To understand where the boundaries and risks lie for pipeline induction bending it is important to understand the characteristics of the various types of linepipe and how they relate to the induction bending process.
For HFW linepipe in the size range DN100 to DN600, wall thickness up to 14.3mm and grades X42to X80, the pipeline designer should have every confidence that induction bends can be produced with material properties equivalent to the motherpipe. Linepipe manufactured in modern HFW pipe mills is produced from thermo-mechanically control rolled steel strip with chemistries to meetgrade and high speed seam weldability requirements. HFW pipe chemistry is generally well suited to the requirements for the induction bending process. This can partly be explained in that modern HFW linepipe mills utilize in-line induction heating for the weld seam annealing heat treatment process. This annealing treatment - albeit at a different temperature and speed - is not dissimilar tothe induction bending process thermal effect on material properties.
Larger diameter and heavier wall SAW pipe may slow the induction bending process and thereby restrict the range for the various process parameters. This is particularly the case for high X gradematerials where higher temperatures and faster cooling rates derived from faster process speeds arerequired. For large diameter and heavy wall pipes, high strength properties may not be achievablewithout a corresponding increase in pipe chemistry to ensure that the pipe material is sufficientlyresponsive (hardenable) for the lower peak temperature at the pipe bore and the slower rate of cooling.
Achieving high strength properties directly off the induction bending machine tends to be more problematic for seamless pipe compared to the equivalent size and grade of welded pipe.
High strength seamless carbon steel linepipe is manufactured in a manner quite different than thatused to make pipe from rolled plate or strip. Seamless pipe is hot formed to achieve the required pipe diameter and wall thickness; it is then heat treated to achieve the required strength andtoughness. Pipe mills naturally design pipe chemistries to suit the rapid internal and external millquench and heat treatment process. Induction bending is practically limited to external water spraycooling (ie from one side only) at relatively slow speeds and therefore cannot achieve the samequench rate as pipe mills. For lean chemistry high strength seamless pipes with wall thicknessesabove 13mm it may be necessary to perform a full body post bend quench and temper heattreatment otherwise only downgraded material properties may be achieved off the bending process.
As has been demonstrated, chemistry plays and important role in achieving the required pipeline properties – this is particularly the case for high strength induction bends from heavy wall line pipe.
The Offshore Pipeline Standard gives maximum allowable chemistries for variousgrades of line pipe (seamless and welded,) and motherpipe for induction bending. The trend of allowing higher chemistries for higher grades is clearly evident. Theallowable maximum percentage of the principal constituents of carbon and manganese, as well asthe micro-alloying elements of niobium, titanium and vanadium, all increase with strength grade.In addition, it can be seen that for induction bends a higher chemistry is allowable over and abovethat for the equivalent grade seamless pipe; and even more so over that for welded pipe. Thesetrends are most apparent in the consequential increase in the maximum allowable carbon equivalent(CEQ) for each grade and type. The footnote for each table indicates that the maximum allowablechemistry is applicable to quite heavy wall thicknesses.
Pipe Wall thickness
The actual wall thickness compared to the “nominal” wall thickness, and the variations in wallthickness, can be quite different between welded pipe and seamless pipe.
Welded pipe is made from plate and as such will have a very even wall thickness along the pipe andaround the pipe circumference with some thickening in the weld zone. Since pipe mills like toeconomise, it can be expected that the actual wall thickness for welded pipe will almost invariably be at or slightly under the nominal value.
Seamless pipe wall thickness is dependant on the quality of the pipe mill and can be much morevariable that for welded pipe. Wall thickness may vary greatly around the pipe circumference andalong the length of the pipe; and between pipe joints from the same heat. The bore may beeccentric to the outer diameter and give thicker and thinner sides to the pipe; and ridges in the boremay give immediately adjacent thick and thin areas of pipe wall.
On top of all of this of course any mark or blemish is going to further detract from the wallthickness. Expectations of the actual motherpipe wall thickness compared to the nominal valueshould generally be pessimistic – not optimistic!
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