Has the problem of collapsing container stacks and containers lost overboard been overblown?:
Interesting article, especially the new Marin âtraveling waveâ findings is important.
The Marin research found that with a ship in a head sea, the slamming motion can pass along the length of the vessel as a travelling wave through the hull. When this wave reaches the aft end, the lighter structure shows peak accelerations of similar amplitude to the original slamming forward. This can increase the loading on a containerâs lashings by 40-50%, but it remains unclear if all the classification societies calculate or currently allow for such additional forces when approving the Cargo Securing Manual that the crew must use to assess the container stowage before each voyage.
Not mentioned is the fact that a large portion of such a wave is reflected when it reaches the aft end of the ship. Standing waves are then possibly created leading to even higher forces on the stacks.
Do you think, a large ship acts like a vibrating rod, used for personal fitness training?
I do not.
I do not think hulls are made of spring steel. A large part of the flexing energy should be transformed into heat in the complex longitudinal structures.
This may lead to metal fatigue, which has nothing to do with the shipâs behaviorâŚ
âŚuntil she breaks in two pieces.
Itâs a decent article. His point about tending to the lashings during the voyage is definitely something that some do not think about. Once you have started rolling (even gently) in a following sea, they need to be tightened at least once a day. Once the weather is so bad that it is no longer safe to go on deck, you really donât want to be wondering when the last time you cranked down on them was.
Iâve been through some very rough North Atlantic weather over the years and thankfully have never lost a box. It is one of the early questions people ask me if we have just met and are discussing what I do. My answer has always been the same. I do not exceed stack weights or lashing limits and always look for the best possible weather when I can. That is my plan and it has worked well so far. I feel that some chief officers and Masters are afraid to tell a planner that the ship is not capable of carrying certain load profiles. That is what I think is the main reason for these cargo losses. Overloading or poor cargo planning.
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Clearly no expert here, but could visibly see loaded oil barges flex a bit while pushing ahead into weather. No matter what size. Was told that was a good thing, I donât know if it was or not. Everything shook, Barge and tug. Always wondered how those thousands of welds held up.Was not as concerned about the tugs, built like brick shithouses, The barges, not so much.
Yes, that isnât only what I think but the (thin walled) beam theory is used generally for analyzing ship vibrations. See this link, page 3. Slamming will strike a blow sufficiently powerful to cause the whole ship to vibrate. Physics, not me, dictate that a traveling wave that reaches the shipâs end must be reflected and can cause standing waves.
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Anyone who has sailed on a VLCC will recall how you could see the hull flexing in a head sea, you could play a tune by tapping a guard rail with a shifter. Now that we have box boats the same size it is no surprise to me.
I never lost a box or had any move in a stow. I was 250 miles from the centre of a super typhoon north of Luzon and they stayed in place I am glad to say and I never want to get that close again. I have had them damaged rounding Cape Horn and once the sea punched out the floor of a container. It was full of tinned corned beef and the Kiribati crew was most appreciative.
I am pleased I went back to the oil patch before these behemoths appeared.
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gCaptain published this video several years ago. The X16 speed illustrates how much the hull flexes. Too bad the sound was left out; to me the sound of the plates warping is more distubing than the sight.
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On tankers during bad weather you could see a gap widening and narrowing at the end of the catwalk at the midships side. The maximum gap width was about 10-12 cm. There was a sliding arrangement so that things could move easily. Some dropped in a couple of walnuts to be cracked often with disastrous results, pressed into a pulpâŚ
All I can say is I am safely at home and retired. Even at a very young age saw these rigs flex, and was concerned. Captain told me all is well, it is normal. Sailed with some decent fellows, some of my deckies later on noticed it too. Went back to my roots and told them âItâs okâ. Was I right?
The SL-7s would flex like that.
On some ships when the bow slams into a head sea it will cause the whole ship to vibrate noticeably, a low frequency (maybe one second or so). After a few shakes it dies away.
Related to the flexing but I think thatâs what this âwaveâ is referring to.
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Heavy lift transport have to take consideration of both flexing and slamming when calculating and designing seafastening configuration.
Wire lashing stretches when exposed to forces caused by acceleration from rolling and pitching and need to be re-tightened regularly.
