STOP Leaving Pollution in Your Wake

Stern Tube Oil Pollution Is Just One of Many Reasons to Make the Switch to Seawater-Lubricated Stern Tube Bearings.

When the subject of oil pollution comes up between two shipping executives, thoughts often turn to the catastrophic 1989 oil spill in Prince William Sound, Alaska. Or perhaps the 1978 Amoco Cadiz grounding off the coast of Brittany comes to mind. On the other hand, it’s doubtful that stern tube oil pollution (STOP) is even mentioned. Yet even a conservative leakage rate estimate of 6L/day (taken from Class Society Seal Type Approvals) emanating from stern tube oil seals can amount to as much as 80 million liters (21m US gal) annually from normal operations. By comparison, oil pollution from the Exxon Valdez spill was 41.6m liters (11m US gal).

Today’s shipowners are faced with enormous pressure from many angles to run “green.” Stack emissions, oil pollution, invasive species emanating from ballast water, hull coatings that potentially leach toxins into the water, and sewage or gray water are all issues that are becoming increasingly more regulated. These potential hazards have to be managed from an engineering and operational standpoint. Probably the least obvious source of vessel pollution comes from stern tube oil leaks. And the fine imposed by the Coast Guard for such an event may be the least of your worries. Fortunately, removing this potential environmental issue from your menu of regulatory headaches is an easy fix.

The installation of seawater-lubricated stern tube bearings is a smart investment. In the short term, the elimination of stern tube oil pollution as a potential source of fines provides obvious and instant gratification. Long term, amortizing the cost of this simple yet valuable equipment may be a much shorter payout than one might think. Today, more than 600 Thordon-brand, seawater-lubricated stern tube bearings are in use around the globe, with over 100 more on order. That’s just about one percent of the world’s fleet. You have to wonder what the other 99 percent are waiting for.

The Status Quo
Most commercial oceangoing ships employ a propeller shaft typically supported by oil-lubricated metal bearings with the oil contained in the stern tube by forward and aft shaft seals. The seal must leak (aft - into the sea, or forward - into the ship’s bilge) at the shaft/seal interface in order to function properly. It doesn’t take much to damage this arrangement. Fishing nets or mooring ropes caught in the screw are just two of many regular headaches that can allow stern tube oil to flow out into the sea. A typical merchant ship’s stern tube might contain as much as 1500L (396 U.S.) gallons of oil.

Classification societies, maintenance personnel and operators alike agree that oil lubricated seals are going to leak. Consumption from the seal is controlled to the extent necessary to maintain an acceptable life, but by design it is essential to have oil at the mating surfaces. Oil consumption is always lost directly to the sea, and some laboratory testing has produced estimates that as many as 2.5 million gallons are lost annually from vessels. Getting more precise information on this from any owner is understandably difficult, if not impossible.
Dr. David Gilmour of BP-Castrol Marine Lubricants, a large supplier of stern tube oils, has stated that “Environmental legislation can only get tougher, but even well-maintained and managed ships will still leak oil.” Indeed. Therefore, and without a doubt, one of the most serious environmental issues facing shipowners of medium and large commercial vessels is stern tube oil leakage. As environmental regulations get tougher and global enforcement is ramped up even further, shipowners find themselves looking for ways to manage their risk in the face of more than $140 million in criminal fines levied by U.S. Courts alone. Despite this reality, the perceived additional cost of retrofitting or installing seawater systems as original equipment gives many operators real hesitation. That type of thinking is more than shortsighted.

Achieving Zero Discharge
The alternative to an oil-lubricated sealed system that completely eliminates stern tube oil pollution is already a proven technology. Seawater-lubricated open systems use seawater as the lubrication medium in place of oil. The seawater is taken from the sea, pumped through non-metallic shaft bearings and returned to the sea. No stern tube oil is needed. Proven materials and new designs of non-metallic bearings now offer performance similar to metal shaft bearings.

There are other alternatives to the oil-lubricated system and, in practice, these methods involve a more environmentally friendly sealing procedure. In one case, seal manufacturers use multi-lip seals, which reduce the amount of oil that escapes, but shaft seals can still be damaged and oil can still escape into the sea. Some systems also employ biodegradable oils, but these substances are still “oil” and leave a sheen, which is considered pollution.

The third, less conventional, but simpler option uses proven stern tube bearing technology from Thordon Bearings. The simplest way to completely eliminate oil from the stern tube is to use seawater as the lubrication medium and Thordon non-metallic bearings in place of oil and white metal bearings. The seawater enters the forward section of the stern tube just aft of the seal and passes through the forward and then aft bearings prior to reentering the sea. The quality of the seawater supplied to the bearing is critical to long wear life. To ensure that abrasives are removed from the seawater, a water quality package is used. This package employs centrifugal forces to remove particulate from the seawater stream, then collects it and discharges it through a blow downline. Use of seawater-lubricated bearings eliminates the aft seal as well as the storage, sampling and disposal of oil. The potential impact of stern tube oil pollution is zero.

