Unplasticized copolymer tested for high temperature flexible lines

Inner liner material less vulnerable to hydrocarbons

Michael Hallberg
NKT Flexibles

• The photos illustrate the toughness of the rotary bending fatigue test method. After only 4000 cycles, the riser has developed a severe "pigtail." [4,442 bytes]
• Photograph 2. [6,430 bytes]
• Axial load variation during a thermal cycle for a 8-in. PVdF pipe sample. [36,013 bytes]

Traditionally, unbonded flexible pipe for high temperature applications has been based upon the usage of plasticized PVdF (Polyvinyl ideneflouride) as the inner liner material. Recently, severe problems with this particular material have developed because of the loss of plastiziser.

At elevated temperatures hydrocarbons may switch place with the plasticizer molecules causing the PVdF to shrink and become brittle. This has led to a series of failures of both flexible pipe structures and end-fittings.

NKT's flexible pipe solution for high temperature applications is to use an unplasticized PVdF copolymer, which is not susceptible to the shrinkage problem. The individual PVdF molecules are chemically bound to the molecules of the plasticizing component. As such, the hydrocarbons cannot wash out any parts of the PVdF copolymer, thus making it a very stable material.

Qualification, testing

During 1997-1998, NKT carried out a test program in order to qualify the PVdF copolymer in accordance with the requirements stipulated in API Specification 17J and API RP 17B. The qualification program encompassed material and full scale tests performed within a temperature range of -20°C to +130°C. The general material qualification of the PVdF copolymer was likewise undertaken by the raw material supplier, Solvay (Belgium).

All material tests were carried out on un-aged as well as aged PVdF copolymer cutouts of flexible pipe samples manufactured at NKT's factory in Kalundborg, Denmark. The NKT flexible pipe design for both static flowlines and dynamic riser applications are based on the results from the material tests. Some of the most important test results derived from the test program follow are described below.

• Chemical compatibility: Compatibility tests at elevated temperatures with crude oil as well as a number of representative groups of injection chemicals were conducted.

For each of the environments, PVdF samples were exposed for 30 days at 23°C, 60°C, and 100°C. Subsequently, tensile properties were tested and weight changes were recorded.

All of this testing has demonstrated that no short term effects of any of the above environments can be detected. A certain amount of swelling has been observed after conditioning of the PVdF copolymer in crude oil. For a flexible pipe design, this is much less onerous than the material shrinkage experienced in connection with loss of plasticizer in the plasticized grade of PVdF. Further, the swelling will tend to induce compression in the PVdF copolymer, which is much more desirable than tensile stress. From an end-fitting sealing point of view, the swelling will even have a positive effect by inducing additional force to the mail sealing faces.

• Blistering: Blistering resistance tests were conducted by BG Technology in the UK. A series of tests were carried out with temperatures and pressures in the range of 85-130°C and 82-345 bar, respectively. The results showed a good resistance of the material to the blistering phenomena and full compliance with the specific accept criteria. Tests of the mechanical properties of the material after being subjected to the blistering test were carried out and no effect on the material was detected.
• Notch sensitivity: Notch sensitivity tests were conducted by UCL (UK) and by Solvay (Belgium). A series of tests were carried out with temperatures in the range of -20°C to 23°C. The tests showed less-than-expected sensitivity to artificial notches simulating possible and typical irregularities. Of particular importance is the fact that the unplasticized PVdF will not develop the brittleness which is characteristic of the conventional plasticized PVdF.
• Fatigue: Fatigue tests of PVdF samples were carried out at Force Institute (Denmark) and by Solvay. These tests were carried out using both machined samples and samples with full wall thickness which were taken from as-extruded PVdF inner liner in order to test the inherent surface features.

The strain rates varied between 3.5% and 8% and the test samples were subjected to 5,000-1,000,000 cycles to prove the fatigue resistance. This is far in excess of the number of cycles a flexible riser will ever see at these high strain rates. The maximum allowable strain defined for NKT dynamic risers with PVdF inner liner is 3.5% during storage and 2.3% in service.

Following the fatigue tests, the test specimens were subjected to tensile testing and no significant reduction in mechanical properties was detected. The fatigue tests showed that the PVdF copolymer has good fatigue properties.

• Rotary bending fatigue test: A significant milestone in the comprehensive qualification test program for the PVdF copolymer was reached by the successful completion of a rotary bending fatigue test of an 8-in. ID dynamic riser with a PVdF copolymer inner liner.

The purpose of the test was primarily to verify the integrity of the PVdF co-polymer when used as an inner liner in a flexible riser and secondarily as a dynamic fatigue testing of the steel armour wires. The flexible pipe will develop a pigtail, which will further accelerate the test due to severe local overbending of the pipe structure.

In this test, the actual bending radius in several locations was no more than 2.0 meters as opposed to the specified minimum bending radius (MBR) of 3.4 meters.

The period of testing was five months, during which the flexible riser successfully completed more than 1.2 million rotations at full design pressure of 345 bar. The test program was initially based upon 800,000 cycles.

Failure mode of the test was cold-flowing of the PVdF copolymer inner liner upon loss of structural support by the C-formed hoop stress layer in a single location. Due to a fatigue crack in one of the C-armour layers, caused by the excessive overbending of the pipe body, a small opening developed. This allowed the PVdF copolymer inner liner to creep until reaching a stage of leakage. This was an expected failure mode for this particular type of test.

During the dissection of the flexible riser, the PVdF inner liner was carefully examined.

Areas of particular focus included material characteristics, wear and tear, notch sensitivity, crack development, cold flow of the polymer as well as possible fatigue damage of the copolymer resulting from the rotations.

No indications of degradation in the PVdF copolymer were detected.

• Thermal cycling test: The past has shown a limited service life for plasticized PVdF due to the loss of plasticizer. This has, in particular, resulted in insufficient sealing in the end-fitting leading to failure of the flexible pipe.

In June 1998, a high temperature test was initiated at Sintef (Norway) in their thermal cycling test rig as part of a joint industry project (JIP). The JIP is being sponsored by Exxon, Norsk Hydro, Saga, Shell, and Statoil. The test sample consists of an 8-in. ID pipe sample with a PVdF copolymer inner liner.

The PVdF pipe is subjected to cyclic temperature variations between 5°C and 125°C. One temperature cycle has a duration of approximately 48 hours (24 hours at constant low temperature, followed by 24 hours at constant high temperature). The induced axial thermal load in the pipe sample is continuously measured using a load cell. An accompanying figure shows an example of the axial load variation during thermal cycling of the pipe. Also, the axial movement of the inner liner is monitored during the test via inspection holes in the end fittings.

At the time of writing, the test pipe has been subjected to more than 100 temperature cycles without leakage or any detectable axial movements of the PVdF inner liner.

Extensive testing of this new unplasticized PVdF copolymer, in accordance with the requirements stipulated in API Spec 17J and API RP 17B, has confirmed that none of the good properties of the PVdF with regard to chemical compatibility at elevated temperature have degraded, when compared with the traditional grade of plasticized PVdF.

The most important quality of this new grade of PVdF is, however, the fact that there is no plasticizer in the product, thus no plasticizer can be lost during the service life of the flexible pipe. The full scale tests have shown that the PVdF copolymer is, in fact, a very robust and reliable material to use as inner liner in flexible pipes.

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