Investigation of the soundness of structures

A vibrator unit for use in investigating the soundness of structures, said vibrator unit being adapted to be lowered to a location within a bore hole within said structure, having an expansion chamber arranged such that upon extension thereof the vibrator unit is coupled at a selected position within the bore hole to the sides thereof, a vibrator within said unit adapted to vibrate in a direction parallel to the axis of the bore hole and to thereby generate polarized shear waves radiating through a sub-surface ground from the region of the vibrator unit, said vibrator unit comprising:

• a thick-walled cylindrical electrically-insulated shell provided with similar thick-walled end caps tightly and sealingly fitted to the ends of the cylindrical shell;
• a pair of end electrodes respectively fixedly located at opposite ends of said shell and extending thereacross;
• entirely filling the cylindrical space defined between said electrodes and said cylindrical shell, a gel-like silicon material in which is suspended a plurality of fine magnetically sensitive metallic particles;
• means adapted to couple said electrodes to a source of electrical power, passing in fluid tight fashion through one said end cap, and being adapted to provide an electrical current of selected frequency to said electrodes to cause oscillation of said metallic particles between said electrodes to impact an oscillating inertial energy to the gel-like silicon material matrix;
• and wherein said expansion chamber comprises an inflatable capsule attached to one side of the exterior of said cylindrical shell, said capsule being provided with means for connection to a pneumatic source for expansion of said capsule within a bore hole to positively locate said vibrator unit at a predetermined location within said bore hole.

A method of monitoring the soundness, structural integrity and performance of a structure over a period of time, the method consisting of:

• producing a vibration wave over a large frequency range;
• transmitting said vibration wave through said structure from a vibration location to produce vibration responses therein;
• sampling the vibration response at a plurality of detection locations on said structure spaced from said vibration location;
• producing a record of the frequencies at which maxima occur in the frequency spectra;
• and
• performing all the aforesaid steps at predetermined intervals of time to determine any variation in frequencies at which maxima occur in the frequency spectra of vibration response at the same plurality of detection locations during the passage of time such that the variation in frequency constitutes the first sign of incipient failure of said structure and wherein the general location of said incipient failure may be determined relative to the detection location by reference to the individual records of said given detection locations.

An off-shore structure provided with apparatus for continued monitoring of soundness, structural integrity and performance thereof over a period of time, which apparatus comprises:

• at least one vibrator mounted on and coupled to said structure at a vibration location above the water-line and being adapted to produce a vibration with a frequency selected within a frequency range and to transmit said vibration as a wave through said structure from said vibration location;
• a plurality of vibration response detectors coupled to said structure at detection locations thereon spaced from said vibration location above the water-line;
• means coupled to said plurality of detectors for effectively producing a record of the frequencies at which maxima occur in the frequency spectra of vibration response over said range at said detection locations;
• control means for operating said apparatus at predetermined intervals and for producing a record of any variation in the frequencies at which maxima occur in the frequency spectra of vibration response at the same detection locations during the passage of time;
• and wherein said apparatus further comprises elongate gauges extending longitudinally of selected critical members of said structure below the water-line, each said gauge having its ends solid with its respective member and including means for causing a vibration to be set up between the ends of the gauge, and means for detecting the characteristic frequency of said vibration.

An off-shore structure provided with means for monitoring its structural integrity and performance, which means comprise:

• elongate gauges extending longitudinally of selected critical members of the said structure below the water-line, each said gauge having its ends solid with its respected member and including means for causing a vibration to be set up between the ends of the gauge, and means for detecting the characteristic frequency of said vibration;
• means for periodically operating said gauges from a location above the water-line;
• and
• means for recording any variation of the detected characteristic frequencies of said gauges indicative of changes in the longitudinal dimensions of said members and thus of their strain states.