Choice Between Higher Steel Grade and Higher Wall Thickness 3

[Chembloke99]

Hello,

I would like to know from both technical and commercial point of view if I have to make a choice for a subsea pipeline between Higher Steel Grade and Higher Wall Thickness, which one I should go for?

For e.g. , for a 6" pipeline should I go for API 5L X- 52 Wall Thickness - 12.7 mm or

API 5L X 65 and Wall Thickness - 10.97 mm.

Many Thanks.

 

[MikeHalloran]

If I decide for you, how do I get paid?

Doesn't matter; you haven't provided the necessary commercial information.

Actually, you haven't provided nearly enough information.
I.e., minimizing a number representing purchase cost of a unit length of pipe may not minimize the installed cost of the pipeline. ... or the life-cycle cost. Which are you trying to minimize?


Basically, to properly evaluate such a seemingly simple component choice, you have to take a good shot at designing the whole system, for each possible permutation. A spreadsheet should help you manage the complexity of the exercise as the number of variables increases.




Mike Halloran
Pembroke Pines, FL, USA

 

[Chembloke99]

Thanks Mike for your reply. Honestly speaking, I don't think I have the essential skill and experience to design the whole system for different permutations and then evaluate the cost.

Could you please tell me from a purely technical point of view which one is normally preferred in the industry. If you can provide reference of any Recommended Practice or any Standard which is followed, it would be of great help.

Btw, I do have few parameters, which could give an idea about the installation cost, the pipeline is 1km long and it is to be installed in 5m water depth, and the product fluid has got very small corrosivity, so don't you feel the priority should be to minimise the fabrication cost ?

Thanks once again.

 

[MikeHalloran]

The first question you have to answer is whether you can actually get each material in the quantity you need. Fortunately, that's easy; send out an RFQ and see who has it or can get it, when they can get it, etc.

You haven't told us what's to go through the pipe, so we can only guess which industry is involved.



Mike Halloran
Pembroke Pines, FL, USA

 

[SNORGY]

Chembloke99:

With the wall thicknesses you quoted for 6" pipe, the 10.97 mm wall sounds to me like an ANSI / ASME B36.10 dimensional standard and almost implies seamless pipe (SCH 80), whereas the 12.7 mm wall might be ERW. I am not sure what your standard mill tolerance would be for API-5LX (non-ASME dimensions) pipe without specifying it on the purchase order, but if it is 12.5% - as I believe it would also be for B36.10 - then despite the thinner wall, I would be inclined to go for the 10.97 mm if it was indeed seamless.

Just my gut reaction based on the information at hand.

Regards,

SNORGY.

 

[Chembloke99]

Thank you Mike once again for your reply.

Sending out an RFQ would be a time taking process and unfortunately, I have time contraint at the moment.

With regards to product flowing through the pipeline, it is a multiphase fluid with C02 composition as 1%, Inlet P - 200 barg, Temperature - 70 deg C.

 

[zdas04]

You also need to look to see if your company has qualified welding procedures for either the X-52 or the X-67 in 6-inch. I did a job a while back that I didn't check that little fact and had to qualify a new welding procedure for a pipe grade/size. What a pain in the butt. If you have procedures for both grades, take a look at them, some grades and sizes have pre- and post- weld heat treatment requirements that are sometimes difficult to achieve on a pipeline job and can be really expensive.

I don't find the seamless vs. ERW to be as big a thing as some folks do, but on ERW I'm a stickler for making sure that the HAZ on the longitudinal weld is in the top half of the pipe (the HAZ has a tendency to be very susceptible to MIC, nearly every pipe failure I've ever had due to corrosion has been MIC on the longitudinal weld seam). ERW pipe has less ovality than seamless on most sizes (6-inch isn't bad in either variety).

If the welding procedures are roughly the same, and the availability is roughly the same, and the pipeline hydraulics are roughly the same, then I will always go with the lowest strength pipe that I can get to work. High strength has a correlation to high internal stresses which can be a factor in several kinds of stress corrosion.

