An area where the down ratchet has had particularly pernicious effect is main engine shafting. Between 1998 and 1999, there were at least eight stern tube bearing failures on brand new VLCC's. A stern tube bearing failure generally leaves a ship adrift and helpless. The only good thing about these particular failures is that they occurred so rapidly -- in two cases before the ship was delivered -- that the ships had not yet loaded oil. These failures are shown in Table 13.1.
|YARD||HULL NO||DATE||BEARING MATERIAL|
Since all the Table 13.1 failures involved composite rather than white metal bearings, the immediate reaction was that there was something wrong with the composite material. Several owners replaced a newer composite material with a composite material that had proven itself for over 25 years in hundreds of large tankers, and which in the opinion of most of the tanker industry including the authors was superior to white metal. Two of these proven bearings failed almost immediately. Are we to believe a material that almost always lasts 15 or more years in heavy duty at sea service is the cause of dual failures within a few hours of installation?
Attention then turned to alignment. This was a natural assumption. The Class approved alignment procedure used by the yards is very crude. Over the years, the yards had somehow received class permission to bore the stern tube at the block stage, then weld the stern tube block in place, align by piano wire with the ship still on blocks, and hope. In fact, until about 2001 when a new LR limit on misalignment cames into force, there were no truly concrete requirements with respect to alignment. It was suspected correctly that the yards were taking advantage of the rules and the self-aligning characteristics of the composite bearings to be very sloppy in alignment. However in late 1999, LR carefully aligned two shafts using modern strain gauging techniques with the ship afloat at maximum alongside draft. Both these bearings failed before the ship completed trials. Alignment may be lousy but it is clearly not the root cause.
The current "solution" is to use white metal bearings and high volume, forced lubrication in place of the traditional oil bath system. This is a dangerous work-around, not a solution. The repair yards are reporting rapid weardown in the white metal bearings, and in our opinion, it's only a matter of time before they begin failing. High pressure lubrication is an invitation to blown stern-tube seals, and more importantly forces the the crews to make an impossible choice. If a stern tube seal starts leaking on a ship -- and this happens all the time -- the crew's normal response is to reduce the pressure in the stern tube lube oil system to nearly the same as the external sea water pressure. In an old style oil bath lubricating system, this generally halts the leak with nil increase in the chance of a bearing failure. If the crews attempt this with the current forced lubrication system, they face a high risk of a disabling casualty. If they don't adjust the pressure down, they face the certainty of a large fine at the next port, and a very displeased employer. This is the down ratchet in action.
To develop a real solution, we must understand the real cause of the problem. As Table 13.2 shows, over the last twenty five years, shaft diameters have decreased by at least fifteen percent for the same torque. This is a product of both higher strength material and the down ratchet. Since shaft bending goes as diameter to the fourth power, the net effect is that bending within the stern tube bearing has increased by more than 75% for the same propeller weight. At the same time, propellers have become bigger and heavier due to the decrease in Main Engine RPM.
|Hellespont Embassy, 1976||45,000||85||1,010||Smooth turbine torque|
|VLCC 1999||44,640||76||820||80% torque pulses|
And here finally we come to the reason why the composite bearings have failed immediately while the white metal bearings have taken longer. There is only one area where white metal is better than composite but in that area it is far better. That area is heat conductivity. The conductivity of white metal is over 30 times higher than that of the composite bearings. The composite bearing relies on the lubricating fluid to conduct away the 7.5 KW of heat generated in a VLCC stern tube bearing. But most of that heat is generated in the high pressure portion of the bearing where the film thickness is much too thin to do the job. The composite bearing burns out almost immediately. White metal has great deal more ability to conduct the heat away itself so there is no immediate burn out despite the thin film thickness. But that doesn't change the fact that the pressures are very high, in fact far above the yield point of the white metal, rapid weardown will occur, and premature failure is inevitable. We will see a lot of VLCC's dead in the water due to bearing failures. The only real question is: how many will drift ashore?
ABS and others have correctly argued that the shaft must be more flexible than the hull in way of the shaft. Otherwise we will generate unacceptably high reaction forces on the bearings as the hull deflection changes with loading condtions and wave forces. This is true. But it is also true we must reduce the shaft bending in the bearing to avoid high local pressure. The solution is obvious: thicker shafts and correspondingly stiffer aft bodies.
Hellespont decided to go with a 15% thicker shaft than Class requires. This brings the shaft diameter almost back to the standards of the 1970's and reduces bending in the bearing by over 70%. We also put more than 200 tons of extra steel in the aft hull structure. With this system we were able to get the max pressure according to BV down to 50 bar from 165 bar. It also allowed us to go to a two bearing system, to obtain the flexibility we needed with respect to bending moment and shear force at the Main Engine coupling. It is clear to us that the reason why the mid-70's V's and U's have had relatively good shaft performance despite the crude analytical procedures was the shaft diameters that were used. The industry has a choice:
Finally, we must do all this for a range of operating conditions: laden, ballast, aft peak tank full, aft peak tank empty, engine hot, engine cold, RPM at full speed, and RPM during maneuvering and lightering, and make sure that the shafting system can handle all these conditions. This implies running the analysis over and over, but that's what the computer is good at. A particularly difficult case is lightering in warm sea water. The fact that a VLCC has to operate for extended periods of time at 10 to 15 RPM during lightering, an extremely tough situation for shaft lubrication, is completely ignored by the Rules. But the best alignment will be a compromise. An alignment that looks very good in one operating condition can easily be horrible in other conditions. Unless you examine a full range of conditions, you will never know.