US3248937A - Load-measuring device - Google Patents

Description

May 3, 1966 R. P. VINCENT LOAD-MEASURING DEVICE 3 Sheets-Sheet 1 Filed Sept. 9, 1963 RENIC P. VINCENT INVENTOR.

ATTORNEY.

May 3, 1966 R. P. VINCENT 3,248,937

I LOAD-MEASURING DEVICE Filed Sept. 9, 1963 3 Sheets-Sheet 2 RENIC P. VINCENT A INVENTOR.

ATTORNEY.

May 3, 1966 R. P. VINCENT 3,248,937

LOAD-MEASURING DEVICE Filed Sept. 9, 1963 3 Sheets-Sheet 3 54 6 o BALANCING BRIDGE 5 AMPLIFIER POWER SUPPLY PNEUMATIC STRAIN GAGE RENIC P. VINCENT INVENTOR ATTORNEY United States Patent 3,248,937 LOAD-MEASURING DEVICE Renic P. Vincent, Tulsa, Okla, assignor to Pan American Petroleum Corporation, Tulsa, Okla., a corporation of Delaware Filed Sept. 1%3, Ser. No. 307,549 8 Claims. (Cl. 73-141) This invention relates to a device for measuring a load applied to a test member or the like, and more particularly it relates to apparatus for measuring tension in a line. In one embodiment, the invention relates to apparatus for measuring the tension in an anchor line to aid in positioning a vesesl floating over a fixed position on land submerged beneath the anchored Vessel.

In the drilling of oil and gas wells in formations submerged beneath the Water, the drill string is supported from a drilling Vessel and extends downwardly to the formation into which the hole is being drilled. Often times the depth of the water is several hundred feet and winds, tides and currents tend to displace the vessel from its position above the bore hole, causing considerable bending stresses in the drill pipe and other tubular members depending from the vessel to the well site. Continued motion resulting from the rotation of the drill pipe or the forces of wind and Water can result in the failure of these elements by fatigue. To aid in maintaining the drilling vessel in the desired position above the bore hole, a common practce is to anchor the vessel in place with multiple lines extending from anchor winches mounted on the vessel to anchors on the ocean One method used in positioning 'a vessel above the well site employs the measurement of tension in the anchor lines. For example, if the vessel tends to move from its position above the well, an anchor line on one side of the vessel will have an increased tensile load while a line extending from the vessel in the direction of movement will slack ofi, with a resulting decrease in line tension. Thus, the measurement of the tension in the various anchor lines gives an indication of the direction of movement of the vessel from its desired position.

As mentioned above, it is a practice to employ on a drilling Vessel anchor winches upon which are wound multiple layers of cable passing over the winch drum to an anchor on the floor of the sea. One device commonly used for measuring the tension in an anchor line is clamped to the anchor cable and utilizes the cable deflection to actuate a hydraulic cylinder, the movement of which produces pressure variations which are translatable into cable tension. When the anchor line is taken in or paid out, it is generally'necessary to detach the device and then reposition it on the cable after the cable has been wound or unwound on the winch drum.

With such apparatus, the anchor tension is determined on each line and the individual measurements are 'then compared to determine the movement of the vessel.

Various load-measuring devices are well known in the art. Many of these operate on the principle of measuring the strain occurring in a member as a result of a load applied to the member. As long as the loading results in a stress within the elastic limit of the material of which the member is constructed, the strain is proportionate to the applied stress and the loading can be readily determined from a knowledge of the characteristics, e.g., the type of material and dimensions, of the load-bearing member.

The persent invention provides an improved loadmeasuring device which is simple in construction while providing accurate measurement of a load imposed upon a member. The device and apparatus used therewith is rugged and readily useable by unskilled operators, while at the same time being economical and relatively free from maintenance.

In one embodiment, the present invention provides apparatus for measuring tension in a line which comprises a frame, a revolvable drum member mounted on the frame for supporting a line Wound thereupon, a pawl mounted on the frame to engage with the drum and receive a load proportionate to tension applied to a line wound upon the drum, and strain-sensitive means attached to the pawl for sensing strain or dimensional changes occurring in the pawl as a result of a load applied to the pawl and means for indicating the magnitude of such changes.

