Patent Publication Number: US-2010127229-A1

Title: Drawworks

Description:
The present invention relates to a drawworks, a rig comprising such a drawworks, a method of upgrading a drilling rig, a gear system for use in the drawworks, a method of repairing a drawworks with such a gear system, a brake for using in the drawworks, and to a method of repairing a drawworks with such a brake. 
     A drawworks is used in connection the raising and lowering of a variety of loads. In wellbore operations, such as drilling a well for oil or gas, a drawworks is used on a rig or with a derrick to hold and to raise and lower tubulars, e.g., but not limited to, a drill string and associated equipment above, into and/or out of a wellbore. A travelling block with a hook or other similar assembly typically used for the raising and lowering operations is secured in block-and-tackle fashion to a crown block or other limit fixture located at the top of the rig or derrick. Operation of the travelling block is performed by means of a hoist cable or line, one end of which is secured to the rig floor or ground forming a “dead line”, with the other end secured to the drawworks proper and forming a “fast line”. 
     In certain aspects, prior drawworks include a rotatable cylindrical drum upon which cable or fast line is wound by means of a prime mover (motor) and power assembly. The drawworks and travelling block assembly are automatically controlled or operated by an operator, e.g. a “driller”. In association with the raising of the travelling block, the prime mover (motor) is controlled by the operator e.g. with a foot or hand throttle; or the drawworks is automatically controlled by a suitable control system. The drawworks is supplied with one or more suitable brakes—for routine operation and for emergencies. The lines or wirelines are usually wire ropes or steel cables, although other materials have also been used. 
     Drawworks motors are relatively heavy high-horsepower motors. They provide the power to raise and lower loads that can be many hundred ton loads, some exceeding a thousand tons. In a variety of common drawworks systems, a gear system is located outside the drawworks drum and housing, taking up space which can be at a premium, particularly on offshore rigs. In a variety of common drawworks, calliper disc brakes are used which are also located outside the drum or housing. 
     The use of permanent magnet (PM) motors has been suggested for drawworks. The main advantage is that the footprint of the drawworks is considerably reduced since the motor is housed wholly within the drum of the drawworks. 
     The present invention is based on the insight by the applicant that yet further reductions can be made on the size of the drawworks, and in particular by placing at least a part of the gear system and/or brake inside the drum. This insight has given rise to problems not previously encountered in the drawworks field. 
     One particular problem is that traditional drawworks offer a combination of two functions: line-pull and line speed. The former is useful for lifting very heavy loads (e.g. a BOP weighing perhaps as much as one thousand tons); the latter is useful for tripping operations where speed is essential (a typical maximum line speed is about 25 ms −1 ). It is important to preserve this dual functionality if the new kind of drawworks motor is to be useful on drilling rigs. 
     The maximum torque of a PM motor can be increased by increasing its diameter. This means that the diameter of the drum has to be increased to house the motor. However, as the drum diameter increases the line pull is reduced thereby reducing the benefit of increased torque. On the other hand, PM motors have comparatively low RPM limiting line speed and thereby their usefulness for tripping operations. 
     A 2300 kW PM motor mounted in a 1.56 m internal diameter drum generates about 49,000 NM of torque. Transferring such large torques via a gear system, that is a least partially within the drum, to the line poses difficulties. At such torques smaller diameter gears require better manufacturing tolerances which are not economically feasible. 
     These particular problems are addressed by the use of a planetary gear system, and in certain aspects a planetary gear system having two gears. Furthermore better torque transfer is accomplished by mounting the planetary gears on flex pins whereby load is shared substantially equally between the planetary gears. This enables the diameter of the planetary gear system to be reduced without a corresponding increase in the required manufacturing tolerances. 
     According to the present invention there is provided a drawworks comprising a permanent magnet motor mounted inside a drum, said permanent magnet motor arranged to drive said drum via a gear system, characterised in that said gear system is located at least partially within said drum. 
     Further features of the drawworks are set out in claims  2  to  16  to which attention is hereby directed. 
