US3812984A

US3812984A - Transport of woodchips

Info

Publication number: US3812984A
Application number: US00249655A
Authority: US
Inventors: J Cladingboel
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-05-06
Filing date: 1972-05-02
Publication date: 1974-05-28
Anticipated expiration: 1991-05-28
Also published as: CA973116A

Abstract

The invention resides in a method of loading and unloading woodchips for transport by marine vessels. The woodchip is mixed with water and pumped to the hold into which it is discharged at high velocity to achieve a degree of compaction. Water is separated simultaneously from beneath the cargo and pumped out of the hold. For unloading the cargo is disturbed and remixed with water at hold floor level and then pumped from the ship.

Description

United States Patent [191 Cladingboel 1 TRANSPORT OF WOODCHIPS [76] Inventor: James David Cladingboel, 6 Regent St., Mount Waverly, Victoria, Australia [22] Filed: May 2, 1972 [2]] Appl. No.: 249,655

[30] Foreign Application Priority Data May 6, 1971 Australia 28597/71 May 31, 1971 Australia 29476/71 [52] U.S. C1. 214/15 B, 214/152, 302/14 [51] Int. Cl. B63g 53/30 [58] Field of Search 214/14, 15 R, 15 B, 152;

[56] References Cited UNlTED STATES PATENTS McDougall 214/15 B X 11 3,812,984 1451 May 28, 1974 689,741 12/1901 McDougall 214/15 B 1,127,137 2/1915 West 214/15 B 2,884,147 4/1959 Hunt et a1. 1. 214/15 B X 3,445,008 5/1969 Koch 214/14 3,630,400 12/1971 DeKoning 214/15 B X Primary Examiner-Frank E. Werner Attorney, Agent, or Firm1-1arold L. Stowell [57] ABSTRACT The invention resides in a method of loading and unloading woodchips for transport by marine vessels. The woodchip is mixed'with water and pumped to the hold into which it is discharged at high velocity to achieve a degree of compaction, Water is separated simultaneously from beneath the cargo and pumped out of the hold. For unloading the cargo is disturbed and remixed with water at hold floor level and then pumped from the ship.

7 Claims 6 Drawing Figures 'l I TRANSPORT OF WOODCHIPS This invention relates to methods and apparatus for the loading, compacting and discharging of woodchips and like materials into and from the holds or compartments of ships.

The carriage of woodchips in bulk is a relatively new development in International Shipping, but is already a very large trade, and has considerable growth potential not only from a continued increase in world consumption of items derived from woodchips, but also as industrial nations have to go further afield to secure quantities of this raw material from the few remaining areas of the world containing suitable forest resources.

The particular nature of woodchips is such that, in order to secure a reasonably efficient utilization of ship space, the individual particles need to be compressed, or compacted, into the smallest possible volume. One cubic foot of solid timber, after chipping, will occupy a volume of about 2.5 cubic feet, but efficient compaction into a suitable compartment can reduce this volume toabout 2.2 cubic feet. One ton of woodchips therefore averages about 90 cubic feet or more of ship space, as compared with e.g. 8 cubic feet/ton for chrome ore, and about cu.ft/ton for iron'ores.

The current method of loading woodchips consists of conveyor belts from the stockpile areas feeding a single pneumatic loading tower, in which woodchips are blown at high speed down a delivery spout into the holds or compartments of the ship or vessel being loaded. Within certain limitations, this system works quite well, although there is increasing evidence to suggest that such systems, designed originally to handle softwood chips, are less effective in handling the eucalypt hardwood chips becoming available from Australia and, in the near future, Brazil.

The pneumatic delivery of woodchip at high speed is sufficient to achieve a good order of cargo compaction, and an installation of this type is normally capable of loading woodchips at a rate of some 500 or 600 tons per hour of nett operational time.

