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Transcript of SHIP LAUNCHING
• These are designed to take the load thrown on them by the pivoting action, the load being widely distributed in order not to squeeze out the lubricant between the sliding surfaces.
• Shoring may also be found necessary forward in the ship to prevent structural damage at the time the stern lifts.
•> Side launching is often used where the width of water available is considerably restricted.
•> There are in fact some advantages to this method, for example the absence of keel declivity, and the relatively simple cradle and short ground ways which do not extend into the water.
•> The ground ways are arranged transversely, i.e. at right angles to the line of keel.
•> Sliding ways also can be placed transversely with the packing above them forming the cradle, but they are generally arranged longitudinally. In this case where they are parallel to the keel the sliding ways are in groups covering two or three ground ways. Packing again forms the cradle with tie pieces between the groups of sliding ways.
Different types of Ship launching methods
Traditionally, ship launching in the water is done using four main types of methods. These methods are:
• Launching involves the transference of the weight of the ship from the keel blocks, shores, etc., on which it was supported during construction, to a cradle on which it is allowed to slide into the water.
• Normally the vessel is launched end on, stern first, but a number of shipyards located on rivers or other narrow channels are obliged to launch the vessel sideways.
• Vessels have been launched bow first, but this was a rare occurrence as the buoyancy and weight moments, also the braking force, are generally more favorable when the vessel is launched stern first.
Shoring - is the process of supporting a building, vessel, structure, or trench with shores (props) when in danger of collapse or during repairs or alterations.
•> Conventional slipways or berths are relatively solid and reinforced with piles to allow them to sustain the weights of ships built upon them.
•> Keel blocks are arranged so that the height of keel above the ground is 1.25 to 1.5m, giving reasonable access, but not too high so that a large amount of packing is required
•> To transfer the ship from the building blocks to the launching cradle, the commonest practice is to drive wedges into the launching cradle. This lifts the ship and permits the removal of the keel and bilge blocks together with the shores. In large ships it becomes necessary to split the blocks to remove them, but several types of collapsible blocks have been used to overcome this difficulty.
•> One type is the sand box which contains sand to a depth of 80 to 100 mm held in a steel frame located between two of the wooden blocks. This steel frame may be removed and the sand allowed to run out. Another type is a wooden block sawn diagonally, the two halves being bolted so that they collapse on removal of the bolts.
LAUNCHING WAYS AND CRADLE
•> Fixed ground ways or standing ways on which the cradle and ship slide may be straight or have a fore and aft camber. Transversely the ground ways are normally laid straight but can be canted inwards to suit the ship’s rise of floor.
•> The ground ways have a small uniform fore and aft camber say 1 in 400, the ways being the arc of a circle of large radius. This means that the lower part of the ways has a greater declivity (say 1 in 16) than the upper part of the ways (say 1 in 25). As a result a greater buoyancy for the same travel of the ship beyond the way ends is obtained, which will reduce the way end pressures.
> Advantages are increased water resistance slowing the vessel, and a bow height which is not excessive.
•> The slope of ground ways must be adequate to allow the vessel to start sliding; and if too steep, a large amount of shoring will be required to support the bow; also the loads on the releasing arrangements will be high. Straight sliding ways have declivities of the order of 1 in 25 to 1 in 16.
•> Generally two ground ways are fitted, the distance between the ways being about one-third the beam of the ship.
•> Ground ways are laid on supporting blocks and extend down to the low water mark so that they are covered by at least one metre of water at high tide.
•> The sliding ways covering about 80 per cent of the length of the vessel form the lower part of the cradle, the upper part consisting of packing, wedges, and baulks of timber with some packing fitted neatly to the line of the hull in way of the framing.
•> Saddle plates taken under the forefoot of the ship with packing between them and the shell may be fitted to transmit the load to the fore poppets and hence ground ways.
•> The bow sections are relatively full and little support is required above the fore end of the sliding ways.
•> The design of the forward poppets is based on greater pressures than the lubricant between the sliding ways and ground ways could withstand if applied for any length of time. However as the duration of pivoting is small, and the vessel has sufficient momentum to prevent sticking at this stage, these high pressures are permissible.
