Ship Assist and Escort Winches for Dynamic Seas The ARR Winch for Crowley Maritime Tug RESPONSE

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1 (ITS 2004 logo here) International Tug & Salvage Convention Organised by the ABR Company Limited Ship Assist and Escort Winches for Dynamic Seas The ARR Winch for Crowley Maritime Tug RESPONSE Barry Griffin, B.A. Griffin Associates, Inc. USA SYNOPSIS: The concept, design engineering, and first year of operation of the Asymmetric Render Recover (ARR) electric winch system on Crowley Marine Service s Escort Tug RESPONSE is presented. A brief overview of the history of North American hawser winches leading to the definition, engineering, construction, trials, and operation of an all electric winch capable of render-recovery in excess of the maximum indirect bollard pull of escort tugs in tanker arrest maneuvers is detailed. The operational issues leading to the need for ARR performance is demonstrated. Construction photos and videos of winch and tug-tanker operations are included. INTRODUCTION Line handling systems for North American harbor and near-shore assist tugs have evolved in three phases over the past 30 years. During this period merchant ships have become larger, faster, and more specialized. Assist tugs have increased their size, power and performance to keep pace with new maneuvering requirements. The development of tanker escorting and indirect towing created a separate need for specialized tugs, hawsers, and deck machinery. Traditionally, and to a certain extent continuing today, ship assist lines are worked by hand off the tug s bow and stern bitts. As new build tug size and horsepower (HP) increased there became a point where a line with adequate breaking strength became too big and heavy for a man s hand to grip effectively. This size is generally considered to be 75mm diameter. Simultaneous advances in rope technology led to significant increase in fiber strength and abrasion resistance, and reduction in the weight and size of standard assist lines. For comparison, consider that an 80 mm diameter Manila hawser of the 1940 s with a breaking strength of 38 tons can today be replaced with a Ultra High Molecular Weight Polyethylene (UHMW-PE) hawser of the same diameter, lower weight, and a breaking strength of 427 tons. Improvements in assist lines allowed the higher HP tugs to continue working lines over capstans and bitts. It is not uncommon to still see a pack of six or more 2,000-4,000 HP tugs head out to help a major container ship round Bergen Point in New York harbor in a blow, working 75mm and 100mm diameter lines by 1

2 hand on bow bitts. Another common strategy for tugs above 6000 HP is to deploy two lines in parallel to achieve the required breaking strength. The second phase began with the application of Voith and ASD propulsion systems to assist tugs. The maneuverability and force levels now possible created the desire to have direct and instantaneous control of the tug assist line from the pilothouse. Two high power agile tugs with fewer crew could now accomplish the work of four or more traditional tugs. At the same time, crew cost and safety became important issues in new tug design. Man-handling lines was now too slow and too risky. Bow winches were fitted to handle wire and softline on all types of tugs. Traditional wire tow winches were also fitted with synthetic tow hawsers when necessary. This approach continues to this day. As a side note on hawser sizing, it should be mentioned that current hawser winches are almost exclusively fitted with UHMW-PE softlines with initial breaking strengths in the range of five to seven times the tug s static bollard pull. These large safety factors are necessary to handle the substantial gravity loads (proportional to tug displacement) that occur when working ships at high lead angles, as well as loads that would be encountered when working in the indirect towing mode, if present. Lines typically fatigue to one half their initial breaking strength before being retired from service. The initial five to one safety factor therefore maintains at least a two and one half to one safety factor throughout a ropes useful life, which is more in keeping with commercial wire rope towing practice. The current and third phase of winch design addresses the special needs of tanker escort tugs which developed after the Exxon Valdez incident. Current tugs in Valdez and North Puget Sound typically have twice the HP of the previous generation. The following paper discusses the development of a special winch for this service. CLASS 1 ASSIST WINCHES Until recently, winches and ropes for tugs in US flag escort and assist service had changed little since Despite major changes in tug operations, propulsion systems, and hull form, a basic winch satisfies most needs. The standard hawser winch derives from common towing and mooring winch designs with the following special emphasis: Line drum for Spectra/Plasma (UHWM-PE) softlines. Winch frame of stout design to absorb high shock loads. Drum brake with high static holding power band brake(s) Escape function to provide high speed instant hawser payout. Line recovery speeds approaching three knots. 2

