Hook for Recovery of Subsea Pipeline 1

[finbarwright]

We wish to design and fabricate a hook for recovery of a subsea pipeline.

The question I have is whether the rules which apply to lifting appliances offshore also apply to pipelay equipment (specifically to the winch which will be used for recovery).

The reason is that if we have to apply the safety factors normally used for lifting equipment then our hook will be a monster.

I would think that they 'should' apply. However, the head which will be welded to the pipe and which will be used to connect the hook to is designed and built with a safety factor of only 1.5 and is therefore the weak point in the system.

 

[Ussuri]

Maybe it is that the point you are lifting off is under designed? What is your 1.5 FOS applied to?

Have you done a recovery analysis which will take into account the dynamic effects from the vessel motion, hydrodynamic effects, any on-bottom suction, the possibility that it may be buried etc?

My initial response would be the 'hook' needs to have the appropriate factor of safety for lifting because if it fails your load will fall. Consequently, there is a commercial risk attached to it. It is often false economy trying to save a few pounds (dollars) just to optimise a design especially as there are possibly a few unknowns. Can you not buy a suitable hook off the shelf? My Crosby catalogue has hooks up to 1000 tonne, and that is a monster.

My company does a lot of that sort of stuff but its not something I have come across as we have a special department that deals with it. I can maybe investigate if it helps.

 

[StephenA]

Are you designing the whole system or just the lifting eye?

Will you be using a heave compensator which would reduce the surge loading somewhat?

As Ussuri says what does 1.5 FOS apply to?

Does it have to be a hook? Would a spelter socket on the end of the wire rope be sufficient?



Stephen Argles
Land & Marine

http://www.landandmarine.com

 

[finbarwright]

Just to clarify a little the situation.

The recovery analysis has been carried out and the head designed as normal for every project. 1.5 FOS is applied to the SWL of the recovery winch system. I think that the philosphy behind the low FOS is that, unlike a crane hook, the equipment is used infrequently.

The point is not to save a few dollars, it is to avoid use of an overly large hook. The problem is that commercially available hooks are not suitable for the geometry of the pipelay ramp of the lay vessel. We must design and build our own. In fact, by the time we include design engineering, material, labour, load testing, third party verification etc etc, I doubt that it will be cheap.

We are designing only the head and the hook. The rest of the system is already in place and has a long and successful track record. It does not need to be heave compensated. When the highest load is on the system, the pipe is already on board and is inside the tensioners (before they are closed) therefore most of the movement of the pipe is absorbed by the rollers. Having said that, the winch will pay out automatically in case that the load increases above its SWL.

Normally, we would use a spelter socket. In this case, however, we have to recover a pipe laid down by another vessel. In order that the system suits both vessels, we have to use the hook.

 

[Ussuri]

A 1.5 FOS on the SWL of your winch is typically a proof load test factor. That is the load they will test the winch up to. It is also typically around the brake capacity of the winch. So while you winch may only be able to pull 'x' tonnes it could hold 'y' tonnes. So if you designed your hook for a load equal to 1.5 x the SWL of the winch and due to some unforseen event the brake is on and your load snags, you will have ultimately loaded your hook to failure (assuming you are designing to an ultimate limit state, you should claify).

If this is a once off, as the engineer you need to make a judgement call as to what load you design it for.

Ultimately you are responsible and you need to justify your approach to other parties. I would give your self plenty of margin.

IF you have had the recovery analysis done you know the maximum dynamic load you expect to see on the wire. Now if it was a shackle or a wire you are looking at the MBL would be around 3 - 3.5 times the Dynamic Load.

Will you have a warranty surveyor on your project, will you need to provide certificates for load tests etc?

I would maybe think about for two load cases (1) proof test load case (2) operating.

(1) SWL x proof test load factor
(2) Maximum dynamic load

These are the forces you can expect to see, you then need to apply additional factors to that. If you can justify your hook is a 'lift point' DNV suggest a FOS of 1.7