Patent Publication Number: US-6213215-B1

Title: System, vessel, seabed installation and method for producing oil or gas

Description:
FIELD OF INVENTION 
     This invention relates to a system for offshore production of oil or gas, comprising a surface vessel having means for maintaining a desired position and orientation, a bottom installation at the seabed, for at least two production wells, and risers for connecting the bottom installation to the vessel. Moreover, in the following specification there is described a method of producing oil or gas with the system, whereby the method initially comprises deployment of the bottom installation at the seabed and the vessel in position at the sea surface above the bottom installation. 
     BACKGROUND OF THE INVENTION 
     When developing oil or gas fields at sea the time factor is very significant. The time taken from the drilling of the first well being able to produce, until this well and other wells in association therewith are completed and in full production, should be as short as possible. This has to do, inter alia, with the large investments being made in the form of expensive drilling and production equipment. Also this comprehensive equipment as such should be designed and built aiming at the lowest possible total costs. 
     BRIEF SUMMARY OF THE INVENTION 
     One aspect of the present invention is a system for offshore production of oil or gas. The system includes a surface vessel which has an apparatus for maintaining a desired position and orientation, a bottom installation at a seabed for at least two production wells, and at least one riser for connecting the bottom installation to the vessel. The vessel is provided with drilling equipment for wells at the seabed and with process equipment for produced oil or gas. The bottom installation comprises a template having a foundation at the seabed. The bottom installation has a manifold unit. The bottom installation is designed with at least two wellhead sections adapted for installation of associated Christmas trees. The system also includes a drill string adapted to extend from the drilling equipment at the vessel, and a movable operation module adapted to be installed on the bottom installation and to be connected to the lower end of the drill string. The at least one riser includes a product riser adapted to connect the operation module to the process equipment on the vessel. 
     The system may also include a swivel unit having a vertical axis, in association with the riser. The swivel unit may be located on the vessel, and may be incorporated in the operation module. The swivel unit may be motor driven under control by the apparatus for maintaining the vessel at a desired position and orientation. The swivel unit may have a through-going axial hole for the drill string, and may be underneath a blowout preventer which is also incorporated in the operation module. 
     The product riser may extend upwards through the water for connection at the vessel at a point at a distance from equipment, abaft on the vessel. The product riser may extend up through the water in an approximate S-shape with an upwardly curved, elevated middle portion of the product riser. The product riser may be attached along a drilling riser for the drill string. 
     The vessel may be provided with a means for dynamic positioning/orientation. 
     According to another aspect of the present invention a vessel is provided with drilling equipment for wells at a seabed and with process equipment for produced oil and gas. The vessel includes an apparatus for maintaining a desired position and orientation at the sea surface, and an apparatus for controlling a swivel unit in association with the means for maintaining the desired position and orientation, being based on dynamic positioning and orientation. 
     The vessel may be equipped with a swivel unit for the upper end of a product riser. The drilling equipment may include an associated work opening, and be located generally midship, and may also have an associated coupling for the product riser. The process equipment may be located abaft. 
     According to yet another aspect of the present invention a bottom installation includes a template and movable operation module. The template has a foundation at a seabed, and is provided with a manifold unit. The template is designed with at least two wellhead sections adapted for installation of associated christmas trees. The movable operation module is adapted to establish a rotatable connection to a riser extending to a surface vessel. 
     The movable operation module may include a swivel unit which is motor-driven, and may have a through-going axial hole for a drill string. The swivel unit may be located underneath or over a blowout preventer which may be also incorporated in the operation module. 
     The operation module may have at a lower portion an interface of the same type as christmas trees intended for being landed and connected to the wellhead sections. The operation module may be provided with a guide yoke adapted to hold product risers extending laterally and at an inclination upwards, with a clearance in relation to remaining components and equipment of the template. 
