Patent Publication Number: US-2010116507-A1

Title: Mobile equipment for riserless drilling, well intervention, subsea construction and the like from a vessel

Description:
The present invention relates to mobile equipment, and more particularly to the composition thereof, for use during riserless drilling, well intervention, subsea construction and the like from a dynamically positionable vessel having a moonpool. 
     Today, a number of the processes relating to offshore oil and/or gas exploration or production are carried out from specially designed drilling vessels or platforms that have a fixed derrick for use in this connection. In addition, various other vessel types must be hired in, such as platform supply vessels (PSV), anchor handling vessels (AHV) and multipurpose vessels (MPV). These additional vessels are generally dynamically positionable and have moonpools. 
     It would of course be cost-effective if the last-mentioned vessel types also could be used, for example, in connection with the actual drilling during hydrocarbon exploration. This could mean increased utilisation of a hired vessel for the operator and at the same time a reduction in the need to use different vessel types during the overall process. 
     Accordingly, one of the main objects of the present invention is to reduce the use of such costly drilling vessels or platforms during hydrocarbon exploration or production, so that riserless drilling, well intervention, subsea construction and the like can be carried out from, for example, already existing dynamically positionable vessels having a moonpool. This object is achieved with mobile equipment of the type mentioned in the introduction, the equipment comprising, as disclosed in the characterising clause of claim  1 , two masts located at the moonpool, a yoke extending between and movably arranged on the masts, a hydraulic top drive mounted on the yoke, and a drill floor mountable in connection with the moonpool. It is also desirable to have economy of space on and a minimum weight increase of the vessel in question, and that the mobile equipment can be mounted without major changes to and conversion of the vessel, and also to have a low centre of gravity. 
     Compared with a specially designed drilling vessel with fixed derrick, the low centre of gravity means that the equipment is lowered considerably, as about 85% of its total weight is lower than four metres from the main deck of the vessel, and also as the drill floor itself is only built to a level within one metre from the main deck. The space requirement is reduced also in relation to a derrick of standard type. 
     The respective mast can be mounted on a base, preferably of the prefabricated type, and can be laid down on the vessel deck using at least one hydraulic cylinder. Thus, only minor vessel-specific adaptations are required for the vessel in question in relation to the base work and the folding down of the masts. Furthermore, the yoke may be designed to be width-adjustable, thereby permitting alternative positioning of the mast bases. The folding down of the masts with mounted yoke and top drive to a position on the vessel deck provides obvious advantages during relocation from one location to another, and during maintenance and mounting or dismantling of the equipment. 
     The drill floor can be formed so as to be splittable and have a cut-out in the centre for the installation of slips. This permits easy displacement of the drill floor, for example, when the drill floor must be removed to gain access to the whole of the moonpool. The cut-outs otherwise form a through-opening in the drill floor, for example, for passing through a drill string. An iron roughneck can be provided on the drill floor, which is movably arranged thereon, and likewise an air winch, a hydraulic manipulator arm and the like. It will be appreciated that the low overall height of the drill floor gives advantages when devices of this type are to be brought onto the drill floor. Moreover, the mast base can act as a guide for the splittable drill floor. 
    
    
     
       Reference is now made to the attached drawings which show a preferred embodiment of the invention, in which 
         FIG. 1  is a schematic side elevation, in the longitudinal direction, of equipment according to the invention on board a vessel; 
         FIG. 2  shows the equipment illustrated in  FIG. 1 , but seen in the transverse direction; 
         FIG. 3  shows the equipment illustrated in  FIG. 1 , but seen from the top; 
         FIG. 4  is a perspective view of the equipment according to the invention in operative position; and 
         FIG. 5  is a perspective view of the equipment according to the invention in lowered position for maintenance or during relocation of the vessel. 
     
