Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WO 2012/054808 CA 02804898 2013-01-09PCT/US2011/057228
LEG ICE SHIELDS FOR ICE WORTHY JACK-UP DRILLING UNIT
FIELD OF THE INVENTION
[0001] This invention relates to mobile offshore drilling units, often called
"jack-up"
drilling units or rigs that are used in shallow water, typically less than 400
feet, for
drilling for hydrocarbons.
BACKGROUND OF THE INVENTION
[0002] In the never-ending search for hydrocarbons, many oil and gas
reservoirs have
been discovered over the last one hundred and fifty years. Many technologies
have been
developed to find new reservoirs and resources and most areas of the world
have been
scoured looking for new discoveries. Few expect that any large, undiscovered
resources
remain to be found near populated areas and in places that would be easily
accessed.
Instead, new large reserves are being found in more challenging and difficult
to reach
areas.
[0003] One promising area is in the offshore Arctic. However, the Arctic is
remote
and cold where ice on the water creates considerable challenges for
prospecting for and
producing hydrocarbons. Over the years, it has generally been regarded that
six
unprofitable wells must be drilled for every profitable well. If this is
actually true, one
must hope that the unprofitable wells will not be expensive to drill. However,
in the
Arctic, little, if anything, is inexpensive.
[0004] Currently, in the shallow waters of cold weather places like the
Arctic, a
jack-up or mobile offshore drilling unit (MODU) can be used for about 45-90
days in the
short, open-water summer season. Predicting when the drilling season starts
and ends is a
game of chance and many efforts are undertaken to determine when the jack-up
may be
safely towed to the drilling location and drilling may be started. Once
started, there is
considerable urgency to complete the well to avoid having to disconnect and
retreat in the
event of ice incursion before the well is complete. Even during the few weeks
of open
water, ice floes present a significant hazard to jack-up drilling rigs where
the drilling rig
is on location and legs of the jack-up drilling rig are exposed and quite
vulnerable to
damage.
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[0005] Jack-up rigs are mobile, self-elevating, offshore drilling and workover
platforms equipped with legs that are arranged to be lowered to the sea floor
and then to
lift the hull out of the water. Jack-up rigs typically include the drilling
and/or workover
equipment, leg-jacking system, crew quarters, loading and unloading
facilities, storage
areas for bulk and liquid materials, helicopter landing deck and other related
facilities and
equipment.
[0006] A jack-up rig is designed to be towed to the drilling site and jacked-
up out of
the water so that the wave action of the sea only impacts the legs which have
a fairly
small cross section and thus allows the wave action to pass by without
imparting
significant movement to the jack-up rig. However, the legs of a jack-up
provide little
defense against ice floe collisions and an ice floe of any notable size is
capable of causing
structural damage to one or more legs and/or pushing the rig off location. If
this type of
event were to happen before the drilling operations were suspended and
suitable secure
and abandon had been completed, a hydrocarbon leak would possibly occur. Even
a
small risk of such a leak is completely unacceptable in the oil and gas
industry, to the
regulators and to the public.
[0007] Thus, once it is determined that a potentially profitable well has been
drilled
during this short season, a very large, gravity based production system, or
similar
structure may be brought in and set on the sea floor for the long process of
drilling and
producing the hydrocarbons. These gravity based structures are very large and
very
expensive, but are built to withstand the ice forces year around.
BRIEF SUMMARY OF THE DISCLOSURE
[0008] The invention more-particularly relates to an ice worthy jack.up rig
for drilling
for hydrocarbons in potential ice conditions in offshore areas including a
flotation hull
having a relatively flat deck at the upper portion thereof. The flotation hull
further
includes an ice bending shape along the lower portion thereof and extending
around the
periphery of the hull where the ice bending shape extends from an area of the
hull near
the level of the deck and extends downwardly near the bottom of the hull along
with an
ice deflecting portion extending around the perimeter of the bottom of the
hull to direct
ice around the hull and not under the hull. The rig includes at least three
truss form legs
that are positioned within the perimeter of the bottom of the hull wherein the
legs are
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arranged to be lifted up off the seafloor so that the rig may be towed through
shallow
water and also extend to the sea floor and extend further to lift the hull
partially or fully
out of the water. A jack up device is associated with each leg to both lift
the leg from the
sea bottom so that the ice worthy jack up rig may float by the buoyancy of the
hull and
push the legs down to the seafloor and push the hull partially up and out of
the water
when ice floes threaten the rig and fully out of the water when ice is not
present. Leg ice
shields are arranged to protect the truss form legs from ice.
[0009] The invention further relates to a method for drilling wells in ice
prone waters.
The method includes providing a flotation hull having a relatively flat deck
at the upper
portion thereof and an ice bending shape along the lower portion thereof where
the ice
bending shape extends from an area of the hull near the level of the deck and
extends
downwardly near the bottom of the hull and an ice deflecting portion extending
around
the perimeter of the bottom of the hull to direct ice around the hull and not
under the hull.