     According to yet another aspect of the present invention, a system for offshore production of oil or gas is provided which includes a surface vessel equipped with an apparatus for maintaining a desired position and orientation, a bottom installation at a seabed for at least two production wells, and risers for connecting the bottom installation to the vessel. The vessel is provided with drilling equipment for wells at the seabed and with process equipment for produced oil or gas. The bottom installation comprises a template having a foundation at the seabed. The bottom installation is provided with a manifold unit. The bottom installation is designed with at least two wellhead sections adapted for installation of associated christmas trees. The system further comprises a drill string adapted to extend from the drilling equipment on the vessel to a blowout preventer on the bottom installation. The risers include a product riser adapted to connect one or more production wells on the bottom installation to the process equipment on the vessel. 
     According to yet another aspect of the present invention, a method of producing oil or gas with a system including a surface vessel, a bottom installation for at least two production wells and provided with a manifold unit and a plurality of wellhead sections, and at least one riser for connecting the bottom installation to the vessel is provided. According to the method the bottom installation is deployed at a seabed, and the vessel is positioned at the sea surface above the bottom installation. A first well is drilled and completed at one of the plurality of wellhead sections. A second well is initially drilled at a second one of the plurality of wellhead sections. The operation module and riser are set down for landing and connection at the second one of the plurality of wellhead sections and the second well is additionally drilled and completed. Production is established for the first well through the manifold and the operation module at the second one of the plurality of wellhead sections, simultaneously with the additional drilling of the second well. A corresponding third well is drilled from a third one of the plurality of wellhead sections and production is subsequently established through the operation module from the third well. 
     The initially drilling of the second well may include installing both conductor and casing tubes for the second well. 
     Additional new wellheads may be drilled according to the method. If additional new wells are drilled the operation module may be disconnected and removed from the new wellhead sections after drilling, completing and putting into production a desired number of wells. If additional new wells are drilled, the operation module may also be finally located at a dedicated section of the bottom installation separated from the new well head sections. 
     It is an object of this invention to provide a system and a method as well as associated components, in particular in the form of a bottom installation and a special vessel, that in an optimal manner makes it possible to fulfill the objects just mentioned. 
     According to the invention this is obtained by means of the novel and specific solutions being defined in the claims. 
     In short the concept described here is based on the employment of a drilling vessel which is also equipped with process modules, so that the same vessel can provide for the essential functions both for drilling and for the subsequent production of hydrocarbons. This special vessel is equipped for adequate positioning, e.g. with equipment for dynamic positioning, so that it can lie with the bow against wind and weather and rotate therewith, depending in particular on the wind direction. The connections to the bottom installation concerned in most cases will involve a need for a swivel arrangement. This can be located either at the bottom installation, — or more specifically at the well where drilling is going on, or the swivel can be provided on the vessel. According to the invention, however, it is not excluded that the system and the method can operate also without a swivel. The drill string from the vessel will normally have heave compensation, that usually takes place at the vessel. Such heave compensation does not involve any problems or otherwise has no significant influence on the system described here, or the associated method. 
     The invention involves the substantial advantage that it will be possible to start production from an oil or gas field and thereby obtain economic gain at a very early time. Moreover there will quickly be obtained information as to how the field concerned produces, at the same time as drilling and completion of wells take place for exploiting the same reservoir. 
     In connection with the above the drilling vessel can be utilized in an optimal manner in that it has equipment and capacity for processing hydrocarbons, such as crude oil, whereas simultaneously it can perform a drilling operation. Based on production experience from wells being completed and put into production, the drilling programme and completion of subsequent wells can then be optimized, so that the productivity of the field concerned, can be increased as a whole. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various aspects and combinations comprised by the invention, will be explained more closely in the following description with reference to the drawings, wherein: 
     FIG. 1 schematically shows an embodiment of the system according to the invention, where there is included a surface vessel, a bottom installation and risers between the bottom and the surface, 
     FIG. 2 schematically shows an example of an arrangement of the bottom installation as seen in plan view, 
     FIG. 3 shows another example of a bottom installation with an increased number of wellhead sections and two manifolds, 
     FIG. 4 in vertical section more in detail shows an exemplary embodiment of an operation module installed at a wellhead, 
     FIG. 5 shows another example of an operation module installed at a wellhead, 
     FIG. 6 shows another embodiment of the system based on combined risers and a swivel provided on the vessel, 
     FIG. 7 shows a similar arrangement as the one in FIG. 6, but without a swivel, 
     FIG. 7A shows an enlarged view of the produce riser, umbilical, and drilling riser of FIG. 7, 
     FIG. 8 shows a further embodiment of the system, whereby the product riser and the drill string are separately connected at two positions on the bottom installation, and 
     FIG. 8A shows an enlarged view of an example of a bottom installation, 
     FIGS. 9A-9E illustrate a typical development of operation sequences with drilling, completing and production of four wells on a common bottom installation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG.  1  and several of the other figures of drawings the seabed is denoted  1  and the sea surface  2 . A vessel  3  of a specific design has drilling equipment  8  located more or less midship and process equipment  9  for produced oil or gas, located preferably abaft of the drilling equipment  8 . Vessel  3  is equipped with means for positioning and orientation, dynamic positioning being advantageous in this context. 
