Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Method and Apparatus for Towin~ Iceberqs
Back~round of the Invention
This invention relates to a method and apparatus for the
towing of icebergs.
Offshore oil activity in iceberg infested waters exposes
marine facilities and vessels to a risk of iceberg damage.
Offshore platforms, drillships and support vessels can be
damaged by iceberg collision. Subsea installations such as
pipelines, wellheads and moorings can be damaged by the
scouring action of icebergs passing over them. Routine marine
operations can be disrupted by the presence of icebergs in the
oilfield area.
Iceberg towing is a means of reducing the effect of
icebergs on offshore oilfield operations. In a towing
lS procedure a boat, usually a tugboat, attempts to divert the
trajectory of an iceberg by pulling it with an attached line
or hawser. Many proposed methods for the towing of icebergs
have been suggested and a small number of these have been
evaluated experimentally. Only one towing method, that of
encircling an iceberg with a floating polypropylene towline
pulled by a tugboat, has found acceptance in routine offshore
operations. In this method, known as the conventional iceberg
towing method, the ends of the encircling polypropylene
towline are shackled to a steel towing hawser wound out from a
winch on the towing vessel. The polypropylene towline is
pulled tight against the rear face of the iceberg as the
vessel assumes its required heading and engages the tow.
There has been little subsequent advance in iceberg towing
since this procedure was first proven off the coast of
Newfoundland in 1974.
There are several important advantages to this
conventional towing method including the fact that it can be
carried out in adverse weather and requires only simple
equipment and limited training of personnel. An important
safety advantage of this approach is that it eliminates the
necessity of closely approaching the iceberg to be towed. On
the negative side, the tow success rate of this method is
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impaired by the potential loss of towing attachment. This
loss of attachment is due to the tow rope slipping over the
top of the iceberg, either because the iceberg is smooth and
rounded with no grooves or protrusions on which the towline
could register, or because the iceberg rolls in the water and
dumps the towline. If the loss of attachment occurs while the
towline is stretched under tension the line will snap back
towards the vessel and frequently become snarled. When
attachment is lost, reconnection is often slow and difficult.
The steel towing hawser must be winched on board, one end of
the polypropylene tow rope must be freed and the rope
redeployed around the iceberg. The delay associated with
reattachment may be critical when icebergs are in close
proximity to oilfield installations.
Several techniques have been tried to overcome problems
with rope slippage and loss attachment in iceberg towing. One
technique has been to use nets to enclose the iceberg rather
than simply encircling it with a rope. As well, attempts have
been made to apply the towing force to the iceberg at a point
below its water line. This minimizes the rolling moment
induced on the iceberg by the towforce and thus reduces the
likelihood of rope slippage caused by iceberg rolling. In
both cases, however, the hardware required to implement the
concepts has proven cumbersome and difficult to handle at sea.
There have also been attempts to directly attach a towing
device to an iceberg. With this arrangement, the towing
device is embedded into the iceberg or connected to the
iceberg surface such as to have an attachment strong enough to
bear the full towing force, even if the iceberg should roll.
Among direct attachment mechanisms there may be mentioned
suction cup devices, thermally embedded ice anchors, anchors
embedded into the iceberg by drilling below the waterline with
a submerged remotely operated vehicle (ROV), etc. All of
these towing methods have met with very limited success as all
have required the use of either a helicopter or ROV or they
have required the direct boarding or very close approach to
the iceberg to be towed. No direct attachment device suitable
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for routine use from an unassisted workboat in adverse weather
and sea conditions has to date been developed, and the
handling and logistics problems to be overcome before such a
system could be considered functional are very great.
At the present time the conventional iceberg towing
method is the best available. It is, therefore, the object of
the present invention to provide an improved system which
retains the safety and ease of operation of the conventional
technique while decreasing the number of unsuccessful tows
caused by towline slippage.
SummarY of the Invention
This invention relates to an improved conventional
iceberg towing system. With the system of the present
invention the towline is provided with a large, neutrally
buoyant mass which strongly resists any force tending to lift
it from the water.
