US 20050100414 A1
An integrity monitoring system for monitoring degradation in a composite riser string. The system includes composite riser structures incorporating strain and vibration sensors to measure changes in the stiffness strain on a first orientation and on a second orientation. The system can also include monitoring modules attached to each individual riser and devices to transfer the data from the monitoring module to the surface controller. Additionally, the monitor system can provide for an alarm when predetermined warning limits are exceeded.
1. A composite riser assembly comprising:
a surface platform for supporting composite risers;
at least one composite riser supported by said platform, at least one of said one composite risers having a first strain sensor affixed thereto;
a controller located at said surface platform;
said controller being in signal communication with said first strain sensors in said at least one composite riser; and
said controller having a signal device capable of transmitting signals to and receiving signals from said first strain sensors in said at least one composite riser.
2. A riser assembly of
3. A riser assembly of
4. A riser assembly of
5. A riser assembly of
6. A riser assembly of
7. A riser assembly of
8. A riser assembly of
9. A composite structure comprising:
an elongated composite structure defining an axis;
a first strain sensor associated with said composite structure at a first orientation relative to said axis to measure strain in the direction of said first orientation; and
a second strain sensor associated with said composite structure at a second orientation relative to said axis to measure strain in the direction of said second orientation.
10. A composite structure of
11. A composite structure of
12. A composite structure of
13. A composite structure of
14. A composite structure of
15. A composite structure with monitoring device comprising:
an elongated composite structure defining an axis;
a first strain sensor affixed to said composite structure at a first orientation relative to said axis to measure strain in the direction of said first orientation;
a first strain monitoring module in signal communication with said first strain sensor for measuring strain;
said first strain monitoring module associated with said first strain sensors having
a power source,
a central processor unit,
a signal device, and
a communication device.
16. A composite structure of
17. A monitoring module for use with a composite riser comprising:
an input means for inputting strain measurements from a first strain sensor;
a processor means for receiving said strain measurement data; and
a communication means for transmitting the recorded strain data to a receiving unit.
18. A monitoring module for use with a composite riser comprising:
an input means for inputting strain measurements from a first strain sensor in a first orientation and from a second strain sensor in a second orientation;
a processor unit for recording said strain measurements from said strain gauges; and
communication means for outputting said recorded strain measurements.
19. A monitoring module of
20. A monitoring module of
21. A monitoring module of
22. A monitoring device for monitoring strain in a composite riser having at least one strain sensor in operative association therewith comprising:
means to receive strain measurements from at least one strain sensor;
means to store said strain measurement data and the time of measurement; and
means to communicate said strain measurement data to a receiver.
23. A monitoring module of
means to compare said strain measurement data with a predetermined warning value; and
means to transmit an alarm signal when said predetermined warning value is exceeded.
24. A system for monitoring strain in a composite riser having at least one strain associated therewith comprising:
a submersible vehicle having a recorder mounted thereon;
said recorder comprising a central processing unit, a communication means, a means to receive and record strain data received from said strain sensors associated with said composite riser.
25. A system of
a means to storage strain data in said recorder.
26. A system of
a means to generate a signal for transmission to said strain sensors in order to measure strain.
27. A method of monitoring strain in an underwater composite riser comprising:
providing a submersible vehicle with a recorder device;
sending a signal to a strain sensor attached to a composite riser from the recorder of said submersible vessel; and
recording the response to said signal in said recorder device.
28. A method of monitoring strain is an underwater composite riser comprising:
sending a signal to a strain sensor in operative association with a composite riser;
receiving the response to the signal; and
storing the response to the signal.
29. A method of
30. A method of monitoring strain in composite riser comprising:
providing a strain sensor in association with a composite riser;
providing a monitoring module on said composite riser;
placing said monitoring module in signal communication with said strain sensor; and
sending a strain measuring signal to said strain sensor.
31. A method of
transmitting the strain data from said monitoring module to the a surface receiver.
32. A method of
acoustically transmitting the strain data from said monitoring module to a surface receiver.
33. A method of
transmitting the strain data from said monitoring module to the a submersible vehicle.
34. A monitoring module for use with a composite riser comprising:
a mounting device adapted to mount said housing on a composite riser;
at least one strain gauge interface for providing a data interface for a strain gauge;
a processing device, disposed in said housing, for receiving data obtained from a strain gauge and for processing that received data in accordance with predetermined principles;
a storage device, connected to said processing device, for storing at least one of data obtained from a strain gauge and data processed by said processing device;
a communications interface, connected to said processing device, for transmitting data from said monitoring module to a receiver; and
a power interface for receiving power from a power source.
35. A monitoring module of
36. A monitoring module of
37. A monitoring module of
38. A monitoring module of
39. A monitoring module of
40. A riser assembly of
41. A composite structure of
42. A composite structure of
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.
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 of 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 joints 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 of 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 strength. 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, 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, and preferably a value between 10 and 14 million lb/square inch. Damage of the composite riser will manifest itself by reduction of the stiffness that means the elastic modulus is 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 based on monitoring the 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 to 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 frequency of the riser that influences the vibration behavior of the riser is a function of the stiffness and mass to monitor the integrity of the riser. As the stiffness changes, the natural frequency will change and thus the vibration signature will change. Well known technique, but custom curves are required to characterize a specific riser because configuration, cross-section, wall thickness, material selection, etc. will affect vibration response characteristics. Monitoring the changes in the vibration signature, which is commonly done using accelerometers, can provide an indication of the level of damage. 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 qualification program. This will involve 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.
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 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 the individual riser. This 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. If desired, more than one monitoring unit can be employed. 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 strain 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 to store the measured strain values and/or the ratio of measured strain values. Additionally, the moduling unit 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, 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 to warning limits, store the data, 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.
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:
For the fiber optic strain sensors, the same fiber can contain multiple sensors. (See
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
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.
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.
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 vending 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
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.
During the testing, strain was monitored using both fiber optic sensors and strain gauges. In
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.
The measured strain clearly showed that the strain pattern changed over the test duration. Detailed analysis of the changes in the strain pattern demonstrate the absolute value of the strain under load, the residual strain under zetro load and the ratio of the hoop strain to axial strain thus 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 is the joint is decreasing, which can also be measured using vibration monitoring techniques. In another aspect the present invention provides for using the natural frequency of the riser that influences the vibration behavior of the riser is a function of the stiffness and mass to monitor the integrity of the riser. As the stiffness changes, the natural frequency will change and thus the vibration signature will change. Well known technique, but custom curves are required to characterize a specific riser because configuration, cross-section, wall thickness, material selection, etc. will affect vibration response characteristics. Monitoring the changes in the vibration signature, which is commonly done using accelerometers, can provide an indication of the level of damage. 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 qualification program.
While warning limits may be empirically determined as described above, warning limits may also be analytically determined based on predated 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.
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
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 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'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 124 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
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 monitor module 160 new base information updates such as a change in the warning limit and accept downloads of strain data from the monitoring module 150. This arrangement can be repeated for each riser.
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.
While the present invention has been described in relation to various embodiments, the invention is not limited to the illustrated embodiments.