LAUNCHING
Methods of Lifting Ships from Water
1. Dry Docks – Graving Docks
This is the only method for lifting large ships. One of the biggest docks is in Dubai
(1000000 tonne). Another big dock is in ASRY’s shipyard in Bahrain (500000
tonne), while Alexandria Shipyard dry dock is 80000 tonne. It is mainly used for
ship repair. However, sometimes it is used for building ships, or for the assembly
of complete sections, e.g. welding of a ship built in two halves.
2. Floating Docks
It is composed of a floating pontoon subdivided into compartments and side walls.
It can be used for large, medium and small ships. It is rarely used for building
ships (except for welding of a ship built in two halves). Its main advantage is its
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mobility. It is possible to lift a ship longer than the dock and also two ships can be
lifted together.
3. Mechanical Slipway
It is either end or side lifting and launching. End lifting requires enough space for
the slipway and it cannot be used for long ships. It is used for lifting small ships
only, and it is generally has a parking area. The building cost of a mechanical
slipway is concentrated in the civil work required for the slipway and the parking
area, which is very expensive. The winches and trolleys are relatively cheap.
4. Ship Lift
This is the latest developmnt in lifting ships. Now it replaces the mechanical
slipway. The biggest is about 100000 tonne. Therefore it is not used for large
ships.
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It consists of a structural platform that is lifted and lowered exactly vertical,
synchronously by a number of hoists. First, the platform is lowered underwater,
then the ship is floated above the support, and finally the platform with ship
support and ship is lifted and the ship is brought to the level of the quay.
It is important to synchronize the winches. Modern systems use electrical control
and drive systems for the winches. Many shiplifts use a transfer system for ships
so that the vessels can be transported from the water to a parking place where
they can be painted or repaired. One shiplift can serve many parking places, while
a dry docking installation can only dock one ship.
For large vessels the transfer system consists of a number of trolleys or cradles,
supported by high capacity steel wheels. The wheels drive on heavy duty rails.
The transport can be one directional, but in order to serve more parking places,
two directional systems are used.
Launching
Launching consists primarily of transferring the weight of the ship from the fixed
building blocks to the launching cradle, and then allowing the ship and cradle to slide
down the ground ways into the water.
Types of Launching
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1. End Launching
2. Side Launching
3. Mechanical Slipway
4. Dry Dock Launching
5. Semi-Submerged Ways provided with Gate
The water is kept away from the stern of the ship during construction, and the ship
can be placed at a lower level with respect to the water, thus entering the water
earlier and the ship stopped sooner.
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End Launching Arrangement
In cases where the location of the LCG in launching condition is relatively far aft or
where the lines are such that favorable buoyancy moments are produced if bow is
launched first, bow launching can be advantageous.
The usual end launching arrangement consists of a movable portion fitted to the
ship and a stationary portion fitted to the ground.
Support During Construction
Launching Arrangement
Ground Ways
Usually made of heavy timbers but sometimes of reinforced concrete. They may be
straight or cambered longitudinally. They should preferably be placed under the ship’s
longitudinal framing or bulkheads.
The slope of the ways must be sufficient to insure that the vessel will start when
released. If the slope is large the forward portion of the ship will be at a considerable
height above the ground and will require more blocking, shoring and staging. Also the
load on the forward poppet will be great. Relatively small declivities are used for large
ships.
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Sliding Ways
They are placed immediately over the ground ways at least 25 mm inboard of the
ribband. Ordinarily the length of the sliding ways is about 80% of the ship’s length. If
possible the ends of the sliding ways should be located under transverse bulkheads.
Cradle and Poppets
Its purpose is to form a support for the ship during its passage down the ground ways.
It is composed of the sliding ways, wedges, wedge riders, packing and the forward
and aft poppets.
After Poppet
Fore Poppet with Bracket
End Launching Calculations
1. Estimation of Weight
In preliminary calculations this estimate can be only approximate. Many heavy weights
such as boilers and machinery units cannot be handled by the cranes at the building
slip. In case where the stability of the ship afloat is expected to be small, large weights
high in the ship should not be installed before the launch. The height of the items is
also limited by the clearance under the cranes. The launching weight Wl is generally
from 80 to 90% of the complete weight.