     Placing at least a part of the brake within the drum has its own associated problems. For example, it is not practical to mount calliper brakes (traditionally used on drawworks brakes) inside the drum since maintenance becomes too difficult. Furthermore the diameter of the brake disc must be reduced to fit in the drum; the applicant has realised that braking a single smaller diameter disc would generate too much heat too be practical. Accordingly, to be mounted at least partly in the drum the brake should be relatively low maintenance and be able to dissipate the heat generated by braking. 
     These particular problems are addressed by a multi-disc brake comprising a first set of brake discs that rotate with the drum and a second set of brake discs that remain stationary. The two sets of brake discs may be brought into contact with one another to effect braking. This enables the kinetic energy of the drum to be dissipated as heat in a greater mass of material; at the same time the multi-discs are lower maintenance than standard calliper brakes. 
     According to the present invention there is provided a drawworks comprising a permanent magnet motor mounted inside a drum, said permanent magnet motor arranged to drive said drum, characterised by a brake system that is located at least partially within said drum. 
     Further features of the brake system are set out in claims to  21  to  30  to which attention is hereby directed. The brake system features of these claims may stand separately from the gear system features of claims  1  to  16 . In other words the present invention envisages a drawworks comprising a brake system as aforesaid, with or without the gear system features of claims  1  to  16 . 
     There is a need, recognized by the present inventors, for effective and efficient drawworks systems and brakes, gear systems, and motors for them. There is a need, recognized by the present inventors, for drawworks systems whose footprint is significantly reduced as compared to certain prior drawworks systems. There is a need, recognized by the present inventors, for reduced weight of equipment both for easy transportation for land rig applications and increased variable deck load on offshore vessels and floaters. 
     The present invention, in certain embodiments, provides a drawworks system with a permanent magnet motor located within a drum. In one aspect the motor includes a stationary stator that is secured to a primary central shaft and a rotor that is secured to and rotates with the rotating drum. In certain aspects the primary shaft has cooling channels therethrough through which a heat exchange fluid is circulated which can be any suitable fluid, e.g., but not limited to water, freon, liquid nitrogen, or antifreeze. 
     The present invention discloses, in certain aspects, a drawworks with a gear system which is located at least partially within the drawworks drum and, in certain particular aspects, a gear system that is entirely enclosed, partially within a system housing and partially within a drum. The present invention discloses, in certain embodiments, systems including: a rig; a derrick on the rig; a drawworks (any according to the present invention); a motor for powering the drawworks, the motor having a motor shaft, power cables for providing electrical power to the motor, a portion of each of the plurality of power cables passing through the shaft; and a plurality of channels passing through the shaft, the channels for the passage therethrough of a heat exchange fluid for the exchange of heat to cool the motor. 
     The present invention discloses, in certain aspects, drawworks having an “inside-out” permanent magnet motor. 
     The present invention discloses, in certain aspects, drawworks having a brake system located within a system housing. Such a brake system, in certain aspects, has a plurality of interleaved brake discs. Alternatively, systems according to the present invention have a brake system exterior to a system housing. 
     The present invention discloses, in certain aspects, drawworks having a gear system with planetary gears secured to gear carriers with flexpins that provide even load distribution on the planetary gears. 
     The present invention discloses, in certain aspects, drawworks having a gear system coupled to a motor with a splined connection for transferring high torque between the two parts and for easier assembly of the two parts. 
     The present invention discloses, in certain aspects, drawworks having a gear system in which gear shifting is effected by selectively moving a shifting sleeve in a two-step system to more efficiently use the power of the motor. 
     The present invention discloses, in certain aspects, drawworks having a brake system with a stationary brake hub that is connected to the systems primary shaft with a splined connection. Using the splined connection facilitates assembly and efficiently transfers high torque. 
     The present invention discloses, in certain aspects, drawworks with a torque arrestor connected to the systems primary shaft with a splined connection which efficiently transfers torque on a shaft to the exterior of the system. 
     The present invention discloses, in certain aspects, methods for moving an item in a rig system, the rig system for use in wellbore operations, the rig system as any described herein with a drawworks according to the present invention; the method including: raising or lowering the item by running the drawworks. 
    
