WO2002088516A1

WO2002088516A1 - Subsea drilling riser disconnect system and method

Info

Publication number: WO2002088516A1
Application number: PCT/EP2002/004850
Authority: WO
Inventors: Peter Eric Azancot
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2001-04-30
Filing date: 2002-04-26
Publication date: 2002-11-07
Also published as: GB2391889A; GB0323869D0; MY141181A

Abstract

A system for drilling a subsea well, comprising an offshore platform positioned above the well, and a conduit providing fluid communication between the offshore platform and the well. The conduit comprises a lower portion including a subsea blowout preventor (26) and an upper portion including a second blowout preventor (15) arranged at the platform (10) and a drilling riser disconnectable from said lower portion of the conduit. A drill string extends from the offshore platform (10) through the conduit into the well. A subsea disconnect system is provided for selectively disconnecting the drilling riser (18) from said lower portion of the conduit.

Description

SUBSEA DRILLING RISER DISCONNECT SYSTEM AND METHOD

The present invention relates to a method and system for drilling a well into an earth formation below a marine environment .

Exploration and production of hydrocarbons from subsea reservoirs is an expensive and time-consuming process. The drilling and production processes often require allocation of expensive assets, such as drilling and production platforms located offshore. There are a number of problems associated with offshore drilling and production not found in land operations.

Primary among these is the marine environment. Unlike the surface environment, much of the offshore drilling control equipment is located on the seabed and not subject to direct control and monitoring - one simply cannot see the equipment without the use of vision equipped ROVs .

The mechanics of drilling in a marine environment also differ from land operations. Drilling operations utilize a weighted drilling fluid, known as "mud" which is pumped down the drill string and circulated back to the surface through an annulus between the drill string and the borehole wall. The drilling mud cools the drill bit as it rotates and cuts into the earth formation. It also provides a medium for returning drilling cuttings created by the drill it to the earth's surface via the annulus. The weight of the drilling mud in the annulus further operates to control pressure in the borehole and help prevent blowouts. Lastly, additives in the mud are designed to form a cake on the inside walls of the borehole to provide borehole stability and to prevent formation fluids from entering the borehole prior to desired production. It will be appreciated that during land operations, the drilling mud and cuttings may be readily returned to the surface via the borehole annulus. Such is not the case in offshore operations.

Offshore operations require location of a drilling platform in waters located generally above the reservoir of interest. The depth of the water may range from several hundred feet to well in excess of a mile. A drill string must travel from the surface of the platform, down to equipment located on the seabed and then into the bore of interest prior to actually beginning cutting operations. Unlike land operations, there is no annulus between the floor of the seabed and the drilling platform at the surface. Accordingly, a drilling "riser" comprised of generally cylindrical elements is provided for from a wellhead located at the seabed to the surface drilling platform above the water level. The riser operates to protect the drilling string during operations and acts as an artificial annulus.

The risers are formed from large (on the order of 21 inches) diameter metal tubular goods linked together. Buoyancy elements, often manufactured from syntactic foam or metal, may be affixed the external surface of the drilling riser along its length to provide essentially neutral buoyancy. The syntactic foam buoyancy elements are typically 6-12 feet in length. The specific foam chemistry and diameter of the float are selected in accordance with the specific environmental conditions to be encountered in operations. Riser joints may be as long as 75 feet or more in length and multiple buoyancy elements may be affixed to a single riser joint. The buoyancy elements are generally manufactured onshore and shipped, together with the riser joints, to the drilling platform prior to use. The buoyancy elements are usually installed on the riser prior to riser installation. The foam floats may be affixed about the riser elements any number of ways as will be discussed with reference to the preferred embodiments of the invention.