At present, woodchips are normally carried in specialized ships equipped with their own cranes and highcapacity grabs. These vessels also carry hoppers and conveyor belts on board, so that vessels are selfsufficient in the recovery of cargo from their holds, and its assembly by the conveyor belts to one or more points along vessels sides where transfer of the woodchip cargo is effected to a shore-based conveyor system, and thence to a stockpile area.

This method of discharging woodchip and like materials works quite well, and normally achieves rates of between 500 and 700 tons per hour. However it has the disadvantages of extremely high capital costs: for the cranes, each of which needs to be about l0 tons lifting capacity; for the special grabs needed to handle woodchips efficiently; and for the deck-mounted systems of hoppers and conveyor belts.

The high capital cost of these items inhibits any great plurality of these installations, and it is common to find a woodchip carrying vessel of six holds fitted with only three deck cranes and grabs. Under these circumstances, a vessel is only 50 percent efficient in the discharge of cargo.

The disadvantages of the current, conventional, systems can be summarized briefly as follows:

1. High capital costof pneumatic systems, loading tower and conveyor belts.

2. Delays caused by loading only one hold or compartment at any one time. Either vessels must move to place each hold in turn beneath the loading tower, or the whole loading device must be enabled to move along the length of the ship. in either event, four, five or even seven such moves are necessary during the loading of a'ship, with consequent loss of time.

3. The need for conveyor belts from stockpile to loading point,'which may besome distance off- ,shore to allow an adequate depth for ship operation, together with the frequent necessity to cover such conveyor belts to protect the woodchip against wind and rain.

4. The'maintenance cost of the pneumatic equipment, loading tower and associated conveyor systems.

5. The relative inefficiency of shipborne cranes, as

described above.

6. The high degree of maintenance of theshipborne equipment, exposed on deck at all times to salt water andsalt air.

7. The relatively high incidence of woodchip damage, i.e. degradation of particle size, which occurs in pneumatic systems, and which appears to be aggravated in the case of hardwood chips.

It is a general object of this invention to provide a new method of loading woodchip and like materials into the holds or compartments of ships, which greatly reduces installed capital cost, and similarly reduces maintenance costs of the loading equipment.

Another object of this invention is to provide a method of loading woodchip and like materials at a plurality of loading points thus enabling a continuous loading rate greater than that obtainable by present methods.

Another object of this invention is to provide a high degree of cargo compaction in the.loading of woodchip.

A further main object of this invention is to provide new method of discharging woodchip and like materials-from ships, which greatlyreduces the capital cost of installation, and similarly reduces the cost of operating and maintaining the discharging equipment.

Another object of this invention is to provide a method for the discharge of woodchip and like material from ships or vessels or barges having no specialized cranes or grabs or conveyors, or from such vessels as have no cargo handling gear whatsoever.

Another object of this invention is to provide a system of discharging woodchip and like material which is sufficiently economical and simple to justify installation into relatively small ships, where the high cost of cranes, grabs, conveyors and hoppers would be prohibitive.

Another object of this invention is to create a system of discharging woodchip and like materials from a vessel which can be installed with no constructional changes, or with only minor constructional changes, to the vessel.

According to one of its aspects the invention provides a method of loading woodchip into a marine vessel, comprising the steps of:

a. forming a pumpable mixture of woodchip and water,

b. pumping the mixture at a controlled rate through at least one pipeline to at least one hold of the vessel,

c. discharging the mixture into the hold in a controlled direction at a velocity sufficient to achieve at least some degree of compaction of the woodchip,

d. simultaneously with steps (b) and (c), draining water from the mixture towards the floor of the hold and pumping said water from the hold at a controlled rate while preventing access of the woodchip to the pump inlet.

