Flexible pipes have been used to transport oil and gas in subsea applications for more than 40 years. Since 2010, and following a significant qualification work performed within Technip flexible pipe technology center, it has been possible to use a low plasticized high molecular weight PVDF with enhanced mechanical properties as monolayer pressure sheath in flexible pipe applications. This qualification work covered both static and dynamic applications. A large number of projects have already taken advantage of the monolayer design for high temperature applications. Indeed, more than 80km of flexible pipe with this PVDF grade have already been manufactured.
This paper will present the work that has been carried out during the last 5 years in order to extend the qualified domain of use of this PVDF grade:
-Maximum design temperature extended from 130°C to 150°C,
-Minimum design temperature extended from −20°C to −30°C,
-Design pressure extended up to 17.5kPsi.
A specific focus will be made on the results obtained on several tests performed at lab scale and on full scale flexible pipes and which have been used to support this extension of the domain of use.
Such tests include the verification of the PVDF grade to sustain rapid gas decompression after exposure to gas medium at high pressure (> 17.5kPsi) and high temperature (150°C). Permeation tests have also been performed at 150°C.
Four full scale tests have been performed on a 6″ flexible pipe with design pressure equal to 17.5kPsi. The first test was performed on a flexible pipe sample submitted to a hydrostatic pressure test at a pressure reaching 27.5kPsi. The second test was performed on a flexible subjected to a fatigue test (tension and angle variation) simulating flexible pipe loading in operating conditions. A burst test was performed on a third flexible pipe sample. During this test, a burst pressure equal to 37.9kPsi was reached. Finally, a fourth flexible pipe sample was tested through the realization of a large number of large bending cycles while keeping the flexible under internal pressure.
The possibility to use this PVDF grade with enhanced mechanical properties down to −30°C has been demonstrated through the realization of thermal cycles between 150°C and −30°C on two flexible pipe samples. This test is realized in order to confirm sealing efficiency towards thermal stresses which occur during shutdowns. The result of a specific full scale test performed, after ageing at 150°C, in extreme conditions of bending and temperature (lower than −30°C) will also be presented.
This extension of the qualified domain of use enables the use of flexible pipes in harsher applications, and also further substantiates the robustness for the use of this grade within the initial qualification domain.