Abstract:
This invention relates to the remote monitoring of the floating roofs of large storage tanks, including tanks used for storing liquid petroleum products or other chemicals. The invention is used to monitor the position and flexure of the roof and other conditions such as vibration; the presence of vapours or liquids from water, hydrocarbons, or other chemicals; the presence of snow; or the presence of intruders. The invention can be connected to a monitoring system using wired or wireless means and can be used for routine status monitoring or for notifying plant operators in the event of alarm conditions. The invention can be completely self-contained and is suitable for encapsulation and use in harsh environments.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to the remote monitoring of the floating roofs of large storage tanks, including tanks used for storing liquid petroleum products or other chemicals. The invention is used to monitor the position and flexure of the roof and other conditions such as vibration; the presence of vapors or liquids from water, hydrocarbons, or other chemicals; the presence of snow; or the presence of intruders. The invention can be connected to a monitoring system using wired or wireless means and can be used for routine status monitoring or for notifying plant operators in the event of alarm conditions. The invention can be completely self-contained and is suitable for encapsulation and use in harsh environments. 
       BACKGROUND OF THE INVENTION 
       [0002]    Large storage tanks are often cylindrical and have a circular floating roof. The roof floats on the surface of the liquid, thereby decreasing the vapor space inside of the tank. A floating roof may be required for reasons of safety or for pollution reduction. The floating roof has a perimeter seal to seal it to the wall of the tank that helps to prevent the escape of the contained liquid or vapors from that liquid. 
         [0003]    The floating roof is a large dynamic structure having a diameter of up to 100 meters and weighing several tons. This dynamic structure is subjected to changing forces from environmental conditions (temperature, wind, rain, snow, etc.) that affect the tank, the lid, or the contained liquid; convective forces within the liquid; or forces that occur when liquid is added or removed, including friction from the seal. The roof will flex and possibly tilt in response to these changes, which may result in the loss of the contained liquid or vapor. In extreme cases, the roof may tilt enough to cause it to sink into the tank. 
         [0004]    The industry is therefore quite interested in monitoring systems that can be used to improve safety, reduce environmental concerns, or avoid major problems such as seal failure or a sunken roof. 
         [0005]    There are existing patents that address the application of electronic monitoring or control to storage tank systems. For example, U.S. Pat. No. 4,596,266 (Kinghorn, et al., 1986) describes an electronic safety valve and system for controlling the roof drain on a hydrocarbon storage tanks for the purpose of allowing water drainage while preventing the escape of the contained liquid. U.S. Pat. No. 4,176,553 (Wood, 1979) describes a system for measuring the level of a liquid in a storage tank having a predetermined reference level. Although these patents are applied to storage tanks, they are essentially unrelated to the invention described herein. There is no existing art that addresses the roof monitoring system described herein. 
         [0006]    U.S. Pat. No. 6,700,503 (Masar, et al. 2004) describes a means for wireless remote monitoring and graphically displaying the liquid level inside of a tank. The invention described herein does not incorporate a graphical display. 
         [0007]    WIPO PCT filing 94/26627 (Christensen, 1994) describes a system for estimating the inclination of a storage tank roof by using float-based liquid level sensors and reed switches whereas the invention described herein uses solid-state micro electro-mechanical devices to directly measure inclination. 
         [0008]    There is a considerable body of literature, standards, and patents that describe wireless sensor networks. A representative book that describes the current art is  Protocols and Architectures for Wireless Sensor Networks  by Karl Holger and Andreas Willig (Wiley, 2005). 
