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.

The demand for energy is steadily increasing and, at least for the coming decades, the world has to rely on oil and gas to address this need. Most of the easiest accessible offshore petroleum reservoirs have been discovered and a great part developed over the last six decades. Thus, development of new oil and gas fields faces a lot of challenges as most of them are in remote areas, in deep waters and/or in areas with extreme environments like the Arctic region. One of the major trends in the offshore petroleum industry points towards deeper waters (e.g. outside West Africa, the Brazilian Pre-Salt developments and in the Gulf of Mexico). This trend also includes increased use of subsea installations instead of platforms, more subsea processing and increased use of pipelines to transport the hydrocarbons to shore or into a pipeline grid. This paper addresses some of the challenges pipeline design, installation and operation may face in deep and ultra-deep waters. The main design challenge is related to the high external pressure that may cause collapse of the pipeline. This potential failure mode is normally dealt with by increasing the pipe wall thickness, but at ultra-deep water depths this may require a very thick walled pipe that becomes very costly, difficult to manufacture and hard to install due to its weight. One approach to overcome this is to improve some of the parameters that determine the collapse resistance by an improved manufacturing process. Other approaches are to ensure a minimum internal pressure is maintained in the pipeline during all phases, or to install a buoyant pipeline that is anchored at a moderate water depth rather that laying on the sea bed.

Recent development plans envisage the transport of hydrocarbons at temperature and pressure conditions far more severe than in past projects. Technical feasibility of certain inter field lines was put in doubt as a consequence of application of design guidelines currently in force. This fact gave rise to a critical review of design criteria. The HotPipe Project is a Joint Industry Research and Development Project, whose overall objective is to prepare a DNV Recommended Practice to be used in structural design of high temperature/high pressure pipelines. The DNV-RP will cover most practical cases where pipelines are subjected to high internal pressure and high temperature (HP/HT). The design criteria are based on the application of reliability methods to calibrate the partial safety factors in compliance with the safety philosophy established by DNV OS F101. The overall objective was pursued by performing the following subprojects: • Pipe Capacity: Experimental tests were carried out and FE models were developed aiming at establishing the failure mechanisms of thick pipes subjected to internal pressure, bending moment and axial compression under monotonic and cyclic loading conditions. • Pipe Response: Analytical tools and FE models were developed for studying the localization of buckling pattern for the envisaged pipeline scenarios i.e. pipelines laid on even sea bed, pipelines laid on uneven seabed and buried pipelines. • Mitigation Measures. Study of relevant mitigation measures and associated criteria for preventing/reducing/controlling additional pipeline bending. • Design Guideline. Preparation of the design guideline. The Joint Industry Project Hotpipe did recently finish the work and issued an internal confidential project guideline. This is in the process of being converted into a public DNV Recommended Practice, ref. DNV-RP-F110, which will be published later this year. It provides procedures and criteria to fulfill this functional requirement, to ensure the integrity of the pipeline in the post buckling condition. This paper will describe the procedures and criteria in the project guideline. It is expected to be identical in the coming RP issued for industry hearing. This paper describes the main structure and the covered design scenarios of the DNV-RP, particularly: • Pipelines exposed on even seabed, where thermal expansion may be accommodated by lateral snaking; • Pipelines on uneven seabed corresponding to even seabed; and • Pipelines on bottom of trenches/covered by natural or artificial backfill.

The offshore pipeline industry is planning new gas trunklines at water depth ever reached before (up to 3500 m). In such conditions, external hydrostatic pressure becomes the dominating loading condition for the pipeline design. In particular, pipe geometric imperfections as the cross section ovality, combined load effects as axial and bending loads superimposed to the external pressure, material properties as compressive yield strength in the circumferential direction and across the wall thickness etc., significantly interfere in the definition of the demanding, in such projects, minimum wall thickness requirements. This paper discusses the findings of a series of ultra deep-water studies carried out in the framework of Snamprogetti corporate R&D. In particular, the pipe sectional capacity, required to sustain design loads, is analysed in relation to: • The fabrication technology i.e. the effect of cold expansion/compression (UOE/UOC) of TMCP plates on the mechanical and geometrical pipe characteristics; • The line pipe material i.e. the effect of the shape of the actual stress-strain curve and the Y/T ratio on the sectional performance, under combined loads; • The load combination i.e. the effect of the axial force and bending moment on the limit capacity against collapse and ovalisation buckling failure modes, under the considerable external pressure. International design guidelines are analysed in this respect, and experimental findings are compared with the ones from the application of proposed limit state equations and from dedicated FE simulations.

Whereas the wall thickness for most pipelines is governed by internal pressure, the wall thickness of pipelines at very deep waters may be governed by external pressure and the failure mode is collapse. This paper will firstly summarise the work performed in the early 90ties in the SUPERB project that constitutes the basis for the collapse equation adopted in DNV Rules for Submarine Pipeline Systems. This work documented a comparison between various expressions for collapse prediction (Timoshenco, Murphy and Langner (Shell) and Haugsmaa (BSI)) to available experimental results. This work made it possible to select the formulation deemed to be most appropriate as a design equation as well as calibrating safety factors. Secondly, the paper will discuss the well documented detrimental effect that pipe forming can have on the compressive yield strength in the hoop direction and thus the collapse capacity of pipes. This effect led to the introduction of the so-called fabrication factor in DNV-OS-F101 that reduces the compressive yield strength by 7–15 per cent for pipes manufactured using cold forming. However, DNV-OS-F101 states “The fabrication factor may be improved through heat treatment or external cold sizing (compression), if documented” and the paper will summarise various published work, experimental and analyses, that has, during the last 15 years, been performed in several pipeline projects to document the beneficial effect that mainly light heat treatment but also optimised forming in the UOE process have on the compressive yield stress and collapse capacity.