Page 223 - Pipelines and Risers

196 Chapter I2

pipeline. A User Subroutine is used to specify a distributed load to the pipeline to get the right
submerged weight. This load is applied to the pipeline at the same time as the buoyancy load
computed by PB. The magnitudc of this load is equal to the difference in buoyancy load
caused by the fact that the outer diameter of the steel pipe is smaller than the outer diameter
for a pipe covered with concrete and coating. The magnitude of the distributed load specified
in the user subroutine is computed as explained below.

bd = ba - bpb (12.18)
D, 'n
b, =- Pg (12.19)
4
Dm 'n
b =- Pg (12.20)
pb 4
where:
D,= Outer diameter of pipe with concrete and coating
D,= Outer diameter of steel pipe
b, = Actual buoyancy loadlm
bpb = Buoyancy load (PB)/m
bd = Distributed load from user subroutine/m

The User Subroutine DLOAD has been used. The subroutine can be used to define the
variation of a distributed load magnitude as a function of position, time, element number, etc.
This subroutine is made such that the calculated distributed load only will be applied to
elements beneath the still water surface.

12.4.9 Theoretical Aspects of Pipe Rotation
Severe pipe rotation has been experienced during deepwater pipelaying, but the reasons
causing the phenomenon are not understood in the industry. While analytical models have
demonstrated the influence of residual curvature on pipe rotation, 3D FE simulations of the
pipelay process are needed to predict rotation.
This section, which is taken from Damsleth et al. (1999), deals with the consequences of the
plastic strain that can occur in the outer fibres of the pipe wall as it passes over the stinger
during laying. Endal et al. (1995) have shown that the pipe twists, i.e. it rotates around its
axis. They also show that, provided the plastic strain is small, the on-bottom configuration is
straight and flat as for an entirely elastic process. Thus the main consequence is the rotation
during pipelaying. They also state that pipeline twist acts only in the elastic sagbend (or
underbend) section and characterize it as a typical instability phenomenon. These aspects will
be reviewed here as we elaborate their theoretical approach, the main modification being the
inclusion of the gravitational potential energy.

During installation, the pipe extends from the horizontal tension machine, bends over the
stinger and, while sloping downward through the water, bends gradually in the opposite
direction onto the horizontal seabed. The tensioner provides the upper support for the pipe
while the seabed provides the lower support where residual tension is balanced by friction.
Customary terms used to describe this S-lay pipe configuration are overbend, inflection point
and underbend, or sagbend.