Note: Descriptions are shown in the official language in which they were submitted.
1 1 ~2~42
This invention relates to a marine dr;lling unit of the
floating barge type, and more particularly to a unit of said type for
use in arctic waters of onl~ moderate depth such as are encountered on
a continental shelf, for example in the ~eaufort Sea area north of
Canada and Alaska.
In shallow arctic waters it has a7ready been found
practicable to accommodate drilling rigs by building artificial islands
of one type or another on the sea bottom, or on a specially built berm,
which resist the forces of moving ice that forms on top of the water~
exploratory drilling into the sea bottom can readily be carried out
from drilling rigs located on such islands.
In deeper waters where islands cannot be built economically~
floating rigs have to be used for drilling, and numerous types of drill
ships and drilling barges have been proposed for such work, of which
some have been designed to deal with some of the problems created by
ice encountered during operation. It is accepted, however, that in
water deep enough to be subject to movement of icebergs all drilling
vessels, having a displacement of several thousand tonnes at most, must
be prepared to move out of the path of approaching icebergs, these
frequently have displacements amounting to several million tonnes.
In arctic waters of modest depths, for example in the
Beaufort Sea area where the continental shelf underlies the water many
kilometres out from the shoreline, creating water depths mostly in the
range from 10 to 75 metres, the problem of iceberg or ice island
approach occurs infrequently. However, an ice cover extending many
kilometres, which forms in this area to thicknesses of one to two
metres in autumn and early winter over the waters which open up in the
- short summer season, creates problems for floating rigs because the ice
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covering water depths greater than 25 metres is almost constantly
moving, under influence oF such factors as wind, ocean currents, and
the coriolis force. Because a floating drilling rig must remain
relatively stationary over its drill hole, to prevent an encroaching
ice cover from moving a rig off its location it is necessary to break
this ice cover as it approaches a floating drilling rig; the most
obvious expedient to achieve this is the use oF auxiliary ice-breaking
ships, which can break the cover and even large ice ridges therein.
This expedient is costly~ and more economic expedients are sought.
Attempts have been made to design drilling vessels which
themselves can break sheet ice as it encroaches on a vessel. To date,
none o-f these have been found satisfactory -For the modest water depths
that are encountered over the continental shelf. Thus for example the
"Big Buoy" spar buoy designs, having a large diameter submerged caisson
to support a working deck on a smaller diame-ter surface piercing
column, are unsuitable because they have too deep a draft to be
accommodated in the modest depths over the cont-inental shel-F area. It
has also been suggested that a conical collar around the surface
piercing column at the ice-water line of such designs be used to assist
in breaking the ice; such arrangement may be beneficial but in no way
does it reduce the draft requirements of the designs, which remain
entirely unsuitable for use in water of modest depth. Similarly the
structures suggested and disclosed by B. G. Gerwic~, in Can. P.
1,074,628 and USP 4,04~3,943, are entirely unsuitable for water of
modest depth. These structures utilize a truncated conical or an
hour-glass shaped hull at the ice-water line to assist in breaking ice,
and enhance the breaking action by active heaving oF the structures in
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the water; means are provided ~or vertically moving the structures so
that the con;cal wall thereof obliquely contacts surrounding ice w;th
sufficient dynamic force to break the ice. Essential to the vertical
heaving action are mooring lines which are substantially vertical in
the region close to the hull and are alternately tensioned and relaxed
to provide vertical motion (heaving) of the structures. With a minimum
draft of about 50 metres and a requirement for vertical mooring lines,
it is obvious that these structures are entirely impracticable in
waters of modest depth, for example the range from 25 to 75 metres.
It has been determined, by study oF relative horizontal
movement of models of apex-down truncated cone shape vessels against
sheet ice equivalent in thickness to one to two metre thicknesses oF
ice encroaching on an anchored f~ll scale vessel, that the total load
on the mooring system of such vessels is made up oF a breaking
component (force required to break ice), comprising about 20% to 40% of
the to-tal, and a clearing component (force required to resist movement
of the vessel by broken ice moving around past the vessel as additional
ice encroaches thereagainst) comprising correspondingly about 80% to
60% of the total load. This factor prevails with apex-down conical
vessels which exert a downward force on ice to break it and
consequently have an upward vertical component of tension placed on the
mooring lines as well as a horizontal component of tension caused by
the horizontal movement of the ice. It is an unexpected and surprising
condition that the clearing component of the mooring line load
?5 constitutes such a large proportion of the total load. The Factor
plays a significant role in the development and construction of the
present invention.
