Global buckling of submarine pipelines may happen when the expansion due to temperature and pressure in the pipeline is restrained by the pipe/soil resistance. Global buckling may occur for both trenched pipelines (upheaval buckling) and for exposed pipelines (lateral buckling). Global buckling has been a major offshore pipeline design topic for High Pressure/ High Temperature (HPHT) since the mid 1980s, when the first buckles observed.
In 1996 the Hotpipe project was initiated by Statoil with the purpose of developing design criteria for global buckling. In 2001 the first revision of the Hotpipe Guideline was issued and in 2007 this was published as a recommended practice DNV-RP-F110. In 2002 another initiative was launched with the Safebuck Joint Industry Project to carry out research and develop global buckling criteria for exposed pipelines. This work was intended to complement the work performed by Hotpipe by addressing some of the issues more relevant to deepwater flowlines. The latest revision of the Safebuck Guideline was issued in Dec 2008. Several papers have in parts been discussing global buckling in the past but only two documents provide consistent design guidance; the public DNV-RP-F110 (outcome of the Hotpipe project) and the Safebuck JIP Design Guideline (confidential to JIP participants). In 2009 Safebuck and DNV initiated a process with the purpose of merging these two documents. The advantages of this merger will be several; taking the best parts from the two documents, will remove the confusion in the industry with two alternative design guidelines. The combined document will be published in the public domain as a new revision of DNV-RP-F110.
This paper will discuss the individual advantages of the design concepts in the two different codes and the benefit of combining these. Finally, some elaboration will be given on the structure of the future Safebuck Guideline that may unify how HPHT pipelines will be designed in the future.
Global buckling of pipelines is sometimes referred to as Euler buckling as the pipeline may be considered as a straight, slender, stress free bar in compression. As long ago as 1757, Leonhard Euler formulated the capacity of a slender bar and one may therefore believe that this phenomenon is fully understood (Bradly 2007). Uncertainties do, however, contribute to make global buckling a challenge to pipeline engineers. First, as often experienced in history, engineers were skeptical about " academic problems?? and for a long time did not believe this to be an actual problem; even when there were several indications that this was a real behavior of pipelines.
In the mid 1980s the first major upheaval buckles (vertical buckles of buried pipelines) were experienced. Over the next decade, pipeline design approached upheaval buckling with much success. However, the overburden employed to stabilise the pipelines was often excessive. In the mid 1990s computers became fast enough to model the actual pipeline configurations and more detailed design methodologies were applied. This lead to a general decrease in the overburden design but with patchy success in mitigating upheaval buckles and upheaval buckles started to appear again.
Global (lateral) buckling of exposed pipelines was identified as a potential problem for pipelines resting on the seabed by Palmer (1974). For a long time this was also considered to be an academic problem only; or possibly relevant for " hot pipelines??. It was a common understanding that hot was at least 50ºC above ambient temperature and buckling was not an issue for big trunk lines. Today we know that many existing trunk lines have buckled. True enough, these buckles are long, smooth and have not impaired the integrity of the pipelines. Generally, most pipelines with concrete coating due to on bottom stability requirements will for a certain environmental condition be very light. This means that they are likely to be susceptible to global buckling, but will be exposed to relatively low lateral restraint in the buckled condition. Hence, " hot?? and " high pressure?? must also be linked to the lateral resistance, i.e. the submerged weight of the pipeline.