CA1044079A - Iceberg towing system - Google Patents
Iceberg towing systemInfo
- Publication number
- CA1044079A CA1044079A CA269,728A CA269728A CA1044079A CA 1044079 A CA1044079 A CA 1044079A CA 269728 A CA269728 A CA 269728A CA 1044079 A CA1044079 A CA 1044079A
- Authority
- CA
- Canada
- Prior art keywords
- line
- iceberg
- tow
- length
- load line
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
- B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING
- B63B35/00—Vessels or similar floating structures specially adapted for specific purposes and not otherwise provided for
- B63B35/08—Ice-breakers or other vessels or floating structures for operation in ice-infested waters; Ice-breakers, or other vessels or floating structures having equipment specially adapted therefor
- B63B35/086—Vessels for displacing icebergs, or related methods
Abstract
Abstract of the Disclosure A method of towing an iceberg comprising paying out a floating tow line from a vessel as the vessel approaches and passes around an iceberg, the tow line having a length of negatively buoyant load line and a length of yieldable line shorter than the load line attached together in parallel as an intermediate section of the tow line, catching the end of the tow line initially payed out, bring the length of the yieldable line into contact with the rear of the iceberg, by drawing both ends of the two lines, while allowing the load line to sink against a submerged portion of the iceberg, pulling the ends of the tow line in a direction away from the iceberg to cause the yieldable line to yield, and drawing the load line after yielding of the yieldable line tautly against the submerged portion of the iceberg, so as to tow the iceberg by a force exerted from the tow line via the load line to the iceberg.
Description
This invention relates to a new towing method for diverting icebergs and to an unique towing apparatus used in .:
- practicing the towing method. -.
With increased sub-sea oil exploration in oceans which experience iceberg activity, it becomes highly important to avoid accidents caused by the ice fragments. For example, -one of the most promising sectors of the offshore resource area lies in the northern Labrador region where icebergs and extreme weather conditions-combine.to present.hazardous envi-r.onmenta~
constraints.upon any offshor.e.~peration. .In addition to posin-g: ,.;.
--- technical problems, these environmental factors increase.the mobilization cost of such operations by restricting drilling activity to the.period between June.and November..:
Icebergs represent the mos~ unique of these environ- ¦~
..
mental hazards. Ranging between l0 tons~-and-100 mil-l-ion-tons, - ~
the glacier ice fragments drift through prime potential oil ~...... ... ~ .
, .
and gas acreage-, with peak-flux in May-j June and July. .:
As well as presenting tactical hazards during explora-tion, the unique and seemingl~v random iceberg incidence demands ~.
20 new approaches to both the development and production phases , of offshore exploration. To cope with the potential hazard, ~
one or more of the following strategies are usually considered: ;
(a) strengthening or reinforcement of subsea man-made structures .~.
(i.e., designs to withstand iceberg impact); (b) avoidance . .
, , (sub-sea completions, tunnelling or the use of manoeuverable ;
drill ships); (c) diversion (towing); (d) demolition (iceberg ~
draft reduction). `::
i'.' -' One of the mo~t common strategies for preventing :
iceberg collisions, both for an oil and gas drilling platform ~!
30 and with the well head on the seabed, is avoidanceO While this . .
can mean disconnection of drill ships and simply moving off : .
site, it involves disconnection of the Blow Out Preventer stack .. ....
..^.
~L
- 1 - ~F~ ~'.'' '',.~
7"~
and for semisubmersibles, 'breaking of anchor lines. Diversion or towing of the icebergs is often used to minimize the 'I
necessity of resorting to the scheme of avoidance.
Iceberg towing, originally believed to have been demonstrated off the coast of Newfoundland, is now a routine operation. However, recent experience has shown that the towing techniques presently being used are not satisfactory '' for certain classifications of icebergs. Towing involves ; ;
attaching a typically four~-inch-test polypropylene-rope -10 -iaround the'iceberg at the waterLine:-and applying between;
- 10,000 and-70,000 pounds~force~to~direct -it away~from'a - -drilling operation and into currents' which will draw it '~
safely-around the drill ship.
The method fails, however,-for smaller, unstable and/or dome shaped-~icebergs.~- Either the-~smaller bergs---caps-ize-- --- - ' when the load is applied at the waterline or, in the case of the domed bergsj the towing hawser slips over-the top of the berg.' In some such circumstances, drilling must be suspended ' and, the second strategy, that o~ avoidance is resorted to. -' When, as is sometimes the case, a drilling platform is forced off site quickly in-the technique of avoidance, damage to the drill stem can cause very expensive delays. ';~
Moreover, if the draft of the iceberg is typically within ¦~
about forty feet of the sea bottom, the wellhead must be cemented in for protection during the iceberg pass. Thus, for either drill ships or semisubmersible platform drilling, the iceberg towing failures can result in expensive down time ~
associated with the avoidance option. ~ ;
Until now it has been impossible to tow the full 1' 30 spectrum of icebergs encountered. This limitation is overcome ¦ , by the present invention which provides for the towing force being applied close to the centre of mass of the berg, thus ..
