A s in most businesses, winch and deck ma- chinery builders often focus their efforts on specialty areas of the marine market in which they command a deep knowledge of the engineering and customer require- ments necessary to provide reliable long term service in their chosen niche. This knowledge derives from years of innovation, trial and error, and finally the methodical adherence to the design rules, carefully carved in stone tablets, that provide some measure of certainty and secur ity as new engineers and projects enter the scene. Perhaps this process has not changed since humankind first sketched in sand and committed the result to animal skins. What is significant and relevant to the world of deck machinery as we move ahead in an age of increasing complexity, safety and security are the use of new materials such as HMPE fiber ropes, the success of marine rated variable frequency electric drives in many machines as a viable alterna tive and improvement over hydraulic systems, new computer based design tools and production machinery, and of course our ability to communicate quickly and globally, including in the following design example, our ability to observe in real time the inner workings of our cus tomers’ deck machiner y , By Barry Griffin and Gary Nishimura HOW TO DESIGN A BIG NEW WINCH This photo shows the Markey- supplied winch for Crowley's Response, with the below-deck components in grey. Photo courtesy of Markey Machinery. winches, and ropes over the Internet. When the customer arrives in the office with a new set of plans and opportunities beyond the current state of any company’s current machinery or knowledge, one can say either “Too risky, no thanks, perhaps we’d like to study it further, etc” or “L et’ s get going!” Both reac tions are legiti- mate and understood as appropriate among customers and suppliers with long-term relationships. Markey Machinery Company accepted the opportunity and challe nge to design a new class of winch for ship handling service on the open ocean. The following is intended to illustrate t o the marine community the way deck machinery engineers are solving today’ s c hallenges. Industry Recommendations Markey Machinery is in the final design sta ge of winches to be installed on new, purpose-built tugs to be stationed at the Costa Azul LNG terminal in Ensenada, Mexico and operated by Moran T owing and their partner Gr upo Boluda. The winches will be placed on tugs designed by Robert Allan Ltd. and intended to maintain an average of 70 tons line pull in 3 meter sea s with a 10 sec ond period. T wo electric drive motors are capable of delivering 760 hp during the inhaul DESIGN METHODOLOGY OF WINCHESFOR USE IN DYNAMIC SEAS BY BARRY GRIFFIN AND GARY NISHIMURA HOW TO DESIGN A www.pacmar.com • July 2007 • Pacific Maritime 1
As in most businesses, winch and deck ma-chinery builders often focus their effortson specialty areas of the marine market inwhich they command a deep knowledgeof the engineering and customer require-
ments necessary to provide reliable long term service intheir chosen niche. This knowledge derives from yearsof innovation, trial and error, and finally the methodicaladherence to the design rules, carefully carved in stonetablets, that provide some measure of certainty and security
as new engineers and projects enter the scene. Perhaps this process has not changed since humankind first sketchedin sand and committed the result to animal skins. What issignificant and relevant to the world of deck machinery aswe move ahead in an age of increasing complexity, safetyand security are the use of new materials such as HMPEfiber ropes, the success of marine rated variable frequencyelectric drives in many machines as a viable alternative andimprovement over hydraulic systems, new computer baseddesign tools and production machinery, and of course ourability to communicate quickly and globally, including inthe following design example, our ability to observe in realtime the inner workings of our customers’ deck machinery,
By Barry Griffin and Gary Nishimura
HOW TO DESIGN A
BIG NEW WINCH
This photo shows the Markey-supplied winch for Crowley'sResponse, with the below-deckcomponents in grey. Photocourtesy of Markey Machinery.
winches, and ropes over the Internet.When the customer arrives in the office with a new set
of plans and opportunities beyond the current state of anycompany’s current machinery or knowledge, one can sayeither “Too risky, no thanks, perhaps we’d like to study itfurther, etc” or “Let’s get going!” Both reactions are legiti-mate and understood as appropriate among customers andsuppliers with long-term relationships. Markey MachineryCompany accepted the opportunity and challenge to designa new class of winch for ship handling service on the openocean. The following is intended to illustrate to the marinecommunity the way deck machinery engineers are solvingtoday’s challenges.
Industry Recommendations
Markey Machinery is in the final design stage of winchesto be installed on new, purpose-built tugs to be stationedat the Costa Azul LNG terminal in Ensenada, Mexico andoperated by Moran Towing and their partner Grupo Boluda.The winches will be placed on tugs designed by Robert AllanLtd. and intended to maintain an average of 70 tons line pullin 3 meter seas with a 10 second period. Two electric drivemotors are capable of delivering 760 hp during the inhaul
DESIGN METHODOLOGY OF WINCHESFOR USE IN DYNAMIC SEAS
winches become more complex, so do the controls in the wheelhouse. Photo courtesy of Markey Machinery.
and expand, they continue to push thelimits of winch and rope technology,traditionally based on calm-water as-sumptions.
