Unbounded flexible risers - Dropped Object

[robb4]

Dearl all,

I'm writing a specification for the purchase of unbounded flexible riser (about OD 16", ID 8" in a water depth of 1400m).

I'm thinking about the protection from dropped object, how can I fix the requirements in terms of Impact Energy and Object Diameter?

The only reference I found is ISO 13628-1 that select these values for other structures than multiwell structures:
- 20 kJ for 500 mm diameter
- 5 kJ for 100 mm diameter

Are these values applicable also for flexible riser? Are there any other reference that I can use (without performing dropped object study)?

The standard for flexible (API RP 17B) recommends that the dropped object should be considered (operating and abnormal conditions) but no guidelines (or values) are fixed to evaluate the impact energy.

Thank you and sorry for so long post.

 

[SRossiter]

Maybe the dropped object study is the best way to go. DNV generally recommends FEA be performed.

Steven Rossiter MEng - University of Sheffield (Aerospace) - www.bpp-tech.com

 

[Clumpweight]

This was posted by another correspondent some while back

"If the water is deep enough the object will reach terminal velocity. Examples of impact energies are:

- Drill collar - 1050KJ
- BOP - 1290KJ
- Drill Casing - 164KJ

Hence they are substantial and need to be considered.

The main loads therefore are a result of:

- Kinetic Energy
- Momentum
- Mass and Velocity

The effect on the item can be calculated by a number of denting models for pipelines for example, i.e. based on the proportion of impact energy absorbed by local deformation of the pipe wall. General guidelines for UK sector can be found in HSE OTH561." This, however, tends to concentrate on impact damage from overtrawling

Offshore Technology Design Manuals may be of some help which can be found at

http://www.bentham.com/

A Flexible Riser Design Manual
M H Patel, J A Witz and Z Tan
ISBN 1 874612 17 X Price £610.00 (yikes!)

Trust this may be of some help.