Road crossings and high design factor

The rules for allowable stress at road crossings are given in Clause 5.7.3(c) of AS 2885.1.  Recently I had an interesting query which illustrated a bit of a gap in these rules.

Basically the combined equivalent stress should be calculated according to API RP 1102 and must not exceed 72% SMYS at formed road and track crossings but may be up to 90% SMYS at informal crossings where (for example) a farmer may drive a vehicle across the pipeline on infrequent occasions.  All of that is fine for design of a new pipeline.

The problem arises when someone wants to put a road or track over an existing pipeline at a location where the wall thickness corresponds to a design factor of 0.8.  There is just no way to achieve strict compliance with the stress rules – the hoop stress alone may be up to 80% SMYS so a combined equivalent stress of only 72% SMYS is clearly impossible.   The pragmatic thing to do is to build a bridging slab over the pipeline to isolate it from the vehicle loads (and it may also provide external interference protection).  If the slab is properly designed to distribute the load away from the pipe that will be a perfectly satisfactory way of protecting the pipe against high stresses.

But it still won’t comply with the current words in the Standard.  It’s a legalistic problem, not a practical problem.   The combined stress will exceed the 72% limit set by the Standard for a formed road crossing, even though the pipe will experience no greater load than in the adjacent paddock.

Now that we have increasing numbers of pipelines built with design factor of 0.8 this is a question that might arise more often.  Common sense should prevail – put in a bridging slab and satisfy yourself that the pipe stress state will not be adversely affected.  For anyone who is worried about black-letter compliance  I suggest you refer to Clause 1.6.2 of Part 0 which deals with departures from the Standard.

A final comment:  It’s fair to say that the principle behind these rules for transverse external loads is that there is a wealth of history of successful operation with pipelines designed for up to 72% SMYS at road crossings.  Operating at 80% SMYS obviously reduces the margin for additional loads.  It just seems prudent to set a lower stress level for situations (such as road and rail crossings) where the pipe is in a more complex stress state.  At busy road and rail crossings there is the added complication of possible fatigue.  The increased limit of 90% for informal crossings is consistent with the higher stresses usually tolerated for occasional loads, and the level of comfort with this is increased by  recognition that even under gross overload the most serious failure mode is likely to be ovalling of the pipe, not loss of containment.  There is a little more on this in an earlier post.

Posted in Operations, Pipeline design, Standards, Uncategorized | 2 Comments

APIA Convention 2013

I’m at the APIA Convention, shortly before going to the closing dinner.  For those who are here or have been to Conventions in the past it needs no description.  For those who haven’t it is an amazing combination of conference, trade show, networking opportunity and big party.  I never have enough time to connect with all the people I would like to talk to, but it’s still the best opportunity each year.

From my perspective a highlight of the Convention this year has been the weighting given to human factors in the selection of presentations.  As well as a diversity of great presentations on engineering and business topics there were two presentations from the EPCRC RP4 program (sociology of safety), another two that gave a psychological perspective on safety, and others included knowledge transfer, engineer training, project managers vs. operations, and an interesting story on fatigue issues.  That’s about one third of the papers.  It’s becoming normal to talk about organisational culture and its implications for safety which can only be a good thing.  You can’t eliminate the potential for failure by engineering alone; people will have unique ways of making things go wrong.

Some of the papers also emphasised the fundamental difference between safety in the WHS sense (personnel safety) and public or operational safety (preventing catastrophic pipeline failures).  This distinction has been clear to me for so long that I’m always a bit surprised when I hear of people who don’t grasp it but I think the message is spreading.  They are very different.  Both  are important but they require quite different approaches.

This is a great industry to be part of.

Posted in Conferences | 2 Comments

Temperature de-rating

Easing back into writing after a long period of zero inspiration has been helped by some advice from Phil Venton in response to a query he received about the temperature de-rating rules in AS 2885.1:

I was going through the pipeline temperature de-rating section in AS2885.1.  Can you please explain the data/reasoning behind this equation and why there is a different maximum temperature which requires de-rating for the pipelines and flanges to AS2885 and flanges to ASME B16.5?

