Mid-depth disconnectable risers
Publication Date: 2013-Jan-10
The IP.com Prior Art Database
Floating vessels performing drilling operations in deepwater regions may sometimes encounter severe environmental conditions which can substantially affect operability and reliability of the drilling vessel, drilling equipment and the mooring system. This requires the vessel to have the ability to quickly disconnect the mooring system. However, before the mooring system can be disconnected, the drilling process needs to stop and the marine drilling riser that connected the vessel to the sea floor (through the BOP) needs to be retrieved, so that it will not interfere with the disconnection of the mooring system. The time to fully retrieve a deepwater drilling riser is very long. For example, in a water depth of 1000 m, it may take 20 hours or longer to fully retrieve the riser. A few alternative concepts exist that could be used for the purpose of shortening the time of riser retrieval. One concept is to disconnect the riser at mid-depth, retrieve only the upper riser section to the vessel and leave the lower riser section connected to BOP (e.g., Patents 4,234,047 and 5,657,823). A buoyancy can is attached to the top of the lower riser section to keep it upright. The buoyancy can mounted on top of the deepwater riser provides little lateral restoring force and allows for large lateral motion, which could render the riser reconnection operation impossible under certain conditions (e.g., strong currents). A new approach is required for the mid-depth riser disconnection concept to limit the lateral offset of the lower riser so that riser reconnection is possible under such conditions. Key words: drilling riser, disconnectable, rapid disconnection, mid-depth, mid-height, reconnection
The new concept we propose is to use both a buoyancy can and a means of stationkeeping (such as a spread mooring or a steel truss structure) that is connected to the lower riser section to provide a lateral restoring force to it to limit its lateral offset, which will enable the reconnection of the upper riser with the lower riser when environmental conditions improve.
Figure 1 is a schematic illustrating the key components of the concept, under normal drilling operation:
This configuration shows the floating drilling vessel (1) with a drilling derrick (2) and a turret (3) that connects the mooring lines (4) which hold the vessel in place.
The marine drilling riser is made up of two sections: the upper section (5) and the lower section (6). The two sections are connected via a coupling joint (7). A shut-in valve (7a) at the top end of the lower riser (6) ensures the lower riser section can be shut-off. Buoyancy can (8) is attached to the top of the lower riser to provide positive buoyancy to lower the riser tensioner requirements during drilling and hold the lower riser upright after disconnection. The lower riser section (6) is held in place laterally with a station-keeping device, such as a spread mooring (9) or a structure (e.g, a steel truss). Without such station-keeping device, the buoyancy can alone will not be sufficient to keep the top of the lower riser essentially in place laterally for reconnection under adverse environmental conditions such as relatively higher current or mild current conditions which could lead to riser VIV.
The lower riser section (6) is connected to the lower marine riser package (LMRP, 10), which connects to the blowout preventer (BOP, 11) at the sea floor (12).
The following is a description of the steps taken under this concept in order to disconnect the floating vessel performing drilli...