    
    
     As already mentioned above, the present invention is intended for use on existing PSVs, AHVs, MPVs and similar vessels that are dynamically positionable and have a moonpool. However, this does not prevent the equipment from being equally suited to be put into service on newly built vessels. Another condition that applies, for example, during drilling and well intervention, is that the vessels have sufficient storage tank capacity for drilling mud, bulk barite/bentonite and bulk cement. This is in addition to the need for accommodation for drill personnel, for example, 15 beds. The bed requirement is slightly smaller in connection with subsea construction operations only. 
     The equipment according to the invention is specifically intended to be used during riserless drilling, well intervention, subsea construction and the like from a dynamically positionable vessel with a moonpool  8 . The main components of the equipment are two masts  1  that are located on either side of the moonpool  8 , a yoke  6  extending between and movably arranged on the masts, a hydraulic top drive  7  mounted on the yoke and a drill floor  3 ,  4  that is positionable over the moonpool  8 . 
     In  FIGS. 1 and 2  there is by way of example only shown one drill pipe  14  extending downwards from the hydraulic top drive  7 , and one subsea accessory  15  that is mounted on the lower end of the drill pipe. Again by way of example, the subsea accessory may consist of a guide base  16  and a conductor pipe  17  of suitable diameter. 
     Each individual mast  1  is of any suitable design known in the field having sufficient height and load capacity. The mast height is selected for the handling of, for example, drill pipes having a length of 9-13 metres. The lower end of the mast is attached to a mast base  2 . The base is dimensioned for distribution of the load from the mast in a way that does not result in overloading and resultant damage to the vessel deck  9  and its underlying supporting structure. The base is attached by welding or bolting. 
     To permit the masts and the equipment located thereon to be laid down in a horizontal position on the vessel deck  9 , see  FIG. 5 , at least one hydraulic cylinder  11  is arranged between the base  2  and the mast  1  which can be secured using a suitable swivel link  10 . The cylinder is fastened to the base and mast in any suitable manner, for example, using a pivot joint. Such a possibility of laying down the masts horizontally on or close to the vessel deck is of course advantageous for the vessel&#39;s stability during relocation from one location to another. 
     The yoke  6  is of any suitable type and is, as mentioned above, arranged for movement up and down along the masts  1  during the actual working procedures. Such a movement along the masts is well known in the field and may be effected (not shown in the drawings) in that the respective mast is in the form of a separate hydraulic mast cylinder, is formed having a chain drive, a rack drive and the like. It is in addition assumed that the yoke is of a width-adjustable type, which, for example, may mean variable positioning of the mast bases  2 . 
     A hydraulic top drive  7  is mounted on the yoke  2 . Moreover, as shown in  FIG. 2 , there m is also provided a shackle device  12  and lifting gear  13  below the top drive. Heave compensation of any suitable type may also be provided, in a non-illustrated manner, in connection with the top drive. 
     The drill floor  3 ,  4  is advantageously splittable, which means that the whole moonpool  8 , when required, can be uncovered by pushing the drill floor halves apart without first optionally removing, for example, the drill string. It will be understood that the drill floor halves can be supported displaceably on the mast bases  2 . Each drill floor half is made having a cut-out  12 . Thus, a through-opening is formed in the drill floor when positioned over the moonpool. A standard drill floor usually has a maximum opening of about 1.32 metres. Slips (not shown) are also mounted in the drill floor opening in the respective half  3 ,  4  of the drill floor. Otherwise, the drill floor comprises an iron roughneck  5 . The iron roughneck is advantageously movably arranged on the drill floor, for example, using non-illustrated rails. As the drill floor is at a level close to the vessel deck  9 , this allows easy handling of other tools (not shown in the drawings) such as a manipulator arm, an air winch and others, on the drill deck, for example, using a skid. 
     Apart from the components of the equipment according to the invention as discussed above, it may briefly be mentioned that there is, for example, a need for a hydraulic unit, a power generator if the vessel lacks capacity, an operator cabin, a ramp for handling pipes between the equipment and the vessel deck, a movable pipe handling crane for lifting pipes between storage site on the vessel deck and ramp, equipment containers, a ROV unit, and during drilling and well intervention, mud pumps, and a mobile cement mixer. 
     The aim is that the vessel&#39;s own systems should be used as much as possible so as not to unnecessarily take up deck space on board the vessel. The equipment according to the invention also requires only a minimum of adaptations of the vessel itself, normally solely in connection with the mast bases. 
