# Impact force during lowering of a load

## Homework Statement

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.

## Homework 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.

## 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?

Nidum
Gold Member
Hi Nidum, thanks for the reply but I am not an Engineer with so much back ground knowledge that I can understand these documents. Can you give some advice?

Nidum
Gold Member
This is a at best a difficult problem to deal with .

See if you can post physical descriptions or pictures of the fixed column and of typical loads . It may be possible to get at least ball park answers for the impact loads .

Hi Nidum,
The fixed column is a pipe 3 m in diameter with a wall thickness of 30 mm. It stands 40 m heigh of which 10 m sits in the seabed and the top of the column is 12 m above the waterline. The object which is to be lowered onto this column is 4 m in diameter, a meter high and weighs 60 ton.The crane tip sits at 75 m above the waterline with a 40 mm cable quadrauple reeved to a hoist block.

So the column is a pipe with a 30 mm wall. Typical loads are not known, what do you mean by this?

Nidum
Gold Member
Typical loads are not known, what do you mean by this?

Bit open ended really but possibly machinery or other equipment . Open , crated or palletised , big or small - just an idea of what we are dealing with .

The load is a cilindrical cap like object which closes off the fixed pipe. The cap is 4 m in diameter and is lifted at 3 lift points on the top. The weight is 60 ton.

Nidum
Gold Member
There is no way of actually getting a one value answer but it should be possible to make an estimate of the probable range of the impact forces .

I have to do other things now . Pick up on this later .

Martin7919
Thank! I hope you can help me before tomorrow before I have to go to school again.

Anyone anything to share which helps me further? A code/practise which I can use or...?

Nidum
Gold Member
This is not the same question as the one originally posted and you now have added information which was not in the original statement of either question .

Sorry but I am a bit confused .

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Yes it is, you asked how it looked like. However, I am looking for a method of solving. Otherwise asume it as being connected to the crane wire directly. No three point rigging in between.

Nidum
Gold Member
(1)
For a small case study I have to determine the lateral and vertical impact loads during the installation of a object onto another object.
This is done by a cranevessel, using its dedicated lifting crane. However, I am not familiar with this type of installation though.

The case study is presented hereafter. A weight of approximately 65 tonnes is to be lowered with the crane onto the red marked object in a seastate of Hs = 0.5 m.
During the lowering, the crane boom outreach is 21 m from its rotation centerline and located 40 m away from the vessel centre of motion with its boom tip at a height of 60 m above the centre of motion.

(2)
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.

(?)
The fixed column is a pipe 3 m in diameter with a wall thickness of 30 mm. It stands 40 m heigh of which 10 m sits in the seabed and the top of the column is 12 m above the waterline. The object which is to be lowered onto this column is 4 m in diameter, a meter high and weighs 60 ton.The crane tip sits at 75 m above the waterline with a 40 mm cable quadrauple reeved to a hoist block.

I get it, my apologies. It concerns case (2). I hope you can help me. The top end of the stationay object sits above the water and the lifted object needs to be lowered onto this object for which the impact load is to be calculated.

Anybody got any idea, I really want to understand how I can solve this problem :(.

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Nidum
Gold Member
I have looked again at the questions . The problem is that there is not enough information supplied to be able to do any meaningful calculations .

To calculate impact loads you need to know the closing velocity between the colliding objects and the impact response characteristics of the objects .

I would like to help but I don't see any way of proceeding at present .