     There are shown risers for connecting vessel  3  to a bottom installation being generally denoted  5 , at the seabed  1 . More definitely there is shown a drilling riser or marine riser  22  with a drill string  21 , and a product riser  6 , which in this example is arranged in a bight or curve  16  over a guide element  16 A suspended from vessel  3  and with a possibility of heave compensation as indicated at  16 B. 
     The bottom installation  5  is based on a template  10  which has a foundation in the seabed  1  and among other things carries a manifold unit  11  that can have a design being known per se. Besides the template  10  has a number of wellhead sections or positions as shown more clearly in FIG.  2 . In FIG. 1 there is purely schematically illustrated a Christmas tree  14 A being located at one of the wellhead sections ( 13 A in FIG.  2 ). The foundation of template  10  is generally indicated at  10 A, and can be made in a conventional way. 
     At one of the wellhead sections ( 13  in FIG. 2) of the template, there is installed a particular operation module  20  that constitutes an essential component in several of the embodiments of the invention. As main parts of module  20  there are included a swivel unit  30  and a blowout preventer  25 . In this case valve  25  is mounted on top of swivel  30 . 
     For the purpose of drilling operations, vessel  3  is maintained in position above bottom installation  5  by means of dynamic positioning or other means, so that drill string  21  with associated riser  22  can extend as near vertical as possible down from drilling equipment  8  to the blowout preventer  25 . As illustrated in FIG. 1 drilling equipment  8  can comprise a usual derrick, drive works and so forth as well as a moon pool as known per se in drilling vessels and platforms. 
     Even though vessel  3  is maintained in such position that the drill string  21  will have the desired vertical direction, wind and weather will result in turning of vessel  3  so that it preferably at all times has the bow facing the wind or waves influencing the vessel. So as to attend to such turning movements in the system, the swivel unit  30  mentioned above, is provided together with blowout preventer  25  in the operation module  20  at the bottom installation  5 . As will be apparent in more detail from FIG. 4, to be discussed below, the product riser at its connection to operation module  20  thus will be able to take part in the turning movements of the vessel without involving any risk of undue mechanical stresses or torques in the structure of bottom installation  5 . Swivel unit  30  serves to make possible the required relative rotational movements, and therefore advantageously has a vertical axis coinciding in the principle with the axis of the drill string. 
     When considering now FIG. 4 the two main parts of operation module  20  are found, namely the blowout preventer  25  (partially shown) and the actual swivel unit  30 , the outer members of which are adapted to rotate about the vertical axis mentioned above, so as to follow the turning movements of the vessel at the sea surface. Product riser  6  together with an umbilical  6 A extends laterally out from swivel  30  and further at an inclination up through the water. In order to keep riser  6  and umbilical  6 A somewhat together and under control so that they do not move into conflict with other components and equipment at the bottom installation, there is provided a guide yoke  39 . The actual swivel can be considered to be subdivided into a product swivel part  30 A and a control swivel part  30 B, whereby the umbilical  6 A is connected to the latter swivel part. As known per se, there can here be the question of transfer of electric power and signals as well as hydraulic pressure. 