As with land drilling operations, subsea drilling operations must provide for means for shutting the well down in the occurrence of a well event that cannot be controlled utilizing drilling mud. A series of blowout preventors (BOPs) are used to control well flow in such instances. The BOP stack typically consists of multiple BOPs connected to each other and the wellhead and may include shear ram or annular BOPs. In land operations, the BOP stack is typically just below the rotary table and may be easily monitored and operated in response to a significant well event. Such is not the case, however, in subsea drilling operations. The BOP stack is often located on the seafloor and requires various umbilical and control lines to monitor conditions and operate the BOP stack. It will be appreciated that the umbilicals and control lines must likewise traverse the distance between the offshore platform and the subsea wellhead. The riser, umbilicals, control lines and other subsea elements, including buoyancy elements, are subjected to ocean currents along their respective lengths, causing lateral deflection in the riser from the seabed to the surface platform. A riser and control lines may be subjected to varying and differential ocean currents along its length resulting in complex lateral deflection of the riser. This results in a number of problems. The continued deflection of the riser may result is stress points along its length and ultimately weaken the riser. Radical lateral deflection in the riser could result in excessive drill string contact with the inside riser wall resulting in further weakening of the riser. Metaocean conditions, such as winter storms, hurricanes or typhoons add yet another degree of complexity to offshore drilling operations. Drilling operations are typically suspended and the crew evacuated in such instances. In the case of fixed offshore platforms or compliant tower platforms, the riser is often left in place as it is supported by a conductor system that extends from near surface to near sea bed. Floating offshore platforms present different problems in that there are no conductors to support the riser system, which depends instead, on a combination of flotation cells and topside tensioners to support the riser system. Should a metaocean condition occur, the crew is similarly evacuated, leaving the riser system subject to current stresses, as well as wind and wave stresses placed on the floating platform. To prevent damage from occurring, the riser system is often disconnected from the sea floor BOP stack and tripped, together with the control lines to the platform surface. After the condition abates, the riser system, as well as the umbilicals and control lines must be reconnected to the sea floor BOP stack and a series of time-consuming safety tests run before drilling can resume.

It will be appreciated that the time required to disconnect and subsequently reconnect results in significant lost rig time, particularly in the case of offshore platforms. Reconnection typically includes running the riser and associated umbilicals down to the seafloor BOP stack. These are typically reconnected utilizing ROVs . This process can take several hours or days followed by days of testing. Accordingly, there exists a need for a more cost-effective means of disconnecting and reconnecting drilling riser systems. In accordance with the invention there is provided a system for drilling a well into an earth formation below a marine environment, comprising: an offshore platform positioned above the well; a conduit providing fluid communication between the offshore platform and the well, the conduit comprising a lower portion including a subsea blowout preventor and an upper portion including a second blowout preventor arranged at the platform and a drilling riser disconnectable from said lower portion of the conduit; - a drill string extending from the offshore platform through the conduit into the well; and a subsea disconnect system for selectively disconnecting the drilling riser from said lower portion of the conduit. It is thereby achieved that in case of an emergency situation, for example during adverse weather conditions, the offshore platform can be disconnected from the well and moved away from the well location. This is done by retrieving the drill string from the well and from the subsea blowout preventor (BOP) , closing the subsea blowout preventor, and disconnecting the drilling riser from the lower portion of the conduit by means of the subsea disconnect system.

Preferably the subsea BOP is a minimal BOP as it does not need to have the full functionality of a conventional subsea BOP. For example, in a preferred embodiment it is sufficient if the subsea BOP is a ram-type BOP such as a shear ram-type BOP, while the second BOP (at the offshore platform) is a surface BOP stack including an annular BOP. This embodiment is particularly attractive for wells of smaller diameter than conventional wells, as such slender wells require only a small diameter drilling riser which is capable of withstanding relatively high internal pressures. An additional advantage is that such small diameter riser is relatively light so that less stringent load bearing capabilities are imposed on the offshore platform. Suitably the system further comprises a subsea control system for monitoring and controlling the subsea blowout preventor, and a surface control system in communication with the subsea control system. The subsea control system is preferably arranged to also control the subsea disconnect- system.

