Influence of Pressure in Pipeline Design: Effective Axial Force

Abstract

This paper discusses use of the effective axial force concept in offshore pipeline design in general and in DNV codes in particular. The concept of effective axial force or effective tension has been known and used in the pipeline and riser industry for some decades. However, recently a discussion about this was initiated and doubt on how to treat the internal pressure raised. Hopefully this paper will contribute to explain the use of this concept and remove the doubts in the industry, if it exists at all. The concept of effective axial force allows calculation of the global behaviour without considering the effects of internal and/or external pressure in detail. In particular, global buckling, so-called Euler buckling, can be calculated as in air by applying the concept of effective axial force. The effective axial force is also used in the DNV-RP-F105 “Free spanning pipelines” to adjust the natural frequencies of free spans due to the change in geometrical stiffness caused by the axial force and pressure effects. A recent paper claimed, however, that the effect was the opposite of the one given in the DNV-RP-F105 and may cause confusion about what is the appropriate way of handling the pressure effects. It is generally accepted that global buckling of pipelines is governed by the effective axial force. However, in the DNV Pipeline Standard DNV-OS-F101, also the local buckling criterion is expressed by use of the effective axial force concept which easily could be misunderstood. Local buckling is, of course, governed by the local stresses, the true stresses, in the pipe steel wall. Thus, it seems unreasonable to include the effective axial force and not the true axial force as used in the former DNV Pipeline Standard DNV’96. The reason for this is explained in detail in this paper. This paper gives an introduction to the concept of effective axial force. Further it explains how this concept is applied in modern offshore pipeline design. Finally the background for using the effective axial force in some of the DNV pipeline codes is given.

Nonlinear elastic–plastic stress investigation for two interacting 3‐D cracks in offshore pipelines

“…The internal pressure is one of primary loadings experienced by offshore pipelines as well as ground pipelines in operation . For pipeline design, the effect of the internal pressure cannot be neglected because it changes the effective axial force, which is one of dominating factors resulting in global buckling of pipelines . In this section, the nonlinear elastic–plastic analyses focus on two circumferential cracks in the cracked pipeline subjected to the combined loadings such as the tensile loading combined with the internal pressure.…”

“…30 For pipeline design, the effect of the internal pressure cannot be neglected because it changes the effective axial force, which is one of dominating factors resulting in global buckling of pipelines. 33 In this section, the nonlinear elastic-plastic analyses focus on two circumferential cracks in the cracked pipeline subjected to the combined loadings such as the tensile loading combined with the internal pressure. The influence of the hoop stress (induced by internal pressure) on the CTOD_s evolutions is investigated in details.…”

Modal response of short pipeline spans on partial elastic foundations

“…Compressive or tensile axial forces increase or decrease, respectively, the bending moments in the pipe (Bazant and Cedolin, 1991), and pressure loading causes a net transverse load on the tensile side of the pipe during bending (Fyrileiv and Collberg, 2005;. The combined second-order load effects from axial loading and transverse loads from pressure are generally accounted for by use of the effective axial force concept (Fyrileiv and Collberg, 2005;Sparks, 1984;, which is treated as an ordinary axial force when geometric stiffness effects are considered. The effective axial force S eff for an axially restrained pipeline is obtained as (DNV-OS-F101, 2012; DNV-RP-F105, 2006)…”

Upheaval buckling of pipelines triggered by the internal pressure resulting from the transportation of oil and gas: theoretical discussions and geometrically nonlinear analysis using Finite Element Method.

“…Given the limited amount of discussion about the internal pressure in the context of buckling of pipelines, the general objective of the present work is justified. Using different approaches for considering the internal pressure, the simple and advanced numerical simulations of the present work allow to discuss the works of Sparks (1984) and Fyrileiv and Collberg (2005), which deal with the concept of effective axial force, as well as the work of Gay Neto, Pimenta and Martins (2017), which deals with a specific formulation for applying the internal pressure as a distributed load dependent on pipe curvature. Such formulation, which constitutes an advance of the formulation proposed by Dvorkin and Toscano (2001) in the scope of finite elements, by the way, will be tested through the practical numerical simulations of the present work.…”

“…Sparks (1984) is one of the main authors who deal with the effective axial force. Fyrileiv and Collberg (2005), in their turn, summarize the knowledge about the topic. For the next discussions, based on the studies of the previous authors, consider the pipe segment shown in Figure 3 and the following additional notation: ds is the pipe segment length, T T is the true axial force in the pipe wall (as a result of the integration of the stresses on the pipe cross section),…”