In conventional iceberg towing, the tow force is applied
to a floating towline encircling the iceberg and is
transmitted to the rear face of the iceberg, acting at or
about the waterline. Loss of connection occurs when the
towline slips over the iceberg either because the iceberg is
smooth and the towline fails to register at the waterline and
slides upward or because the iceberg rolls. Since the centre
of mass of the iceberg lies well below the waterline the
towforce generates a large rolling moment. In conventional
iceberg towing, loss of connection always starts with the
towline increasing its elevation relative to its initial
position near the waterline.
This problem is overcome in the present invention by
providing a towline of very large neutrally buoyant mass.
When the towline has been weighed down sufficiently the line
cannot begin to slip upwards on the iceberg until enough tow
force has been applied to lift the added weight. The weight
of the towline used to prevent loss of connection is
determined by the available towforce, iceberg roll stability,
and the degree of slope of the rear facing surface of tne
iceberg at the waterline.
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According to a preferred embodiment of the invention, the
weight necessary to keep the towline at or near the waterline
is provided by seawater entrapped in containment bags. With
this method, the mass of the seawater moving with the towline
assembly is high while the dry weight of the towline assembly
remains low to facilitate handling during deployment and
recovery.
Thus, in accordance with the improved system according to
the preferred embodiment of the invention, a number, e.g. 2 to
5, of large discrete water filled bags are attached to the
towline by shackles at evenly spaced intervals. These bags
are spaced in accordance with the observed size of the iceberg
to be towed such that the distance between the outermost bags
approximately equals the-width of the rear face of the
iceberg.
The bags are adapted for filling with water at time of
deployment and for emptying of water when they are retrieved.
In another embodiment of the invention the bags are left
filled with water and are moored in the ocean between uses.
Filling of the bags can be achieved by pumping them full of
seawater with a high capacity pump or by towing them behind
the tow vessel such that water flows into and expands the bag.
In a preferred embodiment of the invention the bags are
configured to give a self-filling action upon immersion in the
ocean. This action can be provided using a bag open to water
at the top in combination with top end flotation, bottom end
ballasting and one or more wall stiffening members to expand
the bag to its full capacity.
In a preferred embodiment of the invention a recovery
strap is attached to the bottom of each water bag for the
purposes of emptying it of water at time of retrieval. The
strap is preferably extended to a point on the towline at an
appropriate position ahead of each bag. Then, as the towline
is being retrieved, the recovery strap reaches the stern of
the vessel before the bag does. When the recovery strap is
retrieved on deck it is attached to an auxiliary winch and
pulled in separately from the towline. Tnis upends the bag,
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causing the water to be dumped out and at the same time
pulling the bag onto the deck. Once the bag is brought on
deck, it is disconnected from the towline and can then be
stored until next needed.
According to a further embodiment of the invention,
instead of a number of discrete water filled bags, a single
long cylindrical containment bag can be used in the manner of
a boom which is filled with seawater. This long containment
bag is attached to the towline at several points along its
length. This long cylindrical containment bag can be filled
by pumping it full of seawater with a high capacity pump or by
towing it behind the tow vessel such that water flows into and
expands the elongated bag. Restrictions along the bag can be
used to control the flow of water along the length of the bag
when the bag is subjected to asymmetrical lifting or towing
forces. This embodiment would be well suited for mooring at
sea between tows, in which configuration it would require less
frequent filling and draining.
According to another embodiment of the invention, an
iceberg towing boom can be assembled from rigid containers
filled with ballast sufficient to make them approximately
neutrally buoyant. As for example, metal or plastic
containers filled partly with concrete or rocks and partly
with air. For example, the containers can be made from a
rigid plastic, such as polyethylene. The containers could be
spaced along a rope or cable forming a boom which could be
assembled at a shore location and then towed when needed to an
offshore oil location. The flexible boom would be deployed
around an iceberg in a manner similar to that used to deploy a
conventional polypropylene towline. The boom could be
constructed sturdily to resist storm damage or ice contact
damage and would be left in the ocean for extended per~ods
during which a potential iceberg towing requirement existed.
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Brief Description of the Drawinqs
Figure 1 is a schematic illustration of a towing system
for icebergs;
Figures 2a and 2b are schematic illustrations of towing
forces on an iceberg;
Figure 3 is a pictorial view of a towing arrangement
according to the invention;
Figure 4 is a pictorial view of a further embodiment of
the invention; and
Figures 5a-5d are schematic illustrations of a self
filling water bag arrangement.