Suitably the riser system traverses the distance from the seafloor to the offshore platform. The riser system is then connected to the surface BOP stack, which may include both ram and annular BOPs, which are supported by the platform utilizing known tensioning devices. The surface BOP stack may be readily monitored and controlled using a system that is located on the platform, as opposed to a subsurface BOP stack. The integrity of the surface BOP may be readily tested and, if required, repairs may be readily made to the surface BOP stack. With respect to the subsea ram BOP, it is operated and monitored utilizing a subsea control module located adjacent to the subsea BOP. The module is comprised of a power, processing and communications section. The power section includes stored pressurized air that may be used to pneumatically activate the BOP. The processing system may be comprised of a suitable programmable controller or a microprocessor in an environmentally suitable enclosure. Electrical power to the module is provided by means of batteries and is used to operate the circuitry for the module. A hydraulic system and pump may be used as a power source to operate the subsea BOP. The pressurized air or hydraulic fluid and pump are designed such that they may be regularly serviced or replaced using ROVs.

Where the subsea BOP is not operated by ROVs, it is in communications with the surface utilizing known acoustic transponder system that may be used to send command information to the subsea BOP or send information on the status of the subsea BOP to the surface system. In operation, the operator of the platform decides to disconnect the riser and sends a signal through the water utilizing the acoustic transponder system. The acoustic transponder system may be powered utilizing a battery system of other suitable self-contained means. The use of an acoustic transponder system and the self contained power system effectively eliminates the need for umbilicals for the subsea BOP, reducing cost and time associated with the running of the umbilical lines during an installation or disconnect. Alternatively, an ROV may be landed on the module to activate the necessary controls for the BOP and utilize the stored gas as a power source for operation.

The method of disconnecting the present system begins with the tripping of the drill pipe to remove the bit and drill collar and any associated tools or logging equipment from the subsea formation and the riser system. The operator transmits a set of coded commands from the platform through the water utilizing the acoustic transponder system. The acoustic signals are received by the acoustic transponder system located on or near the subsea BOP. The acoustic signal is converted to an electrical signal and transmitted to a control module , where it is interpreted and acted on by the processor or controller located in the module and the appropriate signals are sent to the BOP control system to close the rams, thereby sealing off the hole. The processor or controller monitors the status of the BOP operation and issues a control signal to the surface via the acoustics transponder system that the rams have been activated and the well is sealed off. In the event of a problem with the BOP activating, an ROV may be dispatched to complete operation of the BOP. Once the subsea BOP has been closed, the operator unlatches one portion of the disconnect system from the other. This may be accomplished from the surface or with ROV intervention. Once disconnected, the riser system and the platform are independent of the subsea BOP and may be relocated or tripped

The reconnection of the riser system begins with the repositioning, if required, of the offshore platform. In the prior paragraph, the drilling riser remained connected to the platform following disconnect. However, it will be appreciated that the drilling riser may be likewise tripped and disassembled following disconnect. In such instances the drilling riser is reconnected to the surface BOP stack and extended down to the subsea BOP in sections. Once it is in the general area of the subsea BOP, the riser connection is made up once again, either entirely from the surface or utilizing ROVs. An ROV it may also be used to check the status of the module with respect to power and communications. Once the riser connection is made up, the operator can begin to test to surface BOP systems for integrity. Upon completion of the surface BOP tests, the subsea BOP may be tested and upon satisfactory completion of such tests, the rams may be opened. The rams are reopened by issuing a transponder signal from the platform that is received by the subsea BOP acoustic transponder and transferred to the control module. The control module the BOP controls and the air pressure or hydraulic fluid may be used to open the BOP rams .

An alternative embodiment of the present invention calls for the place of the connector at some distance below the ocean surface. When the riser is disconnected following sealing of the subsea BOP, the riser length remains connected to the subsea BOP and only a portion is relocated or tripped with the platform. The riser section left subsea may be supported by flotation elements that are placed about the riser sections. The subsea floatation elements may also be used to reduce subsea vortex induced vibration. Upon reconnection, the end of the riser section may be located by means of acoustic transponder or ROV and reconnected with the riser system extended from the platform. Thus, the number of riser sections that need be run down to the connection is decreased and the time to reconnect reduced. Again, this embodiment does not require umbilicals for the operation of the subsea BOP.