An additional design consideration with seawater-lubricated bearings is that the propeller shaft requires corrosion protection from the seawater. For economic and technical reasons, the shaft is typically steel, which will corrode in (salt) seawater. To prevent this, bronze or stainless steel liners are generally placed over the shaft. Between the liners a waterproof flexible coating is typically fitted to protect the shaft. Thor-Coat, a new two-part epoxy shaft coating, was specifically developed to complement seawater-lubricated propeller shaft bearing systems with the objective of providing corrosion protection for a minimum ten-year period. Thor-Coat addresses the issue of current shaft coating systems not being sufficiently reliable to allow extension of shaft withdrawal periods beyond five years.

Follow the Money
Craig Carter, Director of Marketing and Customer Service, readily admits that the cost of a Thordon bearing far exceeds that of the conventional oil-lubricated unit. That cost differential can run anywhere from $50,000 to $100,000. But, he adds, “That’s only part of the story.” Shipowners using Thordon bearings are already realizing significant savings from multiple cost centers. The benefits of using seawater-lubricated Thordon COMPAC bearings include:
1 reduced operating costs (no aft seal)
2 greater time spans between required seal maintenance
3 proven performance and reliability
4 fitting and monitoring methods to meet class society approvals
5 reduced legal and business risks from oil pollution

Probably the best reason to employ Thordon COMPAC bearings can be found in research presented at the 2007 RINA (Royal Institute of Naval Architects) conference. There, the benefits of water-lubricated propeller shaft bearings received more than just a little attention. The presenters from Fincantieri, a large shipyard in Italy, stated that “Traditionally, the shaft line is oil-lubricated and located inside the tube case with a diameter larger than the shaft itself. An alternative solution is presented with water lubrication, which offers some consequent benefits. First of all, the inflow water meets a smaller diameter and so the wake peaks on the propeller plane are reduced. Furthermore, the water through frictional effect is trailed in rotation towards the propeller with a significant benefit for propulsion efficiency (about two percent). The water-lubricated shaft line is also practically maintenance-free and represents a ‘green’ solution as the risk of oil leakage is avoided.”

For those operators needing reasons beyond the obvious secure feeling of knowing that at least one source of ship “emissions” is gone forever, consider the fact that Disney Cruise Lines has two cruise ships equipped with COMPAC bearings. And no wonder: Working with Lloyds Register, they haven’t had to pull a shaft in ten years and they might not have to do it for another five. Ordinarily, the withdrawal interval for oil-lubricated bearings is five years. Thordon’s Carter insists, “We expect 15 to 20+ years of service life, but it depends on the ship’s operational profile.”

Good Company
Currently there are over 600 commercial ships operating with Thordon seawater-lubricated bearings with the first commercial installation taking place back in 1983. Still, this represents a very small percentage of the world’s commercial fleet. Just a few of the more prominent operators using Thordon Bearings today are listed in the table below.

The world’s largest cruise ship operator, Carnival Corporation, through its subsidiary Princess Cruises, has COMPAC bearings currently installed on nine of its ships with five more on order. Mr. Chris Joly, Principal Manager, Marine Engineering, for Carnival Corporation recently stated, “Sea-water-lubricated bearings are the present for many of our ships; I would like to see them for all of our future ships.”

2015: Standard Equipment?
The opportunity for shipowners to employ worry-free shaft lines for the life of the vessel is at hand. Ships trading in the world’s oceans and seas can now eliminate both operational and accidental stern tube oil pollution while reducing maintenance costs and saving money over the service life of the ship.

Improvements in design and technology have resulted in Thordon bearings offering improved wear life, fitting and monitoring methods to meet class society approvals. The performance of seawater-lubricated COMPAC bearings to date has been comparable to oil-lubricated white metal stern tube bearings. They also eliminate any risk of criminal, civil and administrative penalties and other adverse reactions such as bad public relations for the shipowner that may occur from oil leaking from the stern tube.

Thordon COMPAC bearings are guaranteed for 15 years. That guarantee is available for commercial newbuild vessels with shaft diameters of 300mm (12”) or greater and COMPAC bearings enclosed in stern tubes. With that guarantee comes the promise of zero pollution risk, high abrasion resistance, an elastomer bearing that withstands shock loads and edge loading, a proven long bearing wear life and global availability.

Right now, a Thordon seawater stern tube bearing is merely one of many options that a shipowner could choose for his existing vessels or future planned tonnage. That metric could change. And for anyone who doubts that possibility, just ask an operator who is currently shopping for a million-dollar ballast water treatment system for his fleet of vessels. The legislation necessary to implement that requirement is coming, and it is coming soon. So, too, will the requirement for stern tube bearings that no longer pollute the seas. With only one percent of the world’s collective global fleet now employing such devices, you might want to consider ordering yours now – in advance of the coming rush.