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A Review of Best Available Technology in Tanker Escort Tugs

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A Review of Best Available Technology in Tanker Escort Tugs A Review of Best Available Technology in Tanker Escort Tugs A Presentation to the Board of Directors Prince William Sound Regional Citizens’ Advisory Council Sept. 19, 2013 Outline • Terms of Reference • Some Historic Context • Escort Tug Inventory • Escort Tug Performance • Vessel Performance Assessment • Gap Analysis – BAT • Summary Terms of Reference • Compile Present Vessel Inventory • SERVS Fleet • RoW Fleet • Various tug types • Analysis of Vessel Performance • Basic performance parameters for escorting • Indirect performance predictions • Best Available Technology Gap Analysis • RoW vs. SERVS • Draft Final Report • Final Report What IS an Escort Tug??? • What makes it different from other tugs? • What creates the performance differences? • What does an “Escort Tug” Rating by Class mean? To be defined by Class as an “Escort Tug”… • The hull of the tug shall be designed to provide adequate hydrodynamic lift and drag forces when in indirect towing mode. Due attention shall be paid to the balance between hydrodynamic forces, towline pull and propulsion forces • The towing winch shall have a load reducing system in order to prevent overload caused by dynamic oscillation in the towing line, and • The propulsors shall be able to provide ample thrust for manoeuvring at higher speeds for tug being in any oblique angular position • The vessel shall be designed so that forces are in equilibrium with a minimum use of propulsive force except for providing forward thrust and balancing transverse forces during escorting service • In case of loss of propulsion, the remaining forces shall be so balanced that the resulting turning moment will turn [yaw] the escort tug to a safer position with reduced heel Therefore, an “Escort Tug” must have… 1. Hull and Appendages able to generate indirect forces with ample stability and freeboard 2. A winch which carries the load on power, not on a brake, and which automatically reduces excess load 3. Omni-directional propulsion 4. A tow-point located for “Fail-Safe” operation • NOTE…this is much more than just your average “Tractor Tug” A Brief History of Escort Tugs…! • The Exxon Valdez and OPA ‘90 • In the beginning…there was only Voith! • Successful Pitch was made to End Users, not Operators Excellent tugs like the Garth Foss reinforced this perception Escort Tugs… • Early ASD tugs were ill-conceived for Escort • Low Freeboard Aft • High Towing Points • No skegs to create lift In spite of high BP these big tugs could NOT effectively provide escorting. The industry was convinced that “ASD’s don’t work for escort”! The Evolution of Escort Tugs - ca. 1998 • Next generation of dedicated VSP escort tug: “Ajax” • High efficiency, very stable hull form • 41 metres L o.a. • 10,000 bhp • 92 tonnes B.P. • 150 tonnes Fs Same power and VSP drives as big Foss and Crowley tugs, but better performance in a slightly smaller hull The Evolution of Escort Tugs “Fin-Forward” VSP tugs: • Example of innovative, original thinking • First application of dedicated skeg-first operation • Prototype for Crowley’s ‘Protector’ class tugs Some limitations: - No towing capability - Uni-directional operation The Evolution of Escort Tugs (2001) • Omni-Directional, Advanced VSP Escort Tugs: • Advanced hull form • High lift skeg with “Turbo-fin” • Same indirect performance as ‘Ajax’ • Smaller hull • Less Power • Works very well in both directions • No towing limitations Omni-Directional VSP Tugs The Evolution of Escort Tugs - the Acceptance of ASD Tugs for Escorting • Largely the result of private research by Robert Allan Ltd • Hull Forms and Appendages evaluated • Numerous Model Tests • Extensive CFD Analysis The Evolution of Escort Tugs • Comparison of Escort Vessel Designs: VWT vs. ASD Characteristics and Performance - RAL Simulation Studies/Tests ca. 1996 ASD tug VSP tug Power 7460 kw 7460 kw Free running speed 15.3 kn. 15.0kn. Bollard pull 127.0 t 86.1 t Fs max. @ 5 knots 120.9 t * 83.9 t Fs max. @ 8 knots 131.8 t * 98.0 t Fs max. @ 10 knots 147.5 t * 132.9 t Fb max. @ 5 knots 160.1 t * 139.7 t Fb max. @ 8 knots 179.4 t * 127.9 t Fb max. @ 10 knots 186.9 t * 156.0 t * to max. thrust limit ASD Escort Tugs… • The RAstar Class: • High Performance ASD tugs • Sponsoned Hull Form • Deep skegs forward • Optimized Towing Position • Since 2005 more than 15 RAstar Escort Tugs built for major oil or LNG terminals worldwide RAstar Class ASD Escort Tugs • Indirect Performance equivalent to or better than Voith tugs of same Bollard Pull (BP) • Significantly less expensive than VSP tug of same BP • More multi-functional than VSP • Concepts now very well-proven in service • Hull for optimal escort also provides exceptional sea-keeping Seakeeping Improvements – RAstar hull RAstar “ASD” tug vs. conventional “Existing” wall-sided Z-drive Tug So where does the SERVS Fleet stand today? • Classification ? • Design ? • Performance ? • Equipment ? • What has transpired in B.A.T. since these tugs were built? The SERVS Fleet The SERVS Fleet • Of