A better understanding of the present invention may be gained by a reading of the following Detailed Description which is given by way of example, taken together with the accompanying figures in which: Figure 1 is a depiction of a prior art drilling system utilizing a subsea BOP stack;

Figure 2 is a depiction of a prior art subsea BOP stack; Figure 3 is a depiction of a drilling system utilizing a preferred embodiment of the present invention; and

Figure 4 is a depiction of an alternative embodiment of the present invention.

In the Figures, like reference numerals relate to like components.

Figure 1 is a simplified depiction of a prior art offshore platform drilling system. A floating platform 10 is positioned at the desired location and maintains the position by way of mooring lines (not shown) and/or dynamic positioning systems (not shown) . In this example, the floating platform is a semi-submersible rig. The simplified drawing depicts a drilling rig 12, which typically includes a number of systems, including rotary table, kelley, pipe handling equipment and other equipment not shown. Unlike land-based operations, offshore operations do not have a natural annulus reaching from the wellhead back to the surface. Accordingly, an artificial annulus is created through the use of a drilling riser 18, which extends from the surface down to the wellhead 24 located on the seafloor 22. The riser is typically located below the rotary table and is suspended from the platform 10, utilizing a tensioning system 16. A spider and gimbal system is installed to permit the riser system 18 to flex in response to wind, wave and current conditions acting on not only the riser 18 but the platform as well. The riser 18 is itself comprised of multiple joints connected together at the surface using suitable means, such as flanges and bolts (not shown) . The riser 18 system further might include telescoping joints (not shown) to further permit the riser 18 system to flex in response to environmental conditions. The riser 18 further includes umbilical or control lines 20A and 20B that are typically secured to the riser 18 and made up as the riser is made up and lowered into the ocean. The function of the control lines 20A and 20B is to provide control signals and power for operation of the subsea BOP stack 26, as will be discussed in further detail below. The riser 18 sections and control lines 20A and 20B are made up until the riser 18 reaches the wellhead 24. The riser 18 is not connected to the wellhead 24 itself but through a subsea blowout preventor (BOP) stack 26 that is sealingly connected to top of the wellhead 24. The wellhead 24 further includes a casing hanger (not shown) for the purposes of suspending casing in the borehole to provide structural support during drilling operations. The topside of subsea BOP stack 26 terminates in a connector 32 that mates with the riser 18 system flex joint connector 34. The control lines 20A and 20B are generally connected via flow lines or manifolds to the subsea BOP power system for operations.

In operation, the drill string, including bit, drill collar and any attendant equipment, such as bent subs, drilling motors, pulsers and Logging While Drilling (LWD) is lowered through the spider and gimbal system 14, through the riser 18 system, through the subsea BOP 25 and into the wellhead 24 to perform drilling operations. Drilling mud is circulated down the interior of the drill string (not shown) and back up the annulus formed by the borehole and the drill string, the subsea BOP stack 26, through the riser 18 and back into the mud pits (not shown) on the platform 10 for analysis and cleaning.

Figure 2 is a detailed view of a typical subsea BOP stack 26. In fact, the subsea BOP stack 26 is typically made up of multiple BOPs. The subsea BOP stack 26 stack is mated to the wellhead 24 by bolting or other mechanical means and is generally installed prior to the riser system 18 being lowered. The subsea BOP stack 26 depicted in Figure 2 is representative of a guidelineless BOP stack manufactured and sold by the Cameron division of Cooper Industries, Inc. It is comprised of multiple ram-type BOPs 28 that are in series with each other. Fig. 2 depicts four ram BOPs 28 mounted in series in the wellhead 24. The ram BOPs 28 are themselves connected to an annular BOP 30, such that the drill string may be lowered through the annular 30 and ram 28 BOPs. The connector 32 atop the subsea BOP stack 26 is depicted in partial cut-away, with the riser 18 and flex joint 34 in which it terminates. The control lines 20A and 20B may be connected by means of a manifold, or, with ROV assistance, flowlines to one or more control pods 36. The control pods 36 may include programmable controllers or processors to process commands issued by the operator and sent down the control lines 20A and 20B, monitor the BOP stack 26 and the individual BOPs therein 28, 30. The control pods 36 may derive their power through the control lines and/or may further include a separate power, e.g., battery back up. In the illustrated system, the control pods 36, may themselves be replaced or serviced by ROV. The control pods 36 supply electrical, hydraulics, and/or pneumatic power to the individual BOPs 28 by means of control lines 38 within the subsea BOP stack 26. The entire subsea BOP stack 26 is generally supported by a framework 40 that not only lends structural integrity to the system but also operates to protect the components during installation and servicing. The process of disconnecting the riser 18 system from the wellhead 24 begins with the withdrawal of the drill string. Commands are issued by the operator and transferred to the control pods 36 by means of control lines 20A and 20B to close the ram 28 and annular 30 BOPs. The appropriate power is applied to the BOPs through control lines 38 to close off the hole. The riser 18 system is then unlatched by disconnecting the flex joint from the top of the subsea BOP stack 26. The riser 18 system and the control lines 20A and 20B are then relocated or tripped and disassembled.

