Patent Publication Number: US-7721611-B2

Title: Composite riser with integrity monitoring apparatus and method

Description:
This is a divisional application of U.S. patent application Ser. No. 10/704,079 filed on Nov. 7, 2003. 

   TECHNICAL FIELD OF THE INVENTION 
   The present invention relates to composite structures, apparatus to monitor the integrity of composite structures, and a method to monitor changes in stiffness. The present invention relates to using displacement, strain and vibration sensors to monitor changes in the riser stiffness. In particular, the invention has particular application to composite risers used in offshore oil and gas production. 
   BACKGROUND OF THE INVENTION 
   In offshore oil and gas drilling, production, and completion operations a platform at the surface of the ocean is connected to the well head on the sea floor by risers. A riser is a tubular member through which drilling tools, tubing, and other components used in oil and gas exploration pass. The current practice is to make the risers from steel. More recently, it has been proposed that the risers be made from composite materials. Risers made from a composite material offer the advantage of being lighter in weight than steel risers. Thus, composite risers have the advantage of requiring a smaller surface platform to support the same length of composite riser than would be required with a steel riser. 
   Offshore oil and gas exploration is progressively moving to deeper and deeper water. Thus, the weight savings advantage of the composite riser become more significant as the water depth in which wells are drilled becomes greater. Some well heads are on the sea floor more than 5,000 feet below the surface of the ocean. 
   A concern with any deep water oil and gas exploration is maintaining the integrity of the riser system. Breaches in the riser system can result in the escape of drilling muds, oil and/or gas into the sea. 
   The use of composite risers in actual field applications is relatively new. Thus, there is little long-term experience concerning the reliability of composite risers. Clearly, failure or breach of a riser is to be avoided. The present invention provides an apparatus and method for monitoring the integrity of composite risers by monitoring changes in the riser stiffness. Monitoring of the stiffness of the risers can allow identification of weakened risers and allow their replacement prior to failure. A change in the stiffness is monitored using strain sensors or vibration sensors. 
   Stiffness is defined as a measure of the amount of deformation per unit load. When a riser joint is new, it will have certain stiffness value and therefore when the joint is subjected to a certain load, the joint will deform to a certain level, which can be measured using displacement gauges or strain sensors. The strain is defined as the displacement per unit length of the section over which the displacement is measured. The virgin stiffness of a riser joint can be predicted using numerical solutions and the amount of strain when the riser joint is subjected to a specific load can also be predicted using numerical solutions such as finite element analysis. When the riser is damaged, the stiffness will be reduced and the amount of deformation for the same load will be increased. 
   Stiffness of the composite riser is an important design parameter because high stiffness results in high loads when the riser stretches as the platform moves and low stiffness is not desirable because it can result in clashing between different risers. The axial stiffness of the riser is related to the elastic modulus of the riser, the cross sectional area and the length of the riser string. The length of the riser string is defined by the water depth and the cross sectional area is mainly established to ensure that the riser can withstand the design loads such as pressure, tension and bending loads. The elastic modulus is affected by the fibers used to manufacture the composite riser and the layout of the different laminates. While the currently used material, steel, has a fixed elastic modulus of 30 million lb/square inch (206.85 million kPa), composite risers can have different values. The present invention can be used with composite risers, the elastic axial modulus of which is between 5 to 15 million lb/square inch (34.475 and 103.425 million kPa), and preferably a value between 10 and 14 million lb/square inch (68.95 and 96.53 million kPa). Damage to the composite riser will manifest itself by a reduction of the riser&#39;s stiffness, indicating that the elastic modulus of the riser has been reduced. 
   It is also noted that the composite riser joint will fail when the strain in the riser reaches a specific value. This value is in the order of 0.5% for the carbon fiber composite risers being considered for offshore applications. An object of the present invention is to monitor riser strain either (1) on a continuous basis to assess the extent of damage and also the variation of loading, or (2) by monitoring for the maximum strain experienced in the riser until it reaches a specific value which is lower than the strain at which failure is expected. This will ensure sufficient time to remove the damaged joint prior to its failure. In another aspect, the present invention provides for using the natural vibration frequency of the riser to monitor the integrity of the riser. As the stiffness of the riser changes, its natural frequency, which is a function of the riser&#39;s stiffness and mass, will change and thus the riser&#39;s vibration signature will change. Although this is a well known technique, individual testing and the generation of custom strain curves is required to characterize a specific riser because configuration, cross-section, wall thickness, material selection, etc. will affect vibration response characteristics. Monitoring the changes in a riser&#39;s vibration signature, which is commonly done using accelerometers, can provide an indication of the level of damage to that riser. Because of the complexity of the composite structure, theoretical predictions of the relationship between level of damage and changes in strains or vibration signature are difficult. Therefore, calibration curves need to be developed as part of the riser qualification program. Developing these curves involves testing some composite joints to induce damage. In one embodiment of the invention, fiber optics are used as the strain sensors and a test method is provided demonstrating the qualification of the riser when strain monitoring is used. 