David

 

[Chembloke99]

Thanks a lot Snorgy for your help. Yes, the pipe is indeed seamless. Can you please provide me some reference from any standard or recommended practice where I can obtain detail information on this topic and which I can quote while preparing the report.

 

[Chembloke99]

Thanks a lot Zdas for your guidance. I can understand the things which are to be kept in mind while deciding on material grade selection and also your reasoning behind choosing lower grade steel .

Many Thanks once again.

 

[SJones]

As a matter of interest, how much corrosion allowance is incorporated in the wall thickness calculations?

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[Struct71]

Is pipe weight an issue?
Is the material available?

 

[BigInch]

You should assume a higher material strength will give the cheapest overall installation. That will typically give you the lowest cost/lb installed due to very similar costs for X grade pipe steel, but much greater savings on shipping, handling, fabrication and installation costs. Offshore, extra concrete weight coating is cheaper than X60 steel used as ballest.

If higher steel strength wasn't USUALLY the best way to go, there would have been much more emphisis on developing lower grade steel over the past 100 years and today we would be talking about using X5 or X10 instead of X60 and X65 and X80 and X100.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Chembloke99]

Thank you all for your help.

@ BigInch: Your advice is well taken.

@S Jones: The Corrosion Allowance is taken as 3.2 mm.

@Struct 71: The pipe weight is not an issue and the material is also avaiable.

 

[Chembloke99]

@ Big Inch

Sir, do you mean to say that, their is greater saving on higher grade steel from installation point of view? With due respect to you, from what I know, lower grade steel have lower installation cost due to the fact that it is difficult to weld higher grade steel and consequently lay rate is lower compared to laying the lower grade steel.

Though, while laying in deep water the situation changes, as the vessel is laying at maximum lay tension, and reduction in pipe weight would result in lower lay tension. So we prefer higher grade steel. But in this job, the installation is not supposed to be carried out in deep water. The water depth is just 5m.

 

[BigInch]

You may find some instances where the reverse is true, but generally yes I do mean to say that higher strength metals will typically give an overall lower cost installation. While the higher strengths may generally be more difficult to weld, not even that is true all the time. High strength metal pipes of X80 (and I think even X100) have been successfully welded together in the field for 1000's of miles all by automatic welding machines... with very excellent quality and high production levels achieved.

Sometimes project economics depend highly on when a pipeline can be put in service, hence sometimes it may increse your company's overall revenue stream to use locally available B pipe, rather than wait an extra 90 days for delivery of higher strength pipe. A $ 1,000,000/day revenue stream starting even 30 days earlier may be worth considering using API 5L Gr B pipe, if you can get your hands on it today.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[SNORGY]

Chembloke99:

The 10.97 mm jumped out at me because I work with so much 6" pipe that I immediately recognized it as SCH 80. A reasonably good set of tables for pipe to ANSI B36.10 dimensions is found in Crane TP-410/410M in Appendix B. The same type of data occurs in The Piping Handbook by Nayyar, which I believe also tabulates the common wall thicknesses for ERW pipe. The CSA Z245.1 Code in earlier releases used to have a pretty comprehensive table for ERW pipe wall thicknesses, but I think they dropped it in their latest release.

Not sure if that is what you were asking me about...hope it helps though.

Regards,

SNORGY.

 

[BigInch]

Since the question is poised for pipelines, there may further be no reason to chose a particular standard wall thickness, as if the quantity meets the minimum requirements for a special mill order, it may be possible to order the pipe fabricated to ANY custom wall thickness (standard or not) you'd like to have.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[BigInch]

A possibility, BTW, which sometimes makes it only that much more difficult to increase pressure by rounding off to the next highest std wt. If you can order your pipe to the nearest 0.001", that "choose a std wt reason" doesn't hold water either.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[Chembloke99]

@ SNORGY

Thanks SNORGY for your help. Those reference are of nice help.

@ BigInch

Thanks for your advice. I have understood your point.

 

[eadwine]

One quick question, "What does the owners piping specificaation call for in this application"? If the specification will not work in the application, then most owners require an "Engineering Change Order" to be created via it's "Management of Change" procedures.