LNG escort tugs take shape
The vessels measure 92 ft x 32 ft x 13 ft 9 in and have an estimated bollard pull of 110,000 lb ahead and 105,000 lb astern.
To be named Freedom and Liberty, they were designed to fulfill BTT's contract with Tractebel LNG Shipping North America LLC, which brings LNG ships into Boston Harbor.
The tugs will assist the LNG tankers during inbound and outbound transits and will be used to turn the ships 180 degrees in the basin east of the Tobin Bridge and back them under the bridge to their Mystic River berth. In addition to ship docking, the tugs will be used for barge handling.
The propulsion units will be Rolls Royce (Ulstein) 1650H Z Drives equipped with 86 in stainless steel propellers and Kort nozzles lined with stainless steel. The Z Drives will be of modular type, permitting removal through soft patches on the main deck.
The bow of each vessel is fitted with a Markey type DEPCF-50 75 hp electric hawser winch, for 500 ft of 9 inch synthetic line. The winch features line-pulls up to 44,000 lbs, line-speeds up to 393 ft/min., and a drum brake holding capacity of 400,000 lbs.
The Markey level-wind spools the line in such a way as to minimize pull-down during high-tension operations, and a line-tension display system provides the operator with an active display of line-tension while executing ship-docking or escort maneuvers.
The winch is provided with all electrical controls, including controls that allow the operator to freewheel the drum at any time, whether loaded or unloaded, and Markey's constant-tension "hands-free" render-recover system.
The stern of each vessel is fitted with a Markey type TES-32 75 HP electric towing winch, for 2100 ft of 2 inch wire plus three layers flange-margin for synthetic line. The two-speed towing winch features line-pulls up to 156,000 lbs., line-speeds up to 86 ft/min., and a drum brake holding capacity of 400,000 lbs. A standard Markey towing winch level-wind spools the wire-rope smoothly and consistently. Special features of this towing winch include an integrated towing bitt for special operations that require manual line-handling and tie-off, a warping head with line-speeds to 60 ft/min., and a super-compact design as required for stern installation on z-drive tugs. The Winch is provided with all electrical controls, including controls that allow the operator to freewheel the drum at any time, whether loaded or unloaded, and Markey's constant-tension "hands-free" render-recover system. The Markey Electrical Drive & Control System is based on two 75 HP AC Variable Frequency Drives which can be switched between either the Bow Hawser Winch, or the Stern Towing Winch, providing complete redundancy. BTT has strict fendering requirements for its ship assist and barge handling tugs. The upper bow course will be Viking 14" molded square fender. Below it will be Viking 17" Enhanced Soft Loop to the deck level; Viking 12"x14" square fender at the deck line; Viking 17" Enhanced soft loop below the deck line to the waterline; and two courses of 18" Flanged Viking Fender below the waterline. Viking 12" black rubber "D" fender will be fitted at the main deck from the bow fenders at the sides and around the stern. Additional courses of 8" "D" fender will be strategically located at different points on the hull and tires will be used to cover the forward sides. Stainless steel pins will be used to fasten the D fendering.