Abstract
The last four decades have witnessed a significant change in the oil and gas offshore industry where field developments have moved from shallow water (10 metres to 100 metres) to deep water (100 metres to 1000 metres) and currently ultra deep water, 1000 metre and deeper. The subsea structures, components and pipelines have to stand such high external hydrostatic pressure which becomes in some cases the governing design criteria rather than the well pressure and temperature. This study focuses on pipe induction bends, which is a vital element in any ultra deep water subsea system. Bends are mainly used in the tie-in spools to connect pipeline end structures to manifolds or to well head trees. A significant amount of work has been conducted to study the behaviour of pipe bends under internal pressure in the last 150 years for its widely used applications in power plants, processing facilities and generic industrial use, however, minimal work has been conducted for pipe bends under external pressure for its rare applications. Oil and gas explorations and developments started to increase the demand of better understanding the pipe bend behaviour under high external pressure. As a result of the lack of knowledge and experimental work in this area, codes and standards have either considered it as a piping component or recommended quite stringent design criteria that more fits shallow water applications but is almost impossible to comply with deep water applications. Designers end up with required pipe wall thicknesses that exceed the manufacturing capabilities and the practical welding limits. This research investigates the failure modes of pipe bends subjected to external pressure and bending moments and presents a guideline on how to model, analyse and design these vital components for the subsea oil and gas fields developments. A limit state design criteria is proposed to enable the design of pipe bends, the proposed limit state design is in alignment with the current limit state design for pipelines.