     FIG. 4 also shows components belonging to the actual well or wellhead, namely tubes (e.g. of dimension  18 {fraction (3/4+L ″)}) that constitute the wellhead 7 itself, whereas an inner casing hanger is shown at 7A. With a wellhead coupling 30C the swivel 30 is installed on wellhead 7. Thus coupling 30A belongs to the radially inner and stationary part of swivel 30.    
     Whereas the rotary movement in swivel  30  in an arrangement as shown in FIG. 1, can take place exclusively under the influence of vessel  3 , i.e. through product riser  6  being extended up for connection relatively far abaft on vessel  3 , it is preferred in many cases to provide for a motor-driven rotary movement in swivel  30 . For this purpose there is provided a crown wheel  36  that is stationary together with the inner swivel parts as mentioned, and a rotation motor having a gear  35  for cooperation with crown wheel  36  and thereby for rotating the outer swivel parts when motor/gear  36  is driven in rotation. This will take place by means of electric or hydraulic power being supplied through umbilical  6 A. The angular position to which the swivel is turned, will be controlled from the vessel  3  (FIG. 1) preferably under control by the dynamic positioning means mentioned above, that maintains the vessel at the desired position and orientation. 
     Drill string  21  with its riser  22  has not been indicated in FIG. 4, but cooperate with blowout preventer  25  in a conventional manner. Valve  25  is mounted on an upper extension of wellhead  7 , so that rotationally it is stationary on the bottom installation. The drill string riser being rigidly connected to the top of blowout preventer  25  (see FIG. 1) thus in a usual way has a rotary coupling to vessel  3  at its upper end. A vertical bore  33  for the drill string through blowout preventer  25  continues vertically and centrally through swivel unit  30  and further down into the well. 
     As stationary components associated with the operation module  20  in FIG. 4, there is also shown a laterally projecting coupling  31  intended for establishing connection to other equipment units on bottom installation  5 , in particular a manifold  11  as shown both in FIG.  1  and in FIG.  2 . Coupling  31  is provided for a production pipe  32 A for transferring the hydrocarbon product concerned, from the bottom installation to the vessel through swivel  30  and product riser  6 . Besides there is shown a conduit or passage  32 B for electric control/monitoring signals and hydraulics, that via swivel  30  continues up to the vessel through umbilical GA. In the embodiment illustrated in FIGS. 1 and 4, where swivel unit  30  is located under blowout preventer  25 , a quick disconnection may be required in an emergency situation. For this purpose FIG. 4 shows valves at  34  and  34 A for the product riser  6  and the umbilical  6 A, respectively. Correspondingly there may in a way known per se be possible to provide for quick disconnection of the upper part of blowout preventer  25  (not shown). 
     In the embodiment as described thus far, it has decisive significance that swivel unit  30  is incorporated in an independent module (operation module  20 ) that can have an integrated design and can be used over and over again in a standardized production system for subsea production. Such a swivel or operation module should be adapted to be installed and retrieved by means of normal subsea installation methods. Moreover it must be possible to lock the swivel into existing equipment profiles, e.g. the conventional locking profiles on a wellhead  7 , as shown in FIG. 4, and in addition this must be able to be connected to existing pipe line and connector profiles, so that the unit or module can be utilized in a flexible manner in view of various connections on a bottom installation  5 , among other things taking into account successive steps in the form of drilling, completing and production thereof. 
     In connection with the explanation immediately above, it is also important that production coupling  31  on the operation module swivel  30  can be connected to and locked to a cooperating and standardized coupling profile on the manifold ( 11  in FIG.  1  and  2 ). With the above described installation and locking of swivel  30  to wellhead  7  by means of the wellhead connector or coupling  30 C, there is obtained a well-defined landing and locked position so that making the coupling at  31  against manifold  11  will be simple. 
     The solution described here means that swivel unit  30  and operation module  20  as a whole are incorporated in the pressurized part of the wellhead and through which drilling takes place. As known per se this means that the structure at all points that are subjected to such stresses, has the required mechanical strength. 
     As an alternative to what has been described above with reference to FIGS. 1 and 4, the operation module with the swivel can be installed and locked to the actual template structure or the conductor (e.g. 30″). Such a solution would have as a consequence that the swivel module will not be exposed towards the drilling system or the blowout preventer, but will be mounted outside this system. 
