Design Methodology and Numerical Analysis of a Cable Ferry

Design Methodology and Numerical Analysis of a Cable Ferry Dean M. Steinke1 (M), Ryan S. Nicoll1 (V), Tony Thompson2 (M), Bruce Paterson3 (M) 1. 2. 3.

Dynamic Systems Analysis Ltd., 101-19 Dallas Road, Victoria, BC, Canada EYE Marine, Suite 1, 327 Prince Albert Road, Dartmouth, NS, Canada British Columbia Ferry Services, Inc., 500-1321 Blanshard Street, Victoria, BC, Canada

Cable ferry systems offer many advantages including simplicity, low maintenance, low operations risk, and lower energy use when compared to freely maneuvering ferries. In this paper, a proposed ferry system on a long 2000m crossing is assessed using advanced time domain analysis with verification by model scale tests. The results are used to ensure a safe and reliable design with similar level of service to the incumbent ferry system on the existing route.

KEY WORDS: Cable ferry; passenger vehicle; roll-on/rolloff; hydrodynamics; mooring; dynamic analysis; time domain simulation

INTRODUCTION Cabled ferry systems have been in use for centuries. They are used all over the world to due to their simplicity and frequent advantages over freely maneuvering ferries. Cable ferries are connected to both shores of a crossing with a cable or cables that facilitate ferry transit. A modern vehicle and passenger carrying cable ferry is typically connected using wire ropes and propelled by a winch onboard the ferry. Cable ferry routes typically vary from 100 to 1500 m with the most common range from 800 to 1000 m. Figure 1 shows the Henry Nase cable ferry operating in New Brunswick on a 615 m crossing at Gondola Point. Aprons on either end of the ferry are used for loading and unloading of vehicles on each end of the crossing. Figure 2 shows a cable ferry berthing. The figure indicates the cable which the ferry

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uses to guide and pull itself through the crossing. The cable transit shown here is 615m and is located at Gondola Point. Cable ferries are used for a number of reasons. They are simple structurally due to their flat bottomed barge shape and require relatively simple mechanical systems. The mechanical systems are minimized by the use of a winch drive system, replacing rudders and propellers. In addition, fuel consumption is low because the propulsive efficiency of the winch drive system is greater than a propeller. The reduced complexity requires a smaller crew and lower operational and maintenance requirements results in reduced operational costs. Lastly, the cables guide the ferry and so navigation and complex berthing operations are simplified, which reduce operational risks. Most cable ferries operate on shorter routes in protected coastal areas or on rivers or lakes and therefore can be designed by marine engineers and naval architects based on common practice, experience, and standard analysis methods. However, longer crossings require additional consideration. The longer the

crossing, the more severe the potential environmental loads due to increased exposure and fetch, which in turn can result in encountering higher wind speeds and wave conditions. As a result, the motion of the ferry and resulting stresses on the cable system must be considered to ensure safety and reliability, and to achieve an acceptable level of service specified by the operator throughout the lifetime of the ferry.

Figure 1: Henry Nase Ferry operating in New Brunswick, Canada; crossing distance 615 m

Figure 3: Buckley Bay to Denman Island ferry route; crossing length 2000 m

CABLE FERRY OPERATIONS Ferries are used around the world as a simple and efficient system where traffic demands will not warrant the construction of a bridge or where a fixed structure is not practical or would impair the passage of other marine traffic. The cable ferry is limited by applications where access roads and terminals can be arranged in a direct line to facilitate loading and unloading. Since the ferry follows the track of the fixed cables and there are no navigating requirements other than docking and maintaining watch, the crew can often be reduced consistent with safety requirements to further reduce costs. Figure 2: Cable ferry docking with traction cable indicated British Columbia Ferry Services, Inc., (BC Ferries) is presently engaged in the replacement of a ferry offering service between Buckley Bay on Vancouver Island and Denman Island, on Canada’s west coast. The existing ferry operates on a straight route, approximately 2000 m in length, in a relatively wellprotected area, as shown in Figure 3. Because of the aforementioned benefits, a cable ferry has been proposed for use on this route. However, when designing such a system, the cables have a significant impact on many aspects of the design of the vessel, and many additional considerations arise. For example, relative motion between the ferry and the docking structure are significantly affected by cables. The cable fatigue life must be considered, and ferry seakeeping behavior is affected by the cables. This paper reviews work directed by BC Ferries to assess the design. An emphasis is placed on the numerical analysis work completed by Dynamic Systems Analysis Ltd. (DSA) and E.Y.E. Marine Consultants (EYE). This work included assessment of level of service, cable loads, docking loads, and ferry motions for the proposed ferry. The unique aspects of the project are emphasized.

