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This application claims the benefit of U.S. Provisional Application No. 60/092,908, filed Jul. 15, 1998 and U.S. Provisional Application No. 60/095,188, filed Aug. 3, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to subsea oilfield tubing operations and systems, and more particularly to operations and systems in which tubing is used for subsea wellbores in marine and offshore drilling and wellbore locations. 
     2. Background of the Art 
     Oilfield wellbores are drilled by rotating a drill bit conveyed into the wellbore by a drill string. The drill string includes a drilling assembly (also referred to as the “bottom hole assembly” or “BHA”) and tubing that carries the drill bit. The tubing may be coiled tubing or jointed pipe. The drilling assembly usually includes a drilling motor or “mud motor” that rotates the drill bit and a variety of sensors for taking measurements of a variety of drilling, formation and BHA parameters. A suitable drilling fluid (commonly referred to as the “mud”) is supplied or pumped under pressure from the surface down the tubing. The drilling fluid drives the mud motor and discharges at the bottom of the drill bit. The drilling fluid returns uphole via the annulus between the drill string and the wellbore and is returned to the surface work station via a return line. 
     For drilling wellbores under water (referred to in the industry as “offshore” or “subsea” drilling), a supply of tubing is carried at the surface work station (for example, located on a vessel or platform). A rig, which may have one or more tubing injectors, is used to move the tubing into and out of (trip) the wellbore. U.S. Pat. No. 08/911,787, assigned to the assignee of this application, provides certain methods of injecting tubing into subsea wellbore, which is incorporated herein by reference as if fully set forth herein. A riser, which is formed by joining sections of casings or pipes, maybe deployed between the surface work station and the wellhead equipment. The riser is utilized to guide the tubing toward the wellhead. The riser also serves as a conduit for the fluid returning from the wellhead to the sea surface. The riser is substantially larger in diameter than the wellbore and is designed so as not to leak the drilling fluid into the surrounding water. To deploy the riser, sections of pipe (usually 30-40 feet long) are serially connected at the drilling platform and deployed under water. Such large diameter jointed pipes or tubing are very heavy and thus impose significant loads on the surface work station and in particular the rigs and injectors used to deploy the riser. 
     One suitable injector for deploying the riser is shown in U.S. Pat. No. 5,850,874, commonly assigned to the applicant. While the high speed operation of such injectors can be useful in reducing the time for deployment of the tubular riser, holding the upper reach of a long string of riser against slippage in the injector and against human error in the operation of the injector can be a problem. Once the injector loses its hold on the riser, it is free to fall to the sea bed, with resultant damage to the riser and other subsea equipment. Similar problems can arise with drill strings or other tubing strings in subsea operations in deep water and/or in deep wellbores. Such drill strings are thus also long and heavy, so that they too must be securely held in the injector. Failure to do so will result in the drill string dropping into the wellbore, which may be difficult or perhaps impossible to retrieve. 
     In an alternative design to the above-noted tubular riser for conveying the return fluid from the subsea wellhead to the surface work station, the return line may be separate and spaced apart from the drill sting tubing. Such return lines are typically smaller and lighter than the jointed pipe/tubing riser, and indeed may be constructed of a flexible, non-metallic material. However, such construction results in the return line leaving the drill string tubing unprotected from the elements of the subsea environment. Indeed, the return line may actually come to interfere with the movement of the tubing toward and away from the subsea wellbore, if the surface work station is a ship or other moveable platform that allows the return line and the tubing to become twisted or wrapped together, upon angular movement of the platform. It is known that the water currents near the sea surface can cause great turbulence in the drilling equipment that extends from the drilling vessel to the wellbore. It is also known that sea water corrodes the drilling equipment that extends from the drilling vessel to the wellbore. 
     A riser that extends the full distance from the surface to the wellhead to hold drill fluid protects the drilling equipment extending from the vessel to the wellbore both mechanically, such as from upper level turbulence, and chemically, such as from corrosion. Applicants, however, have found that such turbulence is relatively minor past 150-200 feet from the sea surface and that corrosion is also relatively small after such depths. 
