Impact loads during lowering onto a fixed stucture

[Martin7919]

1. The problem statement, all variables and given/known data
An offshore crane is lowering a load with a constant speed of 0.2 m/s. The crane tip has a maximum vertical and horizontal acceleration, as a result of a seastate, of 2.0 m/s2 (vertical) and 0.8 m/s2. The crane needs to lower a 200 ton object onto a stationary object. The stationary object is fixed to the seabed and sticks out of the water. No wave slamming forces are present.

2. Relevant equations
Determine the maximum load at the crane tip.
Determine the vertical and horizontal impact load upon lowering of the 200 ton object to the stationary object.

3. The attempt at a solution
Fvertical = m × a
m = 200000 kg
a = 2 m/s2
Fvertical = 200000 × 2 = 400000 N = 40.775 ton

Fhorizontal = m × a
m = 200000 kg
a = 0.5 m/s2
Fhorizontal = 200000 × 0.5 = 100000 = 10.194 ton

Am I missing something in this?

 

[LittleInch]

Your acceleration figure is the acceleration of the load as such the first solution is valid for the additional transient load seen by the crane hook during the lifting operation ( i.e.the equivalent of an additional 40 tonnes to your 200 tonnes.

However your impact load deceleration figure isn't listed. Depending on the ultimate max velocity that the object is travelling when it impacts with the structure, this deceleration could be very high. All depends on what timescale and over what distance the load stops moving. A short time / short distance and the impact load could be 3 ,4 or more times the mass of the load (!)

So to get any idea what this could be you need to work out the time period your load is subject to the acceleration listed relative to the structure to calculate max velocity downwards (0.2m/s + integration of the acceleration).

Then decide what is the load landing on and how far can it deflect.

200 tonnes is not to be sniffed at and that's why lifting isn't often done with any sort of sea state present or without a heave compensated crane.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[LRJ]

Out of interest, what are the formulae needed to perform this calculation? I noticed in another thread started by the OP there was reference to the effect deflection would have too. What formulae would calculate the deflection (an equation involving M and EI)?