Dropped Object Impact on a Submerged Flat Steel Plate

[Ussuri]

As part of the design of a subsea structure a critical load case to be assessed is the dropped object case. This is critical to ensure the integrity of the equipment contained within the structure.

Impact protection is often provided with roof panels fabricated from flat steel plate. The analysis of dropped object is based on a given impact energies, which in my area of the world are defined as 5kJ impact over a 100mm diameter, 20kJ impact over a 500mm diameter and 50kJ impact over a 700mm diameter (these are defined in NORSOK and ISO 13628). It is well know these are not related to any real life objects, and the impact energy of a dropped subsea tree could be in the thousands of kJ. The industry standard approach to demonstrate a safe design is to do the impact calculations on the assumption of an in-air impact.

However, in reality these plates are submerged, so the presence of the water will dictate the behavior (deflection) of the plate under impact. In order for an impacted plate to deflect the surrounding water must be moved, so the resistance of the plate to impact is not just a function of the plate shear and tensile capacity, but also the added mass (inertia) of the surrounding water.

I have been trying to find out if anyone has done research on this subject. There is plenty about impact on submerged pipelines (which are resting on the seabed) but nothing I can find about flat plates suspended in water. I have been through the normal online searches and paper searches but I have been unsuccessful.

Are any members aware of research on this topic, or something similar?

 

[Erik Panos Kostson]

Yes, you are correct; it is not the same as having the plate in air.

The most important thing is that vibration modes/frequencies, will be altered in the water due to the added mass effect which lowers the eigenfrequencies. Additional damping also arises from the water, and other effects. I am not sure that you will find an analytical solution for your exact problem. Paidousis has written a good book in fluid structure interactions, you might want to see if there is a chapter on this specific problem (if not there might be a reference).

Many times, one has to do FEA that might account for these effects (advanced FEA codes can possibly do that, e.g., MFLUID function in Nastran).

In any case if you calculate/design it for air that should be conservative since there is more damping in water thus in theory (SMSS) in water there should be a possibly lower vibrational amplitude due to a sudden impact (should be then OK in water); another parameter for more pulsed impacts (not sudden load impact where the dynamic amplification is 2 times the static one), is the ratio of the pulse duration to the eigen-mode (and frequency) in the impact direction of the plate, which is of course altered due to the water added mass. Of course validation and verification is always key, so do not rely on my assumptions here, test it out.

 

[WARose]

Interesting problem. One big question I have is: how deeply submerged are we talking? I ask because if it's deep enough....then it really becomes a simple buoyancy problem as the water takes away all the impact energy. If not, it could get pretty tricky.

But to answer your question: that doesn't ring any bells with me. (in terms of research done.)

 

[Ussuri]

Granted, if it works in air then it will work in water. However, we can make designs work in air at 50kJ but at 100/200/300kJ or more then it becomes impractical to design for this. So industry approach is not to bother. But if the effect of the water was included then I imagine you could make a case that the submerged panel will provide protection against a much higher impact, how much higher, I don't know.

In my geographic area we are looking at water depth ranges from 30m to 500m. Once the object is submerged it just continues to fall until it strikes something.

 

[SlideRuleEra]

Ussuri - As a alternate approach, consider the falling object instead of the plate. Buoyancy and drag as the object moves though water combine to keep a falling object's terminal velocity pretty low, and the falling object gets to terminal velocity quickly.

I have no idea how to estimate the way an underwater plate will respond to a constant speed impact... but, I'll bet that the effect of water on the plate is small compared to the effect of water on the falling object.

From the internet, seems that falling spheres are the easiest falling objects of to analyze. Google on "terminal velocity of objects falling in water".

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[SAIL3]

a complex problem indeed....maybe one could determine the deflection of the pl in air...it's velocity/acceleration during deflection...then, with this velocity, assume the water in contact with the pl has this velocity and calculate it's force on the pl...this force would then counteract the force of the impact...it would give an upper-bound (conservative).....

 

[rb1957]

i guess you "could" approach the problem that way (using the velocity of a plate in air) but the water environment makes this (IMHO) a red herring. The water is going to affect the object (slowing it considerably) and the plate (both supporting it and also applying pressure waves).

but I wonder if we're over-thinking it ... the structure has to support impact energies, not specific dropped masses. Subjecting the plate to the same impact energy in air or in water is likely very similar, there'd be some argument to say that the air case is conservative. If that's what SAIL3 meant, then I guess we agree !?

another day in paradise, or is paradise one day closer ?

 

[BUGGAR]

In the design of ship fenders, we sometimes include the water cushioning effect of a flat-faced wharf. I'm out of date on this but there may be some useful info there. NAVFAC has free fender design guides but they may be pretty "cook book" for what you need.
"Mythbusters" did some great stuff with projectiles shot into water.

