Distance measuring

Apparatus for measuring and indicating the settlement of the floor of a large liquid containing vessel such as an oil storage tank. The apparatus comprises sonic echo sounding means positioned at or movable between each of a multiplicity of spaced points disposed within the vessel and above its floor in known relationship one with another, and ranging means for furnishing for each point, and upon the basis of the echo sounding means output, a measure of the distance from the point to the vessel floor and an indicator (preferably visual) for indicating from the ranging results the settlement of the floor below those points. In preferred embodiments, the surface of the liquid within the vessel is utilized as a datum and the arrangement may be adapted to provide an indication of the volume of liquid within the vessel.

BACKGROUND OF THE INVENTION 
This invention relates to measurement or monitoring functions and in 
particular to measurement or monitoring functions associated with large 
liquid-containing vessels such as oil storage tanks. 
One vital measurement or monitoring function is that of base plate 
deformation in such vessels, which deformation arises as follows. 
One presently accepted common method of building large vessels for 
containing liquid, for example, storage tanks for the bulk storage of oil, 
uses a construction technique in which the foundations for the tank 
consist of a "ring" of compacted rock or concrete. Such a tank is known as 
a "soil supported" tank. 
Unfortunately, soil supported tanks suffer from differential settlement of 
a number of kinds (axial, peripheral and base plate), and whilst the tank 
design can allow for some such settlement, it cannot cope with excessive 
amounts. Accordingly, in order to monitor the actual settlement, various 
measurements have to be taken of the tank's dimensions over a test period 
(commonly of at least a year) during which it is slowly filled, meter by 
meter, with liquid (commonly water) not only to check that the tank does 
not leak but also to assist in the satisfactory consolidation of the 
foundation sub-soil. Indeed such measurements have to be checked 
throughout the period that the tank is in service and the life of a 
typical bulk oil storage tank may well be in excess of twenty years. 
Measurement of axial and peripheral settlement can relatively easily be 
effected from outside the tank, but measurement of base plate settlement 
cannot, and presently causes much difficulty (part of which is that 
current methods are too simple to give good; detailed, results). 
SUMMARY OF THE INVENTION 
It is one object of this invention to provide a novel method and apparatus 
for measuring storage tank base plate settlement or distortions allowing 
the obtention of very detailed results and yet being at the same time 
relatively simple and cheap. 
According to one aspect of this invention, a monitoring or measuring 
arrangement comprises means, disposed within or capable of being 
positioned within a liquid containing vessel, for effecting a sonic echo 
sounding from each of a number of spaced points disposed within the vessel 
and above its floor, whereby a measure is obtained related to the distance 
of each point to the vessel floor. 
Preferably said sonic echo sounding means is or are positioned within said 
vessel to be normally below the surface level of liquid in said tank. 
Preferably again the arrangement is such that there is used as a datum 
level the surface of liquid in said vessel, all distances being determined 
either actually or in effect from that surface datum level. 
The measurements thus obtained may be utilised to provide a pattern of 
measurements relating to the behaviour of the floor of said vessel either 
within the period of hydrostatic testing and/or soil consolidation during 
initial commissioning and filling of the vessel, or during service. By 
this means various settlements or distortions of the floor (e.g. axial 
differential settlement and peripheral differential settlement) may be 
monitored or indicated. 
In addition, or alternatively, the measurements thus obtained may be 
utilised to provide a measure of the volume of liquid contained within the 
vessel which, unlike a simple measurement of the height of the surface of 
the liquid, tends to take into account such last mentioned settlements or 
distortions. 
In another aspect of this invention there is provided a method of detecting 
the settlement or distortions of the floor (base plate) of a 
liquid-containing vessel, such as an oil storage tank, in which method 
sonic echo sounding from each of a number of spaced points disposed within 
the vessel and above its floor in known relationship one with another is 
used to furnish a measure of the distance from each point to the vessel 
floor, and from the results there is detected the settlement (if any) of 
the floor below those points. 
In some embodiments, the invention provides apparatus for detecting the 
settlement of the floor (base plate) of a liquid-containing vessel such as 
an oil storage tank, which apparatus comprises: sonic echo sounding means 
positioned at or movable between each of a multiplicity of spaced points 
disposed within the vessel and above its floor in known relationship one 
with another: ranging means to furnish for each point upon the basis of 
the echo sounding means output a measure of the distance from the point to 
the vessel floor; and indicator means for indicating from the ranging 
results the settlement (if any) of the floor below those points. 
The invention uses sonic echo sounding--that is to say, echo sounding using 
radiated sound as opposed to, for instance, radar. The frequency of the 
sound is an important factor in determining the accuracy of the results. 
