ship yard practices

                                   

DESCRIPTION OF CONSTRUCTION STAGE IN SHIPBUILDING

•      Treatment of steel in a shipyard

•      In the shipyard corrosion protection is of prime importance.

•      Pre-Heating

•      The plates & profiles first pass through a pre-heater, this raises the temperature of the metal ready for blasting & removes any surface moisture.

•      Blast Cleaning

•      Sheets & profiles are thoroughly blast cleaned.The blast chamber removes rust and mill scale  and provides a finish to internationally recognised preparation grades

•      Rust grades

•      Four rust grades are specified. These are defined by precise written descriptions

•      and photographic examples in ISO 8501-1documentation. They vary from A: mill scale,to D: where the mill scale has rusted away and general pitting is visible.

•      Preparation grades

•      Surface preparation by blast cleaning is designated by the international standard ISO8501. Four grades are specified, ranging from Sa 1: light blast cleaning, to Sa 3: blast cleaning to visually clean steel.

•      Paint spray chamber, voc-system and filter

•      The various plate and profile widths are automatically identified and are coated in a continuous process with a weld-primer coating

•      thickness of approximately 15 - 25μm.

•      Paint dust and solvents (if no water based paint is used) are treated according to the local requirements in an automatic filter unit and aVOC-treatment plant.

•      Drying chamber & slat conveyor

•      This chamber can be heated by the exhausted air from the pre-heater . Additional circulation of high quantities of air accelerates the drying process.

•      Lying on the support points of the conveyor cross slats the wet primer remains undamaged as the plates pass through the drying chamber.

•      Marking

•      Each sheet has its own unique identification to allow subsequent allocation and control. The mark is spray painted by computer controlled nozzles.

•      Edge cleaning

•      To ensure optimum weld quality, the edges of the profiles are blasted to remove paint from the weld area .

•      Airblast units for this operation are suitable for smaller profiles and often precede a gas-cutting machine.

•      Conveying

•      The plates are now conveyed to be fabricated into ship segments

•      Building and repair activity has changed considerably over the years not least with the introduction of prefabrication techniques.

•      New technology providing exact measurement has allowed the larger and heavier section to become the norm rather than the exception

COMPUTER AIDED DESIGN / ENGINEERING / MANUFACTURE

Since the late 1970s, developments in computer hardware based on microchip technology have made available to industry very powerful computer systems at moderate cost. This technology has led to a significant advance in the capability of systems for computer aided design, computer aided engineering and computer aided manufacture (CAD/CAE/CAM) which are commonly available to industry. Such systems have steadily replaced manual draughting and numerically controlled parts programming in many shipyards. Computer based shipbuilding systems developed during the late 1960s and early 1970s and used for numerically controlled production machines and loftwork , have now been enhanced to extend their scope to the design and drawing office functions and provide interfaces with a comprehensive

range of other shipyard systems.

The CAD/CAE/CAM systems are based on a 3D Ship Product Model in

which the geometry and the attributes of all elements of the ship, derived from the contract design and classification society structural Requirements are stored.

This model can be visualized at all stages and can be exploited to

obtain information for production of the ship.

It has been common practice for the drawing office to contain a material

ordering department, which was able to lift the necessary requirements

from the drawings and process them. Also, the office worked in close conjunction with the loft and planning production office. With the introduction of CAD/CAE/CAM systems to the drawing office these functions have been increased and improved. From the 3D ship product model, the precision of the structural drawings generated enables them to be used with greater confidence than was possible with manual drawings and the requisitioning information can be stored on the computer to be interfaced with the shipyards commercial systems for purchasing and material control. Subassembly, assembly and block drawings can be created in 2-dimensional and 3-dimensional form and a library of standard production sequences and production facilities can be called up so that the draughtsman can ensure the structural design uses the yards resources efficiently and follows established and cost effective practices. Weld lengths and types, steel weights and detailed parts lists can be processed from the information on the drawing and passed to the production control systems. A 3-dimensional steel assembly can be rotated by a draughtsman on screen to assess the best orientation for maximum downhand welding. The use of 3-dimensional drawings is particularly valuable in the area of outfit drawings where items

like pipework and trunking can be ‘sighted’ in the 3-dimensional mode and more accurately measured before being created in the 2-dimensional drawing.

