General introduction of Ship launching with marine airbags

The traditional ship launching methods include end on launching , side launching and dry dock floating. Most airbag ship launching are end on launching.

To launch a vessel with marine airbags, people should set up holding system first , then lift vessel from keel blocks , remove blocks and move the vessel with airbags into water at last.

      Set up holding system for airbag ship launching                  Jack up vessel from keel blocks

Generally airbag ship launching is carried out on a ramp, with suitable angle the vessel can move into water by the sliding force from its gravity once the holding system is released.

There are two methods of holding system releasing corresponds to “free launching” and “controlled launching” separately. The free launching means the connection of holding system with vessel will be unlashed when vessel is launched£¬it needs wide water area.

     Free ship launching with airbags                                                  Ship floats after launching

In case the launch water area is limited, the controlled launch is preferred. Winches will be released slowly to control the movement of vessel, once vessel floats, it will be stopped by winches’ pulling.    

Applicability of airbag ship launching

The airbag launching feasibility of a specified vessel should be calculated first but there are some simple rules may help people to know launch feasibility quickly.

The majority of airbag ship launching is end launching but side launching is feasible  only when some rigorous conditions are satisfied.

Vessels whose DWT below 70,000 tonnes may be launched with airbags.Regarding bigger vessels only by calculation people can know the launch feasibility. 

Airbag ship docking/beaching introduction

Airbag ship docking means people haul out vessels to the land with pulling winches and rolling airbags. Comparing with traditional trolley docking method, This system has less demand of ramp/slipway and also the pulling winch.

Generally people will ground or nearly ground vessel then insert airbags to lift the bow first in low tide time. When water level increasing, vessel will be pulled forward thus more rooms underneath vessel become to be available for more balloons. Step by step vessel will be entirely supported by balloons.

When vessel arrives at its final position, by deflating balloons slowly vessel sits on keel blocks steadily.

Facilities for airbag ship launching and beaching

Winches

Winches are deployed to hold vessels before they can be launches or pull vessel out of water. Generally speaking winches deployed in airbag ship launching or beaching projects are electrical, low speed type. Their pulling speed is below 10 m/minute.

For safety reason winches should be equipped with one extra set of manual brake. In case of electric brake system breaks down or power loses accidentally people can brake the wire ropes with manual brake.

Ground Anchors (winches foundations)

Ground anchors are designed to fix winches. Ground anchors can be cast concrete structure or buried heavy objects. The safe load of ground anchor should have at least 1.5 times safe coefficient.
To enable the installation of winch and pulleys, ground anchors should have the connection pad eyes.

Wire ropes

Wire ropes should have at least 3 times safe coefficient. The use of rotation-resistant wire ropes will not likely be required.

Pulleys (Blocks)

Pulleys are deployed in airbag ship launching or ship beaching projects to magnify winches’ pulling force. Generally traveling blocks will connect with vessel, standing blocks will connect to pad eyes on ground anchor.
There are many techniques to reeve blocks. But most important purpose is to avoid the twist of blocks, especially the traveling blocks.

Conventional wire ropes will twist or unlay slightly when they are tensed. To eliminate odd-part reeving where the wire rope dead end is on the traveling block is very important because wire rope torque, from the application of load, is greatest at the rope dead end.

Air compressors

To pump airbags, the air compressor needs to have 7-8 bar inflating pressure and not less than 2 CBM per minute flow rate.

Airbag Arrangement

Airbag arrangement is very important for safe ship launching or docking. All conditions include vessel dimensions, the weight distribution, launch slipway and water level shoule be considered to decide airbag arrangement.

Vessel Length

All air bags should be positioned center to center with the vessel. More airbags deployed for the project is better for the safety. The maximum space of airbags should not exceed 6 meters. The minimum spaces of airbags are decided by airbags’ diameters.The follow chart shows minimum spaces of different airbags. 

Vessel Width

When airbag’s length is longer than vessel’s width, airbags should be positioned center to center with vessel.

When airbags’ length is shorter than vessel’s width but longer than 1/2 vessel’s width, airbags should be positioned as crossover type.

When airbags’ length is shorter than 1/2 vessel’s width, airbags should be arranged as two rows. We don’t suggest arrange airbag in three rows even for extra wide vessel or platform launch.

          Cross over type of airbag arrangement                                                 Airbags in two rows

Vessel Gravity Distribution

It is easy to understand that under heavier part of vessel, there should be more air bags.

Launching Slipway and Water Level

The water level and slipway affect the working height ,space of airbags arrangement.

Slipway/Ramp design

The slipway's parameters which include length, width, slope angle, bearing capacity and elevation should be designed base on calculation results. They are decided by vessel's parameters and water level.
The length and width of airbag launching slipway’s can refer to traditional end launching slipway design.

The bearing capacity of airbag launching slipway can be lower than traditional slipway because airbags get more contact area than traditional launch trolleys.

Generally slipway for airbag launching and haul out will have shore part and under water part. The shore part has small slope angle and the underwater part has bigger angle which can give launched vessel more slide forward force and enable airbags roll out of bottom of vessel. The length, angle and structure of underwater part are very important for the success of launching and airbags’ safety.

If possible concrete slipway is the best for airbag launching and haul out. For temporary vessel launching, strong enough ground is acceptable but slope angle should satisfy the demand.

Basic information needed for design of ship launching procedure

1.  Type of vessel?
2.   LOA  and length of vessel?
3.   Molded breadth of vessel?
4.   Vessel's actual launched weight (gravity weight)/weight without cargo?
5.   Block coefficient of vessel
9.   Draft of stern and bow
10. Gravity center position
11. Type of ground ship is built on? (Dirt, concrete, etc.)
12. Height of wooden blocks that support vessel
13. Angle degree of slipway? (Land, not under water)
14. Water survey data of end of slipway area
15. Tide schedule at site
16.  Distance from vessel stern to the end of slipway
17. Pictures of vessel ( stern,bow) and slipway

Risks Analysis

There are risks of ship launchng with airbags.When accidents occur the marine airbag is most often blamed. But according to our investigation more than 80% accidents are caused by inappropriate air bag arrangement and bad launch conditions.

The main factors that affect launch safety are launch ramp, water level and facilities. See details at:

Risk of ship launching with airbags-Launch Ramp

Risk of ship launching with airbags-Water Level

Risk of ship launching with airbags-Holding System

 Related information:

Ship launching airbags                  Calculation of ship launching with air bags

Vessel haul out procedure             Risks of ship launching with airbags