We were asked to produce detailed fatigue analysis of three sub-sea mining machines in a very tight timescale. Due to the large size of the machines, we applied initial fatigue screening analyses to global shell models. These models were used to identify potential regions of high fatigue damage. Once we had identified these critical regions, we analysed local solid sub-models. This approach meant that the appropriate amount of detail was applied only to areas where it was required: thus, saving a significant amount of time and reducing costs.
Our client, a global manufacturer of subsea vehicles and ROV, had designed a brand-new suite of subsea mining machines to extract mineral-rich deposits from the seabed. Weighing several hundred tonnes and operating at over a mile down on the seabed (close to volcanic vents) for weeks at a time, meant it was essential to ensure that the machines were structurally robust enough to deal with the harsh environment and the high number of load cycles they would be exposed to. All of the vehicles had to be assessed to DNVGL-RP-C203; this assessment represented part of a significant project milestone which had to be achieved within an incredibly short timescale.
We were asked to support a large number of tasks related to the project. A critical task we took responsibility for, was the detailed fatigue analysis of all the vehicles. The scope of work involved the initial derivation of all the applicable loads and the estimated number of cycles. As the client had not designed this type of machine before, PDL engineers drew upon their vast range of fatigue experience to support the generation of justifiable and logical cases. Given that a number of machines (3) needed to be assessed and tight timescales adhered to, it was vital that an accurate, consistent and efficient approach was taken. PDL engineers used their standard processes and procedures to ensure that this was the case. In addition, PDL engineers supported many other key tasks on the project.
Due to the urgent nature of the work, we deployed a number of our engineers to work through several items in parallel. We spent some time at the client’s site, which was most beneficial during the initial analysis planning stage. Once we completed this preliminary stage, we conducted most of the work from our offices, where access to our high-performance computer cluster allowed analyses to be solved quickly and efficiently.
Due to the large size of the mining machines, as well as the local details within the design, a global and local analysis approach was taken. We applied an initial fatigue screening analysis to global shell models of the vehicles; these were used to identify potential regions of high fatigue damage. Once we had identified these critical regions, local solid sub-models were used to capture the effects of details such as welds. The analysis work was completed using a combination of ANSYS Workbench and ANSYS Mechanical within the ANSYS Parametric Design Language (APDL) graphical user interface (also referred to as ANSYS Classic). Using APDL allowed us to utilise some customised scripts for the fatigue assessment to DNVGL-RP-C203. By using scripts to output the damage plots automatically, we could easily compare the results to the relevant allowables. This approach meant that the appropriate amount of detail was applied only to regions where it was required: saving a significant amount of time. Following completion of the analysis, all work was written up within detailed fatigue reports for each machine which were subsequently reviewed and approved by DNV.
- We ensured that the machines were suitably designed and robust for the operating environment to the relevant code standards
- Our accurate modelling allowed any changes to be made at the design stage prior to manufacture saving time and money
- Our methodology reduced time to generate the required documentation and obtain certification
- Using the global and local analysis methodology saved a significant amount of time and reduced our client’s costs