Ship Lifting Methods: Dry Docks and Slipways

Launching Methods of Lifting Ships from Water

• Dry docks (graving docks): Only method for lifting large ships
• Dubai has one of the biggest docks at 1,000,000 tonnes
• ASRY's shipyard in Bahrain has a large dock at 500,000 tonne
• Alexandria Shipyard dry dock is 80,000 tonne
• These docks are mainly for ship repair, ship building, or assembly of sections

Floating Docks & Mechanical Slipway

• Floating docks consist of a floating pontoon subdivided into compartments and side walls
• Floating docks can be used for large, medium and small ships
• Floating docks are rarely used for building ships (except for welding halves) where their mobility is advantageous
• Mechanical slipways are either end or side lifting and launching
• End lifting needs space
• Mechanical slipways are only for small ships
• Mechanical slipways are generally a parking area
• The building cost of mechanical slipways is concentrated in civil work for the slipway and parking area, so they are expensive, while winches and trolleys are cheap

Ship Lift

• Ship lifts are the latest development in lifting ships, replacing the mechanical slipway
• The largest ship lift can handle about 100,000 tonne, so they are not used for large ships
• Structural platforms lift and lower ships vertically and synchronously using hoists
• The platform is lowered underwater, allowing the ship to float above the support, then lifted to quay level
• Synchronization of winches is crucial, and modern systems use electrical control and drive
• Shiifts use transfer systems, transporting vessels to parking for painting or repair
• One shiplift can serve many parking places while dry docking installations can dock just one ship
• Transfer systems utilize trolleys or cradles on steel wheels driving on heavy-duty rails

Launching

• Launching involves transferring weight from fixed building blocks to a launching cradle, allowing the ship and cradle to slide into the water
• End launching, side launching, mechanical slipway, dry dock launching, and semi-submerged ways are types of launching where the gate provided is kept away from the stern during construction, placing the ship lower relative to the water for earlier water entry

End Launching Arrangement & Ground Ways

• End launching can be useful when the LCG in launching trim is far aft or when hull lines create suitable buoyancy if the bow launches first
• A movable portion fitted to the ship and a stationary portion fitted to the ground make up the usual end launching arrangement
• Ground ways are usually made of heavy timbers or reinforced concrete, straight or cambered longitudinally, and placed under the longitudinal framing or bulkheads
• The slope of the ways must be sufficient for the vessel to start, but large slopes put the forward portion of the ship at a considerable height above the ground and require more blocking, shoring, and staging and a large load on the forward poppet, so relatively small declivities are used for large ships

Sliding Ways, Cradle and Poppets

• Sliding ways are placed over the ground ways at least 25 mm inboard of the ribband
• Sliding ways are typically about 80% of the ship's length; locating ends under transverse bulkheads is ideal
• The cradle and poppets purpose is to support the ship during its passage down the ground ways
• They are composed of the sliding ways, wedges, wedge riders, packing and the forward and aft poppets

End Launching Calculations

• Preliminary end launching calculation estimates can only be approximate due to the difficulty moving heavy equipment with cranes
• If the ship's afloat stability is expected to be small, heavy weights high in the ship should not be installed before launch, and the height of items is limited by crane clearance
• Launching weight (W₁) is generally 80 to 90% of the complete weight
• Calculating the longitudinal and vertical positions of the center of gravity is important; LCG affects pivoting, way end pressure, and trim, and KG influences stability during and after launch and ballast is used to adjust C.G. to decrease way-end pressure or improve stability

Condition Afloat

• The estimated launching weight and C.G. determine the condition afloat
• Mean draft is taken from displacement curve, trim is calculated from trimming moment (LCB and LCG), and the end drafts are calculated
• From the draft to the cradle's bottom at its fore end and the water depth (H), it is assessed whether the ship will float off or drop off way end and the total drop = 2 (TF − б) and H > 2 (TF − δ)

Buoyancy During Launching, Pivoting and Tipping

• Buoyancy during launching formulas are determined relative to declivity
• As the ship moves into the water, the buoyancy of the after portion increases until the stern lifts and pivots about the forward end of the cradle
• The pivoting condition is B.b = W.a
• The maximum load on the forward poppet is the difference between the ship's weight and buoyancy as the ship continues down the ways where the load decreases and becomes zero when the ship leaves the ways
• If buoyancy is insufficient after the center of gravity has passed the end of the ways, the ship will tip downward at the stern creating heavy pressure on the way ends and bottom of the ship where tipping occurs when W.c = B.e
• To prevent tipping, provide sufficient water depth over the end of ways, otherwise extend the ways further into the water or increase inclination
• Insufficient buoyancy before pivoting can cause excessive pressure