     SUMMARY OF THE INVENTION 
     The methods and apparatus of this invention overcome many of these tubing handling problems encountered in subsea tubing handling operations. For the problem of securely holding the upper reach of heavy tubular strings suspended from the surface work station, whether the string be the riser, the drill string or any other oilfield work string or whether it is a string of coiled tubing or jointed pipe, this invention provides an automatic safety device to prevent the loss of such string. This safety device supplements the rig or injector, by providing an automatic stop at the surface work station to grip and hold the string if the rig or injector does not. Indeed, such safety device is even useable at on-shore and shallow water drilling sites having shorter lengths of string and thus are at less risk of lost pipe for mechanical (if not operator error) reasons. 
     The present invention further provides for the reduction in the overall weight of the drill string and/or work string formed from continuous or coiled tubing suspended from the surface work station toward a work site in a wellbore. Such string has a first length or segment of coiled tubing shorter than the total length needed to reach from the surface work station to the wellbore work site, and second or upper length of coiled tubing to make up the difference having characteristics different from the first or lower length of coiled tubing. A tubular connector is provided to secure the lengths together so as to preserve the overall mechanical and pressure integrity of the string. Thus, the string can be designed to have a lighter and more flexible lower segment and a stronger (and perhaps larger) upper segment. Other differences in characteristics as between the length of tubing are also contemplated. 
     Similarly, the invention enables the use of a separate and distinct return line (rather than a riser) without the problems of leaving the drill string unprotected and avoiding the tendency of the return line and the drill string to wrap together. For the latter problem, the work moveable platform is provided with a turntable or other moveable device for passage of both the drill/work string and the return line thereto at spaced apart locations and then holding the string and return line in a predetermined spaced relationship. This reduces the tendency of these members to twist about each other. 
     The present invention further eliminates the need for the complete, full length riser. In the present invention a relatively short (about 200 feet) large diameter tubing (referred to herein as an “isolation tube”) may be deployed below the drilling surface work platform to negate the impact of turbulence and the corrosive effect of the sea water near the sea surface. The isolation tubing may be formed of a lighter gage material than a conventional riser and is filled with a suitable non-corrosive, non-water soluble fluid whose fluid density is less than that of the sea water. Such a fluid remains within the isolation tubing. A separate return line carries the return fluid from the wellhead to the surface work station. 
     Examples of the more important features of the invention have been summarized rather broadly so that in order that the detailed description thereof that follows may be better understood, and so that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For detailed understanding of the present invention, reference should be made to the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals: 
     FIG. 1 shows a schematic diagram of a section of the riserless subsea drilling system of this invention wherein a relatively short isolation tube is deployed below the vessel to mechanically and chemically protect the drilling equipment extending from the sea surface to the wellhead; 
     FIG. 2 shows a schematic diagram of a device for maintaining the fluid return line from wrapping around the drill string when the drilling vessel rotates during the drilling operations; 
     FIG. 3A is a schematic diagram of the automatic safety apparatus of this invention shown in its open mode of operation; 
     FIG. 3B is a schematic diagram similar to FIG. 3A showing the apparatus in its tubing engaging mode of operation; 
     FIG. 3C illustrates the uniform application of force on the tubing by various engagement members; and 
     FIGS. 4A-4B are schematic diagrams of the multiple segment coiled tubing drill string of this invention, with FIG. 4A showing drilling with a drill string of one segment of coiled tubing, FIG. 4B showing drilling with a multiple segment drill string. 
