NO347359B1

NO347359B1 - System and method of automatic release of underwater equipment

Info

Publication number: NO347359B1
Application number: NO20211522A
Authority: NO
Inventors: Stig Bakke
Original assignee: Resqunit As
Priority date: 2021-12-16
Filing date: 2021-12-16
Publication date: 2023-09-25
Also published as: NO20211522A1; WO2023113616A2; WO2023113616A3

Description

SYSTEM AND METHOD OF AUTOMATIC RELEASE OF

UNDERWATER EQUIPMENT

FIELD

The present invention relates to a system and method for automatically releasing parts of underwater equipment if the equipment becomes lost. The invention is particularly useful in fishing, however it also finds application in any underwater environment.

BACKGROUND

Many different pieces of equipment are used in the fishing industry to catch fish and crustaceans. When trying to catch seafood such as fish, lobster, langoustine, crabs, crayfish etc. structures such as ‘lobster pots’ are lowered into the water and rest on the seabed. The crustaceans, herein after referred to as ‘the catch’ are enticed into the structure. The structure then closes on the catch, trapping the catch therein, until the structure is recovered to the surface to remove the catch for sale/consumption.

In some cases the structure may be tethered to a floating buoy such that the structure can be easily recovered when required. Tethering may be by rope, cable or another moveable tethering device. In some instances, the structure may detach from the tether to which it is attached. This is typically due to severe weather causing stress on the tethering system. Also, the tether may snap or it may be cut by the propeller of a boat. In some cases, if the structure sinks deeper than expected, the tether may be too short and the buoy therefore remains underwater and is therefore not visible from the surface. The structure may continue to trap the catch – so called ghost fishing. The trapped catch then remains in the structure until it die, or in very rare cases, the structure is found and recovered. It is inhumane to trap the catch in a lost structure for an extended period of time until the catch inevitably dies. Additionally, the trapped catch is wasted in the sense that the catch has suffered confinement without any benefit to humans, i.e. if the catch was instead to be recovered and provide food for humans. It is generally accepted that sealife should be released if it is not being used for a particular purpose, such as for research purposes or as a foodsource.

Lost structures used in fishing are a pollutant to the seas, oceans and lakes in which they are used. The structures may cause damage to the aquatic environment. Furthermore, sealife can become caught up in lost fishing structures. Fishing structures are often designed to be reusable many hundreds of times, and so it is highly undesirable and expensive to lose such structures. It is particularly difficult to locate lost fishing equipment in vast seas, and so most lost fishing equipment is not recovered. It is prohibitively expensive and time consuming to send equipment into the water to locate and recover lost equipment, however there is a growing need to recover lost structures for environmental, animal welfare and economic reasons.

Patent document CN105660550A discloses a deep-sea organism trapping device with doors closed through driving of a turbine worm motor. The deep-sea organism trapping device comprises a box with doors and an organism induction mechanism for detecting whether organisms enter the box or not. A door closing mechanism and a control drive mechanism are also provided.

Patent document US10,232,914B1 discloses a pressure activated time-delayed release device and method. The device includes a body having a drive chamber and a compression chamber disposed therein, a drive piston disposed within the drive chamber of the body, and a compression piston disposed within the compression chamber of the body. A throttle mechanism is disposed between the drive piston and the compression piston. The throttle mechanism controls a release of viscous fluid from the drive chamber into the compression chamber. The drive piston and compression piston are connected so as to cause the compression piston to move substantially simultaneously with the drive piston to compress a compressible element disposed within the compression chamber. The device moves from an unactuated state to an actuated state when the device is exposed to a predetermined water pressure for a predetermined period of time.

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art.

The object is achieved through features, which are specified in the description below and in the claims that follow.

SUMMARY

According to a first aspect of the invention, there is provided a submersible catching device for catching fish, seafood or crustaceans, the device comprising: a structure configured to receive a catch; a door moveable between a closed configuration in which the door is secured to the structure such that the catch cannot escape and an open configuration in which the catch can escape; a release mechanism comprising a pressure sensitive device, a locking mechanism and a timer circuit; where the timer circuit comprises an electronic timer configured to start counting down from a pre-set time when the pressure sensitive device is exposed to a pre-determined external pressure; and the locking mechanism is configured to be in operative engagement with the door when the door is in the closed configuration, and to release the door from the closed configuration when the timer times-out; such that, in use, when the catching device reaches a pre-determined depth the catch is automatically released from within the device when the timer times out.

