Patent Publication Number: US-9404347-B1

Title: Apparatus and method for connecting a riser from an offshore rig to a subsea structure

Description:
BACKGROUND 
     1. Field of the Disclosure 
     This disclosure relates to a subsea wellbore system and in particular to a system and methods for attaching a riser to a subsea wellhead equipment. 
     2. Background of the Art 
     Subsea wells (also referred to as wellbores or boreholes) for the production of oil and gas are drilled into subsea formations from an offshore rig (such as a vessel). Often, the water depth exceeds 5000 feet. A riser, a hollow large diameter (e.g., 12-20 inches) flexible longitudinal tubular member, is connected between drilling equipment on the offshore rig and the wellhead equipment installed on the sea floor above the wellbore. During installation, the riser is filled with the sea water. To drill the wellbore, a drill string including a drill pipe attached to a bottomhole assembly having a drill bit at end thereof is conveyed into the wellbore through the riser. During installation of the riser, if the underwater eddy currents are strong enough, they can deflect the riser from the vertical between the offshore rig and the wellhead, which can make it difficult or unfeasible to connect the bottom end of the riser to the wellhead equipment. It is known that in the Gulf of Mexico, relatively strong sustained loop eddy currents exist, often between 2000-4000 feet depth. Such currents have at times severely disrupted oil and gas drilling activities, one of the reasons being the inability to connect the riser to the wellhead equipment. Therefore, there is a need to provide apparatus and methods for connecting a riser from an offshore rig to wellhead equipment when the riser is deflected from a vertical under the water. 
     The disclosure herein provides apparatus and methods for connecting a riser from an offshore rig to a wellhead equipment when the riser is deflected from the vertical. 
     SUMMARY 
     In one aspect, a method of connecting a riser from an offshore rig to a wellhead equipment placed on a sea bed is disclosed, wherein the wellhead equipment includes a first connector thereon. The method, in one non-limiting embodiment, includes lowering a riser of sufficient length having a second connector at a lower end thereof that is configured to connect to the first connector of the wellhead equipment, wherein the riser is in fluid communication with and is filled with the sea water (first fluid); closing the riser proximate to the lower end of the riser after lowering the riser; displacing the first fluid in the riser with a second fluid that is heavier than the first fluid to straighten the riser and the second connector; and connecting the second connector at the lower end of the riser to the first connector of the wellhead equipment. 
     In another aspect, an apparatus for connecting a riser from an offshore rig to a wellhead equipment is disclosed that includes a first connector thereon. The apparatus, in one non-limiting embodiment, includes a riser of sufficient length having a second connector at a lower end thereof configured to connect to the first connector of the wellhead equipment, wherein the riser is lowered from the offshore rig toward the connector of the wellhead equipment and is filled with the sea water; a plug closing the riser at a selected location in the riser; and a tubular inside the riser for supplying a fluid heavier than the sea water to displace the sea water in the riser with the heavier fluid. 
     Examples of the more important features of the apparatus and methods disclosed herein are summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features that will be described hereinafter and which will form the subject of the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed understanding of the apparatus and methods disclosed herein, reference should be made to the accompanying drawings and the detailed description thereof, wherein like elements are generally represented by same numerals and wherein: 
         FIG. 1  shows an exemplary offshore oil well system that includes an offshore rig, wellhead equipment and a riser between the offshore rig and the wellhead equipment, wherein the riser has been deflected from the vertical to an extent that makes it impractical to connect the riser to the wellhead equipment; and 
         FIG. 2  shows the offshore oil well system of  FIG. 1 , wherein a tubular has been conveyed from the offshore rig into the riser, the riser has been plugged proximate to the bottom of the riser and a heavier fluid is replacing at least some of the sea water in the riser with a heavier fluid to straighten the riser to aid in connecting the riser to the well bore equipment. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an exemplary offshore well system  100  for performing wellbore operations, including, but not limited to drilling and completion operations.  FIG. 1  shows an exemplary offshore rig, such as a ship or vessel  110  or another maneuverable structure, carrying a rig  112  for performing various wellbore operations, stationed at the sea surface  103 . Wellhead equipment  120 , such as a x-mas tree including a blow-out preventor  122 , is shown placed above a wellbore  101  formed in a subsea formation  102 . Cemented casings  106  and  108  with cement  109  are shown outside of the wellbore  101  to stabilize the earth formation below the sea floor  104  to selected depths. The wellhead equipment  120  includes a connector  125  to which a riser  130  may be connected as described below. The riser  130  includes at its bottom end  132  a connector  135  configured to mate with the connector  125  of the wellhead equipment  120 . The riser  130  is typically a flexible hollow longitudinal member of relatively large diameter (typically between 12-20 inches). The riser  130  is open at the end  136  of the connector  135  and is thus in fluid communication and filled with the sea water  105 . To perform a wellbore operation, the riser  130  with the connector  135  attached to its bottom end is lowered from the offshore rig  110  toward the connector  125  of the wellhead equipment  120 . Since the riser is in fluid communication with the sea water  105 , it is filled with the sea water during its deployment. When strong underwater currents  140  are present, they tend to bend or deflect the riser to an angle “A” such as shown at location  134  of the riser  130 . If the deflection angle A is relatively large, it deflects the face  136  of the connector  135  to a relatively large angle that makes it difficult and in extreme cases not feasible for an operator to straighten the connector  135  to latch it onto the connector  125  of the wellhead equipment  120 . An exemplary, non-limiting apparatus and method for connecting the deflected riser connector  135  to the stationary wellhead equipment connector  125  is described in reference to  FIG. 2  below. 
