Shock loading

[hydrac]

I've got a payload being lowered over the side of a ship. I'm interested in the shock loading experienced by the cable due to wave interactions. So far, I know I need to consider the impulse force F=mv/t due to the payload being possibly lifted by a wave and then dropped to the trough, so find the delta_v assuming t= 1/2 the period. Do I need to consider any forces due to the wave "pulling" the payload down? From what I read, this doesn't tend to be a factor. Is there anything else I might be missing?

 

[C2it]

DAF = 2 ??

 

[BigInch]

Meaning ...
Design factor = 2,
Design Load = static load * 2, not (static load + impluse load)*2
and ...
Wire Rope Load = 2 * Design Load
... Right?

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[BigInch]

Meaning ...
Design factor = 2,
Design Load = static load * 2, not (static load + impluse load)*2
and ...
Wire Rope Load = 2 * Design Load
... Right?

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[hydrac]

Yeah, I've been told that most things are designed with a safety factor of 2. I've also been told that 2*Design Load is the rule of thumb that is used. I was asked to derive the math behind what the impulse felt is, which is why I figured I'd ask around.

 

[BigInch]

2G = 2 x Gravitational Acceleration

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[JRMacGregor]

If your load is wide and flat, it will experience a greater DAF (dynamic amplification factor) or shock load as it passes through the "splash zone" (air/water interface) than a slender, vertical structure. If the sea is totally flat, then the DAF will be minimal.

You also need to take care of hydrodynamic added mass on the submerged object.

The DAF for your lift will depend on the sea state that the lift is being done in (small waves, less impact on the load), the motions of the crane vessel in that sea state (which induces accelerations into the crane wire), and on the shape/weight characteristics of the load (see above). The motions of the crane vessel depend on the size/shape/heading of the vessel as well as the sea state.

The DAF for a difficult lift can easily be more than 2. If you are lucky and the sea state is low, the DAF may be as low as 1.3.

Lloyds Register have Code for Lifting Applicance in a Marine Environment (CLAME) and DNV have similar rules for offshore lifting. You can see from these rules how the safe lifting capacity of an offshore crane (expressed in terms of the static load) is reduced with increasing sea state.

And of course you know that the total load predicted to be experienced by the crane (combination of static load and dynamic amplification) should not exceed the stated limits of the crane (the "crane curves").

 

[Ussuri]

Is this for checking snatch loading on the rigging if it goes slack, or for assesing the slam on the structure as it passes through the air/water interface?

The DAF of 2 is commonly used as that is the point at which you may get slack rigging (ie upward hydrodynamic load is equal to the submerged weight of the object).

Design calculations for deployment through the splash zone are typically done to ensure that you do not have a slack rigging condition. If you do have a slack rigging condition the load applied can be way in excess of a factor of 2 (object is in freefall)

For most lifts it is easier to design so that you dont have slack rigging (resticting seastate, ballasting, tilted lift etc) than to accomodate it.

If you dont not have slack rigging, then you do not have a 'shock' load, instead you have a hydrodynamic load that varies with phase.

As you go through the splashzone your structure will be subject to an upward slam force. This may be greater than the upward hydrodynamic load (or not - depends on your geometry) but you would still still try and design such that even under slam pressures the rigging does not become slack.

 

[Shinexx]

See DNV-RP-H103 (Modelling and analysis of marine operations)