   SUMMARY OF THE INVENTION 
   In one aspect, the present invention relates to a composite structure adapted for the measurement of changes in the stiffness of the composite structure. In a preferred embodiment, the composite structure is a composite riser having a metal liner with metal composite interfaces attached to each end. The riser is covered with one or more composite structural members. The riser includes at least one strain gauge attached to the riser. Preferably, the riser includes a first strain sensor oriented in a first orientation and a second strain sensor oriented in a second orientation. These strain sensors can be of any known design; however, in the preferred embodiment the strain sensors are fiber optic strain gauges and electromagnetic sensors (steel elements) which are embedded in the riser during fabrication. 
   The strain gauges can be positioned in areas of interest. Typically, these areas of interest will be the areas most likely affected by internal damage to the composites; for example, the area where the composite structure and the metal connector interfaces are joined. This area is called the metal-composite interface (MCI). 
   In another embodiment, the present invention relates to monitoring changes in the composite riser stiffness using vibration monitors (e.g. accelerometers) that will allow for determining changes in the natural frequency and mode shape of the composite structure. 
   In another embodiment, the present invention relates to a monitoring system for a riser assembly. In this embodiment, a plurality of risers extend from the well head on the sea floor to the surface platform. In this embodiment, the strain sensors and the vibration monitors located in each riser are connected to a control unit on the surface platform. The control unit on the surface platform has a means to generate a signal to the individual strain sensor in each riser, to measure the strain and vibration response in each riser, and to record the measured strain and natural frequency. Preferably, the measured strain and/or natural frequency are recorded together with the time that the strain and/or the vibration responses are measured as well as the riser in which the responses were measured. Alternatively, the strain and/or vibration responses in only selected risers can be monitored. 
   In another embodiment of the present invention, a monitoring module is provided on an individual riser, although if desired, more than one monitoring unit can be employed. The use of a self-contained monitoring module obviates the need to connect the risers to the surface via a transmission line. The monitoring module has a power source, a processor unit, a communication device, and a signal device. The processor unit of the module has the capability of initiating the signal unit to send a signal to the sensor on the riser. The processor also includes an interface or other device to receive the measured data from the sensors, memory to store the measured data, and preferably signal processing capability to compare the measured data against a predetermined warning value. With a preferred embodiment, the processor unit also includes a signal processing capability to determine the ratio between the measured strains in either the first or second orientation against the strain measured in the other orientation. In yet another embodiment, the processor also includes a means to compare the determined ratio against a predetermined value of the ratio set as a warning limit. Preferably, the monitoring module also includes a memory or other storage means to store the measured strain values and/or the ratio of measured strain values. Additionally, the monitoring module contains a communication device to output the strain data and/or the stored values. The monitor module can also include a capability to initiate an alarm in the event the warning limit is exceeded. 
   The invention also is a control system for performing the monitoring of the strain. The control system components and functions can be integrated at a single location or dispersed to multiple locations. The control system can include an input interface to input data and commands such as riser identification, alarm limits, and commands to initiate measurement; a signal means to send and receive measurement signals to the strain gauges; a processing capability to receive the measured data and process the data as desired, e.g., compare the measured data to warning limits, store the data, and output the data; and a communication device for outputting data in a desired manner. 
   In another embodiment, the invention includes a remotely controlled submersible vehicle. This remotely controlled submersible vehicle includes a recorder device. In one aspect, the recorder includes a processor and a link device. The link device provides a communication link to the monitoring module. The processor includes a mechanism to initiate a download of stored strain measurements data or ratio data of strain measurements from the monitoring module, and a way to store the downloaded data. The recorder also includes a way to output these values when the submersible is recovered at the surface. 
   In another aspect, the recorder unit of the submersible vehicle includes a device to generate a signal to the strain gauges in the riser. The recorder includes a device to record the measured strain from the sensors in the individual risers. This embodiment is especially suited to the use of electromagnetic strain sensors. 
   The method of the present invention can include the steps of sending a signal to a strain and/or vibration measuring device in operative association with a composite riser, recovering the response to the signal, comparing the response to a warning limit, computing the ratio of response measured in one orientation to that measured in another orientation, comparing the computed ratio to a warning limit, outputting the data, storing the data, and initiating an alarm. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood in light of the detailed description when read in conjunction with the drawings. Any drawings in detailed description represent certain embodiments of the invention and are not intended to be limiting of the invention. In the drawings: 