3 It is useful to group these basic winches into a Class 1 category when discussing their capabilities, as shown in Figure 1. Typical winch performance provides a line pull equal to 20% to 40% of the tug s static bollard pull, and line payout and retrieval speeds approaching 20% of the tug s free running speed. Remote pilothouse controls allow simple and direct proportional payout and retrieval of the tug hawser, in addition to automatic and manual control of the massive drum friction brake(s). Pressing the large and high visible ESCAPE button allows the brakes to instantly release, even under full load, and free the drum to payout the hawser at high speed. 3

4 Most assist tugs are fitted with 600 to 1200 feet of hawser. Therefore it can be appreciated that upon escape at speed, little time remains before coming to the end of your rope. For this reason a separate brake override lever is provided to hopefully stop the drum before this occurs. Unfortunately any subsequent differential in speed between the tug and ship creates a significant shock load 4

5 when the brake is set and the hawser comes taut. For example, I have seen tension spikes over 300 tons when this occurs on a 4400 HP harbor tug at an indirect towing speed of six knots and a probable tug to ship speed of two knots. Attenuating these and other inadvertent loads led to the development of the Class 2 winch. CLASS 2 ASSIST WINCHES Crowley Maritime conceived the original Class 2 winch, or Render- Recover operational concept for use on Crowley s tanker escort and assist tugs. Their idea was to develop a winch that would increase the performance, speed, and safety of tug escort and assist operations by automating certain winch functions. The primary goal was to allow the winch to automatically payout and recover the softline connecting the tug to the assisted vessel during extended tethered transits, thus reducing operator fatigue and wear and tear on the tug and towing hawser (see Photo 1). They were clear in distinguishing the future need for rendering in excess of the traditional constant tension winch, and thus the notion of a Render-Recover winch with potentially higher retarding of payout forces was envisioned. The Render-Recover term implies asymmetry as opposed to the symmetrical payout and haulback of constant tension machines, such as the familiar ship mooring winch. VMS asked Markey Machinery Company to develop and retrofit their concept on two existing tugs with traditional Class 1 winches. The system has worked better than expected, and has become a standard feature on over 20 US flag tugs now fitted with the Markey system. As shown in Figure 1, the Class 2 winch provides adjustable automatic constant tension inhaul up to the innate power limits of the winch drive. It is important to note that no constant tension winch can provide significantly more power in dynamic braking or load attenuation than it can provide hauling in the load, unless some other means is provided to add braking power. This limitation led to the development of the Class 3, or Asymmetric Render Recovery winch. CLASS 3 ARR WINCHES Crowley Maritime and Vessel Management Services, Inc. along with B.A.Griffin Associates and Markey Machinery recently completed the development and deployment of the first Class 3 hawser winch on Crowley Maritime s 7200 HP Voith tug RESPONSE for assignments in North Puget Sound. As shown in Figure 1, the ARR winch system has significant performance improvements over previous winch systems, including: 5

6 Tow Hawser Render and Recover during Indirect Towing. Increase in winch pull from 50% to 250% of static bollard pull. (see Photo 2) Improved maneuvering speed. Increase in winch speed from 20% to 50% of free running speed. Improved shock load control. Rapid and precise dissipation of 1600 HP of system energy. Improved control of hawser spooling on the winch drum. Pilothouse control of drum spooling to 1.4 million pound side load. High overall system efficiency and redundancy. 250 HP combined total HP, using two motors and VFD-AC drives. Low cost per winch HP. Ten times the line pull and five times the line speed with three times the overall cost. RATIONALE Crowley Maritime wanted to significantly increase tanker escort tug performance and vessel safety in the dynamic sea conditions often encountered in the North Puget Sound. Relatively shallow inland seas combine with a 100 Km fetch to Pacific Westerlies to create steep short seas of three meters with six to eight second periods. Several oil terminals are roughly perpendicular to the weather, thereby compounding the assist issues when sailing a tanker broadside to the seas. Adding a Nylon surge pennant midway in the standard UHMW-PE hawser was briefly considered as a means of reducing the extreme shock loads that occur as the tug pitches in head seas at full bollard pull. The Nylon method has been used successfully in escort modes. However, Crowley was looking for a more universal solution. In these seas, and for the purpose of this discussion, several operational challenges arise: Connecting and maintaining the hawser during hour long tethered escorts. Quickly applying steering and braking maneuvers from non-ideal positions. Putting a tanker to sea from the terminal as weather deteriorates. Simultaneous control of heading, propulsion, and winch in high energy maneuvers. Uncertainty of hawser tension and deployed length. Re-establishing hawser control following a hawser abort event. Preventing pull-down and burying of the hawser on the winch drum. Crowley asked the following question: Could the existing Render Recover system concept be improved to address the expected operational requirements, including full indirect towing to 300% of bollard pull? 6

