Patent Publication Number: US-2012030887-A1

Title: Telescoping-Tube System For Crew Transfer

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of maritime equipment and more particularly to a telescoping-tube system for crew transfer between a vessel and a stationary platform. 
     BACKGROUND OF THE INVENTION 
     The use of large marine-based oil rigs has increased the need for a safe, efficient and cost effective method for the transfer of crew members between support vessels and stationary oil rig platforms. Currently, crew transfer is usually accomplished using helicopters, which is costly, subject to weather restrictions, subject to airspace restrictions and capable of transporting only a limited number of personnel per flight. Location of oil rig platforms at extended distances from shore based facilities adds to the disadvantages of this method. 
     In addition to helicopters, cranes operating on oil rig platforms are often used to lift a basket containing personnel to transfer the personnel between a support vessel and the oil rig platform. Cranes require crane operators and handlers stationed to open and lock the baskets. And there is operational risk to the crew being transferred as well as the handlers. Also, only a limited number of personnel can be transferred at a time, such that the process is both time consuming and inefficient. Crew transfer operations at present are labor intensive and present certain risks, especially if performed in higher sea states. 
     The prior art related to crew transfer apparatus includes a transfer system developed by Lockheed Martin. The apparatus, named “Viking,” includes a flexible ladder apparatus (Selstair®) that is permanently mounted on an oil rig as well as a walkway, identified as a CEWay™, for bridging the gap between the support vessel and the flexible ladder. The CEWay™ includes a fixed-length walkway having an end mounted on a sled that runs on tracks mounted on the support vessel. This system utilizes the flexible ladder for vertical crew transfer and the walkway for horizontal or near horizontal crew transfer. 
     Additional approaches for connecting a static structure to a dynamic structure, as disclosed in the following references, have various drawbacks. 
     Publ. U.S. Pat. Appl. No. 2006/0191457 A1 discloses a telescoping gangway that includes a pair of relatively flat walkway sections supported by a hydraulic cylinder. The gangway provides no personnel protection from weather conditions and does not accommodate relative motion between the boat, on which the device is mounted, and a stationary structure. Furthermore, there is no provision for efficient storage of the gangway when not in use. 
     U.S. Pat. No. 6,131,224 discloses a flat rigid connecting bridge and a pair of multi-directional trunnion and roller assemblies for accommodating relative motion between a static and a dynamic structure. A trunnion and roller assembly is attached to the static structure and to the dynamic structure and the trunnion and roller assemblies support the connecting bridge. This system does not protect the crew from weather conditions. Furthermore, storage is not provided for the device when it&#39;s idle. 
     U.S. Pat. No. 2,641,785 discloses a rigid walkway that is supported by a crane. A first end of the walkway is connected to a dock by a pivoting hinge connection and the second end of the walkway connected to a boat by a ball joint. The rigid configuration of the walkway prevents compact storage when the device is not in use. 
     U.S. Pat. No. 4,333,196 discloses a rigid walkway for coupling a stationary platform to a vessel. The first end of the walkway is pivotably coupled to the stationary platform. The second end of the walkway rests on the vessel. The rigid configuration of the walkway prevents compact storage when the device is not in use. 
     The prior art devices are not able to provide safe operation in higher sea states. This results in relatively high costs and potential operational interruptions and reliability problems. Nor do the prior art devices provide for compact storage of the devices on a vessel when the devices are not in use. Simply put, the prior art does not provide a safe and effective solution for transferring crew members between a vessel and a stationary platform. 
     SUMMARY OF THE INVENTION 
     The present invention provides a crew-transfer system for transferring crew between a first platform, such as a floating vessel, and second platform, such as oil rig, that avoids at least some of the disadvantages of the prior art. 
     In the illustrative embodiment, the system comprises: a telescoping-tube assembly comprising three nestable/extendable tubes, a first coupling that movably couples a proximal end of the telescoping-tube assembly to a moving platform (e.g., a floating vessel, etc) and a second coupling that reversibly and movably couples the distal end of the telescoping-tube assembly to a stationary platform (e.g., oil rig, etc.). 
     The tubes are of sufficient internal diameter to accommodate a crew member who must transit between the two platforms. A ladder is advantageously disposed in each tube to enable a crew member to traverse the tubes, especially when one platform is at a different height than the other. In the illustrative embodiment, linear actuators are used to extend/retract the three tubes. 
     The largest (i.e., outermost) of the tubes is operatively coupled to a lift mechanism that raises and lowers the distal end of the telescoping-tube assembly. Once raised, the tubes of the telescoping-tube assembly are deployed (extended) via the linear actuators. 
     After the tubes are raised and extended, the smallest (i.e., innermost) tube is reversibly coupled to the stationary platform by the second coupling. In the illustrative embodiment, the second coupling comprises a male portion that depends from the end of the innermost tube and a female portion that is disposed on the stationary platform. The second coupling is capable of temporarily locking, to ensure that the distal end of the telescoping-tube assembly remains coupled to the stationary platform until transfer operations are complete. 
