Consider the pipeline has been spiralled over its complete length of several
kilometers. Some “transport” ropes can be attachedto reduce the tension in the
pipeline during its voyage.
When the anchor lines are detached from the centre point the spiral can
be towed to the installation location. The pipeline can be propelled by the centre
barge and the auxiliary boats (See figure 6 ) or by some tugboats.
Although the spiralled pipe can withstand for a short period of time rough waves (up to
a certain limit the spiral can stand waves up to 4-5 meter) it is always good to
consider the weather forecasts when the pipe is being transported.
Because the spiral is exposed to the waves that collide with the spiral when it is
exposed to open water we have to consider fatigue. Fatigue is an important factor to
reckon with. The wave height and the frequency that wave heads come by are the
parameters that will affect the heat affected zones (HAZ’s) of the welded pipes.
Fatigue crack propagation Tests have shown that fatigue occurrence with welded
pipe is strongly depending on the existence of planar defects and the frequent
slamming of the waves has a larger impact for higher grade steel then for the lower
grades. Fatigue crack propagation with the spiral pipeline that is for a longer period on open sea needs more investigation. Although the O-lay method needs a relative
small weather window, the O-lay method is considered at this moment for
geographical area’s where the weather and sea state can be forecasted for the period
of transport and installation. Workable conditions are when the wave height during
transport and installation are preferable below 1,5 meters.
For logistic reasons it is recommended to choose a pipe yard that is in the vicinity of
installation location since the expected speed of travel of the spiral with maximum 5
knots is relatively low.