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Most of the cable ferries have evolved to using a single drive cable on the centerline of the ferry. This cable is anchored at both ends by substantial ground anchors with allowable pull greater than the breaking load of the cable. The cable is lead through fairleads on either end of the ferry and around a pair of bullwheels. A rendering of a typical hydraulically driven bull wheel system is shown in Figure 4. At least one of these bullwheels is powered, however both can be powered if redundancy or additional pull is required. Cable ferries can use additional guide cables to control drift or to increase the level of safety in the event of a cable break. Depending on the traction load required, more than one drive cable may be necessary. Most cable ferries utilize a hydraulic drive motor directly coupled to the shaft which can deliver the required torque at a speed of about 50 RPM. There are electric drives that are battery operated as well as mechanical drive systems. The hydraulic drive is relatively efficient with an estimated drive efficiency of about 80% for a well matched pump and motor. It is this high efficiency that makes the system attractive to operators as fuel costs are substantially reduced. A 42 m 24 car ferry can reliably maintain a speed of 7 knots with a 50 kW hydraulic motor in normal full loaded operating conditions.

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The lateral drift of the ferry from the direct route is caused by wind and current loads. The drift is controlled by the tension on the cables. It is generally necessary to pretension the cable to reduce the drift and to provide enough friction on the drive bullwheels to prevent slippage. Pretension may also be required to guide the ferry when docking at a terminal. In general, the pretension of 1/5th of the breaking load is not to be exceeded. It is not necessary to lift the cables off the bottom, although this occurs over deeper sections of the crossing. When the ferry is subjected to an external lateral force the cable catenary is reduced and naturally increases the tension in the cables until a balance of environmental forces against resultant cable tension is achieved and the system is in equilibrium.

accommodations and machinery spaces and supports a raised control station (bridge) providing good visibility to the docking areas and the car deck. The central deckhouse facilitates fast and safe loading and unloading by maintaining straight lanes in conjunction with wide loading ramps with dual lane loading/unloading.

Figure 5: Profile of concept ferry Table 1: Proposed ferry physical properties and characteristics Length OA 80 m Beam (WL)

16 m

Depth

2.1 m

Draft

1.5-1.6 m

Mass

526,000 kg

Roll radius of gyration

6.41 m

Yaw radius of gyration

19.6 m

Pitch radius of gyration

19.6 m

Vehicle capacity (AEQ = Automobile 50 AEQ Equivalent unit, 6.5m) Complement 150 Vessel life Figure 4: Rendered model of a typical hydraulically powered traction winch drive system complete with bullwheels Analysis of the cable ferry system by equalizing of the resultant forces is a quasi static approach. The resulting forces have a safety factor of about 3 applied to compensate for dynamics and wear that are not considered in quasi static methods. An important control in using quasi-static methods is to correlate the stretch of the cable to the predicted drift and catenary shape to ensure that the stress/strain relationship remains correct. While the quasi-static methods have been used successfully on many ferries in service with great success, the method should not be relied upon when dynamic conditions become significant. On the subject ferry it was determined that combination of a large ferry with a longer transit and potentially higher transverse winds warranted the use of dynamic methods to accurately determine the cable sizes and forces.

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The proposed ferry is propelled by a bull wheel around which the traction cable wraps. In the proposed design, all machinery is located above the main deck for easy and safe access. Two additional cables on either side of the ferry provide additional guidance during the crossing, as shown Figure 6. Figure 19 (at end of paper) shows how the cables interact with the seabed during the crossing. The pretension in the guide and traction cables for this application will be around 200 kN, although it is possible to operate at pretensions near 100 kN as well. A 1.625 inch 6x19 IWRC wire rope, with a mass of ~7.0 kg/m, will be used for the guide and traction cables.

CONCEPT OVERVIEW The cable ferry concept shown in Figure 5 has been proposed for use between Vancouver Island and Denman Island. Key parameters for the proposed ferry are provided in Table 1. The route length is approximately 2000 m. The ferry will carry approximately 50 vehicles or 50 Automobile Equivalent (AEQ) units on a roll-on / roll-off deck. The ferry is about 80 m long, with a centerline deckhouse that contains passenger

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Figure 6: Cable ferry general arrangeme:

Design Methodology and Numerical Analysis of a Cable Ferry Design Methodology and Numerical Analysis of a Cable Ferry

The bull wheel must have sufficient power to pull the ferry through the crossing at the required transit speed, which may be slightly slower than a conventional ferry as there is less time lost in docking. For the project under consideration, the ferry is expected to have a service speed of 7.5 knots with 300kW power available for propulsion. The ferry must also have sufficient power to pull itself into the dock during heavy weather conditions. The ferry will berth with the terminal on a floating pontoon that is connected to a hinged ramp. The floating pontoon structure, held laterally and in yaw by piles. The docking system schematic, shown in Figure 7 (piles not shown), illustrates how the cables pass through the pontoon to shore where they are anchored. Figure 20 shows the ferry next to the pontoon, and the passage of the cable through the pontoon and ferry.

