Page 213 - Pipelines and Risers

186 Chapter 12

A= Ramberg-Osgood equation coefficient
B= Ramberg-Osgood equation exponent

The method described above is for typical standard S-lay, J-lay or reeling method installation
of an offshore pipeline. The analysis can be carried out both by static analysis or dynamic in
order to determine the effect of the weather conditions. For special consideration of local
constraints on the pipeline in terms of structures or similar other simulation tools may be used
in terms of more generalized computer programs (ANSYS, ABAQUS). More generalized
computer software tolls may also be used if special installation methods should be used,
where OFFPIPE not is found to be applicable.

12.3.2 Code Requirements
For pipeline installation analysis code requirements may be related to the pipeline curvature
on the stinger and in the sagbend, for S-laying. A typical code is the Statoil Specification F-
SD-101. For a carbon steel material comply with MI-5L- X65, the code requirement are
listed below:

Pipeline overbend (stinger) 0.20 %
Pipeline sagbend (spanning section) 0.15 %

In line with the tendency of allowing higher strains a level of 0.23 % may be used for the
pipeline overbend. This is based on the recommendation in Statoil F-SD-101, Amendment 1.

It should be indicated the allowable strain for installation may be developed using limit-state
based design as discussed in Chapter 4.

12.4 Physical Background for Installation

12.4.1 S-lay Method
Different technologies and equipment are adopted to install pipelines offshore. One of these
methods is the S-lay method.

The lay-vessel can be either a normal vessel or a semi-submersible vessel. What makes the
lay-vessel special is that it has a long ramp extension or “stinger” at the stem. At the vessel
there is a near horizontal ramp. This ramp includes equipment like welding stations and
tension machines. When the pipeline is welded the pipeline is fed into the sea by moving the
vessel forward on its anchors. A number of rollers are placed at the stinger and vessel. These
rollers support the pipeline when it moves from the vessel and into the sea. The rollers placed
on the stinger and the vessel, together with the tension machines, create a curved support for
the pipeline. The pipeline is bend over the curved support on its way into the sea and this part
of the pipeline is named “overbend”, see Figure 12.5. The stinger radius controls the overbend
curvature.