Friday, March 5, 2010

Ship Launching Revolution-Marine airbag ship launching

The ship launching history is more than thousands years. Nowadays the most commonly used ship launching method is still the oldest end-on launching, in which the vessel slides down an inclined slipway with stern first. Another ship launching method is side launch which is often used on narrow water area by launching the ship broadside. The float-out method means building a ship in basins or dry stocks and then admitting water into the dock to float the ship. The traditional ship launching methods need the fabricated slipways or dry stocks and supporting facilities so the engineering period is long and expensive. When shipyards suffer from the production problem of short of slipways or expensive investment of new fixture they need a solution for sure. For salvage of stranded vessels people also need a less equipments and landform limited ship launching method.
Evergreen airbag launching system is an innovative flexible ship launching method and need the least supporting fixture. It demands the ramp for ship launching only enough bearing capability and neatness. The main instruments of Evergreen airbag launching system are marine airbags, air- compressor and regular winches. The sausage shape marine airbags are made up of rubber layers with rubber dipped synthetic-tyre-cords as reinforcement material. Their diameters range from 1 to 2 meters and lengths range from 6-20 meters. One marine airbag for 1.5 meter diameter and 18 meter length can hold over 200 tons weight. Marine airbags act as the air springs and rolling logs when they conveying ships to water.
Ship launching with marine airbags have a more than ten years history in China. But only in these years this technique develops into an integrated system with the advancement of manufacture and establishment of theory system. It is estimated that more than 80 percent new built ship whose DWT below 60,000 are launched by marine airbags in China. In fact Evergreen airbag ship launching system is not only a good alternative but also indispensable to some given marine projects. A typical ship launching project completed by Evergreen airbag launching system will well illustrate the character of this system.
At July 2009, a new built 20,000DWT ship on a no fixture ramp was successfully launched by Evergreen airbag launching system in Chinese Jiangsu province. This cargo ship is 150 meter long and 21.6 meter width with 7800 tons light weight. For some untold reasons, it was built upon a 200 meter long concrete ramp without fixture or anchor block. The angle of concrete ramp is 0.86 degree and its connection of river water is unsettled mud. This inland river is 350 meter wide with 7-11 meter deep water. The distance between the ship stern and end of ramp is 15 meters. Because there was no anchor block on the ramp, the winch was not available. Then two major difficulties came out: how to start the movement of ship launching and how to avoid the unwanted slip when ship was lifted from the wooden blocks.
In low tide period, mud of riverside was cleaned by an excavator grab and some sand bags were placed to fulfill pits on the riverbed. Total 34 pieces marine airbags with 1.5 m diameter and 18 m effective length were placed between wooden blocks according to the calculation. The charge of airbags was strictly executed according to the scheme which demands the sequence and exact working pressure of each airbags. From the calculation and computer simulation, the ship would not slip at maximum 0.57 degree when it was raised up by airbags. So during the “raise-up” process the principle was control the ship’s gradient angle not exceed 0.57 degree. Airbags were inflated from the stern to midship one after the other until exceeded the gravity center position. Then inflation from bow to midship was carried out to rise up the whole ship. After all supporting wooden blocks removed, airbags at the bow was deflated to adjust the ship’s gradient angle to be 0.45 degree which would make the ship to be safer from the unexpected slipping.
When the tide reached needed level airbags were inflated and deflated respectively to adjust the ship’s gradient angle to be 0.57 degree again. And an excavator grab whose shovel was wrapped with tyres began to push the bulbous bow to give a start-up force. After twice push, the ship started to move slowly by riding rolling airbags and the movement was self-accelerated until stern floated because of ship’s increasing gradient angle. It took 62 seconds from ship’s movement start to total float while the preparation works took more than 12 hours.
This launching project exhibits the convenience and efficiency of Evergreen airbag launching system which can save most time and investment comparing with traditional slipway launching. Because of the advantages Evergreen airbag launching system has been applied to ship launching, ship landing, heavy conveying, wrack salvage, etc. For example, the Chinese invaluable 800-year-old ship “Nanhai No.1” was landed and conveyed by marine airbags and it is said the airbag launching system is one of the most important factors which ensure the success of salvage project .And for gravity quay building, more than 60% power plants use this system to launch prefabricated caissons in China. For marine salvage, Evergreen airbag launching system is widely used by American salvage companies to launch or refloat wrecks.

Conclusion
Evergreen airbag launching system is simple but powerful. It is less limited by equipments and landform and can save most investment and time to complete marine projects. With the development of theoretical system integrity and computer simulation technology, Evergreen airbag launching system will be accurately controllable and becomes into the cutting-edge solution for launching projects.

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