     In the illustrative embodiment, the linear actuators, in conjunction with a control system, selectively lock two of the extended tubes of the telescoping-tube assembly. In particular, the linear actuators are controlled to lock two adjacent tubes while leaving the remaining tube free to move relative to an adjacent tube. The locking functionality operates in conjunction with a motion-sensing system, which senses the presence of a crew member in a particular tube and is used to determine the crew member&#39;s direction of travel. Based on information from the motion-sensing system, both the tube in which the crew member is then positioned and the adjacent telescoping tube along the crew member&#39;s direction of travel are locked. This ensures that the crew member travels along a rigid connection between adjacent telescoping tubes. The system also ensures that the third tube (in the illustrative three-tube system) is free to move relative to an adjacent tube. This enables the telescoping-tube assembly to move in the axial direction (i.e., along its length) to accommodate relative translational movement between the first and second platform as can be caused by wave motion. 
     In preferred embodiments, the first and second couplings have a sufficient number of degrees of freedom to accommodate rotational motions of one or both of the platforms. Typically, only one of the platforms (e.g., a floating vessel, etc.) is subject to such motion, which is a natural consequence of wave motion. Specifically, the platform(s) will experience rotational motions in the pitch, roll and yaw directions. Providing first and second couplings that accommodate these rotational motions serves to significantly mitigate the stresses that would otherwise be induced in the telescoping-tube assembly when in use. 
     In the illustrative embodiment, the first coupling and the second coupling are each implemented as a ball and socket joint. The ball and socket joint will provide at least three degrees of freedom (of motion). In the absence of additional stabilization, implementing both the first and second coupling as a ball and socket joint would render the telescoping-tube assembly unstable. In particular, the telescoping tube assembly would be able to “roll” (i.e., rotate about its longitudinal axis). As a consequence, one coupling (preferably the second coupling) is limited to two degrees of freedom (pitch and yaw). This can be accomplished in various ways, such as by modifying the ball and socket joint, or using cables that prevent the telescoping tube assembly from rolling, or via other features that, by virtue of their attachment to the telescoping-tube assembly, limit the movement thereof. 
     As previously noted, in additional to rotational motions, waves will cause a floating vessel to move towards or away from the stationary platform. In other words, wave motion will result in translational movement of a floating vessel. This movement is accommodated, as previously noted, by ensuring that one of tubes of the telescoping-tube assembly is free to slide (i.e., extend or retract) with respect to one of the other tubes of the assembly. 
     A set of cables optionally connects the largest tube to the deck of the vessel. Additional cables optionally connect the end of the extended telescoping-tube assembly to the stationary platform. As previously discussed, depending upon how the first and second couplings are implemented, these cables might be required to prevent “roll” of the telescoping-tube assembly. In some embodiments, a cable connects the support vessel and the stationary platform. This cable can be used to limit the relative translational movement of the floating vessel and the stationary platform. 
     In some embodiments, the present invention provides a crew-transfer system comprising: 
     a telescoping-tube assembly, a first coupling that movably couples a proximal end of the telescoping-tube assembly to a first platform, and a second coupling that movably and reversibly couples a distal end of the telescoping-tube assembly to a second platform, wherein the telescoping-tube assembly comprises:
         (a) at least three tubes; and   (b) a first mechanism that selectively locks two adjacent tubes of the at least three tubes, wherein the tubes are selected for locking based on:
           (i) a location of a user within the telescoping-tube assembly; and   (ii) a direction of travel of the user therethrough, wherein the locked tubes include the tube in which the user resides and the tube to which the user is next heading.   
               

     In some additional embodiments, the present invention provides a crew-transfer system comprising: 
     a telescoping-tube assembly, a first coupling that couples a proximal end of the telescoping-tube assembly to a first platform that, in use, is subjected to forces that cause the first platform to move in at least three different directions, and a second coupling that reversibly couples a distal end of the telescoping-tube assembly to a second platform, and wherein the telescoping-tube assembly comprises:
         (a) at least three tubes that allow ingress and egress for a user;   (b) a motion-sensing system for sensing the presence and direction of movement of the user through the tubes; and   (c) a first mechanism for extending and retracting the at least three tubes;       

     and wherein the first coupling is structured to accommodate the movement of the first platform by providing three degrees of freedom of movement between the telescoping-tube assembly and the first platform. 
     In yet some further embodiments, the present invention provides a crew transfer system comprising: 
     a telescoping-tube assembly, a first coupling that couples a proximal end of the telescoping-tube assembly to a first platform, and a second coupling that reversibly couples a distal end of the telescoping-tube assembly to a second platform, wherein the telescoping-tube assembly comprises:
         (A) at least three tubes that allow ingress and egress for a user;   (B) a motion-sensing system for sensing the presence and direction of movement of the user through the tubes; and   (C) one or more mechanisms that:
           (i) extend and retracting the at least three tubes;   (ii) selectively locks two adjacent tubes of the at least three tubes, wherein the tubes are selected for locking based on:
               (a) a location of a user within the telescoping-tube assembly; and   (b) a direction of travel of the user therethrough, wherein the locked tubes include the tube in which the user resides and the tube to which the user is next heading.   
               
               

     In still further embodiments, the present invention provides a crew-transfer system that couples to a first platform and reversibly couples to a second platform, wherein the crew-transfer system comprises: 
     a telescoping-tube assembly, the telescoping-tube assembly comprising:
         (A) a first tube, second tube, and third tube that are suitably dimensioned to permit passage therethrough by a human user;   (B) a motion-sensing system for sensing the presence of the user and a direction of movement of the user through the tubes; and   (C) a plurality of linear actuators selected from the group consisting of mechanically-coupled rodless cylinders or magnetically-coupled rodless cylinders, wherein the linear actuators are disposed between:
           (i) an inner surface of the first tube and an outer surface of the second tube; and (ii) an inner surface of the second tube and an outer surface of the third tube.   