In another embodiment, the present invention provides a novel load-measuring device which may be employed as the strain-sensitive means attached to the pawl as mentioned above. This device comprises a fluid-carrying conduit positioned in said pawl member, a variable orfice connected to sad conduit, the variable orifice comprising a seat member and a disc member movable in respect of said seat member to vary the opening of said orifice, said disc member being connected to the pawl member and movable in response to strain in the pawl member resulting from the application of a load thereto, to vary the opening in the orifice, and means for measuring variations in the flow characteristics of a fluid, such as air or other gas, passed through the variable orifice which variations result from the strain occurring in the pawl member.

According to the invention, an anchor winch comprising a winch drum upon which a line is wound, with a gear-like member, or ratchet, is attached to one end of the drum which is supported upon a frame bolted to the deck of a vessel. A pawl secured to the frame engages with the ratchet to prevent the drum from turning and paying out line. When so engaged with the ratchet, a load is transmitted to the pawl which is proportionate to the tension in the anchor line. With the most common method of mounting the pawl, the force acting on the cable is transmitted to the pawl as a compressive load. The magnitude of this compressive load will, in addition to variations in the line tension, depend upon the drum diameter, the number of layers of cable on the drum, the angle formed by the pawl with a line passing through the center of the drum and the point on the drum which is contacted by the pawl. However, with a knowledge of these factors, the load applied to the pawl can easily be correlated with the tension in the line.

My invention will be better understood by reference to the following description and the accompanying drawings of preferred embodiments of the invention wherein:

FIGURE 1 diagrammatically illustrates a drilling vessel anchored in place over a well site and positioned by means of anchor winches and lines; and

FIGURE 2 schematically illustrates a typical anchor winch and pawl employed to measure line tension and for positioning the drilling vessel; and

FIGURE 3 is a partial sectional view of a pneumatic load-measuring device positioned on a winch pawl; and

FIGURE 4 is a sectional view of the device of FIG- URE 3 taken at line 44; and

FIGURE 5 schematically illustrates the pneumatic circuit employed in conjunction with the device of FIGURE 3; and

FIGURE 6 illustrates a winch pawl on which are mounted electrical strain gauges; and

FIGURE 7 diagrammatically illustrates an electrical circuit employed with the device of FIGURE 6.

Referring to the drawings, in FIGURE 1 a drilling vessel 11 is positioned over a well site 12 located in forma tion 13 submerged under a body of water 14. A derrick 16 mounted on the deck of the vessel supports a drill string 17 extending downwardly to the well site. The drill string has on its lower end a drill bit (not shown) for boring a hole into the formation and may be rotated by conventional means. The vessel is secured in place over the well site by anchor lines 18 attached to anchors 19 set in the submerged land to maintain the vessel in the desired position. Mounted on the deck of the vessel are anchor winches 21 which may be of the conventional type, and one or more rotating winch drums may be employed on a particular winch, with separate cables wound upon each drum. Rotation of a winch drum causes the cable wound thereupon to be paid out or taken in, depending upon the direction of rotation. A gear member 20, or ratchet, is attached to each of the drums and a pawl 22 is pivotally mounted on the frame 21a of the winch to engage with the ratchet to prevent the winch drum from turning and paying out line. When the anchors are set the cable extending to each anchor is under tension suflicient to maintain the vessel in a desired position over the well site, and the vessel position may be changed by varying the tension in the cables. The tensile load on a particular anchor cable is transmitted through the drum and ratchet to the pawl engaged with the ratchet. The load transmitted to the pawl will be in proportion to the load on the cable. If it is desired to pay out line from a particular drum, the pawl is disengaged from the ratchet to permit the drum to unwind. A conventional braking means may be employed with each of the drums to control the payout of line. When the desired position has been attained the pawl is engaged with the ratchet, and the brake is released to measure the load transmitted to the pawl by means of load-measuring means 26 and indicators 30 connected thereto.