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2. Centre of Gravity
It is necessary to estimate the longitudinal and vertical positions of the centre of
gravity at the time of launching. LCG is important in the calculations for pivoting, way
end pressure and trim of the vessel. KG is of importance in the calculations of the
stability during pivoting and after launching. Sometimes ballast is used to alter the
C.G. position to decrease the way-end pressure or to improve stability.
3. Condition Afloat
It is determined from the estimated launching weight and C.G. The mean draft is taken
from the displacement curve and the trim is calculated from the trimming moment (i.e.
LCB and LCG) and the MCT 1cm. Hence the end drafts could be calculated.
From the draft to the bottom of the cradle at its fore end and the estimated depth of
water H over the end of the ways, it can be ascertained whether the ship will float off
or drop off the way ends.
The total drop = 2 (TF – δ)
H > 2 (TF – δ)
4. Buoyancy During Launching
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Let
β =
α =
E =
S1 =
H =
way declivity
keel declivity
height of keel at A above the water level
horizontal travel of ship
vertical distance of A1 below water level
The draft at the after perpendicular before the stern starts to lift
(
)
( )
The corresponding draft at the forward perpendicular
By means of equation (1) and Bonjean’s curves as shown above the buoyancy and
LCB can be calculated for any travel until the stern starts to lift.
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5. Pivoting Condition
As the ship moves farther into the water, the buoyancy of the after portion increases
until it is sufficient to raise the stern, i.e. the ship pivots about the forward end of the
cradle (the forward poppet).
The pivoting condition is therefore at:
B.b = W.a
At the pivoting position the maximum load on the forward poppet is the difference
between the ship’s weight and buoyancy (W – B). As the ship continues to move down
the ways, the load decreases and becomes zero when the ship leaves the ways.
To calculate the drafts after the stern starts to lift, the TF is found as already described;
buoyancy and moment of buoyancy are then calculated for several trims about the
fore poppet. At all times after the stern starts to lift the moments of weight and
buoyancy about the fore poppet must be equal. When the curves of moment about the
fore poppet cut, there is the correct trim; buoyancy can be read off and LCB
calculated.
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6. Tipping Condition
If the buoyancy is insufficient after the centre of gravity of the ship has passed the end
of the ways, the ship will tip downward at the stern and thus causes a heavy
concentration of pressure on the way ends, and also on the bottom of the ship.
Tipping occurs when
W .c = B . e
In order to prevent tipping, a sufficient depth of water over the end of ways should be
provided. If the available rise of tide is not enough for this purpose, the ways may be
extended farther out into the water, or they may be given a greater inclination. Even if
tipping is prevented, insufficient buoyancy before pivoting may still cause an
excessive pressure on the way ends and on the ship’s bottom.
A complete set of launching curves is shown in the following figure:
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Factors Affecting Launching Calculations
1. Slope and Elevation of Ground Ways (β)
When the slope of the ways and keel is reduced by elevating the outer end of ways,
the pivoting load is decreased, but the way-end pressure is raised. Increasing the
slope of the ways and keel by lowering the outer end of ways will reduce way-end
pressure but will result in an increased load on the forward poppets.
The slope of the ground ways depends primarily on the frictional resistance of the
launching lubricant.
In the figure Wl is the launching weight of the ship
The frictional resistance of the lubricant
Where μ is the initial coefficient of friction.
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For movement down the ways F1 must be > F3
2. Slope and Elevation of Keel (α)
At the bow the keel should be at a sufficient elevation above the ground ways to allow
for the dip of the vessel’s fore foot during pivoting. However, a large distance between
the keel and the ground ways at the bow, will result in small draft at the forward end of
the cradle when the ship leaves the ways. Hence the drop at the end of the ways will
be great.
The stern should be placed as far as outboard as practicable. Having chosen this
location the elevation of the keel at the after perpendicular should be such that work
around the stern is not seriously affected by the tide.
The height of the keel above the ground amidships should be about 1.70 m in order to
provide reasonable working conditions. For large ships, a slope of the keel which is
about
in/ft less than the slope of the ground ways will provide satisfactory launching
conditions.
