Tuesday, 15 January 2013

What you don't know won't affect you?














The manufacture of steel tube in the global market is set up around petroleum and structural applications. The requirements for these industries are quite different to that of screw piling, where significant torque is applied to the tube.

Many years ago we procured tube in accordance with AS1163 (Structural Steel).  We soon revised this when it became clear that the standard did not guarantee a continuous seam to any given length of tube.

The photograph shows a section of the tube in question
The application of a torque significantly below calculated 
capacity has caused this failure.


















Fortunately these issues were exposed during testing before they became permanent works.

In response to this we moved to utilising an American Petroleum Institute standard (API5L) that ensures all product is hydrostatically and radiographically tested, guaranteeing a continuous Electric Resistance Welded (ERW) seam to all lengths of tube.

However, our understanding of this continues to evolve (The API5L standard does not give suitable comfort to our team with respect to the steel’s strength.) Yield tests may be longitudinal according to the code dependent on the tube size – evidently not suitable for application of torque and in permanent works scenarios around bending moments. We now specify both transverse and longitudinal tensile strength testing on all our material.

There are a number of other areas that should be considered:
  • Grade of steel – weld ability and ductility?
  • Certification – what tensile strength can you use in design calculations and comply with NZ3404?
  • Elongation Value
  • Wall thickness tolerance
All of these specific requirements make it somewhat of a minefield to buy product from supplier’s stock or from ‘pre-loved’ applications.

Unfortunately there is no recognised standard that considers screw piling that the consultant can refer to, providing comfort that a suitable material is being used. We are continuing to evolve a specification around best practice. This is available on our website here.

3 comments:

  1. What a great idea, I am always fascinated by the inner workings of things like this. Do you use API 5L X52 PSL 2 or API 5L X65 PSL 2? I am surprised that you use ERW as I tend to think the HAZ might struggle to cope with the loadings. Seamless might be safer. AS/NZS 3404.1 I believe defers to AS/NZS 1554.1-7in relation to grades and strengths of materials. 1554.1 allows welding up to 500 MPa.
    For higher strength materials e.g. ASTM A514, with a yield strength of 690 MPa, weld procedures are developed from AS/NZS 1554.4 Welding of high strength quenched and tempered steels.
    Elongation Value I know nothing about. But wall thickness should be able to be reduced as yield strength increases.
    It is an interesting subject and I look forward to reading more about this ‘black art.’
    Cheers, Peter Lochead

    ReplyDelete
  2. Thanks Peter - you raise some interesting points.
    We typically specify a minimum yield strength of 350 MPa - Suppliers often then provide X52 to deliver this.
    ERW is used the world over for screw piling. We have not heard of or witnessed any issues around the heat affected zone(HAZ)before. The key is the continuity of the seam to ensure structural integrity.

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