    
     
       For a better understanding of the present invention reference will now be made, by way of example only, to the accompanying drawings in which: 
         FIG. 1  is a side cross-section view of a drawworks according to the present invention; 
         FIG. 1A  is an end view of the drawworks of  FIG. 1  (left end as viewed in  FIG. 1 ); 
         FIG. 1B  is an end view of the drawworks of  FIG. 1  (right end as viewed in  FIG. 1 ); 
         FIG. 1C  is a cross-section view of the drawworks of  FIG. 1  with parts that rotate shaded; 
         FIG. 2  is a cross-section view of a motor part of the drawworks of  FIG. 1 ; 
         FIG. 3  is a cross-section view of a torque arrestor of the drawworks of  FIG. 1 ; 
         FIG. 4A  is a cross-section view of a brake part of the drawworks of  FIG. 1 ; 
         FIG. 4B  is a cross-section view of part of the brake as shown in  FIG. 4A ; 
         FIG. 4C  is a cross-section view of part of the brake as shown in  FIG. 4A ; 
         FIG. 4D  is an end view of part of the brake as shown in  FIG. 4A ; 
         FIG. 4E  is an enlargement of part of the brake shown in  FIG. 4C ; 
         FIG. 4F  is an enlargement of part of the brake shown in  FIG. 4D ; 
         FIG. 4G  is a side view of a brake disc of the brake shown in  FIG. 4A ; 
         FIG. 4H  is a front view of a brake disc of  FIG. 4G ; 
         FIG. 4I  is a side view of a brake disc of the brake shown in  FIG. 4A ; 
         FIG. 4J  is a front view of a brake disc of  FIG. 4I ; 
         FIG. 5  is an isometric view of a gear system of the drawworks of  FIG. 1 ; 
         FIG. 5A  is a cross-section view of the gear system along line A-A of  FIG. 5J ; 
         FIG. 5B  is a cross-section view of the gear system along line B-B of  FIG. 5J ; 
         FIG. 5C  is a cross-section view of the gear system along line C-C of  FIG. 5H ; 
         FIG. 5D  is a cross-section view of the gear system along line D-D of  FIG. 5G ; 
         FIG. 5E  is a cross-section view of the gear system along line E-E of  FIG. 5J ; 
         FIG. 5F  is a cross-section view of the gear system along line F-F of  FIG. 5J ; 
         FIG. 5G  is an end view of the gear system of  FIG. 5 ; 
         FIG. 5H  is an end view of the gear system opposite the end shown in  FIG. 5G ; 
         FIG. 5I  is an enlargement of part of the gear system shown in  FIG. 5G ; 
         FIG. 5J  is a side view of the gear system of  FIG. 5 ; 
         FIG. 5K  is a side view of the gear system opposite the side shown in  FIG. 5J ; 
         FIG. 5L  is an enlargement of part of the system shown in  FIG. 5J ; 
         FIG. 5M  is an enlargement of part of the system shown in  FIG. 5K ; 
         FIG. 5N  is a cross-section view of a gear selection mechanism part of the gear system of  FIG. 5 ; 
         FIG. 5O  is a cross-section view of a gear selection mechanism part of the gear system of  FIG. 5 , with some parts omitted for clarity; 
         FIGS. 5P and 5Q  show the gear selection mechanism in different positions; 
         FIG. 6  is a graph of hook load versus block speed for a drawworks according to the present invention; 
         FIG. 7  is a graph of torque versus motor speed for a drawworks according to the present invention in high gear; and 
         FIG. 8  is a graph of torque versus motor speed for a drawworks according to the present invention in low gear. 
     