       FIG. 2  shows the offshore well system  100  of  FIG. 1  in the process of straightening the riser  130  and thus the connector  135  for attachment with the connector  125  of the wellhead equipment  120 . To straighten the riser  130  from the deflected position at location  134  in  FIG. 1 , a plug  140  is placed at a suitable or selected location in the riser  130 , which may be proximate to the bottom end  132  of the riser  130 . The plug  140  blocks or prevents fluid communication between the sea water  105  and the riser  130 . The riser  130 , however, remains filled with the sea water  105   a . Any suitable plug known in the art may be utilized, including, but not limited to a packer and an inflatable bridge plug. An open ended tubular  150 , such as a drill pipe, is conveyed from the rig  110  into the riser  130  to a suitable depth above the plug  140 . A fluid  160  heavier than the sea water is supplied into the tubular  150  from the rig  110  by a supply unit  168 , such as a pump unit, to cause a desired amount of the sea water  105   a  previously present in the tubular  150  and the space  155  between the tubular  150  and the riser  130  (also referred to as the annulus between the tubular  150  and the riser  130 ) to move to the surface as shown by arrows  165 . As the heavier fluid  160  displaces the sea water  105   a , the riser  130  starts to straighten. The extent to which and the speed at which the riser will straighten will depend upon the weight of the fluid  160  relative to the sea water weight and the supply rate of the fluid  160  into the riser  130 . Thus, in aspects, the type and weight of the fluid may be selected based on the deflection of the riser  130  (computed or measured) from the vertical. Sensors  180  may be placed along the riser  130  to provide measurements for determining the deflection along the riser  130 . The sensor data may be transmitted to a controller or processor  190  on the rig  110  via conductors  181  placed along the riser  130  or wirelessly. Any other method of monitoring the deflection of the riser  120  may also be utilized, including, but not limited to using surface measurements and using remotely-operated vehicles. The deflection along the riser  130  may be continuously monitored before, during and after the placement of the tubular  150  in the riser  130 . The change in deflection may be monitored in-situ as the heavier fluid  160  is supplied into the tubular  150  to control the supply rate of the fluid  160 . The extent of the deflection may be used to determine the type and density of the displacement fluid  160  and the supply rate. Once the connector  125  has been straightened to a desired extent, such as shown in  FIG. 2 , the vessel  110  may be maneuvered in any of the directions  192  while still monitoring the deflection to position the connector  135  above the connector  125 . The vessel speed may be adjusted to limit the combined deflective force on the riser  130  below a selected limit for safe operations. Latching of the connector  135  to connector  125  may be accomplished by an operator sent underwater in an enclosed vehicle (not shown) and by maneuvering mechanical or robotic devices. Alternatively, attachment may be accomplished remotely from the surface by an operator and/or by the controller  190 . Any other method of latching known in the art may be used for connecting the connectors  135  to connector  125 . After connecting the riser to the subsea equipment, the tubular  150  and the plug  140  are removed from the riser. 
     The foregoing disclosure is directed to certain exemplary embodiments and methods. Various modifications will be apparent to those skilled in the art. It is intended that all such modifications within the scope of the appended claims be embraced by the foregoing disclosure. The words “comprising” and “comprises” as used in the claims are to be interpreted to mean “including but not limited to”. Also, the abstract is not to be used to limit the scope of the claims.