Towing winches: Brute machines for offshore tugs
Mar 22, 2007
Aside from those massive diesel engines, the real brute force found at the heart of every offshore tugboat is its towing winch. Almost nothing is towed in offshore waters without a towing winch. And no matter how modern the rest of a tug's equipment, the towing winch has changed little from its original design.
The winches that get all the headlines today are those that use so-called ‘soft lines' on the bow of tractor-style tugs. These are meant for assisting ships with docking and maneuvering in inshore waters.
But in the offshore tugboat world, winches are always located on a tug's stern and they are wound with thousands of feet of heavy wire rope. It would be rare for any barge, dead ship, dry dock, drilling rig or any other large object to be towed any distance without a so-called towing machine capable of handling a couple thousand feet of two-inch diameter wire rope.
Since it was first introduced in the 1930s, along with the introduction of wire rope for towing, the towing winch has undergone some fundamental engineering changes, but its essential purpose, function and design remain the same as the first commercial model installed decades ago.
"The towing winches of today are little changed from their predecessors," said Allen Craft, vice president of marketing for Intercontinental Engineering (Intercon). "Our customers today are not asking for new bells and whistles. They want the same rugged, heavy, dependable machine that they bought 20 years ago."
Intercon of Kansas City and Markey Machinery of Seattle, are the most active providers of towing winches in the U.S. market today. Others in the business include JonRie InterTech of New Jersey, Timberland/Almon Johnson of Ontario, Canada, and Rapp Hydema US, Inc., of Seattle.
Located on the aft deck, often just beneath the aft end of the boat deck and thereby slightly out of the weather and well forward of the tug's pivot point, is a large, usually-black machine called the towing winch. Its bulk is dominated by a metal spool on which steel wire rope as thick as two to three inches can be wrapped, layer upon layer, typically as much as eight or nine layers thick. The machinery and supporting steelwork for this spool takes up an even greater amount of space. A typical towing winch for 2 1/4-inch wire has a footprint of maybe 15 feet square with a weight in the vicinity of 50,000 pounds. The machine will stand well over the height of a typical crewmember and it will come with a price tag approaching $750,000 or more. Just the wire used on such a winch, say 2,300 feet of 2 1/4-inch wire, might weigh eight to 10 tons.
Just aft of the spool with its wire is a level-winding device consisting of a couple of vertical posts that move back and forth with the wire between the posts, much like the guide on a fishing reel, ensuring that wire being reeled back in is laid down in an orderly and efficient manner. Slightly further aft might be a set of guides to ensure the wire does not veer off to the side. And still further aft might be a set of H-bitts, either standing alone and welded to the deck or constructed as part of the winch body. The H-bitts, which may also serve as the wire guides, are not necessarily part of the towing winch operation, but they can be used for emergency use as well as for general shipboard operations.
Different manufacturers of towing winches offer different engineering applications. For the overall drive power, for example, one company might offer a chain drive from a dedicated diesel engine, while another might turn its winch with a hydraulic drive and still another might use electric motors. One company might power its big band-type brake with hydraulic power, while another will use compressed air.
As an example of an engineering improvement, Brandon Durar, president of JonRie InterTech, an East Coast manufacturer, said he is currently building a towing winch for a mid-Atlantic towing company with an independently powered and controlled level-winding system. While the traditional level winder moves back and forth through the action of a worm gear chain driven from the winch drum, JonRie's level winder is powered by an independent hydraulic motor with separate control that can stop and start the winder to adjust the wire-spooling operation.
Another example: Markey Machinery recently provided a pair of hydraulic wire winches for Boston Towing and Transportation with full render-recovery features for the wire, similar to hawser winches found on some ship-assist tractor-style tugs. Although there is not much call for such a feature for standard towing operations, those particular winches were developed for use in ship docking and ‘fire wire' work involving LNG (liquefied natural gas) tankers in Boston.
The majority of towing winches produced by Intercon are direct diesel drive with a sprocket chain linking the dedicated drive engine and the winch itself. With the drive engine located in a protected area, the chain drive might stretch over a distance as long as six feet, sometimes moving up at an angle from a below-deck machinery space to the winch.
Some new towing winches today come with an electronic abort system with pilothouse control. The abort system releases the winch brake and allows the drive motor to freewheel in such a way that will pay out all the wire on the drum.
"The whole point is to release that wire and get rid of it — to disconnect yourself from the tow," said Durar. "And once that wire starts to run, there's usually nothing that you can do to stop it, even as regards the beginning of the wire where it is clamped to the drum."
When the ill-fated 130-foot tug Valour, operated by Maritrans Operating Partners, had to disconnect itself from its 500-foot barge before sinking off the mid-Atlantic coast this past winter, its captain elected to let the towing winch spool out the entire 2,000 feet of wire. That wire, still attached to bridles at the bow of the barge, later had to be cut away before the barge could be towed into a harbor.
While abort-system controls might belong in the pilothouse, it is debatable whether there is a place for full winch controls in the pilothouse since in most cases, the winch can not be directly observed from there. A traditional place for winch controls, in addition to directly at the winch, is at the aft end of the boat deck, often situated in a small, full-height doghouse. From there the tug skipper or mate can control movements of the winch, observing the entire deck and the tow while also controlling movements of the tug through auxiliary controls.
Despite their incredible strength and durability, towing wires can still break. Most tugboat skippers are quick to tell stories of incidents involving parted towing wires. For that reason, some companies prefer to install double-drum towing winches that offer an auxiliary drum of wire for use in the event of an emergency involving the first wire.
"When you part a wire, you need to know that you have that back-up capability with an extra wire ready to go," said Chris Roehrig, who has been towing oil barges on the East Coast for many years. "For that extra security, especially when you are working with oil barges, you just can't beat having two drums of wire back there."
It is said that the first modern (non-steam) towing machine with wire rope in a commercial vessel was installed in the diesel-electric tug Edmond J. Moran in 1939. With the onset of World War II, the demand for ocean towing capability increased dramatically, and most new tugs built after that were equipped with towing winches, many of them designed and fabricated by the late engineer and equipment manufacturer, Almon A. Johnson.
As tugboats grew ever more powerful, along with the size and weight of objects being towed, the use of towing winches with wire rope towing hawsers became commonplace. Among the newest towing winches were those labeled ‘automatic,' meaning that they could maintain a constant towline tension and towline length (scope) according to pre-determined settings. Some towing winches would allow a towline to pay out or slip in stressful conditions, but could only retrieve the line under manual operation. Many winches were also built with spring-loaded mechanisms that absorbed shocks or surges of tension in a line, which are often exerted by sea conditions.
Line pull of today's winches — the pulling power of the winch by itself — often exceeds the bollard pull of the tug, as exerted by its engines. Line pull can be envisioned as the power that a winch might have in pulling an object on land towards the tugboat, as if the tug had its stern tied to a pier with no help from its propellers. For example, Intercon's Model SD225 winch for 2 1/4 wire comes with a line pull of up to 200,000 pounds, according to company specifications, while most good-sized tugboats today have a reported 100,000 to 150,000 pounds of bollard pull.
They are bigger, heavier and far more powerful, but according to the manufacturers, towing winches are slow to change. "It's all about towing very large objects on ocean passages, and the technology for that does not change very rapidly," said Durar.