    
    
       FIGS. 1-1C  show a drawworks system  10  according to the present invention which includes a primary shaft  20  supported by supports  12  on a base  14 ; a motor  60  encompassing the primary shaft  20 ; a gear system  50  coupled to a rotor  62  of the motor  60 ; a housing  30  to which are connected the gear system  50  and the rotor  62  of the motor  60 ; a brake system  70  connected to the housing  30 ; and a drum  40  connected to the housing  30 . Fluid conducting channels  20   a ,  20   b ,  20   c ,  20   d ,  20   e , and  20   f  (see  FIGS. 1 and 2 ) provide passageways for heat exchange fluid for cooling the motor  60 . The channels  20   c  and  20   d  extend through the stator  68 . 
     The drum  40  holds rope, line or cable to be reeled in by and payed out from the system  10 . 
     The brake system  70  in this embodiment is within the housing  30 . This housing, part of the planetary gear (described below), is connected to the drum  40  and rotates at the same speed as the drum. The motor  60  is within the drum  40  and comprises a permanent magnet motor having 24 poles and an output power of about 2300 kW. The gear system  50  is partially within the drum  40  and partially within the housing  30 . Optionally, the brake system is located exterior to the housing. 
     A coupling  64  connects the gear system  50  to the rotor  62  of the motor  60 . A coupling  66  connects the rotor  62  of the motor  60  to the brake system  70 . The torque arrestor  80  is connected to the primary shaft  20  and is secured to a part  12   a  of a support  12 . Bearing housings  16  on the supports  12  support the primary shaft  20 . A bushing  18  encompasses the torque arrestor  80 . A main bearing  19  of the drum  40  encompasses the shaft  20 . 
     The housing  30  has lugs  32  with holes  34  therethrough. The base  14  has corresponding lugs  15  with holes  17  therethrough. Bolts (not shown) in the holes  34  and  17  hold the drum  40  and housing  30  immobile (e.g. during maintenance). 
     It is within the scope of the present invention for the motor  60  to be any suitable permanent magnet motor, including, but not limited to motors as disclosed in pending U.S. application Ser. No. 11/709,940 filed Feb. 22, 2007 and incorporated fully herein for all purposes. Further details of a suitable permanent magnet motor can be found in IADC/SPE: SPE-99078-PP ‘Utilizing Permanent Magnet Motor Technology on Larger Drilling Equipment for Improved Safety and Better Control’, Kverneland, H. et al. IADC/SPE Drilling Conference, Miami, 21-23 Feb. 2006; and in SPE-112312-PP ‘New Large Capacity Compact Drawworks for New Builds and Upgrade Jobs’, Kverneland, H. et al. IADC/SPE drilling Conference, Orlando, 4-6 Mar. 2008. Reference is specifically made to the features of the motors disclosed in these two papers. As shown in  FIG. 1 , the motor  60  has a stator  68  with windings  69  secured to the primary shaft  20 . The rotor  62  has permanent magnets  63  secured thereto. The stator  68  is connected to the primary shaft  20  either with a flange connection or with a shrink-fitted connection. 
     The rotor  62  rotates on bearings  161  between the rotor  62  and the primary shaft  20 . 
     The torque arrestor  80  transfers torque from the wire and drum via the shaft  20  to the base  14 . In one aspect the connection between the torque arrestor  80  and the primary shaft  20  is a splined connection with splines of the torque arrestor  80  meshing with corresponding splines of the primary shaft  20 . In certain aspects this insures that the primary shaft  20  and the torque arrestor  80  have the same torsional stiffness for proper load shearing in the spline. 
       FIGS. 4A-4J  show the brake system  70  and details of its structure and parts. The brake system weighs about 3220 kg and has an outer diameter of 1400 mm and a length of about 580 mm. A stationary brake hub  71   a  is secured to the primary shaft  20  via a splined structure that includes splines  20   s  on the primary shaft  20  which engage with splines  71   s  on the stationary brake hub  71   a . A rotating brake hub  70   b  has lugs  70   c  which are bolted with bolts  71  extending through the lugs  70   c  to housing lugs  33  and, thus, the rotating brake hub  70   b  rotates with the housing  30 . 
     A plurality of discs  72   a  connected to the stationary brake hub  71   a  are interleaved with a corresponding plurality of discs  72   b  which are connected to the rotating brake hub  70   b.    
     End plates  73   a ,  73   b  are at opposite ends of the brake system  70  and are bolted with bolts  74   a ,  74   b , respectively, to the stationary brake hub  71   a.    