~: ,. -. , .. , . , :.-Q7~ -avoiding the capsizing of the berg or slippage of the hawser over its top. Before describing the invention in detail, it is important to appreciate the scale of operations and the `~
interrelationship between the mobilization portion of the towing and the actual tow itself. Even small icebergs of less than 100,000 tons are massive in comparison to the towing vessel and, in general, the icebergs towed are at least ten times the mass of the towing vessel. Moreover, even the smallest iceberg-chunks of the order of-five-t~ ten tons consti-tute an operational--haza-rd *~ the-=èxposed--s-i-de~o-f drill-ships. From this,~ we see~-that-any-`-efficient towing- - ~
technique must be equally effective for`icebergs with weights ranging over many decades.
In addition to having the above range capacity `
requirement,- any towing apparatus must be able~~to~be quickIy deployed and recovered. Because of the relative size of the iceberg to the drill ship, this means quickly circumnavigating the iceberg while paying out one-half mile of necessarily -bulky gear as fast as possible. A net-like apparatus, while ensuring a secure tow, is impractical because of mobilization difficulties (tangling, etc.). The system which is described in the following paragraphs overcomes the above deployment and towing constraints.
The inventive method of towing the iceberg is com- !': .
prised of the following steps. A floating tow line is payed `
.~
out from a vessel as the vessel approaches and passes around the iceberg. The tow line has a length of negatively buoyant ;~
load li~e and a length of yieldable line shorter than the load line attached together in parallel as an intermediate section of the tow line. The end o~ the tow line which was initially payed out is caught after the vessel has passed completely around the iceberg. The length of yieldable line -., i". . , I ' ' ~ Q ~5 is then brought into contact with the rear of the iceberg by drawing both ends of the tow line, while the load line is allowed to sink against a submerged portion of the iceberg.
The ends of the tow line are then pulled in the direction away from the iceberg to break or stretch the yieldable line to the maximum extent allowed by the load line, while drawing the load line tautly against the submerged portion of the -iceberg. In this manner the iceberg can be towed by the force exerted ~rom the tow line via the load line-to the 10 - iceberg, but at a submerged positionj;closer to the cente-r _-~
of--mass of the iceberg than-would otherwise be provided~by ------a force exerted at the surface.-- Preferably the yieldable ;
line is made of elastomeric material.
A further aspect of the invention is the iceberg towing configuration which-is compr-ised--of- a--pair--of tow lines, one (front) end of each being adapted for attachment to a towing means, such-as the winched steel--tow-line of a tender vessel, and a length of yieldable line joining the other (rear) ends of the tow lines. A length of load line joins the same rear ends of the tow lines, the load line being longer than the yieldable line in its substantially unstrained condition.
Where the yieldable line is fabricated of elastomer, the load line should be shorter than the length of elastomer when stretched to its failure length. Preferably, the load line is about 150% of the length of the elastomer in its unstretched condition. -~
The term yieldable line has been used generica~ly to denote a length of line which has one of two cha~acteristics:
either elastomeric properties which allow it to regain its original length after stretching, or the ability to break when stressed during the application of force on the tow line when it is wrapped around a portion of an iceberg. Obviously, : ....... . , ~ . , . . , :
.':
the line which breaks cannot be reused and must be replaced;
nevertheless, the basic principles of the invention will be operat.ional upon its use as with the elastomer line.
Presently the preferred form of the invention uses an elastomer line. Consequently, the description below will be directed to that embodiment, although it should be -.
understood that the yieldable line which breaks is intended :.
as a second form of the invention. The~breaking yieldable `~.
line.:.can have one or more..short..chipped..in sections which~
10 -are analogous to one or more.-fusible links, rather ~han-be .`.
completely made of breakable:material.. -The-fuse section can - . .-be replaced later for- reuse of the-yieldable line, if desired. ~:
A better understanding of the invention will be ;:~ -obtained by reference to the-description-below,--and-the-~ -.
following drawings, in which~
Figure 1 is a view of a number of sections of the ' iceberg towing apparatus, Figure 2 is a plan view of the apparatus in storage `.
on a ship, and . -20 Figures 3, 4 and 5 depict in perspective the position .
. ~ ; ., and condition of the apparatus when placed in position and when the iceberg is being towed. . .