As shown in FIGURE 2, the tension in the anchor cable tends to unwind the cable wound upon the win-ch drum, which unwinding is resisted by the anchor pawl. The load transmitted to the pawl is determined by the factors mentioned above, and, in the embodiment shown, the pawl is compressed longitudinally when engaged with the ratchet, so that the pawl is subjected to elastic deformation and tends to shorten along its length. The compressive load on the pawl is sensed by a strain-sensitive means 25 positioned on the pawl to measure the strain resulting from the compressive load. In this embodiment, the strain-sensitive element is positioned on the pawl to measure the longitudinal strain in the pawl. The configuration of the pawl will deter-mine the precise positioning of the strain-sensitive element and the manner of engagement with the ratchet will determine the direction of the strain. Various types of strain'sensitive elements may be employed as load-measuring devices for this purpose. The strain-sensitive element25 shown in FIGURE 2 is a pneumatic strain gauge (hereinafter described), although other known strain-sensitive elements, such as wire-grid or semi-conductor strain gauges may be employed. Basically, such electrical strain gauges measure changes in electrical resistivity which occur with the application of stress. Typically, piezoresistive materials, such as silicon or germanium crystals, or other such semiconductor materials may be utilized. Methods for attaching the strain gauges to the pawl and for measuring the resulting changes in electrical characteristics are Well known to those skilled in the art and need not be described in detail herein.

One type of load-measuring device which has been found to be particularly useful in measuring the strain occurring in winch pawls is shown in FIGURES 3 and 4. Such a device comprises a fluid-carrying conduit 31, preferably positioned in the interior of the pawl member and aligned with the longitudinal axis of the pawl. As

shown, the conduit 31 is secured in a longitudinal cavity formed in the pawl member. Connected to the conduit 31 is an inlet 32 and an outlet 33. O-rings 34 provide a fluid-tight seal between the conduit 31 and the walls of the pawl cavity. A variable orifice 36 is connected to the conduit 31 near the outlet end thereof to restrict the flow of fluid passed through the conduit. The variable orifice comprises a seat member 37, located near the outlet end of the conduit, and a disc member 38 which is movable in respect to the seat member to vary the opening through which fluid passed through the conduit escapes into the pawl cavity. Thus, movement of the disc member causes a variation in the pressure drop of fiuid passed through the orifice. Advantageously, the variable orifice comprises a ball type valve as shown, although a cone or other type disc member may be employed. The disc member is in contact with the pawl member to move longitudinally in response to strain in the pawl member resulting from the application of a load thereto, and as shown, the disc member moves closer to the seat member with the application of an increased load on the pawl to restrain the free exhaust of fiuid from the conduit and to increase the pressure drop in the system.

As shown in FIGURE 3, the disc member may be directly coupled to the pawl member by means of a pin or rod 39 screwed into the pawl, with the disc member being secured to the rod by welding or other suitable means and positioned downstream of the seat member. Also, another arrangement which has been found extremely satisfactory is to position the disc member within the conduit 31 upstream of a cap attached at the outlet end of the conduit and provided with a seat member. In this latter arrangement, the rod member 39 is secured at the end of the pawl cavity as described above and extends through the seat member into the conduit cavity to contact the disc member and cause the disc member to move away from the seat member when the pawl is placed in compression.

In the operation of the above-described pneumatic loadmeasuring device, air is passed through the inlet 32 at a constant pressure and flows through the conduit 31 and the variable orifice 36 into the pawl cavity, from which it escapes through outlet 33. Free exhaust of the air from the outlet is restrained by the disc member and the seat member, and compressive strain or load on the pawl causes the pawl to shorten and the rod member 39 moves the ball in respect of the seat member by a distance which is proportionate to the strain or compressive load on the pawl. This movement varies'the orifice opening to affect the escape of air through the outlet 33. Measurement of the resulting variations in air pressure gives a quantitative indication of the load applied to the pawl. For example, a series of measurements made with varying loads applied to winch pawls gave the following results with p.s.i.g. air supplied to the above-described pneumatic devices attached to the pawls:

Outlet air Compressive load applied to pawl, lbs.: pressure, p.s.i.g. 10,000 72 20,000 57 30,000 44 40,000 34 50,000 26 60,000 21 A typical pneumatic circuit employed with the abovedescribed apparatus is shown in FIGURE 5. Pressured air from a source, typically an air compressor 40, flows by way of line 41 through a water drain and filter 42 and an air filter 43 which remove foreign material, such as scale and water, from the air stream to a pressure regulator 44 which is set to deliver 100 pounds air pressure to the system. Downstream of the air regulator, the air flows to the pneumatic strain gauge 48 positioned in a winch pawl. Air to the pawl is metered through an adjustable metering orifice 46, typically a needle valve and a pressure gauge 47. The air then is passed to the strain gauges 48 from whence it is exhausted to the atmosphere. The pressure gauge is calibrated to read in terms of tensile load on the anchor cable upon the winch on which the pawl is mounted.