     Springs  75  are disposed within channels  76   a  in a spring hub  76 . The springs  75  urge the spring hub  76  so that an end  76   b  of the spring hub  76  pushes the brake discs together to effect braking action (springs urging the spring hub to the right as shown in  FIG. 4B ). Brake fluid under pressure within an inner chamber  77  of the spring hub  76  normally prevents the springs  75  from urging the spring hub  76  toward the brake discs. When braking action is desired, the brake fluid is evacuated from the chamber  77  via outlets  77   a , thus permitting the springs  75  to move the spring hub  76  to compress the brake discs against one another. The brake fluid under pressure is supplied from a fluid pressure source (not shown) and braking is controlled by a control apparatus (not shown). 
     The discs  72   b  have outer splines  72   r  which mesh with and slide in corresponding splines  72   s  of a sliding spline  78   b . The discs  72   a  have inner splines  72   t  which mesh with and slide between splines  72   u  of a sliding spline  78   a . Under action of the springs  75 , the discs  72   a  and the discs  72   b  slide in their respective splines until they are ‘bunched’ together. In this way braking action takes place on both sides of the rotating discs  72   b . When the brake is released fluid pressure is re-applied to the spring hub  76 , and each disc  72   a ,  72   b  is returned to its original position under a restoring force provided springs (not shown). In this original position the discs are spaced apart from one another so that the discs  72   b  may rotate freely between the discs  72   a.    
     The use of the springs  75  to apply the brakes insures a fail-safe operation of the brakes. If there is a failure of brake fluid pressure, e.g. in the event of a pressure failure, the brakes will be applied and the drum will stop. 
       FIGS. 5-5Q  show a gear system  50  and parts thereof according to the present invention and parts thereof. The overall length of the gear system  50  is about 1.36 m (including coupling  64 ) and the maximum diameter is 1.7 m. That part of the gear system (i.e. up to the flange adjacent the lifting lugs in  FIG. 5 ) that fits inside the drum  40  has in outer diameter of 1.56 m. The weight of the gear system  50  is approximately 8500 kg. The coupling  64  provides a splined coupling between the gear system  50  and the motor  60 . As shown in  FIG. 1C , the gear system  50  rotates with the rotor  62  and the housing  30 . 
     A rotating gear housing  53  rotates around the primary shaft  20  and houses the various gears described below. The rotating gear housing  53  also rotates around a stationary end cover  52  which is secured to the primary shaft  20  with a splined connection which includes splines  52   s  on the end cover  52  which mesh with corresponding splines  20   r  on the primary shaft  20 . A hollow gear shaft  54  encircles the primary shaft  20  and is connected to the end cover  52  with hollow dowel pins  52   p . A lube oil outlet  56  that extends through the end cover  52  is in fluid communication with the interior of the rotating gear housing  53  via a channel  52   n . Lube oil for the gear system flows through the lube oil outlet  56 . 
     A gear shift sleeve  57  encompasses the hollow gear shaft  54  and is movable toward and away from the end cover  52  to shift the gears. Two actuators  58   c  move the sleeve  57 . The gear system  50  is provided with lifting lugs  50   l  and  50   m . A breather is used (not shown) to vent the interior of the gear system to reduce condensation therein. 
     A gear coupling actuator  58  includes two cylinders  58   c  and the sleeve  57 . 
     Within the rotating gear housing  53  are a first planet wheels  151   a ; a first planet wheel carrier  152   a ; a first planet wheel carrier support  153   a ; a second planet wheels  151   b ; a second planet wheel carrier  152   b ; a second planet wheel support  153   b ; a first sun wheel  154   a ; and a second sun wheel  154   b . Flexpins  155   a  connect the first planet wheels  151   a  to the first planet wheel carrier  152   a ; and flexpins  155   b  connect the second planet wheels  151   b  to the second planet wheel carrier  152   b . The flexpins provide a double cantilevered mount for each planet wheel whereby translation (i.e. movement without skewing) of the planet wheels relative to the respective planet carrier is permitted. The flexpin comprises a central shaft mounted to a planet carrier. Each planet wheel is mounted to the other end of the central shaft. Further details of each flexpin can be seen in U.S. Pat. No. 3,303,713 to which reference is specifically made in this respect. 
     Proximity switches  156   a ,  156   b  (see  FIG. 5J ,  FIG. 5K ) provide signals indicating what gear the gear system is in. 
     Bearings  255   a - 255   m  facilitate movement of the parts between which they are located. 
