## Impact loads during lowering onto a fixed stucture

## Impact loads during lowering onto a fixed stucture

(OP)

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?

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?

## RE: Impact loads during lowering onto a fixed stucture

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.

## RE: Impact loads during lowering onto a fixed stucture