Figure 7: Ramp and pontoon general arrangement

CONECEPT VALIDATION Prior to commencing detailed dynamic analysis of the ferry, a validation of the ferry concept was completed. The selection process consisted of checking weights, intact stability, and damaged stability against regulatory standards. The approximate weights of the cables were included in the lightship weight. A preliminary bulkhead arrangement was determined to confirm that the floodable length standards were met. Structural calculations were performed to confirm the weights and longitudinal strength were acceptable. Finally a preliminary resistance calculation was prepared to identify that service demands and speed requirements would be met. A hull optimization process was carried out that included a preliminary evaluation of the ferry motions in waves and at speed using a strip theory analysis. The vessel’s docking system, as shown in Figure 7, facilitates a non-flat bottomed ferry to be used. This is in contrast to the docking system shown in Figure 2, which relies on a flat bottom. Since a non-flat-bottomed ferry may be used, the vessel could be designed to incorporate minimal deadrise to improve the longitudinal directional stability of the barge type hull (addition of appendages such as bilge keels were considered). Substantial overhangs to minimize deck wetness were incorporated and consideration of the guide cables and traction cable propulsion system were incorporated. All of the validation work confirmed that the ferry exceeded regulatory standards and would result in a safe and seaworthy vessel.

DYNAMIC ANALYSIS REQUIREMENTS

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To comprehensively assess the proposed cable ferry system and its associated structures requires analyzing the interplay between the cables, the ferry, the shore works (i.e. pontoon, ramp, piles), and the mechanical systems on board the ferry. The global dynamics of this system must be understood to accurately assess the loads and behavior of the ferry during transit and berthing. BC Ferries initiated a project to determine loads and the behavior of the cable ferry under a range of environmental conditions through dynamic analysis. This analysis was used to support the detailed design of the ferry, the shore structure, and allow for specification of system components (wire rope size, bull winch power capacity, etc). The dynamic analysis of a cable ferry is unique in several respects. The ferry can be considered both a moored platform and a moving vessel, so consideration to its motion must take into account coupled vessel and cable dynamics. The assessment of the station-keeping behavior of moored platforms is well established. Experience, standards, and methodologies exist which provide guidelines for their analysis. Common mooring standards include: API RP 2SK – “Recommended Practice for Design and Analysis of Stationkeeping Systems for Floating Structures”; DNV OS-E301 – “Position Mooring”; and; ISO 19901-7 – “Stationkeeping systems for floating offshore structures and mobile offshore units”. However, these standards provide analysis methodologies focused on offshore platforms and not coastal marine applications. Assessment of the dynamic behavior of barge shaped vessels is likewise well established in codes and practice, but the design techniques used for these vessels do not typically include analysis of the vessels attached to cables. Thus analysis of a cable ferry must draw upon aspects of both of these fields of practice to ensure accurate analysis and achieve acceptable risk levels for operators. Regardless of the approach or technique used, whether numerical or through physical model tests, the dynamic analysis must allow for determination of: i. Vessel / pontoon impact loads ii. Relative motion of the pontoon and ferry while docked iii. Loads on shore structures when ferry is docked iv. Loads in cables (safety factors) v. Loads on cable anchor points vi. Powering requirements for ferry bull wheel system vii. Braking load requirements viii. Excursion from transit centerline during extreme conditions ix. Cable fatigue life x. Operability given expected environmental conditions at the site xi. Ferry motions (accelerations and roll) during transit and docking xii. Loads on cable fairleads on ferry xiii. Loads on cables in damaged condition (e.g. one cable has been disconnected) xiv. Assessing modal frequencies of cable/ferry with respect to environmental loads xv. Assessing the effect of modifying cable pretensions on maximum loads and excursion from transit centerline

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METHOD OVERVIEW For the project, the nonlinear time domain simulation software, ProteusDS, was used by DSA to simulate the coupled ferrycable system dynamics. The software is used for coupled analysis of offshore structures, moorings, and a variety of ocean equipment and technologies (Steinke, 2008; Nicoll, 2011; Nicoll, 2012). It was selected for use for several reasons. First, it can model the cable loads using a nonlinear finite element cablebeam model, which can incorporate axial, bending, torsional, and viscous drag effects. Second, it incorporates bathymetric variation of the crossing and the resulting change in cable loads during interaction with the seabed from friction and contact forces. Third, the software can model rigid body floating vessels along with winch controller systems. Lastly, the software allows for modeling and assessment of ferry-dock rigid body interaction, including cable and contact loads.

Ferry Hydrodynamic Model The seakeeping analysis software ShipMo3D was used to generate a hull-specific hydrodynamic database to incorporate the ferry hydrodynamic effects in the analysis (McTaggart 2012; McTaggart 2010). The hydrodynamic database allows the computation of wave radiation, diffraction, and excitation loads as a function of forward speed of the ferry in the time domain. The software uses a panel method approach in conjunction with potential flow theory to determine wave excitation, radiation, and diffraction forces on a ship hull. This data was generated for the ferry hull using a panel method code instead of a strip theory approach due to the relatively shallow draft and low length-tobeam ratio; comparison of results between a strip theory program and ShipMo3D with model testing results confirmed that the strip theory approach under-predicts the maximum roll angles. The hull mesh, with panel size limited to 0.2 m2 is shown in Figure 8. This fine mesh was found to be required due to the shallow draft (0.61m) which led to large irregular frequencies in the potential flow solution. Removal of the irregular frequencies was accomplished by decreasing the panel size in the mesh.

