The steel Mobile Arctic Caisson has had a flexible Engineering/Scientific data acquisition system installed for operational and performance monitoring. The instrumentation used, ifs installation, commissioning "and operation are described. A number of system enhancements have been made, demonstrating the need to provide system flexibility.
Gulf Canada?s 'Molikpaq? (Figure 1) is a bottom founded mobile Arctic structure capable of year round operation in water depths from 15 to 40 metres. In form it consists of a continuous steel caisson ring supporting a drilling deck on rubber bearings. The core is filled with sand for resistance to ice loadings with provisions for keeping the sand core and water ballast from freezing. The MAC was delivered in the spring of 1984 and has operated two complete w-inter drilling seasons. An instrumentation programme was developed to estimate the structural soundness and stability of the composite structure (MAC, core, berm and Sea bed foundation) and to verify the overall safety of the structure. Preparations were initiated early in the project with over 500 channels of data being specified. The sensors were concentrated on the sides predominantly subject to ice loadings (North, Northeast and East). The architecture of the Data acquisition system (see Figure 4) concentrated most instrumentation in a central signal conditioning room in the inner deck area with operator control from the main control room on top of the accommodation module, below the helideck about 100m away. This was done to minimize signal cable lengths and to take up a minimum of space in the control room. The control room equipment fit into a single standard rack approximately 2 ft by 2ft and 6ft high with no back access. Into this limited space, the main minicomputer, CRT display, printer, D/A controller, power supply and two 9 track tape recorders were placed.
The 500+ channels of conditioned data were to be scanned at a rate of one complete scan every two seconds with 12 bit resolution. The equipment was protected against expected power surges and was to be self starting after a power failure. A backup data logger was provided on approximately 100 channels, mostly stability related signals. This system was considerably slower with a scanning rate of one scan every 10 seconds. Each system had self checking features and alarm annunciators to identify faulty components.
The sensors consisted of the following:
282 Ailtech weldable strain gauges supplied with integrally connected cable with water blocking and low temperature flexibility and microwelded to the caisson structure.
40 total pressure cells (full bridge strain gauge transducers)
12 pore pressure cells (full bridge strain gauge transducers)
8 extensometers; caisson (linear potentiometer biaxial wire line)
8 tiltmeters;caisson (force balance servo accelerometer)
2 extensometers; guidepipe (potentiometer wire line)
1 tiltmeter; guidepipe (force balance servo accelerometer)
36 inplace inclinometers (biaxial)
30 electrical piezometers
31 ice load panels.