For most purposes it will be necessary to "measure" distances to within at 
least one inch (about 2.5 cms), so that the frequency of the sound must be 
such that the sound wavelength is less than this. On a very rough basis, 
sound travels through liquids at about 5000 ft/sec (about 1500 m/sec) so, 
using the expression velocity equals frequency times wavelength it is 
clear that the frequency must be greater than 5000/(1/12), or about 60000 
Hz, which is well into the ultrasound region. 
The sound employed to allow the sonic echo sounding may be generated and 
received by any convenient apparatus, typically an electric 
signal-to-sound (and vice versa) transducer. It is common these days for 
ultrasound to be generated using a piezoelectric transducer and such a 
transducer, using as the piezoelectric material a quartz crystal or a 
piece of polarised plastics materials such as polyvinylidene fluoride 
(PVDF) is perfectly satisfactory. Naturally the transducer is, in use, 
connected up to a suitable source of electrical signals (for generating 
the ultra-sonic sound when it is operated in the transmitter mode) and to 
suitable equipment for detecting its output (when it is operated in 
receiver mode). 
It will be appreciated that where liquids of a highly volatile, inflammable 
nature are concerned care must be exercised to avoid sparking. If 
therefore electric transducers are used within the vessel these should 
either be encapsulated or otherwise "spark-proofed" or suitably low 
voltages should be used. Alternatively it may be desirable to avoid 
electric signals within the tank altogether in which case pneumatically or 
mechanically operated transducers may be used within the tank with 
acoustic/electrical conversion (if required) outside of the tank. 
The number of points at which the echo sounding is effected depends upon 
the definition desired of the resulting "picture" of the vessel floor 
(which in turn depends upon the size of the floor) and upon the cost. By 
way of example, however, for an oil storage tank 100 ft (about 30 m) in 
diameter, acceptable results are obtained if the points are about 3 ft (90 
cm)--say, from 2 to 5 ft (from 60 to 150 cm)--apart. 
The disposition, one from another, of said spaced points at which 
echo-sounding is effected are preferably evenly distributed over the floor 
area and preferably regularly disposed in a grid pattern. One preferred 
grid pattern is a radial grid pattern centred on the floor centre, said 
spaced points being regularly spaced outwardly from the floor centre along 
radii, each radius being equiangularly spaced from its immediate 
neighbours. In such a case, a floor picture of acceptable definition is 
obtained for a 100 ft diameter tank if the points are spaced radially in 
multiples of 3 ft and angularly in multiples of 10.degree.. 
Whilst said points may be at different heights above the vessel floor, 
preferably all of said spaced points are at, or near, the same height. 
However, their actual height becomes of less importance if the surface of 
the liquid (or in other words, the liquid interface) within the vessel is 
used as a datum level, all distances being determined either actually or 
in effect from that surface datum level: the computation required depends 
upon whether the echo sounding points are below, on or above the surface 
level (as discussed further hereinafter) but it is relatively simple to 
relate all floor depths to the surface, so making unnecessary a knowledge 
of the exact height of each echo sounding point. 
The points where the echo sounding is effected may, as stated hereinbefore, 
be below, at or above the surface of the liquid in the vessel. However, if 
the points are above the surface then it is difficult to receive any echo 
from the vessel floor, whilst if the points are on the surface then it is 
additionally necessary for the echo sounding means at those points so to 
be mounted that they can move up and down with the surface. Preferably, 
therefore, the points are all below the surface, and most conveniently 
they are all in a plane roughly two to three feet (60 to 90 cm) above the 
vessel floor. 
The means for effecting the echo sounding may be provided as a static 
arrangement, wherein each point has echo sounding means permanently 
positioned thereat. However, transducers tend not to be cheap, and rather 
than employ a large number in such a static arrangement, it may be more 
convenient to employ a much smaller number and move them from one set of 
points to another--as the "measurement" proceeds. This is made easier if 
the points are in a regular array. For example, using the preferred radial 
array there need only be sufficient transducers in one set to fill the 
positions in one radius; after the readings have been taken there the set 
is swung sideways into place along the next radius; and so on. Indeed one 
transducer alone may be used, the arrangement being such that one 
transducer is moved from spaced point to spaced point as the "measurement" 
proceeds. 
Preferably however, the echo sounding means are disposed along a 
substantially rigid boom pivotally mounted at the vessel floor's centre so 
that it can sweep over the floor as the hand of a clock sweeps over the 
clock face. The boom may be a single arm boom (with an appropriate 
counterweight), or it may be a "double arm" boom, extending on either side 
of a central pivot point--indeed, if desired, it may be triple or 
quadruple arm boom (and so on) though a double arm boom is presently 
preferred. 
