Page 204 - Pipelines and Risers

177

Chapter 12

Installation Design

12.1 Introduction
Marine pipeline installation is performed by specialized lay-vessels. There are several
methods to install a pipeline, the most common methods being S-lay, J-lay and reeling.

Depending on the method, a marine pipeline is exposed to different loads during installation
from a lay-vessel. These loads are hydrostatic pressure, tension and bending. An installation
analysis is conducted to estimate the minimum lay-tension for the pipeline for a given radius
of curvature to ensure that the load effects on the pipeline is within the strength design
criteria.

A commonly used FEM computer program for installation analysis is OFFPIPE. This
program can give indicative global results for most situations but not the effects of
stresdstrain concentration and point loads due to change in stiffeners.

This chapter also describes a finite element model for pipeline installation analysis. The
model should be able to compute static load effects on a pipeline during installation, based on
the layramp geometry, pipeline design data and water depth for the pipeline to be installed.

The established model should be a tool for analyzing the static configuration of a pipeline
during installation. The static configuration of the pipeline is the shape of the pipeline from
the lay-vessel to the seabed when it is in static equilibrium. The model should also be capable
of analyzing the load effects on the pipeline when a section like a valve is installed. The
model should also be capable of letting the pipeline slide over the stinger. A pipeline cross
section will then move from the lay-vessel, over the stinger and through the sagbend to the
seabed.

The purpose of developing this finite element model for pipeline installation is to calculate the
load effects on a pipeline during the installation of an in-line valve. These analyses do not
involve the response due to environmental loads.