7 It was my assignment as the winch and hawser system engineer to answer this question, and eventually lead the project team that built, installed and commissioned the winch and control system. Review of existing winches revealed the following issues and led directly to the final design decisions. Electric winches offer the highest overall system efficiency. The existing and proposed hydraulic winches could provide high pull and high speed but with 20% less power when asked to provide both power and speed. Auxiliary HP is typically limited on tugs of this type, so efficiency is an important issue. It was decided early on to focus on an all-electric system with redundant auxiliary electric generators and winch motors, capable of remote and parallel control from the pilothouse. After one year of service the electric drive has proven to be much more powerful, significantly quieter, and more accurate to control than our previous best hydraulic system. In addition, the control computer associated with the electric motor system integrates well with the overall computer control of the winch. This overall control allows the winch motors, the three speed automatic gear transmission, the torque measuring load cell, and the independent dynamic brake to provide nearly ten times the power and operational envelope than previous systems. The electric drives dissipate regenerative braking energy into resistive grid banks sufficient to dissipate the full winch power when rendering the hawser at full drive performance. Synthetic hawsers are difficult to spool at high load. No existing highpull softline winch appeared to address the question of how the line would spool, lay, or bury on the winch drum at such high loads, especially at the winch side flanges, with the typical 12 strand UHMW-PE hawsers in current use. We have had some success with what is termed open weave spooling, in which the rope windings cross over one another in a proprietary pattern. Narrow drums without spooling gear are another option, although these drums compromise capacity or full drum performance. It was decided to provide independent control of the drum spooling system to allow the Captain to override the automatic spooling by 7

8 moving the normal winch control lever to port or starboard to create a similar motion of the spooling head. This system has worked very well and provides the ability to lay the hawser on the drum with complete control even under full escort indirect towing forces. Typical winch brakes are not suitable for dynamic conditions. Traditional band or caliper drum brakes lack sufficient control and thermal capacity for continuous response to the anticipated high speed and long duration shock loads. It was decided to rely on a low inertia and water cooled dynamic braking system instead of the typical static band brakes. We have used dynamic brakes before on anchor handling tugs for dissipating the anchor lowering loads. We also investigated their use on marine cranes before a final decision was made. The eventual design employs redundant fresh water cooled disc brakes, capable of continuously dissipating a combined 1600 HP (see Photo 3). Each brake contains a series of plastic and copper friction disc elements. The discs are unusual in that their static friction coefficient is lower than their dynamic coefficient, or just the reverse of common winch braking materials and drum surfaces. This difference eliminates the stick-slip jerking behavior of typical winch brakes when attempting to payout heavily loaded line using the brake alone. This static/dynamic coefficient relationship and the inherent linear nature of disc brakes enables the total braking force to be directly proportional to the air pressure used to compress the discs together. In the ARR winch the electrical command signal from the pilothouse or winch computer directly controls an air pressure actuator mounted close to the brake, which in turn creates the required amount of braking effort. Heat generated is carried away by the multiple freshwater cooling jackets and dissipated in a sea water heat exchanger. The Asymmetric function occurs when the hawser tension exceeds the power capacity of the electric winch drive grid bank. Within milliseconds the dynamic brake applies a restraining load equal to the electric drive capability, releases the electric drive, and continues to apply braking until the load lessens to a level where the electric drive can be safely and automatically engaged. For utmost safety, the electric and pneumatic control of the brake is derived from the tugs 24 Volt DC battery bank service and ship engine starting air supply, in addition to being wired to the pilothouse control with direct copper wire connections. This provides a high level of 8