       FIG. 1  is a cross-sectional view of a composite riser of the present invention; 
       FIG. 2  is a cross-sectional view of a composite riser of the present invention; 
       FIG. 3  is a schematic representation of orientation of separate fiber optic strain sensors of the present invention; 
       FIG. 4  is a schematic representation illustrating the use of a single fiber optic strain sensor for both axial and hoop measurement; 
       FIG. 5  is a riser string and control system of one embodiment of the present invention; 
       FIG. 6  is a side view of a riser with electromagnetic strain sensors in another embodiment of the invention; 
       FIG. 7  is an illustration of one embodiment of a monitoring module and submersible vehicle of the present invention; 
       FIG. 8  is a graph of strain percentage for various test sequences; 
       FIG. 9  is a graph of the ratio of hoop to axial strain for various test sequences; 
       FIG. 10  is a schematic illustration of the control system of the present invention; 
       FIG. 11  is a schematic illustration of alternate embodiments of the distribution of control functions; 
       FIG. 12  is a schematic illustration of two embodiments monitor module attached to a riser, and a remote vehicle for monitoring the risers; and 
       FIG. 13  is a schematic illustration of a monitoring module. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a cross-sectional view of one embodiment of a riser of the present invention. The figure is not to scale for purposes of illustration. Composite riser  20  has an inner liner  22  which defines passageway  24 . Liner  22  is preferably of a metal such as steel, aluminum or titanium. Adjacent to liner  22  is shear ply  26 . Shear ply  26  is a rubber of polymeric material. Further, the shear ply is preferably fluid impermeable. Placed over shear ply  22  is the main structural layer  28 . The main structural layer  28  is of a composite material. Covering the outer side of structural layer  28  is a fluid impermeable layer  30  preferably made of rubber that is covered by a scuff absorbing layer  32 . In this embodiment, two fiber-optic strain sensors  34  and  36  are embedded in the riser below the outer fluid impermeable layer  30 . Preferably, they are embedded in the area of the metal-composite interface. It will be understood by those skilled in the art that the specific design of the riser is not limited to the illustrated design. In a preferred embodiment, the composite riser has an elastic axial modulus of from 5 to 15 million pounds per square inch, and more preferably a value from 10 to 14 million pounds per square inch. Risers with an elastic modulus within these ranges can be provided by known techniques and methods of construction using finite analysis to design the composite structure. 
   For the fiber optic strain sensors, the same fiber can contain multiple sensors. (See  FIG. 4 .) These sensors are generally formed by machining a grating (Bragg grating) in the fiber. When laying the optical fiber, some of the sensors will be positioned to monitor the axial strains (See  FIG. 1 , sensor  34 ;  FIG. 2 , sensor  34 ;  FIG. 3 , sensor  58 ; and  FIG. 4 , sensor  67 .) while the others are positioned to monitor the hoop strain (See  FIGS. 1 , sensor  36 ;  FIG. 2 , sensor  36 ;  FIG. 3 , sensor  64 ; and  FIG. 4 , sensor  68 ). In order to monitor these sensors, the ends of the fiber containing the sensors pass through fluid impermeable layer  30  and the scuff barrier  32  to the outside for connection to the monitoring device. Typically the composite riser  20  will be constructed by winding the composite fibers over the liner. Normally in such construction there are fibers which are positioned longitudinal or substantially parallel to the axis  25  of passageway  24  and also fibers, usually referred to as hoop fibers, in one or more directions running in a direction substantially offset from the axis, such as circumferential, spiral, helical, etc. Preferably, the fiber optic strain gauges are embedded in the riser during production of the riser. Thus, it is convenient for them to be positioned in orientations corresponding to the orientation of longitudinal fibers and to the hoop fibers. Preferably, one of the strain gauges is oriented substantially parallel to the axis of the riser to measure axial strain. The other strain gauge is preferably positioned and embedded along the orientation of one of the hoop fibers. In the hoop orientation the strain sensor will be available to measure the hoop strain. Preferably, the orientation of the strain sensor embedded in the hoop direction is substantially perpendicular to the axis of the riser. In a less desired embodiment, the orientation of the strain sensor embedded in the hoop direction is at an angle within 30 degrees of the perpendicular to the axis. The orientation of the other strain gauge should be substantially longitudinal and preferably is parallel or not more than 20 degrees from being parallel to the axis of the riser. The preferred location for the fiber optic strain gauge is in the main structural layer but they can be positioned elsewhere if desired. 
     FIG. 2  is a simplified cross-sectional view of composite riser  20 . On the interior of the riser is metal liner  22  along axis  25 . On each end of the liner are attached metal composite interface portions  40  and  42 . Metal composite interfaces  40  and  42  are provided with metal connectors  44  and  46  respectively. In this example, flanges are shown, but other commonly used oilfield connectors such as pin and box threaded joints can be considered. These metal connectors can contain holes  48  through which bolts or other fasteners can be passed to connect two or more risers together. The layers surrounding the liner  22  and the metal composite interfaces  40  and  42  are generally indicated as  50 . The details of the layers have been omitted for purposes of clarity. In this illustrated embodiment, two longitudinally oriented strain gauges  34  and  34 ′ are provided. These are illustrated as extending some length along the riser axis. The particular length and number of these first strain gauges is a matter of choice. Also, if desired the various first strain gauges can be installed at different depths within the structural composite layers  50 . 