     A drilling procedure that is the most demanding of the processes involving use of the mobile equipment according to the invention, can, for example, be carried out in the following way:
         Loose drilling implements, casing pipes, wellhead, mud, cement, barite and bentonite are loaded onto the vessel.   The vessel is moved to the drilling location and dynamically positioned above the drilling site.   The ROV is deployed and lowered towards the seabed. A basket of is transponders is run down on the air winch. Pipes are taken from the storage space on the vessel deck using the pipe handling crane and laid on the ramp. A drilling assembly, including drill bit, direction-measuring tool, drill collar and drill pipe, is taken from the ramp using the lifting gear, put down in the slips and made up by the iron roughneck. The running in of pipes is stopped when the drilling assembly has come down to immediately above the seabed.   Drilling of holes of 80.7 cm. The position of the vessel and the drilling assembly is verified. After location at the bottom, the hole is drilled to the planned depth, usually 65-75 metres below the seabed. The section is drilled with seawater and viscous water-based mud plugs that are pumped by the mud pumps. Seawater is drawn directly from the sea, whilst the viscous mud plugs are pumped from the mud storage tanks. The string is rotated by the top drive, and new drill pipes are introduced constantly during the downward drilling. To be more specific, the string is placed/suspended in the slips, the top drive is unscrewed, a new drill pipe that is lying ready on the ramp is fetched, the new pipe is placed on the top of the string that is in the slips, the pipes are screwed together by the iron roughneck, the top drive is screwed into the new pipe, the string is released from the slips, and the drilling continues until the new pipe has been drilled down. When the planned depth has been reached, the hole is displaced with made-up mud that is pumped from the mud storage tank. The string is pulled out, whilst the individual pipes are unscrewed from each other and placed in the storage space on the vessel deck. That is to say that the string is pulled up until a screw joint is at the right height above the drill floor, the string is put in the slips, the iron roughneck is moved forward and unscrews the pipe, the pipe is lifted out of the pipe box that is in the slips, the pipe hanging in the lifting gear is moved to the ramp by manipulator arms for guiding to the ramp, the lifting gear is released when the pipe has been laid down on the ramp. The drilling is observed by the ROV.   The installation of conductor pipe of 76.2 cm. The conductor pipe is laid on the ramp using a pipe handling crane. The pipe is moved using the lifting gear and is put in the slips. A new pipe is brought and set in, and the last pipe to be set in is a conductor pipe housing. The pipes generally have a length of about 12 metres. The conductor pipe of 76.2 cm is installed/run on the drill pipe. Entering of the hole of 91.4 cm is monitored by the ROV. If there is a need for adjustment during the entering of the hole, the vessel is moved. The conductor pipe is placed on the bottom of the hole, and then the top of the conductor pipe housing is about 1.5 metres above the seabed. The conductor string is subsequently cemented by pumping cement through the running string, i.e., the drill string, down the string and out from the bottom thereof, and then upwards on the outside until the cement comes out on the seabed. The cement is displaced using seawater. After displacement, the cement is on the outside of the whole conductor pipe string and 5 metres inside the bottom thereof. At the bottom of the conductor pipe there is a return valve which prevents the cement from flowing back into the conductor pipe, there is higher pressure on the outside than on the inside because of the density of the cement mix. The cement is mixed in the mobile cement mixer. Cement is blown from the vessel&#39;s bulk cement storage tanks to a service tank on the mixer. When the cement mix has the desired density, the cement mix is pumped down in the string. The conductor pipe is held under tension until the cement has set, about 6 hours. The running string/drill pipe is then released and can be pulled up and laid out on the vessel deck.   After the conductor pipe of 76.2 cm has been installed and cemented, a hole of 66 cm is usually drilled down from the bottom of the conductor pipe. When this additional hole has been completed, a casing pipe of 50.8 cm is installed and cemented. The drilling is carried out in the same as described above, as is the installation and cementing. The top pipe of the casing string is in the form of a wellhead. Before the vessel leaves the completed borehole, a trawl-proof protective cover is placed on the wellhead top. The cover can be put in place using the air winch.   If no more processes are to be carried out using the mobile equipment according to the invention, the vessel then goes ashore for dismantling of the equipment, which takes about two days.   When the costly drill rig later comes to the same location that has been prepared as described above, the BOP (blow-out preventer) and the riser can be run directly, i.e., connected to the already mounted wellhead, with parallel retrieval of the protective cover using the air winch. Thus, substantial cost savings are made compared with normal operations, where the actual drilling rig deals with all drilling of the borehole.