               

     In a further aspect of the invention, the present invention provides a method for transferring crew members comprising:
         extending, from a first platform to a second platform, a plurality of telescoping tubes; temporarily coupling, to the second platform, an end of a distal-most tube of the telescoping tubes;   sensing a presence of a user moving within the telescoping tubes;   determining a direction of travel of the user; and   locking both the telescoping tube in which the user resides at the time of sensing and an adjacent telescoping tube in the direction in which the user is traveling.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  depicts a simplified overall schematic side view of a crew transfer system in accordance with the illustrative embodiment of the present invention. The system is depicted mounted on a vessel in a quiescent state ready for deployment. 
         FIG. 1B  depicts the system of  FIG. 1A  during the process of deployment. 
         FIG. 1C  depicts the system of  FIGS. 1A and 1B  after the deployment of the system has been completed with the system ready for crew transfer. 
         FIG. 2  depicts an end view the system of  FIG. 1C . 
         FIG. 3  depicts a view of the system taken along the line  3 - 3  of  FIG. 1C . 
         FIG. 4  depicts a cross-sectional view of the telescoping-tube assembly taken along the line  4 - 4  of  FIG. 2  and also depicts a simplified schematic of a control system. 
         FIG. 5  depicts the first coupling, which couples the telescoping-tube assembly to a first platform, such as a vessel. 
         FIG. 6  depicts the second coupling, which couples the telescoping-tube assembly to a second platform, such as an oil rig. 
         FIG. 7  is a view taken along the line  7 - 7  of  FIG. 6 . 
         FIG. 8  depicts a simplified cross-sectional view taken along the line  8 - 8  of  FIG. 6 . 
         FIG. 9  depicts a simplified cross-sectional view taken along the line  9 - 9  of  FIG. 6 . 
         FIG. 10  depicts a simplified cross-sectional view of a rodless pneumatic linear actuator. 
         FIG. 11  depicts a simplified cross-sectional view of a magneto-rheological (MR) linear actuator. 
         FIG. 12  depicts a simplified cross-sectional view of a magnetically coupled rodless linear actuator. 
         FIG. 13  depicts a simplified elevation view of an electro-mechanical ball screw linear actuator. 
         FIG. 14  depicts the manner in which the telescoping-tube assembly adapts to changes in the relative position of the first platform and the second platform. 
         FIG. 15  is a simplified flow diagram depicting a process in accordance with the illustrative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1A through 1C  provide an overview of the structure and operation of telescoping-tube system for crew transfer  100  in accordance with the illustrative embodiment of the present invention. The system  100  creates a temporary connection that enables safe and efficient crew transfer between a first platform and a second platform. 
     The system is advantageously used in applications in which at least one of the platforms is subject to forces that cause it to move, such as when one of the platforms is floating and therefore subject to wave motion. One particularly useful application—and the context for the illustrative embodiment—is the creation of a temporary connection for crew transfer between a floating vessel and a stationary platform such as oil rig. Of course, crew transfer system  100  can also be used for crew transfer operations when both platforms are moving or when neither platform is moving. But in the latter case, much of the capability that is provided by system  100  would not be required. 
       FIG. 1A  depicts system  100  in a quiescent state, before deployment of system  100  and showing only a single tension line  106  connecting vessel  102  and stationary platform  104 .  FIG. 1B  depicts system  100  in an actuated state wherein telescoping-tube assembly  108  is partially extended but before the end of telescoping-tube assembly  108  has made contact with stationary platform  104 .  FIG. 1C  depicts full connection between vessel  102  and stationary platform  104  for crew transfer. 
     Referring now to  FIGS. 1A-1C  and  FIG. 2 , the salient features of system  100  include: telescoping-tube assembly  108 , first coupling  112 , lift mechanism  114 , and male portion  124  and female portion  180  of a second coupling. 
     Telescoping-tube assembly  108 , which has posterior end  109  and anterior end  110 , comprises three tubes  118 ,  120 , and  122 , wherein tube  118  has the largest diameter and tube  122  has the smallest diameter. The tubes are suitably dimensioned to enable personnel to move freely therein. The tubes are capable of nesting (as shown in  FIG. 1A ) and extending ( FIG. 1C  shows the tubes fully extended). As depicted in  FIG. 2 , tube  118  includes ports  208  and  210  that enable crew members to enter or exit telescoping-tube assembly  108  for transit from or to support vessel  102 . Port  212  of tube  122  permits crew members to exit or enter telescoping-tube assembly  108  for transit to or from stationary platform  104 . The telescoping tubes are preferably made from a material that is relatively light in weight, relatively stiff, and highly resistant to corrosion; for example, fiberglass. 
     Telescoping-tube assembly  108  is movably coupled to vessel  102  proximal to posterior end  109  via first coupling  112 . When in use, the telescoping-tube assembly is reversibly and movably coupled to stationary platform  104  via a second coupling. The second coupling comprises male portion  124  and female portion  180 . The male portion includes a spherical protuberance that depends from an end of the innermost tube  122 . The female portion comprises receiver assembly  180 , which is disposed on stationary platform  104 . As used in this specification, including the appended claims, the term “movably coupled” means coupled in a way that permits relative movement between the items that are coupled by the coupling. 
