BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention concerns measurement of loads in an anchor chain.
2. Description of the Prior Art
Prior art methods for measuring loads in anchor chains have included placing load cells directly on a chain link to measure load in the chain when mooring an offshore structure such as an offshore platform or vessel. U.S. Pat. No. 5,845,893 discloses an extensiometer mounted on a latch housing to measure chain force in an anchor chain when it is held by a latch mechanism.
Identification of Objects of the Invention
A primary object of the invention is to provide a force measuring arrangement in the support load path for the measurement of anchor chain load.
Another object of the invention is to provide an arrangement for measuring the compressive force between an anchor chain retainer and a support arm.
Another object of the invention is to provide an arrangement for indirectly measuring the anchor chain load by measuring the deflection of an inner portion of a support arm with respect to the position of an outer portion of a support arm which reacts the chain load.
SUMMARY OF THE INVENTION
The objects identified above along with other features and advantages are incorporated in an arrangement for measuring the load of an anchor chain by measuring the reactive load in structures which support the chain. In a first embodiment, contacting load cells are placed between a chain retainer and arms of a trunnion block for directly measuring the load of the chain. In a second embodiment non-contracting sensors are provided for measuring deflection of inner portions of the trunnion arms with respect to fixed portions of the trunnion arms as an indicator of the chain load transferred to the trunnion arms.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows resistance compression load cells mounted directly in the load path between the chain retainer and the trunnion block.
FIG. 2 shows non-contact sensors on ends of the trunnion block which measure the relative deflection between an indicator rod attached to the center section of the trunnion block and a non-contact sensor mounted to the end of the trunnion.
FIGS. 3 and 4 show an optical sensor where an optical beam is emitted from the sensor toward a reflective target, such that if the sensor housing is under load, the target rotates causing the beam to be reflected back to the sensor at an angle where the measurement of that angle is a measure of the load in the trunnion housing.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates a first embodiment of the invention for measuring the load in an anchor chain. The load F in the anchor chain 10 is reacted by chain retainer 14 on link 10A. Load cells 15 placed between abutting surfaces 16, 17 are compressed by the load between chain retainer 14 and trunnion block 12. The trunnion block 12 is supported on an offshore structure at spaced positions indicated by the arrows S. Because the downward force on trunnion block 12 caused by the weight of the chain retainer 14 is known, the downward force F caused by the weight of the chain 10 and retaining force placed on it is determinable from the measurement of the load cells 15. Load cells appropriate for the arrangement of FIG. 1 are commercially available from Scientific Marine Services, Inc. The load cells include electrical leads (not shown) for communication to a remote signal panel.
FIG. 2 illustrates a second embodiment of the invention where non-contact sensors 20 are mounted in housings 22 which are mounted at the exterior opening of slots 24 formed in trunnion block walls. Indicator rods 26 are fixed at an inner end 28 to the wall of the trunnion block 12 and extend to an outer end 30 placed within the sensor housing 22. The outer ends 30 are free to move within sensor housing 22 when the inner end 28 deflects a short distance when load F is reacted by chain retainer 14 and trunnion block 12. The inner end deflects, because the effective load path through trunnion blocks 12 is inwardly of supports S. The sensors 20 can be any device that senses the deflection of one member (e.g. the end 30 of rod 24) with respect to another (e.g. the sensor housing 22). Such sensors 20 can alternatively be based on capacitive, or eddy current, or optical measurements. Example commercially available sensors are Accumeasure System 1500 Capacitive Gauging System, MTI 2000 Fotonic Sensor or Microtrak 7000 Laser Dispacement Sensor, which are manufactured by MTI Instruments, Inc. and SUNX GP-A Eddy Current Displacement Sensors from Matsushita Electric Works UK. Electrical leads 21 provide communication to sensors 20.
FIGS. 3 and 4 illustrate another alternative arrangement for measuring the load F on chain 10 that uses a laser-based triangulation distance measurement system to measure target rotation. Sensor housings 50 are installed in the trunnion block arms 12. A laser displacement sensor 56 is mounted at the outer end of the housing 50, and a reflective target 54 is placed at the inner end of the housing 50. As load of chain 10 is reacted by the chain retainer 14 and the trunnion block 12, the inner portion of the trunnion blocks deflects or rotates a small distance with respect to the outer end at supports S. FIG. 4 shows the operation of laser displacement sensor 56 that produces a sending light beam 60 toward target 54. Target 54 reflects the beam 62 toward the sensor 56. As illustrated, if the reflective target has been rotated as a result of chain load, the returning beam 62 is reflected at a new angle α with respect to the sending light beam. The sensor 56 measures the angle change. A conversion of that angle information into chain load information is made remotely. An example of a commercially available sensor is the Microtrak 7000 Laser Dispacement Sensor manufactured by MTI Instruments, Inc. Electrical leads 64 to sensor 56 connect to a processing unit (not shown) for data collection and processing.
Patent Grant
6925890