•> For the ship to start sliding on release of the holding arrangements it is necessary for the ship to overcome the coefficient of friction of the launching lubricant.
•> To do this the slope of the ways under the vessel’s centre of gravity must exceed the lubricant’s coefficient of friction.
•> An estimate of the frictional resistance of the grease must be made before building the ship since the declivity of the keel is dependent to a large extent on the slope of the ways.
•> Formerly melted tallow was applied to the ways, allowed to harden, then covered with a coat of soft soap. Since the mid 1900’s patent mineral based greases have been applied to the ground ways, these greases being virtually unaffected by temperature changes and insoluble in water whilst adhering firmly to the ways.
•> To prevent the petroleum-based grease from soaking into the sliding ways a base coat may be applied to them. Standing ways which extend into the water may be dried out at low tide prior to the launch and the base coat and grease applied.
•> Small ships may be released by knocking away a diagonal dog-shore fitted between the sliding and standing ways.
•> There are several types available, hydraulic, mechanical, and electrical-mechanical triggers having been used.
•> Electrical-mechanical triggers are commonly used for rapid simultaneous release in modern practice; the hydraulic trigger is less easily installed and less safe. The electrical-mechanical is generally located near amidships and a small pit is provided in the berth to accommodate the falling levers. A number of triggers will be fitted depending on the size of the vessel to be launched; in the case of the 75 000-tonne bulk carrier for which a launching sequence is given below six triggers were fitted for the launch. These triggers are in effect a simple system of levers which allow the large loads acting down the ways to be balanced by a small load on the releasing gear.
•> The principle is often compared with that of a simple mechanical reduction gearing. Simultaneous release of the triggers is achieved by means of catches held by solenoids wired in a common circuit. These are released immediately the circuit current is reversed.
•> As a guide to the procedure leading up to the launch, the following example is given for the launch of a 75 000-tonne bulk carrier.
•> The launch ways have been built up as the ship is erected from aft; the ways have been greased and the cradle erected.
•> If the vessel fails to start under the action of gravity, the initial movement may be aided by hydraulic starting rams which are provided at the head of the cradle.
•> The commonest arrangement is to use chain drags which are generally arranged symmetrically on either side of the ship.
•> Each chain drag is laid in the form of a horseshoe with its rounded portion away from the water, so that as the ship moves down the ways the forward portion of the drag is pulled through the remainder of the pile.
•> As the ship is released and moves aft the tracing lines are broken in turn, the work done absorbing some of the ship’s energy.
•> To further increase the resistance to motion of the ship wooden masks may be fitted at the stern of the ship.
•> One or two shipyards are forced to provide arrangements for slewing the vessel once it has left the ways, as the river into which the ships are launched is very narrow in relation to the ship’s length.
•> Chain drags, weights, or anchors may be placed in the water to one side of the building berth for this purpose.
•> Once the vessel is clear of the ways the slack of the lines is taken up and the stern swings so that the ship is pointing up river.
• > One of the features of side launching is the drop where the ground ways are not extended into the water; consequently large angles of heel occur when the vessel strikes the water.
• > As a result it is necessary to carry out careful stability calculations and close any openings before side launching a vessel. It is true of course that stability calculations are also required for a conventional end launch.
> The greatest advantage of the building dock is the relative simplicity of the task of getting the vessel waterborne.
> Calculations are needed to check the stability and loads exerted by the blocks during flooding, the whole problem being similar to that of un-docking a vessel which has been dry-docked for survey or other reasons.
> In some shipyards conventional berths are fitted at their river end with what is virtually a pair of dock gates.
> In many instances it also permits higher tides over the way ends when the gates are opened for a launch.
• >> Whilst large ships may be extruded out of building halls on to a slipway (Chapter 11) and large sections transferred by similar means to the head of the slipway and raised on to it for joining and launching, smaller complete ships may be transferred to a ship lift for launching.
• >> Rail systems are incorporated into the building hall and lead out to the open ship lift.
• >> The best known of these ship lift systems is the patented ‘Syncrolift’ originally used for slipping ships for repairs and surveys but now also used by some shipyards for launching new ships.