According to another of its aspects the invention provides a method of unloading woodchip from a hold of a marine vessel comprising the steps of:

a. providing a pump inlet at or near the floor of the hold,

b. removing any obstruction preventing access of the woodchip to the pump inlet,

c. forming a pumpable water/woodchip mixture at least adjacent the pump inlet by admitting water to said hold through jet nozzles arranged adjacent said pump inlet, 2

(1. drawing said mixture into the pump inlet and pumping it from the hold while continuing to form water/woodchip mixture by admitting water to said hold through jet nozzles arranged at points progressively more remote from said pump inlet, said steps being carried out in any suitable order. According to yet another of its aspects the invention provides a vessel for the transport of woodchip comprising at least one hold; a pump mounted within said hold adjacent the floor of the hold; movable means to prevent or permit access of woodchip to said pump; a plurality of water jets ar- 1 ranged at or near the floor of the hold, some of said jets being adapted to discharge water at high velocity towards said pump and some of said jets being arranged to discharge water at high velocity in a vertical direction; and means to control said jets to enable those nearer to the pump to be brought into action earlier than those farther from'the pump.

In order that the invention may be more readily understood, it will now be described by way of example with reference to the accompanying drawings wherein:

FIG. I is a perspective view of a ship being loaded with woodchip by the method of the invention.

FIG. 2 is a crosssectional view of a hold of the ship on line 22 of FIG. 1.

FIG. 3 is a plan view of the hold on line 33 of FIG. 2.

FIG. 4 is a cross-sectional view on line 44 of FIG. 3 illustrating the arrangement of a pump in the hold.

FIG. 5 is a cross-sectional view on line 5--5 of FIG. 3 illustrating diagrammatically certain stages in the unloading of woodchip.

FIG. 6 is a diagrammatic view illustrating the separation of woodchip from water during the unloading process.

FIG. 1 illustrates a ship designed for bulk carriage of goods having its engines and other equipment towards the stem. The ship has five holds 10 which are preferably loaded with woodchips simultaneously by the method of this invention. For reasons which will become apparent hereinafter each hold is filled by means of an individual pump 12 and pipelinel4. In each hold, preferably against the rear transverse bulkhead of that hold, there is provided a direct-coupled submersible pump 16 which is preferably a centrifugal pump having its axial inlet directed downwardly and spaced a short distance from the floor of the hold.

The pump inlet is surrounded by a horizontal pressure plate 18 which is supported on legs 20 with its lower surface a distance above the level of the hold floor 22 which is at least slightly less than the maximum dimension of particle which the pump is capable of handling. The gap between the periphery of the pressure plate and the hold floor 22 is adapted to be covered by a screen 24 which in turn is adapted to be removed from its position on the floor of the hold and drawn upwardly through the cargo by means of a hawser 26 for a purpose to be described hereinafter.

As best seen in FIGS. 2 and 5 the screen is approximately semi-circular in shape and extends over a depth of approximately half the height of the hold. The top of the screen is covered to prevent woodchip reaching the pump during loading.

The casing of pump 16 is provided with two vertically spaced

sensors

28, 29 each of which'is adapted to produce a signal according to.whether or not the water level in the hold is above it or below it. The floor of the holdis provided with a number of vertical water jets 30 which are adapted to discharge water at high velocity upwardly into the cargo and a number of horizontal water jets 32 each of which is adapted to discharge water at high velocity horizontally across the floor of the hold towards the inletof pump 16.

Each pipeline 14 is connected to one or more monitors 34 mounted adjacent the hatch opening of each hold. Preferably there are two such monitors for each hold and they are mounted in known manner to enable them to be moved out of their operating position to enable the hatch cover to be removed and replaced in position to close the hold. Each monitor 34 is universally mounted to permit its discharge to be directed over a wide area of the hold. The sequence of operations in loading a cargo of woodchip is as follows:

At a suitable point on shore, preferably located as close to a moored ship as possible, woodchip is fed into a mixing tank and a. pumpable mixture of woodchip and water is formed. For most applications the water may be either fresh or salt. The mixture is removed from the tank by means of a plurality of pumps 12 preferably equal in number to the number of holds 10 of the ship.