     FIG. 4C illustrates the different internal dimensions of various segments of tubing. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic diagram of a section of a riserless subsea drilling system within the scope of the invention having a relatively short isolation tubing  260  projecting below the surface work station, such as vessel  101 . The Applicants have found that heavy turbulence usually occurs up to about 200 feet below the sea level. The typical jointed pipe/tubing riser (not shown) utilized in the prior art systems serves as a barrier to such turbulences. Since the riser is filled with the drilling fluid, it also serves to chemically protect the tubing from the corrosive affects of the sea water which is most prevalent up to 300 feet depth. In the present invention, for deep water drilling, an isolation tube, such as  260 , may be deployed below the vessel  101 . The tubing  260  is of lighter gage material than the conventional riser and is preferably filled with non-corrosive, non-water soluble, environmentally friendly fluid  261 , which is lighter in density than the sea water. The fluid  261  is buoyed in sea water and thus remains within the isolation tube  260 . Oilfield tubing, such as drill sting tubing from a suitable supply, such as reel  180  for continuous or coiled tubing, is surrounded by the isolation tube  260 . A return fluid line  132  and control/gas injection lines  134  may also be routed through the isolation tube  260 . The isolation tube  260  is of a size, rigidity and strength to mechanically protect the tubing  142 , return line  132 , and the gas injection/control line  134  from the water turbulences, while the fluid  261  chemically protects such elements from the corrosive effects of the sea water. The isolation tube  260  is easier to install than a full-length riser, is much shorter and thus less expensive and depending upon the length may utilize only a fraction of the fluid  261  compared to a deep sea riser. 
     The isolation tube  260  is also much lighter than a full-length riser and thus imposes less load on the vessel  101  and the rig, which may include one or more injectors  190 . In addition, the fluid  261  held in the isolation tube  260  may have properties other than anti-corrosive properties. For example, it may alternatively or in addition, have anti-fouling, anti-freeze and/or lubricating properties. 
     The drilling vessel  101  tends to rotate about its axis over time, which can cause the return line  132 , which is separated and spaced apart from the tubing  142  and the gas injection/control line  134  to wrap around the tubing. To prevent this, a device such as that shown at  270 , shown in FIG. 2, is mounted on the vessel  101 . The device  270  has a through opening  272  which allows the passage of the tubing  142 . A slot  274  made around the opening  272  may be used to pass the return line  132  and the gas injection/control line  134  between the vessel  101  and the wellhead equipment  130 . The slot  274  may cover 360° or may include a stop  276  that enables the line  132  to move about the tubing  142  substantially 360 degrees. The lines  132  and  134  may be held together or spaced apart. The device  270  may also be made to rotate about the tubing  142 . In either embodiment, the device  270  keeps the tubing  142  in a predetermined spaced relation to the return line  132  and gas injection/control line  134 . 
     In certain instances during wellbore operations, it is desirable to stop the movement of the tubing due to some emergency, such as the detection of a kick, insufficient pressure in the wellbore or equipment failure, etc. A brake may be used for such purpose. The prior art brakes abruptly apply force on the tubing which often severely damages the tubing or in some cases breaks the tubing. Continuous tubing may exceed 10,000 feet in length. If the tubing is severely damaged or broken, it must be replaced. Replacement of the tubing is very expensive and also requires tripping the tubing string out of the wellbore, which can cause several hours of down time and for deep sea operations can cost several thousand dollars per hour. Therefore, it is desirable to have a brake system that can effectively stop the tubing movement without causing a catastrophic failure of the tubing. The present invention provides a safety braking system and method for controllably and effectively braking and holding the tubing. An embodiment of such as system is shown in FIGS. 3A-3C. 
     FIG. 3A is a schematic illustration of a tubing deployment safety system  600  that includes an opening receiving tubing  602  and one or more tubing engagement members  614   a - 614   b  moveably mounted on a frame or support member  610  on the surface work station. Each engagement member  614   a - 614   b  includes an associated activation mechanism  616   a - 616   b . For example, member  614   a  includes a gripping face and is associated with an activation mechanism member  616   a . The activation mechanism  616   a - 616   b  is coupled to power unit  620 . A suitable controller or control unit  630  controls the operation of the power unit  620 . The controller  630  receives input from one or more sensors S 1 -Sn and in response thereto and other instruction received or stored therein operates the power unit to engage or disengage the engagement members  614   a - 614   b . During normal operation, the system  600  remains in disengaged position, i.e., the engagement members  614   a - 614   b  are not engaged with the tubing. There remains a gap G 1  between the tubing  602  and the engagement members. One of the parameters monitored by the controller is preferably the actual motion of the tubing  602  compared to a predetermined limit. Other parameters may include the detection of a kick or pressure at the wellhead or in the wellbore. The controller  630  activates the power unit  620 , which provides the required power to the activation mechanism  616   a - 616   b , which moves the engagement members toward the tubing  602 . The force applied on the engagement members is controllably or progressively increased until the tubing  602  stops. 
