Page 236 - Pipelines and Risers

Instal[alion Design 209

overbend and in the sagbend. The increase in bending moment induces higher strains in the
adjacent pipeline. If the strain in the adjacent pipeline in the sagbend exceeds the design
criteria a higher lay-tension can be applied to the pipeline to reduce the strain.

When a valve is placed at a support, the adjacent pipeline will be lifted as a result of the
contact between the valve and the support. This also leads to an increased bending moment
locally. The result of these two effects is that the strain in the adjacent pipeline increases. To
reduce the increased bending moment because the pipeline is lifted, the pipeline can be
tapered.

An example design analysis was performed by Martinsen (1998).

12.6 Two Medium Pipeline Design Concept

12.6.1 Introduction

The design and construction of pipelines and flowlines is one of the key issues for the
development of deepwater production and transportation facilities. The installation of large
diameter trunklines has been limited to around 600m (Rivett, 1997). Smaller diameter
flowlines have been installed in as much as lOOOm depth. New challenges presented by
projects currently undertaken in even deeper water are challenging the present pipeline
technology and have stimulated the development of new concepts (Damsleth and S. Dretvik,
1998, Walker and Tam, 1998).

It is known that linepipe material cost takes a large portion of the CAPEX of pipeline
projects. Using present technology, installation design for external pressure would govern
wall thickness selection for deepwater pipelines. There is a need to develop new design
concepts to avoid this situation (Palmer 1997) and make deepwater pipelines as commercially
competitive as their shallow water counterparts.

Until a few years ago, pipeline design has based on simplified capacity equations and some
special purpose computer programs for installation and on-bottom stability design. Recently,
use of nonlinear finite element simulations and limit-state design has become acceptable
practice (Bai and Damsleth, 1997, 1998) in situations where design criteria has significant
cost impact. The technological advances in finite element simulation have permjtted project
specific optimizations that have saved up to 16% of the pipeline CAPEX development
(Home, 1999) for pipelines in water depths of 350m. The potential for optimization can be
even greater for deeper water pipelines.

This section presents a new design concept for deepwater installation, which is called Two
Medium Pipeline (Bai et a1 1999).