         [0009]    U.S. Pat. No. 7,339,957 (Hitt, 2008) describes how transmission time slots are allocated to transmitting nodes in a system of wireless environmental sensors and actuators for the purpose of controlling irrigation systems. Although the invention described herein utilizes environmental sensors, it does use actuators nor does it rely upon a slotted communications protocol. 
         [0010]    U.S. Pat. No. 7,386,352 (Davis, et al., 2008) describes a modular sensor network node architecture where the node architecture has each sensor coupled to its own small microprocessor so that it can be “plugged” into a sensor node containing a master microprocessor. The invention described herein is not modular and requires only a single microprocessor. 
         [0011]    U.S. Pat. No. 7,468,661 (Petite et al. 2008) describes a system and method for monitoring and controlling remote devices. This patent describes a variety of application areas but does not address storage tank monitoring. In its Detailed Description, this patent “describes the data structure of messages sent and received using the invention”. Throughout its claims, the communications system requires a retransmission device (e.g., “one or more retransmitting receivers”) or a “computer program that formats and stores select information for retrieval on demand”. The invention described herein does not require any of these components. 
       BRIEF SUMMARY OF THE INVENTION 
       [0012]    The present invention provides a new apparatus for monitoring floating tank roofs. The invention comprises one or more intelligent Sensor Units and one or more Communication Units. The Sensor Unit integrates multiple sensors into a self-contained unit that can be completely encapsulated for use in harsh environments. Additionally, the unit may have a magnetic base for rapid installation on steel tank roofs. The Communication Unit communicates with the Sensor Units and with an external Monitoring System. The Communication Unit can be used to relay information from the Sensor Units to or from a Monitoring System and may contain a microprocessor for sensor fusion or for computing alarm conditions. The external Monitoring System uses existing art and is not considered further. 
         [0013]    The Sensor Unit can incorporate batteries and/or solar cells for as a power source and communicates with the Communication Unit using a wireless communications link. Therefore, the invention does not require any external wiring, thereby simplifying deployment and improving reliability. It may also be encapsulated, thereby further improving reliability. 
         [0014]    The Sensor Unit is comprised of several functional units including a microcontroller; a wireless communication module, an inclinometer or tilt sensor; and a liquid sensor. Any or all of several other functional modules may be incorporated into a the Sensor Unit: a vapor sensor (such as a hydrocarbon sensor); a temperature sensor; a position sensor that uses the Global Positioning System (GPS) or differential GPS; a proximity sensor; and a microelectromechanical (MEMS) accelerometer. 
         [0015]    There is no known existing apparatus that is similar to the current invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1 : Conceptual Plan View of the System Deployed on a Tank Roof 
           [0017]      FIG. 2 : Functional Block Diagram of the Sensor Unit 
           [0018]      FIG. 3 : Pictorial Drawing of the Sensor Unit 
           [0019]      FIG. 4 : Liquid Level Measurement Technique 
           [0020]      FIG. 5 : Use of a Channel or Dome to Protect the Vapor Sensor from Immersion 
           [0021]      FIG. 6 : Submerged Sensor Unit 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    With reference to the conceptual plan view of a deployed system in  FIG. 1 , the invention consists of one or more encapsulated intelligent Sensor Units  10  and one or more Communication Units  12 . The Sensor Units  10  are deployed on the floating roof of the tank whereas the Communication Units  12  are normally located near the top of the wall of the tank  14 . 
         [0023]    The Sensor Units  10  communicate with one or more Communication Units  12  via wireless means. As shown in  FIG. 1 , a plurality of Communication Units  12  can be employed, thereby adding redundancy to the system and improving the overall system reliability. 
