Pipeline Stability

[JT100]

I'm currently peforming a subsea pipeline stability analysis. In previous anaylses I have performed, increasing the lateral friction coefficient increases the stability- as to be expected. However, in several cases the stability is decreasing when I increase the friction. It is in relatively shallow water (35m) and I'm thinking this may have something to do with it, but cant come up with a reason- although I'm sure its fairly straightforward- could anyone explain this for me so that I can start kicking myself......

 

[BigInch]

It certainly doesn't sound reasonable. I have never seen a calculation method that would tend to decrease stability while friction is increasing, maintaining other variables constant. You are only increasing the friction variable, while maintaining water depth and the rest as constants, right? If you are using a new program, better check it.

Isn't it still U.S. std. practice to bury (petroleum lines) when water depths are less than 200 ft?

http://virtualpipeline.spaces.msn.com

 

[narenr]

Lateral stability is directly proportional to lateral friction, if you are using direct friction values. In certain methods (partial burial cases) when you use passive soil resistance, the friction increases till the breakout value and then reduces. Check your inputs particularly Cl value. What program and what method [2D, DnV RPF109, DnV RPE305 or AGA?]are you using?

As you go shallower, the hydrodynamic forces increases and also the wave theories changes, refer Shore protection manual for applicability, some programs do this automatically.

Narendranath R
Pipeline engineering is made easy with state of the art computer software, visit

http://www.narendranath.itgo.com.

 

[JT100]

I am using a verified in-house excel spreadsheet to calculate an on-bottom stability safety factor. The method is based on static analysis, 2D Morison’s equations approach to DNV ’76, DNV ’81 or DNV Class Notes 30.5. Yes I am only changing the friction factor, as a check I increased the water depth and then changed the friction factor and this resulted in the stability increasing, as expected. By the sounds of things its looking like this may be a problem with the spreadsheet? Thanks for your replies.