   Two second strain sensors  36  and  36 ′ are shown in the hoop orientation. These strain gauges are helically wrapped about the axis  25  and within the outer layers  50 . Like the first strain sensors  34 , second strain sensors  36  can be positioned at various depths. Also, one or more second strain sensors can be employed. As illustrated in  FIG. 2 , second strain sensors  36  and  36 ′ are wrapped in a helical fashion or about the axis. The preferred orientation for the second strain sensors is along the circumference of the risers, i.e. 90 degrees off of the axis  25 . 
   The fiber optic strain gauges are preferably embedded in the structural layer  28 . The strain gauges are also preferably positioned such that they are adjacent to the portions of the riser  20  most likely to be damaged or to fail, which is typically the metal-composite interface area. 
     FIG. 3  illustrates the fiber optic axial sensors  54  and the hoop sensor  56  that can be used for measuring the axial strain and the hoop strain.  FIG. 3  shows the use of a separate fiber for each strain sensor. Axial strain sensor  54  has an axial fiber optic strain sensor portion  58 , a fiber optic tail portion  60  connecting the axial strain sensor portion  58  to lead  62  for connecting to monitoring equipment. Hoop strain sensor  56  can have the same construction as axial strain sensor  54 , except that the hoop strain sensor portion  64  is positioned substantially perpendicular to the axis  25 .  FIG. 4  illustrates the use of single optical fiber  66  having a strain sensor section  67  and a hoop strain sensor section  68 , to measure both axial and hoop strain. If desired more than several sensors can be provided per optical fiber to provide for redundancy as well as temperature compensation. 
     FIG. 5  illustrates another embodiment of the present invention.  FIG. 5  illustrates riser string  70  composed of a number of individual risers  20 . The top of the riser string  70  is connected to a surface platform  72  on the surface  74  of the ocean. The lower portion of the riser string  70  is connected to the wellhead  76  on the sea bed floor  78 . In this embodiment, a transmission line  80  extends from the surface platform  72  along the riser string  70  and is connected to leads  84  and  86  to the first and second strain gauges in the separate riser sections  20 . In the illustration, each riser  20  has its strain gauges connected to the transmission line  80 . The transmission line  80  can be attached to the outside of the riser string or embedded in the risers  20 . However, only a selected riser joint  20  can be monitored if desired. In a preferred embodiment, each riser joint  20  is monitored. 
   Transmission line  80  is connected to controller  82 . Signals can be sent from controller  82  to the various strain gauges on the various risers  20  and the measured strain data on one or more selected strain gauges is returned. Transmission line  80  may be a single common line for a plurality of risers  20 , or may be a bundle of transmission lines, one for each riser. Well known electrical addressing techniques may be used in the case of a common transmission line  80  for communicating with a selected one of a plurality of risers connected to that line. Measured strain can be displayed to the user, recorded in a databank, or compared against a preset warning level, which if reached, causes an alarm signal, such as a light, sound, etc. to be activated. Preferably, the controller  82  records the date, time and measured data for each riser and the identification of the riser. This provides a historical record of measured data to be used to improve riser design, predict the life cycles, and to identify risers in need of preventative replacement. 
     FIG. 6  illustrates another embodiment of the present invention. A transmission line  80  extending along the length of riser string  70  has certain drawbacks, including the difficulty of installation and protection from damage. Thus, in another embodiment of the present invention, a monitoring module  90  is provided. The monitoring module  90  is provided with a means to attach it to the riser, such as a collar  92  for mounting on riser  20 . In the illustrated embodiment, collar  92  has a first arm  94  hingedly connected to a second arm  96  by a hinge. Arms  94  and  96  at their free ends  100  and  102  are provided with holes through which a bolt  104  can pass. In a preferred embodiment, a spring  106  is provided on the outside of one of the free ends. Spring  106  serves to bias arms  94  and  96  against the outside of riser  20 , to compensate for any decrease in riser diameter as it is subjected to increasing pressure the further it is extended into the sea. Of course other types of connections are equally suitable such as clamp, fasteners or even glue. The module  90  is provided with connectors  108  and  110  to connect to the leads of first and second strain sensor. Thus, the strain sensors are connected to a signal device  111  and control device module  112 . Control device  112  has attached to it output/input communication device  114  which is described further below. Control device  112  can be a battery powered computer processor  116 . 
