Patent Application
20200398961
The invention relates to the field of cable ferries, cable tugs, cable freighters, cable shipping or any other marine vessel using cables for guidance and/or propulsion, whether primarily propelled using cables or primarily propeller or jet-driven and cable guided.
Cable ferries, also referred to as chain ferries, have long been used to transport vehicles and people for relatively short distances across bodies of water. Such ferries are guided by cables, ropes or chains, all of which are referred to herein as cables. Currently the cables used for cable ferries generally are made of stranded steel wire rope with 6 or 8 main strands wrapped around an Independent Wire Rope Core to form a cable not less than 1″ diameter and typically in the 1½″ to 1¾″ size range.
Cable ferries are increasingly being used for salt water applications and for longer ferry routes. That is largely because a vessel that uses a cable to propel itself is more energy-efficient than the same vessel using a propeller, jet or any other method to achieve motion and guidance. Cable ferries therefore use a fraction of the energy compared to an equivalent conventional ship with resultant lower greenhouse gas emissions. They cost less to build, use less crew and have advantages in docking. Cable driven vessels are also much quieter when in motion than propeller-driven vessels. Less noise means a healthier experience for passengers and crew and less disturbance for marine mammals. Given the inherent advantages of cable ferries, but their current limitations of distance and the impediment they pose to other vessel traffic, it is a worthwhile to create new technology to allow more widespread use of cable ferries and to allow the emergence of other general shipping that may use submarine cables for propulsion.
A current example of a cable ferry application is a recently introduced cable ferry service crossing Baynes Sound to Denman Island from Vancouver Island in British Columbia, Canada, operated by BC Ferries (“the Denman Island Ferry”). This cable ferry route is believed to be currently the longest cable ferry in the world at almost 2000 meters in length. Knowledgeable ferry operators believe that this is approaching the maximum cable ferry length, due to the tensile forces that must be applied to tension a steel cable in order to draw it taught enough so that it will serve to adequately guide and propel the ferry with enough excess strength to sustain the additional forces acting on the ferry from wind and current while also providing a safety factor. The terminal anchor axial and vertical forces and the problems imposed on the crew to handle the heavy weights involved with servicing the cable systems make it impractical to consider using cable ferries which use steel wire ropes for the drive and guide cables on routes that have long distances between the ferry terminals and where the water is too deep for the cable to lie on the seabed. For the Denman Island Ferry, the cable lies on the bottom of the body of water being crossed for most of its length in order to reduce the requisite tension on the cable. In a fully extended cable system with a typical 1½ inch cable, for a clear span of over 1 km cable tensions become enormous. Using a larger diameter cable to obtain more tensile strength also adds to the weight of the cable which exacerbates the problem.
Contact with the sea-bottom by the cables causes wear on the cable and environmental damage from cable scouring of the bottom. The bottom material must somehow be constantly removed from the cable to prevent wear on the machinery and the cable itself. In some parts of the Denman Ferry route, the cable is quite deep and results in an excessively steep angle of approach of the cable to the ferry which results in inefficiency. A steep approach angle at one end of the route increases cable weight and friction while reducing the magnitude of the horizontal vector that provides ferry movement. High tensile forces cause correspondingly high constricting force to be applied to the ferry's bull wheels, which may require that they be specially designed, and increases wear on them as cable tensions increase with route length. Other problems are caused by a cable that contacts the bottom when the ferry route has a rocky, irregular or steep bottom terrain. Here the problem can be more extreme wear on the cable from the abrasive bottom or even cable snagging.
There are challenges to controlling long lengths of cable due to the physical weight of, and friction on, the homogenous steel cable typically used. As the ferry transits the route, the cable catenary rises very quickly from the bottom and the cable tension likewise dramatically increases as a result. This effect is pronounced with the increased droop required to limit the overall cable tension where the ferry route is long, deep and the cable is not supported by the bottom.
There is therefore a general need to provide a modified cable construction which will reduce the foregoing problems, and in particular to allow cable ferries and other shipping to be deployed on longer crossings than is currently possible.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.
The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools and methods which are meant to be exemplary and illustrative, not limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated, while other embodiments are directed to other improvements.
Cables for propulsion and/or guidance of cable ferries are disclosed which permit their use on longer ferry routes. One embodiment provides the use of cables for propulsion and/or guidance of cable ferries where the unit weight, specific gravity, density and/or diameter of the cable is variable over the length of the cable. According to one embodiment, heavier or denser cable is used adjacent to the terminals, while lighter or less dense cable is used for a middle section. Where sections have positive buoyancy, the cable may be tethered to one or more anchors. Where sections have negative buoyancy, the cable may be tethered to one or more floats. In this way control of the positioning of the cable and its dynamic qualities are achieved. Cable control, namely the positioning of the cable and its dynamic qualities, is thereby achieved through the physical qualities of the cable itself using a variable specific gravity and/or diameter (“VSGD”) cable. In other aspects cable control is achieved by applying an external force to a cable which may or may not include VSGD cable sections. According to this aspect, cable control is achieved by applying external forces to the cable through weights, floats or an attachment to the bottom of the body of water.