Today CAD/CAM software programs for ship construction are available which can be run on ordinary PCs having sufficient memory. These programs can be used to design a ship according to one’s own requirements. Besides hull design and production of plans it is also possible to calculate hydrostatics and stability, internal structural design, Strength, wind and sea resistance and power requirements.

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WATERTIGHT DOORS

                                                WATERTIGHT DOORS

We need water-tight doors, to exclude the possibility of one or more of the vessel’s spaces becoming flooded & compromising the water-tight integrity of the vessel.

In order to maintain the efficiency of a watertight bulkhead it is desirable that it remains intact. However in some instances it becomes necessary to provide access between compartments on either side of a watertight bulkhead and watertight doors are fitted for this purpose. A particular example of this in cargo ships is the direct means of access required between the engine room and the shaft tunnel. In passenger ships watertight doors are more frequently found where they allow passengers to pass between

one point of the accommodation and another.

Where a doorway is cut in the lower part of a watertight bulkhead care must be taken to maintain the strength of the bulkhead. The opening is to be framed and reinforced, if the vertical stiffeners are cut in way of the opening. If the stiffener spacing is increased to accommodate the opening, the scantlings of the stiffeners on either side of the opening are increased to give an equivalent strength to that of an unpierced bulkhead. The actual opening is kept as small as possible, the access to the shaft tunnel being about 1000 to 1250 mm high and about 700 mm wide. In passenger accommodation

the openings would be somewhat larger.

Mild steel or cast steel watertight doors fitted below the water line are either of the vertical or horizontal sliding type. A swinging hinged type of door could prove impossible to close in the event of flooding and is not permitted.

Hinged water tight doors may be allowed in passenger ships and in watertight bulkheads above decks which are placed 2.2m or more above the waterline. Similar doors may be fitted in weather decks openings in cargo ships.

Hinged watertight doors are not permitted below the waterline.

These may be similar to the weathertight doors fitted in superstructures,  but are to have gunmetal pins in the hinges.

                                  Hinged Door

                                               

Sliding W/T doors may be hand operated, but in most modern ships they are hydraulically controlled.

Power-operated sliding doors shall be capable of being closed simultaneously from the central operating console at the navigation bridge in not more than 60 s with the ship in the upright position.

•     The means of operation whether by power or by hand of any power operated sliding watertight door shall be capable of closing the door with the ship listed to 15 degrees either  way.

•     Watertight door controls, including hydraulic piping and electric cables, shall be kept as close as practicable to the bulkhead in which the doors are fitted, in order to minimize the likelihood of them being involved in any damage which the ship may sustain.

•     All power-operated sliding watertight doors shall be provided with means of indication which will show at all remote operating positions whether the doors are open or closed.

•     Each power-operated sliding watertight door shall have a vertical or horizontal motion

Sliding Doors

                                                           

                                                            Vertically Operated Sliding Door

•     It shall be provided with an individual hand-operated mechanism. It shall be possible to open and close the door by hand at the door itself from either side, and in addition, close the door from an accessible position above the bulkhead deck with an all round crank motion or some other movement providing the same degree of safety acceptable to the Administration.

•     Direction of rotation or other movement is to be clearly indicated at all operating positions. The time necessary for the complete closure of the door, when operating by hand gear, shall not exceed 90 s with the ship in the upright position

•     Watertight doors for passenger ships are tested to a head of water equivalent to their depth below the bulkhead deck before they are installed in the ship. When in place,these and other watertight doors are hose tested.

•     It shall be provided with controls for opening and closing the door by power from both sides of the door and also for closing the door by power from the central operating console at the navigation bridge; It shall be provided with an audible alarm, distinct from any other alarm in the area, which will sound whenever the door is closed remotely by power and which shall sound for at least 5 s but no more than 10 s before the door begins to move and shall continue sounding until the door is completely closed.