Referring now to Figure 1, the apparatus is shown ~.. :.
having tow lines 1 and 2. Connected as an intermediate section . .
of the tow line is elastomer 3.. In parallel with the elastomer, ...
a load line 4 is attached to the ends of the respective tow lines 1 and 2. Preferably the ends of the elastomer and loa~
line are connected to the respective tow lines by means of swivels and shackles 5 and 6. The swivels and shackles allow :;.
orientation of the elastomer and load line along various contours of the iceberg without tangling, and will also allow a certain amount of twisting of t~e relative part~ while avoiding 0~7q~
tangling.
The relative portions of the towing apparatus will be stored aboard ship as shown in Figure 2 and it has been '' found most convenient to store the parts separately. On ' board ship each tow line is stored along the length of the boat 7. The load line 4 and elastomer 3 can be stored along the axis of the boat as shown. A steel towing cable 10 is '~
shown connected to one end of the tow line 1, over a winch 11. ';
Before the tow,'the load 'line '4-and elastomer 3 can~jointly~ ~-be shackled into the mid section of the separated--tow li~es.
10 In this manner, all components can be easily-stored aboard '~
ship with minimal probability of tangling, yet payed out smoothly in proper sequence by the wlnch.
Referring now to Figures 3, 4 and 5~j Figure 3 depicts a vessel 7 which has already released the towing apparatus ~`
as it passed iceberg 8. The apparatus may be seen to be comprised of tow lines 1 and 2 and elastomer 3, joined at swivel and shackles 5 and 6. The two ends of the tow line are connected to towing cable 11. The tow line and elastomer ' would normally have positive buoyancy in order that they can float at the surface of the sea around the iceberg.
A load line 4, having negative buoyancy, hangs below elastomer 3, and is coupled to swivel and shackles 5 and 6. In Figure 3,' the vessel 7 has not yet begun drawing the tow lines.
In Figure 4, the vessel 7 has begun drawing tow lines 1 and 2 around iceberg 8. The elastomer 3 has drawn 1~
up to the iceberg ~ut is not yet stretched to any great '~' extent. The load line 4 is in contact with the iceberg and has seated itself below a crag section 9. 1~-In figure ~k the vessel 7 is towing the iceberg 80 The tow lines 1 and 2 have pulled the load line 4 taut, having ; . , . ,~i ~ ,~ .. : ..
. : , .; . . ..
:. :., .,., ~ ~ .: , .
9/7~
been caught on crag 9. The elastomer 3 has stretched to accommodate the towing stress bein~ applied directly from tow .
lines 1 and 2 to load line 4.
:: .
Where instead of elastomer 3 a breakable line has been used, upon the tow line being pulled taut, the breakable `
line breaks and as with the elastomer, towing stress is applied directly from the tow lines to the load line. The breakable .
line should be designed to break with about 1000 pounds of ~
, .. .
stresC~ typically.
: It is:thus.clear--that the load line-4-exert~,-towing pressure ~n-iceberg~8~substantially .below;the surface-i-of.-the sea, arbitrarily closer to the centre of mass of the iceberg .
than should it have been applied at the surface as with prior towing techniques. .
It is preferred-that the elastomer-be made-,of-natural ': ..
rubber compound with a specific gravity of 0.98. It has been found that this form of elastomer expands to.about 150% of its unstressed length with approximately 170 pounds per square .
inch tension. WheA the cross section of the elastomer is .`.
20 one inch thick by six inches wide, approximately lC,00 pounds .
tensile stress will be applied to a natural rubber elastomer :... -for the lS0~ extension.
For the e~,bodiment of yieldable line which was a breakable or fusible section, positively buoyant polypropelyne . .
rope can be used which has a narrow diameter clipped iF'i (or `d shackled) short section at its mid point. The narrow diameter section can be, for example l/2 to 1 inch in diameter, in . :
order that it break at suitable tensi~n, say 1000 pounds.
It is preferred that the load line be formed of nylon 30 because its physical properties are uniquely well suited to ...
this applicati,on, although other materials which suit the .
purpose could be used. Its density is only slightly negatively -- 7 -- ..
... . . . . . . . . . . . .. ... . .
;~
buoyant, having a specific-gravity of about 1.14, so that a minimum of positive buoyancy is required at its junction points :.
with the tow lines.
In addition, the strength characteristics of bulk ~`.
nylon are such as to minimize hysteresis so that elastic recovery is almost complete after each tow. Nylon does not immediately return to its original length, but creeps back slowly. However, it almost instantly recovers approximately `
50% of:the stretch imparted.and wi~l, during:the fi~st -: ;
10 twent~-fourJhours,:-recover--a-iotal--~Of approximate:ly-85~- o~- ~f the amount-of--the s.tretch...
