World's Biggest Side Launch of Ship in Gujarat
Ashok Kumar Khetan
Head Production,
L & T Ship Building
The process of putting ship's hull into water from a building berth is termed as 'launch'. The building berth is the area wherein a ship is finally assembled and launched. Launch is considered to be one of the significant stages in the process of shipbuilding. In most of the cases, this is also linked to stage payment from the owner to the shipbuilder.
The type of building berth and its equipment depend on the type, dimensions, and structural features of the intended ship to be built and local conditions such as availability of waterfront, variation in water levels and other ground/ climatic conditions.
Ships are launched in different ways commensurate to building berths. Accordingly, the ship:
- may be floated out after filling the dry dock in case of launch from dry dock;
- may be pulled out after lift platform is lowered in to water in case of a ship lift system;
- maybe end/ side launched on cradles down inclined ways by gravity, or along rails on trolleys/ carrier wedges.
- Small ships may be hoisted and launched directly by a crane.
Each of these launching means have its associated requirement of capital investment and recurring expenditure along with operational advantages and difficulties for its effective utilization. A correct choice therefore, is extremely important at the time of setting up of such a facility in the shipyard.
Launching at Larsen & Toubro Shipyard, Hazira
Larsen & Toubro Shipyard (LTSHB) at Hazira is located on the bank of River Tapi flowing down to Arabian Sea. The existing water frontage provides extremely low draft at zero tide level and has wide variation of tide levels during the year. Availability of very narrow width of waterfront led LTSHB to set-up side launch building berth facility. This facility is, therefore, being used for final assembly and launch of all vessels built in this shipyard.
LTSHB has so far has built and delivered two Roll Dock Vessels to its Dutch customers. These ships with length overall of 140 meters were side launched from this facility at launch weight of approximately 5600 metric tons.
The side launch of Big Lift vessel with length overall of 154 meters and launch weight of 8480 tons approximately from this building berth on 1 September, 2011 is considered as probably the biggest side launch in the world so far.
Challenges associated with Side Launch
The principle of normal side launch, wherein launch ways move along with the vessel until it floats normally, was followed in case of Big Lift side launch on 1st September, 2011. The calculations for launching involved :
- Determining the launching weight and centre of gravity of the ship after considering ballasting/ de-ballasting so that ship post launch floats near to even keel.
- Determining position of slides under the hull in line with launching rails, to spread the ship’s weight as evenly as possible over the slides.
- Supporting the hull on strong positions and weight transfer to the slides such that it remains within permissible limits.
- Calculating total surface of the slides and its longitudinal centre.
- Adjusting the position of ballasts on the ship such that new centre of gravity is vertically above longitudinal centre of slides surfaces.
Side launch is considered to be probably most complex and risky means of launching vessels (especially larger ones) due to involved means and requirement of their synchronization.
A large no of launching ways (20 for Roll Dock and 29 for Big lift), a wide variation of ship’s weight distribution along its length, limit on permissible weight carrying capacity of launching rails and synchronized vessel’s slide on these large number of launching ways requires detailed precise calculations, planning and execution of the operation. Reliability of hydraulic jacks for transfer of Vessel’s weight from keel blocks to launching ways and hydraulic holding device’s simultaneous release of launching ways along with the vessel is considered to be of paramount importance in success of this operation. The availability of tide window only on specific dates for shorter durations, to meet the higher launch draft demand (in case of large vessels) further increases the requirement of operation with precision. Launch of any type is always associated with risk; however failure during side launch is considered to be more catastrophic.
The Big Lift vessel has large over hang at its aft end (35% of ship’s approximate length). Therefore, very little area was available to support and transfer its weight in this region to launch slides. Further, especial support arrangement in the form of pillars had to be inserted to cope up with the height difference of hull and slides in this region. In order to safeguard the propeller and rudder that are already fitted prior to the launch, these pillars had to be designed in pieces so that they have natural collapse once the hull is water borne.
The large number of top wedges, expected to slide down along with the vessel, have to be disconnected from the vessel once it is water borne and recovered from water with minimum effort without causing any damage to the vessel. This required the top wedges to be designed with only little positive buoyancy and floating at even keel.
Conclusion
After three successful launches of two different types of vessels ranging from launch weight of 5600 to 8500 tons, L&T shipyard is quite confident to continue utilizing its side launch facility very effectively and has developed expertise in providing assistance to other shipyards in this regard. The shipyard has also incorporated certain measures to execute launch operation in its facility more economically.
Ashok Kumar Khetan is Head Production at Larsen & Toubro Shipyard, Hazira. He is a graduate in Mechanical Engineering, Post graduate in ship design/ construction from IIT Delhi and St. Petersburg. He has long and illustrious career in Ship Design and Construction spanning over more than three decades.