The reconnection of the riser 18 to the subsea BOP stack 26 begins with the repositioning of the riser 18 system or making it up and lowering to the subsea BOP stack 26 from the platform 10. The connectors 34 on the flex joint and 32 atop the subsea BOP stack 26 are mated with the assistance of ROVs. The control lines 20A and 20B must then be connected to the appropriate manifold lines on the BOP stack 26 to come into communication with the control pods 36 and/or control lines 38. While seemingly straightforward, the positioning of the riser 18 system and the reconnection of the control lines 20A and 20B is often performed at depths of thousands of feet. The BOP stack must then be tested for integrity and the ability to properly operate. It will be appreciated that the test procedure must test not only the individual BOPs, but must also test the BOPs in combination with each other. This can be a time consuming process. Only after the entire BOP stack 26 has been tested can mud be circulated back downhole to control pressure, the drill string lowered through the riser 18 system, BOP stack 26 and wellhead 24 and drilling recommenced. The problems associated with locating the BOP stack on the seafloor have been addressed to a degree. The idea of using a surface BOP attached to the wellhead has been addressed by several companies. The article, Surface BOPs Free Modest Semis for Immodest Depths, Offshore Engineer, March 2001, pp. 31-34 discusses the problems related to subsea BOPs and how to address them. One design proposed by Unocal calls for the entire BOP stack to be located at the surface and connected via riser directly to the wellhead, as illustrated in the article, page 32, the lower figure depicted therein. A similar illustration of the use of surface versus subsea BOP stacks is also set forth at page 32 of the article. As noted therein, this particular design is useful in adverse ocean environments.

A second design discussed at pages 33-34 of the article, depicts a suspension system for a surface BOP riser system as used by Shell/Woodside . It is stated that the Shell design decreases bending moments by allowing for lateral movement. The article further addresses the need for a disconnect capability when using a surface BOP. As noted, therein, quite often the general practice is to plug and abandon the well.

The preferred embodiment of the present invention addresses the need for disconnect in the event of a metaocean condition while minimizing the time required for the disconnect/reconnect operation. In Fig. 3, the platform 10 is again shown at the surface, having a surface BOP stack 15 supported by a tensioner system 16. It will be appreciated that the tensioner system 16 may be of types generally known in the industry as illustrated in the above referenced article. As with prior art systems, the riser 18 is comprised of multiple riser joints connected together to extend from the platform 10 down toward the wellhead 24. It should be noted, however, the riser 18 system of the present invention does not include control lines as in the prior art. It will be appreciated that by eliminating control lines, the cost of the riser joint is reduced, as well as the time required to connect the joints together and test the integrity of the control lines once connected. As before, the riser 18 system may include various telescoping or flexure connections and may be supported by buoyancy elements (not shown) together with the tensioner system 16. As in prior systems, the riser 18 system terminates at a flexible joint connector 34 and mates with connector 32 mounted atop the subsea BOP stack 26, which is in turn, mounted on the wellhead 24. Unlike prior art systems, the subsea BOP stack 26 is a minimal BOP stack comprised of at least one BOP, preferably, a ram-type, as will discussed further below. In a preferred embodiment (not shown) the subsea BOP stack is supported by a framework that is attached to the ram BOP by means of flange bolted to the framework. The framework itself is attached to a wellhead connector which is designed to sealingly connect with the wellhead to provide an open bore when the rams are open. The BOP stack further includes accumulator tanks which may be used to store pressurized gas to pneumatically operate the BOP through piping. The accumulator tanks are slaved to a battery driven control unit which may include a programmable controller or processor, and is designed to not only operate the BOP, but to monitor the status of the BOP stack through sensors on the accumulator tanks and BOP. The control unit is further adapted to permit an ROV to land on the control unit and perform service tasks, such as control maintenance or battery replacement.