The mixture passes through pipelines 12 to the ship where each pipeline divides to feed two monitors 34. The monitors are of standard construction having a venturi-like outlet which increases the velocity of the mixture as it is discharged. Each of the monitors is manned by an operator, who directs the mixture into the hold in such manner as to fill the parts of the hold remote from pump 16 before commencing to fill the hold in the region of the pump. The high velocity of the mixture entering the hold and the weight of water seeping through the cargo ensures a degree of compaction of the woodchip.

As the woodchip builds up on the floor of the hold, water drains from it and commences to fill the hold. The lower sensor 28 associated with each pump controls the pump in such manner that, as soon as a sensor 28 is submerged in water it starts the pump to commence pumping the water out through pipeline 36 connected to the outlet of pump 16. If at any time during the filling of the hold the water level drops below the lower sensor 28 the pump 16 will be temporarily shut off. Conversely, if the rate of filling of the hold is so high that water is fed in faster than it can be pumped out, the upper sensor 29 operates to shut-off the appropriate loading pump 12 until such time as the water level again drops below upper sensor 29.

The water removed fromthe cargo may be merely pumped over the side or may be returned to the mixing tank for further use. To assist in compaction of the cargo the main engine of the vessel may, if so desired, be kept running during loading to give a mechanicalvibratory and a tamping effect.

If so desired, provision may be made in the electrical system to over-ride the lower sensor 28 to remove more of the water from the hold after loading has been completed. It will be appreciated that for quite a long period after loading has been completed further quantities of water will gradually percolate down through the cargo towards the floor of the hold.

To remove the cargo from the hold at the discharge port it is necessary to reform the pumpable woodchip and water mixture. For greatest efiiciency the necessary actions are therefore preferably commenced sometime before the ship is moored in position at its unloading berth. The hatch covers are removed and the monitors 34 are placed in position. Discharge pipeline 36 is also placed in position and connected to pump 16. Screen 24 is pulled up through the cargo by means of hawser 26 thereby disturbing and freeing the woodchip in the region of the pump to assist in forming the water/woodchip mixture. The hold is filled with water at least up to the height of the lower sensor 28 either from the ships ballast tanks or by water drawn in through the ships seacocks.

When the ship has reached its mooring position and has been connected to means for transporting the woodchip to a shore-based place of storage the pump is started and the jets nearest to the pump are turned on to discharge water at high velocity, some towards the pump and some vertically upwards to release woodchip for inclusion in the water/woodchip mixture.

It is found that woodchip has a high capacity to form a bridge and to resist collapsing when part of the material supporting it is removed. The initial-stage of discharge of the cargo therefore results in a cavity beneath the woodchip in the region of the pump in a shape which may be similar to that indicated by line 50 in FIG. 5. As unloading proceeds, jets further from the pump are broughtinto operation to extend the cavity beneath the cargo by freeing further quantities of woodchip.

From time to time the cargo in various areas collapses onto the floor of the hold and is swept towards the pump in a water/woodchip mixture by means of the horizontal jets. Concurrently water is supplied to the hold from the monitors 341 and the force of these jets coupled with the weight of the water seeping down through the cargo. assists in collapsing the cargo to permit it to be pumped from the hold.

In some cases the water/woodchip mixture may be pumped to a location on shore where the water is separated from the woodchip and either discharged to waste or, if required, is returned to the ship for further use.

In other cases it will be found desirable to take the woodchip ashore in a substantially dry condition. In such case an arrangement similar to that illustrated in FIG. 6 may be used. In this figure discharge pipeline 36 is shown feeding the mixture to a screen 54 having a steep angle to permit the woodchip to flow to the hopper 56 of a belt conveyor 58. The water leaves the woodchip and passes through screen 54 into a trough 60 from which it can be either discharged over the side 62 of the vessel or recirculated to carry further quantities of woodchip to screen 54.