   Preferably, the processor  116  is programmed to initiate a signal or prompt the signal device to send a signal to the first and second strain sensors at a predetermined time or on command. The processor may be any type of computer, microcomputer, microprocessor, or digital or analog signal processor. The strain data from each sensor in response of the signal is received and processed by the processor  116 . In one embodiment, the signal received can be compared against a predetermined strain data value corresponding to a warning limit. Preferably, the strain data is stored in a memory for later download. In a preferred embodiment, the memory is located inside the module  90 . The processor is also connected to one or more output/input communication device  114 . The output/input communication device can be in the form of acoustic transceiver, a hard connection to the transmission line, optical link or other means. In one embodiment, the strain data is stored in module  90  until a submersible vehicle  120  aligns with the communication device for inputting and outputting stored data from the control device  112 . The stored strain data can be downloaded to a recorder  126  on the submersible vehicle  120 . The submersible vehicle  120  can then be recovered at the surface and the data obtained from the module extracted for use. 
   In another embodiment, the control device  112  can also include an acoustic generator  127  as a communication device. Strain data values can then be transmitted directly to the surface acoustically. Alternatively, strain data values can be stored until downloaded to the remote vehicle  120 . Preferably, even in the situation where strain data values are stored an immediate action is desirable in the event that the warning limit is exceeded, in which case an acoustic signal is transmitted to the surface to activate an alarm on the surface platform. 
   The monitoring module  90  can be provided with a capability or fixture for aligning the submersible  120 , such as projection  122 , to assist in aligning the communication terminal  114  of the monitoring module  90  in position to communicate with the communication device  124  of the recorder  126  of the submersible  120 . The submersible vehicle can also have an alignment means such as recesses  129  to receive projections  122 . The submersible may be of any known design for submersible vehicle and preferably is remotely controlled from the surface platform. The submersible  120  is equipped with a recorder  126 . The recorder  126  can include a control element to signal the control device  112  of the monitoring module  90  to download data. In one embodiment, the submersible is positioned such that the communication means  124  of the submersible and communication device  114  of the monitoring module  90  are in communication and strain data is downloaded to the recorder  126  on the submersible for later recovery and processing at the surface. One type of self contained monitoring module system is disclosed in U.S. Pat. No. 4,663,628. Details of the internal operation of monitoring module  90  are omitted as the construction and programming of microprocessor based data collection and storage systems is well known. 
   Alternatively, the submersible can include a control element  130  to directly initiate a signal to the strain sensor and then record the response strain measurement. In this embodiment, the monitoring module is not required. Instead, the submersible aligns with the leads to the fiber optic strain gauges and transmits a strain signal and records the response. 
   In another embodiment the strain sensor may be a piezoelectric strain sensor. Currently, these have the disadvantage that with the current technology they are rather bulky and are not as conveniently incorporated into the composite riser as are the fiber optic strain sensors. The piezoelectric strain sensors are connected to leads and the operation is like that as described in relation to the fiber optic strain sensors. The disadvantages of piezoelectric sensors may change over time rendering this type of sensor more desirable for use in implementations employing the present invention. 
   In yet another embodiment of the invention, the strain sensors are magnetic. Magnetic strain measurements have the advantage that a power supply mounted in a monitoring module is not needed. As illustrated in  FIG. 7 , first magnetic strain sensor  131  and second magnetic strain sensor  132  are strips of metal adhered or embedded into a composite riser. The magnetic gauge can be a wire of magnetic material bonded within the structure, or it can be a strip of magnetic material with a reduced cross-sectional area in the midportion of the strip which increases the sensitivity of the gauge. These magnetic gauges are passive in the sense that no direct connection to a circuit is required, and magnetic detection equipment is employed in conjunction with gauge. This detection equipment generates a magnetic field and measures the difference in the field caused by the gauge. The detection equipment can be contained in the submersible vehicle. Strain is measured by measuring the change in the magnetic field associated with changes in the magnetic sensor caused by strain. Thus, these magnetic strips can be adhered to the composite riser and the magnetic field monitored and recorded by a remote vehicle. Magnetic gauges may also be used with a monitoring module to simplify the attachment of the monitoring module and to obviate the need for electrical or optical connections to the module. 
   In yet another embodiment, the strain gauge can be a resistance gauge or an acoustic gauge. An acoustic strain gauge is shown in U.S. Pat. No. 5,675,089 entitled “Passive Strain Gauge” and is incorporated herein by reference. 
   In yet another embodiment, accelerometers are used to measure the vibration response for determining strain data. The vibration signal can be analyzed by any number of means including frequency transform using fast Fourier transform algorithmic analysis to detect variations in natural frequency and shift in phase angle. 
   Testing for Setting Warning Values 
   For each composite riser design, testing of the riser should be performed and measurements of changes in axial displacement, axial and hoop strains, and vibration signature during pressure testing recorded. This testing allows one to empirically determine values to be employed as warning limits in the monitoring of integrity in the operational environment. Preferably, the strain sensors are installed in the test riser at selected locations during fabrication. The accelerometers are mounted on the riser joint after fabrication. This test riser is then subjected to a sequence of increasingly severe loads that are intended to create damage in the test specimen. An example of such testing protocol is described below and is summarized in Table 1. 
   