Figure 8: Ferry hull mesh as shown in ShipMo3D

Wind Model In fetch-limited coastal regions, wind loads are an important source of loading. A model of the ferry topside was created for use in the time domain simulations. The topside geometry is

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roughly represented, and a drag coefficient due to wind velocity in the principal degrees of freedom (surge, sway, heave) is used. Figure 9 shows the topside geometry used in the simulations. Wind loading is applied based on the relative velocity at each discrete panel in the mesh. A wind shear profile was used with lower wind speed at the water surface. Key ferry simulation properties are given in Table 2.

Figure 9: Ferry topside wind loading model Table 2 Ferry simulation parameters Parameter name Value Mass 526 x 103 kg Roll radius of gyration 6.41 m Pitch radius of gyration 19.6 m Yaw radius of gyration 19.6 m Vertical location of center of 1.984 m gravity from baseline Superstructure surge wind drag 1.15 coefficient Superstructure sway wind drag 0.97 coefficient Draft 0.61 m

Finite-Element Cable Model The cables were represented using a cubic finite element cable model. The mechanical properties given in Error! Reference source not found. were calculated from data provided by the manufacturer, and recommended values for axial rigidity for stranded wire rope from DNV (DNV, 2010). Bending and torsional stiffness values for the cable were specified; however, the axial rigidity, drag, and weight are the most critical parameters for the dynamic simulation. The contact forces between the seabed and cable are computed using a seabed stiffness and damping coefficient and cable penetration into the soil. For all simulations, the cables were connected to the ferry via a tangentially sliding connection at the bow and stern of the ferry, as shown in Figure 10. The sliding connection represents the swivel sheaves on the ferry. The connection type use stiffness and damping to keep the cable laterally aligned to the connection point but does not provide any resistance to relative motion in the cable tangential direction at the connection point. Sliding friction in the fairleads is not significant and was neglected. The sliding connections are the primary means of transmitting cable reaction loads to the ferry.

Design Methodology and Numerical Analysis of a Cable Ferry Design Methodology and Numerical Analysis of a Cable Ferry

Table 3 Cable properties for simulation (final cable selected had a diameter of 0.041 m and a mass per unit length of 7 kg/m) Parameter name Value Diameter 0.038 m Mass per unit length

5.92 kg/m

Axial rigidity/EA

0.106 x 109 N

Flexural rigidity/EI

8.2 kNm^2

Torsional rigidity/GJ

8.2 kNm^2

Normal drag coefficient

1

Tangential drag coefficient

0.01

Normal added mass coefficient 1

ferry. The traction winch which increases the tension in the traction cable to propel the ferry is indicated. During transit simulations, the dock structure was neglected and the cables were pinned to the static pontoon dock connection location. During crossing, the pontoon is too far away to significantly influence the ferry motion and the reduced complexity increased execution speeds of the time domain model. For docking simulations, the cables were pinned to locations on shore that correspond to shore anchor locations. In addition, several sliding connections were used to constrain the guide and traction cables to guide the lines along the pontoon before termination at the shore anchors as illustrated in Figure 11.

Winch Model In order to stop or drive the ferry forward, a thrust force is required. A PID (proportional-integral-derivative) controller that observed the forward speed of the ferry and applied a thrust force on the ferry with equal and opposite reaction force on the traction cable to adjust the speed accordingly was used. The controller used integral control exclusively to ensure an accurate steady state forward speed. This automatic controller ensured the ferry reached a relatively steady desired forward speed value during transit in spite of environmental loading conditions. The applied force was limited knowing the winch power and load capacity. The power was limited to 300kW in the simulations using the controller. This value was selected based on previous experience and resistance calculations. The actual traction and braking power needed during a given simulation was computed through a post-processing step after a simulation was completed. The braking or traction power required to stop or drive the ferry is the thrust force produced by the controller multiplied by the forward speed of the ferry.

Simulation Measurements In the time-domain software, virtual instruments were placed at specific locations on the ferry model to measure key pieces of information throughout the numerical simulation; the key data was written to text files for post-processing. For example, acceleration probes were placed at several points on the deck at the bow, stern, and passenger lounge near the center of the ferry to record linear acceleration seen at each point through the simulation. This was done because the bow and stern will be the location of maximum heave acceleration on the ferry due to ferry roll and pitch effects.

Figure 10: Time domain model setup schematic showing placement of sliding connections on the ferry which model the swivel sheaves where the cable loads are transferred to the cable

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In addition, air gap probes were placed at several locations around the bottom of the hull of the ferry. The air gap probes are placed at various locations on the hull surface location underwater and indicate the distance between the sea surface and the probe location. The air gap probes are used to give an indication on whether large portions of the hull are exposed to air due to shorter length waves in storm conditions. This is important for structural reasons in addition to assessing the validity of the numerical model.