these, only the PWS/ETT Class really qualify for definition as an “Escort Tug” • The PRT Class tugs perform escort duties but do not work in the indirect mode • None of the conventionally-propelled tugs were considered further in this study…but why not? Typical Tug Types vs. Escort Capabilities Typical Tug Types vs. Escort Capabilities Conventional Conventional ASD with Hull Class requires Screw Hull w. ASD VWT Designed for Rotor Tug Propulsion propulsion Escort uses thrusters Hull & Appendages capable of producing high lateral instead of forces drives Towing Winch with overload protection through full load range of operation Propulsors able to direct thrust in any direction at high speeds Forces must be in equilibrium with minimum effort from thrusters Have a "Fail‐Safe" tow‐point location Have sufficient freeboard and stability to support all the above Have controllable pitch drives to protect from engine overload SERVS Fleet Escort Capabilities: ESCORT TUG CHARACTERISTICS Class requires ETT PRT Invader Sea Swift ERV Hull & Appendages capable of producing high lateral forces Towing Winch with overload protection through full load range of operation Propulsors able to direct thrust in any direction at high speeds Forces must be in equilibrium with minimum effort from thrusters Have a "Fail‐Safe" tow‐point location Have sufficient freeboard and stability to support all the above Have controllable pitch drives to protect from engine overload Major Escort Tugs (>35m.) - RoW • Z-Tractor - 1 • Rotor Tugs - 2 • VWT - 9 • ASD - 14 Total = 26 (exc. SERVS tugs) Escort Tugs - Worldwide Escort Vessel Performance - Measures of Merit • Design Considerations: • Length / Beam ratio • Displacement / Length ratio (Disp. / (.01 x Lwl^2) • Performance Parameters: • BP per unit power • Indirect Steering Force (Fs) • per unit lateral area • per Lwl x draft • Indirect Braking Force (Fb) • per unit lateral area • per Lwl x draft • Speed / Length ratio (Vk / Lwl^0.5) Escort Vessel Performance - Hull Proportions: L/B ratio The SERVS vessel proportions, represented by L/B ratio, are fairly typical of the fleet, although the PRT tugs are rather more slender than the trend for other ASD tugs. This should make for a faster boat Escort Vessel Performance - Hull Proportions: Disp / Length ratio The Displacement/Length ratios for the ASD tugs demonstrate reasonable consistency over the size range examined, and the PRT tugs are a close match. The VSP tugs, rather surprisingly, are more scattered in this criteria, and the ETT tugs are much lower than the fleet norm. This indicates only that the ETT tugs are lighter for their size than other tugs of this type, and therefore should be faster Escort Vessel Performance - Power Efficiency ratio (BP/kW) The BP/Power ratios illustrate that both the PRT and the ETT tugs are fairly representative of the performance achieved in their respective propulsion types, but not in the upper performance category Escort Vessel Performance - Speed / Length Ratio The Speed/Length ratios for all tugs are quite consistent, so in spite of the more slender form of the PRT tugs no speed advantage is apparent. The performance of both tug types is average or slightly below Escort Vessel Performance - Indirect Performance: Fs / Lateral Area Fs / L x draft The Indirect Performance criteria reveal that both Classes of SERVS tugs fall below the trend lines for their respective classes. In the case of the PRT tugs the performance is dramatically below par, with a force per unit area capability of only 0.4 tonnes/m2 whereas the trend line for ASD escort tugs of this size suggests that a value of 0.75 t/m2 should be expected Vessel Performance Comparisons • Escort Stability • Compliant with ABS Class Rules • Would not fully satisfy more stringent Class Rules (e.g. DnV) • Not “unsafe” in any respect • Seakeeping • Compared ETT & PRT to RAstar 40-100 & AVT 43-100 • AVT 43-100 has the best all-round performance of the hulls examined, with the ETT a close second. The RAstar 40-100 and the PRT, with almost identical responses, exhibit acceptable seakeeping responses for relatively small vessels operating in such high sea states. • the ETT and PRT vessels have seakeeping performance only slightly less than the best available today BAT Performance Summary - VSP Tugs Table 7.1 BAT Performance for VSP Propelled Escort Tugs Performance Parameter Units BAT ‐ 2013 ETT % Variance Reasons for ETT deficiency with an input power of 10,192 BHP into VSP 36GII/270, Kg/kW 12.65 12.36 Voith predict a bollard pull of 96.2 tonnes with today’s Bollard Pull / Unit Power ‐2% new blade design The lower value in the ETT can be explained by any of lbs/BHP 20.81 20.34 several different sources: ‐ Old blade design ‐ Quality of the guard ‐ Pitch adjustments during the trial not optimal lower than expected speed suggests that either the hull form is fuller than normal (which is not obvious), and/or Speed/Length ratio 1.34 1.23 ‐8% drag associated with VSP guard plate and associated struts may be higher than normal Sponsoned hull form provides more stability to resist Indirect Steering Force per Tonnes/ Sq.M 0.90 0.83 ‐8% heeling forces.
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