Published at the MTS Underwater Intervention 2017 and 2018 conferences, as well as in the MTS Journal in 2019, this work presents the results of a 5 year study to investigate the use of a honeycomb structure in deep sea submersible window seats as a means to maintain stiffness and decrease weight. As viewing area increases, the window seat required to support a traditional submersible window becomes quite large and a significant weight penalty is incurred due to the discrepancies in yield strength and stiffness between acrylic submersible windows and typical submersible hull materials. This study examined the effect of supporting the window using a honeycomb structure with window bearing surface 'defects' (honeycomb holes) far in excess of those allowable by the ASME-PVHO-1 standard. The study showed that it is feasible to support a window with a honeycomb window seat geometry without jeopardizing the integrity of the acrylic window itself. The study was conducted as a joint venture between OutsideInnovation and the Roatan Institute of Deepsea Exploration.
FEA STUDY
The first research into this topic was conducted using FEA studies of several potential honeycomb configurations. The one shown in the image above is the most traditional, but not necessarily the best for this application due to the requirements for maximum weight reduction and integrity of the acrylic window. It quickly becomes apparent that the maximum allowable hole size is required to determine the optimal packing configuration and density in the window seat design.
LAB TESTING
Following the success of the field test both window seats were tested in a laboratory setting with strain gauges and a displacement sensor to provide measurements to compare to the FEA results. The windows were cycled in a hydraulic press and measurements made to look for residual strain and permanent deformation. After this loading was applied, the top window showed signs of wear and the bottom window remained unharmed. Measured values were in family with the FEA results. Finally, hydrostatic loading was used to determine the STCP of the windows. Final values were within 75% of those specified in ASME-PVHO-1.
TEST WINDOW SEATS
The two window seats shown above were built for field and lab testing. The traditional honeycomb was forgone at this stage due to budget constraints and the desire for a fast fail approach. Using the larger hole patterns provided more information for determining the maximum allowable hole size, but risked under-performing compared to a traditional window. This risk was deemed acceptable at the time. Ultimately, the window seat in the top image was tested on the submersible Idabel on 100+ dives to an average depth of 1500 ft with no adverse effects. This is a low number of total cycles, but an intriguing result nonetheless.