     To couple telescoping-tube assembly  108  to stationary platform  104 , lift mechanism  114 , which is operatively coupled to the telescoping-tube assembly, raises distal end  110  of the telescoping-tube assembly. The angle of lift (i.e., the angle formed between telescoping-tube assembly  108  and deck  170  of vessel  102 ) is sufficient, when the tubes of the telescoping-tube assembly are extended, to couple the telescoping-tube assembly  108  to stationary platform  104 . A typical angle-of-lift is about 60 degrees, but it could be more or less than this figure as a function of the relative heights of vessel  102  and platform  104 . In the illustrative embodiment of system  100 , lift mechanism  114  is a conventional hydraulic cylinder that is coupled to tube  118  of telescoping-tube assembly  108 . In some other embodiments, the lift mechanism is a conventional pneumatic cylinder. 
     Lift mechanism must be coupled to deck  170  and telescoping-tube assembly  108  in a manner that assures that the telescoping-tube assembly is not over constrained. If over constrained, such as by the use of a rigid coupling between deck  170  and the lift mechanism, stresses would be induced in telescoping-tube assembly  108  during use, effectively defeating the stress-mitigating functionality of the first and second couplings. It is also important that the coupling between the lift mechanism, deck, and telescoping-tube assembly doesn&#39;t result in over constraining the telescoping-tube assembly. This could result in instability during lifting/lowering due to sideward tipping forces. 
     A coupling arrangement wherein a pin joint is used to couple the lift mechanism  114  to deck  170  and a conventional rod-end fitting (having end of travel stops) couples the lift mechanism to tube  118  is suitable for appropriately constraining the telescoping-tube assembly. The conventional rod end fitting has a ball and socket portion that accommodates a degree of misalignment and rotation in various planes. 
     Once tube  122  of telescoping-tube assembly  108  is coupled to stationary platform  104 , there is no longer a need for the lifting force provided by lift mechanism  114 . As a consequence, hydraulic (or pneumatic) pressure will be released after coupling, enabling telescoping-tube assembly  108  to “telescope,” thereby reducing induced stresses. In some embodiments, the pin joint (for coupling lift mechanism  114  to deck  170 ) is designed so that after the pressure to lift mechanism  114  is relieved, the pin joint accommodates motion in various planes. In some other embodiments, a lock-unlock mechanism is used at the lift mechanism/deck coupling. During lifting/lowering of telescoping-tube assembly  108 , the mechanism is locked, which permits only pivoting (pitching) motion of lift mechanism  114 . Otherwise, the lock-unlock mechanism is in an unlocked state, which permits motion in the pitch direction as well as movement in other rotational directions. In conjunction with this specification, it is within the capabilities of those skilled in the art to couple lift mechanism  114  to deck  170  and telescoping-telescoping tube assembly  108  in a manner that avoids over- or under-constraining the system. 
     In the illustrative embodiment, cables  202  and  204  connect tube  118  to vessel  102  and cables  206 A and  206 B connect tube  122  to stationary platform  104 . The combination of lift mechanism  114  and cables  202 ,  204 ,  206 A,  206 B provide vertical and lateral positioning and stabilization for telescoping-tube assembly  108 . 
     Cables  202 ,  204 ,  206 A,  206 B must be suitable to withstand the tension (resulting from the relative movement of vessel  102  and stationary platform  104 ) that is required to stabilize telescoping-tube assembly  108 . The cables must also be corrosion resistant. Suitable materials for these cables include, without limitation, corrosion-resistant stainless steel, etc. In some embodiments, these cables are elastic/stretchable/resilient, such as a Bungee cable. It is within the capabilities of those skilled in the art to design or specify cables suitable for use in conjunction with the illustrative embodiment of the present invention. 
     It is to be understood that  FIG. 2  provides a general indication of the orientation/direction of cables  202 ,  204 ,  206 A,  206 B. As appropriate, those skilled in the art will be capable of deciding whether it is necessary to use a larger number of cables, having the general orientations illustrated, for the purpose of stabilizing telescoping-tube assembly  108 . In some embodiments, depending on the particular rotational degrees of freedom provided by the coupling arrangement for the deck  170 /lift mechanism  114 /telescoping-tube assembly  108  interface or provided by the first and/or second couplings, cables  202 ,  204 ,  206 A,  206 B can be omitted. This is a matter of selecting couplings for system  100  such that the system is appropriately constrained, which is within the capabilities of those skilled in the art. Specifically, appropriate selection of couplings will ensure that (1) the amount of stress induced in telescoping-tube assembly  108  during crew-transfers operations is desirably low; and (2) the telescoping-tube assembly is prevented from “rolling.” 
     This specification now proceeds with further details of the structure and operation of telescoping-tube assembly  108  and the first and second couplings. 
     Telescoping-tube assembly  108 . As is depicted in  FIG. 3  (via an end view from the perspective indicated at line  3 - 3  in  FIG. 1C ) and  FIG. 4  (via a side cross-sectional view from the perspective indicated at line  4 - 4  in  FIG. 2 ), telescoping tubes  118 ,  120 ,  122  include respective ladders  306 ,  308 ,  310 . Each ladder comprises rung support  304  and a plurality of rungs  302 . These ladders enable a crew member to traverse telescoping-tube assembly  108  and travel between support vessel  102  and stationary platform  104  in a safe manner while being protected from weather conditions. 
     With continuing reference to  FIGS. 3 and 4 , telescoping-tube assembly  108  includes a first mechanism for extending or retracting telescoping tubes  118 ,  120 ,  122 . In the illustrative embodiment, that mechanism comprises a plurality of linear actuators  314 ,  316 ,  318 ,  320 ,  322 , and  324 . 