     The shifting sleeve  57  has three positions—two end positions, Low and High; and a neutral (free) position. When a sleeve is activated, it goes to one of the two end positions—Low gear or High gear. For maintenance purposes, the sleeves are manually put in the neutral position (a “fake” end position) so that the drum can be manually rotated.  FIG. 5O  shows the shifting sleeve  57  in the neutral position. The shifting sleeve  57  comprises two sets of teeth: a first set of teeth  300  is positioned on the inner surface of the shifting sleeve  57  and the teeth are oriented so that axial movement of the sleeve in one direction (to the right in  FIG. 5O ) brings the first set of teeth  300  into engagement with a corresponding set of teeth  302  on the second sun wheel  154   b  to prevent that sun wheel rotating (this position is shown in  FIG. 5P ). Since the second sun wheel  154   b  is fixed to the first planet wheel carrier  152   a , the latter is also prevented from rotating. In this position the gear system is in high gear for moving low loads at high speed and the drum  40  is driven by the planet gears  151   a.    
     A second set of teeth  304  is positioned on the outer surface of the shifting sleeve  57  and the teeth are oriented so that axial movement of the sleeve in the opposite direction (to the left in  FIG. 5O ) brings the second set of teeth  304  into engagement with a corresponding set of teeth  306  on the second planet wheel carrier  152   b  to prevent it rotating. In this position the gear system is in low gear for moving heavy loads at low speed and the drum  40  is driven by the planet gears  151   b . In the neutral position neither the first of teeth  300  nor the second set of teeth  304  is in contact with the teeth  302  or  306  and thereby the drum  40  can be rotated manually for maintenance purposes. 
     It is within the scope of the present invention to employ gears with any suitable gear ratios. In one particular aspect the two-step planetary gear system as shown provides a 1:3.77 gear ratio for heavy loads and a 1:11.43 gear ratio for tripping pipe. In certain aspects the gear systems according to the present invention are lubricated and cooled with hydraulic oil or with gear oil. It is within the scope of the present invention to have two or more gears and two or more different gear ratios including but not limited to, gear ratios for a high speed mode and for a high torque mode. Also, a gear ratio can be provided for a medium speed mode. Furthermore it is within the scope of the present invention for the planetary gear system to have only one gear. 
     A position pin  157  is mechanically connected to the sleeve  57  and moves in and out when the sleeve  57  is pushed in and out by the two hydraulic cylinders  58   c . The two cylinders  58   c  are connected hydraulically in parallel so that both move simultaneously.  FIGS. 5N and 5O  show the gear apparatus in a neutral position (gear not engaged). As explained above with the gear in high speed mode, the sleeve  57  is its very right position (see  FIG. 5P ), and the second sun wheel  154   b  is blocked. The two cylinders  58   c  are pressurized on the piston side, so the piston rods are fully extended. Only the first sun gear  154   a  is now engaged. The position pin  157  is in its very right position, and the proximity switch  156   a  gives a positive feedback to the drawwork control system, confirming the high gear, high speed position. 
     The drawwork drum  40  is (and must be) at standstill during gear shifting operations. The brake system  70  must be applied during gear shifting operations. The control system prevents the possibility of gear change if the fail safe brakes are not applied. When shifting gears from low load, high speed mode to high load, low speed mode, hydraulic pressure is applied to the rod side of the two pistons  58   c , moving the sleeve  57  towards left. 
     When the two cylinders are in a left end position, i.e. both cylinder rods fully retracted, the sleeve  57  is in its very left position, and the planet wheel carrier  152   a  is locked. Both first sun gear  154   a  and second sun gear  154   b  are now engaged, and the gear is in high load mode. Position pin  157  is now in left position, and the proximity switch  156   b  gives a positive feedback to the control system, confirming the low gear, low speed position. 
     When a positive feedback is given from the proximity switch  156   b , the brake can now be released and the drawworks operated. If there is no positive feedback from any of the two proximity switches  156   a  or  156   b , the brakes will not be released, and the drawworks can not be operated. Only in Service Mode is it possible to operate the brakes without having a positive feedback from the one of the proximity switches. 
     A particular advantage of the present invention is the reduction in weight and footprint of the drawworks. An apparatus according to the invention is manufactured by the applicant under the trade mark MAGNAHOIST. Table 1 below shows a comparison between the dimensions, footprint and weight of a MAGNAHOIST compared to other equivalent power capacity drawworks currently available from National Oilwell Varco (NOV). 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Width 
                 Length 
                 Height 
                 Footprint 
                 Weight 
               