Welds may crack from vibrations caused by slamming, but more so by the deck of the HLV flexing differently from the cargo.
That is the reason why we seldom weld seafastenings both to the deck and the cargo. (There are exceptions)
Daily inspection of lashings and welded seafastenings are recommended on the C of A issued by the Warranty Surveyor.
Typical seafastening of a rig (or other floating cargoes) on a HLV. Strongbox welded to the cargo and standard Seafastenings welded to the deck:
The most complex heavy transport I have been involved with was the concrete Ekofisk Barrier halves from Rotterdam to Norway in the autumn 1988. (140 m. diam & 27000 m.t each half):
The HLV would flex, but prestressed concrete does no bend. The cribbing had to take up any flexing, so they consisted of a mixture of steel, Iron Wood and Soft Wood, placed according to the calculated load from limited flexing of the deck.There was also a limitation put on; max. allowed roll of 5 degr. to avoid slamming against the 40 m. overhanging âendsâ of the barrier half.
As Project Mariner for the Warranty Surveyor I signed C of A for departure of the first transport based on a very good forecast and joined for the ride.
Within hours after departure we were hit by an unforecasted weather front with B10-12 force wind in the German Bight:
We avoided excess roll by heaving to and keeping the seas 2 points off Port bow.
When the weather calmed down I went down on deck to see if there were any visible crack and was greatly relieved when I saw none. A bottle of water that someone had left on top of the scouring skirt was still there.
On closer inspection by the main Contractor after arrival in Ă lfjorden it was confirmed that there were no cracks.
What could you do with the 27000 m.t cracked half of a concrete barrier, except sinking it in a deep fjord?
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Not too much.
For a PCTC of 200 m length, 32.2 m width, 10.6 m draft, depth of 38 m and Î v is 1.6*Î, the natural frequency of the ship is 2.12 Hz. Second order frequency is 4.2 Hz and the third 6.02 Hz.
The Î is 1.6 times larger because the hull is coupled with the water mass surrounding the ship. The travelling wave doesnât only âseeâ the ship mass but also the surrounding water mass.
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Thatâs about right 2 Hz. IIRC it wasnât just when the ship was pounding or slamming, just digging the bow deep into a swell and then popping out would do it. Could not just feel it but hear it as well.
Not much that I recall on the PCTCs, most pronounced on a MSC 208 meter RO/RO.
It was fun to stand at one end of the tunnel on boxboats and watch the far end door moving around in rough seas. There was easily 3 or 4 feet of movement.
The Keystone (later ATC) tankers would vibrate like a guitar string from slamming. It was very noticeable in the steering grear room. Every panel in the accomodation would squeak and rattle as well.
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Itâs rather incredible how much movement is allowed for in structures. When I first started traveling for business, many years ago as a smoker, I would select the very last aisle seat on airplanes and look ahead in the airplane tube and see how much it twisted and bent when taking off and turning. Very plastic, that aluminum stuff.
My brother in law worked in the now-fallen World Trade Center towers. The fact placards in the building said the towers swayed 15â (5m +/-) out of vertical.
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On the subject of vibration, a base amount exists to varying degrees from the operation of the plant regardless of sea condition. The pitch changes depending on how much of the hull and which part of it is submerged as when climbing over large swells or waves. I never gave it any thought until I noticed the lack of it when sailing in a ship powered by turbines.
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The hull of a ship is a highly elastic structure and sensitive to vibrations. These vibrating forces may be broadly divided into three kinds:
- The impact of the waves.
- The unbalanced moving weights of the engines.
- Certain inequalities in the thrust of the propellers.
Vibrations due to the shock of the waves may be disregarded as being too infrequent to cause any discomfort. The other two can cause acoustic hinder.
With the introduction of the steam turbine vibration from the unbalanced moving weights of the engines was absolutely eliminated, the moving parts being perfectly balanced and therefore incapable of producing those mechanical couples which in the reciprocating engine, send a rhythmical series of tremors through the whole structure of the ship.
However, the other unbalanced engine room equipment like auxiliary (diesel) powered engines will be responsible for generating the necessary vibrations.
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