The structure may be a lobster trap or lobster pot or creel.

The release mechanism may further comprise: a motor connected to a battery; a rotatable shaft connected to the locking mechanism; and a power switch configured in use to start the timer counting down on upon activation of the power switch; wherein the pressure sensitive device is configured to activate the power switch when the pressure sensitive device is exposed to a pre-determined external pressure; the timer circuit is configured to turn on the motor upon time-out of the timer; and the rotatable shaft is operatively connected to the motor such that, in use, when the motor is turned on the rotatable shaft rotates, thereby rotating the locking mechanism out of engagement with the door.

The pressure sensitive device may comprise an elastically deformable pressure sensitive membrane comprising an inner surface and an outer surface, wherein the pressure sensitive membrane is configured in the release mechanism such that, in use, the outer surface is exposed to an external hydrostatic pressure, such that an increased external hydrostatic pressure elastically deforms the pressure sensitive membrane. The pressure sensitive membrane provides protection of components from the harmful effects of salt water. In this regard, moving components protected by the pressure sensitive membrane cannot become clogged with salt from the salt water or become damaged by the salt water.

The release mechanism may further comprise an activator arranged in operative engagement with the inner surface of the pressure sensitive membrane, such that, in use, elastic deformation of the pressure sensitive membrane causes movement of the activator between a first position in which the power switch is not activated, and a second position in which the power switch is activated.

The release mechanism may further comprise a pressure chamber configured to provide an internal pressure to the pressure sensitive device, thereby, in use, opposing the external pressure experienced by the pressure sensitive device when the device is exposed to an external pressure up to the pre-determined pressure. The pressure chamber may be provided with an internal pressure of, or close to, atmospheric pressure.

The electronic timer may be configured to be settable by a user of the device.

The door may further comprise a floatation element configured with a buoyancy such that, in use; when the door is in the closed configuration and the device is submerged in water, the device does not float to the surface; and when the door is released from the closed configuration, the door has sufficient buoyancy to float to the surface.

The device may further comprise a door tether with a first end and a second end, wherein the first end is attached to the structure and the second end is attached to the door, such that when the door is released from the closed configuration the door tether connects the door to the structure.

According to a second aspect of the invention, there is provided a method for automatically opening a lost submersible catching device, comprising the steps of: providing a submersible catching device according to the first aspect of the invention; submerging the catching device in water; exposing the pressure sensitive device to the pre-determined external pressure; letting the timer time-out; and automatically releasing the door from the closed configuration by moving the locking mechanism out of operative engagement with the door.

The step of exposing the pressure sensitive device to a pre-determined external pressure may further comprise sinking the device to a pre-determined depth.

According to a third aspect of the invention, there is provided a release mechanism for releasing a door of a submersible device, the release mechanism comprising: a pressure sensitive device, a locking mechanism and a timer circuit; where the timer circuit comprises an electronic timer configured to start counting down from a pre-set time when the pressure sensitive device is exposed to a pre-determined external pressure; the locking mechanism is configured to be in operative engagement with the door when the door is in the closed configuration, and to release the door from the closed configuration when the timer times-out; such that, in use, when the release mechanism sinks to a pre-determined depth the release mechanism releases the door a pre-determined amount of time after the release mechanism has reached the predetermined depth.

The release mechanism may further comprise: a motor electrically connected to a battery; a rotatable shaft mechanically connected to the locking mechanism; and a power switch configured in use to start the timer counting down on upon activation of the power switch; wherein the pressure sensitive device is configured to activate the power switch when the pressure sensitive device is exposed to a pre-determined external pressure; the timer circuit is configured to turn on the motor upon time-out of the timer; and the rotatable shaft is operatively connected to the motor such that, in use, when the motor is turned on the rotatable shaft rotates, thereby rotating the locking mechanism out of engagement with the door.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the following drawings, in which:

Fig.1 shows a release mechanism for an underwater structure;

Fig. 2 shows an exploded view of the release mechanism of Fig.1;

Fig. 3 shows a cross-section view of a door for an underwater structure;

Fig. 4 shows a rear view of the door of Fig.3;

Fig. 5 show a rear isometric view of the door of Fig.3; and

Fig. 6 shows a front isometric view of the door of Fig.3.