     In the illustrative embodiment, three linear actuators are disposed between any two tubes, arranged about 90 degrees apart. More specifically, linear actuators  314 ,  316 , and  318  are disposed between the interior surface of tube  118  and the exterior surface of tube  120 . And linear actuators  320 ,  322 , and  324  are disposed between the interior surface of tube  120  and the exterior surface of tube  122 . 
     The manner in which the linear actuators operate to extend/retract the tubes is described later in this specification in conjunction with  FIGS. 10-13 . 
     In the illustrative embodiment, the first mechanism (e.g., the linear actuators, etc.) serves a dual purpose. That is, in addition to being used to extend/retract the tubes, the first mechanism is also used to selectively “couple” or “lock” adjacent tubes to prevent relative movement between them. In some other embodiments, these two functions can be performed via separate mechanisms. This “locking” functionality and its significance are discussed in further detail later in this specification. 
     In the illustrative embodiment, the linear actuators are mechanically-coupled, rodless, pneumatic cylinders. Each such actuator includes a cylinder body and a mechanically-coupled piston yoke. For example, as is shown in  FIG. 3 , actuator  314  includes cylinder body  326  and mechanically-coupled piston yoke  330 . Cylinder body  326  is mounted on inner surface  328  of telescoping tube  118 . The movable portion of the rodless cylinder (i.e., piston yoke  330 ) is connected to adjacent tube  120 . 
     Suitable mechanically-coupled, rodless, pneumatic cylinders include ULTRAN brand rodless cylinders, available from Bimba Manufacturing in Monee, Ill. Other types of actuators that are capable of providing the same functionality as the mechanically-coupled rodless pneumatic cylinders of the illustrative embodiment may suitably be used in alternative embodiments. Those skilled in the art are familiar with linear actuators and will be able to appropriately specify them for use in conjunction with the illustrative embodiment and alternative embodiments. 
     The first mechanism requires a source of power to extend/retract the tubes or selectively lock the tubes. In the illustrative embodiment, a plurality of electrically-operated control valves  416 ,  418 ,  420 , and  422  and source  428  of pneumatic power ( FIG. 4 ) serve this purpose. In particular, the first mechanism—linear actuators  316 ,  318 ,  320 ,  322 , and  324 —is driven by pressurized air that is selectively delivered via the control valves. 
     Telescoping-tube assembly  108  also includes a motion-sensing system that senses crew-member motion within telescoping tubes  118 ,  120 , and  122 . Referring now to  FIG. 4 , in the illustrative embodiment, the motion-sensing system includes a plurality of motion sensors  402 ,  404 ,  406 ,  408 ,  410 ,  412 , two of which are disposed in each of telescoping tubes  118 ,  120 , and  122 . In the illustrative embodiment, the motion sensors are infra-red motion sensors. Other types of motion sensors (e.g., optical, etc.) may suitably be used. Those skilled in the art will know how to select and use other types of motion sensors for use in conjunction with the present invention. 
     As a crew member climbs through telescoping tubes  118 ,  120 , and  122 , motion sensors  402 ,  404 ,  406 ,  408 ,  410 , and  412  sense, in sequence, the presence of the crew member. In this fashion, the location of the crew member can be resolved to a specific tube at any moment. Furthermore, the crew member&#39;s direction of travel through telescoping tubes  118 ,  120 ,  122  can be determined based on the sequential “tripping” of the sensors. That is, if sensor  406  indicates motion and then sensor  408  indicates motion, the crew member&#39;s direction of travel is towards the “right” in  FIG. 4 . If, on the other hand, sensor  406  indicates motion and then sensor  404  indicates motion, the crew member&#39;s direction of travel is towards the “left” in  FIG. 4 . 
     The motion-sensing system also includes control computer  414 . In conjunction with suitable circuitry, etc., computer  414  receives signals from the motion sensors (i.e., indicative of a crew member&#39;s location and direction of motion) and generates one or more control signals based thereon. The control signal(s) are transmitted to one or more of electrically-operated control valves  416 ,  418 ,  420 , and  422 . 
     The control valves, which are fluidically coupled to source  428  of pneumatic power, are arranged to actuate linear actuators  314 ,  316 ,  318 ,  320 ,  322 ,  324 . In the illustrative embodiment, each of the linear actuators is fluidically coupled to two of the electrically-operated control valves. For example, linear actuators  314 ,  316 ,  318  are each fluidically coupled to control valves  416  and  418 . For simplicity and clarity,  FIG. 4  depicts control valve  416  as being coupled only to actuator  314 . The control signal(s) that are generated based on the output of the motion sensors are essentially a “lock/unlock” command for the linear actuators. It is within the capabilities of those skilled in the art to design or specify pneumatic connections and electrically-operated valves suitable for use in conjunction with the illustrative embodiment of the present invention. 
     Based on the input from the motion-sensing system and via operation of the linear actuators, the tube through which the crew member is moving and the adjacent telescoping tube in the crew member&#39;s direction of motion are coupled or locked together to prevent relative motion therebetween. More specifically, computer  414  generates signals that ultimately cause the appropriate control valves to actuate one or the other triad of linear actuators (i.e., linear actuators  314 ,  316 ,  318  or linear actuators  320 ,  322 ,  324 ) as required for coupling the particular two tubes together. 