The control of switching on the jets at increasing distances from the pump during unloading may be either automatic, for example on a controlled time cycle, or may be carried out manually according to the judgment of the crew of the vessel.

in the standard construction of bulk carriers, engines and crew accommodation are located towards the stern of the vessel. Consequently such vessels have a natural tendency not to float on an even keel but to trim down by the stern when they are not loaded. This attitude may be easily maintained during both loading and discharging operations. lt is for this reason that it is preferred to locate the pump 16 against the after bulkhead of each hold so that removal of the last pieces of cargo or contained matter is facilitated.

Sensors

28 and 29 also control the pump and the supply of water to theholds during unloading. If there is a great difference between the weights in various holds at any given moment hogging or sagging of the ship can result with possible structural damage. Consequently the upper sensors 29 are used to shut off the supply of water to the hold if the water level tends to rise above these sensors. The lower sensors 28 shut off the pump if the watersupply is such that continued operation of the pump would cause the water level to drop to a stage where the pump is running dry.

As in loading, it is necessary to over-ride the lower sensors 28 during unloading to enable the last of the cargo to be pumped ashore.

ln some cases it may be necessary to mechanically disturb the cargo during unloading. This may be performed by running the ships main engines or in some cases suitable mechanical devices may be immersed in the cargo. Additional disturbance of the load may be achieved by placing further water jets in the walls of the hold directed into the cargo.

For loading the vessel there must be complimentary and simultaneous operation of both shore equipment and ship-borne equipment. During unloading the ship is self-sufficient except possibly for the provision of shore. power and/or water.

The invention enables the convention or conveyor systems for loading and unloading to be replaced by pipelines with the advantages of lower capital cost and no need for protection from the weather. In the case of a loading facility the pipelines may be laid on or under the seabed to an offshore loading terminal. The invention also enables the woodchip to be discharged from the ship in a transportable form mixed with water or in a substantially dry form. In the latter case existing installations for taking the woodchip ashore may be used but the removal of the cargo from the hold is greatly facilitated. The system of the invention is thus compatable with existing systems.

Performance and economy taking the instance of a vessel of six holds and hatches, being loaded at a distance some 1,500 feet offshore with 30,000 longtons woodchip.

A 12 inches diameter pipeline, delivering woodchip and water mixture of 30 percent solid content into each hold at a velocity of 18 feet per second would achieve a delivery of about 400 tons woodchip per hour/per hold, enabling loading to be completed in about 12 hours.

The cost of six pumps, mixing tanks and delivery pipelines would be between 20 percent and 30 percent of the cost of pneumatic equipment which would take more than 4 times as long for the same task.

As regards power requirements, operating six pumps under the conditions outlined above would absorb about 1,500 horsepower, similar to that required, over a longer period, for penumatic equipment. Power for shipborne pumps, handling the contained water, would be approximately 1,800 horsepower, but would be net by ships own generating capacity.

The vessel hypothesized above would have in each of the six holds, two submersible pumps with 150 horsepower direct-coupled electric motors (also fully submersible and of a generally available type) capable of discharging from each hold about 6,000 gallons per minute of water, or about 5,000 gallons per minute of woodchip and water mixture.

During loading at the rate of 400 tons per hour of woodchip, some 3,000 gallons per minute of water is also delivered, therefore pumps on board need to operate for about 50 percent of loading time to handle the return or discharge of such contained water.

Twelve shipborne pumps are capable of discharging 30,000 tons of woodchip at the same speed as it was loaded; but at hold-floor level there is very little control over the solid content of the mixture, and uniform solids concentrations of 30 percent cannot be achieved. However, average solids content must fall below 10 percent before pumps lose their superiority in speed over cranes and grabs.

in terms of cost to the ship, 12 pumps and associated equipment as described are only about 25 percent of current costs of 3 deck cranes, grabs and hopper/conveyor equipment.