For clarity reasons, some elements may in some of the figures be without reference numerals. A person skilled in the art will understand that the figures are just principal drawings. The relative proportions of individual elements may also be distorted.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig.1 shows a cross-sectional view of a release mechanism 100 for releasing a door of an underwater structure (not shown) when the structure has become lost. Fig.2 shows an exploded view of the release mechanism 100 shown in Fig. 1. Referring to Figs.1 and 2, the main components of the release mechanism 100 are a pressure chamber 110, a motor 120, a rotatable locking hook mechanism 130, an activation mechanism 140 and a battery 150.

The pressure chamber 110 forms the main housing of the release mechanism 100 and comprises a first shell portion 111 and a second shell portion 112, which are arranged in a locking configuration via a snap-lock created by a press fit portion 112’ of the second shell portion 112 locking against a recessed portion 111’ of the first shell portion 111. Between the first and second shell portions 111, 112 there is positioned an o-ring 113 creating a pressure-sealed environment within the pressure chamber 110 created by the first and second shell portions 111, 112. The pressure chamber 110 may be filled with fluid at a particular pressure before the first and second shell portions 111, 112 are joined by the interference fit and sealed off by the o-ring 113.

The motor 120 in the presently described example is an electrically driven motor, with electricity being provided by the battery 160.

The rotatable locking hook mechanism 130 comprises a hook 131 mounted on a rotatable shaft 132 operatively connected to the motor 120 such that the motor 120 can turn the rotatable shaft 132 and hook 131. In this connection, the rotatable shaft 132 passes from outside the pressure chamber 110 where the hook 131 is located to inside the pressure chamber 110 where the motor 120 is located. To provide sealing of the rotatable shaft 132 to ensure the pressure within the pressure chamber 110 is not undesirably altered, the rotatable shaft 132 is provided with a shaft seal 133.

Furthermore, the rotatable shaft 132 is provided with a coupling 134 configured to couple the rotatable shaft 132 to the motor 120 via a gearbox 135, so that the motor 120 can rotate the hook 131, the purpose of which will be described later. In the example shown, the gearbox 135 provides a gear exchange rate of 1000/1. It will be understood that other suitable gear exchange rates will be possible depending on the configuration.

The activation mechanism 140 comprises a pressure sensitive membrane 141 configured such that the pressure sensitive membrane 141 moves in response to a differential pressure being applied thereacross.

As shown in Figs.1 and 2, the pressure sensitive membrane 141 in the present example is a dish shape with an outer surface 141’ exposed to the pressure of the environment in which the release mechanism 100 is placed, for example the hydrostatic pressure of the depth of water at which the release mechanism 100 is placed. The pressure sensitive membrane 141 further comprises an inner surface 141’’ which is, in the present example, positioned in contact with an activator 142. In this regard, the pressure sensitive membrane 141 protects the activator 142 from exposure to salt water. The activator 142 is shaped and configured on a first face 142’ to register with the inner surface 141’’ of the pressure sensitive membrane 141, such that movement of the pressure sensitive membrane 141 will cause movement of the activator 142. The activator 142 comprises a mushroom head 142’’’ which is configured to abut against a corresponding abutment surface 112’’’ of the second shell portion 112. This configuration arrests the lateral displacement of the activator 142 at the abutment surface 112’’’, the purpose of which will be explained later. Additionally, the activator 142 is exposed on a second face 142’’, opposite the first face 142’, to the fluid pressure within the pressure chamber 110. In this connection, an increased pressure in the environment in which the release mechanism 100 is placed causes a pressure differential across the membrane 141 and activator 142. It will be understood that, although not shown in Figs.1 and 2, the activator 142 may be provided with a biasing spring, thereby biasing the activator 142 towards the membrane 141, to alter the pressure at which the membrane 141 will deform and the activator 142 will be pushed. In this way, the pressure differential required to move the activator 142 can be adjusted. In this way, the membrane 141 and activator 142 can be configured to activate at a certain hydrostatic pressure, corresponding to particular depth.