     From what has been described above it will be clearly apparent to experts in this field that operation module  20  when installed at a bottom installation occupies a section or position that is traditionally intended for a Christmas tree. In this connection it is important, as also discussed above, that the coupling profile or interface of the swivel/operation module against the manifold is identical to what is present at the Christmas tree concerned, that will be installed for the actual production, namely when mounted upon termination of drilling of the well concerned. Thus the solution described does not involve introduction of new coupling points or interface points, beyond that which is already standard for a subsea production system. 
     FIG. 2 shows an example of an arrangement of the main components at a bottom installation  5  as already mentioned above. Manifold  11  constitutes a central component and at both longitudinal sides thereof there are shown wellhead sections  13 ,  13 A,  13 B and  13 C. At the latter two positions or sections there are installed Christmas trees  14 B and  14 C. Wellhead sections  13  is here utilized for the operation module  20  mentioned above, that is connected to surface vessel  3  through product riser  6  and umbilical  6 A. 
     In the arrangement of FIG. 3 the capacity has been expanded in relation to the arrangement of FIG. 2, since bottom installation  5  has been supplemented with a more or less adjacent bottom installation  15 . Between these there are shown pipeline and umbilical connections at  15 C, with associated coupling modules  15 A and  15 B being each connected to one of the bottom installations  15  and  5 . 
     It is obvious that the invention can be employed also in other arrangements or configurations of bottom installations, than the two being illustrated as examples in FIGS. 2 and 3. 
     In FIG. 5 there is shown an alternative design to the one in FIG. 4, i.e. with an operation module  50  having a swivel unit  40  located above the blowout preventer  47 . From drilling riser  42  a through-going axial hole  43  for the drill string continues down through swivel unit  40  and blowout preventer  47 . The swivel itself is divided into a product swivel part  40 A and a control swivel part  40 B corresponding to the embodiment of FIG.  4 . Moreover in FIG. 5 there is shown a coupling  41  for connecting operation module  50  to a manifold at the subsea installation concerned, and corresponding connections  42 A and  42 B leading to the stationary inner parts of the actual swivel  40 . The outer, rotatable parts of swivel unit  40  are provided with a guide yoke  49  for the product riser  44  and the umbilical  44 B. Finally there is indicated a rotation motor  45  with an associated gear for the rotary movement of swivel  40 . With the embodiment of FIG. 5 the riser attachments are removed from the actual subsea structure. The connections  42 A and  42 B from coupling  41  are extended upwards along and are attached to blowout preventer  47 . In an emergency situation the embodiment of FIG. 5 will be based upon quick disconnection from the upper part of blowout preventer  47  at the same time as connections  42 A and  42 B are disconnected in the same region. For this purpose there is shown a quick disconnect connector  46  for the upper part of blowout preventer  47 . If a situation as mentioned above arises, that results in deviation of the vessel from its position during drilling, the valves in blowout preventer  47  will be activated and then disconnection from blowout preventer  47  takes place by means of connector  46 . As an illustration at this point it can be mentioned that blowout preventer  47  typically is a module having a height of about 20 meters and a weight of the order of magnitude 200 tons. 
     In the embodiment of the system according to the invention as shown in FIG. 6, the required swivel function is located at the surface vessel  63 , as shown at  60 , i.e. at the upper end of product riser  66 . Moreover FIG. 6 like the preceding figures of drawings, shows a template  10  with a foundation  10 A at the seabed  1 , a manifold  11  and a Christmas tree  14 A as well as a blowout preventer  67  and drilling riser  62  with drill string  61 . Such a design gives good access to swivel unit  60 . Product riser  66  and an umbilical (not shown) are attached to the outside of drilling riser  62 . In this case blowout preventer  67  will also comprise coupling means (not shown) for establishing connection to manifold  11 . 