         [0024]    With reference to the functional block diagram in  FIG. 2 , said Sensor Unit  10  minimally comprises a power module  26  and the following core functional modules: a microcontroller  16 ; a communications module  18 ; an inclinometer or tilt sensor  28 ; and a liquid sensor  20 . Optional functional modules that can be incorporated to enhance the utility of the Sensor Unit include: a vapor sensor  30 ; a temperature sensor  22 ; a position sensor  32  that employs the Global Positioning System (GPS) or differential GPS (dGPS); a proximity sensor  24  that can be used to detect birds or other intruders; and a microelectromechanical (MEMS) accelerometer  34  to detect vibrations and changes in position. 
         [0025]    With reference to  FIG. 2 , the connecting lines between the microcontroller  16  and the modules ( 18  through  34 ) indicate communication links and are shown as being bidirectional but unidirectional connections are also possible. The communication link for the power module  26  is optional. 
         [0026]    The Sensor Unit  10  is powered by a power module  26  employing batteries, solar cells, or a combination thereof. The Communication Unit  12  can be line powered or be powered by a power module employing batteries, solar cells, or a combination thereof. 
         [0027]    For the purpose of identifying each Sensor Unit  10 , each Sensor Unit  10  is uniquely identified by one or more identification numbers: an electronic identification number that is unique to a particular deployment and is set up during system configuration; an electronic identification number that is unique and is set up before system configuration; or an identification number that is based on the GPS position of the Sensor Unit  10 . 
         [0028]    In the current embodiment, the Communication Unit  12  is comprised of a Texas Instruments MSP430 microcontroller; a Texas Instruments CC2500 communications module for communicating with the sensor units; a Cirrus Logic CS8900A Ethernet Controller; and a power module containing solar cells and a rechargeable battery pack. The Communication Unit  12  communicates with the Sensor Units  10  and with an external Monitoring System. The Communication Unit  12  can be used to relay information from the Sensor Units  10  to or from a Monitoring System. The microprocessor  16  is re-programmable and can easily have software added to it for supporting sensor fusion or for computing alarm conditions. The Communication Unit  12  uses known technologies and is not described further. 
         [0029]    With reference to  FIG. 3 , the Sensor Unit  10  is preferentially encapsulated for use in harsh environments, including but not limited to chemical plants, petrochemical plants, and marine environments. The alternative to encapsulation is mechanical sealing systems, such as enclosures sealed with gaskets. The Sensor Unit is weatherproof and immersible. For illustrative purposes, the communications module  18 , the position sensor  32 , and a solar panel (a possible component of the power module  26 ) are shown covered by a transparent dome  36  in  FIG. 3 . 
         [0030]    Preferentially, each Sensor Unit  10  incorporates a magnetic base  38  for rapid attachment to ferrous metal structures such as the floating lid of a storage tank. It may also be mounted using adhesives or mechanical means including fasteners or clamps. 
         [0031]    The Sensor Unit  10  may incorporate a GPS or dGPS position sensor module  32  to facilitate rapid and inexpensive installation. In this scenario, the Sensor Units  10  can be installed without regard for the specific location of any other particular Sensor Unit  10 . Subsequent information received from said position sensor module  32  allows the position of the Sensor Unit  10  to be determined after installation. 
         [0032]    In the current embodiment of the Sensor Unit, the core functional modules displayed in  FIG. 2  are implemented using: a Texas Instruments MSP430 microcontroller; a Texas Instruments CC2500 communications module, an Analog Devices ADIS16209 digital inclinometer, and an ultrasonic liquid sensor that is described next. 
         [0033]    With reference to  FIG. 4 , the preferred embodiment of the ultrasonic liquid sensor is comprised of: an ultrasonic transducer  40 ; an electronics module for the transducer that uses the existing art; and an acoustically-reflective surface  42  that may be the surface of the tank roof. In response to a command signal from the microcontroller  16 , the transducer transmits an acoustic pulse  44  toward said reflective surface  42  located at a known distance d 2    50 . The first reflection from said pulse  44  will be received by the transducer  40  after a propagation delay of 
         [0000]    
       