In this connection, the membrane 141 and activator 142 can be configured to activate at, for example, 1m water depth or 3m water depth or whatever water depth it is expected that the release mechanism will operate at, herein after referred to as its “operating depth”. It will be known by the person skilled in the art that at 1m water depth, a hydrostatic pressure of around 10kPa is experienced, and at 3m water depth around 30kPa is experienced. It is therefore possible to configure the release mechanism to activate at, for example, 10kPa or 30kPa, corresponding to 1m water depth or 3m water depth, or alternatively, any other hydrostatic pressure corresponding to any “operating depth”. The activation pressure is herein referred to as “operating pressure”.

The pressure differential required to activate the membrane 141 and activator 142 may be adjustable, for example by adjustment of the pressure within the pressure chamber, thereby requiring that a higher hydrostatic pressure to be applied to the membrane 141 to cause movement of the membrane 141 and activator 142. Increasing the pressure inside the pressure chamber may be performed by injecting fluid via a port (not shown) located in the first shell 111 for example. Alternatively, if a spring is used to provide bias to the activator 142, the spring stiffness may be adjusted to adjust the pressure differential required to activate the membrane 141 and activator 142.

The activation mechanism 140 further comprises a power switch 143 arranged to be contacted by the activator 142 when the activator 142 is moved by the pressure differential. In this regard, the pressure sensitive membrane 141 also protects the power switch 143 from exposure to salt water. In a preferred example, the power switch 143 provides a spring bias to the activator 142 to resist the external pressure. It will be appreciated that in some examples, the pressure sensitive membrane 141 may be configured to contact the power switch 143 directly, rather than via the activator 142 as in the present example. When the activator 142, or the membrane 141 in absence of the activator 142, contacts the power switch 143, the power switch 143 is pressed and an associated electronic circuit is completed. As shown in Figs.1 and 2, the power switch 143 is configured on a circuit board 144 which is arranged with suitable electronics, the suitable configuration of which is well within the abilities of a person skilled in the art. As previously discussed, the mushroom head 142’’’ of the activator 142 is configured to abut against a corresponding abutment surface 112’’’ of the second shell portion 112. This protects the power switch 143 and circuit board 144 from excessive pressure differentials, which could damage the power switch 143 and/or the circuit board 144.

The circuit board 144 further comprises a timer circuit (not shown) comprising an electronic timer which is configurable by the user. The user may configure the electronic timer to count down from 24 hours, or 48 hours, or any another period of time. The user may select the countdown time based on the expected time that the release mechanism will be at the operating depth/pressure.The power switch 143 in combination with the circuit board 144 and battery 150 activates the electronic timer of the timer circuit to begin the elapsing of time to count down from the period of time set by the user. In the presently described example, the circuit board 144 and electronic timer are configured to not start the motor 120 until the period of time set by the user has elapsed in the electronic timer. In this way, the motor 120 cannot be started until it is confirmed by the timeout of the timer that the structure is indeed lost and that the catch should therefore be released.

As previously explained, the timer circuit is configured such that elapsing of the timer duration starts when the release mechanism is exposed to the operating pressure, such that the timer duration only starts elapsing when the release mechanism reaches the operating depth. This ensures that the timer is not inadvertently started prematurely, which could result in releasing the catch when the structure is not lost.

Still referring to Figs.1 and 2, upon timeout of the timer, the motor 120 is turned on by the circuit board 144 and battery 150, and by the operative connection previously described the rotatable shaft 132 is turned to turn the rotatable hook 131.

The pressure sensitive membrane 141 is held within the second shell 112 by a retainer ring 145 and a retainer lock 146. These components allow for easy assembly of the release mechanism 100, and allow access to the membrane 141, activator 142 and biassing spring (if present), for servicing and repair, or for adjustment of the biasing strength of the biasing spring.

The arrangement of components can be summarised for ease of understanding as follows: the hydrostatic pressure outside of the release mechanism 100 increases as the release mechanism 100 is sunk to its operating depth where it experiences the operating pressure which causes the pressure differential across the membrane 141 and activator 142 and overcomes the internal pressure in the pressure chamber 110 and any spring biassing the activator 142. This pushes the power switch 143 which in turn starts elapsing of the electronic timer of the timer circuit. Once the pre-set period of time of the electronic timer has elapsed, the motor 120 powered by the battery 150 is activated. The motor 120 rotates the rotatable shaft 132, which turns the hook 131 from a locked configuration (shown in Figs.1 and 2) to an unlocked configuration (for example 90 degrees, 180 degrees or 270 degrees from that shown in Figs.1 and 2).