The LNG industry is currentlydriving the need for ship handling
capabilities in unprotected waters.Several offshore terminals have startedconstruction, and many others have been proposed and are awaiting ap- proval. Their profitability will dependon routine service of LNG tankers inexposed ocean conditions. The largewaves frequently encountered at manysites will induce significant relativemotions between the tug and tanker,making conventional winch and ropetechnology inadequate. These appli-cations will typically require a newgeneration of equipment, designed
specifically to accommodate thesedynamic environments. But withlimited information, on boththe environmental conditionsand relative ship motions, howdoes one design for these newapplications?
A natural instinct is tocompare a new situation with previous experiences- in thiscase, a physical sense of themotions induced by waves un-der a boat, a rough idea of theforce required to move an object
through water, and a feel for thetension present in a line, basedon its behavior. These experi-ences allow educated guesses by those working in similarconditions. Talking with othersin the industry, we polled thevast experiences of naval archi-tects, marine engineers, and tugoperators, gathering a range ofopinions and recommendations.In many cases, the preliminarydesign process would stop here.With the seasoned experiencefrom multiple perspectivesin the industry, there is oftenenough evidence to justify afinal design without going anyfurther. However, in this case,
we were surprised by the widely variedfeedback, with total power estimatesranging from 115 hp to 2,000 hp. Inaddition, this must be weighed againstthe cost and consequences of failure.
Liquefied natural gas, as an energycarrier, poses unique challenges. Thefuel is a gas at atmospheric conditions,making it more difficult to contain
D e c k M a c h i n e r y
cycle, and water-cooled slip brakes areused to maintain a maximum of 100tons during payout.
Our design effort for this new andunique winch application began witha survey of opinions and recommen-
dations from the industry, which gavewidely varied feedback. We also drewfrom past experiences gained from theasymmetrical render-recover winchesinstalled on the Crowley Response,the Bulldog of Crescent Towing,and the Edward J. Moran of Morantowing. While these projects sharedmany design elements, their dynamic performance was intended to maintainline tension for maneuvering and repo-sitioning purposes, not to accommodatedynamic sea states. Nevertheless,the Crowley Response, operating in
the Strait of Juan de Fuca with a 250hp winch drive, is routinely tested in
energetic seas. Although the sea statedynamics are somewhat less demandingthan those at Costa Azul, the abilitiesand limitations of the Response in theseconditions provided important feedbackfor preliminary design estimates.
To further refine the performancerequirements, a 1:24 scale, self-pro- pelled ASD tug model was connected
to a rigid panel, extending below thewaterline to simulate the tanker. Nearly100 tests were run in a 100-foot longtowing tank with a wavemaker at oneend. These results were also comparedwith a second model test, based on
a larger model and tested in a largerand deeper wave tank. After extensiveanalysis of the data, a maximum speedof 1.5 meters per second and a maximumacceleration of 1.5 meters per secondsquared were found. Using the formulafor power, maintaining 70 tons line pullat this speed requires more than 1,250hp. Utilizing the asymmetrical opera-tion, this was reduced to 760 hp. Theresult is a relatively low power winch,capable of averaging high line tensionsin dynamic sea states.
Tugs for LNG Terminals
Historically, shiphandling opera-
tions have been focused around portsand harbors, in somewhat protectedwaters. Although these locations hadtheir share of challenges, they seldominclude large, unsuppressed waves. Asa result, there has been relatively littleattention paid to the operational require-ments of ship handling in open seas.However, as these operations evolve
than liquid fuels, and increasing therisk of explosive mixtures collect-ing in confined areas. Under storageconditions, it also has a relatively lowdensity, requiring larger, but lighterships that are more difficult to controlin windy conditions. On a global scale,the LNG business is emerging during aturbulent time of energy concerns. Asa major component of today’s availableenergy forms, it is intertwined with the political, economic, environmental,and social repercussions of worldwideenergy use and supply. These factorsmake LNG handling a challenging, yetcritical operation. Combined with thelack of definition for the winch design,it was clear that further analysis would be required.
In this application, a fundamentalrequirement to a successful design isa good understanding of the relativemotion between the tug and the tankerin dynamic seas. Starting with simpleapproximations based on wave heightand period, some rough calculations es-tablish a general order of magnitude forthe required speed and power. The tankeris assumed to be stationary, greatly
simplifying the problem. Assumingthe waveform can be represented by asine wave, the speed of the tug in thevertical direction is approximated bythe derivative of the position functionas follows.
Vertical Position = .5 (Wave Height)Sin (b t)
Vertical Speed = .5 b (Wave Height)Cos (b t)
Where b = 2 / Wave Period, t =time, and Wave Height is measuredfrom trough to peak.