     
       
         
             
             
             
           
             
               TABLE 1 
             
             
                 
             
             
               Load Sequence 
               Load Case 
               Comment 
             
             
                 
             
           
          
             
                1 
               Pressure to 427.5 bar (6200 psi) and hold for 5 min. 
                 
             
             
                2 
               Pressure to 427.5 bar and hold for 15 min. 
             
             
                3 (FPT 1) 
               Pressure to 315 bar (4500 psi) and hold for 5 min. 
               Baseline measurement 
             
             
                4 (FPT 2) 
               Pressure to 315 bar. 
             
             
                5 
               Axial load to 2060 kN without internal pressure. 
             
             
                6 
               Axial load to 2060 kN without internal pressure. 
             
             
                7 
               Axial load to 2060 kN with 30 bar internal pressure. 
             
             
                8 
               Axial load to 2060 kN with 30 bar internal pressure. 
             
             
                9 (FPT 3) 
               Pressure to 315 bar. 
             
             
               10 
               Axial load 2550 kN with 30 bar internal pressure and 
               First extreme axial load 
             
             
                 
               hold at max. load for 5 min. 
               sequence. 
             
             
               11 
               Cyclic axial load between 2060 kN and 2550 kN for 
               First cyclic load sequence. 
             
             
                 
               101 cycles 0.1 Hz, with 30 bar internal pressure. 
             
             
               12 (FPT 4) 
               Pressure to 315 bar. 
             
             
               13 
               Axial load 4500 kN with 30 bar internal pressure. 
             
             
               14 
               Cyclic axial load between 3500 kN and 4500 kN for 
             
             
                 
               101 cycles 0.1 Hz, with 30 bar internal pressure. 
             
             
               15 (FPT 5) 
               Pressure to 315 bar. 
             
             
               16 
               Axial load 5000 kN with 30 bar internal pressure. 
             
             
               17 
               Cyclic axial load between 4000 kN and 5000 kN for 
             
             
                 
               109 cycles 0.1 Hz, with 30 bar internal pressure. 
             
             
               18 (FPT 6) 
               Pressure to 315 bar. 
             
             
               19 
               Axial load 5800 kN with 30 bar internal pressure. 
             
             
               20 
               Cyclic axial load between 4800 kN and 5800 kN for 50 
             
             
                 
               cycles 0.1 Hz, with 30 bar internal pressure. 
             
             
               21 
               Cyclic axial load between 4700 kN and 5900 kN for 20 
             
             
                 
               cycles 0.1 Hz, with 30 bar internal pressure. 
             
             
               22 
               Cyclic axial load between 4600 kN and 6000 kN for 20 
               Max axial load higher than 
             
             
                 
               cycles 0.1 Hz, with 30 bar internal pressure. 
               predicted failure load of 
             
             
                 
                 
               5925 kN (1330 kips). 
             
             
               23 
               Cyclic axial load between 4400 kN and 6200 kN for 20 
             
             
                 
               cycles 0.1 Hz, with 30 bar internal pressure. 
             
             
               24 (FPT 7) 
               Pressure to 315 bar. 
             
             
               25 
               Axial load 6500 kN with 30 bar internal pressure. 
               Failure after 4:20 min at 
             
             
                 
                 
               6500 kN steady load. 
             
             
               26 (FPT 8) 
               Pressure to 315 bar. 
             
             
               27 
               Axial load 2060 kN with 30 bar internal pressure. 
               Same as 7 and 8. 
             
             
                 
             
          
         
       
     
   