9 certainty in the ability to release the hawser under all possible conditions. The feedback from the tug crew indicates that the dynamic brake is the most significant advance over previous designs for two reasons: 1. The shock loads that occur in dynamic pitching conditions are absorbed more smoothly because the dynamic brake can be set more precisely and reliably to a known level. 2. The brake allows the winch to render line when towing indirectly at high force levels, reducing the need to escape and recover under conditions that previously created shock loads. In the event of a recovery from abort, the brake can be brought on smoothly, while allowing the tug to slowly catch up to the tanker. Existing hawser tension monitoring is inadequate. Knowledge of hawser tension is increasingly important information for escort tug operations, especially when towing in the indirect mode. Existing hawser tension monitors rely on brake pin load cells, or interpretation of motor currents or pressures. Both systems are susceptible to shock damage, and calibration and environmental errors. The decision was made to place the winch system load cell in the gear train between the winch drum and the drive-brake system. Consequently, the load cell continuously and directly measures drive torque, which is used both to control the ARR functions and to display hawser tension, as calculated from a separately determined amount of line payed out. Operator controls are inadequate for high power winches. Anyone who has worked in a pilothouse at night or in other demanding conditions, knows how easy it is to pull the wrong lever. The decision was made to clearly separate and define the ARR control functions to lessen these potential errors. Operator errors have special importance for a winch capable of pulling over three times the static bollard pull of the tug (see Photo 4 and 5). It was decided to separate the controls into a Manual or Primary group, all located to the operator right, and an Electrical Drive, or Secondary group to the operators left. The Manual group controls all the failsafe, Escape, and dynamic brake functions, all of which are powered by primary ships power and directly connected to their corresponding actuators on the winch. 9

10 The Electrical group controls the winch electrical drive system, the paralleling of the tugs generators, the speed and direction on the winch, the override control of the hawser spooling system, and the level of Render-Recovery. Although the overall winch system is more complex than previous designs, experience so far has shown that separating the controls has probably reduced errors. DESIGN, CONSTRUCTION AND INSTALLATION Tug winches are long term investments, with typical life spans of 20 or more years. Construction quality and detail are important issues for longevity, safety, and competitiveness. In most cases, the goods at the end of the wire or tow hawser are a customer s valuable possessions. For these reasons, the design and construction compromises that can be made on utility winches, fishing equipment, and even some mooring winches do not generally find favor among tug owners and crews. Fabricated frames with bolt on components are inadequate for these applications. Tug winches must therefore be true machines built using heavy welded and stress relieved frames, with machined surfaces incorporating robust and conservative internal components (see Photo 1). For example, the completed weight of the ARR winch was over 60 Tons. The ARR winch was designed to drop into a reinforced rectangular hole in the main deck, and therefore allow compact and unit mounting of the electric motors, transmission, and dynamic brake system below deck (see Photo 2). The electrical 175 Hp and 75 HP Dual Motor drive enclosures and dynamic brake cooling system were installed independently in close proximity to the winch in a forward, well ventilated compartment, thus limiting the overall outfitting cost. CONCLUSION AND FUTURE DEVELOPMENT The Response is in its second year of operation. The ability of the dynamic brake to ease shock loading is the safety feature most appreciated by the crew. It has likewise been discovered that the ARR system allows the tug to steam away, or shear off, at three knots differential speed while applying full bollard pull. On the other hand, because the winch drive can overpower the tug s static bollard pull, the tug can approach the ship at up to 0.4 knots while applying full static backing force. Taken together these discoveries allow the tug to simultaneously work the ship and seek a new position. We are exploring what advantages this feature might offer in tight quarters and rapid response situations. 10

11 Application of the ARR concept to wire towing winches is a promising outcome of this project. We have recently provided our first two Class 2 Electric Towing Winches for 700 Meters of 50 mm. wire for a pair of 4000 HP tugs. This winch Renders and Recovers to 80% of static bollard pull. The customer reports their long time Senior Captain was amazed that all he had to do to bring in the tow was back off on the throttles and wait for the barge to magically appear at the stern, paying easily in and out during swells along the way. This project has shown that current winch technology can provide a wide range of performance options for dynamic conditions. UHMW-PE rope can be operated successfully at high load on a winch with provisions for proper control of spooling. The electric drive has overtaken hydraulics as the preferred winch drive system for most applications. Computer controls can be successfully combined with primary failsafe controls. The author would like to thank Crowley Maritime for their vision and support in funding this advance in winch and control technology. In addition credit must be given to Mr. Ron Greene, Senior Engineer at Markey Machinery Company, without whose creative and analytical skills the winch could not have been built. PHOTO RECORD 11

12 PHOTO 1 ARR WINCH TETHERED TANKER ESCORT 12

13 PHOTO 2 ARR WINCH INDIRECT MANEUVER PHOTO3 ARR DYNAMIC BRAKE 13

14 PHOTO 4 ARR WINCH PILOTHOUSE CONTROLS PHOTO 5 ARR WINCH OPERATION 14

15 PHOTO 6 ARR WINCH ASSEMBLY PHOTO 7 ARR WINCH INSTALLATION 15