At least three truss form legs are positioned within the perimeter of the
bottom of the hull
where each leg comprises a plurality of vertically oriented posts connected by
cross
members and ice shields are arranged between the posts to protect the cross
members
from ice. Each leg is jacked down in a manner that feet on the bottom of the
legs engages
the sea floor and lifts the hull up and fully out of the water when ice is not
threatening the
rig while the rig is drilling a well on a drill site. The hull is further
lowered into the water
into an ice defensive configuration so that the ice bending shape extends
above and below
the sea surface to bend ice that comes against the rig to cause the ice to
submerge under
the water and endure bending forces that break the ice where the ice flows
past the rig.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] A more complete understanding of the present invention and benefits
thereof
may be acquired by referring to the follow description taken in conjunction
with the
accompanying drawings in which:
[0011] Figure 1 is an elevation view of a first embodiment of the present
invention
where the drilling rig is floating in the water and available to be towed to a
well drilling
site;
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[0012] Figure 2 is an elevation view of the first embodiment of the present
invention
where the drilling rig is jacked up out of the water for open water drilling
through a moon
pool;
[0013] Figure 3 is a second elevation view of the first embodiment of the
present
invention where the drilling rig is partially lowered into the ice/water
interface, but still
supported by its legs, in a defensive configuration for drilling during
potential ice
conditions;
[0014] Figure 4 is an enlarged fragmentary elevation view showing one end of
the
first embodiment of the present invention in the Figure 3 configuration with
ice moving
against the rig;
[0015] Figure 5 is a top view of a leg of the drilling rig with a first
embodiment of the
leg ice shields in place;
[0016] Figure 6 is a top view of a leg of the drilling rig with a second
embodiment of
the leg ice shields in place;
[0017] Figure 7 is an elevation view of the drilling rig with the first
embodiment leg
ice shields in place; and
[0018] Figure 8 is an elevation view of the drilling rig with the second
embodiment
leg ice shields in place.
DETAILED DESCRIPTION
[0019] Turning now to the detailed description of the prefened arrangement or
arrangements of the present invention, it should be understood that the
inventive features
and concepts may be manifested in other arrangements and that the scope of the
invention
is not limited to the embodiments described or illustrated. The scope of the
invention is
intended only to be limited by the scope of the claims that follow.
[0020] As shown in Figure 1, an ice worthy jack-up rig is generally indicated
by the
arrow 10. In Figure 1, jack-up rig 10 is shown with its hull 20 floating in
the sea and legs
25 in a lifted arrangement where much of the length of the legs 25 extend
above the deck
21 of the hull 20. The legs 25 are constructed of long posts with cross
members
connecting the posts in a truss arrangement. On the deck 21 is derrick 30
which is used
to drill wells. In the configuration shown in Figure 1, the jack-up rig 10 may
be towed
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from one prospect field to another and to and from shore bases for maintenance
and other
shore service.
[0021] When the jack-up rig 10 is towed to a drilling site in generally
shallow water,
the legs 25 are lowered through the openings 27 in hull 20 until the feet 26
at the bottom
ends of the legs 25 engage the seafloor 15 as shown in Figure 2. In a
preferred
embodiment, the feet 26 are connected to spud cans 28 to secure the rig 10 to
the
seafloor. Once the feet 26 engage the seafloor 15, jacking rigs within
openings 27 push
the legs 25 down and therefore, the hull 20 is lifted out of the water. With
the hull 20
fully jacked-up and out of the water, any wave action and heavy seas more
easily break
past the legs 25 as compared to the effect of waves against a large buoyant
object like the
hull 20. As shown in Figure 2, well drilling operations may commence in the
ordinary
course while there is no ice in the area. However, when ice begins to form on
the sea
surface 12, the risk of an ice floe contacting and damaging the legs 25 or
simply
bulldozing the jack-up rig 10 off the drilling site becomes a significant
concern for
conventional jack-up rigs and such rigs are typically removed from drill sites
by the end
of the open water season. The ice-worthy jack-up drilling rig 10 is designed
to resist ice
floes by assuming an ice defensive, hull-in-water configuration as shown in
Figure 3. In
Figure 3, ice tends to dampen waves and rough seas, so the sea surface 12
appears less
threatening, however, the hazards of the marine environment have only altered,
and not
lessened.
[0022] When the ice-worthy jack-up rig 10 assumes its ice defensive, hull-in-
water
configuration, the hull 20 is lowered into the water to contact same, but not
to the extent
that the hull 20 would begin to float. A significant portion of the weight of
the rig 10
preferably remains on the legs 25 to hold the position of the rig 10 on the
drill site against
any pressure an ice flow might bring. The rig 10 is lowered so that inwardly
sloped,
ice-bending surface 41 bridges the sea surface 12 or ice/water interface to
engage any
floating ice that may come upon the rig 10.