     FIG. 3B shows the safety system  600  in the engaged position. A sensor Sp may be provided to determine the amount of the force being applied by the engagement members on the tubing  602 . The controller  630  may be programmed to utilize this feedback in operating the power unit  620 , thereby providing a closed loop control system. 
     FIG. 3C shows that the force F 1  is uniformly applied on the tubing by all of the various engagement members, four of which are illustrated, for example, by their forces F 1  in FIG.  3 C. The controller  630  preferably is microprocessor based system or a general purpose computer that is capable of handling the desired instructions. The controller can vary the application of the force as to the brakes to avoid “skidding” wherein the tubing is essentially unrestrained. This is done by reducing the applied force when skidding is detected so as to increase the frictional force between the engagement members  614   a - 614   b  and the tubing  602 . 
     The engagement members may be of any number or type, including wedges having resilient liners, such as an elastomer or any other composite material, facing the tubing  602 . There may only be one engagement member, such as an annular device with an internally inflating bladder. The bladder surrounds the tubing  602  and when the bladder is activated, it inflates radially or inward, i.e., toward the tubing  602 , thereby engaging the tubing. The length of the bladder is selected to provide the desired gripping force. Similarly, the surface area of the engagement members  614   a - 614   b  is selected to provide the required gripping force. More than one bladder or sets of engagement members may be utilized arranged longitudinally along the tubing  602 . The activation mechanism  616   a - 616   b  may be pneumatically, hydraulically, electrically, electromagnetically operated or by any other suitable method. The safety apparatus  600  may be disposed at the rig or for subsea applications, under water or at the surface, or even at a land well. 
     The activation mechanisms  616   a - 616   b  move their corresponding engagement member between a first or disengaged position, spaced laterally away from the tubing  602 , and a second or engaged position in pressurized engagement with the tubing. In the first position, the engagement members allow for the movement of the tubing into and out of the wellbore. In the second position of the engagement members, the activation mechanism controllably increases the force applied by the members to the tubing so as to slow or stop the movement of the tubing. At least one of the sensors senses a parameter indicative of an operating condition of the tubing. More particularly, when the apparatus is used in conjunction with an injector, such as injector  190 , the sensor senses a parameter indicative of the operation of the tubing selected from the group of operating parameters consisting of the speed and movement of the tubing (including downward movement into the wellbore or upward movement, such as in an underbalanced or blow-out situation), the gripping force of the tubing by the injector and the slippage or differential speed of the tubing relative to the operation of the injector. The safety apparatus  600  of this invention is useable both with coiled or jointed tubing that is employed for any oilfield operation purpose such as a riser, drill string or a work string. 
     As described above, coiled or reeled tubing is frequently used as the conveying member of a drilling string utilized for drilling wellbores. The coiled tubings currently utilized are continuous flexible metallic tubulars having uniform external diameters so that they may be moved by commonly available tubing injectors, which are usually designed only to handle continuous tubings with uniform outside diameter. The length of the tubing depends upon the total depth of the proposed wellbore. If the wellbore is to be drilled to 15,000 feet, then the tubing used is at least 15,000 feet. Very deep wellbores thus require very long tubings, which then require equally large reels. Reels of 40 feet diameter are being used in some instances. Such reels are expensive to make, difficult to transport and require large rig surface area, which is at a premium especially for offshore platforms and vessels. 
     Injectors, such as described herein above, have adjustable openings and can accommodate different diameter tubings. In one aspect, the present invention utilizes multiple field connectable tubings of the same or different outside diameters. In this manner, shorter reeled tubings may be utilized which can be carried by different lateral segments of a single large reel or more than one smaller reel. 