     The crew member thus traverses adjacent telescoping tubes that are locked together, while relative motion is permitted between unlocked tube(s). For example, when a crew member travels from tube  118  to tube  120 , tubes  118  and  120  are coupled/locked and tubes  120  and  122  are uncoupled/unlocked. Once the crew member reaches tube  120  and is determined to be moving toward tube  122 , the tube  118 /tube  120  coupling is released and tubes  120  and  122  are locked. The freedom of relative motion of the telescoping tubes (in the axial direction) that are not being currently traversed enables the telescoping-tube assembly  108  to accommodate relative motion between vessel  102  and the stationary platform  104 . This selective coupling/uncoupling is described further later in this specification in conjunction with  FIGS. 10 and 14 . System  100  thereby provides safety to crew members and also accommodates the motion of support vessel  102  without inducing undue stresses in the system. 
     Three tubes are considered to be a minimum number of tubes for telescoping-tube assembly  108 . Using a minimum of three tubes ensures that telescoping-tube assembly  108  can always accommodate relative motion between vessel  102  and stationary platform  104 . That is, some portion of telescoping tube assembly  108  will be able to freely extend or retract. In conjunction with this specification, those skilled in the art will be capable of making and using alternative embodiments (not illustrated) in which telescoping-tube assembly  108  includes a larger number of tubes, each similar to the tubes that have been shown. The use of a greater number of tubes accommodates, for example, relatively greater distances between support vessel  102  and stationary platform  104 . 
     First and Second Couplings. First coupling  112  includes a male portion that depends from telescoping-tube assembly  108  and a female portion that is disposed on support vessel  102 . In the illustrative embodiment, these two portions effectively create a ball joint. With reference to  FIG. 5 , the male portion comprises spherical protuberance or ball  502  and stem  504 . The stem extends from “lower” surface  506  of tube  118 , near the end thereof. Ball  502  depends from stem  504 . 
     The female portion comprises base support member  508 , which is disposed on deck  170  of the first platform (e.g., the support vessel, etc.). Socket  510  is formed in base support member  508 . Ball  502  is received by socket  510 . 
     The ball joint formed by ball  502  and socket  510  accommodates roll, pitch and yaw motions of the first platform. This substantially reduces the stresses that would otherwise be induced in telescoping-tube assembly  108  when it is coupled to stationary platform  104 . It is notable that, in the illustrative embodiment, first coupling  112  does not permit relative translational motion between telescoping tube assembly  108  and support vessel  102 . This is because ball  502  is free to “spin” relatively unencumbered in socket  510 , but it cannot “translate;” that is, it cannot move along a linear path. In the illustrative embodiment, linear movement, such as occurs when vessel  102  moves towards or away from stationary platform  104 , is accommodated by the at least one free-to-move portion of telescoping-tube assembly  108 . 
     In alternative embodiments, first coupling  112  is configured differently such that it can allow for translational motion, in addition to accommodating roll, pitch, and yaw motions. For example, in some of such alternative embodiments, a similar ball joint coupling is mounted on a “sled” that is free to slide back and forth, thereby accommodating relative translational movement between vessel  102  and stationary platform  104 . 
       FIGS. 6-9  and the accompanying description provide detail of the second coupling, which movably and reversibly couples telescoping-tube assembly  108  to the second platform (e.g., stationary platform  104 , etc.)  FIG. 6  depicts male portion  124  of the second coupling coupled to the female portion  180 .  FIG. 7  depicts a top view of female portion  180  from the perspective indicated at line  7 - 7  in  FIG. 6  (male portion  124  not depicted).  FIG. 8  depicts a cross-sectional view through female portion  180  from the perspective indicated at line  8 - 8  in  FIG. 6  (male portion  124  not depicted).  FIG. 9  depicts a cross-sectional view through female portion  180  from the perspective indicated at line  9 - 9 . 
     Male portion  124  of the second coupling includes ball  602  mounted on stem  604 . The stem extends from the lower surface of tube  122  proximal to end  212  thereof. This arrangement is the same as ball  502  and stem  504  previously described. In the illustrative embodiment, female portion  180  of the second coupling is hereinafter referenced as receiver assembly  180 . The receiver assembly includes a socket (see  FIG. 8 , socket  810 ), which is formed in base  612  that is mounted on deck  614  of stationary platform  104 . 
     During operation, the linear actuators and lift mechanism  114  move end  212  of tube  122  to position ball  602  to drop into socket  810 . Remotely-controlled socket cover  616  (see  FIGS. 6 and 7 ) slides over ball  602 , thereby effectively locking end  212  of telescoping tube  122  to deck  614  of stationary platform  104 . As depicted in  FIG. 9 , socket cover  616  includes dovetail guide  920 , which is slideably mounted in guide way  922  formed in base  612 . Motion of socket cover  616  in the directions indicated by arrow  636  in  FIG. 6  is controlled by linear actuator  628  driven by electric motor  630 . The motor is controlled by wireless remote control unit  632  and brake  634 . 
     Slot  702  ( FIG. 7 ) in socket cover  616  is suitably dimensioned to prevent the ball from dislodging from socket  810  while accommodating movement of stem  604 . Such movement, which includes movement in the roll, pitch, and yaw directions, will occur as vessel  102  (and telescoping-tube assembly  108  situated thereon) moves in response to wave motion. 
