Calculating Impact Force for 65 Tonne Object Lowered by Crane: Case Study

[Martin7919]

Homework Statement

I am a student and have a question:

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.

How can I calculate the impact force as a result from lowering the object, weighing 65 tonnes, onto the stationary object (red object) which is fixed to the seabed.

Homework Equations

F = m x a

The Attempt at a Solution

m = 65 tonnes
a = ?
F = ?

 

[CWatters]

I suspect this question requires specialist knowledge rather than general physics knowledge..

In a calm sea, if the crane lowered the weight very slowly there would be very little impact force. However you mention a seastate hs = 0.5. Presumably this causes the boat carrying the crane to rock which raises and lowers the weight potentially increasing the impact velocity even if the crane is hardly moving? That would effect the impact forces but without knowledge about how the seastate effects the motion of the weight I can't see a way to proceed.

Is the red object floating? Does that also move up and down with the waves?

There is another problem. It's not possible to calculate the impact force between two colliding objects without knowing more about their physical properties. Are they very hard or very soft objects? For example if you fall over and bang your head on the ground the force you experience will depend on the hardness of the ground. Is it concrete or mud? They say you should never bang two hammers together because being very hard the forces at even low speeds can be very high causing the iron to shatter.

So from a general physics perspective I'm not sure you have sufficient information to answer the question. Perhaps there is data/assumptions that are specific to this industry you can use?

 

[Martin7919]

Hi CWatters,

Thanks for the reply! I know that some specialist knowledge is required. However, I do not have this at hand. I know that, in a sea-state, the vessel has a certain response to it in terms of heave, pitch, roll and yaw. Heave, pitch and roll around the center of motion will result in a certain crane tip motion as a result of its distance to that center of motion. Hence, the greater the outreach of the crane (i.e. the location of the crane tip), the greater the impact of the vessel motions on the acceleration of the crane tip. Since the object is connected, via rigging, to the crane tip, the object would see nearly identical accelerations in vertical direction I think. That is, if you consider that no cable pay-out at that time is taken into account. Otherwise, I would think, one should take into account this payout acceleration also, isn't it?

However, I do not know how to calculate or estimate the crane tip motions. Do you or anyone know if there is a certain code which allows you to do this by approximation?

To answer your question, the red object is not floating. Hence, there is no object to object impact (i.e. both objects moving to each other). So the red object is stationary and the 65 tonne weighing object is lowered to this stationary object.

Regards,
Martijn

 

[Martin7919]

Any idea on the above?