This was happened due to unproper fulfilment of SWL test during 5-yearly survey. Crew was trying to lift up tested cargo with derrick in park position.

No sign of any impact damage

Maybe it was hit by incoming cargo or container ...... have seen swinging loaded boxes do a lot of damage to vessels, just a thought

@Robert,
Your attempts were well understood by me, much appreciated.

Assuming that's how it happened, it seems that hook stowage "box" became damaged in the process and the hook was susequently secured to something on deck. I am surprised that the normal securing mechanism would be strong enough to produce this result. Also noted from other underway pics that when stowed both the hook and jib wires appear to be slackened (apart from, perhaps the two wires to the jib mid-point.

Swtk: I know it was your idea originally. I am merely trying to explain why your original idea is probably correct, and also answer questions directed at me.

My idea Robert.
I started my comment with: "what could have happened"
I have seen overload protections being by-passed on several occasions as well.
My experience with a -then- high SWL was with a Stuelcken derrick with an SWL of 225 tons.
That was normally secured for the sea voyage by tensioning the runner tackle which pulled everything in the system taut.
That had to be done carefully as was explained to me by the bosun, as high reaction forces would be introduced in the ships construction.
Later, when I was able to lay my hands on the forces diagram of that Stuelcken derrick I saw how right he was then. I saw reaction forces up to 1000 tons!
During a cargo gear inspection I found a 25 tons derrick bent in the horizontal plane. That happened when the crew used the derrick to pull a load horizontally in position. Yes, a new derrick had to be made.
It is pretty easy to overload lifting appliances if you do not pay attention when you use them.
Well, may-be we will hear from somebody what happened.

Denis and Bart Hakxe. I can't say this was the reason. However, I had 30 years offshore experience plus some brief deep sea trawling experience. In this time I spent a considerable time working with and occasionally operating 58 ton cranes. I also had considerable experience with rigging and lifting, this includes equipment designed to lift up to 1000 tons, and operating equipment rated at a SWL of 500 tons. I stand by my original explanation, because as I have said if the crane hook is tensioned even up to its limit's, the load is no longer being shared between the boom, the boom mounting point and the top of the pedestal. Rather all the load is concentrated on the top of the pedestal (column). The crane was never designed to be loaded in such a way. As such I think it was easily overloaded in this way. As for the alarms going off, it may be that the alarms were configured in such a way, that the weight sensors were not solely confined to the winch used to hoist and lower the block, but rather shared between points which would normally be under load. However, that would be unusual. It’s also not unknown for load limiters to be bypassed. I have been aware of this happening on quite a few occasions, with consequent severe damage to the crane. I am by no means an expert on cranes, however, I am at loss to see what else could cause the buckling. I would love to hear other opinions on the subject.

@ Robert
How do you know it happened exactly like that? Maybe the column buckled already during the heavy lift operation & then they stored the crane like it is.

Robert,
I'm sorry but that doesn't make any sense to me.
I'm very curious what happened here but I don't think it happened because the crane driver kept tensioning the runner wire with the boom in the boomrest.

For the boom it doesn't matter much whether it's supported by the hanger wires or by the boomrest in this position.
When the boom is hanging in it's wires all that tension also ends up at the sheeves on the top of the cranehousing, the cranehousing is made for this and can handle quite a bit more than the normal load in the crane. So with the crane in the boomrest and the hangerwires slack (that's the only time you start tensioning up the runner wire) there is actually less force on the top of the cranehousing than when the crane is in normal use.

Furthermore: if somebody puts so much tension on the wire to buckle the housing I'm pretty sure some alarms would have gone off and some overload protection would have kicked in waaaay before this happened.

I don't know what caused it, and unless somebody of the crew comes forward I don't think we'll know for sure.

Denis. Because the boom was on the rest, the load was no longer being shared between the point where the boom is mounted to the pedestal and the the top of the pedestal. Instead all the load was concentrated on the top of the pedestal. The cranes column is designed to withstand a certain SWL if the load is shared between the boom mounting point, the boom and the top of the column (pedestal).

@swtk How can the crane's column (which is designed to withstand certain SWLs at certain boom outreach angles) can simply buckle just cause of tensioning the fixed hook?

I think SWTK has got the right answer here. No doubt the crane operator got too enthusiastic in tensioning the hook. I suppose he quickly left the cab, in the hope nobody would notice what he had done.

What could have happened is that during stowing operations, with the hook attached to a fixed point, the man in the crane continued tensioning the runner wire.
Since the opening in the crane housing for the cabin creates a discontinuity in that crane housing it will buckle in the area as visible on the photo.

She probably shipped a green sea. LOL

Blimey, how could they do that? It's supposed to be vertical. You can see where it's buckled.
http://www.shipfriends.gr/forum/gallery/image/65444-patras-tugs-06012016/

Does not look like a casualty to me. The crane is in stowage position, I think.