     FIG. 8  shows a graph of the sequence of loading tests to cause progressive damage to the composite riser. The x axis of  FIG. 8  is the load sequence number for Table 1, and the y axis is pressure in bars. In an actual test performed by the inventors, the test specimen failed at load sequence  25  at an axial load 6,500 kN. Failure was detected by a loud bang and by a drop in the load from 6,500 kN to 5,500 kN. On visual inspection the riser had numerous small cracks on the outer surface at the middle of the riser and towards one end. The riser joint was cut open and it was found that the composite had delaminated between the two ends with visible cracks in the matrix in the hoop layers in the trap locks. Despite this amount of damage the riser integrity remained mostly intact. This was demonstrated by the subsequent ability of the specimen to withstand load sequences  26  and  27  that includes a pressure test of 315 bar and axial test 2,060 kN. 
   During the testing, strain was monitored using both fiber optic sensors and strain gauges. In  FIG. 9 , the x axis shows the FPT sequence number from Table 1, while the measured axial strain during eight pressure cycles is shown on the y axis.  FIG. 9  shows the changes in the axial strain when the joint is loaded and also the residual axial and hoop strains at zero loads. These results indicate the changes in the strains as a measure of damage. 
   The measured strain clearly shows that the strain pattern changed over the test duration. Importantly, it was discovered that the ratio of the hoop strain to axial strain serves as an excellent indicator of progressive damage.  FIG. 10  presents the changes in the strain ratio after different FPTs (the x axis shows the FPT sequence number from Table 1) as measured by fiber optic sensors embedded in the composite joint. As shown in  FIG. 9 , an indication of failure occurred when the longitudinal (axial) strain increased by about 100% (from 0.115 at the reference FPT to 0.2% for the FPT prior to when the failure was observed, see sequence number 7). Even when the strain increased by 100%, the hoop and axial load capacities were not compromised indicating that the riser still had sufficient capacity to be retrieved without compromising the safety of the riser. As a safety measure, a realistic criterion may be preferably set at a change in the strain of 50% for removal of the joint from service or other predetermined value. One benefit of the present invention is that historical data can be used to adjust the warning value based on in-service experience. Alternatively, as shown in  FIG. 9 , the residual axial or hoop strains at zero loads can also be used as an indicator of damage development as these values increase after severe loading cycles. 
   The measured strain clearly showed that the strain pattern changed over the test duration. Detailed analysis of the changes in the strain pattern demonstrate that the absolute value of the strain under load, the residual strain under zero load, and the ratio of the hoop strain to axial strain each serve as an excellent indicator of progressive damage. 
   The changes in the axial strain under constant load, as the joint is progressively damaged, means that the stiffness in the joint is decreasing, which can also be measured using vibration monitoring techniques. In another aspect, the present invention provides for using the natural vibration frequency of the riser to monitor the integrity of the riser. As the stiffness of the riser changes, its natural frequency, which is a function of the riser&#39;s stiffness and mass, will change and thus the riser&#39;s vibration signature will change. Although this is a well known technique, individual testing and the generation of custom strain curves is required to characterize a specific riser because configuration, cross-section, wall thickness, material selection, etc. will affect vibration response characteristics. Monitoring the changes in a riser&#39;s vibration signature, which is commonly done using accelerometers, can provide an indication of the level of damage to that riser. Because of the complexity of the composite structure, theoretical predictions of the relationship between level of damage and changes in strains or vibration signature are difficult. Therefore, calibration curves need to be developed as part of the riser qualification program. 
   While warning limits may be empirically determined as described above, warning limits may also be analytically determined based on predicted behavior of the structure so long as adequate models are available. What is pertinent for the current disclosure is not the details of well known modeling techniques, but, instead, how warning limits are utilized. 
   Control System 
   The control and monitoring functions can be consolidated at the controller  82  on the surface platform  72 , or divided among the monitoring modules  90  on the composite risers  20  and the recorder  126  of the submersible vehicle  120 . The control system and method will be discussed first as an overall system and method in reference to  FIG. 11 . It is understood that the specific components and functions can be implemented in different manners by different devices at different locations in the system. The functions can be performed by a computer, microcomputer or microcomputer based system programmed to perform the functions operating in conjunction with peripheral devices. Alternatively, some functions can be conducted by a circuit or device having specific functionality rather than a programmed computer. 
   In a preferred embodiment, an input device, block  140 , such as communications port or interface is provided to input basic information into the processor. This information can include, an identification assigned to each individual riser to be monitored, clock settings, timing sequence for testing, and warning limits. The strain measurement sequence can be initiated on command inputted by the operator, or automatically based on a timing program or by input from sensors triggered by certain events, such as environmental conditions indicative of severe weather which could produce severe strain on the riser string. This function can be performed by a means to initiate measurement such as a keyboard, timing program, or inputted sensor signal, block  142 . 
   The system includes a strain measurement signal generator and receiver of the return measured strain value, block  144 . This can be performed by known strain measuring equipment for the type of gauge being employed. The measured strain in each orientation is inputted into the control system. The control unit preferably includes a visual output device, block  146 , such as a display screen, printout, or other means to allow the operator to view the results. In a preferred embodiment, the processor also includes a capability to correlate the measured strain data, block  150 , with the time at which the measurement was taken and a means for storage of that information, block  148 . Additionally, it is preferred that the control system include a capability for calculating the ratio of strain data measured, block  150 , in either the first or second direction against the strain measured in the other orientation. The ratio value is preferably stored together with the time that the measurements used to compute the ratio were taken. In a preferred embodiment, an input means such as a keyboard or a ROM chip is provided for input of the predetermined warning value for strain data in one or more of the first orientation, second orientation, and/or strain ratio indicative of a strain threshold on the riser predictive of damage or failure. The controlled processor preferably includes a means such as program code to compare the measured strain against the predetermined warning value, block  150 . 