In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following detailed descriptions.
Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive.
Throughout the following description specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.
“Cable ferry” means any ferry, ship, barge, tug or any other vessel that is guided by or uses cables, ropes, chains or any other type of line(s) for guidance and/or propulsion.
“Cable shipping” means any ferry, ship, barge, tug or any other vessel that is guided by or uses cables, ropes, chains or any other type of line(s) for guidance and/or propulsion.
“Cable control” means the control of the positioning of the cable and its dynamic qualities.
“Specific weight” (also referred to as the unit weight) is the weight per unit volume of a material. The density of a material is the mass per unit volume, while the specific or unit weight is the force of gravity on the unit mass of the material.
“Specific gravity” is the ratio of the density of a substance to the density of a reference substance, namely the density of water at 4 degrees Celsius which has a specific gravity of 1; equivalently, it is the ratio of the mass of a substance to the mass of a reference substance for the same given volume.
“Tether” means a line attached to the cable on one end and with the other end attached or anchored to the bottom, to a weight or a float.
“VSGD” is an acronym for Variable Specific Gravity and Diameter and means that the unit weight, specific gravity, density and/or diameter of the cable is variable over the length of the cable.
In the drawings a thick black line is used to designate VSGD cable sections that vary in unit weight, specific gravity, density, and/or diameter from the rest of the cable.
In
Modern synthetic ropes with breaking loads equal to or exceeding that of the same diameter steel wire rope are now readily available. Rope manufacturers such as Samson Ropes and Cortland Ropes offer cables made up of a number of different fibers to give the finished cable the requisite strength, weight, abrasion resistance and cyclic bending fatigue characteristics required for the intended application. Cables can be custom manufactured with varying diameters, either as a continuous rope or using splices, and in almost any length. For the cable ferry application, there are the advantages disclosed herein that result from having a cable that has a variable buoyancy, whether neutral, positive or negative buoyancy, and also is able to take repetitive cyclic bending as the cable passes over the drive bullwheel and the deck guide sheaves.
Buoyancy of the cable section 28 can be increased by incorporating into the cable large masses of low density material such as cellular rubber or closed cell expanded polyethylene, such as disclosed in U.S. Pat. Nos. 2,403,693 and 2,818,905, or by incorporating filament braid flotation members as disclosed in U.S. Pat. No. 3,155,768 in connection with a buoyant electrical cable. U.S. Pat. No. 3,806,568 provides a method of manufacturing a continuous cable which has a uniform diameter and sections of different strength and weight in sea water, without splicing different cable sections. Other VSGD cables can be made from synthetic rope or steel incorporating the desired characteristics.
A notional cable ferry route, with a length of 7600 feet in very deep water was studied. For the notional ferry route it was assumed that the cable will allow a deep sea vessel drawing up to 35 feet to sail over it except when the ferry is in passage. Therefore it is necessary for the portion of the cable in the area of the ship passage to be no shallower than 80 feet from the surface when the ferry is not in passage. Given the end forces on the cable in such a configuration, it is clear that a 2″ diameter conventional wire rope cable is unsuitable for a route of that length and depth, applying a safety factor of 5 to the breaking force of the cable. The nominal breaking load for a 2″ wire rope is 360,000 lbs which, after applying the factor of safety of 5 translates to a maximum allowable force of 79,000 lbs. The end force is generated by the physical weight of the cable, the initial tension in the cable induced by the tensioning winch and the resistance of the water on the hull of the ferry as it pulls itself along the cable. The hull resistance is proportional to the load (number of vehicles) on the ferry and its speed through the water. It is not difficult to conclude that the terminal anchor axial and vertical forces and the problems imposed on the ferry crew and equipment to handle the heavy weights involved when handling long cables make it impractical to consider steel wire ropes for the drive and guide cables for the proposed route.
Use of a cable having a constant cross section along its entire length but using synthetic rope cable for a middle section of the cable was therefore examined. Synthetic cables can also be manufactured with varying density and diameters along their length and without the need for splicing. Calculations were done to determine the shape of the cable catenary having ends with a unit weight the same as a 2″ diameter steel wire rope (7.4 lbs/ft) and with the longer middle section of 2″ diameter synthetic rope with a unit weight of less than 1 lb/ft. The 2″ diameter cable displaces a volume of water that weighs more than 1 lb. and therefore the middle section of the cable will arch upwardly towards the surface, being restrained only by the heavy ends. Using a 690 ft length of 2″ steel wire rope the cable found equilibrium with the lighter mid-section when the lower end of the heavy section was submerged at 190 ft and the top of the upward arc of the synthetic mid-section was 85 ft below the surface. This is illustrated where the ferry is docked, with same and exaggerated vertical scales, in
Therefore the solution was considered to use a dense, high specific gravity section of cable in the waters approaching each terminal and then using a much lighter cable to connect the ends of the heavy cables, and thus provide a continuous set of drive and guide cables for the ferry. As the drive cable must come into physical contact with the friction-drive bullwheel and idler, the transitions between the heavier end sections of the cable and the lighter centre section have to be smooth and symmetrical. The transition may take the form of a splice, with appropriate further adjustment to the allowable safe working loads for the cables. The technology surrounding synthetic ropes is now sufficiently developed that the applicable safety factor multiplicands are available, or could soon be calculated, and a suitable splice manufactured and tested. A preferred option is a continuous synthetic rope cable with heavier ends and less dense middle sections.