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Dock Model To examine the interaction of the ferry with the dock, further time domain simulations were completed. The dock model was represented by an articulated body, or a serial concatenation of rigid bodies connected by joints. The dolphins were held fixed rigidly against the seafloor. The floating pontoon was connected to the dolphins by a joint that allowed heave, pitch, and roll. The ramp was neglected but the mass of the ramp was lumped in with the floating pontoon. The general setup of the model is shown in Figure 11 and Figure 20.

load with relative heading to the ferry between steady state simulation and the wind tunnel test data can be seen in Figure 13; the maximum wind load of approximately of 108kN was seen in the sway direction at a wind speed of 55 knots. It was assumed that the general layout and geometry of the wind tunnel model matched the configuration used in the time domain model. The surge and sway drag coefficients used for the time domain model were determined by using the same wind velocity and shear profile and matching the total absolute forces provided from the test data. For the same wind shear profile and various wind headings, surge and sway loads between wind tunnel model tests and numerical simulation were in excellent agreement. However, the time domain model did not reproduce the expected heave load and yaw moments; however, these loads were not significant compared to hydrostatic stiffness and cable reaction loads, which prevent any significant motion or deflection.

Figure 11: Ferry and pontoon model for time domain docking simulations Environmental loads on the pontoon were computed with the use of a ShipMo3D hydrodynamic model which includes wave radiation and diffraction effects. The resulting loads include hydrostatic buoyancy, wave excitation, current, and wind loads. In order to model the effects of impacts during docking procedures, a directional elastic force constraint was placed between the contact point of the pontoon and the ferry to represent the fender. The contact force is only applied as the ferry and pontoon interfere in a contact event. The contact force uses a constant stiffness. Limited damping was used to help ensure a conservative result in computing the impact load on the pontoon. The total yaw moment and reaction forces were computed at the geometric centroid of the dock.

VERIFICATION WITH PHYSICAL DATA The wind and hull resistance coefficients were validated with physical model test data. The dynamic time domain ferry response in waves was also compared with model scale tests in a wave basin facility to ensure validity. Data of wind loads produced from wind tunnel tests on a scale model abovewaterline superstructure of the ferry was provided by Oceanic Consulting Corporation (Oceanic) and BMT Fluid Mechanics, as shown in Figure 12. The corresponding superstructure surge and sway wind coefficients to be used in the time domain simulation models were established by comparison of absolute wind load with the wind tunnel test data. The variation in wind

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Figure 12: Wind tunnel testing completed by BMT Fluid Mechanics In the model scale tests, a planar motion mechanism (PMM) was used to drive the scale hull model through the water at a range of forward speeds and various static hull yaw angles, as shown in Error! Reference source not found.. The hull resistance tests indicated skin friction forces on the hull that resisted forward speed while the drag tests gave information of lateral drag loads that would occur from relative currents. In general, surge viscous loads are important as they will influence the power required to drive the ferry forward and sway viscous loads are important as they influence the lateral deflection of the system. The yaw angle was varied slightly to give an indication of the load variation that may occur in the event that the guide and traction cables constrain the ferry to some offset yaw angle as the ferry moves through the crossing. A comparison was made in steady state conditions in the time domain model. In general, the loads are in agreement for small yaw angles. However, due to the simplicity of the hull resistance time domain model, no yaw moment was produced as the yaw angle increased. In addition, the forward surge resistance decreased as the yaw angle increased in simulation, while the model test data indicated an increase in surge resistance over the range tested.

Design Methodology and Numerical Analysis of a Cable Ferry Design Methodology and Numerical Analysis of a Cable Ferry

Surge drag loads must eventually drop to zero once the yaw angle reaches 90 degrees and so the model scale test results highlight the more complex flow effects at moderate yaw angles. However, time domain crossing simulations indicated the dynamic yaw response of the ferry remains within 5 degrees of the forward direction and so the difference in surge drag is not significant. Furthermore, the lack of yaw drag moment was not a significant concern due to the significant yaw stiffness imparted by the cable system.

position and orientation relative to a wavemaker in a wave basin facility as shown in Figure 15. A top view of the beam wave loading case can be seen in Figure 16, which shows a combination of the model scale test and a snapshot from the time domain simulation model. In the time domain model, the mooring lines used had the same material properties as the full scale cable system. However, to ensure physically consistent results, additional linear springs were added at the anchor points of the cables as well as a pretension value that corresponded to the model test system. The time domain model used a JONSWAP spectrum as in the model test case.

Figure 15: Model basin testing of cable ferry hull

Figure 13: Steady state simulation (line) and wind tunnel test loads (dots) as a function of relative wind direction to the ferry The model scale tests provided data corresponding to lateral current loads of 1m/s. A comparison was made in steady state conditions in the time domain model. The surge and sway lateral drag loads compare reasonably well over a wide range of yaw angles. The yaw moment was not accurately represented in the simulation model though it is not significant due to the substantial yaw stiffness of the cable system used by the ferry

Figure 14: Resistance and current drag testing using PMM To assess the seakeeping response of the ferry, model tests of the ferry in waves were carried out by Oceanic. The experiment model scale setup consisted of a soft mooring to maintain ferry

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An initial comparison of the wave basin tests and the time domain model showed the roll response of the time domain model in beam waves was less than the seakeeping model tests. Sensitivity studies indicated that the roll wave radiation parameters used in the time domain simulation, which produce a damping effect, were too large. The wave radiation data was scaled to better match the model test results and ensure realistic roll behavior. All of the tests were performed at a significant wave height of 1.2m and peak period of 5s. The focus was on comparison of standard deviation of response between the model tests and time domain simulation results. Accelerations at several locations on the deck and motion of the center of gravity were very similar.

