The aim of one of TWI's biggest-ever mechanical testing jobs was to confirm the integrity of large-diameter pipe as it undergoes installation from a lay barge to the bed of the North Sea.
'We're intending to introduce a new type of fabricated pipe on our Pickerill development in the North Sea', said Sid Mendall of Member company Arco British. 'It's made by the electric resistance welding process and we need to demonstrate to ourselves and to the Department of Energy that it has satisfactory properties'.
In preparation for the test, a length of the pipe was strain gauged at midspan on its top and bottom external surfaces, and mounted in one of TWI's test rigs between supports 4.5m apart.
The specimen was loaded using two hydraulic actuators at 1.5m centres beneath the pipe until the designated strain was reached. It was held under load for 30 minutes to simulate a pipe-joining period on the lay barge. Finally it was loaded rapidly to produce twice the already established strain over several seconds, a test to simulate the action of a large wave.
'Because a pipe like this has never been used subsea before, it's necessary to demonstrate that the materials will withstand the strains involved and that there will be no problems with weld integrity,' clarified Mendall.
Strain levels in the test pessimistically reflect what could arise in practice. The test simulates a laying, pulling up and relaying operation, which could be necessary if defects are found after laying.
After the test, Mendall added: 'The pipe has withstood two loading cycles likely to be imposed during installation. The complete test is entirely satisfactory showing no problems with weld integrity. We think the pipe is now proven for installation by the lay barge method'.