Prior to installing the pawls on the winches, the strain gauges in pawls are calibrated and the pressure gauges are also calibrated to read anchor cable tension, with multiple scale provided on each gauge face. This is necessary since the winch drums may contain different numbers of layers of cable after the anchor is set. The number of layers of cable on the drum affects the length of the lever arm, which in turn affects the load measured at the pawl. Typically, the gauge is calibrated in units of 1,000 pounds. A sufiicient number of scales are provided on the gauges, consistent with the desired accuracy, for the number of layers of cable to be wound on the drum. For example, where a typical winch drum may have capacity of twelve layers of Fir-inch cable, and the anchor system normally operated with from three to ten layers wound upon the drum, a single scale may be provided for each two layers of cable. For instance, if the indicated reading of scale 5-6 is 30,000 pounds and there are six layers of cable on the drum, the actual tension will be 30,750 pounds, while if there are only five layers on the drum, the actual tension wil lbe only 29,250 pounds. The advantage of the simplified gauge face outweighs the effect of the small error of 2.5 percent introduced.

Initially the pressure regulator is set so that the instrument is receiving air at the desired pressure, e.g., 100 p.s.i., as indicated by the gauge on the regulator. With the pawls lifted out of engagement with the winch ratchet and under zero strain, each metering orifice 46 is adjusted so that the corresponding gauge reads zero anchor tension. The pawls are then engaged with their respective ratchets and the winch brake released so that the pull of the anchor cable is resisted by the pawls. Anchor line tension on each of the cables will then be indicated by the appropriate pressure gauge.

Pneumatic load-measuring devices of the type described above have been successfully employed with an accuracy in the range of about 1-3 percent when used on anchor pawls having a cross-sectional area of approximately six square inches, with compressive loads as great as 60,000 pounds applied to the pawls. They have been found to be rugged and simple to operate and maintain, while providing consistently accurate readings of anchor tension.

An alternative arrangement is shown in FIGURE 6 wherein one or more electrical strain gauges 51 adhered to the surfaces of the pawl are employed in lieu of the aforementioned pneumatic strain gauge. Typically, four of the electrical strain gauges are cemented to the pawl so as to produce an electrical signal corresponding to the load applied to the pawl, with an average of the readings from the four electrical gauges being used as to obtain the load applied to the pawl. In this arrangement, the number and position of the electrical strain gauges will also vary according to the configuration of the pawl, however, an appropriate arrangement may be selected in the manner well known to those skilled in this art. The electrical conductors 52 from each of the electrical strain gauges are connected by switches 50 into a bridge circuit 53 which is connected to an amplifier 54 and an indicating device 56 (or recording device if desired), and interconnected power supply 57 provides electrical energy for operating this device. The bridge circuit includes a variable resistor 58 which permits the circuit to be balanced to give a zero reading on the indicator when zero strain is applied to the pawl. When the pawl is loaded by engaging with the winch ratchet, the resulting strain creates a corresponding electrical signal in the respective strain gauge attached to each pawl and the amount of the strain is indicated.

As mentioned above, prior to installation, the electrical strain gauges on the pawls may be calibrated, and the indicator dial may be marked with the appropriate number of scales to read directly in the load applied to the anchor cable. These load-measuring devices are employed in a manner similar to the above pneumatic device.

It is apparent from the foregoing description that various modifications and alterations in the details of the construction may be made to the apparatus of the invention without departing from the spirit and scope thereof. For example, the pneumatic strain gauge may be constructed with various type discs employed in the variable orifice, and the pressure measured by the pressure gauge 47 may either decrease or increase as the disc member moves in respect of the seat member. Further, the pressure gauge 47, or other measuring device, may be positioned downstream of the pneumatic strain gauge with a very small fixed orifice provided at the atmospheric outlet of the system.