         
           
             
               t 
               S 
             
             = 
             
               
                 2 
                  
                 
                     
                 
                  
                 
                   d 
                   1 
                 
               
               
                 v 
                 A 
               
             
           
         
       
     
         [0034]    where t −S  is the round-trip propagation delay for the pulse; d 1    48  is the distance to the surface  46  of the liquid; and v A  is the velocity of acoustic propagation in the ambient atmosphere. Since we can use well-known methods to measure the propagation delay t −S , and we know d 2  and v A , we can use this equation to accurately determine the depth (d 2 −d 1 ) of the liquid. If d 1  is computed to be approximately equal to d 2 , then no measurable amount of liquid is present. 
         [0035]    The utility of the invention can be enhanced by additionally considering a second reflection due to the acoustic pulse. The round-trip propagation delay, t 2 , of said second pulse is given by 
         [0000]    
       
         
           
             
               t 
               2 
             
             = 
             
               
                 
                   2 
                    
                   
                       
                   
                    
                   
                     d 
                     1 
                   
                 
                 
                   v 
                   A 
                 
               
               + 
               
                 
                   2 
                    
                   
                     ( 
                     
                       
                         d 
                         2 
                       
                       - 
                       
                         d 
                         1 
                       
                     
                     ) 
                   
                 
                 
                   v 
                   L 
                 
               
             
           
         
       
     
         [0000]    where v L  is the velocity of acoustic propagation in the liquid. Since we can use the previous equation to determine d 1  and we know v A  and d 2 , we can use this new equation to determine v L . In many practical application areas, such as the storage of petrochemicals, the computed value of v L  can be used to determine if the liquid that is detected by the liquid sensor is the stored liquid, water, or a combination thereof. This type of liquid sensor and the techniques and the specific components required for its implementation are known in the existing state of the art. 
         [0036]    The current embodiment of the Sensor Unit includes the following optional sensor modules: a MicroChemical MiCS 5524 hydrocarbon vapor sensor, a Texas Instruments TMP275 temperature sensor, and a Tyco A1037-A GPS module. 
         [0037]    Since the Sensor Unit is designed to be immersible, the vapor sensor should be protected from contact with possibly damaging liquids. With reference to the conceptual cross-sectional diagram in  FIG. 5 , this is accomplished by placing the vapor sensor  52  inside of a hollow channel or dome  54 , which is shown as in  FIG. 5 . As the liquid level rises into said channel or dome, the atmospheric pressure inside of the channel or dome  54  increases. As shown in  FIG. 6 , this increase in atmospheric pressure in the entrained atmosphere  60  prevents the liquid  62  from rising into the channel or dome  54  far enough to reach the vapor sensor. 
         [0038]    With reference to  FIG. 5 , the channel or dome  54  preferentially contains a means, such as a rotary or piezoelectric fan  56  or a pump  58 , for circulating the ambient atmosphere across the surface of the vapor sensor  52 . The operation of said circulation device  56  or  58  is controlled by the microcontroller  16 . In  FIG. 5 , the conceptual flow of atmosphere is indicated by curved arrows. 
         [0039]    Communications among the system components (the Sensor Units  10 , the Communication Units  12 , and the external Monitoring System) may be initiated using one or more of the methods described in the following four paragraphs. In all cases, communications between any Sensor Unit  10  and the Monitoring System must pass through a Communication Unit  12 . 
         [0040]    The Monitoring System can send a request to one or more Sensor Units  10 . The Sensor Units  10  will subsequently reply with the requested information. This type of communications is referred to as polling. 
         [0041]    The Sensor Units  10  can send periodic status messages to the Monitoring System. These periodic messages can contain information from the sensors; alarm status; and/or information regarding the state of the Sensor Unit  10 , such as battery condition. This type of communications is referred to as periodic. 
         [0042]    The Sensor Units  10  can send messages to the Monitoring System in the event that an alarm condition has been detected by one or more Sensor Units  10 . This type of communication is referred to as event-driven. 
         [0043]    The Sensor Units  10  can communicate with each other to exchange sensor status and/or they can act as communications relays to improve the reliability or the range of the wireless communication system. This type of communication is referred to as local. Communications between the Sensor Units  10  and/or Communication Units  12  can use mesh networking protocols to improve reliability.