The invention gathers the data obtained by echo sounding from each of the 
chosen points, and uses it to furnish a measure of the distance from each 
point to the vessel's floor beneath the point. The data itself is in the 
form of electronic pulses fed to and received from the echo sounding means 
and in effect constitutes times--times of generating pulses, and times of 
receiving echoes from either the floor or the surface; from these times 
may be derived the required distance information. Conveniently the data 
may be recorded onto magnetic tape. However, quite how this distance 
measure is calculated, and whether it be an absolute or a relative 
measure, naturally depends upon the particular embodiment of the 
invention--and, especially, upon the level of the chosen points. In the 
preferred case, with all the points in a plane in the liquid about two to 
three feet above the floor (and thus forty to forty-five feet below the 
liquid surface when the tank is full) the distance between the floor and 
the surface is represented by the sum of the echo-return times for the 
two, while if the points are on the surface, or above the surface, then 
the distance is represented respectively by the (sole) time and the 
difference of the two times. The ranging means referred to hereinbefore is 
the means--timing, computational and so on--that performs whatever actions 
are necessary to derive the required distance measure from the echo data. 
From the derived measure of the floor distance may be detected (and 
indicated) any settlement. The indication may take any convenient 
format--including for example the sounding of an aural alarm--but 
preferably this is in visible form. Thus, the data may be manipulated so 
that it is viewable as contour lines, as a floor section profile, or just 
as a set of figures, and may be so viewed either as printed out on paper 
or as displayed on a TV type screen. Moreover the successive sets of data 
produced may be analysed to determine the rate at which settlement is 
occurring. 
In operation, the invention will conveniently involve the emplacement 
within a vessel--specifically, within each of a series of vessels such as 
oil storage tanks--of sonic echo sounding means and the associated 
supporting (and moving) apparatus (for example, the floor-sweeping boom 
arrangement referred to herein), followed by the connecting of the 
sounding means to activating and data reading and recording means whereby 
there may be obtained the necessary information to allow the furnishing of 
the required sounding-point-to-floor measure, which itself allows the 
detection/indication of any floor settlement. It is envisaged that each 
combination of sounding means and the supporting (and moving) apparatus 
need not have its own activating and data reading and storing means, but 
may share such means common to a whole series of such combinations. Thus, 
in an oil storage tank depot for instance, up to 50 tanks might each 
contain its own sounding means combination, a single 
activating/reading/storing means being moved sequentially from tank to 
tank to gather the data, thereafter bringing that data back to some 
control point for analysis. 
The boom echo sounding means mounting arrangement described herein for use 
submerged in the liquid is itself novel and inventive, and in another 
aspect this invention provides such a device for use in or with the base 
plate detecting method and apparatus of the invention. Thus, the invention 
provides, for mounting within and coaxially of a large vessel such as an 
oil storage tank, an echo sounding means support and movement arrangement 
which comprises: a longitudinally-extended boom member adapted to be 
pivotally mounted at or near one end centrally of the vessel floor for 
movement in a plane parallel thereto; means for causing the boom to move 
in that plane, rotating about its pivot mounting; and a plurality of 
ultrasonic echo sounding means mounted along the boom, some pointing 
towards the vessel floor and some pointing away therefrom but at least one 
pointing in each of these directions. 
The boom may be a single arm boom, in which case it will preferably have 
either a counterweight or a boom arm tip support (such as a wheel), or a 
multiple arm boom, the arms balancing each other. The boom arm or arms 
normally extend substantially the full effective internal radius of the 
vessel, and is conveniently a framework of the cantilever type. The boom 
is advantageously mounted or mountable on a pillar which pillar is itself 
mounted or mountable on the vessel floor centrally thereof; either the 
boom can pivot on the pillar or the pillar can pivot on the floor. The 
rotation-causing means is advantageously a pneumatic (to avoid the 
possibility of sparks) stepping motor (the angular step being adjusted to 
fit the required angular distance for the echo sounding points). The 
number of echo sounding means (preferably piezoelectric transducers) 
depend upon the size of the vessel (and thus the length of each boom arm); 
it is generally satisfactory to have a floor-directed transducer every 
three or four feet and one directed in the opposite direction towards the 
liquid surface every five to ten feet. 
It will be appreciated that by utilising what may be termed "differential 
measurement" utilising the liquid surface as datum, errors due, for 
example, to tilting of the boom tend to be cancelled.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The tank of FIGS. 1, 2 and 3 comprises conventional walls (11), baseplate 
(12) and girder-supported roof (13). It is built upon foundations which 
consist of a ring (14) of crushed rock with an inner filling (15) and 
outer ramp (16) of compacted sand, the whole being supported on the ground 
(generally 17). The tank, which is full of liquid 18 with a surface 19, is 
150 ft in diameter and 50 ft tall. 