I claim:

1. Apparatus for measuring tension in a line, which apparatus comprises a frame; a revolvable drum member mounted on said frame for supporting a line wound thereupon; a pawl member mounted on said frame to engage With said drum member and receive a load proportionate to tension applied to a line wound upon said drum member; strain-sensitive means attached to said pawl member for sensing strain variations occurring in said pawl member as a result of a load being applied to said pawl member and for indicating the magnitude of said variations.

2. The apparatus of claim 1 wherein said strain-sensitive means comprises an electrical strain gauge.

3. The apparatus of claim 1 wherein said strain-sensitive means comprises a flow conduit, a variable orifice in said conduit and a pressure-sensitive device connected to said conduit for measuring variations in the pressure of a measuring fluid passed through said variable orifice.

4. The apparatus of claim 1 wherein said strain-sensitive means comprises a fluid-carrying conduit attached to said pawl member; a variable orifice connected to said conduit, said variable orifice comprising a seat member and a disc member movable in respect of said seat member; said disc member being connected to said pawl member and movable in response to strain in said pawl member resulting from the application of a load thereto to vary the opening in said orifice; and means for measuring variations in the flow characteristics of a fluid passed through said variable orifice resulting from said strain.

5. Apparatus for measuring the tension in an anchor line used for positioning a vessel floating on the surface of a body of water, which apparatus comprises an anchor winch secured to said vessel; an anchor line wound upon said winch; said winch comprising a winch drum, a gear member attached to said drum and a winch pawl for engaging with said gear member to prevent said drum from turning; strain-sensitive means attached to said pawl for sensing a load applied to said pawl as a result of tension in said line; and means for indicating the magnitude of strain variations occurring in said pawl.

6. The apparatus of claim 5 wherein said strain-sensitive means comprises a fluid-carrying conduit positioned in said pawl member; a variable orifice connected to said conduit; said variable orifice comprising a seat member and a disc member movable in respect of said seat member to vary the opening in said orifice; said disc member being connected to said pawl member and movable in response to strain in said pawl member resulting from the application of a load thereto to vary the opening in said orifice; and wherein said indicating means comprises means for measuring variations in the flow characteristics of a fluid passed through said variable orifice resulting from said strain.

7. The apparatus of claim 5 wherein said strain-sensitive means comprises an electrical strain gauge.

8. An apparatus for measuring tension in a line which comprises:

a frame,

a revolvable drum means mounted on said frame for supporting a line wound thereupon;

a pawl member mounted on said frame to engage With said drum member and receive a load proportionate to tension applied to a line wound on said drum member, said pawl member having a cavity therein;

a fluid conduit at least partially positioned within said cavity of said pawl member, a portion of said conduit being aligned in the direction of the load applied to said pawl member, said portion terminating at a first end opening into said cavity, the other end of said conduit extending exterior of said pawl member,

a disc member movable with respect to said first end of said conduit to vary the size of the opening of said first end;

a rod element in contact with said pawl member and movable in response to strain in said pawl member,

said rod element being aligned in the direction of the load applied thereto and in contact with said disc member, causing said disc to move;

pressure measuring means connected to said conduit for measuring variations in the pressure drop across the opening of said first end caused by movement of said disc member.

References Cited by the Examiner UNITED STATES PATENTS 2,210,759 8/1940 Hansen 73-143 2,455,285 11/1948 Versaw 7388 3,069,902 12/1962 Crosby 73-136 15 RICHARD C. QUEISSER, Primary Examiner.

CHARLES A. RUEHL, Assistant Examiner.

Claims (1)

1. APPARATUS FOR MEASURING TENSION IN A LINE, WHICH APPARATUS COMPRISES A FRAME; A REVOLVABLE DRUM MEMBER MOUNTED ON SAID FRAME FOR SUPPORTING A LINE WOUND THEREUPON; A PAWL MEMBER MOUNTED ON SAID FRAME TO ENGAGE WITH SAID DRUM MEMBER AND RECEIVE A LOAD PROPORTIONATE TO TENSION APPLIED TO A LINE WOUND UPON SAID DRUM MEMBER; STRAIN-SENSITIVE MEANS ATTACHED TO SAID PAWL MEMBER FOR SENSING STRAIN VARIATIONS OCCURRING IN SAID PAWL MEMBER AS A RESULT OF A LOAD BEING APPLIED TO SAID PAWL MEMBER AND FOR INDICATING THE MAGNITUDE OF SAID VARIATIONS.

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