Qualification of Enhanced Collapse Capacity UOE Deepwater Linepipe

Abstract

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.