GB2195770A

GB2195770A - Inductive deformation transducer

Info

Publication number: GB2195770A
Application number: GB08624030A
Authority: GB
Inventors: Magne Michaelsen; Knut E Ulvund
Original assignee: Norsk Agip AS
Current assignee: Norsk Agip AS
Priority date: 1985-08-28
Filing date: 1986-10-07
Publication date: 1988-04-13
Also published as: NO157357B; GB8624030D0; NO853389L; NO157357C

Abstract

The transducer is especially suitable for use on offshore constructions at great depths. An inductive sensor element 5 and a magnetic measuring element 9 are attached independently of each other to a structure 1, of which the deformation is to be measured. The surrounding sea water can circulate freely between the elements 5, 9. The sensor element and the measuring element are attached to respective first, second main parts 4, 6 of bronze. These are secured by stainless steel bolts 3 to respective plates 2 which are welded to the steel structure 1. An eccentric bolt 8 is rotatable for calibration. The measuring element is attached to a calibration part 7 by a bolt 10 of which the length is determined such that the temperature expansion coefficient for the whole transducer equals that of the structure. <IMAGE>

Description

SPECIFICATION Inductive deformation transmitter The present invention relates to a deformation transmitter for measuring deformation of structures down to large depths, such as offshore structures, loading buoys, pipelines and underwater production systems. The transmitter is provided with calibrator and is to be installed permanently to temporarily. The measuring and supervision of the structure elements which are subjected to stresses, as well as cracks and the development thereof in such elements, is of great importance for detecting an uncontrolled development of possible damages at an early point of time.

Today there are used various types of deformation transmitters, based on principles related to strain gauges with water sealing, swinging strings and inductive transmitters.

As regards structures in sea water down to large depths, it has appeared that the field of application for the above mentioned transmitters is limited by one or more of the following items, the lifetime of the structure (10 to 40 years) being taken into consideration: - they absorb forces due to deformations in the structure and consequently put up large requirements as regards attachment to the structure, - they load the structure by ad pting forces from the structure in the measuring point and consequently influence the result of the measurement, - they are influenced by the water depth, which in turn limits the use on large depths, - they inherit no possibility for recalibration after installation, - they inherit no compensation for temperature changes, -they cannot be moved or replaced after installation, -they are not water tight at large depths, -they exhibit specific side sensitivity (in orthogonal plane relative to the direction of measurement).

By using the inductive deformation transmitter according to the invention, which can be calibrated, there is achieved a large sensitivity as regards absolute and relative elongation, as well as deformation such as crack opening there being especially achieved insensitivity for variations of depths and for loading at the measuring point both during measurement and calibration, as well as a possibility for displacement (or replacement) of the transmitter after installation. Preferably, the transmitter has built into it a passive temperature compensation system. The side sensitivity is very low.

These advantageous properties are achieved in an embodiment of the transmitter as stated in the following patent claims.

A deformation between the two attachment points will detected as a variation in the magnetic field between the sensor element a second magnetic material (measuring element).

Any liquid or gas which does not contain magnetic particles, preferably sea water, could freely encompass the sensor element and the measuring element without influencing the magnetic field.

Thereby is achieved a friction free connection between the two main parts of the transmitter, the sensitivity not being influenced by variations of depth. The calibration of the transmitter takes place by turning an eccentric bolt, such that there is achieved a preset variation as regards distance between the sensor element of the deformation transmitter and the measuring element. This calibration can bee effected statisticly (stepwise setting of the eccentric bolt) and dynamiciy (continuous rotation of the eccentric bolt).

By combining materials having higher and lower temperature expansion coefficients than the coefficient of the structure, there is achieved a passive temperature compensation which eliminates variation in the sensitivity of the transmitter as a function of temperature.

Since the two main parts of the deformation transmitter do not have a mechanical connection with each other, and since the transmitter can be calibrated in situ, the deformation transmitter can be moved or replaced very easily without any deprivation of the accuracy.

The inductive deformation transmitter including calibrator, according to the invention, is disclosed on the appended drawing.

The two main parts of the transmitter, which preferably are made of bronze (due to reasons related to technical production and corrosion), are attached to the steel structure (1) for example through two welded doubling plates (2), by means of two attachment screws (3) of sea water resistant material, for example stainless special steel. The first main part is a fixed part (4) for the anchoring of the inductive sensor element (5) which preferably has a capsule of stainless special steel.

The second main part (6) has an attachment bracket on which a movable calibration part (also of stainless steel, as an example) can be displaced by turning an eccentric calibration bolt (8).

The measuring element (9) is attached to the calibration part (7) through an extension bolt (10), the lenght of which is determined so that the temperature expansion coefficient for the total transmitter is made equal to the temperature expansion coefficient of the structure. The measuring element (9) is also made of a sea water resistant material, which in addition is magnetic. Possibly, there may be used a magnetic material which is protected by a non-corrosive layer ("coating"). Because both the sensor element (5) and the measuring element (9) consist of such material, an open construction is allowed, so that sea water can freely encompass both elements direct. Consequently, pressure problems are completely eliminated.

In the illustrated embodiment the first main part (4) is given a form with a guiding pipe (11) which embraces the extension bolt (10).

The purpose of this arrangement is to achieve a good parallell matching of the two opposite surfaces of the sensor element (5) and the measuring element (9). In order to move without friction in the guiding pipe, the extension bolt (10) is consequently provided with not illustrated bearings of for example teflon material.

Claims

1. Deformation transmitter for detecting and measuring deformation, including absolute and relative elongation, as well as crack opening between two attachment points on a construction (1), comprising a first main part having an inductive sensor element (5) and a second part having a measuring element (9) consisting of a magnetic material, said two main parts being connected with each one of the two attachment points, characterized in that the two main parts are mounted on the construction (1) by means of a fixed screw or clamp connection, respectively, and without any further mutual mechanical connection between the sensor element (5) and the measuring element (9), whereby no forces are exchanged between the transmitter and the structure (1), and in that the two elements (5, 9) both are located in direct and open contact with the surrounding environment, whereby large pressures and changes thereof will not influence the transmitter.

2. Deformation transmitter as claimed in claim 1, characterized in that the second main part (6) is provided with an eccentric calibration bolt (8) which can rotate so that the measuring element (9) can move controlled in relation to the sensor element (5).

3. Deformation transmitter as claimed in claim 1 or 2, characterized in that the measuring (9) is mounted at the end of an extension bolt (10), said extension bolt being constructed with such a lenght and of such material that the total temperature extension coefficient of the transmitter will be equal to the temperature extension coefficient of the structure (1).

4. Deformation transmitter as claimed in claim 3, characterized in that the first main part is provided with a controlled pipe (11) provided around the extension bolt (10) for parallel alignement of the measuring surfaces of the sensor element (5) and the measuring element (9), the extension bolt (10) being allowed to slide without friction in the guiding pipe (11) on teflon bearings.

5. Deformation transmitter as claimed in any of the preceding claims, characterized in that the two main parts are clamped or screwed on to the construction (1) via doubling plates (2) which are welded to the structure.