Generally, high and oftentimes phenomenal abrasion -.
resistance is experienced when using nylon hawsers. Both spun nylon and continuous filament yarn type nylon have been found to be substantially abrasion resistant, with the former, spun nylon, being more abrasion resistant than continuous filament fabric of equal weight. Thus, the natural pliability, inherent toughness and flex resistance all contribute to low wear against the ice surface. Nylon will stretch under load to -~
approximately 115% of its original length. Failure will not normally occur, however, until it has been stretched to its -125% original length. In the present application, the nylon operates well below the 125% mark, where the stress-strain .
curve is linear. The extendability of the nylon section then is insignificant compared with that of the elastic rubber section.
Unstrained, the rubber elastomer section is preferably approximately half the length of the nylon section so that, from the geometry, the rubber section will never need to be stretched beyond twice its unloaded length.
Steel cable can also be used as the load line, although its strength per ~nit weight is not as great as nylon, -- 8 -- .
,. .
711~
and it is somewhat more difficult to handle.
It is preferred that the tow lines be fabricated of polypropylene rope, which is positively buoyant. Preferably each tow line is about 1000 feet long, and the load line is about 500 feet long, although these dimensions can vary !~'.
considerably, depending on the maximum and minimum sizes of iceberg expected to be towed. The polypropylene rope is preferably 3 to 4 inches in thickness, although other thicknesses can be-used depending on the expected~tension.
- 10 The small--spherica-l buoys--(not shown but similar to 1,~
those commonly used in-fishing nets) can be used to-offset~
the weight of the shackles marked 5 and 6. These standard shackles are usually either stainless steel or cast iron with -.;
a strength--rating in the order of 25-tons. The buoyancy- at-- -these two shackle-points (and elsewhere if found-necessary) is adjusted to maintain the "floating" members 1, 2 and 3 '-' : ' in a near surface position.
If the tow line is not positively buoyant, then such floats could be attached thereto, and furthermore if the load line is not negatively buoyant, ballast can be attached thereto. ~owever floats and ballast should be used only when necessitated by field conditions since they make handling r~
during deployment and recovery more difficult.
It can be seen that the inventive configuration combines the three essential characteristics required for a successful improvement over the existing towing equipment and procedure. These are: deployability to substantially the full range of ice~erg sizes, and compatability with the existing equipment and procedures.
- Icebergs of widely varying cross-section can be accomodated by increasing the lengths of the elastomeric and load line sections while maintaining their relative lengths.
:.
_ g _ Finally, this configuration overcomes the adaptability problem common to all systems that use two or more parallel load bearing memhers at the same time. Such geometrics depend on load distributions that cannot be assured because of the .
unknown iceberg geometry -- especially the below water shape.
This problem is overcome by the present structures and method ~:
which apply the main load to the sîngle member below the water line.
--- --10 i -- -~
, .
-~",'' ' ;' ....
-- 1 0 -- '
- practicing the towing method. -.
With increased sub-sea oil exploration in oceans which experience iceberg activity, it becomes highly important to avoid accidents caused by the ice fragments. For example, -one of the most promising sectors of the offshore resource area lies in the northern Labrador region where icebergs and extreme weather conditions-combine.to present.hazardous envi-r.onmenta~
constraints.upon any offshor.e.~peration. .In addition to posin-g: ,.;.
--- technical problems, these environmental factors increase.the mobilization cost of such operations by restricting drilling activity to the.period between June.and November..:
Icebergs represent the mos~ unique of these environ- ¦~
..
mental hazards. Ranging between l0 tons~-and-100 mil-l-ion-tons, - ~
the glacier ice fragments drift through prime potential oil ~...... ... ~ .
, .
and gas acreage-, with peak-flux in May-j June and July. .:
As well as presenting tactical hazards during explora-tion, the unique and seemingl~v random iceberg incidence demands ~.
20 new approaches to both the development and production phases , of offshore exploration. To cope with the potential hazard, ~
one or more of the following strategies are usually considered: ;
(a) strengthening or reinforcement of subsea man-made structures .~.
(i.e., designs to withstand iceberg impact); (b) avoidance . .
, , (sub-sea completions, tunnelling or the use of manoeuverable ;
drill ships); (c) diversion (towing); (d) demolition (iceberg ~
draft reduction). `::
i'.' -' One of the mo~t common strategies for preventing :
iceberg collisions, both for an oil and gas drilling platform ~!
30 and with the well head on the seabed, is avoidanceO While this . .
can mean disconnection of drill ships and simply moving off : .
site, it involves disconnection of the Blow Out Preventer stack .. ....
..^.
~L
- 1 - ~F~ ~'.'' '',.~
7"~
and for semisubmersibles, 'breaking of anchor lines. Diversion or towing of the icebergs is often used to minimize the 'I
necessity of resorting to the scheme of avoidance.