Factors Affecting Launching Calculations - Slope and Ways

• Reducing the slope of the ways and keel by elevating the outer end decreases pivoting load but increases way-end pressure
• Increasing the slope of the ways and keel by lowering the outer end reduces way-end pressure but increases load on the forward poppets
• The slope of the ground ways depends on the frictional resistance of the launching lubricant
• For movement down the ways, W₁ sinβ > μW₁ cos β is requried

Slope and Elevation of Keel and Wave Length

• 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 balance must be struck to avoid small draft at the forward end of the cradle when the ship leaves the ways
• The stern should be outboard as far as practicable
• The height of the keel above the ground amidships should be about 1.70 m with a slope of the keel about 1/16 in/ft less than the slope of the ground ways to provide satisfactory launching conditions
• Ground ways should be long enough to provide water depth adequate for low way-end pressure and to prevent tipping in addition to allowing the vessel to leave the ground ways

Height of Tide and Way-End Pressure

• The height of tide effects launching, as the stern lifts earlier, increasing moment against tipping, while reducing load on the after end of ways
• Way-end pressure is the difference between the weight and the buoyancy divided by the length in contact which gives a mean load per unit length, divided by the width of ways which gives a mean pressure
• Experience has shown that the mean pressure, for many greases, should not exceed about 2.5 tonf / ft² (27 tonnef / m²)

Load on Ways and Way-End Pressure Solutions

• Load on lc = W-B and is assumed to be trapezoidal
• Where b is the width of each sliding way and n is the number of ways and where W-B=(pa+pf)/2 lc.n.b (1) using the distance and load in conjunction
• Further solving determines that the way-end pressure pa=2(W-B)/nblc

Way End Pressure Solved

• It is found that pf is negative and so it is assumed the load acts over a distance 3x instead of lc
• Tipping occurs when x = 0, so, cambered ways are needed to provide a low as possible declivity which prevents stacking
• Cambered ways difficulty can be overcome by applying them according to radius

Geometry of Cambered Ways

• Circular arcs are required
• When the ship moves down ways the way declivity increases by an angle 0 which is the increase in declivity at any travel
• The horizontal travel from the initial position and height from keel helps to calculate more accurate drafts during launching

Forces During Launching

• The general equation of motion is F = M.a where M is mass
• Using the equation, with the variables for the ship being launched, can help determine the ships motion
• Determining the energy for a graph of forces is achieved during integral calculus

Frictional Resistance & Water Resistance & Checking Arrangements

• As mentioned before B should be greater than µ of the lubricant. Values of u 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).
• Water resistance analyses determine the coefficient k from k = B^3/C
• At yards located on restricted waters safe launchings may depend largely upon the use of adequate arresting or checking arrangements

Masks & Rope Stops Slewing

• A mask is simply a large flat surface at right angles to the direction of travel. It is located at the stern and some yards have fitted them along the side shell
• Masks increase the ship's water resistance
• Rope stops are broken as the vessel is launched
• A heavy chain cable is anchored to the ground on both sides of the vessel where chains are led
• Slewing is done best where basin length is insufficient, allows for the water resistance where weights are placed and is a combination of translation and rotation

Chain Drags - Side Launching

• Chain drags consist of placing drags alongside the vessel that are connected to the vessel, mostly through wire or concrete
• The bending stresses experienced by a ship during launching are those produced by hogging and sagging
• Side launching is done on rivers or narrow channels and has several advantages like eliminating keel declivity structure, underwater structure, simple, and does not require internal shoring due to the low pressures

Successful Launching

• Key aspects for a successful launch consists of starting, leaving evenly, postitve clearence and great depth
• The vessel must be stable during launching with minimal angle because of factors like metacentric height
• The roll must be minimal
• Launching basins width prevents a vessel from striking the opposite side

End Launching

• End launching on airbags was developed in China in 1998 for vessels up to 7000 tonnes
• Cylindrical airbags are placed between blocks, inflated, blocks removed except near the bow, and the ship is rolled into the water
• To be successful, remove any burrs
• To be successful, remove any debris and be accurate to a level of 80 mm and a slope of 1/7
• It is important to maintain good speed and to use a slow winch to stop and limit the force