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# Fatigue Calculation for Pipelay

## Fatigue Calculation for Pipelay

(OP)
Hi,

I been running analysis for pipelay.
Nowdays Fatigue calculation has become a norm in the industry.
However I still cant grasp the fundamental.

In installation phase, we look into fatigue analysis so we can determine the allowable lay duration for each pipe length or pipe weld.
Sometime the vessel goes into slow lay mode and the fatigue becoming a concern of the client.

I been using DNP-RP-C203 to estimate my fatigue calculation.

From the said code and input from my pipelay analysis software (OFFPIPE), I can calculate the Allowable Cycle, N.
I just need the Stress Range from the analysis software output.

My question is how do you used the calculated Life Cycle, N?

Hope to hear from you guys soon.
Can this value be converted into meaningful value such as time in seconds?

Replies continue below

(OP)

### RE: Fatigue Calculation for Pipelay

Consult DNV-OS-F101, Section 5, Part D, 808 and 811.

Steve Jones
Corrosion Management Consultant

All answers are personal opinions only and are in no way connected with any employer.

### RE: Fatigue Calculation for Pipelay

(OP)
SJones,

Thanks for the reference.
However I cant related the practicality of the fatigue calculation to the installation.
Thus why I write this thread.

Could you simplify for me?

Now that I already able to calculate the Life Cycle, N.
This Life cycle is calculated by the Stress Range which is based on my dynamic analysis of the installation.
And I also know the duration of the Stress Range, time taken for extreme stresses to occurs between minimum and maximum stress.
If I said that the time taken is for 1 life cycle, can I just let Time Allowed for Lay = Time Taken for 1 life cycle x Allowable Life Cycle N.
OR considering the Guidance Node of DNV-OS-F101, Section 5, Part D 811: Time Allowed for Lay = 10% x Time Taken for 1 life cycle x Allowable Life Cycle N.

This is by assuming that the cycles of Extreme Stresses is occurring repeating which is conservative.

I would appreciate some guidance.

Thanks.

### RE: Fatigue Calculation for Pipelay

I can't advise on the specific goal of your analysis. Normally, the fatigue assessment is used to evaluate how much of the notional fatigue life of the pipeline could be consumed during pipelay, hence the proposed apportionment of life percentages between pipelay, installation and operation outlined in OS-F101. The combined life of the three is the '1.0' in equation 5.32 It's not about time, it's about cumulative damage. The referenced guidance note in the version of F101 that I have says nothing about time. Try a Google search on "Palmgren-Miner" to review the concept.

Steve Jones
Corrosion Management Consultant

All answers are personal opinions only and are in no way connected with any employer.

### RE: Fatigue Calculation for Pipelay

(OP)
Hi SJones,

I do agreed with you.

However I has been witness that in Installation Phase, it is becoming a norm on translating the Allowable Cycle into Allowable Duration which make me wonder if the approach is correct. Hence why I have say didn't have a full grasp on this subject.

Further more in Malaysia, Fatigue Life has not be considered in the Detail Design by the pipeline designer. Thus I could not get the Allowable Fatigue Cycle for Installation phase, i.e. 10% of Pipeline Fatigue Life (which should be agreed in detail design phase).

Clients are asking such translation of Allowable Cycle into Allowable Duration due to justification of slow lay duration allow before the pipeline is having fatigue.

The concept of Time Allowed for Lay = 10% x Time Taken for 1 life cycle x Allowable Life Cycle N is the most logically way I can think of and seen but it yet to be proven by an actual supported basis, only logical argument.

If only I could have some basis and an actual example.

### RE: Fatigue Calculation for Pipelay

Just work on the stress ranges and numbers of cycles that will occur during the various laying scenarios and see how much fatigue life that they consume and whether they fit the overall design fatigue life of the pipeline that includes laying, installation and operation. The "allowed time" will be the maximum that satisfies the defined pipelay fatigue life fraction with the given stress range distributions used in the time periods of the scenarios; or maybe I'm just being too simple.

Steve Jones
Corrosion Management Consultant

All answers are personal opinions only and are in no way connected with any employer.

### RE: Fatigue Calculation for Pipelay

(OP)
SJones,

If that so, I guess my previous method is correct.
1. Stress Range = From Analysis Output
2. Duration for every cycle = From Analysis, taken as duration between the extreme stress occurrence.
3. Allowable Fatigue Cycle for Installation = 10% * Fatigue Cycle (from S-N Cycle at analysis Stress Range)

Thus
Allowable Duration = Allowable Fatigue Cycle for Installation * Duration for every cycle

### RE: Fatigue Calculation for Pipelay

Are you using regular wave in your analysis? if so, results will be conservative. Aslo, fatigue damage in DNV is for one full cycle (Max-Min-Max) not half cycle (Max-Min).

### RE: Fatigue Calculation for Pipelay

(OP)
Hi PipelineEngr,

My analysis is run using Irregular Wave (ISSC).
My Stress Range should be come from half cycle now.
So my implementation should change to the following:
SJones,

If that so, I guess my previous method is correct.
1. Stress Range = From Analysis Output
2. Duration for every half cycle = From Analysis, taken as duration between the extreme stress occurrence.
3. Allowable Fatigue Half Cycle for Installation = 10% * Fatigue Cycle (from S-N Cycle at analysis Stress Range) / 2

Thus
Allowable Duration = Allowable Fatigue Half Cycle for Installation * Duration for every Half cycle

### RE: Fatigue Calculation for Pipelay

Try moving the discussion to the Pipelines, Piping & Fluid Mechanics forum with a link to avoid duplication and a closure note here. There are some serious pipeliners there that may have some further advice.

Steve Jones
Corrosion Management Consultant

All answers are personal opinions only and are in no way connected with any employer.

### RE: Fatigue Calculation for Pipelay

You may test following procedure:

1. extract stress histogram from dynamic analysis runned for reasonable time T
2. calculate stress range (i.e. rainflow counting method or similar method - I am not familiar with offpipe so i do not know if offpipe could be of any help in this regard)
3. divide stress rang into stress bins (i.e 0-10MPa 1000 Cycle, 10-20MPa 500 Cycles, etc)
4. caculate fatigue damage for above stress range using appropriate S-N curve(divide cycles by two since rainflow counts half cycles)
5. allowable installation time = allowable damage/calculated damage x T

Good luck

### RE: Fatigue Calculation for Pipelay

Hi IrwanHamid,

I also got stuck up with Fatigue life calculation during pipeline installation. Basically I extracted the stress range from the dynamic analysis and found the allowable no:of cycles. But ideally speaking when you run the analysis for three hours duration, what period we need to consider to find the actual fatigue life based on the allowable no:of cycles.

Fatigue life = Allowable no:of cycles * Duration of cycle.

Thanks.

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