Determination of Wall Thickness and Allowable Bending Strain of Deepwater Pipelines and Flowlines
Authors: Kopp F. , Peek R. 
Published: October 2008

Two recently updated design rules, API RP 1111 - Limit State Design1, and DNV Rules for Submarine Pipeline Design - OS F1012, have given engineers on pipeline and flowline projects useful tools to help satisfy the ever present drive to reduce costs of pipelines and flowlines, without compromising pipeline integrity. To cost-effectively install deepwater pipelines and flowlines, it will be necessary to allow for bending strains during construction, and perhaps even during operation, that exceed yield. Both design rules allow for plastic strains under certain conditions, but there are differences, that become evident especially for the combination of high external pressure and bending strains beyond yield. These differences are highlighted using nondimensional pressure-bending interaction diagrams. With examples of design of a pipe-in-pipe flowline and crude oil and gas export pipelines in waterdepths to 6000 ft, it is shown how one can use a pragmatic approach to selecting wall thickness and allowable bending strains during construction and operation. It is shown how use of fitness-for-purpose acceptance criteria for weld flaws will impact selection of allowable bending strains.

Introduction

Significant effort has been undertaken by organizations such as API and DNV in the past several years to progress design of pipelines and flowlines. In conventional design, one would consider safe states, for instance the stresses remaining below yield by an adequate margin. As long as the safe state is not exceeded under design loading, the design is pronounced adequate. However, often this safe state can be exceeded significantly before the integrity of the pipeline or flowline is lost. In other words, the safe state can be exceeded before a "limit state" is reached. Limit states designs essentially arise from challenging conventional design rules. Instead of checking that a safe state can be maintained, the designer checks that margins against all possible failure modes (limit states) are adequate, while at the same time maximizing the economy of the design. Both the recently updated European and U.S. design rules for pipelines and flowlines allow the designer to use the limit states design approach.

One of the important features of both design rules is that the designer is allowed, under certain conditions, to use a "strain-based" design rather than a "stress-based" design. This type of design is appropriate when the pipe is subjected to loading conditions in which the deformations that develop are more or less independent of the resistance that the pipe offers to such deformations. A typical example of this is reeling of a pipe, where the strains to bend the pipe around the reel depend on the radius of the reel and the diameter of the pipe only.

There is one more important factor that needs to be considered in applying strain based limit states design. Five years ago, the Department of Offshore Transportation (D.O.T) in the U.S. allowed use of the Appendix to API 11043 "Alternative Acceptance Criteria for Welds".