Hydro-Elastic Loads

gCaptain has in interesting article about research in Denmark by Ingrid Marie Vincent Andersen which suggests the stresses in ship structures due to waves may be significantly larger than previously calculated by classification society rules.

“It is believed that the hydro-elastic effects and the effect of hull girder flexibility are capable of significantly amplifying the hull girder stresses and thus contribute to fatigue damage as well as to the extreme hull girder loading in container ships,” Andersen notes in her PhD thesis.

In her research, she studied ships in the 8000-9000 TEU range and discovered, “the hull girder vibrations due to hydro-elastic effects is capable of doubling the stress response amidships in some cases – also in the extreme loading cases.”​

http://gcaptain.com/danish-naval-architect-uncovers-important-clues-mol-comforts-demise/

 

This sets up an intersting engineering morals question, similar to the racing multi-hull breakup problem.

Merchant ships are not analysized for shock load whipping. And should they be? Is the added expendature of steel and production time (i.e. no skip welding) across all the ships in the merchant fleet worth the cost compared to the few instances (Fitzgerald, Derbyshire, Comfort, etc) where extreme hull girder and shell plate deflection occur when even the whipping analysis may not be enough to predict loss of sectional comptence? Or should we just expect better weather routing?

As I have said before about the difference in analysis precision and accuracy; we must be careful not to just move the razor line in the chalk line.

 

The new CSR addresses this and updates will include reference to whipping loads to be analysed.

EDIT:

Just checked. The new LR notation will be WDA - Whipping Designs assessment. And also the new FDA SPR - Springing Fatigue design assessment, owing to that analysed using the WDA.

 

The Thesis document written by Ingrid Marie Vincent Andersen on the measurement of full scale hydroelastic loads may be found here:

http://orbit.dtu.dk/files/101604351/S166_Ingrid_Marie_Vincent_Andersen.pdf

A word of caution: This document is over 400 pages long and about 61 Megabytes big.
There are some color glossy photographs included though.

 

Interesting topic this whipping.

I have an old spreadsheet that modifies sinusoidal waveform so I whip it up (no pun intended) to see if I can program it. I was hoping to animate it with different wave velocity to find where it will resonate.

Anyways, I opted to do it manually untill I get the hang of it. I assumed the shock induced velocity is twice the wave speed (speed of ship + speed of wave). Thus, the shock travels faster, reaches the stern and whips up a reverse shock wave to meet the the travelling wave. The result is quite revealing where it meets.

Of course the spreadsheet was based only on resonance and did not include any sophisticated hydroelastic loads and complex waveform. It is at best illustrative in its simplified form.

 

Mmmmh... Where does this assumption come from?

 

The speed of the ship meeting a wave travelling in the opposite direction. It is just an assumption for a start, the length of the wave is equal to the length of the ship. If I vary the wave length, it will resonate at some other point.

 

Finally found some time to do the programming. I simplified the wave form and draw the bow to the left because this is how Excel calculates. Left to right. I limited the iterations to two wave cycles. Adding more would show how the harmonics are developed, but this would complicate the spreadsheet as wave decays have to be factored in. There are no dimensions, only proportions and ratio. The speed is the length of the wave in proportion to the length of ship (S/L ratio).

To reset, just enter the number zero in the red cell. To cycle, pick the slider button on the right. The first wave will start followed by the shock induced wave. This shock induced wave velocity is faster than the regular wave because it is the combined velocity of the ship speed plus the oncoming wave. As the shock induced wave exits the stern, a reverse shock wave is induced and moves towards the bow. The sum of all these waves is shown by the dotted line in the graph. In the ribbon graph, only the sum is graphed. A little bit exaggerated but it shows how the hull is tortured.

What is interesting is that with the shock induced wave, the stern pressure is reinforced and amplifies the wave profile accentuating the stern whip at ¾ cycle .

That there is also twice the bending other than hogging and sagging condition because of the meeting of the travelling wave and shock waves.

The peak stress occurs at nearly twice the pressure at or near the midship. This is caused by the travelling wave and the shock wave colliding at 1.9 or 2.4 wavelength. It varies a little depending on the wavelength or more specifically, the S/L ratio. Tried this at 1.00 and 1.33.

The principle is no different from our high school physics experiment. A brick is tied to a long rope and the free end of the rope is flicked to induce a travelling wave. If the brick is light, the travelling wave will dislodge the brick and dissipate the energy. If the brick is heavy, the travelling wave will return (reverse) to the free end of the rope. Flicking the rope before the return wave has arrived will create a pronounced peak at the middle of the rope.

 

A bit off-topic bur also strength issue, for small craft. We continue our testing of boats with accelerometers and pressure gauges installed on the bottom. Here is a screenshot from yesterday's results. What is interesting - the actual pressures are quite low compared to classification societies rules and ISO12215... I really believe those loads travel from document to document, but originate from well known torpedo boat tests. Need to be verified when applied to catamarans, to contemporary hulls using methods available today.

 

I assume you collected the data digitally, so what was the sample rate? I doubt you measured your peak event and so the only thing that data is good for is stochastic analysis to determine your high confidence limiting event. (i.e. 99.95% of impacts will be less than xx,xxx psi )

FWIW, I have used pen, electrostatic, and digital strip recorders trying to catch hydrodynamic impact loads. Electrostatic seemed to work the best, with no pen inertia and little data drop out. It was just going over 100's of feet of chart looking for your significant event. Also you just couldn't turn it on and walk away and it didn't lend itself to FFT and stochastics.

 

Sample rate for pressure - 20Hz.

 

Too slow; if you really want to measure the peak pressure you need to be in the speed of sound range...so ~45 kHz for a 12mm skin.

 

I don''t think it should relate to thickness of skin, but to wave encounter frequency - yes, it should.

 

So...do you know the phase of the encounter to the skin? I don't think so.

Edit afterthought... I wish I could show people what real hydrostatic shock looked like...10,000 of psi landed in 1/10,000 of a second , but the real energy amount of ft-lbs of energy imparted to the hull. Even if you don't impart enough energy to bend the hull material, you can still cause enough movement to cause loss of section competence.

 

Do You know what frequency was used by Jasper in his 1949 paper, and how that record was processed/filtered?