Page 43 - Pipelines and Risers

Introduction 17

popular. This method can also be used with intermittent rockdumping, by permitting the line
to snake and then to rockdump, this reduces the likelihood of upheaval buckling.

The methods employed in calculating upheaval and lateral buckling as well as pullover
response are detailed in references Nystrom et al (1997), Tomes et a1 (1998).

1.4.7 Pipeline Installation
There are various methods of installing pipelines and risers. The methods of installation
which determine the type of analysis performed are discussed as follows:
Pipelaying by lay vessel;
Pipelaying by reel ship;
Pipeline installation by tow or pull method.

- Pipelaying by lay vessel
This method involves joining pipe joints on the lay vessel, where at a number of work stations
welding, inspection and field joint coating take place (see Figure 1.7).

Pipelaying progresses with the lay vessel moving forward on its anchors. The pipe is placed
on the seabed in a controlled S-bend shape. The curvature in the upper section, or overbend, is
controlled by a supporting structure, called a stinger, fitted with rollers to minimize damage to
the pipe.

The curvature in the lower portion is controlled by application of tension on the vessel using
special machines.

The pipeline designer must analyze the pipelay configuration to establish that correct tension
capacity and barge geometry are set up and that the pipe will not be damaged or overstressed
during the lay process.

The appropriate analysis can be performed by a range of methods from simple catenary
analysis to give approximate solutions, to precise analysis using finite element analysis. The
main objective of the analysis is to identify stress levels in two main areas. The first is on the
stinger where the pipe can undergo high bending especially at the last support. Since the
curvature can now be controlled, the pipeline codes generally allow a small safety factor.

The second high stress area is in the sag bend where the pipe is subject to bending under its
own weight. The curvature at the sag bend varies with pipeline lay tension, and consequently
is less controllable than the overbend.

In a11 cases the barge geometry and tension are optimized to produce stress levels in the pipe
wall within specified limits.