This design has a very beneficial effect on the reaction forces at the terminal anchors and the depth of the catenary loop. Several of the synthetic fibers have unit weights that are less than the weight of water that their volumes displaces and they will generate an upward buoyant force. It was concluded that using a lighter mid-section to the cable is feasible, permitting the consideration for cable ferry routes having much greater distances between ferry terminals without the cables becoming too heavy and unworkable.
Some of the modern synthetic ropes have comparable mechanical strength properties to the same diameter steel wire rope while being considerably lighter. Samson, Amsteel Blue Dyneema 2⅛″ diameter synthetic rope has a breaking load of 396,000 lbs. Applying a factor of safety of 5 would give a safe working load for this synthetic rope of 79,000 lbs—the same value as the 2″ diameter steel wire rope. The elastic stretch of the Dyneema synthetic rope at 20% of breaking load is less than 1 percent which is comparable with the elasticity in the steel wire rope. There is a significant difference in the unit weight of the two cables. The steel cable weighs 7.3 lbs/ft while the synthetic cable weighs only 0.91 lbs/ft. The volume of water displaced by each linear foot of 2″ dia. cable which has a cross section area of 3.142 sq. in. is 37.7 cu ins per linear foot. The synthetic cable under consideration generates 1.39 lbs of upward buoyancy and the cable will float. When the geometric forces at the two cable end anchors with the lighter synthetic cable are calculated, using a 7600 foot spacing between the cable end terminal anchors and the same Factor of Safety of 5, it is found that that while the droop before taking into account the buoyancy forces is the same as for the steel wire, the resultant forces acting on each anchor is reduced to 8,600 lbs. When the buoyancy of the cable is taken into consideration the cable floats on the surface and the forces at the anchors are considerably lower.
With reference to
In the embodiments shown in
The foregoing cable tethering method permits both the depth profile and direction of the cable to be selected as shown in
Weights 32 can be used to draw the cable 18 nearer to the bottom but without touching the bottom, as shown in
Floats 34 may be preferable in shallow water where bottom tethers 38 to anchors 32 are not suitable due to the limitations that shallow water places on the tether length to anchors with the corresponding limit to the elasticity of those tethers. See also
As shown in
Bottom attachments can be effected by connecting elastic tethers 38 between the cable 18 and anchors 32 placed accordingly and set in the bottom as shown in
With control in the xyz planes to orient the cable, long cable routes, even across intercontinental shipping routes can be created to support barge or other towing operations, freighters, and any other vessels traveling routinely on a particular marine traffic lane to provide the advantages of efficiency and other advantages of cable ferries. Examples of routine traffic lanes using barge traffic are where barges are delivering chips to pulp mills or freight to island communities, as well as raw log booms, and providing supplies along tourist routes.
With the use of the foregoing VSGD cables, cable profiles can be designed to fit the requirements of each individual ferry route. This can be applied to cable ferry routes where it is desirable to have the cable fully suspended from shore to shore. VSGD cables can be used on longer cable ferry routes that will not necessarily require bottom contact to limit tension on the cable, though bottom contact may be allowed, and in some cases, be desirable. The specific gravity of the cable, typically near the section at the center of the route, can to be reduced and thereby decrease the overall tension being exerted on the cable which will allow longer cable spans and therefore longer cable ferry routes. A significant advantage to having a cable suspended clear of the bottom includes much longer cable life and less wear on mechanical parts. It will allow for cable ferries to be used on longer routes than has been possible before using uniform, typically plastic wrapped steel wire rope cables.
By varying both specific gravity and the diameter of a cable, and by using tethers to floats and anchors in some cases, the specific challenges of cable ferry routes can be overcome, including, but not limited to, route length, current, environmental sensitivity, vessel traffic, and ferry propulsion efficiency. Additional applications of the VSGD cables are described as follows.
As shown in
Steep or irregular bottom structure 60 (
Other vessel traffic crossing the route may be a design consideration for some potential cable ferry routes. Large ships 62 can pass over the cable through a marked navigation channel as shown in
Varying cable diameter may also be used to minimize the side forces caused by current acting on the cable. Cable diameter as well as density may be varied with modern materials. Varying cable diameter and density would be useful where there is a cross-current caused by tidal flow, rivers or other circumstances, shown in a top view in
Floats and anchors may be used for a cable ferry featuring traction winches mounted on either side of the ferry. In top view,
While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. For illustrative purposes one cable only is shown for the cable ferries. The same concepts apply to cable ferries using multiple drive and/or guide cables. There are two ships illustrated though normally only one ship would serve the system. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are consistent with the broadest interpretation of the specification as a whole.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CA2019/050163 | 2/7/2019 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62633231 | Feb 2018 | US |
