Page 229 - Pipelines and Risers

202 Chapter 12

rotation where Tees and other fixtures are to be installed in the line. This phenomenon need
not become an installation problem provided the rotation can be predicted and controlled.

It is difficult to quantify pipeline twist for the construction phase since the behavior of the
pipeline during . installation is specific to the pipe characteristics and the installation
configuration. While design codes provide criteria for maximum overbend strain to avoid
pipeline twist, the resulting lay configuration may be too costly. Or, strain concentrations due
to coatings, under-matched welds, buckle arrestors and other in-line components may produce
permanent overbend curvature that could cause pipe rotation.

Therefore, non-linear 3D FE models using elasto-plastic beam and frictiodcontact elements
are used to analyze the load history of the pipeline during the pipelay process that accounts
for the complex interaction between constant as well as time and position varying loads
involving all 6 degrees of freedom. The FE model can simulate pipeline rotation to determine
whether control measures are necessary as well as demonstrate the effectiveness of correction
measures.

The three figures below illustrate the twist phenomenon during laying of a 2.4 km section of
deepwater pipeline with a 0.5 m/s lateral current. Pipelay initiation was by dead man anchor
so that the end was free to rotate. Rotational friction on the seabed is ignored in this case.

Figure 12.17 shows the situation after 2.4 km of pipelay indicating total strain and permanent
strain in the vertical plane after the pipe has been subject to elasto-plastic bending over the
stinger during laying. Figure 12.18 shows a net rotation of 60 degrees of the free end due to
the twist effect for an elasto-plastic pipe material and 0 degrees for a completely elastic pipe.
Figure 12.19 shows the resulting rotational moment along the elasto-plastic pipe that
compared to the near-.zero moment of the elastic pipe. The only difference between the curves
is due to the 0.1% residual strain of the elasto-plastic pipe material, demonstrating that plastic
strain, combined with a lateral disturbing force, is the source of the pipe rotation.