Session: 04-01-05 Flexible Pipes V

Submission Number: 175236

Improving Subsea Connection Execution by Applying the Annulus Condition in API 17J 5th Edition 

Flexible pipe is an unbonded, multilayered structure whose bending and torsional stiffness vary with several parameters. These include layer breakdown, applied loads (i.e., hydrostatic pressure, axial tension, internal pressure and temperature) and sealing of the sheaths, which defines the annulus condition.

When the annulus is considered dry, it implies that the surrounding sheaths are sealed. This results in elevated contact pressure profiles between the steel layers within the flexible pipe cross-section. In contrast, a flooded annulus typically generates significantly lower contact pressures, as the local pressure may equal the external hydrostatic pressure.

Contact pressures are therefore a first-order parameter in deriving the hysteretic bending stiffness, particularly the friction moment. For a single flexible pipe subjected to identical loads, a substantial difference in friction moment exists between a dry and a flooded annulus, with the sealed (dry) condition producing a much higher moment. The same principle applies to the torsional behavior of the flexible pipe.

This mechanical behavior becomes critical during the installation of flexible pipes in the deepwater environment. Several challenges may arise at either the first or second end during tie-in operations. The high stiffness induced by hydrostatic pressure can lead to well-known issues among industry specialists, such as

·       Excessive loading on bend restrictors during the upending sequence of the first end

·       The appearance of characteristic “camel hump” shapes at the second end of the subsea flowline

·       Significant tilting effects from the flexible pipe body toward vertical connectors

·       Torsional effects exacerbated by the alignment of tensile armors

·       Loads transmitted to the subsea production system once the tie-in connection is completed

Therefore, annulus condition is a critical factor influencing the installation configuration and methodology and clarifying it is essential to ensure proper setup and to reduce the risk of damage to the product and bend restrictor.

The API 17J 5th Edition, Specification for Unbonded Flexible Pipe, in effect since June 2025, introduces a new annulus classification philosophy, notably allowing deliberate flooding for “Class 0” annuli. Applying Class 0 annuli can simplify subsea connection operations. Controlled annulus flooding can reduce the mechanical loads on subsea assets, mitigating exposure to high bending and torsional stresses. Moreover, it eliminates the uncertainty associated with the annulus condition, thereby reducing risks linked to sudden changes in annulus state.

Presenting Author: Shahreyar Shahyarfar TechnipFMC

Presenting Author Biography: Shahreyar Shahyarfar is the Global Installation Analysis Manager based in Perth, Western Australia, with over 26 years of experience in SURF installation engineering, project engineering management, and installation analysis. He has worked across Europe, the Middle East, and APAC, focusing on the installation of flexible pipes, umbilicals, subsea hardware, and rigid pipelines.

Authors:

Shahreyar Shahyarfar TechnipFMC
Thierry Gavouyere TechnipFMC
Guillaume Micas TechnipFMC
Mustafa Zavar-Houssen TechnipFMC
Renaud Phelut TechnipFMC