The design analyses methods that are commonly utilised for assessing concrete block mattress hydrodynamic stability are highly theoretical. This may lead to potentially over-conservative designs.
The physical model testing was driven by the need of one of Atteris’ clients to confirm the stability of existing mattresses installed on rock berms to stabilise a flowline. A re-baselining of the project metocean data presented more onerous hydrodynamic loading than allowed for at the design stage, with the result the original design approach concluded the mattresses would be unstable under extreme loading.
The main aim of the testing program has been to better understand the mattress failure modes by subjecting the subsea system to a combination of extreme hydrodynamic loading and lateral loading via the flowline. Of particular interest to Atteris has been the mattress’ capacity to resist the loads applied by a flowline when both are subjected to hydrodynamic loading.
The video shows:
- steady current testing showing the rock on the berm became unstable before the mattress itself,
- regular wave test with small incremental mattress displacements with each wave once the load on the pipe reaches a critical level,
- irregular wave tests with a single large wave in the wave train (in combination with pipe load) causing a large lateral displacement of the mattress.
Atteris has used the test results to demonstrate that the global stability of the existing design is acceptable; thus eliminating the need for the planned high-cost project to place a large volume of quarry rock over the mattresses.
Atteris thanks University of Western Australia’s COEL staff for their great support during the testing program.