Iceberg towing, originally believed to have been demonstrated off the coast of Newfoundland, is now a routine operation. However, recent experience has shown that the towing techniques presently being used are not satisfactory '' for certain classifications of icebergs. Towing involves ; ;
attaching a typically four~-inch-test polypropylene-rope -10 -iaround the'iceberg at the waterLine:-and applying between;
- 10,000 and-70,000 pounds~force~to~direct -it away~from'a - -drilling operation and into currents' which will draw it '~
safely-around the drill ship.
The method fails, however,-for smaller, unstable and/or dome shaped-~icebergs.~- Either the-~smaller bergs---caps-ize-- --- - ' when the load is applied at the waterline or, in the case of the domed bergsj the towing hawser slips over-the top of the berg.' In some such circumstances, drilling must be suspended ' and, the second strategy, that o~ avoidance is resorted to. -' When, as is sometimes the case, a drilling platform is forced off site quickly in-the technique of avoidance, damage to the drill stem can cause very expensive delays. ';~
Moreover, if the draft of the iceberg is typically within ¦~
about forty feet of the sea bottom, the wellhead must be cemented in for protection during the iceberg pass. Thus, for either drill ships or semisubmersible platform drilling, the iceberg towing failures can result in expensive down time ~
associated with the avoidance option. ~ ;
Until now it has been impossible to tow the full 1' 30 spectrum of icebergs encountered. This limitation is overcome ¦ , by the present invention which provides for the towing force being applied close to the centre of mass of the berg, thus ..
~: ,. -. , .. , . , :.-Q7~ -avoiding the capsizing of the berg or slippage of the hawser over its top. Before describing the invention in detail, it is important to appreciate the scale of operations and the `~
interrelationship between the mobilization portion of the towing and the actual tow itself. Even small icebergs of less than 100,000 tons are massive in comparison to the towing vessel and, in general, the icebergs towed are at least ten times the mass of the towing vessel. Moreover, even the smallest iceberg-chunks of the order of-five-t~ ten tons consti-tute an operational--haza-rd *~ the-=èxposed--s-i-de~o-f drill-ships. From this,~ we see~-that-any-`-efficient towing- - ~
technique must be equally effective for`icebergs with weights ranging over many decades.
In addition to having the above range capacity `
requirement,- any towing apparatus must be able~~to~be quickIy deployed and recovered. Because of the relative size of the iceberg to the drill ship, this means quickly circumnavigating the iceberg while paying out one-half mile of necessarily -bulky gear as fast as possible. A net-like apparatus, while ensuring a secure tow, is impractical because of mobilization difficulties (tangling, etc.). The system which is described in the following paragraphs overcomes the above deployment and towing constraints.
The inventive method of towing the iceberg is com- !': .
prised of the following steps. A floating tow line is payed `
.~
out from a vessel as the vessel approaches and passes around the iceberg. The tow line has a length of negatively buoyant ;~
load li~e and a length of yieldable line shorter than the load line attached together in parallel as an intermediate section of the tow line. The end o~ the tow line which was initially payed out is caught after the vessel has passed completely around the iceberg. The length of yieldable line -., i". . , I ' ' ~ Q ~5 is then brought into contact with the rear of the iceberg by drawing both ends of the tow line, while the load line is allowed to sink against a submerged portion of the iceberg.
The ends of the tow line are then pulled in the direction away from the iceberg to break or stretch the yieldable line to the maximum extent allowed by the load line, while drawing the load line tautly against the submerged portion of the -iceberg. In this manner the iceberg can be towed by the force exerted ~rom the tow line via the load line-to the 10 - iceberg, but at a submerged positionj;closer to the cente-r _-~
of--mass of the iceberg than-would otherwise be provided~by ------a force exerted at the surface.-- Preferably the yieldable ;
line is made of elastomeric material.
A further aspect of the invention is the iceberg towing configuration which-is compr-ised--of- a--pair--of tow lines, one (front) end of each being adapted for attachment to a towing means, such-as the winched steel--tow-line of a tender vessel, and a length of yieldable line joining the other (rear) ends of the tow lines. A length of load line joins the same rear ends of the tow lines, the load line being longer than the yieldable line in its substantially unstrained condition.
Where the yieldable line is fabricated of elastomer, the load line should be shorter than the length of elastomer when stretched to its failure length. Preferably, the load line is about 150% of the length of the elastomer in its unstretched condition. -~
The term yieldable line has been used generica~ly to denote a length of line which has one of two cha~acteristics:
either elastomeric properties which allow it to regain its original length after stretching, or the ability to break when stressed during the application of force on the tow line when it is wrapped around a portion of an iceberg. Obviously, : ....... . , ~ . , . . , :
.':
the line which breaks cannot be reused and must be replaced;
nevertheless, the basic principles of the invention will be operat.ional upon its use as with the elastomer line.