In addition to the above, two singular side benefits accrue from the use of this invention as described above. Firstly, the act of vigorously agitating woodchip in water, as is done both in loading and discharging, where woodchip passes through centrifugal pumps, serves to remove some part of the lignins, tannins and other impurities, thereby beneficiating the raw material.

Even repeated passage of woodchip through centrifugal pumps has very little damaging effect on the particles, and particles also benefit from the cushioning ef fect of the water in passing through pipelines. Size degradation is therefore largely eliminated in this method of handling.

Secondly, a vessel fitted with pumps in each compartment capable of handling about 6,000 gallons per minute of water. is in a very good position to protect itself from the effects of any collision or grounding which damages the hull below the waterline.

The screens placed around the slurry pumps immediately prior to loading ar not removed until vessel has reached her discharging port in order that water discharging capability of these pumps is always available to the ship. Removal of these screens immediately prior to discharge has the secondary benefit of loosening the compacted woodchip in the immediate vicinity of each pump as they are winched upwards through the cargo mass.

Thus there has been disclosed a novel, economical and high-speed alternative to conventional methods of loading and discharging woodchips and like materials which, by their particular nature and characteristics pose unusual material handling problems.

I claim:

1. A method of loading woodchip into a marine vessel comprising the steps of:

a. forming a pumpable mixture of woodchip and water,

d. simultaneously with steps (b) and (c), draining water from the mixture towards the floor of the hold and pumpingsaid water from the hold at a controlled rate while preventing access of the woodchip to the pump inlet,

wherein the woodchip and water mixture fed to a particular hold is pumped by an individual loading pump and wherein the pumping of said individual loading pump is'discontinued if the water level in the hold rises above a predetermined level, and the pumping of the drained water from the hold is discontinued if the water level in the hold drops below a predetermined level.

2. A method of unloading woodchip from a hold of a marine vessel comprising the steps of:

a. providing a pump inlet at or near the floor of the hold,

c. forming a pumpablewater/woodchip mixture at least adjacent the pump inlet by admitting water to said hold through jet nozzles arranged adjacent said pump inlet,

d, drawing said mixture into the pump inlet and pumping it from the hold while continuing to form water/woodchip mixture by admitting water to said hold through jet nozzles arranged at points progres-. sively more remote from said pump inlet,

said steps being carried out in any suitable order, and

further including the steps of discontinuing the supply of water to the hold if the water level therein .rises above a predetermined limit and discontinuing the pumping of the woodchip and water mixture from the hold if the water level therein falls below a predetermined limit.

3. Apparatus for loading woodchip into a marine vessel comprising a vessel having means to form a pumpable mixture of water and woodchip, at least two loading pumps connected to said vessel; an individual pipeline from each of said pumps adapted to transport the mixture to an individual hold of the vessel; means on said vessel associated with each hold of the vessel adapted to be connected to one of said pipelines and adapted to discharge the mixture into the hold in a controlled direction; a discharge pump mounted within each hold at or adjacent the floor of the hold and adapted to pump water from the hold as the mixture is pumped in; means to prevent access of the woodchip to the pump during loading; and sensing means to sense the water level in the hold to enable the pumping of the associated loading pump to be discontinued if the water level in the hold rises above a predetermined limit and to enable the pumping of said discharge pump to be discontinued if the water level in the hold falls below a predetermined level.

4. A method of unloading woodchip from a hold of a marine vessel comprising the steps of:

a. providing a pump inlet at or near the floor of the hold,

b providing a screen for said pump inlet which permits access of water to the pump but which prevents woodchip reaching the pump,

0. withdrawing said screen from the position in which it protects the pump inlet upwardly through said cargo thereby disturbing and freeing at least some of the woodchip in the area of the hold surrounding the pump inlet,

d. forming a pumpable water/woodchip mixture at least adjacent the pump inlet by admitting water to said hold through jet nozzles arranged adjacent said pump inlet,

e. drawing said mixture into the pump inlet and pumping it from the hold while continuing to form water/woodchip mixture by admitting water to said hold through jet nozzles arranged at points progressively more remote from said pump inlet, said steps being carried out in any suitable order.