Very reasonable question!  The relevant part of AS 2885.1 is Clause 3.4.3.  I’ve touched on this before but in a vague sort of way.  Phil’s response is much clearer and went like this (slightly edited):

The matter relates to the basis by which pipes and flanges are designed, and the change in properties of materials in response to changes in temperature.

The yield strength of steel declines more or less continuously with temperature, whereas the tensile strength does not start to degrade until perhaps 200°C.

Pipe is designed using yield strength.  In principle the yield strength should be reduced for any temperature higher than the reference temperature, however given the complexity that this introduces to typical pipelines, most of which have a design temperature less than 60°C, it seemed reasonable that the nameplate yield strength should be used for design at temperatures <65°C, while for temperatures ≥65°C the yield strength should be reduced at the nominated rate, assuming a starting temperature of 23°C.  (The note at the end of Clause 3.4.3 points out that 65°C is a convenient temperature for this purpose since it covers the discharge temperature from most gas pipeline compressor stations.)

Flanges are designed to a pressure equipment standard (ASME VIII or AS 1210).  These standards require the use of the lesser of the tensile strength/3.5 (/2.35 in some instances) or the yield strength/1.5.  It happens that the factored tensile strength governs the flange design to about 120°C.  Above this temperature (approximately) the factored yield strength is less than the factored tensile strength.

Consequently the basis for each temperature de-rating requirement is entirely rational provided you appreciate the principles, and recognise that the authors of the Standard have made rationalisations to provide practical rules for design.

(Thanks to Phil Venton for this clear and useful explanation of rules that at face value can appear arbitrary.)

Posted in Pipeline design, Standards | 4 Comments

AS 2885.1 review status

Three months of writers’ block …      It has at least revealed that there is an almost unchanged stream of traffic to this blog even when nothing new is going on, which is gratifying in its own way.

Back in June I invited expressions of interest for a working group to review AS 2885.1 in preparation for a submission to Standards Australia in support of the next revision.  That working group has now been constituted and had its first meeting last week.  I  don’t intend to fuel rumour by making premature comment on possible changes but it is worth reporting that the review process is now well under way.

The plan is broadly similar to the review undertaken for the 2007 revision.  A range of issue papers will be prepared.  When the working group is satisfied with each paper it will be circulated to the industry for wider feedback.  The scope is wide, from typographical errors to the underlying philosophy and everything in between.

If you have any burning concerns about the Standard, now is the time to raise them.  Get in touch with me via the “About” links above.  Errors, inconsistencies, imprecision, omissions, new information … we need to know about all of them.

People always want to know when the next revision will be published.  We have a rough program for the initial stages but it is too early to be precise about the end date.  Suffice to note that holding your breath for a couple of years is bad for your health.

Posted in Standards | 2 Comments

Arkansas failure cause

A couple of days ago US media reported interim results of investigation into the failure in Mayflower, Arkansas that I have mentioned previously here and here.  Quoting in full the statement send by ExxonMobil to media (with highlighting added):

ExxonMobil Pipeline Company (EMPCo) and the Pipeline and Hazardous Materials Safety Administration (PHMSA) have received the results of an independent assessment conducted by Hurst Metallurgical Research Laboratory, Inc. on the Pegasus Pipeline segment that failed on March 29, 2013.  We are currently in the process of reviewing and analyzing the data.  Based on the metallurgical analysis, the independent laboratory concluded that the root cause of the failure can be attributed to original manufacturing defects – namely hook cracks near the seam Additional contributing factors include atypical pipe properties, such as extremely low impact toughness and elongation properties across the ERW seam.  There are no findings that indicate internal or external corrosion contributed to the failure. While we now know the root cause of the failure, we are still conducting supplemental testing, which will help us understand all factors associated with the pipe failure and allow for the verification of the integrity of the Pegasus Pipeline.  These tests will help us determine the mitigation steps we need to take to ensure a similar incident does not occur again.

Although the pipe was many decades old, and manufactured by a process no longer used, this does illustrate why the industry remains obsessive about pipe quality.