   The system preferably includes an alarm generating means such as a computer program which initiates an alarm  152  perceptible to the operator such as a visual display, sound, or other indicator. In the embodiment where a monitoring module is attached to the individual risers, this alarm means can include an acoustic signal generator in the monitoring module which sends acoustic signals to a receiver connected to the controller on the surface platform. The method of the present invention in a preferred embodiment involves the steps of inputting to the processor base data, which preferably includes warning limits, initiating strain measurement, conducting strain measurement, collecting strain data, and outputting the strain data. Preferably, the method also includes comparing the strain data against predetermined warning limits, outputting an alarm signal if the warning limit is exceeded. Additionally, the method also includes storing of the strain data. 
   When the control system includes monitoring modules on the individual risers, a submersible vehicle may be beneficially employed. Use of a UAV (Underwater Autonomous Vehicle) is desirable as it eliminates a need for a transmission line from each monitor to the surface. Also, the submersible is preferred in order to conserve power in the monitoring module&#39;s power system. It is also preferred that the control system include a storage device to store data and allow for a database of the measured strain for each riser and details of the riser construction. Suitable types of storage devices are well known and include semiconductor memory, RAM FLASH, etc. An output device  154  is provided to output in electronic, optic, magnetic, or other form this information which can then be either transferred to another computer processor, or visually displayed. Retention of a historical record can be desirably used to improve riser design and to perfect and refine appropriate warning limits. 
   The monitoring system can be constructed in many different manners, and in a preferred embodiment, one or more monitoring modules  160  are attached to each riser  20  or selected risers within the string as illustrated schematically in  FIG. 12 . The monitoring module  160  contains a central processor unit  162 , a communications device  164  to provide communication with the remote controlled submersible vehicle or to provide acoustic communication, optical communication or other communication with the surface platform. Processor unit  162  may be any suitable type of computer, computer module, microcomputer, microprocessor, or digital signal processor. The module further includes a power supply  166  such as a battery to power the unit, a signal device  168  and a memory device  170 . The signal device  168  transmits and receives signals to and from the strain sensors. 
   The central processing unit  162  can be programmed in many different fashions to satisfy the needs of the user. Preferably, the unit has stored in memory an identification of the riser to which it is attached. This identification is used to correlate the output data of the strain or vibration sensors with the particular riser. The processor is programmed to receive command signals and/or a stored timing routine. The processor generates a signal to the signal device which initiates the delivery of a signal to the strain sensor, the return signal is received by the signaling device and the strain value is compared to the warning limit. Similarly, the strain measured in the second orientation is compared against warning limits. The ratio of the strain measured in the first orientation with that measured in the second orientation within a predetermined time is computed and compared against the stored warning limit. If the warning limit is exceeded, the processor can generate a command to the communication device to send an alarm signal to the surface. It is not necessary to make the comparison to the warning limits. Preferably, all measurements made are then stored in the memory device  170 . Preferably, the data stored includes the time of the measurement, strain measured in the first direction, strain measured in the second direction, and a ratio of the strain measured in the two orientations. The processor is further programmed to download the stored data upon receipt of a command from the recorder unit  180  in the submersible vehicle or from the surface controller. The recorder unit  180  contains a processor  182 , a communication device  184 , and a memory device  186 . The recorder can be powered by the power supply of the submersible vehicle. The submersible vehicle can also include lights and video equipment commonly used for underwater visual inspection. The recorder  180  can input into monitoring module  160  new base information updates such as a change in the warning limit and accept downloads of strain data from the monitoring module  160 . This arrangement can be repeated for each riser. 
     FIG. 12  shows another embodiment in the lower half of the figure. One or more alignment devices  190  is preferably provided adjacent to the strain sensors. The use of an alignment device is useful when the strain sensors are magnetic sensors. The alignment device allows for the consistent positioning of a submersible vehicle with an embedded magnet sensor, thereby allowing the submersible vehicle to align with the strain sensors and take measurements. In this embodiment, the recorder  180  includes a strain signal device  188 , for example, a magnetic field generator and sensor to measure strain in embedded magnetic strain sensors  131  and  132  (see  FIG. 7 ). Preferably, the downloaded data includes the stored strain measurement data as well as identification of the riser. The data stored in the memory of the recorder is recovered when the vehicle is brought to the surface. The various steps of the measuring and the functioning of the system can be performed either by the surface controller, by the modules, or by the recorder in the submersible vehicle if employed. 
   Further details of the internal operation of the monitoring modules is omitted for simplicity because the electronic and microcomputer based systems for recording and storing data are well known in the art. For example see U.S. Pat. No. 4,663,628. Accordingly, what is pertinent to the current disclosure is the functions performed by the module, how the modules are accessed and/or interconnected and where and how the modules are placed. Similarly, exterior structural characteristics of the modules is not discussed as this is well known. What is pertinent to this disclosure is that the modules must be rugged and be able to withstand the harsh environment and pressure to which they will be subject without an unacceptable rate of loss of stored data. 
     FIG. 13  is a schematic illustration of a monitoring system. Processor  200  is provided, and is powered by a power source  202 , for example a battery, the processor has ROM and RAM memory  204 , and can be connected to a storage device  206 . The processor is connected to at least one signal generator  208 , and strain gauge interface  210 . Preferably the processor  200  has a connector interface  212 , and a communication device  214 . The communication device inputs from and outputs to receiver  216  data. A command interface  218  can be provided for receiving commands from a command input device such as a microcomputer. 
   While the present invention has been described in relation to various embodiments, the invention is not limited to the illustrated embodiments.