Re-design of Environmental Monitoring System Containments

Client

GEMS.

Assignment

Re-design of Environmental Monitoring System Containments.

Offshore Energy re-designed subsea pod containments for the housing of electronic water and tidal monitoring equipment. Client and operational requirements were taken into the design basis, including a clear polycarbonate face plate to inspect the electronics status without opening the pod. The design was undertaken in line with PD5500 pressure vessel code.

Requirement

Offshore Energy was commissioned to look at the re-design of subsea pod containments for the housing of electronic water and tidal monitoring equipment. Client and operational requirements were taken into the design basis, including a clear polycarbonate face plate to inspect the electronics operational status. The design was undertaken in line with PD5500 pressure vessel code. 
A finite element model was developed to determine the imposed stresses and displacements of the component parts of the instrument pod when the assembly is subjected to either an external or internal service pressure of 10bar. The instrument pod assembly was solid modelled using Autodesk Inventor from which a fully meshed tetrahedral finite element assembly was created, loaded and analysed using Femap with NX Nastran.
The images show the post processed NX-Nastran Von-Mises stress contour model of the Instrument Pod pre and post Pressurised for an internal and external pressure loading.

A finite element model was developed to determine the imposed stresses and displacements of the component parts of the instrument pod when the assembly is subjected to either an external or internal service pressure of 10bar. The instrument pod assembly was solid modelled using Autodesk Inventor from which a fully meshed tetrahedral finite element assembly was created, loaded and analysed using Femap with NX Nastran. The images show the post processed NX-Nastran Von-Mises stress contour model of the instrument pod pre- and post--pressurised for an internal and external pressure loading.

The problem we faced was significant operational downtime and poor data integrity, which resulted from repeated failures with the existing buoy electronics containment canisters. Along with difficulty with diagnostics, maintenance and replacement of the existing canisters in-situ, this resulted in poor reliability of the data collection buoys.

The existing canisters were designed to house the full data collection and transmission hardware for all buoy variants (three types of instruments) together with their battery packs and charging systems. This resulted in a complex, large and heavy unit installed low down in the body of the buoy that was difficult to access and handle. If any component within the canister failed, the entire canister had to be either removed and repaired, or replaced. Service or replacement of the canister with the buoy in the water was impossible, and even with the buoy removed from the water (on deck) it was time consuming and not particularly successful due to the hostile environment.

The Solution

The system was redesigned to be more modular with one standard data collection and transmission hardware canister or pod that could be configured for each type of instrument. Due to its compact size and weight, this was designed to be mounted higher in the buoy and easily handled and exchanged via quick release electrical and mechanical connections without the necessity to remove the buoy from the water. The end of the pod was designed with a clear acrylic inspection plate, to enable the service crew to see the pod status visually via an internal LED / LCD display without having to remove it from the buoy. This allowed the crew to immediately determine the buoy status without unnecessarily removing a healthy unit.

The battery pack and charging regulators were separated and moved to another pod that could be changed without changing the healthy data collection and transmission hardware pods. The re-designed modular system:

  • Dramatically increased the system reliability
  • Provided simple and fast diagnostics
  • Enabled only the faulty unit to be replaced, not the full system
  • Enabled servicing to take place with the buoy in the water
  • Significantly reduced the servicing and maintenance time

    The re-design of the pod included a finite element model which was developed to determine the imposed stresses and displacements of the component parts of the instrument pod when the assembly is subjected to either an external or internal service pressure of 10bar. The instrument pod assembly was solid modelled using Autodesk Inventor from which a fully meshed tetrahedral finite element assembly was created, loaded and analysed using Femap with NX Nastran.

    The images show the post processed NX-Nastran Von-Mises stress contour model of the instrument pod pre- and post-pressurised for an internal and external pressure loading.

       

    GA and detailed manufacturing drawings were produced for the fabrication sub-contractor and there after the design dossier

    All design, manufacturing and testing was undertaken in the UK. The pods were functionally tested to meet their design criteria (electronically and mechanically tested to 2 x depth rating). The design package, manufacturing drawing and two completed test pods were delivered to the client on time and to their satisfaction.

     

    The Vessel fabrication in line with section 3.5.1 and section 3.6.2 of PD5500 for internal and external pressure capability. 

    The photographs show the finished tested pods prior to handover to the client.