Presently the preferred form of the invention uses an elastomer line. Consequently, the description below will be directed to that embodiment, although it should be -.
understood that the yieldable line which breaks is intended :.
as a second form of the invention. The~breaking yieldable `~.
line.:.can have one or more..short..chipped..in sections which~
10 -are analogous to one or more.-fusible links, rather ~han-be .`.
completely made of breakable:material.. -The-fuse section can - . .-be replaced later for- reuse of the-yieldable line, if desired. ~:
A better understanding of the invention will be ;:~ -obtained by reference to the-description-below,--and-the-~ -.
following drawings, in which~
Figure 1 is a view of a number of sections of the ' iceberg towing apparatus, Figure 2 is a plan view of the apparatus in storage `.
on a ship, and . -20 Figures 3, 4 and 5 depict in perspective the position .
. ~ ; ., and condition of the apparatus when placed in position and when the iceberg is being towed. . .
Referring now to Figure 1, the apparatus is shown ~.. :.
having tow lines 1 and 2. Connected as an intermediate section . .
of the tow line is elastomer 3.. In parallel with the elastomer, ...
a load line 4 is attached to the ends of the respective tow lines 1 and 2. Preferably the ends of the elastomer and loa~
line are connected to the respective tow lines by means of swivels and shackles 5 and 6. The swivels and shackles allow :;.
orientation of the elastomer and load line along various contours of the iceberg without tangling, and will also allow a certain amount of twisting of t~e relative part~ while avoiding 0~7q~
tangling.
The relative portions of the towing apparatus will be stored aboard ship as shown in Figure 2 and it has been '' found most convenient to store the parts separately. On ' board ship each tow line is stored along the length of the boat 7. The load line 4 and elastomer 3 can be stored along the axis of the boat as shown. A steel towing cable 10 is '~
shown connected to one end of the tow line 1, over a winch 11. ';
Before the tow,'the load 'line '4-and elastomer 3 can~jointly~ ~-be shackled into the mid section of the separated--tow li~es.
10 In this manner, all components can be easily-stored aboard '~
ship with minimal probability of tangling, yet payed out smoothly in proper sequence by the wlnch.
Referring now to Figures 3, 4 and 5~j Figure 3 depicts a vessel 7 which has already released the towing apparatus ~`
as it passed iceberg 8. The apparatus may be seen to be comprised of tow lines 1 and 2 and elastomer 3, joined at swivel and shackles 5 and 6. The two ends of the tow line are connected to towing cable 11. The tow line and elastomer ' would normally have positive buoyancy in order that they can float at the surface of the sea around the iceberg.
A load line 4, having negative buoyancy, hangs below elastomer 3, and is coupled to swivel and shackles 5 and 6. In Figure 3,' the vessel 7 has not yet begun drawing the tow lines.
In Figure 4, the vessel 7 has begun drawing tow lines 1 and 2 around iceberg 8. The elastomer 3 has drawn 1~
up to the iceberg ~ut is not yet stretched to any great '~' extent. The load line 4 is in contact with the iceberg and has seated itself below a crag section 9. 1~-In figure ~k the vessel 7 is towing the iceberg 80 The tow lines 1 and 2 have pulled the load line 4 taut, having ; . , . ,~i ~ ,~ .. : ..
. : , .; . . ..
:. :., .,., ~ ~ .: , .
9/7~
been caught on crag 9. The elastomer 3 has stretched to accommodate the towing stress bein~ applied directly from tow .
lines 1 and 2 to load line 4.
:: .
Where instead of elastomer 3 a breakable line has been used, upon the tow line being pulled taut, the breakable `
line breaks and as with the elastomer, towing stress is applied directly from the tow lines to the load line. The breakable .
line should be designed to break with about 1000 pounds of ~
, .. .
stresC~ typically.
: It is:thus.clear--that the load line-4-exert~,-towing pressure ~n-iceberg~8~substantially .below;the surface-i-of.-the sea, arbitrarily closer to the centre of mass of the iceberg .
than should it have been applied at the surface as with prior towing techniques. .
It is preferred-that the elastomer-be made-,of-natural ': ..
rubber compound with a specific gravity of 0.98. It has been found that this form of elastomer expands to.about 150% of its unstressed length with approximately 170 pounds per square .
inch tension. WheA the cross section of the elastomer is .`.
20 one inch thick by six inches wide, approximately lC,00 pounds .
tensile stress will be applied to a natural rubber elastomer :... -for the lS0~ extension.