3. Length of the Ways
The ground ways should have a length sufficient enough to provide a depth of water
adequate to insure low way-end pressure and a proper margin against tipping. Further
more, it is desirable that this length should provide a depth of water sufficient to allow
the vessel to leave the ground ways without appreciable drop.
4. Height of Tide
It has a pronounced effect on launching characteristics as follows:
 The stern lifts earlier
 The moment against tipping is increased
 Load on the after end of ways will be reduced
Way-End Pressure
The total load on the ground ways is the difference between the weight and the
buoyancy W - B. Dividing by the length in contact gives a mean load per unit length
and this, divided by the width of ways gives a mean pressure. Experience has shown
that the mean pressure, for many greases, should not exceed about 2.5 ton f / ft2 (27
tonnef / m2) or the grease tends to get squeezed out.
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The pressure on the ways before the stern starts to lift changes as the vessel moves
down the ways.
Load on lc = W – B
∑ M @ A.E.W
B .e - W . c = (W –B) . x
The load may be assumed to be trapezoidal as shown in figure
(
)
( )
Where b is the width of each sliding way and n is the number of ways (maximum
nominal width of one ground way = 2.5 m).
(
)
[
(
)
]
( )
Solving (1) and (2) for pf and pa
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(
)
(
)
( ) Way-end pressure
(
)
(
)
( )
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In equation (4) when
is less than ⅓ , pf is negative. Since this is impossible the
load is then assumed to act over a distance 3x instead of lc as shown below:
(
)
Tipping occurs when x = 0 and the way-end pressure then theoretically would be
infinite, if the ways and the ship were rigid. Actually both yield and the pressure is
distributed over an appreciable length.
Cambered Ways
In order to keep the bow of a large ship as low as possible during construction the
declivity of the ways and keel should be small, but yet sufficient to prevent sticking of
the ship on the ways. If straight ways of moderate slope are used, it might be
necessary to extend them to a considerable distance outboard to obtain enough water
over the way end to prevent tipping.
This difficulty may be overcome by the use of the ways which are curved downward
on the arc of large radius. The maximum distance between the chord and the arc is
known as the camber of the ways.
Weight, tonne
10000
13000
23000
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Way Slope
1 : 0.052
1 : 0.046
1 : 0.045
Keel Slope
1 : 0.047
1 : 0.044
1 : 0.042
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Camber 40 – 50 cm (large ships)
Radius (12 – 23 km)
The General Effects of Camber
1. The buoyancy for the same travel will be greater with cambered ways than
with straight ways.
2. This results in reduced way end pressure.
3. Greater load on the fore poppet at pivoting.
4. Water resistance is generally greater. Consequently when checking is
required, less effort is necessary.
5. The drop of the ship from the way end may be greater.
6. Greater clearance between the ship’s fore foot and the ground ways during
pivoting.
7. Increased moment against tipping.
Geometry of Cambered Ways
Theoretically a circular arc is required but mathematically it is simpler to deal with a
parabolic arc.
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⁄
( )
The radius of curvature R
(
)
| |
As the ship moves down the ways the way declivity increases by an angle θ, i.e. θ is
the increase in declivity at any travel.
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Calculation of End Drafts During Launching
S1 = Horizontal travel from initial position
E = Height of keel at A.P. above water level
hA = Drop due to way declivity + drop due to camber – E
Drop due to way declivity = S1 tan β
S1 β
Drop due to camber = y =
For x = S1
Drop due to camber =
(
)
(
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Forces Acting on a Ship During Launching
The general equation of motion is:
F = M .a
Where M = mass of ship and cradle
a = acceleration or retardation
F = F1 - F2 - F3 - F4
F1 = component of weight along ways = (W –B) sin β
F2 = frictional resistance of lubricant = μ (W – B) cos β
F3 = resistance of water = k v2
F4 = resistance of checking arrangement = μd wd
k
= coefficient of water resistance
v
= velocity of the ship
μd = coefficient of resistance of chain drags, if used
wd = weight of drags in action at any point of travel
This differential equation cannot be solved mathematically because of the presence
of B. A component force diagram can be built up as shown below:
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Energy
The energy absorbed by any component of force can be obtained by graphical
integration of the various forces with respect to the distance traveled.
