Karianne Pran

Ship hull structure monitoring using fibre optic sensors

We discuss the need for ship hull monitoring and the roles such a system may fill during the different stages of a ships lifetime. We have found that fibre optic sensors are well suited for this application and present the fibre Bragg grating technology that has been employed in the composite hull embedded sensor system project (CHESS). Signal processing is the key to real-time structure monitoring, and we comment on the modular signal processing system that is being developed at FFI. Finally we present the CHESS installation on a new Norwegian naval vessel and show some results obtained with this system in systematic sea-keeping tests. The CHESS measurements on the new Norwegian fast patrol boat have been instrumental in the design verification of the vessel.

Transient load monitoring on a composite hull ship using distributed fiber optic Bragg grating sensors

We describe strain data recorded using fiber optic Bragg grating sensors mounted on the hull of a GRP composite ship. Twelve gratings were attached to the structure, in three arrays of four elements. The electro-optic system used was able to monitor a single set of four elements at a time. The preliminary results indicate the usefulness of distributed fiber Bragg grating sensor systems for monitoring transient loading events on such structures.

Wavelength multiplexed fibre Bragg grating system for high-strain health monitoring applications

We have developed and demonstrated a fibre Bragg grating strain gauge system for high strain applications using spatial and wavelength multiplexing. The system takes advantage of a priori knowledge about the strain levels and relative phases between the different sensor signals to accommodate many sensors per fibre in an application where we expect to measure several thousand microstrains. In combination with scanning Fabry-Perot filter interrogation, this principle has been utilized in the design of a system for health monitoring of a naval surface effect ship and the system has been used in long-term load monitoring on the vessel.

Wet deck slamming experiments with a FRP sandwich panel using a network of 16 fibre optic Bragg grating strain sensors

Drop test experiments have been performed with a FRP sandwich panel instrumented with a network of 16 fibre optic (FO) Bragg strain sensors, together with conventional electrical strain gauges for control and verification. The drop tests simulate slamming loads on the wet deck of a surface effect ship (SES). The objectives were to show the possibility of using a network of FO sensors to monitor strain during a slamming impact, and to test out a technique for signal processing. The strain measurements provided both peak strain data and served as a base for frequency analysis. The results showed that the FO strain sensors performed satisfactorily and were in general agreement with the conventional strain gauges used. The FO interrogation system was, however, not designed with sufficiently large dynamic range for the most extreme drop sequences. The peak strain in the panel was found to increase almost proportionally with the drop velocity, or drop height, and the wet fundamental frequency increased with increasing drop angle. Furthermore, the frequency decreased with increasing drop velocity.

Instrumentation of a high-speed surface effect ship for structural response characterization during sea trials

We report on the instrumentation of a high-speed air-cushion catamaran (Surface Effect Ship) with more than 50 fiber optic Bragg grating strain gauges, as well as conventional resistive strain gauges, accelerometers, a Motion Reference Unit and Global Positioning System. A bow mounted wave radar was used to characterize the sea-state in order to estimate the wave loads on the hull. The relatively large number of strain gauges enabled us to determine the global deformation modes of the hull as well as local stress concentrations. This instrumentation was installed on a new Norwegian naval vessel and employed during sea-keeping tests in smooth and rough seas off the Norwegian coast. The measurements enable a detailed characterization of the vessels dynamic response to wave loading and comparison with Finite Element Analysis modeling of the ship. The experimental results provide invaluable information for the subsequent development of a system for health monitoring of the structure. We present the instrumentation layout and selected results.

Fiber optic system for ship hull monitoring

The authors describe a system which will, at about 10 installations in continuous operation after 2002, be the first fiber optic sensor system for ship hull monitoring with a wide user base. The experiences made with these systems in the coming years will provide valuable information on the design, reliability and possible use of such systems, and provide a basis for further development.

Composite plate drop test using a 16-channel fibre Bragg grating strain sensor system

Under the Composite Hull Embedded Sensor System (CHESS) program, which is a co-operative program run jointly by FFI and NRL, a drop test with a composite sandwich plate has been performed [1]. The plate was designed to have similar mechanical properties to a bottom panel in the Norwegian Navy’s new fast patrol boat (MTB).

Fibre Bragg grating smart bolt monitoring creep in bolted GRP composite

We have carried out a long-term test of the tension in bolted glass fibre reinforced composite plates that shows that significant tension is left in such joints after one year, and that re-tightening is an option. The tests have provided valuable information that would be very difficult to obtain with conventional techniques, and are a success despite practical difficulties: The data are strongly affected by the thermal expansion of the composite, and we see large contributions from the insufficient adherence of the acrylate coating to glass fibre. With minor redesign the principle should be applicable to field monitoring.

Application of wavelets for transient detection and characterization in health monitoring systems

We demonstrate the use of a wavelet-based system for detection and characterization of transients. Traditional matched filtering is implemented in the wavelet transform domain, providing us with efficient algorithms with potential for reduced noise and improved signal separation. These O(N) algorithms have been applied to experimental data from a sea trial, demonstrating their potential for online analysis of sensor data in health monitoring systems.