Figure 1 shows a typical conventional towing system for
icebergs. For this purpose the iceberg 10 is pulled by a
thick polypropylene towline 11 and a typical towline for this
purpose is one about 12cm diameter and 800m long. The ends of
the towline 11 connect together by a shackle 12 which is
connected to a heavy duty steel towing hawser 13 which is
wound on to the main winch of an anchor handling tug 14.
With this technique, a marker buoy 16 is positioned and
is connected by a buoy line 15 to towline lla. The tug 14a
then moves in the direction shown around the iceberg 10 while
paying out the polypropylene towline from the rear of the
vessel. The tug continues around the iceberg 10 to locations
14b and 14c and then continues back to the location of the
marker buoy 16 where the ends of the towline are connected
together by the shackle 12. The tow vessel 14 then pays out
additional steel hawser 13 from its main winch until it is a
safe distance from the iceberg 10. At this time the tug is
ready to commence the tow.
Figure 2a shows the typical problems encountered in a
conventional iceberg tow. The towline floats on the surface
of the water 19. Here, the iceberg 10 is shown with a sloping
registration surface 20. It can be seen that the towing force
2la parallel to the waterline acting against the surface of
the iceberg creates a tangential force 21b tending to lift the
towline out of the water and upwards along the surface of the
iceberg. Furthermore, the tow force imparts a rolling moment
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21c about the centre of the iceberg 10a. The problem with
this arrangement as described above is the ease with which
attachment to the iceberg is lost either by the towline
slipping upwards along the surface or the iceberg rolling.
Figure 2b shows the action of the present invention when
used as an enhancement to conventional towing. Here it can be
seen that the tangential component of the force 2ld exerted by
the waterbag 24 opposes the upward tangential force 21b. This
prevents the towline from slipping further upwards and reduces
the rolling moment 21c applied to the iceberg 10.
Figure 3 shows an arrangement according to the present
invention in which water bags 24 are connected to towline 11.
These water bags 24 are quite large, each typically weighing
15 tons or more. Two to five of these bags are typically
positioned along the rear face of an iceberg. Each bag 24 is
held by strapping 23 connected to a mounting collar 22 on the
towline 11. Flotation collars 25 are also positioned on the
towline on each side of collar 22.
The bottom end of each water bag 24 contains ballast
weights in the form of dense metal or other heavy material.
Connected to the bottom of each bag 24 is a recovery strap 26
which is connected at its free end 27 to the towline 11. This
strap 26 is typically about 25 meters long and is connected at
a significant distance from the bag attachment point 22.
During hauling of the towline 11 the recovery strap 26 reaches
the stern of the tug well before the bag 24. The strap 26 is
long enough to be pulled well forward on the work deck so as
to permit the crew to unshackle it from a mid deck position
rather than near the stern. While the strap is being freed,
the recovery of the towline is paused. Once free from the
towline the recovery strap is wound in with an auxiliary
winch, upending the bag and dumping its contained seawater
while at the same time pulling it onto the deck. Once on the
deck, the bag is disconnected from the towline and is stored
until the next tow. This procedure is followed for each bag
until all are on board.
An elongated bag arrangement is shown in Figure 4, with
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the elongated bag 31 mounted from the towline 11 by mounting
straps 30. These mounting straps 30 are connected by shackles
34 to straps 35 surrounding the elongated water bag 31. The
end of the water bag 31 includes a water inlet/outlet valve
32. Restrictions located at points 33 prevent water from
sloshing along the length of the bag under asymmetrical
lifting or towing forces.
Figure 5a-5d shows the action of a self-filling water
bag. In Figure 5a the bag 24 is in its fully collapsed
position at time of initial deployment in the ocean. Figure
5b shows the bag starting to fill with water which pours in
through the opening 38 which is below the water surface.
Filling action is provided by the force of the ballast weight
36 sinking and pulling against the buoyancy of the flotation
collars 25. This action stretches out the bag increasing its
capacity and causing it to pull in water. The wall stiffening
member 37 keeps the sides of the bag from collapsing during
filling and ensures the bag fills quickly with its maximum
capacity of water. Figure 5c shows the bag partially filled
with water 39 flowing in opening 38, while Figure 5d shows the
fully filled water bag.