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It is well recognized in ~he art that ice encroachment on a
moored vessel may come from any direction, and that the most practical
shape in plan view for such vessels to counter the encroachment is
circular. However an exactly circular plan for the hull of a large
vessel is unwarranted because the practical material of construction
for marine vessels is straight edge flat steel plates, welded together
to provide the closest possible proximity to the desired contour.
Consequently the perimeter~ in plan v1ew, of the hull of the marine
drilling unit to which the present invention applies is a substantially
circular polygon, preferably a regular polygon of from 12 to 36 sides,
more preferably 20 to 30 sides, and the shape is referred to herein as
substantially circular polygonal. The dihedral angle between plates
around the perimeter of a regular polygon hull, having 24 sides for
! example, would be 165 degrees; that for a similar hull with 30 sides
would be 168 degrees.
The sum of the horizontal components of the ice breaking and
ice clearing forces when trans-ferred to the mooring lines is
s;gnificantly greater than the sum o-f the vertical components, as the
weight of the vessel assists in reducing the verticat ice breaking and
clearing forces. Hence the mooring lines are more effectively employed
when they are deployed to anchors which are located at a radius around
the centre of the vessel significantly greater than the radius of the
perimeter of the vessel. Such deployment is achieved in waters of
modest depth without resorting to unduly long mooring lines, but as a
result the lines extend -for a considerable dis-tance around the vessel
at depths not much greater than the points where they are attached to,
or achieve their closest proximity to, the underside of the hull. As a
24~
consequence, where the attachment points are near the water surface,
there can be a severe problem of fouling of the lines by broken ice
which must clear past the vessel as ice cover continuously encroaches
thereon. The vessel's downward breaking action on the ice forces
broken ice below the water surface wherel if the ice is ko travel past
the vessel, it must move under the ice cover adjacent the vessel or
around the vessel's submerged hull and into the area beyond the hull in
the direction of ice movement or deeper still directly beneath the
vessel itself where there is greater likelihood of fouling the mooring
lines. The present invention is intended to preclude or substantially
eliminate the likelihood of broken ice moving directly beneath a vessel
where fouling of, or damage to, its mooring lines by ice is most likely
to occurO
The invention is directed to improvements in the ice
clearing capability of moored ~loating barge type vessels which are
moored in modest depth water covered by moving ice cover. More
particularly, the invention consists, in a marine drilling unit
of the floating barge type with draft no greater than 20 metres, ~or
operation in ice-covered water, having a working deck to accommodate
drilling equipment, a marine hull to support said deck, a central
moonpool penetrating the deck and hull to accommodate a marine
drillpipe, and mooring lines attached at their top to said unit via the
bottom of said hull and at their bottoms to anchors surrounding said
unit at some distance, of the improvements which comprise:
(i) a hull having
(a) a substantially circular polygonal perimeter in plan view,
(b) a matching ~irst polygonal wall substantially in khe form
- o~ an apex-down truncated cone extend;ng From below the deck, at
Z
an angle to the horizontal of from substantially 20 degrees to
substantially 55 degrees3 preferrably from 30 to 45 degrees, to
a level at least below the normal bottom level of ice impinging
against the hull,
(c) a selcond matching polygonal wall substantially in the form
of an apex-down truncated cone extending from and below said
first wall, at an angle to the horizontal frorn at least as great
as the angle of the first polygonal wall to substantially 65
degrees, preferrably substantially 50 degrees9 to a level below
the bottom of the first wall by a distance at least as great as
the normal thickness oF ice impinging against the hull,
(d) a matching third polygonal wall substantially in the form
of a cyl;nder extending substankially vertically from and below
said second wall for a distance at least as great as the normal
thickness of ice impinging against the hull,
(e) a matching polygonal deFlector wall substantially in the
form of an apex-up truncated cone extending From and below said
third wall, at an angle to the horizontal of from substantially
20 degrees to substantially 65 degrees, preferrably
substantially 50 degrees, to a level below the bottom of said
third wall by a distance at least as great as the normal
thickness of ice impinging agains-t the hull, and
(f) a substantially flat annular bottom closing said deflector
wall and surrounding said moonpool; and
(ii) a substantially circular guard member secured to the flat bottom
of said hull, surrounding the points at which the mooring lines
achieve their closest prox;mi-ty to the bot-tom of the hull/ and
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extending below the flat bottom of the hull for a distance to
place the lowest edge of said member at a depth below the water
surface at least five times the normal thickness o-F ice
impinging against the hull and not greater than 20 metres.
