Bending steel pipe.
Considering that, within limits, everything can be deformed in an elastic way, steel pipe can be
deformed in its elastic area and return to its original form after the applied forces are taken
Also concrete can be bend elastically. The difference between the steel and the concrete
is that concrete doesn’t have a yield point elongation or plastic deformation and therefore
will break at its yield point.
For steel the elasticity modulus is 210.000MPa while for normal concrete this modulus is between 20.000 and 30.000MPa. Although for ordinary concrete the yield point is very low, the concrete can have a larger deformation (strain) then the steel before it will break. As is shown in the below diagram.
By using additives in the concrete it is even possible to achieve more then 4% of tensile strain. This severe deformation is much more then what is expected from steel.
Developments in concrete go even further. In the laboratories of the University of Michigan, tests shows that they developed a type of concrete that is “self-healing” due to the fact that the micro cracks in the concrete are recovering and the concrete gets back its original strength after researchers subjected them to a 3% tensile strain.
For the spiraling pipeline with concrete weight coating it is interesting to find the constraints of this elastic bending. According these data the concrete is not the determining factor when we are bending steel with a concrete coating in the elastic area of the steel.
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.
On average you can 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 itself will not get into an oval shape. Ovalisation will occur when a pipe is bend beyond the boundaries of elastic deformation. Excessive ovalisation is not acceptable due to 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 pipe to lay on the seabed.
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.
Normally the weight of the pipe to be installed is roughly 1.3 x weight of of the water that is replaced by the pipe. The limitation in pipe diameter when applying the O-lay method arise because of the size of the floaters that have to be used when the pipe diameter becomes to large. At this moment we found that when floaters are needed to keep the pipe afloat a maximum diameter of 32” is a workable diameter. In case the weigth of the pipe is such that it is floating by itself the pipe diameter has no limitation for the O-lay methodology.