The control unit communicates with the platform by means of an acoustic transponder system mounted on the framework. The transponder is powered by and under the direction of the control unit. Coded control signals are issued from a similar transponder on the platform. The code selected is preferably one that allows for easy error correction that may result from extraneous noise introduced into the marine environment from drilling operations, maritime vessels or other sources. Further, the frequency (ies) for the coded control signals is selected to permit efficient transmission of the control signals between the platform 10 and subsea BOP stack. Transponder systems of this type include the Nautica system which has been successfully utilized in a deep sea environment .

The operation of the present invention begins with the decision to disconnect the drilling riser 18 from the wellhead 24 in response to metaocean conditions, such as hurricanes, typhoons or emergency situations that may require temporary platform abandonment. The drill string (not shown) is tripped from the formation below the wellhead 24, through wellhead 24, BOP 26, through riser connectors 32 and 34, riser 18 system, and finally through the surface BOP stack 15. An acoustic signal is issued by the platform 10 and received by the acoustic transponder on the subsea BOP stack 26. It will be appreciated that in emergency situations, the rams may be closed with the drill string still engaged, effectively shearing the drill string and closing the hole. The drill string may be recovered from the hole following subsequent reconnection. The acoustic transponder generates an electric signal corresponding to the acoustic signal received and transmits it to the control unit. The control unit receives the signal and decodes the signal into a set of control instructions. The control unit, in response to the control instructions, then activates the appropriate relays to activate valves to transfer compressed air from the accumulator tanks to ram BOP 26 pneumatic actuators to close the bore. Alternatively, the control unit could activate a hydraulic pump (not shown) to transfer fluid to the actuators, in this instance hydraulic actuators. It will be appreciated that the BOP 26 may be instrumented to generate a signal indicating the state of the BOP 26, i.e., fully open, fully closed, or something in between. This signal may be transferred back to the control unit and transmitted to the platform 10 by the acoustic transponder. Thus, the operator can receive a confirmation of the actuation of the subsea BOP stack 26 prior to taking any further action. Following closing of subsea BOP stack 26, the surface BOP stack 15 is then closed essentially isolating the riser 18. The riser 18 may then be disconnected from the BOP stack 26 with the assistance of an ROV and the riser may be tripped and/or the platform relocated. The process of reconnection is essentially the reverse, beginning with the relocation of the platform 10 to its appropriate position for reconnection. The riser 18 is made up and lowered down to the subsea BOP stack 26. As noted above, the riser 18 system of the present invention does not utilize control lines. This will reduce the time required to make up riser joint connections and decrease the cost of the joint itself. The riser 18 with flex joint connector 34 is then connected to the subsea BOP stack 26 connector 32 with the aid of an ROV. Following inspection of the connection, the operator begins testing of the surface BOP stack 15. This is typically accomplished by testing the functioning of the BOPs individually and in combination. It will be appreciated that the control of the surface BOP stack 15 is under surface control and the BOPs may be readily monitored and tested. Should repairs need to be made on the BOPs that comprise the surface BOP stack, they may be performed readily, as opposed to in a subsea environment as is done in the prior art.

