Example sensor arrangement
Without hull stress monitoring systems (HSMS) there would be no way of recording the dynamic level of stress absorbed by a vessel's hull or the fatigue limit.
Safety-One, a small company based in Kristiansand, has developed a new generation integrated HSMS, Sensfib 2000, using fiber optics both in sensors and signal transmission. The system has attracted widescale interest, and gained the Seatrade "Safety at Sea" award this May in London.
Development of the fiber optic technology will be of interest to the offshore industry with respect to ship hull constructions, and is particularly relevant to hulls of existing ships undergoing conversion to FPSOs.
FPSOs and most offshore installations, in contrast to other ships' hulls, absorb stress from one direction only during their entire operational lifetime.
Existing loading instruments calculate and display static stress values for cargo and ballasting conditions in calm water only. Limits are stipulated by classification.
Sensfib 2000 displays continuously the actual level of stress, static and dynamic, absorbed by the hull at each sensor location (usually located in most exposed areas), and can relate historic values to fatigue limits.
Fatigue is a function of the dynamic stress amplitude and the effective number of load cycles. Sensfib 2000 records both dynamic stress and numbers of load cycles.
Furthermore, Sensfib 2000 can record global stress - to give early warning of a disaster that may happen, unless action is taken - and local stress in the most exposed areas, to warn of local fatigue damage.
The fiber optic technology allows any number of sensors to be fitted anywhere in a hull including inside oil or gas tanks, as fiber optics present no risk in explosion risk areas. The sensors are small with no moving parts, they require no physical calibration and have a very long expected lifetime.
Sensfib 2000 can integrate other data systems (for instance weather, speed/torque, wave, GPS) and can be incorporated into other systems (i.e. complete bridge systems). The data can be transmitted ship/shore as required, via satellite or manually. The system has a patent pending.
The fiber optic technology developed can record stretch, compression, pressure, vibration and temperature, which gives it great potential for developing further applications in the industry. Sensfib 2000's fiber optic cable can be arranged as a highway for all onboard signal transmissions. Safety-One also offers fiber optic lighting on deck, in tanks and in accommodation areas.
This new product has been developed in co-operation with Statoil, DNV and the Ugland group. The prototype system and three sensors were installed last November and early this year a further 15 sensors were installed. The system has been recording stress data extremely successfully.
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