To lift concrete blocks, generally airbags need higher pressure than lifting vessel up because vessels have bigger bottom/weight ratio. For big caisson lifting, air bags' pressure can reach 0.3 MPa.
Moving caisson with marine airbags needs winches to provide pulling and restraint force. That calculation is easy once people know the friction coefficient of airbags with ground is about 0.03. The anchor points for pulling and holding are usually reserved at bottom slab of caissons.
Move caisson to submersible barge is the complicated process. The submersible barge needs to keep same level when caisson is moved to the deck that means the barge will be ballasted first then deballasted. Generally people will do the job during tide rising phase.
Assumptions: To calculate airbag and launching barge bearing force during heavy being transported on board, we take two assumptions: the transported heavy bottom will not bend, airbag bearing force is linear with its working height.
Tolerance: The first assumption is good satisfied regarding concrete structure transport with air bags but the second assumption is not. When air bags are compressed their pressure and contact area will increase nonlinearly. By accurate calculation we find the second assumption will cause 20% tolerance. In the design of heavy transport the safe coefficient is always above three so the second assumption is acceptable.
Equations:
If the gravity distribution of caissons is uniform the calculation of marine airbags’ supporting force is easy to be decided. When there is excentricity of caisson, the calculation will come to a little bit complicated however it can be solved based the basic principle.
Calculation instance
A cuboid concrete structure: Length 60m, Weight 4800 KN, Submersible barge: L=63 m, △l=0.2 m
Compare Air bags heavy transport with trolley heavy transport:
A typical calculation result of excentric caisson supported by marine airbags is illustrated as follows:
From the calculation result people may find that pressure of fifth airbag is more than 4 times of the first one. So before perform transportation, people must check if the proposed maximun pressure of airbags is in the safe working scope to make sure the project’s safety. According to the calculation, people also can know how to arrange the airbags. Different from the uniform caisson, arrangement of airbags for excentric caisson should be arranged as follow drawing to avoid the incline of caisson.
More detailed calculation results can be provided upon request.
