Note: Descriptions are shown in the official language in which they were submitted.
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RISER FOR COIL TUBING/WIRE LINE INJECTION
Field of Invention
[0001] The present invention is directed to the interfacing of a Self
Supporting Riser (SSR) to
a vessel subject to high vessel motions of pitch and roll. The small vessel
employs a unique
stabilization system for supporting a coil tubing/wire line injector and
supporting equipment on
the vessel.
Background of the Invention
[0002] It has been the practice for the intervention in deep wells that
recover hydrocarbons
from fossil hydrocarbon reservoirs deep below the Gulf of Mexico and other
offshore areas to
use very large vessels of various designs upon which the equipment for
intervention into the
wells are supported. These vessels cost millions of dollars and have day rates
that frequently can
not be cost justified to perform work-over coil tubing/wire line procedures.
Summary of the Invention
[0003] The present invention is directed to a system including a self
supporting riser (SSR)
which is connected to a well to provide fluid communication to fossil
hydrocarbon reservoirs
deep below the seafloor. The SSR is constructed of a plurality of joints
comprising regular joints
and specialty joints that define the SSR and are selected to optimize the SSR
for a well in a
specific location. A unique aspect of the invention is further directed to a
small vessel subject to
high vessel motions that penults a coil tubing/wire line system to be mounted
on a stabilizer
system mounted on the vessel.
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Brief Description of the Drawings
[0004] Figure 1 is a schematic view of a Self Supporting Riser (SSR) connected
to a well for
producing hydrocarbons from a fossil hydrocarbon reservoir deep below the
seafloor, and a small
vessel subject to high heave, pitch and roll outfitted for downhole
intervention through the SSR.
[0005] Figure 2 is a schematic view of another embodiment of the stabilizer
system for the
intervention system to inject a coil tubing/wire line into the SSR.
[0006] Figure 3 is a schematic view of the detail of the telescoping section
of the riser
connection from the SSR to the vessel near its midpoint.
[0007] Figure 4 is a schematic view of the detail of the telescoping section
of the riser
connection from the SSR to the vessel at the upward position of heave of the
vessel.
[0008] Figure 5 is a schematic view of the detail of the telescoping section
of the riser
connection from the SSR to the vessel at the downward position of heave of the
vessel.
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Description of the Embodiments of the Invention
[0009] Referring to Figure 1, reference is made to Serial number 12/714,919
and the drawing
therein for a more detail description, a novel Riser Vessel Interface System
(RVI) 60 facilitates
using the SSR for downhole intervention and workover through the SSR 10 using
relatively
small vessels. The nature of an SSR is such that it may be relatively
sensitive to the magnitude of
externally applied tension and to variations in externally applied tension. It
is the nature of small
vessels that their motions in response to waves and swells are greater than
those of larger vessels
and substantially greater than the motions of platforms or floating production
facilities. The
interface between an SSR and a small vessel therefore requires a greater range
of motion and less
tension variation than is provided by the previous art. The coiled tubing
injector must be
supported by the vessel, and the weight of deployed tubing normally hangs from
the injector.
[0010] Referring to Figure 1, intervention vessel 35, with reel 59 and crane
46, is shown with
coil tube injector 120 assembled on the RIV System 60 (not shown). The desired
downhole
tooling has been attached to the coiled tubing and made ready for operations.
The above
equipment and its specific arrangement provide a novel arc of tubing 61 to be
used in the present
invention to extend the fatigue life of the tubing 63. (see also Figures 14
and 14A of Serial No
12/714,919). This allows the SSR to be fixed to the earth while the reel moves
with the deck of
the vessel. As described in Serial No. 12/714,919, riser extension 64 has a
telescoping
joint/section 65 that is present as a contingency for exceptional heave.
[0011] Figure 2 illustrates the combination of an SSR, a riser extension 64
between a vessel 35
and, a telescoping joint/section 65, and a pitch roll stabilized frame 68 to
engage the SSR to a
vessel subject to high vessel motions. The method for using this combination
allows the system
to function without stroking heave cylinders of the RVT structure The riser
extension with
telescoping provisions 65 is engaged to the top of the SSR by connector 92 and
hangs from the
pitch and roll stabilized frame 68 which is in turn supported by heave
platform 66 that in this
embodiment may be attached to the deck 33 of vessel 35. The attachment point
between the
frame 68 and the riser extension 64 is preferably near the vessel center of
motion and preferably
in a moon pool of the vessel 35.