     As previously discussed, in some embodiments, to prevent telescoping-tube assembly  108  from “rolling,” cables  206 A and  206 B are used (see  FIG. 2 ). In this fashion, the three degrees of freedom that would otherwise be provided by the second coupling is reduced to only two degrees of freedom of movement (i.e., pitch and yaw). Like first coupling  112 , the ability to accommodate this movement substantially reduces the stresses that would otherwise be induced in system  100  when telescoping-tube assembly  108  couples a wave-tossed vessel to a stationary platform. There are many ways to modify the second coupling so that it provides two, rather than three, rotational degrees of freedom. For example, a pin (not depicted) can be inserted through ball  602 . Socket cover  616  can be suitably configured so that once the socket cover engages ball  602 , the pin prevents the telescoping-tube assembly from rolling. Alternatively, the second coupling can be modified so that stem  604  extends into slot  702  of the socket cover. This would likewise prevent telescoping-tube assembly  108  from rolling, while still accommodating movement in the pitch and yaw directions. 
     In the illustrative embodiment, the second coupling, like first coupling  112 , cannot accommodate translational movement. In some alternative embodiments, the second coupling is configured to accommodate translational movement, such as discussed above in conjunction with the first coupling. 
     Linear Actuators.  FIG. 10  provides additional detail of the linear actuators previously discussed (i.e., linear actuators  314 ,  316 ,  318 ,  320 ,  322 ,  324 ). As depicted in  FIG. 10 , rodless cylinder type actuator  1002  includes cylinder body  1050  with a central bore  1052  and a piston  1056  which slides in bore  1052 . Piston  1056  divides bore  1052  into first chamber  1058  and second chamber  1060 . Yoke  1054  is rigidly coupled to the exterior of tube  120  and is mechanically coupled to piston  1056 . In some embodiments, the yoke and piston are a unitary element; in some other embodiments, the yoke and piston are distinct elements that are mechanically coupled to one another (e.g., attached to one another, etc.) Sealing of first and second chambers  1058 ,  1060  is accomplished via an elongated elastomeric seal  1062  that flexes to accommodate motion of yolk  1054 /piston  1056  and a flexible steel backing strap or band  1064  that runs through slot  1066  in yoke  1054 . 
     Extension/Retraction Functionality. Application of pneumatic pressure to first chamber  1058  via port  1070  drives piston  1056  and yoke  1054  toward the “right” to second chamber  1060 . This, in turn, causes tube  120  to move to the “right.” If telescoping-tube apparatus  108  is oriented as depicted in  FIG. 4 , such rightward movement would cause tube  120  to deploy; that is, to “telescope” out of tube  118 . Application of pneumatic pressure to second chamber  1060  via port  1072  drives piston  1056  and yoke member  1054  toward first chamber  1058 . This, in turn, causes tube  120  to move to the “left.” If telescoping-tube apparatus  108  is oriented as depicted in  FIG. 4 , such leftward movement would cause tube  120  to retract into tube  118 . 
     Coupling/Locking Functionality. Closing the control valves that supply pressure to first and second chambers  1058 ,  1060  locks piston  1056  and yolk  1054  in place and locks adjacent tubes (e.g., tubes  118  and  120 ) to one another. In this state, there can be no relative movement between locked tubes. Note that this does not mean that locked tubes do not move; it means that there is no relative movement between these tubes. For example, if tubes  120  and  122  are locked, which means that tube  118  is not locked to tube  120 , it is possible for tubes  120  and  122  to collectively move toward  118  when vessel  102  (due to wave motion) moves closer to platform  104 . But what is important is to prevent relative motion between the locked tubes so that a crew member can safely transit from one (locked) tube to the next. 
     Floating Functionality. Releasing the pressure in chamber  1052  allows piston  1056  and yoke  1054  to “float.” In such case, relative motion between tube  118  and tube  120  is permitted. Since, in the illustrative embodiment, tube  118  is the outermost tube and is not movable, tube  120  will move to accommodate relative movement between the first platform (e.g., vessel  102 ) and the second platform (e.g., stationary platform  104 ) towards or away from one another by extending or retracting. 
     The floating functionality is depicted via the representations shown in  FIG. 14 . For clarity, only telescoping-tube assembly  108  is depicted in these Figures. The “upper” representation in  FIG. 14  depicts the telescoping-tube assembly fully deployed. Tubes  120  and  122  are assumed to be locked and relative motion is permitted between tubes  118  and  120 . Tube  118  is assumed to be coupled to a vessel (not shown) and tube  122  is reversibly coupled to a stationary platform (not shown). 
     In the lower representation of  FIG. 14 , the vessel on which telescoping-tube assembly  108  resides has moved closer to the stationary platform. Since relative motion is permitted between tubes  118  and  120 , as the vessel moves closer to the platform, tube  120  “retracts” into tube  118 . Actually, movement is to the “right” in  FIG. 14 . That is, tube  120  does not move; rather, the vessel and tube  118  move to the right by a distance “D.” This has the effect of shortening the telescoping-tube assembly by the length “L.” In this fashion, the length of telescoping-tube assembly  108  is “automatically” altered to accommodate a change in the relative position of the first platform and the second platform. 
     In alternative embodiments of the invention, other cylinders which may be utilized include: magnetically coupled rodless pneumatic, magnetically coupled rodless hydraulic cylinders, cylinders using magneto-rheological (MR) fluid, as well as electro-mechanical actuators. 
     It is within the capabilities of those skilled in the art to design or specify pneumatic, hydraulic connections and electrical connections and electrically operated valves suitable for use in conjunction with the illustrative embodiment of the present invention and the alternative embodiments of the present invention. While rodless type cylinders as described above provide advantages in reduction of size relative to conventional rod type pneumatic or hydraulic cylinders, it should be understood that conventional pneumatic and hydraulic actuator cylinders using magneto-rheological (MR) fluid maybe utilized as well as electro-mechanical actuators may be utilized. 