Posted in Uncategorized | Leave a comment

What is a pipeline?

It was recently pointed out to me that AS 2885 does not have a definition of “pipeline”.  No-one has ever raised that before, perhaps because most of us think we know a pipeline when we see one regardless of any formal definition.  But it made me think about how one should define “pipeline”.  How about this:

  • Cylindrical conduit that conveys fluid under pressure
    • Necessary to start with the very basics
    • Excludes open channels and non-circular ducts
    • “Fluid” may include gas, liquid, multiphase, supercritical or slurry
  • Traverses areas outside the exclusive control of the owner/operator
    • Excludes process piping and in-plant transfer lines
    • Usually but not necessarily of considerable length
  • Has few inlet and/or delivery points
    • Excludes complex distribution networks
    • Production gathering systems may be moderately complex networks, but each element of such a network is usually considered to be a pipeline in its own right
  • May include facilities for pressurising, controlling and measuring flow
    • Excludes processing and storage facilities

For something that I usually think of being as simple as a long round piece of steel with a hole down the middle that’s surprisingly complex isn’t it?  But complexity behind superficial simplicity is pretty typical of pipelines.

This is not meant to be an AS 2885 definition because it covers a lot of pipelines that are outside the scope that standard (eg. water and slurry lines).  Rather it’s just a thought for discussion.

Is it an adequate definition?  Are there legitimate pipelines that it would exclude?  Or non-pipelines that are not excluded?

Posted in Pipeline design, Standards | 12 Comments

AS 2885 Part 1 Committee Invitation

Even though I am chair of the AS 2885 Part 1 committee I don’t propose to make frequent use of this blog for committee purposes.  However the message below needs wide distribution so for once I will use this blog as well as other industry channels.

It’s time for a full review of AS 2885 Part 1.  Although the 2012 edition of the standard was published less than a year ago it was not a comprehensive revision.  There are a number of known issues that need to be addressed and a detailed review will undoubtedly reveal more.

The technical review will be done by a working group constituted under the APIA Research and Standards Committee.  When the working group has agreed on recommended changes the responsibility for final editorial work on the Standard will be transferred to the Standards Australia ME-038-01 committee, although that committee will probably have the same membership as the RSC working group.  It is not essential for members of the working group to be employed by members of the RSC if they bring desirable expertise.

The working group must contain people whose expertise spans the full range of subject matter areas covered by the Standard.  To ensure long-term continuity it should also contain younger members in understudy roles as well as the more senior people.  Hence young engineers with a few years experience and a strong interest in continuing to develop their expertise are encouraged to apply.

Relevant subject matter areas can be identified by scanning Part 1 itself.  Particular areas where expertise is necessary include (but are not limited to) metallurgy and/or materials, fracture control, piping materials and design, stress analysis, corrosion, construction and hydrostatic testing.

Although AS 2885.1 is the Design & Construction part of the standard it has significant implications for pipeline operation.  Hence engineers with a background in operation are encouraged to participate.  Engineers with a construction background are also encouraged as this area has been under-represented in the past.

The working group offers an excellent opportunity for both young and senior engineers to further develop their expertise and to network with colleagues across the industry while making a significant contribution to the quality and safety of Australian pipelines.

The work can be very interesting but does require a serious commitment.  Members of the working group will be expected to attend meetings and to do significant technical review and drafting work between meetings.  Meetings may be for one to three days and may be required a few times per year.  Employer support is required for travel and for time to attend meetings, and possibly for time spent on work outside meetings.

To express interest, please contact me via the “About” link in the header above and include:

  • The subject matter areas in which you have have particular expertise or interest
  • Brief summary of your experience in each area (not more than a couple of paragraphs each)
  • For each subject matter area, whether you consider yourself fully competent or are seeking an understudy position (some mid-career people may be fully competent in some areas and suitable as understudies in others)

Closing date is Friday 19 July.  All expressions of interest will be considered but no-one is guaranteed a position on the working group.  It may be a few weeks after the closing date before the composition of the group is decided.

Posted in Standards | 1 Comment