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Full-scale resonance testing of subsea connectors to predict fatigue capacity

A TWI Industrial Member company, Aker Subsea, wanted to confirm fatigue design calculations for its subsea connectors. TWI carried out full-scale resonance fatigue testing to determine the location of failure and the fatigue strength of the connectors.

Design verification 

Subsea components are often subject to cyclic loading when in service, so operators and connector designers need to understand the fatigue strength of new designs.

Finite element analysis (FEA) is often carried out to predict failure locations. However, in some cases, the location of failure predicted by FEA can be different to those occurring in practice.

Full-scale fatigue testing of components under controlled conditions and realistic cyclic loads are used to validate the results obtained by the finite element models. If there are discrepancies, the models can be updated and reassessed based on the results of full-scale testing.

Connector in the TWI-designed resonance fatigue test machine
Connector in the TWI-designed resonance fatigue test machine

Test and analysis 

Aker Subsea provided four connectors within two 7m long test specimens for testing. The specimen ends were blanked off so that they could be filled with water and pressurised to check that the integrity functioned during the tests.

Usually in a resonance fatigue test, a high internal water pressure is used so that when a through-wall crack develops by fatigue, the internal water pressure is released and the test is stopped automatically. In this case, the design of the connector was such that the internal seals would prevent the release of the internal pressure, so could not be relied upon to stop the test. As an alternative, TWI identified suitable locations for the placement of strain gauges, which would measure a rapid increase in strain when the connector cracked. Data recording software was used to stop the test automatically when the measured strain exceeded set values.

The team tested the connectors in a TWI-designed resonance fatigue test machine. Cracking in both connectors was indicated by the strain gauge readings.  

TWI then opened the connectors and carried out dye penetrant and magnetic particle inspection on the fatigue critical locations. This identified a number of cracks in locations that had not been highlighted by the FEA.

Higher fatigue strength 

The project demonstrated results from full-scale resonance fatigue tests on two subsea connectors, which had never been subjected to testing before. The connectors’ seals allowed the connectors to hold pressure for the full duration of the tests.

The connectors had a higher fatigue strength than the design suggested and the previously identified fatigue critical locations did not crack. Aker Subsea was able to re-evaluate the criteria used in its finite element models and calculations for fatigue life, based on the results of the full-scale fatigue tests.

Fatigue testing of assembled components, such as subsea connectors, is a useful way of verifying connector designs. FEA is a necessary and useful tool to predict fatigue capacity of assembled components but fatigue testing is the bridge and safety barrier between conservative FEA calculations and much more realistic scenarios.

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

Fatigue crack detected with dye penetrant inspection after the fatigue test
Fatigue crack detected with dye penetrant inspection after the fatigue test
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