•      In the case of remote hand operation it is sufficient for the audible alarm to sound only when the door is moving.

•     Administration may require the audible alarm to be supplemented by an intermittent visual signal at the door and  shall have an approximately uniform rate of closure under power.

•     The electrical power required for power-operated sliding watertight doors shall be supplied from the emergency switchboard either directly or by a dedicated distribution board situated above the bulkhead deck

Watertight Tunnels :

 Unless the ship’s machinery is right aft, a watertight tunnel will be fitted to enclose the propeller shaft. The tunnel protects the shaft from the cargo.

To allow the passage for personnel water tight doors are fitted,  openings must be cut only were essential and they should be as small as possible. 1.0 - 1.25m high, 0.7m wide being the usual.

Doors should be of mild steel or cast steel, and they may be arranged to close vertically or horizontally.

A sliding w/t door will be fitted at the forward end leading to the engine room. Also, at the aft end, a trunk leading to the bulkhead deck is often fitted since there must be two means of escape from the shaft tunnel.

The thickness of the tunnel plating and stiffener scantlings are determined in similar manner to that for watertight bulkheads.

The tunnel top is lighter if it is rounded rather than flat, but must be of increased thickness under hatchways unless it is sheathed with timber.

                                   

Cargo W/T Doors

Cargo doors are fitted on ships in certain trades to provide access to ’tween deck spaces e.g. direct loading by forklift truck from the quay into the ’tween deck.

Openings are cut in the side shell plating and arrangements must be made to maintain the strength, particularly in a longitudinal direction. The corners of all openings are to be well rounded to avoid stress concentrations.

Following are illustrated :

1)    A cargo port, manually operated by closely spaced dogs or bolts. This arrangement is typical of the type fitted to facilitate the loading of stores etc.

2)    A patent hydraulically operated sliding door shown in the open & close positions. This type is simple and fast to operate and is self closing since the door is forced against perimeter of the opening assisted by the eccentric path of the guide rollers.

3)    A swing door. This type of door may be fitted at the sides of the ship to give access to the ’tween deck or at the stern to give access for vehicles e.g. Ro-Ro ships. In the latter case the ramps will be a separate item of equipment.

                       

Bow Doors & Ramps

When using the bow doors it is necessary for the bow to run into a specially designed fender. Watertight closure at the bow is usually provided by the ramp in its raised position, or a separate door inside the bow door or visor. This bow door or visor has a spray type seal.

                       

 Stern Doors & Ramps

Many roll on/roll off ships only have stern doors & ramps. The arrangements vary depending on the nature of the service being operated.

At the stern the ramp when in raised position usually forms the W/T closure as shown.

The ramps & doors are normally hydraulically operated & cleated in position.

                                   

HATCH COVERS OPERATION & MAINTENANCE

HATCH COVERS OPERATION & MAINTENANCE

Chain Pull Type Hatch Covers

1.rising track,                                  11.coaming,
2.panel side chains,                         12.sheave,
3.falling track (behind coaming),      13.cross joint cleat,
4.balancing roller,                            14.panel top plate,
5.eccentric wheel,                            15.backhaul wire,
6.screw cleat,                                  16.wire to winch
7.wheel guide rail,                           17.rubber gasket,
8.compression bar,                          18.stowage bay,

9.coaming bar, coaming stiffener,
10.coaming hatch stay,

                                           HATCH COVER CLEATS

                                   

                        HATCH COVER CROSS JOINT WEDGES

                       

           

Hydraulic Type Hatch Covers

           

           

           

                                                HATCH COVER DRAIN VALVE

                                        

                                                                        HATCH COVER PACKINGS

           

                       

Leakage Problems

Most leakage problems occur because of poor maintenance. Although robust, hatch covers will leak if compression surfaces are not aligned correctly and if gaskets are damaged or worn.

Factors affecting watertightness:

Maintenance of the steel-to-steel contact surface

Hatch covers are designed to make steel-to-steel contact between a defined part of the hatch cover and coaming when closed. This steel-to-steel contact determines the amount of compression between the hatch gasket and compression bar. Contact might be nothing more than the hatch skirt sitting on the horizontal coaming plate, although some hatches are fitted with metal landing pads.