Figure 16: Seakeeping setup: wave basin model schematic shown on left and ProteusDS model setup shown on right.

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DEVELOPMENT OF DESIGN BASIS Knowledge of the environmental conditions at the site is needed for the analysis. Wind, wave and current conditions will affect the hydrodynamic loading on the cables as well as the ferry and the terminal. An analysis of environmental conditions was completed by Cascadia Coast Research Ltd. The 1, 50 and 100 year wind, current and wave conditions were determined for the site. Expected 1, 50 and 100 year extreme wind speeds were estimated for 8 directional ranges between 0 and 360 degrees. Values were produced for the terminal location. The USACE Coast Engineering Manual suggests that factor of 1.2 can be applied to determine the wind speeds at the mid-point of the crossing. Extreme current values in the northerly and southerly directions were determined using data from a 3 month deployment of a buoy-mounted ADCP (acoustic doppler current profiler). In this particular region, the tide levels are tidal driven and deterministic. The surface currents are driven by wind and are probabilistic. For this project, current extremes were estimated statistically based on 3 months of surface measurements using a peak over threshold approach. The relatively short data set resulted in a fairly large predicted current relative to the maximum measured value. Circulation modeling approaches might be used to more accurately predict the 1, 50 and 100 year extreme current values. Wave measurements were made at the site using a wave buoy using two deployments. The SWAN (Simulating Waves Nearshore) wave model of Baynes Sound was developed. The model used wind data from a nearby weather station as input. The model was validated by comparing the mid-channel buoy measurements to wind inputs. The model was used to hind-cast wave conditions at each of the terminals and at the crossing midpoint. The waves are strongest at directions of 90 and 135 degrees due to the fetch and winds in these directions. The 1 year return period conditions were used to analyze the possibility of operating in a once-per-year condition. For example, vessel accelerations and roll must be acceptable under these conditions. The 50 year and 100 year extreme conditions were used to assess the system design. This included a damaged conditions scenario, where one guide cable has been released, and the other guide cable and traction cable must take the load. When using dynamic analysis, safety factors for the damaged condition and the intact condition are specified in API RP 2SK as 1.25 and 1.67, respectively. These values were used to justify the use of a safety factor for the mooring design of 1.3 and 2.0, respectively (API, 2008). A safety factor of 5.0 is also applied to the pretension value used in the cables.

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Figure 17: RAO response at 4m/s forward speed and 90 deg absolute wave direction; note the yaw natural period due to the extra cable stiffness in the yaw response. Table 4: Wind, wave and current conditions considered in analysis 1 year 100 year Extreme Wind speed (m/s)

11.53

19.98

28.29

Significant wave height (m)

0.67

1.03

1.03

Peak period (s)

3.39

4.63

4.63

Current speed 0.69 (m/s)

0.97

0.97

FREQUENCY DOMAIN ANALYSIS The numerical wave radiation and diffraction analysis was completed with ShipMo3D. A database of hydrodynamic coefficients was produced that enable wave radiation and diffraction effects to be incorporated into time domain simulation through the use of the Cummins equation (Fossen, 2011). The wave radiation and diffraction loads are resolved

Design Methodology and Numerical Analysis of a Cable Ferry Design Methodology and Numerical Analysis of a Cable Ferry

through frequency domain analysis and are therefore inherently linear. While the draft of the ferry was relatively shallow, the large waterplane area and barge hull form is well-suited for potential flow analysis as wave radiation tends to dominate hull viscous effects in damping vessel motion. The nonlinear time domain model was used to numerically compute a linear constant spring stiffness to represent the cable reaction loads at several locations along the crossing to ensure interaction with the bottom was considered. No linear equivalent damping from the cables was incorporated and this was deemed conservative since it would reduce the ferry motion in the frequency domain results. In addition to generating the hydrodynamic database, the response amplitude operators (RAO) that indicate the ferry motion as a function of incident frequency and ferry forward speed were computed. The resulting RAOs were used to assess ferry motion for various weather directions and forward speeds. The most important result indicated that at maximum forward speed, opposing seas from waves in the 90 degree absolute heading (see Figure 3) could generate unacceptable pitch and roll motion. This is indicated by RAOs in Figure 17. However, this effect is significantly reduced as the ferry forward speed decreases and can therefore be addressed by operational restrictions. Another important result was the cable stiffness indicated a yaw natural period of approximately 15 seconds, as shown in Figure 17. However, this is too far above the expected ocean wave periods to induce any motion. There were relatively minor variations in RAO values along the span of the crossing, which indicates the interaction with the sea floor plays less of a role than pretension in influencing ferry response in the linear range. The RAOs produced were also useful as a further verification of the time domain analysis completed.

(NPD) wind spectrum model was used. The wind model models wind gusts based about the 1 hour mean wind speed. For each simulation case, three wave seeds were used to generate three unique random wave realizations. Each wave seed is used to generate a randomized wave surface based on input parameters of significant wave height, mean wave direction, spreading function and wave period. This was done to ensure that statistically consistent results are being realized and that accurate maxima are determined.