• >> Ship lifts basically consist of platforms which can be lowered into the water and the ship landed on or floated off the platform.
• >> The platform is raised and lowered mechanically or hydraulically and is usually provided with transfer arrangements so that the vessel can be moved on or off the platform either laterally or in line with the platform.
1. Four days before launch a start is made on ramming up the launch
blocks, i.e. driving in the wedges (Figure 15.2) to raise ship off the building
blocks. This is done by a dozen or so men using a long ramming
pole, a gang working either side of the ship.
2. Two days before the launch a start is made on removing the shores.
3. On the morning of the launch everything is removed up to the high water
mark and tumbler shores are put in aft. These are inclined shores which
fall away as the ship starts to move.
4. Every second keel block is then removed, and the vessel is allowed to
5. An evently distributed number of keel blocks are then taken out so
that only about twenty keel blocks are left supporting the ship.
6. Half an hour before the launch the last remaining keel blocks are
7. The bilge blocks are then removed.
8. The full ship’s weight comes on the triggers at the time planned for the
9. Release of triggers on launch by sponsor.
Launching of ships is one of the most important procedures of the entire ship construction process. The ship launching slipway, a structure consisting of a sloping way down to the water from the shipyard, is an essential aspect of the ship launching procedure and has been extensively used for launching newly made or repaired ships.
1. Gravitational type launching
2. Floating-out type launching
3. Mechanical Type launching
4. Airbag launching
1. GRAVITATIONAL TYPE LAUNCHING
The gravitational type launching system is further divided into three main types, which are:
•1. Longitudinal Oiled Slideway Launching
2. Longitudinal Steel-Roller Slideway Launching
•3. Side Oiled Slideway Launching
The longitudinal oiled slideway launching is one of the oldest forms of launching systems. Using this system, the ship slides on a slideway and under its own weight enters the water. Oil or wax is used to assist in the sliding process.
Longitudinal Oiled Slideway Launching
The main advantage of this method is that it uses simple equipment and can be used for vessels with different tonnage and types. However, the coating of oil which is used to slide the ship smoothly can pollute the water. There is also danger of huge pressure on the front part of the ship during the launch.
Longitudinal Steel – Roller Slideway Launching
In this type of ship launching method steel rollers are used instead of oil to reduce friction during sliding. This method uses high-intensity steel rollers, security devices, and steel board for the launching. The steel plate on the wooden slide helps in the sliding process and the slide rails help them to protect from steel balls.
Net bags are used at the end of the slide to collect the steel balls so that they can be used again. This method is most effective and easy to start. However, the initial installation charges are high.
Side Oiled Slideway Launching
The side oiled slideway launching system is also one of the most widely used ship launching systems. This type of system is mainly of two types.
In the first type, the slideway extends into the water and the ship slides into the water using the slideway. In the second method the slideway doesn’t go until the water and the ship along with the frame slides into the water. The ship then becomes steady based on its own buoyancy and stability factors. Such launching requires the ship to have great stability and strength.
2. Floating- Out Type Launching
is carrying out for ships which are built in dry-docks and are launched by filling the dock with water. (Technically this is not a ship launching procedure) The floating-out type launching system is a simple, effective, and safe procedure. Though the initial investment is high, this type of method is most widely used by shipbuilders.
3. Mechanical Type Launching
The mechanical type ship launching system can be divided into following types:
• > Longitudinal mechanized slideway launching
• > Two points longitudinal mechanized chute launching
• > Wedge-shaped mechanized vehicles launching
• > Slope change transverse area mechanized vertical chute launching
• > High-low track slide mechanization launching
• > Mechanized comb slide
• > Lifting ship equipment
All the above mentioned system requires a mechanical feature to launch a ship. The construction and maintenance expenditure is high, whereas the mobility and controlling factors of the systems are not up-to-the-mark. All these methods are mainly used for smaller size vessels and cannot be used for large, high tonnage ships.
4. Air bags Launching
Launching ships using air bags is an innovative and safe technique to launch ships in water. These airbags are usually cylindrical in shape with hemispherical heads at both ends. They are made of reinforced rubber layers and have high load capacity. This method can easily be used in all types and sizes of vessels.