     As a further simplification in relation to the embodiments described above, FIG. 7 shows an example of a system where there is not employed any swivel unit. Product riser  76 A in this embodiment together with umbilical  76 B (see enlarged view in FIG. 7A) are mounted to the drilling riser  72 . It is possible that product riser  76 A and umbilical  76 B can extend at a low helical angle about drilling riser  72  over the length thereof, between blowout preventer  77  and vessel  73 . Thus there is here provided for a fixed connection of product riser  76 A at the subsea installation, which is possible when a flexible design of product riser  76 A is employed, so that this riser can endure some twisting. Typically there may here be the question of one degree per meter length of the riser. Accordingly vessel  73  is permitted to rotate within a given sectorial angle that is also depending on the water depth at the location. Larger water depth will make possible a larger turning movement of vessel  73  during operation. Depending on the circumstances it will be necessary in this embodiment to have a relatively high or increased thruster capacity in vessel  73 , in order to make it possible to keep this within the permitted angular sector during the limited time required for the drilling of a well. 
     In the embodiment without a swivel as illustrated in FIG. 7, it will be expedient to connect product riser  76 A and umbilical  76 B into a dedicated section of the subsea installation. In an emergency disconnect situation, product riser  76 A and umbilical  76 B will be disconnected by means of connectors (not shown) at the subsea installation, preferably in association with blowout preventer  77 . After such emergency disconnection, product riser  76 A and umbilical  76 B will normally be removed or retrieved form the bottom installation. 
     Concerning the specific embodiment in FIG. 7 it is to be noted finally that in areas and under operations with difficult weather conditions, embodiments with a swivel can be the only solution, whereas in other areas having more stable weather, it may be possible to carry out the operations without any swivel incorporated in the system. 
     Immediately above there has been described a variant of the system that in similarity to the embodiment of FIG. 7, is based on a concept that does not employ a swivel. The difference between these two system designs is seen in how the product riser and the control or umbilical cable are extended from the seabed to the vessel. FIG. 7 describes a system where the product riser and the umbilical are attached to the drilling riser and are extended up through the moon pool of the vessel. In the alternative system design the product riser and the umbilical are arranged independently of the drilling riser and are taken in abaft on the vessel. 
     The latter system variant is illustrated in FIG. 8, which in many ways can be compared to the arrangement of FIG.  1 . Thus the surface vessel  83  in FIG. 8 has its drilling equipment  8  connected to blowout preventer  87  at a bottom installation  85  through a drilling riser  82  with a drill string  81 . Also here a product riser  86 , possibly together with an umbilical (not shown) are extended upwards from bottom installation  85  over a heave-compensated guide  86 A to the abaft end of vessel  83 . In this embodiment however, product riser  86  enters the bottom installation at a distance from blowout preventer  87 , i.e. at a specific connecting position or section  90  with a connector device  89 . The arrangement of such a specific bottom installation is schematically shown in FIG. 8A, where otherwise there is incorporated a manifold  11 , three Christmas trees  14 A,  14 B and  14 C as well as the above blowout preventer  87  which occupies one of the four wellhead sections comprised by this bottom installation, in this respect corresponding closely to the arrangement of FIG.  2 . Also as regards many other main features the particular system of FIG. 8 corresponds to aspects of the embodiments described above. 
     The important embodiments comprising a swivel unit, in actual practice for combined production and drilling, typically will comprise the following functions and dimensions: 
     one or two production bores of typical diameter 6″, 
     control functions with 8-16 channels for electrical signals and power as well as hydraulic pressure, 
     the swivel/operation module can be installed and retrieved by means of a subsea tool, 
     a through-going, central hole of approximately 20″ for drilling, 
     production coupling according to common standard to the subsea manifold, 
     arrangement of couplings, conduits or connections according to common standard for connecting control functions to Christmas trees at the bottom installation, 
     locking down the swivel/operation module by means of locking profiles according to common standard. 
     Necessary and sufficient operation steps incorporated in a method for producing oil or gas with a system as described above, are defined in claim  25 . The method is to be explained in the following description in the form of a practical example with reference to FIGS. 9A-9E. This example is associated with the arrangement of FIG.  2  and preferably a swivel/operation module according to FIG. 4 or FIG.  5 . 