     As depicted in  FIG. 11 , in magneto-rheological (MR) fluid actuators, the hydraulic fluid utilized in a conventional hydraulic cylinder is replaced by a magneto-rheological (MR) fluid and an electromagnet  1104  is mounted on cylinder  1102 . Magneto-rheological (MR) fluids are oil-based suspensions of microscopic ferrous particles. When a magnetic field is applied across the fluid, the particles align and resist flow, and quickly become transformed from a fluid to a near solid. The speed of the transformation from a fluid to a near solid is on the order of milliseconds. One source for MR fluids is Lord Corporation of Cary, N.C. The locking capability of the MR fluid actuator provides an alternative to the valve locking system utilized with conventional pneumatic and hydraulic actuators previously described. The elimination of the valve system provides a reduction in the cost and complexity of the system. 
     As is depicted in  FIG. 12 , a magnetically coupled rodless cylinder or linear actuator  1200  is generally similar to rodless cylinder  1002  previously described with the exception that elongated elastomeric seal  1062  and flexible steel band  1064  have been eliminated and piston  1202  is magnetically coupled to piston yoke member  1204 . Actuator  1200  includes cylinder body  1206  with a central bore  1208  and piston  1202  which slides in bore  1208 . Piston  1202  divides bore  1208  into a first chamber  1210  and a second chamber  1212 . Piston  1202  and piston yoke member  1204  each include a strong magnet  1214 ,  1216  and piston yoke member  1204  moves with piston  1202 . Sealing of first and second chambers  1210  and  1212  is accomplished via conventional seals which have not been illustrated. Application of pneumatic pressure or hydraulic pressure to first chamber  1210  drives piston  1202  and piston yoke member  1204  toward chamber  1212 . Similarly, application of pressure to second chamber  1212  drives piston  1202  and the piston yoke member  1204  toward the first chamber  1210 . Closing valves leading to first and second chambers  1210 ,  1212  locks piston  1202  and piston yolk member  1204  in place and locks adjacent tubes for example tubes  118 ,  120 . Additional details of construction of this unit are known in the art and have therefore not been further described. 
     As is shown in  FIG. 13 , a conventional electro-mechanical ball screw actuator  1300  may be used to connect and drive the relative positions of tubes  118 ,  120 . Electromechanical actuator  1300  includes a base  1302  connected to surface  1304  of, for example, tube  118 . Base  1302  supports electric motor  1306  connected to gearbox  1308  which drives ball screw  1310 . End  1312  of ball screw  1310  is connected to, for example tube,  120 . Electric motor  1306  is connected to motor controller  1314  and brake  1316 . Motor controller  1314  is connected to a source of electrical power via conventional electrical connections that have not been illustrated. Additional details of construction of this unit are known in the art and have therefore not been further described. 
     Linear actuators  314 ,  316 ,  318 ,  320 ,  322 ,  324  may include conventional internally-mounted position sensors, not illustrated, for sensing end-of-linear-actuator travel. The end-of-travel sensors provide an electrical signal responsive to full extension and full retraction of telescoping-tube assembly  108 . As is depicted in  FIG. 1  when fully retracted, telescope tube assembly  108  may be stowed on the vessel deck in an efficient manner. 
       FIG. 15  depicts a method  1500  for transferring crew members from a first platform, such as vessel floating in water, to a second platform, which in some embodiments is stationary, such as an oil rig platform. The method includes the following operations:
         Op.  1502 : Extending, from a first platform to a second platform, a plurality of telescoping tubes.   Op.  1504 : Temporarily coupling, to the second platform, an end of a distal-most tube of the telescoping tubes.   Op.  1506 : Sensing a presence of a user moving within the telescoping tubes.   Op.  1508 : Determining a direction of travel of the user.   Op.  1510 : Locking/coupling both the telescoping tube in which the user is present (at the time of sensing) and an adjacent telescoping tube in the direction in which the user is traveling.       

     Operations  1502  through  1510  have been previously discussed in the context of the description of crew transfer system  100 . Additional operations, some optional, will also be conducted. For example, in some embodiments, before the telescoping tubes (e.g., tubes  118 ,  120 , and  122 , etc.) are extended, a cable (see, e.g.,  FIGS. 1A-1C ,  2 : cable  106 ) is used to couple the first and second platforms to each other. Furthermore, since the first platform will often be lower than the second platform, the anterior end of the telescoping tubes is raised using a mechanism (see, e.g.,  FIG. 1B , lift mechanism  114 ). 
     Regarding operation  1510  and as previously discussed, tubes that are not currently being traversed are free to move relative to each other and thereby accommodate movement between the first platform and the second platform. Consider, for example, a user entering a telescoping-tube assembly having three tubes. As the user first enters the assembly (from posterior end  109 ), the first tube and second tube are locked. As the user exits the first tube and enters the second tube, the first tube/second tube “lock” is released and the second tube and third tube are then locked. To the extent that a telescoping tube assembly includes more than three tubes, all tubes other than the tube in which the user presently resides and the adjacent tube in the user&#39;s direction of travel are free to float. 
     It is to be understood that the above-described embodiments are merely illustrative of the present invention and that many variations of the above-described embodiments can be devised by those skilled in the art without departing from the scope of the invention. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.