oil is passed to a partially filled tank 2' through a hose 19. It will be understood that an oil tanker has always a surplus tank volume and when loaded to its loading line thus has two or three tanks which are only half full. As will be seen from Figure 3, the deaerator 18 has a valve 22 with a spark shield 23.

The atomized or finely divided air is passed to the lower end of the hose 17 through a compressed- air line 20. Similar to Figure 2, the trunk 10 is shown in Figure 3 in its operative position.

If the difference in liquid levels between the damaged tank and the receiving tank permits, the air pump, when started, can be caused to operate as a syphon, by cutting of the air supply and interrupting the deaerating operation in the deaerating tank.

Industrial Applicability The advantages afforded by the aforedescribed method lie in its simplicity and the low costs involved. The apparatus required for carrying out the method can be kept in readily accessible places so that they can be readily brought into use when required, thereby to prevent the unintentional discharge of oil from a damaged tank, or at least to secure a true decrease in the amount of oil liable to escape.

-£ EAlr

OMPI

WIPO .,vV

Claims

1. A method of reducing or eliminating the escape of oil from a damaged tank of an oil tanker which has gone aground, with the use of equipment found on board, wherein an underpressure is created in the damaged tank, said method comprising the steps of introducing through the gas space present between the surface of the oil in said tank and the roof thereof a liquid lock in the form of a protable or fixed trunk of given length, said trunk being sealed about the deck opening through which it is introduced into said tank; lowering pump means through said trunk into the oil in said tank; and pumping away said oil.

2. A method according to claim 1, wherein breathing valves associated with said damaged tank are covered in sealing fashion with separate protective hoods? and wherein a portable ejector is connected to one such hood for maintaining or increasing said under¬ pressure in the gas space of said tank.

3. A method according to claim 1 or claim 2 , wherein a portable pressure vessel which includes an immersible trunk is placed in sealing fashion over a hatch cover of the damaged tank; whereafter said vessel is evacuated, the hatch cover removed and the trunk lowered into said tank to a sufficient depth beneath the oil level therein while being in sealed relation¬ ship with the hatch opening, whereafter the pressure in the pressure vessel is re-set and said -vessel removed.

OMPI