The present invention may be more readily understood from
the following description and reference to the accompanying drawîngs in
which Figure 1 sho~s one view, in elevation with a partly cutaway
section, of a typical embodiment of the floating barge of the invention
and Figure 2 shows a plan view below the deck level along the line A-A
of the barge in Figure lo
In Figure 1, a working deck to accommodate marine drilling
equipment is shown as 1, devoid of any such equipment which is
conventional. A conventional moonpool, 2, generally circular in
cross-section, is shown by dotted lines and penetrates the barge
through both the deck and the marine hull bottom, 3. For simplicity of
illustration the barge hull is depicted as being a twelve sided regular
polygon in plan, as is readily seen in Figure 2~ However in preferred
embodiments of the invention a larger number of sides for the regular
polygonal shape is preferred, for example 2~ to 30, the larger number
providing a more nearly circular perimeter. The mar;ne hull of the
barge is composed of five main part~s, each having a perimeter that
approaches a true circular shape as the number of polygonal sides is
increased. Furthermore, three of the five main parts approach
truncated conical shape as their perimeters approach true circular
shape, their shape technically being frusto-pyramidal; for convenience
of description here;n they are referred to as truncated cone sections.
The ~irst of these parts is the apex-down truncated cone section formed
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by panels 4, on the top of which the deck rests, at least in part. The
sloping face of the panels is inciined to the horizontal at an angle of
from substantially 20 to substantially 55 degrees3 most preferrably
substantially 30 degrees~ The barge is ballasted so that the lowest
water line (W/L) on the hull is at least the normal ice thickness above
the bottom edge of these panels~ generally it is somewhat higher than
this, and generally not above the level of the mid point oF the panels,
to maintain appropriate freeboard. The second main part of the hull is
another apex-down truncated cone section, formed by panels 5~ depending
below panels 4 with their sloping face inclined to the horizontal at an
angle of from substantially 20 to substantially 65 degrees and in any
case an angle equal to or greater than that of the first truncated cone
section. The third main part of the hull is a vertical polygonal,
substantially cylindrical, section formed by panels, 6, depending below
panels 5. The fourth main part of the hull is an apex-up truncated
cone section formed by panels, 73 depending below panels 6 with their
sloping face inclined to the horizontal at an angle to the horizontal
of from substantially 40 to substantially 60 degrees. The ~ifth main
part of the hull is the bottom, 3, which is substantially flat below
the bottom of the panels 7 and completes the hull surrounding the
moonpool 2, which opens through it. A number of mooring lines, 8, are
attached to the floating barge, to maintain it in position for
drilling, and are attached directly to the bottom of the hull or, more
generally, via fairleads on the bottom of the hull to winches inside
the hull or on the working deck, with the lines being led inside
through the hull bottom to winches in a conventional manner. Because
the depth of the water, for use in which the drilling unit of the
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1 ~ 624~
present ;nvention is intended, is generally only a modest depth, the
mooring lines usually must run ou~ from the barge to their respective
anchor points on thè marine floor at very shallow angles. In any
event, the manner of attachment of the mooring lines to the barge can
be conventional, but the critical consideration is that, regarding the
poin-ts at w'nich the lines achieve their closest proximity to the hull,
either by attachment directly thereto or by passing therethr~ugh via
fairleads, 10, said points must be within the perimeter of the flat
bottom of the hull and preferrably surrounded by a substantially
circular guard member, 9, which extends, conveniently simply as a
substantially cylindrical wall9 below the flat bottom of the hull for
a distance that precludes moving ice from being trapped by the mooring
lines or between any mooring line and the flat bottom of the huilj to
achieve this desideratum the lowest edge of the guard member must be at
a depth below the water surface at least five times the normal
thickness of ice impinging against the hull, but not greater than
substantially 20 metres.