For example, the energy absorbed by the friction of the lubricant is found from the
relation:
∫
Where S1* is the distance traveled up to the time the cradle leaves the ways. Values
for E3 and E4 are obtained in a similar way.
The energy available for producing velocity is given by the relation:
∫
Where S1 is any distance traveled.
the corresponding velocity
√
Frictional Resistance
As mentioned before β should be greater than μ of the lubricant. Values of μ for
previous launchings are obtained from analyses of observed data. These values may
be used with confidence in a new launching provided the factors affecting the lubricant
to be practically the same (μ = 0.015 – 0.03).
A certain amount of frictional resistance is desirable in that it absorbs a considerable
portion of the ship’s energy so that the checking arrangements need not be as
extensive. The lubricant consists of two layers. Mineral-base greases constitutes the
base coat and commercial lime-soap launching greases constitutes the slip coat. The
former provides a smooth hard bearing surface which prevents the sliding ways from
coming in contact with the ground ways and provides lubrication after the ship starts.
The purpose of the slip coat is to provide a layer of lubricant on top of the base coat
which will have a frictional resistance low enough to insure starting of the ship.
In Japan Steel-Ball system is developed and used instead of the conventional grease
lubricant system. The balls are located at fixed distances by special holders. The
advantages of this system are:
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1. The variable factors affecting the frictional resistance of grease, such as
temperature, humidity and quality of the lubricant are eliminated.
2. The mechanical equipment is reusable.
3. Launching ways breadths can be reduced as the load which the balls can bear
is considerably greater than with conventional lubricants.
Water Resistance
The problem of water resistance is a very difficult one. Analyses of many launchings
have shown that the coefficient k, before the ship leaves the ways, may be determined
from:
C is another coefficient which varies with the buoyancy and B is the buoyancy.
Checking Arrangements
At yards located on restricted waters safe launchings may depend largely upon the
use of adequate arresting or checking arrangements.
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1. Masks
It is simply a large flat surface at right angles to the direction of travel. It is located
as low as practicable and usually at the stern although some yards have fitted
masks in several positions along the side shell.
The mask increases the ship’s water resistance and thus aids in absorbing the
energy developed by the ship’s motion.
2. Rope Stops
The rope stops are broken in succession as the vessel is launched. A heavy chain
cable called the ground chain is anchored to the ground on both sides of the
vessel. The ship chains are led to the ground and are connected to the ground
chain by a large number of manila rope stops.
3. Slewing
This is the most practical method when the length of the launching basin is not
sufficient to allow a free run but is long enough to allow the water resistance to act
for a relatively long time. When the vessel is afloat the resistance of the weights
pulling along the bottom of the launching basin slews the vessel from its initial line
of motion. The motion, in this case, is a combination of translation and rotation.
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4. Chain Drags
This is the most common method and consists of placing chain drags on the
slipway alongside the vessel. These are connected to the vessel through wire drag
ropes attached near the bow (concrete blocks can also be used).
Bending Strength During Launching
The bending stresses experienced by a ship during launching are those produced by:
1. Hogging
This is expected when the stern of the vessel passes beyond the end of the ways
and the travel corresponds to the position of minimum moment against tipping.
During hogging the vessel is supported by buoyancy and by the portion of the
ways which is in contact with the cradle.
2. Sagging
This is expected when the vessel pivots about the forward poppet. During sagging,
the vessel is supported by buoyancy and the forward poppet.
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It is particularly important to investigate launching stresses for vessels with
relatively high length/depth ratios and for vessels with unusual openings or
discontinuities in the strength members.
Side Launching
Side launching is particularly used on rivers or narrow channels where the restricted
width does not permit a sufficient run as required for end launching.
There are, however, advantages of side launching over end launching and it is
sometimes preferred to end launching for the following reasons:
1. The absence of keel declivity simplifies the erection of the hull structure.
2. The construction and maintenance of expensive underwater ground way
structure is eliminated.
