GB2094476A

GB2094476A - Stress monitoring apparatus

Info

Publication number: GB2094476A
Application number: GB8106954A
Authority: GB
Original assignee: University of Southampton
Current assignee: University of Southampton
Priority date: 1981-03-05
Filing date: 1981-03-05
Publication date: 1982-09-15

Abstract

A data processor 10 controls the sampling of a plurality of strain gauge transducer units 14 distributed along the bottom plating structure of a ship. For each sampling cycle, of which there may be one thousand per second, the maximum stress is determined using a curve fitting algorithm and the stress measurements for some period, say the last two minutes, are stored in a memory 12. The data processor operates upon these measurements and determines the probability of a predetermined level being exceeded. The calculated probability is displayed on a display device 18 or an alarm device 20 may be operated when the probability rises to a particular value. The Officer on the bridge of the ship is thus warned when the probability of an unsafe stress level being exceeded becomes too high and can then take remedial action such as reducing speed or even preparing to abandon ship. The invention can be applied to other structure subjected to wind, water, other fluid or mechanical forces fluctuating over a very wide range. <IMAGE>

Description

SPECIFICATION Stress monitoring apparatus The present invention relates to apparatus for monitoring the stresses experienced by a fixed or moving structure which is subjected to fluctuating loads. The invention is particularly, but not exclusively, applicable to structures in the ocean subjected to the forces of waves. The structure may then be a vessel or a fixed oil rig for example.

The invention may also be applied to structures subject to fluctuating aerodynamic or other fluid or mechanically induced stresses.

It is known that the stresses arising at sea from the elements vary over an exceedingly wide range and, when weather conditions are severe, there is a certain probability of a freak wave or gust of wind which can be extremely dangerous for a ship or other structure. Although techniques for calculating probabilities of stress levels being exceeded in such circumstances are well known in themselves, there exists no convenient way of taking advantage of that knowledge in the practical situation of a ship at sea, an off-shore oil rig and so on.

Considerable work has also been done in collecting statistical data on stresses in ships. See for example a paper by Ward and Katory "Data on Midship Bending Stresses from Four Ships" published in International Symposium on the Dynamics of Marine Vehicles and Structures in Waves, April 1974, University College London.

This paper describes research undertaken by the British Ship Research Association. In this research strain gauges were installed in a number of ships and data were collected over substantial periods and recorded on tape for subsequent analysis. The purpose of the analysis was to predict stresses under various weather conditions etc with a view to improving ship design. Again this work did not relate to any system of practical use at sea.

The object of the present invention is to provide an apparatus with a continuous monitoring function whereby it is possible always to be made aware of ruling conditions, to the end that remedial or avoiding action can be taken when there is a certain probability of an unacceptable stress level being exceeded. In some situations remedial action may be taken automatically; in other situations the apparatus may simply give a warning calling for human intervention. The nature of the -action taken will depend upon the nature of the structure in which stresses are monitored. In the case of a ship the first action may simply be to reduce speed. In the case of an oil rig, or at a very high probable stress level in a ship, it may be appropriate to signal a warning to prepare to abandon or actually abandon the ship or rig.

According to the present invention there is provided apparatus for monitoring stresses in a structure comprising at least one strain transducer providing a signal representing the strain in part of the structure, data processing means responsive to the signal from the or each transducer over a period of time to determine in accordance with a predetermined probability function or functions the probability of a predetermined strain and consequent stress level being exceeded, and means for providing an indication of the probability thus determined.

The indication may be given as a continuous digital or analogue readout or intermittent printout of the probability itself which in practical terms may lie in the range 102 to 108. In other words, it is indicated where the probability lies in a range from 1 in a 100 to 1 in a 100 million. An alternative is simply to indicate when the probability increases above one or more threshold values. The probability information may be supplemented with an indication of the trend thereof which will be given simply as the sign of the rate of change of the probability with time or as the algebraic value of the rate of change of probability.

In the simplest application the digital or analogue processing means may sample the strain signal(s) at regular intervals and allocate each processed measurement to one of a plurality of small stress ranges. The processing means also total the number of measurements in each range, so that, a histogram of frequency distribution can be constructed. From the histogram, using known mathematical techniques, it is possible to determine the mean stress value and the standard deviation or other measure of dispersion of the stress values, related to a Gaussian, Rayleigh or any other relevant distribution function appropriate to the structure being monitored and the stresses to which it is subjected. Given the mean and say the standard deviation, it is a staight forward and well known calculation to determine the probability of exceeding any given stress value.

In a more sophisticated approach a double exponential distribution function is employed, e.g.

as described in the aforementioned paper by Ward and Katory.

A practical system embodying the invention will now be described by way of example with reference to the sole figure of the accompanying drawings, showing the main elements of the system in block diagram form. Overall control of the system is effected by a data processor 10, e.g. a microprocessor dedicated to this function. The data processor has at its disposal a memory 12 which can incorporate a ROM for programme information and a RAM for stage of data collected and processed. The data processor communicates with a plurality of transducer stations 1 4 by way of an interface unit 1 6. Two transducer stations are shown by way of example although it is preferred to utilise more, say four stations. Each transducer station comprises a strain gauge, an anaiogue-to-digital converter and means for sampling the analogue-to-digital converter.The strain gauges are spaced along the bottom plating structure of a ship and the output of each gauge is sampled at say one thousand times per second.

The data processor 10 can poll the transducer units 14 in turn in conventional manner to collect the data in this way.

The reason for using sayfourtransducer units spaced along the ship is to obtain a measure of the peak stress which will occur at a position varying along the length of the ship. A simple curve fitting algorithm applied to the four measurements of each sampling cycle will provide an estimate of the peak stress. Accordingly one thousand peak stress measurements will be obtained each second and the data may be stored over a two minute interval so that the last one hundred and twenty thousand measurements are continuously stored.

The data processor 10 is programmed to operate upon statistical information, effectively to provide a prnbabflity or distribution function and then to calculate the probability of a certain danger stress level (or series of levels) being exceeded. The calculated probability or probabilities are continuously displayed on a visual display device 18. In addition, the data processor may be connected to an audible alarm device to trigger an audible alarm when the danger stress level, or a particular one of the series of levels, has a certain probability of being exceeded, considered unsafe. At least the display device 18 and alarm 20 can be installed on the bridge of the ship so that the officer in charge will be warned when it is necessary for him to reduce stresses by reducing speed or even take more drastic action such as preparing to abandon ship.

Claims

1. Apparatus for monitoring stresses in a structure comprising at least one strain transducer providing a signal representing the stress in part ol the structure, data processing means responsive to the signal from the or each transducer over a period of time to determine in accordance with a predetermined probability function or functions the probability of a predetermined stress level being exceeded, and means for providing an indication of the probability thus determined.

2. Apparatus according to claim 1 wherein the means for providing an indication provide in respect of at least one predetermined stress level a digital or analogue read-out of the probability of that level being exceeded.

3. Apparatus according to claim 1 or 2 wherein the means for providing an indication comprise an alarm device providing an alarm signal when the probability of a predetermined stress level being exceeded exceeds a predetermined probability value.