     As initial operations the template and the manifold  11  with wing elements are installed in the usual way, and then a first well is drilled and completed in a traditional manner, namely at wellhead section  13 A as illustrated in FIG.  9 A. There is also shown an installed Christmas tree  14 A. Thus in the situation of FIG. 9A a well is completed and is ready for production. 
     Then drilling of well No.  2  is started and in that connection there are installed in turn in the usual way, a conductor tube (B  30 ″) and a casing/wellhead ( 20″/18 {fraction (3/4+L )}″). The drilling operation takes place in the usual way in an open hole without a BOP and riser in the first instance, and in this phase the resulting drill cuttings from the bore hole will be discharged directly to the surrounding seabed. When the wellhead has been installed in this well No. 2, the swivel/operation module 20 with associated product riser and umbilical, will be installed on the wellhead or possibly on top of the conductor tube structure. Besides, it is possible to install the blowout preventer with its drilling riser so that the drilling operation can continue according to normal practice. Thereupon the first well is connected to module 20 and thereby is connected to the surface system, so that production can start from the first well in parallel to the drilling of well No. 2. This is shown schematically in FIG.    9B . 
     Thus, in this situation well No.  1  is connected through swivel module  20 , the product riser and the umbilical to the process equipment at the surface vessel. The well flow or product is now controlled from well No.  1  through its Christmas tree  14 A and further through the central manifold  11  and from there to swivel module  20 . As explained above it is important in this context that swivel/operation module  20  has the same standard interface or coupling arrangement as a production Christmas tree being preferably of common standard. 
     Upon termination of the drilling of well No.  2  and completion thereof, the blowout preventer and the swivel module  20  are retracted and drilling of well No.  3  is started in the usual way corresponding to the initial drilling of well No.  2 , as discussed above. Also conductor and casing tubes are in turn set and subsequently the blowout preventer with the integrated swivel module  20  is installed on well No.  3 . FIG. 9C shows this situation, where production from and control of wells  1  and  2  takes place through swivel module  20  at the same time as the main part of the drilling of well No.  3  goes on. At this point it is to be noted that in the relatively short period while the drilling of a new well is initiated and up to the installation of the associated casing/wellhead, it can be difficult to obtain a simultaneous and uninterrupted production from the previously completed wells, i.e. the two first wells with their associated Christmas trees  14 A and  14 B, as illustrated in FIG.  9 C. 
     FIG. 9D shows the drilling of well No.  4  at the same time as wells. Nos.  1 ,  2  and  3  deliver their product or well flows through swivel/operation module  20  which is mounted on well No.  4 . 
     Finally FIG. 9E shows a situation where the drilling of all the wells at the bottom installation concerned, are terminated. The four wells now deliver their products to a surface installation through manifold  11 . When such a production phase has been established upon finishing of the drilling, it will accordingly not be required to use the special surface vessel which has both drilling equipment and process modules, as was described and illustrated previously. It is possible however, that when the desired number of wells have been drilled, completed and put into production, the operation module  20  can still be incorporated in the system located on the last completed of the wells. This, however, as a rule is not what is preferred in actual practice. 
     Upon final completion of all wells, the production will normally be directed through pipelines to a platform or a surface installation being located somewhat away from the subsea installation. An alternative solution in this connection is to move the swivel module over to a dedicated section of the bottom installation and to produce directly to a floating installation located in the neighbourhood. In many cases this will lead to significant savings in the form of lower pipeline cost. 
     As shown in FIG. 3 several templates can be tied together in a common bottom installation, and in such case the production from templates being completed and ready, can be delivered through a swivel/operation module located on the template and the well or wellhead section in which drilling is currently taking place. 
     In the embodiments described and illustrated, a template of known form from actual practice has been taken as a starting point, but in this respect the invention is independent of the particular arrangement of template and configuration as well as the number of wellhead sections therein. A detail of interest in this connection is that guide posts being possibly employed at the bottom installation or the template in connection with installation thereof or components thereon, can be designed to be disassembled so that they can be removed in order not to be of any hindrance to the rotary movements of the swivel unit and the associated risers.