     FIGS. 4A-4B schematically illustrate one method of using multiple reeled tubings for oilfield wellbore operations. To drill a wellbore, a relatively large bore  702  is made to shallow depth and casing  703  is installed to avoid hole collapse near the surface. A drill string  705  is then used to drill the wellbore. The drill string includes a drill bit  716  carried by a bottom hole assembly (BHA)  714  which is attached to the bottom end of a reeled tubing  712 . The tubing  712  is reeled on a reel  730 , which is placed at the rig site  701 . In the example of FIGS. 4A-4B the wellbore to be drilled has an upper larger diameter section and a lower smaller diameter section. Referring to FIG. 4A, the wellbore  722  is drilled to a first depth  722   a  with a first drill  716  carried by the first tubing  712  supplied by the reel or spool  730 . 
     FIG. 4B illustrates the use of a second reeled tubing  740  in conjunction with the first tubing  712  to drill the lower section  732  of the wellbore to a second depth  742   a . To drill the wellbore to depth  742   a , the drill string  705  is retrieved. The second drill bit  796  carried by the second tubing  742  is conveyed to the bottom  722   a  of the wellbore  722 . If the length of the second tubing  742  is less than the total depth  722   a  of the downhole work site, the driller attaches the lower end  757  to the first tubing  712  to the upper end  755  of the second tubing  742  with a field connector  757 . The connector  757  may be a separate member that is adapted to attach at one end to the upper end of the tubing  742  and at the other end to the lower end  756  of the first tubing  712 . The connector  757  may include two segments, one segment mounted on one end of each of the tubings  712  and  742 . If the second tubing is longer than the depth  722   a , then the connector  757  is attached after exhausting the second tubing  742 . The drill string, with both the tubings, is then used to continue the drilling of the lower section  732 . Additional tubings of shorter lengths than the total well depth may be used in the manner described above. Such tubings may be carried on a separate reels, which are smaller than a single large reel, easier and less expensive to make and have much smaller foot prints. 
     Alternatively, the tubings may be of same external dimensions and carried on different annular segments of a common reel. However, the segments of tubing may be different internal dimensions such as shown at  780 ,  782  and  784  in FIG.  4 C. The multiple tubings of the present invention offer several advantages over single tubing: as noted above, such tubings may be carried by relatively small reels, which are easier to manufacture and transport and are easier to handle at the rig site, multiple tubings may require smaller power units and if a particular tubing segment suffers a catastrophic failure, only that segment will need to be replaced instead of the entire tubing. Similarly, segments subject to greater wear may be replaced earlier than the other segments of the drill string. 
     Thus, the method of performing oilfield operations (drilling, workover, logging, etc.) with a multiple segment drill string from lengths of coiled tubing involves conveying the drill string to a downhole work site with a first length of coiled tubing shorter than the total distance from the surface work station to the final downhole work site. Thereafter, a second length of coiled tubing is secured to the first length of coiled tubing by sealingly securing the ends of the lengths of tubing. The second length of tubing has different characteristics from that of the first. The drill string having both first and second lengths is then extended to the final downhole work site. The first and second lengths of tubing differ in the characteristics of being of different cross-sectional dimensions, materials of construction, tensile strength, reels on which the tubing was stored and/or the lateral segments of the same reel on which they were stored. The field connector  756  may be of one or several tubular parts and has mechanical tubing connections and hydraulic seals. These connections and seals are such that when the tubular connector is connected to the lengths of tubing, the connector preserves the mechanical and hydraulic integrity of the drill string by providing mechanical strength and pressure ratings substantially equal to that of at least one of the lengths of coiled tubing. In addition, the connector may provide for an electrical and/or optical connection between conductors such as conductors  790  in the lengths of coiled tubing. The multiple segment drill string of this invention is useable not only in marine and offshore applications, but also land based and shallow water drilling, with the surface work station thus being on land or at the surface of the shallow water. 
     While the foregoing disclosure is directed to the preferred embodiments of the invention, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope and spirit of the appended claims be embraced by the foregoing disclosure.

Summary:
A tubing handling for subsea oilfield tubing operations, includes an isolation tube to mechanically and/or chemically protect the drill string and improved passage of the drill string and fluid return line to the drilling vessel that further protects them during drilling use. In addition, the improvements include an automatic safety apparatus to hold against unintended movement of the tubular members under extreme length and weight conditions as well as against human error at the rig. Further, the invention includes multi-segment coiled tubing drill strings that can be adapted to drilling requirements in a deep wellbore.