Page 212 - Pipelines and Risers

Installation Design 185

Limited fabrication length of pipestrings due to the size (length) of the fabrication yard
and the difficulty of controlling a long line during tow out. The maximum line length to
date using this method is 7 km,
The line should be installed in a straight line. A substantial amount of intervention work is
required to install bends in the system.

12.3 Software OFFPIPE and Code Requirements

12.3.1 OFFPIPE
For pipeline installation analysis the fit-for-purpose computer program OFFPIPE may be
used. OFFPIPE is a finite element method program specifically developed for the modeling
and analysis of non-linear structural problems encountered in the installation of offshore
pipelines.

The static analysis carried out in this chapter considers the following 2-dimensional functional
external loads:

0 Tension at lay barge tensioners;
Buoyancy uniformly distributed;
External hydrostatic pressure;
Reaction forces from the lay barge rollers;
Vertical seabed reaction (assumed continuous elastic) foundation.

The material modeling used by the OFFPIPE computer program is a Ramberg-Osgood
material model. This Ramberg-Osgood material model used in OFFPIPE is expressed as
follows:
- =-+ 43
B
K
M
KY MY (12.1)
where:
K= pipeline curvature
M= pipeline bending moment
ICy= 2.";/(E.D)

E= modulus of elasticity of the pipe steel
D= diameter of the pipe steel
I,= cross sectional moment of inertia of the pipe steel
oy = nominal yield stress of the pipe steel