               
                   
                 [mm] 
                 [mm] 
                 [mm] 
                 [m 2 ] 
                 [kg] 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 MagnaHoist 1100 
                 3840 
                 5520 
                 3100 
                 20.1 
                 38,000 
               
               
                 (incl. auxiliary equipment) 
               
               
                 National 1625-UDBE (1) 
                 5775 
                 6760 
                 2960 
                 39.04 
                 60,000 
               
               
                 Drawworks with sand reel and 
               
               
                 Baylor brake 
               
               
                 SSGD 500-3450 
                 4250 
                 7000 
                 4200 
                 29.75 
                 64,000 
               
               
                 GA UDBE is an Oilwell 
                 5080 
                 8290 
                 3000 
                 31.95 
                 61,700 
               
               
                 E-3000 drawworks 
               
               
                 Dreco D3000 AC drawworks 
                 4635 
                 6845 
                 3520 
                 31.7 
                 50,000 
               
               
                   
               
            
           
         
       
     
     As can be seen the weight reduction is between 24% and 41% and the reduction in footprint is between 32% and 48%. 
     The MAGNAHOIST is also smaller and lighter than some lower power drawworks, for example the 1320 UE also available from NOV. The overall length of the MAGNAHOIST is more than 2 metres shorter than the 1320 and it is also slightly smaller in width (3.64 m compared to 4.26 m. The height of the 1320 UE drawworks is approximately 2.9 m, whereas the MagnaHoist is 3.1 m. The total weight of the 1320 UE drawworks including motors, brakes etc is 52.5 tonnes, i.e. 14.5 tonnes heavier than the MAGNAHOIST. 
     The size and weight reduction of the MAGNAHOIST compared to a smaller capacity 1500 kW drawworks is a significant advantage, especially for upgrades on floater and jack-up type rigs where the old drawworks is replaced by a MAGNAHOIST or other drawworks in accordance with the invention. In particular, it is relatively easy to replace a smaller power capacity drawworks with the MAGNAHOIST since the footprint of the latter is smaller. However, the lifting capacity is substantially increased on the existing rig, and at the same time the equipment weight on the rig is reduced. This means that the variable deck load (VDL) capacity is increased, and the rating of the rig increases. 
       FIG. 6  shows a graph of block speed versus hook load for a drawworks according to the invention using 16 lines. It can be see how the two step gear system maintains both the tripping and line pull functionality of a drawworks that incorporates a PM motor. The dashed curve represents maximum hoist loads in low gear. This gear is not used very often; only during high load operations, for example when installing a BOP on the sea bed. Estimated operation time with this gear ratio is less than 20%. The dotted curve shows the actual hook load capacity in high gear. This gear with its pull capacity of 320 tonnes and maximum speed of 1.6 ms −1 , will cover the vast majority of the tripping and drilling operations. 
       FIG. 7  shows a graph of average torque versus motor speed for a drawworks according to the invention in high gear. This graph shows various points during drilling and tripping operations; these operations represent about 80% of the use of a drawworks. The continuous and intermittent torque versus speed characteristics of the drawworks are also shown. It can be seen that the drawworks meets the demands of tripping and drilling that are placed on it for 80% of its working life. 