 

[ldeem]

Since you are designing for a certain impact energy the analysis is a series of force displacement points plotted on a graph - so the area under the curve would have to be greater than or equal to the impact energy. This gives you a static load to use in checking the structure.

I have done a few impact designs and always find the deflection to be rather small. Assuming your case is similar small deflections I would guess that the added energy absorption of moving the water is relatively small. Another way to think of it is as two springs. One spring being the structure and the other the water. The structure spring would be much stiffer than the water spring. Assuming the water is not contained in some way. I am imaging your structure is a roof with open sides. If the roof is actually part of an enclosed structure then accounting for the water spring would be more difficult.

 

[Ussuri]

What rb1957 says is true, the usual design methods are based on energy (specifically kinetic energy and strain energy), rather a prescribed mass and height. Hence 50kJ is 50kJ, there is no reduction due to the effect of the water (or to look at it another way I suppose would be if dropped in air, it will slow when passing through the air/water interface (losing energy?), settle into a submerged free fall situation striking the plate with an residual impact energy of 50kJ (or 100kJ or 500kJ), which is the design load. And the response of the plate is different to the response of the projectile and what happens to the projectile (dropped object) is largely irrelevant as far as the design is concerned. Whether protection is provided depends on the response of the plate, too much deflection allowing plate to strike equipment, plate failure allowing to shear through.

Cushioning is a nice way of terming it and might be a useful lead. My first thought was that water at a fender has a free surface, so the water can displace into as void (into the air , a much lower density material). Subsea there is nowhere for the water to 'displace' into so it is a flow within the mass.

Low velocity impacts are normally measured in impact durations of milliseconds. The plate behavior is normally split into 2 phases, the initial transient phase, directly after impact, where only inertial forces are present in the plate and the global phase where there supports are mobilized. For an in air impact, the transient phase is short lived and the only mass involved is the mass of the plate (which is pretty small), its response is dominated by the global phase. However, adding the water around the plate increases this mass significantly (plate mass plus added mass of the fluid).

So some thoughts I have. How much time would be needed to overcome the added mass of the water? Could it be that the incompressible fluid doesn't actually move much and initially and the object just rebounds?

It could be that a CFD analysis which can handle dynamics/contacts may be how to do it. Complex it is.

Dropped object protection on subsea structures is very common, I would have thought someone would have done some research on this because of the cost and impact associated with a hydrocarbon leak.

 

[Ussuri]

Ideem, thank you. Integrating over the force-displacement curve would give the energy absorbed. Intuitively the presence of the water would allow higher impact energies to be dissipated and the water will also reduce the deflections, which kind of implies the force component associated with a deflection is higher. I have no idea how to calculate the 'force'.

 

[ldeem]

Ussuri - I think you are right the water would contribute to the energy absorption but I think the effect is small. I would run it as if the roof were in air and see what defections you get. If small then probably not worth the effort to try and figure out the water energy absorption.

Blodgett "Design of Welded Structures" has a section on this.

 

[robyengIT]

Blodgett "Design of welded structures" : section 2.8

Blodgett "Design of weldments" : section 3.1

 

[rb1957]

I was thinking that the impact energy applied to the plate was whatever is specified, not the starting energy (potential) of the experiment.

But then youwe think the water will absorb some of the starting potential energy, therefore analyzing a plate in air is conservative ... no?

The difference however could be that in the underwater state a much higher impact energy is required ... something we couldn't apply (sensibly) in the plate-in-air case ?

another day in paradise, or is paradise one day closer ?

 

[Ussuri]

Ideem, I think the presence of the water will have an opposite effect. I think it will hugely increase the amount of energy that can be absorbed, something akin to dropping an object onto concrete.

I agree doing the calcs in air is conservative , and that's why we do it as it gives safe designs. I am very curious as to how conservative though. I am think a lot.

 

[Ussuri]

Buggar, I have had a look at the NAVFAC website and searched for Fender design. I couldn't find any design guides. There is a link to find a NAVFAC design manual which diverts me to the US Dept of Defence. The same search returns 7137 documents. Do you have a reference?

Thanks.

 

[BUGGAR]

I found them in a search for UFC 4-152 (and others - there is a UFC 4-159 etc.)

 

[robyengIT]

marine fender design

 

[WARose]

In my geographic area we are looking at water depth ranges from 30m to 500m. Once the object is submerged it just continues to fall until it strikes something.

Not sure what height (in air) your impacted object is dropped from.....but that sounds to me like it's going to be a 2 part impact/drag/buoyancy problem.

As far as the impact force on the plate goes.....someone has suggested Blodgett's book as a resource. (And it's a good one.)

 

[rb1957]

can you leverage the pipeline analysis you've found ? How do they treat impacts ? A mass dropped at sea-level or an impact energy ?? If a mass dropped at sea-level, then they must have determined the impact conditions (which should be (at least) similar for your case, no?).

another day in paradise, or is paradise one day closer ?