The release mechanism 100 may, in use, be positioned on an underwater structure (not shown) such that the hook 131 in the locked configuration shown in Figs.1 and 2 maintains a door of the structure closed such that the catch cannot escape, and in the unlocked configuration releases a door of the structure such that the catch can escape. For example, the user may plan to locate the release mechanism together with a lobster pot at a water depth of 3m for 24 hours in an attempt to catch lobsters. The user may then set the electronic timer to 28 hours and the operating depth at 3m or operating pressure at around 30kPa. If the lobster pot has not been recovered within 28 hours of locating the structure at 3m water depth (thereby leaving 4 hours continency) any lobsters within the lobster pot will be released.

The same principle as described above may be applied in another way in an alternative example, where the motor 150 is allowed to start upon activation of the power switch 143, as will now be described. In this alternative example, when the power switch 143 is pressed, the electronic timer starts to elapse and count down. The motor 150 may also be started at this time by the circuit board 144. However, in this example, the timer circuit is configured to move the coupling 134 from a disengaged configuration, where the rotatable shaft 132 and motor 150 are not operatively connected, to an engaged configuration, where the rotatable shaft 132 and motor 150 are operatively connected, when the timer ‘times out’. The motor 120, although energised to operate upon the membrane 141 and activator 142 pressing the power switch 143, does not rotate the rotatable shaft 132 because the coupling 134 is not moved to the engaged configuration until the timer has timed out.

Figs.3-6 show a door 200 of an underwater structure used for catching fish, crustaceans or other sea life. The door 200 comprises a frame 210 shaped and configured to register with a corresponding slot or groove in the underwater structure. The frame 210 provides an enclosure within which further components of the door are housed. Within the frame 210 there is a floatation element 220 which is conveniently circular in the presently described example to correspond with the general shape of the door 200, however in other examples it may be any other shape. The floatation element 220 comprises buoyant material in the presently described example. It will be understood that in other examples, buoyancy may be created by other means such as, but not limited to, containment of a liquid lighter than water, or containment of a gas. The door 200 further comprises a slot 230 which is shaped and configured to register with the hook 131 of the release mechanism 100 of Figs.1 and 2. In the locked configuration, the hook 131 is located in the slot 230, thereby holding the door 200 of the structure closed. In the unlocked configuration, the hook 131 is not located in the slot 230, and the door is therefore free to be removed from the structure, as will now be explained.

Since the door 200 is removed from the structure, the catch held within the structure is free to escape. In some examples, the door may further comprise a door tether (not shown) which is wound around a feature of the door so it can be unreeled and is attached at one end to the door 200 and at the other end to the structure. In this connection, when the door 200 is released from the structure by movement of the hook 131 out of the slot 230, the door 200 will float to the surface due to the floatation element 220 and thereby become visible on the surface. In the presently described example shown in Fig.3, the door 200 further comprises first and second pins 240, 240’ which are arranged to allow a door tether (not shown) to be wound around the floatation element 220. The door 200, or components or the door, such as the floatation element 220 may be bright colours, or starkly contrasting colours to those of the sea, lakes or oceans. When the door 200 is located by a person on the surface of the sea, the structure below the sea can be pulled up due to the connection of the door 200 to the structure by the door tether. In this regard, the structure can be recovered to avoid it becoming sea litter, and to allow it to be used again. The door 200 may, in some examples, comprise a visual display 250 which can be read from outside of the structure when the door 200 is assembled on the structure ready for use. In this connection, the visual display 250 may be configured to display information relating to the activation depth or pressure and/or the time remaining until the door will be released if sufficient pressure is provided to the membrane 141.

Although in the presently described example a rotatable locking hook mechanism 130 is described, another type of locking mechanism may be provided in other examples. For example, the locking mechanism may be a pin which is moved into engagement in a corresponding recess or aperture in the door by use of the electric motor. Alternatively, the locking mechanism may comprise additional pressure chambers, such that a fluid pressure can hold the door in the closed configuration and when the pressure is released, for example by venting of the pressure to the surrounding water, the door is released from the closed configuration.