Once the surface BOP stack 15 has been thoroughly tested, a coded acoustic signal is transmitted from the platform 10 to the subsea BOP stack 26. The acoustic signal is received by the acoustic transponder and a corresponding electrical signal transferred to the control unit. The control unit decodes the signal activates the appropriate valves to provide pressurized gas or hydraulic fluid to the actuators located on the BOP 26. The BOP 26 instrumentation generates a signal to indicate the status of the BOP 26. The signal is received by the control unit and transferred back to the platform 10 by means of the acoustic transponder. The operator may then resume drilling operations. An alternative embodiment of the present invention calls for the disconnect of the riser at some point below the surface but well above the seafloor. In Figure 4, the platform 10 is shown as similar to that depicted in Fig. 3. However, at some selected distance below the water surface, for example, 500 feet, the riser 18A terminates in a flex joint connector 64. The flex joint connector mates with connector 66, which is itself connected to a second riser string 18B that extends from connector 66 to the subsea BOP stack 26. The second riser string terminates in a flex connector 34 that mates with a connector 32 located on top of the subsea BOP stack 26. The remainder of the embodiment is similar to that set forth in Figure 3. In this instance, a sufficient number of flotation buoys (not shown) are affixed about riser 18B string to impart enough buoyancy to riser 18B to permit it to maintain a given depth below the ocean surface. The method of disconnect is the very same as that discussed with reference to Fig. 3 with the exception that the disconnect takes place not near the seafloor 22. It will be appreciated that the time required to trip the riser 18A joints will be reduced, thereby reducing associated costs. Likewise the time and costs associated with making up riser 18A and lowering it to connect with riser 18B will be reduced. The riser 18B and connector 64 may be readily located utilizing acoustic transponders and ROVs may be used to reconnect connectors 34 and 64. This second embodiment may be utilized in response to specific ocean conditions, e.g., in areas susceptible to ice floes or icebergs.

Claims

C L A I M S

1. A system for drilling a well into an earth formation below a marine environment, comprising: an offshore platform positioned above the well; a conduit providing fluid communication between the offshore platform and the well, the conduit comprising a lower portion including a subsea blowout preventor and an upper portion including a second blowout preventor arranged at the platform and a drilling riser disconnectable from said lower portion of the conduit; - a drill string extending from the offshore platform through the conduit into the well; and a subsea disconnect system for selectively disconnecting the drilling riser from said lower portion of the conduit.

2. The system of claim 1, wherein the subsea blowout preventor comprises a ram-type blowout preventor.

3. The system of claim 2, wherein the subsea blowout preventor comprises a shear ram-type blowout preventor.

4. The system of any one of claims 1-3, wherein the second blowout preventor comprises an annular preventor.

5. The system of any one of claims 1-4, further comprising a subsea control system for monitoring and controlling the subsea blowout preventor, and a surface control system in communication with the subsea control system.

6. The system of claim 5, wherein the subsea control system is arranged to control the subsea disconnect system.

7. The system of claim 5, wherein said subsea control system comprises:

(a) a pressure monitoring system for measuring pressures in said subsea blowout preventor and generating a signal responsive to said measured pressures;

(b) a power system for operating said disconnect system and generating a signal responsive to the operation of said power system;

(c) a communication system for receiving said signals from said pressure monitoring and power systems, for transmitting said signals to, and receiving data and commands from said surface communications system.

8. The system of claim 7, wherein said pressure monitoring system further measures pressures in the well and in said disconnect system.

9. The system of claim 7, wherein said power system is arranged to operate said subsea blowout preventor.

10. The system of claim 7, wherein said power system includes hydraulic, pneumatic or electrical power sources.

11. The system of claim 7, wherein said communications system includes and acoustic communications system for transmitting and receiving coded signals through the marine environment.

12. The system of any one of claims 1-11, wherein said subsea blowout preventor is operable by a subsea remote operated vehicle.

13. The system of any one of claims 1-12, wherein said subsea disconnect system is operable by a subsea remote operated vehicle.

14. A method of operating the system of any one of claims 1-13, comprising: retrieving the drill string from the well and the subsea blowout preventor; closing the subsea blowout preventor; and operating the subsea disconnect system so as to disconnect the drilling riser from the lower portion of the conduit .

15. The method of claim 14, wherein the second blowout preventor is closed before the subsea disconnect system is operated.