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Flexible Pipe Stress Checks

Flexible Pipe Stress Checks

(OP)
Can someone explain how Von Mises stress is calculated for flexible pipes.

The internal carcass will contain pressure so I'm guessing that this will be used to find hoop stress. However, the other layers will contribute to bending and axial stress...

I'm checking a flexible which is being used for tie-in to a rigid platform. Tension and MBR checks should be straightforward, but I need advice re VM checks.

Any pointers welcomed.
Replies continue below

RE: Flexible Pipe Stress Checks

I don't think that the internal carcass is anything more than a support for the pressure liner.  It is needed for gas lines.  The carcass doesn't contribute to hoop strength from internal pressure.

Do you really mean the pressure vault which lies between the liner and the armour layers?

My understanding is that you need to model each layer separately in a FE model.  Then apply the pressures, moments and forces.

RE: Flexible Pipe Stress Checks

Billsummer is right - the flexible pipe is a multilayer construction the inner most layer of which is the carcass (usually of stainless steel) which supports the fluid barrier (PA11 or PVDF or XLPE).  The pressure is taken by the armouring wires outside that fluid barrier - a shaped interlocking wire for pressure armour and flat low angle tensile armours.

The stresses in the armour wires must be FE modelled in order to establish the VM stress, taking into account the interactions between the different layers and the elements of pressure (internal and external), bending and tension in the pipe.  small scale models should be validated against more complex 3D full scale (full pipe) models and preferably test data.

Suggest you contact one of the manufacturers (Technip / Wellstream / NKTF) or an independent such as Marintek who have their own BFLEX model for flexible pipes.

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