An apparatus for determining various physical quantities such as temperature, pressure, stress, strain, distance and the like in a manner such that a change in the physical quantity of interest results in a measurable change in the frequency of oscillation of a signal generated within the apparatus.
CROSS-REFERENCE
This application is a continuation of U.S. patent application Ser. No. 08/118,090 filed Sep. 8, 1993, now issued as U.S. Pat. No. 5,635,919, which is a continuation of U.S. patent application Ser. No. 07/563,510 filed Aug. 6, 1990, now abandoned.
A method for the direct measurement of large strains in ropes in situ using a plastic optical fiber, for example, perfluorocarbon or polymethyl methacrylate and Optical Time-Domain Reflectometer or other light time-of-flight measurement instrumentation. Protective sheaths and guides are incorporated to protect the plastic optical fiber. In one embodiment, a small rope is braided around the plastic optical fiber to impose lateral compressive forces to restrain the plastic optical fiber from slipping and thus experience the same strain as the rope. Methods are described for making reflective interfaces along the length of the plastic optical fiber and to provide the capability to measure strain within discrete segments of the rope. Interpretation of the data allows one to calculate the accumulated strain at any point in time and to determine if the rope has experienced local damage.
A method is described using optical fiber technology to measure the vibration characteristics of long slender structures subjected to dynamic disturbances imposed by water or wind generated loads. The method is based on making bending strain measurements at selected locations along the length of long slender structures such as marine risers or large ropes using fiber optics technology including Optical Time Domain Reflectometry and Bragg diffraction gratings. Engineering interpretation of information obtained from bending strains determines the vibration characteristics including frequency, amplitude, and wave length. Maximum bending strain measurements assess pending structural damage. One application is measurement of vortex induced vibrations (VIV) response of marine risers. The fiber optics based method is also applicable to the measurement of the bending characteristics of spoolable pipe using plastic optical fibers which can be interpreted to assess the pipe structural integrity and to prevent lock-up during deployment into a small diameter annulus.