       FIG. 8  shows a graph of average torque versus motor speed for a drawworks according to the invention in low gear. This graph shows various points during BOP and casing handling; these operations represent about 20% of the use of a drawworks. The continuous and intermittent torque versus speed characteristics of the drawworks are also shown. It can be seen that the drawworks meets the demands of BOP and casing handling that are placed on it for 20% of its working life. 
     One of the main advantages with the “inside-out” PM-motor in a drawworks  10  according to the invention is that the primary shaft  20  is stationary. Also, the PM-motor shaft is integral to the drawworks shaft, so the possibility for misalignment between motor and drum is reduced. Two spherical plain bearings are used in each end of the stationary motor shaft, reducing the requirement for alignment between the two bearings. This advantage is especially important during installation of the drawworks  10  on a rig: shimming and alignment of the drawworks frame becomes less critical. In a traditional drawworks the main shaft is rotating, requiring the drawworks frame including the bearing pedestals to be properly aligned. Misalignment often results in vibrations and noise in the equipment, which again leads to reduced lifetime on bearings and other main components, increasing the need for maintenance. 
     One advantage of the gear system  50  is that no specific maintenance is required as long as it is properly lubricated and cooled. The lubrication and cooling system consists of a hydraulic power unit including filters, heat exchanger and necessary instrumentation, everything mounted on the drawworks skid. When the drawworks  10  is in operation, lubrication oil is constantly sprayed on all main components in the gear box and circulated back to the hydraulic power unit. The main maintenance issue with the gear box is to make sure that the lubrication oil is properly cooled by the heat exchanger and that the oil is free of particles and water. The gear shifting mechanism and the hydraulic cylinders also needs to be checked periodically, in case of any external leakage in the cylinders. The gear box should provide over 20 years of operation. 
     One particular advantage of mounting the gear system and/or brake at least partially within the drum is that some extra protection is afforded to the gear system and/or brake by the drum. Being mounted in the drum can also help in meeting the necessary ATEX standards for operating in explosive atmospheres. 
     A drawworks according to the invention is particularly advantageous for use on smaller drilling rigs, such as floaters, vessels and semi-submersibles, where rig space is at a particular premium. The drawworks is also useful for upgrading fixed platforms and land rigs. 
     It is envisaged that a drawworks according to the invention may or may not comprise the brake system  70  as described in conjunction with the drawworks  10 . For example a drawworks may be provided that comprises a gear system substantially as described herein mounted at least partially within the drum that uses a conventional calliper type brake system mounted outside the drum. 
     It is also envisaged that a drawworks according to the invention may or may not comprise the gear system  50  as described in conjunction with the drawworks  10 . For example a drawworks may be provided that comprises a brake system substantially as described herein mounted at least partially within the drum that uses a conventional gear system mounted outside the drum. 
     It has been found that a drawworks with a motor of power 2300 kW, and a drum and gear system having dimensions as described herein, functions particularly well. However, it is within the scope of the invention for a drawworks using the principles of the invention to be downsized or upsized according to requirements.