Claims

1. A submersible catching device for catching fish, seafood or crustaceans, the device comprising:

a structure configured to receive a catch;

a door (200) moveable between a closed configuration in which the door (200) is secured to the structure such that the catch cannot escape and an open configuration in which the catch can escape;

a release mechanism (100) comprising a pressure sensitive device (140), a locking mechanism (130) and a timer circuit;

characterised in that the timer circuit comprises an electronic timer configured to start counting down from a pre-set time when the pressure sensitive device (140) is exposed to a pre-determined external pressure; and

the locking mechanism (130) is configured to be in operative engagement with the door (200) when the door (200) is in the closed configuration, and to release the door (200) from the closed configuration when the timer times-out;

such that, in use, when the catching device reaches a pre-determined depth the catch is automatically released from within the device when the timer times out.

2. A submersible catching device according to claim 1, wherein the structure is a lobster trap or lobster pot or creel or device for catching crabs.

3. A submersible device according to claim 1 or 2, wherein the release mechanism (100) further comprises:

a motor (120) connected to a battery (150);

a rotatable shaft (132) connected to the locking mechanism (130); and a power switch (143) configured in use to start the timer counting down on upon activation of the power switch (143);

wherein the pressure sensitive device (140) is configured to activate the power switch (143) when the pressure sensitive device (140) is exposed to a predetermined external pressure;

the timer circuit is configured to turn on the motor (120) upon time-out of the timer; and

the rotatable shaft (132) is operatively connected to the motor (120) such that, in use, when the motor (120) is turned on the rotatable shaft (132) rotates, thereby rotating the locking mechanism (130) out of engagement with the door (200).

4. A submersible catching device according to claim 3, wherein the pressure sensitive device (140) comprises an elastically deformable pressure sensitive membrane (141) comprising an inner surface (141’’) and an outer surface (141’), wherein the pressure sensitive membrane (141) is configured in the release mechanism (100) such that, in use, the outer surface (141’) is exposed to an external hydrostatic pressure, such that an increased external hydrostatic pressure elastically deforms the pressure sensitive membrane (141).

5. A submersible device according to claim 4, wherein the release mechanism (100) further comprises an activator (142) arranged in operative engagement with the inner surface (141’’) of the pressure sensitive membrane (141), such that, in use, elastic deformation of the pressure sensitive membrane (141) causes movement of the activator (142) between a first position in which the power switch (143) is not activated, and a second position in which the power switch (143) is activated.

6. A submersible device according to any preceding claim, wherein the release mechanism (100) further comprises a pressure chamber (110) configured to provide an internal pressure to the pressure sensitive device (140), thereby, in use, opposing the external pressure experienced by the pressure sensitive device (140) when the device (140) is exposed to an external pressure up to the pre-determined pressure.

7. A submersible device according to any preceding claim, wherein the electronic timer is configured to be settable by a user of the device.

8. A submersible device according to any preceding claim, wherein the door (200) further comprises a floatation element (220) configured with a buoyancy such that, in use;

when the door (200) is in the closed configuration and the device is submerged in water, the device does not float to the surface; and

when the door (200) is released from the closed configuration, the door (200) has sufficient buoyancy to float to the surface.

9. A submersible device according to any preceding claim, further comprising a door tether with a first end and a second end, wherein the first end is attached to the structure and the second end is attached to the door (200), such that when the door (200) is released from the closed configuration the door tether connects the door (200) to the structure.

10. A method for automatically opening a lost submersible catching device, comprising the steps of:

a. providing a submersible catching device according to any of claims 1 to 9; b. submerging the catching device in water;

c. exposing the pressure sensitive device (140) to the pre-determined external pressure;

d. letting the timer time-out;

e. automatically releasing the door (200) from the closed configuration by moving the locking mechanism (130) out of operative engagement with the door (200).

11. A method according to claim 10, wherein step c. comprises sinking the device to a pre-determined depth.

12. A release mechanism (100) for releasing a door (200) of a submersible device, the release mechanism (100) comprising:

a pressure sensitive device (140), a locking mechanism (130) and a timer circuit;

characterised in that the timer circuit comprises an electronic timer configured to start counting down from a pre-set time when the pressure sensitive device (140) is exposed to a pre-determined external pressure;

such that, in use, when the release mechanism (100) sinks to a pre-determined depth the release mechanism (100) releases the door (200) a pre-determined amount of time after the release mechanism (100) has reached the predetermined depth.

13. A release mechanism (100) according to claim 12, further comprising:

a motor (120) connected to a battery (150);