Allowable flange loads for occasional conditions

[JFre]

I have a subsea pipeline system and spool that runs through an area with mild seismic activity. The flanges have all been checked against ASME B16.5 allowable pressure for normal operating conditions and pass easily.

During a seismic event, the displacement of the platform means that the equivalent pressure (including external forces and moments) goes over the ASME allowable pressures. Is there an allowance for the max flange loading for short duration, occasional loading like this? Without an increase in the allowable load, I don't see how it would be possible for many subsea flanges (where the riser/platform moves relative to the spool) to be within code limits.

 

[LittleInch]

You normally need to look at the overall piping or pipeline design code to give you this.

The usual assumption is that flanges will be the last thing to break compared to the pipe. However once you start to add bending moments then you need to start doing some analysis on the flange joint itself. It usually comes down to acceptable bolt loads and sealing forces, neither of which is really specified in the B 16.5 code.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JFre]

OK, so rather than using the rated pressure as a limit, you suggest a full ASME flange design calc to ensure the stress and bolt loads are OK.

In terms of acceptable bolt loads and stress, the ASME approach always uses very low allowable stress (33% SMYS) and low bolt loads (<25% SMYS). In practice, offshore flange bolts are usually tightened to around 70% SMYS which already results in a code failure but doesn't yield the bolts or the flange material. It would seem sensible to check the seismic flange loading against SMYS and apply engineering judgement given that there is no code guidance for addressing the flange acceptability.

 

[LittleInch]

Yes.

That's the way it's done usually. It's the moments which kill you on flanges, not pressure rating. Pressure you can go to 1.5 times without an issue as this is what you hydro test to to and no one blinks an eye. But add a load of moment and / or torsion and you get a leaking flange at less than rated pressure. maybe.

You might need something like a compact flange if you run into difficulties. Or an out of rating flange, e.g. a class 900 flange on a class 600 or lower system.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JFre]

Thanks

 

[GD2]

JFre,

Usually, the designer will include the effects of earthquake for piping systems located in regions of earthquake, waves and currents while designing the subsea pipeline. The code sets the allowable stress limits. The designer will run a stress analysis and makes sure that it is in code compliance. the analysis is done not on individual component basis but on a node basis or stress model.

GDD
Canada

 

[1503-44]

It sounds like the pipeline lacks the appropriate flexibility. It must be at least as flexible as the platform it is attached to.

 

[saplanti]

Allowable pressure-temperature table if for all required load combinations. ASME B16.5 allows a factor of 1.5 for hydrostatic test case only, not for other cases.

Additionally, note that B16.5 flanges fail in the ASME VIII flange calculations.

Therefore, you need to keep the loads under or equal to allowable for all load combinations. Sometimes you may be forced to use higher class flanges outside the specification if you cannot add additional support to reduce the load. In this case you need to inform all the parties, get their approval, and show the change in the drawings clearly.

 

[JFre]

It is surprising that B16.5 does not have a factor for extreme/accidental cases. I have seen many flange designs where the loading on the flange is taken close to the allowable pressure for normal design conditions. Any additional temporary loading would therefore take it over that limit and the hydrotest case has already shown that the flange can cope with 1.5 rated pressure. Pipeline design codes often allow lower safety factors during an accidental temporary condition so the same approach for flanges would seem sensible.

I have noticed before that B16.5 flanges fail the ASME VIII calculations. I have always found this is driven by the ASME VIII approach of using low allowable bolt tensions and low allowable stress for the flange material. As soon as you put real world bolt tensions in, the stress is above allowable. However, ASME allows for this. Appendix S states "The maximum allowable stress values for bolting given in Table 3 of Section II, Part D are design values to determine the minimum amount of bolting required. However, a distinction must be kept carefully in mind between the design value and the bolt stress that might actually exist. It is evident that an initial bolt stress higher than the design value may and, in some cases, must be developed in the tightening operation and it is the intent of this Division that such a practice is permissible, provided it includes necessary provision to insure against excessive flange distortion".

Indirectly, ASME is saying that the stress in the flange can go higher than the limits specified in the ASME VIII method provided, it can be demonstrated that the flange does not distort and will not leak. Sadly it gives little guidance on how to demonstrate this. I see the ASME VIII limits as something that should be applied if you are designing a new flange, but not necessarily for assessing an existing flange. My belief is that the calculation approach from ASME is valid but for 'real' world cases, the engineer can look to apply higher allowable stress than the limit of 2/3 SMYS or SMTS/3.5. Previously, I have seen flanges operate successfully where the design checks have been carried out ensuring flange stress < 80% SMYS and the bolt tension is above W1 (bolt load required for working pressure). In fact, it is common practice for installation contractors to tighten flange bolts to approx. 70% SMYS as a default value. This can take the flange body stress over 80% SMYS and yet the practice appears to have worked successfully for many years.