When the horizontal coaming plate or hatch landing pad is worn, pressure on the hatch gasket (rubber packing) increases. If this wear is greater than 4mm, increased pressure on the gasket will cause damage. Landing pad repair is essential.

Maintenance of rubber packing – surface damage

Rubber packing that is physically damaged, cut or chafed should be renewed immediately.

The minimum length of replaced packing should be 1 metre.

Maintenance of rubber packing – permanent set

Rubber packing that is permanently impressed to 75% of its design compression should be completely replaced. The manufacturer will provide details of the design compression. A rule of thumb to estimate design compression is to use 30% of the packing’s thickness.

Permanently impressed rubber packing indicates worn steel-to-steel contact surfaces.

Never replace permanently impressed gaskets without checking the steel-to-steel contact points for wear and doing repairs if they are worn.

Maintenance of rubber packing – aged gaskets

Ozone will age rubber. It becomes hard and loses elasticity. The entire length of aged gasket should be replaced.

Maintenance of the double drainage system

Hatches are designed to drain away water that has penetrated the gasket. Drainage channels should always be cleaned before hatches are closed, and kept free from rust scale and cargo debris. Damaged channels should be repaired immediately and then painted to prevent corrosion. Drainage channels are located along

the cross-joint and on the coaming between the compression bar and the inner coaming.

           

                                   

Maintenance of non-return drain valves

Hatch coaming non-return drain valves are an essential feature of the hatch double drainage system.

They let water that has come through the hatch cover drain away. Damaged, missing or defective non-return drain valves should be repaired or renewed.

Maintenance of rubber seals on hold access doors, access hatches and ventilators

Water can enter the cargo hold through access doors, hatches and ventilator covers.

Maintain them in the same way as you would hatch covers.

Maintenance of hatch cleats

Cleats and wedges hold the hatch in position with adequate gasket compression.

(Cleats are fitted with a rubber washer or ‘grommet’ to aid compression). Compression of the washer determines tension in the cleat. Washers are prone to both physical damage and age hardening (weathering).

When damaged or aged the washer loses its elasticity and should be replaced. Some operators protect the washer and screw threads with a layer of grease or by application of ‘denzo’ tape.

Maintenance of hatch wedges

Cross-joint wedges require less maintenance than cleats and provided the wedge side spring is in place the wedge will work efficiently. Check the springs regularly and replace them if they are damaged or missing.

On a closed hatch, the wedge should make contact with its strike plate on the opposite hatch panel.

If there is a gap, it is likely that the panel is distorted. The steel-to-steel contact may be worn.

Repair and alignment of the hatch panel is necessary. Never repair the strike plate by building it up with weld metal above its original design height.

Maintenance of landing pads

The size and dimensions of a landing pad are dependent on the size and weight of the hatch cover.

Landing pads are normally located adjacent to cleats. The pads are fitted to the top of the coaming and to the side of the hatch panel. Landing pads should always be repaired to their original design height.

Correct adjustment of them can only be achieved during repair when the ship is out of service. Some ships are provided with cassette type landing pads, these are easily replaceable.

Leak Detection Tests

The two most common leak detection tests are the water hose test and the ultrasonic test. Ultrasonic testing is the preferred method because areas of inadequate hatch sealing are accurately located.

Chalk testing (another hatch test) gives only an indication of poor compression and potential leaks. Chalk testing is not a leak detection test.

Light testing is also effective but is potentially dangerous because personnel are in a closed, dark hold looking for light infiltration between panels.

If hatches are found to leak during a test, make the necessary repairs, then test again.

Water hose leak detection test

Water hose tests are used to determine weathertightness of hatch covers. If correctly performed, hose testing will show hatch joints that leak.

The general procedure for hose testing is to apply a powerful jet of water from a 20-30mm diameter hose fitted with a 12mm diameter nozzle held at a distance of 1-1.2 metres from a hatch joint, moving along the joint at a speed of 1 metre every 2 seconds.