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[0012] Figure 3 illustrates the mid heave position. The telescopic function of
riser extension 64
is allowed to extend and retract with telescoping section 65 as the vessel
heaves. The allowable
range of vessel heave with respect to the SSR is as great as the allowable
travel length of the
telescope. Stops can be built into the telescopic joint 65 to limit the
minimum and/or maximum
extension. Hydrostatic lock is prevented by venting the telescopic section so
that fluid can flow
into and out of it as the length and internal volume change. Venting can be
through ports open to
seawater or through a line 55 to a supply of water or other fluid that may be
filtered or otherwise
treated or constituted to reduce corrosion and avoid debris and contamination
and enhance
lubrication. It is apparent that the effective weight below the attachment
point can be increased
or decreased by using line 56 to trim the pressure of fluid in chamber 69 in a
way that causes the
telescopic joint/section to pull on or push against the SSR while flow into
and out of line 56
allows the telescoping section to extend and retract. Chamber 69 is similar in
construction to the
rod end of a hydraulic cylinder. Figure 4 illustrates a heave downward; and
Figure 5 illustrates a
heave upward.
[0013] Friction in the telescopic joint can be reduced by methods such as
filling the volume
with lubricating fluid, securing bearings such as balls between the moving
parts of the telescopic
section, using a liner of material such as ultra high molecular density
polyurethane, or some
combination of similar methods.
[0014] Verticality of the riser extension is aided by any combination of
weight below the point
where it attaches to the pitch and roll stabilized frame, stiffness below the
attachment point, and
active control of the hydraulic cylinders that support the pitch and roll
stabilized frame. The
righting moment due to weight below the attachment point is proportional to
the weight times the
distance between the center of gravity and the attachment point; and the
apparent weight can be
increased by applying pressure through line 56. Stiffness of the riser
extension, in combination
with attachment of the lower end of the extension to the SSR, allows the
stiffness of the riser and
the righting moment of the buoyancy module to counteract any overturning
moment introduced
by the weight of equipment supported above the attachment point. The righting
moment of the
buoyancy module is proportional to the buoyancy force, the angle of
inclination, and the distance
between the center of buoyancy and the effective "hinge point" where the riser
can curve below
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the buoyancy.
[0015] As further described in Serial No. 12/714,919, a valve assembly can be
used to prevent
exchange of fluid between supporting cylinders for the pitch and roll
stabilized frame, thereby
locking the riser extension assembly and its load in position so that it
pitches and/or rolls with
the vessel. Locking the assembly may be advantageous at times when the
extension is not
connected to the SSR.
[0016] As described in Serial No. 12/714,919, the pitch and roll stabilized
frame supports the
riser extension in a way that exerts equal lifting force symmetrically around
the riser extension
regardless of the pitch or roll angle of the vessel. As also described in
Serial No. 12/714,919
active control of verticality can be achieved by trimming the lifting forces
that are otherwise
equally distributed around the riser extension. The load on these cylinders
may be substantial,
but application of a comparatively small force can trim the balance of forces
to compensate for
an offset center of gravity or external horizontal forces on the supported
assembly. The described
hydraulic cylinder arrangement allows control of the relative fluid pressure
in opposing hydraulic
cylinders to balance external forces such as an offset center of gravity. A
feedback signal
proportional to inclination of the riser extension can be used to actively
control the balance of
forces and thereby maintain verticality of the riser extension.
[0017] It is apparent that relative motion between the earth and any tubing
suspended in the
SSR can be avoided by operating the injector to run or pull a length of tubing
equal to the
extension or retraction of the telescopic joint, and that this could be
automated by using a
feedback signal from sensing the relative position of the two elements of the
telescopic joint.
[0018] Maximum pitch and roll angles are limited by the range of motion of the
supporting
hydraulic cylinders and by potential interference with the walls of the moon
pool. It is further
apparent that the outer member of the telescoping joint could be either the
larger or the smaller
of the 2 members, and that the telescopic joint could either be part of the
riser extension or be
part of the SSR. The telescopic joint can include provisions for locking it
anywhere between
maximum and minimum extension, either by ROV or by remotely operated latches.
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[0019] Equipment mounted atop the riser extension remains at a fixed elevation
with respect to
the vessel center of motion while pitch and roll cause the vessel to incline
with respect to this
equipment. The center of gravity is kept as low as is practical while
providing clearance between
the vessel and the outer perimeter of the equipment attached to the riser
extension. This clearance
and the dimensions of the moon pool deteimine the maximum vessel pitch and
roll that can be
accommodated without clashing or other interference. An advantage of this
embodiment is that it
keeps the center of gravity of the supported equipment low to reduce the
overturning moment
due to forces acting on this equipment.
[0020] It is further apparent that a hydraulic connector between the SSR and
the riser extension
is not necessary if one moving part of the telescopic joint is attached to the
SSR and the other to
the riser extension, and there is no lower stop. The vessel can then be
engaged to the SSR by
making up the two halves of the telescopic joint. In this embodiment the
telescopic joint is free to
separate and thereby release the vessel from the SSR if the vessel is forced
far enough off station
to cause the top of the SSR to set down by more than the maximum stroke of the
telescopic joint.
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