Bending steel pipe with a concrete coating

Considering that, within limits, everything can be deformed in an elastic way. Steel pipe with concrete coating can thus be deformed in its elastic area and return to its original form after the applied forces are taken away. For the spiraling pipeline it is interesting to find the constraints of this elastic bending.The differences between steel and concrete during the deformation is that afterv the elastic deformation steel will start flowing and deforms plastcally while the concrete will start to form micro cracks before it finally breaks.

Due to the limitless tests that have been performed on steel and especially on pipeline it is easy to predict what the constraints for bend pipeline will be.
As we can see in the diagram the pipe will deform from the yield point onwards in a way that the steel pipeline will change its form irreversible. Besides that, the structure of the steel will change and harden, the round form of the pipe will become oval.  When the pipeline roundness deformation becomes too large, the inspection tools and pigs that will be utilised in a later stage could have problems passing the deformed
parts of the pipeline.

By using additives the concrete can extend its elasticity. The concrete has a lower E-modulus and by including certain additives the concrete does allow more strain before micro cracks will occur. Therefor the concrete is not the determining factor when it comes to bending to the limits in the elastic area.

To get not into the plastic area of the steel (or of the concrete ) we have to make sure the strain doesn't pass the point where plastic deformation (or plastic yield) starts to occur.

For the pipe’s integrity it is best that deformations remain within the elastic area

Plastic yield occurs in the spiral when:
r/R > Y/E
where r = outside pipe radius
R = spiral radius (more exact, from center of first layer to the pipe axis)
Y = yield stress of the steel
E = modulus of elasticity or Young's Modulus

For a pipe the values of r, E and Y are known. Therefore the R can be calculated.

Example:

For a pipe X65 (API 5L) with OD of 508 mm, E = 2,06*105 N/mm2, Y= 450 N/mm2.

The first winding of the spiral will be minimum 508 X 206000 / 450 > 233 meter.

Generally one could say that the diameter of the first layer of the spiral should be roughly 500 X the diameter of the pipe. As long as the pipeline will stay within its own elastic area, the pipe will not transform into a permanent oval shape. Ovalisation will occur when a pipe is bent beyond the boundaries of elastic deformation. Excessive ovalisation is not acceptable because of the fact of its incompatibility with the use of pigs and TFL tools in a later stage.

With the O-lay method it is possible to prepare any diameter of pipelinewhich has to be installed on the sea-bed.

The advantage of this method is that pipeline will not be plastically deformed during spiralisation and the pipe will always return to its original form after the applied forces are taken away from the pipe.

Source:
http://o-lay.net/method/pipeline-spiraling-stage.html