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Frequency scanning testing of armour wire

TWI Industrial Member company, Exxon Mobil, recently approached TWI to develop a testing programme to determine the effect of loading frequency on fatigue crack growth rate (FCGR) of armour wire in corrosive environment. The work has produced control data that will allow enhanced product design for offshore applications.

Specialised test programme

Armour wire in both flexible risers and flowlines offshore can be subject to corrosion fatigue. As corrosion pits may develop during service, knowledge of the effects of corrosive environment and loading frequency on FGCR is essential for fatigue design of armour wires. TWI developed a frequency scanning test to investigate these effects.

Figure 1 TWI facilities used for the FCGR testing of armour wire.
Figure 1 TWI facilities used for the FCGR testing of armour wire.

Single-edge notch bend specimens with a rectangular cross section of 4 by 11mm were machined from armour wires with a cross section of 4 by 12mm.  FCGR tests in a simulated sour environment were carried out under three-point bending using specialist facilities available in TWI (Fig.1) where environmental conditions can be carefully controlled to represent those in service.

To determine the effect of loading frequency on FCGR, the team carried out frequency scanning FCGR tests where the load range was gradually and automatically decreased after each small increase in crack length to keep stress intensity factor range, DK, constant in each test. The loading frequency was decreased in regular steps after a certain crack extension.

Effect of loading frequency on crack growth rate

TWI successfully determined the effect of loading frequency on FCGR in the sour environment for the armour wire specimens. The figure below shows the FCGR in the sour environment normalised with the FCGR given in BS 7910 for steels in air. It can be seen that the FCGR of armour wire in the sour environment increased sharply with decreasing loading frequency, with a normalised FCGR value being increased from ~3 at 1Hz to ~11 at 0.1Hz, and to ~21 at 0.03Hz [1].

Future developments

Following the success of the tests in the sour environment, we are planning to investigate the use of frequency scanning testing of armour wire specimens in other corrosive environments.

References

[1] Krishnan V K, Asher S, Doynov K and Zhang Y H, 2014: “Flexible armour wires: fatigue load frequency effects and an accelerated pitting methodology”, published in OMAE2014 Conference, 8-13 June, San Francisco, Paper No. OMAE2014-24048.

For more information please email contactus@twi.co.uk

Figure 2 Normalised FCGR versus loading frequency.
Figure 2 Normalised FCGR versus loading frequency.
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