Description
This Report describes the application of differential pressure in the design of unbonded flexible pipes. Differential pressure design principles are well established in subsea rigid pipeline design specifications such as DNV-ST-F101, Submarine Pipeline Systems (2021) and API 1111, Design, Construction, Operation and Maintenance of Offshore Hydrocarbon Pipelines (Limit State Design), Fifth Edition (2015). These standards recognize that the combined effect of internal and external pressure (i.e.,
differential pressure) significantly influences structural integrity, safety margins and project economics.
Although these differential pressure design principles are equally relevant to flexible pipe design, they were historically overlooked. Early flexible pipe applications were primarily in shallow waters, where external hydrostatic pressure was relatively low. In such cases, external pressure was small relative to internal pressure and was often ignored for simplicity. Industry specifications at the time reflected this practice.
As flexible pipe technology has expanded into deepwater environments, notably in Brazil, external pressure has increased dramatically. Differential pressure effects have become an increasingly important design consideration.
These same trends have been observed in the application of other API 17 series subsea equipment. API 17TR12, Consideration of External Pressure in the Design and Pressure Rating of Subsea Equipment, First Edition, provides guidelines for application of differential pressure in the design of pressure-containing subsea equipment. Whilst API 17TR12 provides general guidance, the application of
differential pressure varies across different product types, and the product-specific requirements are detailed in the relevant API 17 series specification.
For unbonded flexible pipes, API 17J, Specification for Unbonded Flexible Pipe, Fifth Edition, updated May 2024, is the relevant design specification. API 17J now facilitates the application of differential pressure in the design of unbonded flexible pipes.
This report provides detailed guidance on how flexible pipe manufacturers implement differential pressure in their design practices in compliance with API 17J. It includes diagrams intended to aid interpretation and to ensure consistent application.