The drawbacks of hose testing are:

. The hold needs to be empty;

. It cannot be performed in sub-zero conditions;

. It requires the deck scupper drains to be open (potentially causing pollution);

. The test cannot pinpoint leaks on the cross-joint or side joint accurately;

. Two people are needed to supervise the test.

Ultrasonic leak detection test

Ultrasonic leak detection is a viable alternative to the hose test for testing hatch covers, access doors and access hatches for water-tightness. This test should only be carried out using class approved equipment and approved test procedures.

The test involves placing (with hatches closed and secure), an electronic signal generator inside the cargo hold. A sensor is then passed around the outside of all compression joints. Readings taken by the sensor indicate points of low compression or potential points of leakage.

Ultrasonic testing overcomes the majority of limitations associated with hose testing and can be carried out when holds are loaded.

The drawbacks of ultrasonic leak detection tests are:

. The equipment requires an experienced and specialist operator to interpret the readings;

. The equipment requires regular calibration;

. The equipment is not normally part of the ship’s equipment.

Chalk testing

When performing a chalk test, the top edge of every compression bar is covered with chalk.

Hatches are then fully closed and reopened. The rubber packing is examined for a chalk mark, which should run continuously along the packings centre. Gaps in the chalk mark indicate lack of compression.

Chalk testing merely indicates if hatch panels are aligned and compression achieved. It will not show whether compression is adequate and therefore it is not a test for water-tightness.

Basic Advice

There are procedures which will help to keep your ship’s hatch covers in good condition.

The following advice can be considered best practice.

Always

• Rectify any steel-to-steel fault before renewal of rubber packing. Renewal will not be effective if steel-to-steel contact points are defective, and expensive rubber packing will be ruined after only a few months of use;

• Replace missing or damaged hatch gaskets (rubber packing) immediately. The minimum length of replaced gasket should be 1 metre;

• Keep hatch coaming tops clean and the double drainage channels free of obstructions. (Open hatch covers to clean coaming tops and the double drainage channels after loading bulk cargo through grain or cement ports);

• Keep cleats and wedges in serviceable condition and correctly adjusted;

• Keep hauling wires and chains adjusted correctly;

• Attach locking pins and chains to open doors and hatches;

• Keep wheels, cleats, hinge pins, haul wires, and chain tension equipment well greased;

• Test hydraulic oil regularly for contamination and deterioration;

• Keep hydraulic systems oil-tight;

• Ensure the oil tank of the hydraulic system is kept filled to the operating level and with the correct oil;

• Clean up oil spills. If the leak cannot be stopped immediately, construct a save-all to contain the oil and empty it regularly;

• Engage tween deck hatch cover cleats when the panels are closed;

• Give notice that maintenance is being performed so that no one tries to operate the hatch;

• Remember that continuing and regular maintenance of hatches is more effective and less expensive than sporadic inspection and major repair.

Never

• Allow grooves to form in the coaming top, especially where the hatch side or end panel rests when the hatch is closed;

• Apply petroleum-based grease or paint to rubber packing;

• Remove the rubber ball from a non-return drain valve;

• Use anything other than the recommended hydraulic oil;

• Leave cleats unfastened when proceeding to sea;

• Attempt to open or close any hatch that has a load or cargo on it;

• Open hatch covers at sea unless absolutely essential;

• Leave open covers unattended when at sea;

• Tighten down the cleats so that the hatch cover is unable to move on the coaming top.

Common False Beliefs about Hatch Covers

• It is the rubber seal that keeps the water out of the cargo.

The double drainage system is as important in keeping water away from cargo.

Renewing a worn rubber seal is all that is needed to keep a hatch watertight.

Worn rubber is usually the result of worn steel-to-steel contact surfaces or a deformed structure.

Rubber renewal alone is futile unless the steel-to-steel contact surface is repaired.

• Watertight is the same as weathertight.

From a hatch cover design perspective, watertight means that water cannot get in or out;

weathertight (as required by the 1966 Load Line Convention) means that water cannot pass through the seal.

• The hatch cover side plate when closed should rest on the coaming top.

