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.

The demand for pipe materials that can withstand increasing hydrostatic pressures continues to grow as oil and gas exploration takes us into deeper and deeper waters. As water depth increases, failure by hydrostatic collapse becomes more likelyas the limiting failure mode for pipeline design. In general, pipes for deepwater applications possess a diameter to thickness ratio in a region where failure is dominated by both instability and plastic collapse. This implies that, prior to failure, the compressive yield strength of the material must be exceeded, followed by ovalisation and further local yielding.To enable effective design work to be performed, and proven during service, it is necessary to ensure that the mechanical and dimensional properties of the pipe material are controlled within very strict boundaries and reliable and accurate formulae are required to take into account these factors. However, mechanical equations for this simple failure mode have not been satisfactory established. Current existing formulations use basic mechanical expressions that are calibrated to some limited test data. However none of these expressions examine the actual mechanics of the failure mechanism. This paper presents an investigation into the mechanics of this specific problem and permits a simple assessment of the effects of geometrical and material data on the collapse pressure of moderately thick tubes. An analytical expression has been generated and correlated to extensive test data and finite element models, proving the accuracy of the expression. The resulting formula also allows a deeper understanding of the effect pipe shape has on collapse and opens to the door for manufacturers to further enhance pipe material performance. Despite the complexity of the collapse mechanisms this work presents a concise yet rigorous and effective analytical approach.The work presented in this paper will form the basis of future investigations that will enhance the understanding of this failure mode in pipelines. This will lead to increased pipeline safety while allowing the boundaries of pipeline application to be challenged, enabling hydrocarbon recovery in deeper and deeper waters.

The local buckling of pipelines under external pressure is comprehensively addressed in section 5 of DNV-OS-F101 Rules for Submarine Pipeline Systems. The equations used, calculate the plastic and elastic components to give an overall collapse pressure. These equations include factors that are controlled by the pipe manufacturer. A key feature of the collapse design formula is that the compressive yield stress of UOE pipes is de-rated by 15 per cent through the use of a fabrication factor, αfab. This de-rating is used to account for the Bauschinger effect caused by the pipe forming process, in particular the final expansion. It is well documented that the cold forming (compression & expansion) and light heat treatment can have a beneficial effect on the compressive strength, leading to higher fabrication factors for UOE linepipe. DNV-OS-F101 states, “The fabrication factor may be improved through heat treatment or external cold sizing (compression), if documented”. The standard does not specify what documentation or quality control is required at the pipe mill to ensure every pipe length has the same collapse resistance to allow the increase in fabrication factor. Tata Steel Tubes Europe (Energy), together with Williams Field Services and Det Norske Veritas have recently concluded a technology qualification process, according to DNV-RP-A203 (Qualification Procedures for new Technology), with the specific aim of detailing the documentation and Quality Control needed to satisfy the requirements of DNV OS F101. This would then allow the use of increased fabrication factors in deepwater linepipe design. A key part of the technology qualification was the an extensive testing program that included small-scale compression tests, full-scale collapse tests and the newly developed ring collapse test procedure, which can be utilised as part of the mill quality control system for more representative assessment of the collapse resistance of linepipe material. This paper presents the systematic qualification process; including pipe manufacture, quality control and verification. It also presents some of the key mill capability requirements for producing deepwater UOE linepipe and additional factors that should be considered when optimising for local buckling resistance. Using this approach collapse pressures of above 585bar were achieved for a 457mm diameter × 31.75mm UOE pipe, equivalent to installation depths of over 5000m.

Thii paper was sebcled for presenialion by the OTC Prqram Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper. as presented, haw not bean r e v i e d by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference or its d i o s m . Eledmnic reproduction, distribution, or storage of any part of this paper for commercial pulposes without the winen consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 3M) words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. AbstractIn May 1995 a multiphase flow meter (MPFM) was installed in a subsea production manifold on the UK South Scott Field operated by Amerada Hess Limited. This achievement was the culmination of a three year collaborative development programme, the objective of which was to develop and prove a prototype MPFM for a commercial subsea application.The incorporation of the MPFM in the South Scott production facilities had two main benefits. It reduced the complexity of the infrastructure required to develop the field by eliminating the need for a dedicated test pipeline and riser system, and significantly reduced the project capital expenditure. Additionally the application provided the opportunity to deliver benefits to the production and reservoir engineering community by allowing real time monitoring of well tests.Despite the comprehensive testing undertaken during the development programme, the South Scott MPFM experienced an electronic component failure early in its operational service life, which resulted in the recovery of the meter and a subsequent failure investigation. This process uncovered a more complex problem associated with one of the sensor units, which required the development of a new technology for a solution. Through the sustained efforts and commitment of the venture partners, the initial setbacks have been overcome and the meter is planned to be operational on the South Scott field in February 1997.