For the e~,bodiment of yieldable line which was a breakable or fusible section, positively buoyant polypropelyne . .
rope can be used which has a narrow diameter clipped iF'i (or `d shackled) short section at its mid point. The narrow diameter section can be, for example l/2 to 1 inch in diameter, in . :
order that it break at suitable tensi~n, say 1000 pounds.
It is preferred that the load line be formed of nylon 30 because its physical properties are uniquely well suited to ...
this applicati,on, although other materials which suit the .
purpose could be used. Its density is only slightly negatively -- 7 -- ..
... . . . . . . . . . . . .. ... . .
;~
buoyant, having a specific-gravity of about 1.14, so that a minimum of positive buoyancy is required at its junction points :.
with the tow lines.
In addition, the strength characteristics of bulk ~`.
nylon are such as to minimize hysteresis so that elastic recovery is almost complete after each tow. Nylon does not immediately return to its original length, but creeps back slowly. However, it almost instantly recovers approximately `
50% of:the stretch imparted.and wi~l, during:the fi~st -: ;
10 twent~-fourJhours,:-recover--a-iotal--~Of approximate:ly-85~- o~- ~f the amount-of--the s.tretch...
Generally, high and oftentimes phenomenal abrasion -.
resistance is experienced when using nylon hawsers. Both spun nylon and continuous filament yarn type nylon have been found to be substantially abrasion resistant, with the former, spun nylon, being more abrasion resistant than continuous filament fabric of equal weight. Thus, the natural pliability, inherent toughness and flex resistance all contribute to low wear against the ice surface. Nylon will stretch under load to -~
approximately 115% of its original length. Failure will not normally occur, however, until it has been stretched to its -125% original length. In the present application, the nylon operates well below the 125% mark, where the stress-strain .
curve is linear. The extendability of the nylon section then is insignificant compared with that of the elastic rubber section.
Unstrained, the rubber elastomer section is preferably approximately half the length of the nylon section so that, from the geometry, the rubber section will never need to be stretched beyond twice its unloaded length.
Steel cable can also be used as the load line, although its strength per ~nit weight is not as great as nylon, -- 8 -- .
,. .
711~
and it is somewhat more difficult to handle.
It is preferred that the tow lines be fabricated of polypropylene rope, which is positively buoyant. Preferably each tow line is about 1000 feet long, and the load line is about 500 feet long, although these dimensions can vary !~'.
considerably, depending on the maximum and minimum sizes of iceberg expected to be towed. The polypropylene rope is preferably 3 to 4 inches in thickness, although other thicknesses can be-used depending on the expected~tension.
- 10 The small--spherica-l buoys--(not shown but similar to 1,~
those commonly used in-fishing nets) can be used to-offset~
the weight of the shackles marked 5 and 6. These standard shackles are usually either stainless steel or cast iron with -.;
a strength--rating in the order of 25-tons. The buoyancy- at-- -these two shackle-points (and elsewhere if found-necessary) is adjusted to maintain the "floating" members 1, 2 and 3 '-' : ' in a near surface position.
If the tow line is not positively buoyant, then such floats could be attached thereto, and furthermore if the load line is not negatively buoyant, ballast can be attached thereto. ~owever floats and ballast should be used only when necessitated by field conditions since they make handling r~
during deployment and recovery more difficult.
It can be seen that the inventive configuration combines the three essential characteristics required for a successful improvement over the existing towing equipment and procedure. These are: deployability to substantially the full range of ice~erg sizes, and compatability with the existing equipment and procedures.
- Icebergs of widely varying cross-section can be accomodated by increasing the lengths of the elastomeric and load line sections while maintaining their relative lengths.
:.
_ g _ Finally, this configuration overcomes the adaptability problem common to all systems that use two or more parallel load bearing memhers at the same time. Such geometrics depend on load distributions that cannot be assured because of the .
unknown iceberg geometry -- especially the below water shape.
This problem is overcome by the present structures and method ~:
which apply the main load to the sîngle member below the water line.
--- --10 i -- -~
, .
-~",'' ' ;' ....
-- 1 0 -- '
Claims (11)
1. An iceberg towing hawser comprising a pair of tow lines, one end of each being adapted for attachment to a towing means, a length of yieldable line joining the other ends of the tow lines, and a length of load line joining said other ends of the tow lines, the load line being longer than the yieldable line in its substantially unstressed condition.
2. A hawser as defined in claim 1, in which each junction of the yieldable line, load line and tow line is constructed of a swivel and shackle.
3. A hawser as defined in claims 1 or 2 further including means for causing the tow lines to be positively buoyant and the load line to be negatively buoyant.
4. A hawser as defined in claims 1 or 2 in which the tow lines are fabricated of positively buoyant rope, the load line is fabricated of negatively buoyant rope and the yieldable line is fabricated of rubber elastomer, the load line being shorter than the length of elastomer when stretched to its failure length.