When a continuous ice cover, which may extend for many
kilometres in all directions around a floating barge of the present
invention~ begins to move and to encroach on the barge~ the Ice cover,
illustrated as 20 in Figure 1, is bent and forced beneath the water
surface, at W/L, until fracture of the ice occurs and pieces, such as s
21, break from the cover and are forced, by their inertia and the
moving, encroaching ice, to submerge under the adjacent hull, the
adjoining cover, or completely under the bottom of the barge. The
contour of the hull, formed by the wall section`panels ~,596, and 7,
forces encroaching ice to bend, submerge, and break off5 and compels
broken pieces such as 21, pushed forward by additional encroaching ice,
either to turn downwards and backwards3 ~hence to float up and
accumulate to some extent under the encroaching ice cover~ or to turn
downwards and thence aside to move around the hull or to float up and
accumulate under the ice cover beside the barge, where they move past
the barge under unbroken ice moving as part of the encroaching ice
cover. The panels 7 adjacent the bottom of the hull 3 form a deflector
wall or skirt which ensures that loose floating ice pieces such as 21,
which are being pushed downwards and generally towards the center line
of the barge by encroaching ice, are turned away from the hull and
generally do not submerge further. The depth to which loose ice is
likely to travel depends.on several factors9 including the thickness of
the impinging ice cover, the speed with which it is pushed by
j encroaching ice behind it, the depth to which loose ice has already
accumulated on the barge hull and under the ice cover adjacent the
barge, and the buoyancy of the ice tending to return it to the surface,
among others. In accordance with the present invention the flat bottom
of a barge hull is at a depth below the water line of at least four
times the normal thickness of ice impinging against the hull. To
2~ provide protection against fouling of the mooring lines by loose ice,
the present invention provides a guard member that surrounds the points
at which the mooring lines attach or come closest to the hull and
extends a distance below the hull bottom to place the lowest edge of
the member at a depth below the water surface at least five times the
normal thickness of ice impinging against the hull without extending to
a depth over substantially 20 metres. This guard member surrounds the
area in ~Ihich the mooring lines approach closest to one another; most
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conveniently it is in the form of a steel ring fastened to the hull
bottom, for example by welding, to d;vert loose ;ce mov;ng towards the
moonpool and mooring line convergence at depths below the water surface
substantially at least five ~times the normal thickness of the ice.
With~ for example~ Beaufort Sea ice thickness of two metres, the bottom
of the guard member would be at a depth of at least 10 metres below the
water l;ne, a depth adequate to provide the des;red protection of the
mooring lines against fouling by loose ice from ice cover substantially
two metres thick.
It is appreciated that, as a winter season progresses in
arctic areas, the normal thickness of ice impinging against any vessel
moored therein tends to ;ncrease significantly with the advent of
pressure ridges in the mov;ng ice cover; the pressure ridges can
increase dramatically in both khickness and frequency as winter
progresses, and make movement of the i~ce cover as threatening as an
iceberg with respect to a moored vessel. ~ However, for a considerable
time after initial freeze up and the formation of continuous ice cover~
for example for the time until the ice cover has developed to a
thickness of up to one metre or even two metres with ridge formation
manageable by auxiliary ice breakers, in water depthsg for example, of
20 to 70 metres9 the floating barge drilling units of the present
invention can easily remain in use and accommodate the random movement
of the ice cover. Within the ranges of hull angle sizes and wall
dimensions disclosed herein, stable drilling barges having a draft less
than 20 metres can readily be built. Such barges can safely and
readily be towed from one location to another in open water and
broken-ice covered water for drilling operations or for winter
~; harboring. By virtue of their ice-breaking and clearing ability,
i 1 ~ 2 ~ ~ ~
they are able to remain on location for drilling for a much longer
season than barges that must be moved when freeze-up starts; in
particular~ the normal Beaufort Sea drilling season from mid-August to
mid-October may be extended into January before ice cover thickness and
ridging make it necessary to suspend drilling and move to harbor until
the next summer season.
It will be obvious to those skilled in the art that numerous
variations may be made in the specific embodiment described without
departing from the scope of the invention claimed herein. Thus the
polygonal hull plan may be covered by a deck that is simply round or
rectangular in plan, with portions of such deck cantilevered over the
top Of the polygonal hullo The hull may be braced and/or
compartmentalized in its interior in any appropriate manner desired,
for example for ballasting, its essential features for the present
invention being merely as set forth herein with respect to its exterior
configuration. The moonpool size and configuration can be
conventional, for example a round moonpool of diameter one-sixth to
one-twelfth of the deck diameter is appropriateO The number of mooring
lines used to hold the barge in place can vary, depending on the ice
2n load it is intended to resist. They are, of course, most
advantageously evenly distributed around the perimeter of the hull,
with generally between eight and sixteen in number being adequate. The
number selected is minimized to reduce risk of fouling by the ice sheet
but at the same time provide adequate hold;ng power with available
mooring line size.
Numerous modifications can be made in the various expedients
described without departing from the scope of the invention ~Ihich is
defined in the following claims.
I