If the weight of a hatch panel is sufficient to cause distortion of its side plate (hatch skirt), then landing pads are fitted to the panel to transfer the weight evenly across coaming top.

• Hatch covers will always leak in heavy weather.

Hatch covers are designed to withstand the rigours of the sea. Provided the cleats are correctly adjusted, hatch gaskets are in good condition and the construction material sound, then hatch covers should not leak, regardless of the weather. Common False Beliefs about Hatch

Cover

• Screwing cleats down hard will ensure watertightness.

No amount of tightening of cleats beyond their correct position will improve hatch watertightness.

Hatch cover manufacturers usually test for watertightness without engaging cleats. The weight of a hatch is sufficient to create the required gasket compression.

The use of hatch cover tape will ensure watertightness.

The use of sealant tape gives a false sense of security. Hatch cover tape is a short-term temporary measure that can be used to stop water from entering cross or side joints. However, the prolonged use of tape increases corrosion in the cross-joint and side plate. In bad weather, sealing tape can and does wash off.

• Drain valves are not important; it does not matter if they are blocked.

Drain valves are an essential feature of the double drainage system as they allow water that haspenetrated the  hatch gasket (rubber packing) to drain away. If the valve is blocked or closed, water will spill from the drainage channel into the cargo hold.

When carrying a cargo on top of a hatch it is not necessary to fasten cleats.

Cleats prevent excessive movement of the hatch as a ship bends and flexes in a seaway.

They allow limited movement to ensure correct contact between the hatch and its coaming, preventing hatch damage. Cargo loaded on the hatch does not secure the hatch to its coaming.

• Tween deck cleats are not essential because the tweendeck covers are not watertight.

Cleats on tween deck covers should always be engaged when the covers are closed. This is because they stop tween deck panels from jumping when a ship pitches, ensuring maintenance of tween deck strength. When cargo is stowed on a tween deck panel, the panel must be secured to the ship’s structure.

Procedures to open and close hatch covers

Prior to opening or closing a hatch, the watch officer should be informed and the condition of the hauling wires or chains checked; the hydraulic system should be topped up.

Never open or close more than one set of hatch covers at a time.

Opening procedures

• Check that the hatch cover panel stowage area is clear of people, equipment and dunnage;

• Disengage all cleats;

• Attach towing or hauling wires; switch the power on and ensure the controls are in neutral;

• Ensure that all personnel are clear of the hatch and its tracking. Position crewmembers

to observe both sides of the hatch;

• Raise hatch covers to the roll position by jacks or by raising the lifting system.

The panels need to clear the hatch guides;

• Check that towing chains are free and do not foul tracks or the coaming top;

• Start to open the hatch, slowly at first, then at normal operating speed until the

hatch is almost open and then reduce to slow speed until fully open. Care must

be taken when opening hatch covers especially when the speed of opening can

be only partially controlled;

• When fully open, secure the hatch with the safety hook or pin before the power

is switched off. If applicable, remove the towing and hauling wires;

• Install portable safety rails, if supplied.

Closing Procedures

• Check that the coaming top is clear of cargo or debris;

• Check and clear drain channels and entrances to the drain valves;

• Check that any damaged wheel tracks, compression bars and landing pads have

been repaired;

• Ensure that the hold is clear of people and that access hatches or entrance doors

are open. Check towing chains are free;

• Attach towing or hauling wires, switch the power on and ensure the controls

are in neutral;

• Remove portable handrails;

• Release hatch locking pins or hooks;

• Avoid injuries by ensuring that all personnel are clear of the hatch.

Position crewmembers to observe both sides of the hatch;

• Check that towing chains are free and do not foul tracks or the coaming top;

• Start to close the hatches slowly at first with the speed of closure being gradually increased to the normal operating speed. As the hatch reaches the closed position the speed should be gradually reduced. Great care must be taken when closing hatch covers;

• Lower hatch covers into guide pockets using jacks or lifting cylinders. Some hatches are lowered automatically;

• Attach cleats before removing the towing wire or switching power off;

• Finally, check no one is in the hold before closing hold access hatches or other hold entry points.