5. A hawser as defined in claims 1 or 2 in which the tow lines are fabricated of positively buoyant rope and the yieldable line is composed of material rupturable when under iceberg towing stress.
6. An iceberg towing apparatus comprising a pair of polypropylene ropes of about 3 to 4 inches in diameter, having one end of each adapted for attachment to a towing vessel, a length of rubber elastomer with specific gravity of about 0.98 joining the ends of the polypropylene ropes, and a length of load line fabricated of nylon or steel joining said other ends of the polypropylene ropes, the load line being substan-tially longer than the unstressed length of rubber elastomer but not longer than about 150% of the unstressed length of rubber elastomer, opposite ends of the rubber elastomer and load line being attached to each said other end of the polypropylene rope by a swivel and shackle.
7. An apparatus as defined in claim 6 in which the polypropylene ropes are each about 1000 feet long, and the rubber elastomer is about one inch thick by six inches wide and about 500 feet long.
8. A method of towing an iceberg comprising (a) paying out a floating tow line from a vessel as the vessel approaches and passes around an iceberg, the tow line having a length of negatively buoyant load line and a length of elastomer shorter than the load line attached together in parallel as an intermediate section of the tow line, (b) catching the end of the tow line initially payed out, (c) bringing the length of elastomer into contact with the rear of the iceberg, by drawing both ends of the two lines, while allowing the load line to sink against a submerged portion of the iceberg, (d) pulling the ends of the tow line in a direction away from the iceberg to stretch the elastomer to the maximum extent allowed by the load line, while drawing the load line tautly against the submerged portion of the iceberg, so as to tow the iceberg by a force exerted from the tow line via the load line to the iceberg.
9. A method of towing an iceberg comprising (a) paying out a floating tow line from a vessel as the vessel approaches and passes around an iceberg, the tow line having a length of negatively buoyant load line and a length of yieldable line shorter than the load line attached together in parallel as an intermediate section of the tow line (b) catching the end of the tow line initially payed out, (c) bringing the length of the yieldable line into contact with the rear of the iceberg, by drawing both ends of the two lines, while allowing the load line to sink against a submerged portion of the iceberg, (d) pulling the ends of the tow line in a direction away from the iceberg to cause the yieldable line to yield, and drawing the load line after yielding of the yieldable line tautly against the submerged portion of the iceberg, so as to tow the iceberg by a force exerted from the tow line via the load line to the iceberg.
10. A method of towing an iceberg as defined in claim 9, in which the yieldable line contains at least one portion breakable under towing stress.
11. A method of towing an iceberg as defined in claims 8 or 9, in which step (a) is comprised of paying out a floating tow line from a vessel, shackling a length of elastomer in parallel with a longer length of load line to the end of the tow line, paying out the elastomer and load line, shackling the end of the elastomer and load line to a second tow line, and paying out the second tow line.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA269,728A CA1044079A (en) | 1977-01-14 | 1977-01-14 | Iceberg towing system |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA269,728A CA1044079A (en) | 1977-01-14 | 1977-01-14 | Iceberg towing system |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1044079A true CA1044079A (en) | 1978-12-12 |
Family
ID=4107729
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA269,728A Expired CA1044079A (en) | 1977-01-14 | 1977-01-14 | Iceberg towing system |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1044079A (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5797343A (en) * | 1993-11-04 | 1998-08-25 | Bukser Og Bjergning As | System for establishing a towing connection between a marine casualty and a salvage vessel |
| RU2743239C1 (en) * | 2020-07-24 | 2021-02-16 | Общество с ограниченной ответственностью "Газпром недра" | Method of transporting a large iceberg |
| RU205321U1 (en) * | 2021-03-24 | 2021-07-08 | Публичное акционерное общество "Нефтяная компания "Роснефть" (ПАО "НК "Роснефть") | DEVICE FOR TOWING ICE FORMATIONS |
-
1977
- 1977-01-14 CA CA269,728A patent/CA1044079A/en not_active Expired
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5797343A (en) * | 1993-11-04 | 1998-08-25 | Bukser Og Bjergning As | System for establishing a towing connection between a marine casualty and a salvage vessel |
| RU2743239C1 (en) * | 2020-07-24 | 2021-02-16 | Общество с ограниченной ответственностью "Газпром недра" | Method of transporting a large iceberg |
| WO2022019801A1 (en) * | 2020-07-24 | 2022-01-27 | Общество с ограниченной ответственностью "Газпром недра" | Method for transporting a large iceberg |
| RU205321U1 (en) * | 2021-03-24 | 2021-07-08 | Публичное акционерное общество "Нефтяная компания "Роснефть" (ПАО "НК "Роснефть") | DEVICE FOR TOWING ICE FORMATIONS |
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