Referring now to the drawings, FIGS. 1 and 2 illustrate a shuttle tanker 10 having an internal mooring turret 11 located towards the bow 12 of the vessel. The vessel may be purpose built or could be a converted tanker such as a “Suezmax” type. The turret is rotationally disposed in relation to the rest of the vessel and is anchored to the sea bed by a plurality of mooring cables 13 such as anchor chains. The mooring arrangement enables the vessel to rotate about a vertical axis of the mooring turret 11. The mooring cables 13 are arranged in angularly spaced groups that radiate from the turret 11 and are supplied from a winch on the turret in a known manner.
Drilling plant is supported on a derrick 14 that is mounted over a moonpool 15 on the stern side of the mooring turret 11 on or around the midship area. The drilling plant comprises a top drive system that connects to a drilling string 16 depending from the derrick 14 through the moonpool 15, and extending inside a drilling riser 17, to a six wellhead drilling template 18 mounted on the sea bed over the existing well. The drilling riser 17 is connected to sub-sea blow out preventers (BOPs) 19 mounted on the sea bed at the drilling template 18.
The region 20 between the stem and the moonpool 15 is fitted with drilling plant and ancillary drilling support equipment, a skid deck, an adjacent gantry crane and an ROV launch. The area 21 between the moonpool 15 and the mooring turret 11 is furnished with production plant.
The drilling well template 18 is connected via a sub-sea pipeline 22 to a sub-sea manifold 23, which in turn is connected to the mooring turret 11 via a salt water injection production riser 24 of conventional configuration. The coupling of the vessel to the production riser 24 enables it to be used for simultaneous drilling and production in a unique manner.
In a simultaneous operations mode, illustrated in FIGS. 1 and 2, bow and stern thrusters (not shown) are used to dynamically position the vessel 10 by rotating it about the stationary mooring turret 11 in order to hold the vessel in a position such that the drilling moonpool 15 is maintained over the appropriate sub-sea wellhead 25. This ensures that drilling operations can continue. The dynamically positionable feature of the vessel also allows the moonpool 15 to be moved between drilling operations in order to permit access to a selected one of multiple wellheads 25 disposed within an arc of the moonpool 15 of the vessel 10, the arc being centred about the vertical axis of the mooring turret 11 as illustrated by the arrows A in FIG. 2.
FIGS. 3 and 4 illustrate the same tanker being used in a stand-by only mode i.e. where the drilling string 16 and riser 17 is disconnected from the drilling template 18 and only production operations are performed. In this mode the vessel 10 is free to rotate about the mooring turret 11 in accordance with the prevailing wind direction as indicated in FIG. 4 where four different vessel positions are shown whilst still being moored by three groups of mooring cables 13. The ability of the vessel to “weathervane” freely in this manner enables the production operations to continue regardless of the prevailing weather conditions.
The drilling plant is configured to perform underbalanced rotary or non-rotary drilling. An example of non-rotary drilling using coiled tubing equipment was described in our U.S. Pat. No. 6,840,322, the content of which is incorporated herein by reference. Two examples of rotary drilling plant are illustrated in FIGS. 5 and 6. In a first embodiment (FIG. 5) the sub-sea riser 30 is configured to permit drilling fluid to return inside the riser and is connected directly to the sub-sea BOP 31. A rotating control device 32 is disposed at the top of the riser 30 and flexible return lines 33 are connected between the riser 30, at a position immediately below the rotating control device 32, and an underbalanced four-phase drilling separator 34 disposed on the vessel, the lines extending through the moonpool 15. The return lines 33 carry the drilling mud and multi-phase hydrocarbon mix to the separator 34 where the hydrocarbons are separated from the mixture. In this arrangement the riser 30 is decoupled from the vessel and associated rig and is therefore not subjected to conventional S-bending loading.
In the embodiment of FIG. 6, a rotating control device 40 is disposed on the sub-sea BOP 31 and the sub-sea riser 30 is connected between the rotating control device 40 and the vessel 10. The drilling mud and fluid mix is conveyed to the vessel via one or more external flow lines 41 to the underbalanced drilling separator 34 on the vessel. The riser 30 can be connected to the vessel conventionally as depicted in FIG. 6 or can be disconnected in the manner shown in FIG. 5.
In operation, underbalanced drilling is performed i.e. the pressure of the drilling fluid is designed to be less than the pressure of the formation fluid at the bottom of the well where the drilling is taking place. This ensures that the hydrocarbon fluids flow into the well and pass up the riser 30 (or external flow lines 41 in the embodiment of FIG. 6) conveying with it solids produced by the drilling process. The returns are passed to the four-phase separator 34 where excess hydrocarbons are separated from the mixture and are transferred to a main production plant 35 for clean up prior to storage in the tanker's cargo tanks.
FIG. 7 illustrates the drilling and production process. The drilling fluid used in underbalanced drilling is a mixture of drilling mud and nitrogen gas. Nitrogen from storage container 50 is introduced into drilling mud from tank 51 before it is passed to a pump in the drilling plant 52. The returns from the drilling operation are passed to the four-phase separator 34 that separates the multi-phase mixture into solids and sediments, drilling mud, nitrogen, hydrogen and crude oil. The drilling mud is returned to the mud storage tank 51, the sediment to storage and the nitrogen and hydrogen to a flare vent 53 for burning off the unwanted gasses. The crude oil from the underbalanced (UB) drilling process is passed to the production process plant 54 where it meets the wellhead fluids returned from the production riser 24. The production process plant comprises a production separator 55 of conventional configuration that separates out the crude oil (and/or other desirable hydrocarbon fluids) and transfers it to onboard cargo holds for storage before subsequent offloading to a dynamically positionable shuttle tanker.
The floating, drilling, production, storage and offloading vessel provides for a unique operation that is versatile, mobile, multipurpose and exhibits operational flexibility. The vessel can give direct access to test sub-sea wells for extended durations and is particularly suitable for intervention in marginal or depleted fields that were previously not considered to be commercially viable using conventional drilling. The vessel can be used for an extended water injection test and allows for the disposal of waster into a sub-sea well. Furthermore, the original structure and features of a converted vessel can be retained so that it can still be employed in the charter market when not being used for direct well interventions. The vessel thus represents a solution to the problem of achieving direct well interventions without the major costs associated with building and operating specialist vessels. Production revenue is generated as soon as the first well is brought on-stream. It is therefore a cost effective alternative to conventional drilling rigs.
Patent Application
20070251695