[0023] The sloped ice-bending surface 41 runs from shoulder 42, which is at
the edge
of the deck 26, down to neckline 44. Ice deflector 45 extends downward from
neckline
44. Thus, when an ice floe, such as shown at 51 comes to the rig 10, the ice-
bending
surface 41 causes the leading edge of the ice floe 51 to submerge under the
sea surface 12
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and apply a significant bending force that breaks large ice floes into
smaller, less
damaging, less hazardous bits of ice. For example, it is conceivable that an
ice floe being
hundreds of feet and maybe miles across could come toward the rig 10. If the
ice floe is
broken into bits that are less than twenty feet in the longest dimension, such
bits are able
to pass around the rig 10 with much less concern.
[0024] Ice has substantial compressive strength being in the range of 4 to 12
MPa,
but is much weaker against bending with typical flexure strength in the range
of 0.3 to 0.5
MPa. As shown, the force of the ice floe 51 moving along the sea surface 12
causes the
leading edge to slide under the sea surface 12 and caused section 52 to break
off. With
the ice floe 51 broken into smaller floes, such as section 52 and bit 53, the
smaller
sections tend to float past and around the rig 10 without applying the impacts
or forces of
a large floe. It is preferred that ice not be forced under the flat of bottom
of the hull 20
and the ice deflector 45 turns ice to flow around the side of the hull 20. If
the ice is really
thick, the ice deflector 45 is arranged to extend downwardly at a steeper
angle than
ice-bending surface 41 and will increase the bending forces on the ice floe.
At the ice
deflector 45, an ice deflector is positioned to extend down from the flat of
bottom of the
hull 20. In an optional arrangement, the turn of the bilge is the flat of
bottom at the
bottom end of the ice deflector 45.
[0025] To additionally resist the forces that an ice floe may impose on the
rig 10, the
feet 26 of the legs may be arranged to connect to cans 28 set in the sea floor
so that when
an ice floe comes against the ice-bending surface 41, the legs 25 actually
hold the hull 20
down and force the bending of the ice floe and resist the lifting force of the
ice floe
which, in an extreme case, may lift the near side of the rig 10 and push the
rig over on its
side by using the feet 26 on the opposite side of the rig 10 as the fulcrum or
pivot. The
cans in the sea floor are known for other applications and the feet 26 would
include
appropriate connections to attach and release from the cans, as desired.
[0026] It should probably be noted that shifting from a conventional open
water
drilling configuration as shown in Figure 2 to a hull-in-water, ice defensive
configuration
shown in Figure 3 may require considerable planning and accommodation
depending on
what aspect of drilling is ongoing at the time. While some equipment can
accommodate
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shifting of the height of the deck 21, other equipment may require
disconnections or
reconfiguration to adapt to a new height off the sea floor 15.
[0027] The ice-worthy jack-up drill rig 10 is designed to operate like a
conventional
jack-up rig in open water, but is also designed to settle to the water in an
ice defensive
position and then re-acquire the conventional stance or configuration when
wave action
becomes a concern. It is the shape of the hull 20 (as well as its strength)
that provides ice
bending and breaking capabilities.
[0028] One of the concerns for operating a jack-up drilling rig in potentially
ice
conditions is the exposed legs 25. To help protect the legs 25 from ice, leg
ice shields 32
are attached between vertical posts 31 of the legs. In Figure 5, the leg ice
shields 32 are
bowed to provide a rounded shape to steer any ice around the legs. In Figure
6, the leg
ice shields 33 are relatively flat. As shown in Figures 7 and 8, the leg ice
shields 32 and
33 are preferably positioned on the legs to extend above, below and across the
waterline
12 but not necessarily cover the entire vertical length of each leg 25.
[0029] While the leg ice shields provide some ice protection from the legs, it
should
be noted that the legs 25 and the openings 27 through which they are connected
to the
hull 20 are within the perimeter of the ice deflector 45 so that the ice floes
are less likely
to contact the legs 25 while the rig 10 is in its defensive ice condition
configuration as
shown in Figure 3 and sometimes called hull-in-water configuration. Moreover,
the rig
does not have to handle every ice floe threat to significantly add value to
oil and gas
companies. If rig 10 can extend the drilling season by as little as a month,
that would be
a fifty percent improvement in some ice prone areas and therefore provide a
very real
cost saving benefit to the industry.
[0030] In closing, it should be noted that the discussion of any reference is
not an
admission that it is prior art to the present invention, especially any
reference that may
have a publication date after the priority date of this application. At the
same time, each
and every claim below is hereby incorporated into this detailed description or
specification as an additional embodiment of the present invention.
[0031] Although the systems and processes described herein have been described
in
detail, it should be understood that various changes, substitutions, and
alterations can be
made without departing from the spirit and scope of the invention as defined
by the
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following claims. Those skilled in the art may be able to study the preferred
embodiments and identify other ways to practice the invention that are not
exactly as
described herein. It is the intent of the inventors that variations and
equivalents of the
invention are within the scope of the claims, while the description, abstract
and drawings
are not to be used to limit the scope of the invention. The invention is
specifically
intended to be as broad as the claims below and their equivalents.
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