Stress Analysis of Offshore Oil Platform Piping Restrained by U-Bolts 1

[Vikoll]

Dear Pipers

I am performing flexibility analysis of piping on offshore oil platforms designed by European company for Asian market. Installation T=+30 deg.C, Design T varies from -40 deg.C to +120 deg.C for different lines.
Design group uses tight U-Bolts (without lugs) with spacing every 3-8 ft depending on the line size as the only mean to restrain piping on two platforms and connecting bridge on a straight runs and at points of direction change. Designers could not provide any information regarding how tight these U-Bolt are in order to estimate the possibility for axial/lateral displacement due to thermal expansion-contraction of piping. They do not perform stress analysis as they base their expertise on a proven record of successful operations of similar platforms without failures for many years
When I do modeling in CAESAR II and check stresses and displacements it becomes very critical to input correct values for coefficient of friction and stiffness of restrains as considering rigid restrains result in severe overstress in EXP case. I found empirical values for stiffness of supporting steel used in similar projects that are consistent with my previous experience, but I am not quite confident to draw a conclusion regarding whether or not this stiffness could be applicable for tight U-bolt type restrains in axial and lateral directions and how to proceed with stress analysis in this case.
My vision on required support configuration and piping routing is quite different from what designers use, so I am wondering if it is a normal procedure and a good engineering practice to use only tight U-Bolts to restrain piping on offshore oil platforms and, if it is a case, how thermal displacements can be controlled if U-Bolts do not let pipe grow, or if they do, how this is achieved if U-Bolts are tight?

Could anyone please enlighten me on an enquiry above and share your valuable expertise or direct me to a right source.

Many Thanks in Advance

Vikoll

 

[DSB123]

Vikoll,
If I were you I would revert back to these "experienced" Designers and ask them have thermal expansion is catered for with tight U-bolts because you will not be able to model the situation accurately. Using tight U-bolts everywhere is not good practice and if the pipework is installed with such restraints expect something to fail.

 

[pennpiper]

There appears to be at least two open issues in this.

1) You stated that there is the "European" Design Firm and there is the "Asian" Market. Who is the "Client"? and Who are you? Are you a third party Consulting Firm doing some Quality Control Checking for the "Client"?

2) Did this "Design" firm use "U" bolts for all pipe sizes? And what pipe sizes are you talking about?

 

[Vikoll]

Thank you DSB123 for your valuable feedback. I totally agree with your opinion. Although almost every engineering company have their own design and pipe support standard, let me ask you what type of support have to be preferably used in offshore oil operations to allow to control thermal growth and properly restrain piping? And secondly, are tight U-bolts totally unsuitable for this environment? If they can be used, how to quantify stiffness and friction of such support?

Regards
Vikoll

 

[Vikoll]

Pennpiper:

Thank you for your questions. You are right:
1) I am a third party consultant doing selective quality control (piping flexibility and stress) for a client
2) I am looking at pipe sizes from 3” to 16” and all of them, even insulated (???), are restrained by tight U-bolts

Please advise
Thanks
Vikoll

 

[BigInch]

U-bolts might act as guides, but not very likely to act as anchors, even if "tight". They do expand and contact with temperature, so they won't remain tight for very long anyway.

"The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward X-CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[Vikoll]

BigInch:

This is one of my primary concerns. Being “tight” during start-up, when pipe is already hot and moved from installed position, U-bolts can still be at lower temperature thus may become “over tighten”, overstressed and eventually fail due to considerably large shear force. What would be your expertise on this?
Thanks
Vikoll

 

[pennpiper]

You stated:
"I am a third party consultant doing selective quality control (piping flexibility and stress) for a client"

This being the case then you need to "Tell the Truth, The Whole Truth and Nothing But the Truth"

As BigInch has pointed "U-Bolts" are a bad choice for this application. You should advise the Client that this is bad Design (tell him why) and require that he have the Design Firm correct the design or you cannot "Sign-Off" on the project.

 

[DLite30]

U-bolts, even if tight, are not going to provide enough axial frictional force to prevent growth due to thermal expansion.

Personally, I would be most concerned with expansion of long runs of pipe and the clamps that are perpendicular to that pipe, where they could see a large horizontal load.

As far as installation I wouldn't worry too much about them over-tightening or breaking from temperature changes, but the key would be to have the contractor or operations re-torque all of the pipe clamps when up at operating temperatures.

 

[BigInch]

Using a typical size U-bolt for any given diameter of pipe, will not restrain the pipe in any manner. The Ubolts will deform, or break; however the pipe wants to do it. Tight bolts will yield when the pipe expands radially with thermal expansion, but that's limited enough that they probably won't actually break. Ubolts are basically only for gravity and wind loads and provide good corrosion initiation points from the moisture they will collect and the wear from transport vibration, and/or scratching they will put on the pipe coating from temperature movements. Offshore I'd consider using them only for short lengths (little or no thermal movements) of 1" pipe and below that was located on a skid with easy access for inspection.

"The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward X-CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[Vikoll]

Dear colleagues:
Thank you very much for your valuable responses. I totally agree with above expressed opinions. My only concern is that “Telling Truth...” will be just my word against their word. It is my understanding, that being an engineer and before advising someone to stop construction and start reviewing support concept from scratch, I have to make sure that my piping model is correct, calculations are done properly, assumptions are reasonable and results are credible. That brings me back to my initial question: how I can quantify friction force and axial/lateral stiffness of U-bolt type restrain? Or can anyone direct me to any literature/papers shedding light on this topic?
I also see U-Bolt failure upstream/downstream long runs after/before pipe changes direction if no means of inherent piping flexibility provided. Thermal displacement of pipe will impose a huge axial strain on such U-bolts.

Please advise and thanks in advance
Vikoll

 

[BigInch]

We're trying to tell you to use 0.

"The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward X-CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[DSB123]

Vikoll,
You seem to be worrying about "your" modelling of the pipework. If you are assessing someone elses Design then why not ask them for their calculations to back-up the use of the U-bolts.
I would not use U-bolts on anything but ambient lines and up to 2"nb personally. On larger size lines the U-bolt diameter can be circa 20mm dia and if you consider that some numptie pipe fitter can "wind them up " you can generate a considerable load acting on the pipe which has to be overcome before movement can occur. I agree that they cannot totally anchor lines along a straight length but they can apply a significant load to the pipe radially. If you are using these on 16"nb pipe then what stresses are you developing in the pipe "locally" ?

As Biginch says - these should only be used for small lines and on relatively short lengths of line which are not subject to large variations in temperature.

 

[Vikoll]

Thank you All for sharing you valuable expertise. Now I have more confidence to proceed the right way.
Vikoll