This represents the vertical motion,which becomes a significant factor whenthe tug is near the tanker and the linelead slopes sharply upward. The powercan then be calculated based on therequired line tension and the maximumspeed from the function above. Thegeneral equation for power is:
Horsepower (hp) = (line tension)(speed) / η
Where hp = (Newton) (meters persecond) / 746 = (lbf) (feet per minute)/ 33000
The result is the winch powerrequired to maintain a specified linetension as the tug moves vertically.This can be a convenient figure forrough sizing; however it neglects manyimportant factors. The horizontal move-ment, or surge, is equally importantat short line lengths, and dominatesthe relative motion when the tug isat greater distances from the tanker.The rolling and pitching motions can be significant in nearly all positions.Additionally, the tug does not exactly
follow the waveform due to the inertiaof the tug and the forces exerted by the propulsion and the line. These effectsare not easily interpreted from thewaveform and are driven by complexhydrodynamic properties of the tughull. The acceleration capabilities ofthe winch must also be consideredsince these motions are not constant.Oversizing the motor can help ensuresufficient line pull when motions aresomewhat unknown. However, thisincreases the inertia of the winch drive,
often reducing its acceleration and itseffective speed in dynamic conditions.Finally, the waveform itself is not a pure sine wave as assumed above.Reflected and rouge waves quickly addirregularities that are nearly impossibleto predict.
Model Testing
To obtain a more comprehensiveunderstanding of the tug motions, aswell as the line tensions from shockloads (resulting from slack line comingtaut), model testing provides valuableinsight. Accelerometers, placed on a
self-propelled tug model, give detailedinformation about the various motions.In addition, force sensors measure theline tensions experienced by the model.Although shock loads are known togenerate high line tensions, we wereamazed to measure an equivalent of552 tons as the model surged forward,and then aft, pulling the line tight. To prevent these dangerous conditions,the winch must inhaul and payout lineto match the relative motion of the tugand tanker. At the same time, it mustalso apply the line tension required for
maneuvering. It can be seen from theformula for power that as the speedincreases, so does the power require-ment for a given line tension. There-fore, with larger waves and shorter periods, even moderate line tensionscan require excessive and unrealistic power levels.
To reduce the power requirements,an asymmetrical operation is used,similar to the techniques employedin deep-sea rod and reel fishing. Infishing, the star-drag pays-out line at
This series Illustrates the force relationship in the hawser between tug and ship in calm water. Graphics by Barry Griffin.
a high tension level. The fisherman then inhauls the line atlow tension. The pull on the fish varies from a high tensionto near zero, averaging somewhere in the middle. On thetug, because it is easier and cheaper to dissipate energy asheat rather than to generate it from fuel, the line tension isincreased during pay-out and reduced during inhaul. Theresult maintains the average line tension, but reduces de-mand from the tug’s engines. In both cases, the maximumline tension is limited by the connection; the line itself, orthe end fittings. Therefore, to increase the average tension,the low tension (inhaul) part of the cycle must be increased.For the winch, this defines the power consumption of thedrive system.
Real World ConditionsTo establish the final requirements of winch performance,
empirical formulas based on the model tests are used. Thetug is seen here as a spring-mass system, where the springrepresents the rope and waves, while the mass includes thetug and the displaced water. Most parameters can be scaledaccurately to represent real world conditions. Physical size,mass, speed, acceleration, and momentum can be easilyinterpreted from the model tests. However, dampening con-stants, related to the wave size and speed, are more difficultto scale since the behavior of a wave differs with its size.Although good, these parameters are not perfect, leaving asmall margin of error.
Numerical Modeling is a promising alternative to con-ventional analysis. Initially, it still requires model, or full-scale testing to establish specific parameters. However, oncedefined, the simulation can be modified to simulate differentscenarios for sea state, tugs, and operational requirements.This allows a very fast analysis of new projects for bidsand proposals. We are currently developing a proprietary program that combines a mooring simulation with a shipmotion simulation. The mooring simulation considers the
spring and mass behavior of the line and the tug, accountingfor the influence of the line tension on the tug motion. Theship motion component considers the dynamic behaviorof the tug in open seas, with relative speed to the passingwaves. By merging these two functions, we hope to form asingle program, capable of simulating any combination ofwinch, tug, and sea state.
From educated guesses, to model tests, to numericalsimulations, approaching the problem from different anglesminimizes the chance of misconceptions and oversights.With multiple analysis giving similar conclusions on per-formance requirements, we are finally ready for the detaileddesign and manufacturing stages. When the first machines
are installed early next year, we’ll begin accumulating realworld experience, using the winch drive to record and sendreal-time operational parameters via the internet. Combinedwith the feedback from tug operators and crews, the designwill see continual development as a new set of stone tabletsare carved for a new and demanding environment.
Barry Griffin is a Harvard graduate with more than 20 years of engineering and sales experience in the marineequipment business. Since 1992 he has logged more than850 ship-days observing winch and vessel operations specializing in high performance winch and rope systemsas a manufacturer’s representative for Markey MachineryCompany, Puget Sound Ropes, Smith-Berger Marine, Ocean
Spar Technology, and PERKO Commercial.
Gary Nishimura is a native to the Pacific Northwest anda recent graduate from the University of California, Davis in Mechanical Engineering and Material Science. He startedwith Markey Machinery just more than a year ago and hasenjoyed his exposure to the marine community and thecomplex challenges of off-shore ship handling.
Note the increase in hawser tension as a result of more tug weight placed in the bight of the line with tug working closer to the ship.