Transit Analysis The nonlinear time domain simulation analysis of the ferry transit completed produced data that was used to guide the ferry structural design, selection of the cables, and design of the ferry terminal. The transit simulations were conducted using the 50 and 100 year return period conditions with coincident extreme wave, wind and current conditions from 90 and 135 degrees. Loading from other directions would be significantly less due to the reduced wave heights. Simulations of the ferry crossing both to and from Buckley Bay were completed. The loading varies depending on the heading of the vessel (following vs. head seas). To complete the simulation at the target speed it was necessary to simulate the winch behavior. The ferry is propelled by increasing the tension in the traction cable. To apply this tension a PID controller was used. The PID controller monitored the ferry forward speed and adjusted the force applied to the cable to mimic the bull wheel system. This controller allowed a desired transit speed to be set for the simulations. Simulations were completed at 3.9 m/s and 1.9 m/s.

TIME DOMAIN ANALYSES Time domain analysis of the ferry while in transit, while stopped during the crossing, and while docking was completed. The transit analysis was used to assess operability of the ferry in the extreme conditions and determined input values for strength analysis. The analysis of the ferry response while stopped during the crossing was used to assess braking system loads. The docking analysis was used to assess the ability of the ferry to offload vehicles in extreme conditions (by examining relative roll) and provide data for detailed design of the terminal facilities.

Wind, Current And Wave Models For each simulation, a JONSWAP wave spectrum was used with 1200 wave segments to ensure a non-repeating random sea condition. A constant uniform current was applied through the water column1. As recommended by several offshore standards (API, 2008; DNV 2010), the Norwegian Petroleum Directorate 1

A log or power law current profile that tends to 0 m/s near the seabed should be used in future projects to better model the current profile. (Soulsby, 1997)

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Figure 18: Dynamic tension shows very large period (~600 sec) from ferry lateral offset and cable uplift during transit, medium period (~50 sec) due to wind load on the ferry in sway, and high frequency (~2sec) due to wave load on the ferry The effect of cable pretension on the cable ferry was also checked with the time domain simulations. Figure 21 (at end of paper) shows the typical drift from the crossing centerline. A maximum lateral drift of 92m was found with a pretension of approximately 200kN. The amount of pretension in the system

Design Methodology and Numerical Analysis of a Cable Ferry Design Methodology and Numerical Analysis of a Cable Ferry

303

affects the excursion distance in extreme conditions from the transit centerline. It also affects the maximum tensions observed, and the ferry roll motion. The maximum tension in the cables during the 100 year storm was found to be 571kN in the beam sea loading case. Several simulations were executed to observe the impact of increasing the pretension in the system. A catenary shape provides some compliance to the cable ferry system, allowing it to respond to changing conditions without severe changes in cable loads (e.g. snap loads), as would be the case in a taut mooring system. However, too much tension results in high cable loads in extreme conditions and a stiff system response. The higher the pretension the more effect the cables have on the vessel dynamics. The dynamic tension in the cables from wind and wave loading during a portion of ferry transit can be seen in Figure 18.

a maximum traction winch force available for braking. The ferry was able to stop within 65 m (less than one ferry length) during 100 year wave and current conditions and with conservatively high 55 knot winds, given a traction braking load of 69 kN.

The ferry roll motion was determined during the transit simulations. Maximum roll angles of between 8 and 12 degrees were observed for the ferry transits. Interestingly, at higher wind speeds, the larger mean cable tensions reduced roll motion of the ferry, resulting in lower roll angles than some simulations with lower wind speeds. Table 5 presents a summary of the transit simulation results.

Damaged Condition Analysis

The key data produced through the transit simulations were: i. Bull wheel / traction winch system power and load limits ii. Loads on fairleads on the ferry iii. Ferry roll motion iv. Lateral deflection during the crossing v. Ferry accelerations at aft/forward port and starboard corners and in the passenger lounge vi. Air gap during crossing vii. Maximum cable reaction loads at anchor points viii. Maximum cable tensions ix. Maximum cable tensions and ferry roll motion were assessed in a damaged state (one guide cable released) Table 5: Summary of transit simulation results Max Vertical Cable Drift Acceleration Condition Pitch (°) Roll (°) Load (m) (m/s2) (kN) 100 Year Storm

429

74.6

0.91

12.66

1.13

Extreme Storm

571

91.8

0.81

9.94

1.46

Damage Case

691

102.8

0.60

8.03

0.66

Braking Analysis The ferry’s capacity to perform an emergency stop in the 100 year wave and current conditions was simulated. The maximum stopping distance in this emergency situation was assessed given

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Emergency stop simulations were completed with the ferry at locations corresponding to 20%, 40%, 60% and 80% of the crossing distance. The brake was applied on the traction cable and the maximum braking loads observed. This corresponds to a friction brake that might be applied to the traction winch system. The maximum loads in the cable were measured, and a maximum of 420 kN was found. The stopping distance did not significantly vary, which in these loading conditions indicates the inertia effect of the cables did not have a significant impact compared to the inertia of the ferry.