3. The launching cradle is less complicated and less expensive.
4. Internal shoring for loading due to pivoting and way end pressure is not
necessary.
Steps of Side Launching
When the holding devices are released the vessel with its cradle will slide down the
ways, reaching a maximum velocity when its C.G. is over the way ends. At this
position, it becomes affected by an unbalance moment which produces tipping about
the way ends. The vessel then will enter the water, roll outward, roll back and finally
stops.
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The Requirements for a Successful Launch
1. The vessel must start
The governing factors will be the weight of the vessel, the declivity of the ways and
the coefficient of friction of the lubricant. Typical ground way inclination is roughly
1 : 0.065 – 0.165.
2. The vessel must leave the ways evenly
Neither the bow nor the stern should lead the other upon passing the way end. The
location of the LCG and the distribution of pressure over the greased surface will
influence this.
Slewing of one of the ends may occur when the resultant of the frictional
resistance forces does not pass through the C.G. of the ship. Non-simultaneous
release of holding arrangements and foreign objects found under the sliding ways
may also result in this slewing action.
3. When leaving the way ends, the vessel must have a positive clearance
between it and the edge of the ways
This is necessary to avoid any possible damage to the shell, bilge keels or other
projections. The velocity of the vessel at the way ends and the amount of static
drop will govern the clearances involved.
4. There must be a sufficient depth of water in the launching basin
This is to prevent damage from the vessel striking the bottom. The amount of static
drop and the maximum heel will influence the maximum momentary draft of any
part of the vessel.
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5. The vessel must be stable during the period of maximum angle of heel
during launching and when afloat at rest
The metacentric height and the range of positive statical stability are the governing
factors for stability. The static drop and depth of water in the basin, which affect the
maximum angle of roll, will also have some effect on stability.
6. The roll back of the vessel after becoming afloat should be sufficiently
small to prevent damage to the side shell from striking the edge of the
slipway
The metacentric height, the velocity at the way ends and the static drop all have an
influence on the roll back.
7. The width of the launching basin must be such as to prevent the vessel
from striking the opposite side
The velocity of the vessel at way ends and the maximum roll outward will affect the
clearance from the opposite side.
End Launching on Air Bags
This innovative technology was developed in China in 1998 for the launching and
hauling of small and medium size ships up to 7000 tonne. It is simply carried out by
inserting cylindrical airbags between the blocks on the building berth under the
ship. The centreline of the airbags is in the transverse direction of the ship. The
airbags are then inflated and the blocks removed, except those near the bow. The
average distance between airbags is 3 m. However, as we go from the bow to the
stern of the ship, the distance becomes closer and closer. The pressure increases
gradually as we go aft, which means more airbags are placed near the water side.
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The launching steps are as follows:
1.
2.
3.
4.
5.
6.
7.
Deflate a little air from the bags near the water.
Inflate the airbags under the bow to increase the keel declivity.
Remove the blocks under the bow.
Push the ship a little by a bulldozer.
Let the ship goes smoothly into the water on the rolling airbags.
Recapture the airbags with the help of a small boat to the ground.
Open the valve, deflate and fold the airbags.
This method overcomes the shortcomings of the fixed launching track, which limits
the productive capability of small and medium sized shipyards. It has the merits of
time and labor saving, flexibility, reliability and safety in operation and
comprehensive economic benefits etc.
The requirements for a successful launch are as follows:
1. All the burrs, welding beading and the like on the ship bottom or appendages
should be ground away to ensure its smooth surface.
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2. The slipway on which the airbags will be rolling should be cleaned and be clear
of sharp ends such as iron nails and stones.
3. The slipway should be leveled and the level error from port to starboard should
be less than 80 mm. The ground caved in should be filled and the ground
bearing capacity should be relatively equalized.
4. The slope of ramp is to be determined according to the size of the ship and is
generally not greater than 1/7.
5. The slipway should extend into water for a certain length.
6. When the ship is relatively big, it is necessary to install a slow speed winch to
stop slipping. Its veering speed is about 9 -13 m/min.
7. The moving speed of a ship must not exceed 6 m/min with the control of
hauling force of winch wire. If the ship weight is less than 200 tonne, the
moving speed can be increased properly.
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