Mounted centrally on the baseplate 12 and coaxial with the tank is a pillar 
(21) and mounted on the piller, for rotation about the axis by a 
pneumatically-actuated stepper motor (not shown), is a double-armed 
strutted boom (22, 23) with a framework of the cantilever type. The lower 
portion (as 24) of each boom arm is approximately horizontal and about 3 
ft above the baseplate 12, and supports spaced along it a number of 
piezoelectric transducers some (as 25) pointing downwards towards the 
baseplate 12 and others (as 26) pointing in the opposite direction, 
upwards towards the roof 13. For clarity there are shown only 6 
transducers per arm pointing down and 3 transducers per arm pointing up. 
Leads 27 to the stepper motor, and to and from the transducers 25, 26 pass 
from the base of the pillar 21, along the floor of the tank, and out 
through a gasketted (sealed) aperture 28 in the tank wall 11. 
In operation (discussed further below) the boom arms 22, 23 are stepped 
round--as most clearly seen from FIG. 2, where by way of illustration 
seven positions defining one full quadrant are shown--and in each position 
the transducers 25, 26 are activated (preferably in sequence) to send and 
receive an ultrasonic pulse to and from both the tank floor 12 and the 
surface 19 of the liquid 18 in the tank. From the times of the received 
echo pulses there is computed the (relative) depth of the liquid--that is, 
the distance from the liquid surface 19 to the baseplate 12--for each 
transducer position, and from this informaton is derived a measure of any 
baseplate settlement that may have occurred. 
The block diagram of FIG. 4 illustrates schematically the operation of the 
invention, and is to be considered together with the "pictorial" output 
tape shown in FIG. 5. The two Figures more or less speak for themselves, 
but may briefly be described as follows. 
The PULSER comprises an oscillator and associated power amplifier to 
constitute a transmitter. This unit provides pulses of 60 KHz signals 
which supply the power to the transducers. These pulses are initiated or 
triggered from the CLOCK. The CLOCK circuit is a synchronizing oscillator 
producing regular pulses which keep the sequence of signals and switches 
locked together in a manner well established in system electronics. 
Suitable DELAY circuits are provided to make allowance for the liquid 
characteristics, tank geometry etc., to ensure rational collecting and 
presenting of the data. The T/R (transmit/receive) SWITCH either passes 
the transmitter signals from the PULSER to the SEQUENCING GATES and thence 
to the transducers (shown in FIG. 3) or it passes the return signals from 
the transducers via the sequencing gates to the RECEIVER. The SEQUENCING 
GATES are triggered in turn by the CLOCK to send successive pulses from 
the PULSER via the T/R SWITCH to the series of transducers (shown in FIG. 
3) in turn. The KEYBOARD is providing to enable ancillary data to be 
entered into the RECORDER (this latter may be a magnetic, paper, or 
similar storage mechanism). All the constituent parts of the functions 
shown in the boxes are well known to electrical engineers concerned with 
this general type of system. 
The block diagram shows switches (the sequencer gates) connecting the 
transmitter (pulser) via the transmit/receive switch to each transducer 
(R, T.sup.1, T.sup.2 etc) in sequence, where R leads to the upper, 
reference, transducers (26 in FIG. 3), and T1, T2, T3 etc. lead to the 
lower, measuring, transducers (25 in FIG. 3). Switches of this type are 
well known in many types of electrical circuits. The gates and the T/R 
switch also connect the return signal to the receiver for amplification 
and onwards transmission to the tape recorder (FIG. 5 shows a typical tape 
pulse format). Each time the clock generates a pulse this opens the next 
gate, fires the transmitter, and the clock pulse and the return pulse from 
that transducer are recorded. The delay .delta., between the clock pulse 
and the return pulse represents the distance of the reflecting surface 
(either the tank floor e.g. .delta.T' or the tank liquid surface, used as 
a reference e.g. .delta.r). 
Provision is made for additional information to be added from a keyboard 
between measurements at each angle of rotation .theta. of the boom. In 
addition, the boom angle can itself be measured and added to the recorded 
data. 
Finally, FIGS. 6A and B show, in extremely exagerated and purely 
illustrative form, a contour line/section display (either VDU or print 
out) of the type derivable from the taped pulse/echo data produced by the 
equipment of FIGS. 4 and 5. The solid line section of FIG. 6B is the 
section on the West-East line through the tank, while the dotted line 
section is that on the Southwest-Northeast line; the contours are at one 
inch intervals, and give a somewhat compressed view of the baseplate 
settlement over the two diameters of the floor.