5. A method of unloading woodchip as claimed in claim 4 including the step of feeding additional water to said hold through at least one high-velocity jet arranged above the upper level of the woodchip in said hold during unloading.

6. A method of unloading woodchip as claimed in claim 4 including the step of separating water from the water and woodchip mixture after the mixture has been pumped from the hold and before the woodchip is conveyed to a storage area.

7. A vessel for the transport of woodchip comprising at least one hold; a pump mounted within said hold adjacent the floor of the hold; a movable screen to prevent or permit access of woodchip to said pump; means for drawing said movable screen upwardly through a cargo in said hold; a plurality of water jets arranged at or near the floor of the hold, some of said jets being adapted to discharge water at high velocity towards said pump and some of said jetsbeing arranged to discharge water at high velocity in a vertical direction and means to control said jets to enable those nearer to the pump to be brought into action earlier than those farther from the pump. V

Claims

1. A method of loading woodchip into a marine vessel comprising the steps of: a. forming a pumpable mixture of woodchip and water, b. pumping the mixture at a controlled rate through at least one pipeline to at least one hold of the vessel, c. discharging the mixture into the hold in a controlled direction at a velocity sufficient to achieve at least some degree of compaction of the woodchip, d. simultaneously with steps (b) and (c), draining water from the mixture towards the floor of the hold and pumping said water from the hold at a controlled rate while preventing access of the woodchip to the pump inlet, wherein the woodchip and water mixture fed to a particular hold is pumped by an individual loading pump and wherein the pumping of said individual loading pump is discontinued if the water level in the hold rises above a predetermined level, and the pumping of the drained water from the hold is discontinued if the water level in the hold drops below a predetermined level.

2. A method of unloading woodchip from a hold of a marine vessel comprising the steps of: a. providing a pump inlet at or near the floor of the hold, b. removing any obstruction preventing access of the woodchip to the pump inlet, c. forming a pumpable water/woodchip mixture at least adjacent the pump inlet by admitting water to said hold through jet nozzles arranged adjacent said pump inlet, d. drawing said mixture into the pump inlet and pumping it from the hold while continuing to form water/woodchip mixture by admitting water to said hold through jet nozzles arranged at points progressively more remote from said pump inlet, said steps being carried out in any suitable order, and further including the steps of discontinuing the supply of water to the hold if the water level therein rises above a predetermined limit and discontinuing the pumping of the woodchip and water mixture from the hold if the water level therein falls below a predetermined limit.

4. A method of unloading woodchip from a hold of a marine vessel comprising the steps of: a. providing a pump inlet at or near the floor of the hold, b. providing a screen for said pump inlet which permits access of water to the pump but which prevents woodchip reaching the pump, c. withdrawing said screen from the position in which it protects the pump inlet upwardly through said cargo thereby disturbing and freeing at least some of the woodchip in the area of the hold surrounding the pump inlet, d. forming a pumpable water/woodchip mixture at least adjacent the pump inlet by admitting water to said hold through jet nozzles arranged adjacent said pump inlet, e. drawing said mixture into the pump inlet and pumping it from the hold while continuing to form water/woodchip mixture by admitting water to said hold through jet nozzles arranged at points progressively more remote from said pump inlet, said steps being carried out in any suitable order.

7. A vessel for the transport of woodchip comprising at least one hold; a pump mounted within said hold adjacent the floor of the hold; a movable screen to prevent or permit access of woodchip to said pump; means for drawing said movable screen upwardly through a cargo in said hold; a plurality of water jets arranged at or near the floor of the hold, some of said jets being adapted to discharge water at high velocity towards said pump and some of said jets being arranged to discharge water at high velocity in a vertical direction and means to control said jets to enable those nearer to the pump to be brought into action earlier than those farther from the pump.