The cable ferry system was tested to determine the response when one of the guide cables has failed or parted due to some unforeseen event. For the analysis a guide cable was removed and the brakes applied to the cables. The tensions in the cables observed when the ferry was at a position corresponding to 40% and 60% of the crossing in the 100 year storm conditions. The tensions in the cables were observed to ensure that at a safety factor of greater than 1.3 was achieved to avoid the remaining cables from breaking. The maximum cable tension measured in the damaged case was 691 kN.

Docking Analysis The docking analysis consisted of two primary sets of test cases. The first set analyzed the effect of the ferry impacting the pontoon, and the second analyzed the interaction of the ferry and dock while the ferry was docked with the pontoon. The setup of the cables, pontoon and ferry is shown in Figure 20. The piles which restrain the pontoon from moving in the simulation are not shown. In the impact study, several simulations were run with the ferry impacting the dock at 2 knots. The simulations started with the ferry 100m from the dock. The PID winch controller was placed under speed control to drive the ferry forward. These tests revealed the impact loads given a bumper stiffness. In these simulations the loads on the dolphins (pile groups) are assessed. This approach allowed for careful assessment of the complex impact and ferry sway loads. In addition, the angle of approach of the ferry is assessed. These tests were designed with the purpose of setting the structural load requirements for construction. The maximum surge force on the pontoons during impact were ~3300 kN and the maximum sway load on the pontoon was 2800 kN. The pontoons must withstand a total yaw impact moment of 47000 kNm. These forces are the maximum instantaneous loads during impact, and the shore structures do not need to withstand these loads for long periods. The second set of simulations conducted for the docking analysis focused on the dynamics of the ferry and dock during extreme conditions. The relative roll of the ferry and the dock

Design Methodology and Numerical Analysis of a Cable Ferry Design Methodology and Numerical Analysis of a Cable Ferry

were assessed to ensure emergency offloading in the 100 year storm event. A wing wall is often used to ensure alignment of the ferry during berthing and to reduce relative roll of the ferry. Simulations were additionally used to assess the effect of wing walls on the sway loads and yaw moments applied to the dock. It was found that the wing wall reduced the relative motion of the ferry by approximately 50%. The heading of the ferry relative to the dock was also measured in the simulations. This was done to ensure that the off-loading ramp on the ferry had sufficient overlap even when the ferry was being impacted by gusting wind and irregular waves. The maximum relative heading between the pontoon was found to be around 8-12° in the 100 year storm and extreme conditions, and 2-3 degrees in the 1 year storm. The relative roll during offloading conditions (1 year storm) was found to be a maximum of around 4 degrees.

FERRY SERVICE Using the data from the analysis, the service expectations of the ferry were estimated. The service expectations were determined based on an analysis of the frequency of occurrence of certain wave sizes, the roll and motion response of the ferry, and the strength of the securing cables. For the Denman Island Ferry, it was determined the ferry would be able to maintain a highly reliable service (99.4%) and that fewer than 35 trips a year out of approximately 12,400 trips would be affected to the extent that a trip might be delayed due to conditions. Two operating modes were assessed, docked and transit. For each of these modes an operation limit was assessed. The limits presented in Table 6 were used to determine at which point operations must cease based on the weather conditions.

FATIGUE ANALYSIS Table 6: Motion and seakeeping criteria An investigation was completed to assess the influence of environmental loading on the fatigue life of the cables. This consisted of checking the fatigue life of the cables due to the dynamic tension that resulted from storm condition wave, current, and wind loading on the ferry as it crossed the channel as well as separately investigating the effects of current load and the resulting Vortex Induced Vibration (VIV) damage. Rather than accurately computing the fatigue life of the cables, a simpler but more conservative approach was used to compute the lower bound of resulting fatigue life to assess if there was any risk of short-term failure due to fatigue. The process in mooring standard API RP 2SK was used to compute the fatigue life in storm conditions as the ferry crossed the channel. The process consists of using the rainflow time domain cycle counting method to establish the expected value of tension range during the crossing. The total fatigue life was computed assuming the ferry is always crossing, the 100 year storm acts 50% of the time, and the extreme storm condition acts the remaining 50% of the time. In spite of these very conservative assumptions, the resulting fatigue life was resolved at 89 years for the wire rope selected. To compute the fatigue life due to vortex shedding on the cables and the resulting VIV that could occur, it was assumed that the cables had uniform and constant tension. It was assumed the maximum wind-induced current velocity of 1m/s (vs a tidal maximum of only 0.2m/s) was constant throughout the water column and that it acted 100% of the time. This is conservative because higher currents will induce faster cable vibration and larger bending stresses. Even with these conservative assumptions, the range of bending stress induced was too small to have any significant impact. The resulting fatigue life due to VIV alone was many orders of magnitude longer than the expected fatigue life due to wind and wave loading. The fatigue analysis did not consider the abrasive damage due to interaction of the cables with the sea bottom, winch equipment, dock infrastructure, or general handling. These effects are expected to be important but the cables can be regularly inspected for excessive wear due to these effects.

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Criteria

Condition

Vertical Acceleration