Problems and failures in shaft line arrangement
As the connection between the source of power and the propeller, the propeller shaft and its associated bearings and seals is despite its simplicity one of the most important systems on a ship. You can read the first article of this series here.
Any problems that are experienced with the shafting system will inevitably result in extra costs for the shipowner and in worst case scenarios the safety of the ship and crew could be compromised. Depending upon the lubrication method for the shaft in the stern tube, the risk of pollution could result in legal penalties as well as a need for immediate repair either in drydock or using one of the specialist underwater repairers.
Although modern ships rarely have a propeller shaft that runs half the length of the vessel as would have been the case when most ships were built with engines and accommodation amidship, the much larger size of modern ships means that the physical length of the shaft may actually be quite similar.
In addition, in order to meet the requirements for improved efficiency, the propeller on modern ships – particularly those built most recently is much larger than would have been the case even a decade ago. The increased diameter of the latest propellers brings with it additional mass and the potential for the propeller to not be totally immersed in part loaded or ballast condition. Another new factor to be considered is the move to ultra-long stroke engines operating at slower speed than was typical for earlier engine designs.
A typical propeller shaft system comprises the shaft itself attached to the propeller and passing through the stern tube plus intermediate shafts and coupling bolts, supporting intermediate bearings, the stern tube seals and a lubrication system. All of these components have their own particular issues that can create problems compromising the whole.
Some problems can be quite easily detected because they gradually worsen and are indicated by way of increased temperatures, lubricant analysis or a need to constantly replenish the header tank of the bearing lubrication system. However, others can be quite sudden and occur without any prior warning. Included in the later are fracture of the shaft or connecting bolts and occasionally the rare case of a break up of the bearing material.
Shaft failures and repairs
Because of the larger size of propellers and the comparatively shorter shaft length in large vessels such as bulk carriers, tankers and container ships, the shafts are much stiffer than in older vessel types. At the same time, the longer length of vessels and optimised construction methods designed to save weight, mean that the ship itself has become more flexible.
The flexing of the vessel, combined with the stresses imposed on the shaft by the propeller has seemingly increased the number of failures attributable to fatigue of the shaft. This can manifest in cracks in the shaft which do not cause complete failure, but which require diligent management to prevent a such from happening. In a significant number of cases the shaft has failed completely leaving the vessel without propulsion. In some cases – the New Zealand ferry Aratere is a good example – the fracture failure occurs outside of the ship resulting in the loss of the propeller.
One a shaft has fractured, it will obviously need to be replaced but equally important is the investigation into the cause of the fracture. This requires metallurgical expertise to determine if the fracture was caused by fatigue or a production fault and if the former, what stresses were involved. Some stresses can be due to the ship operating for long periods outside the parameters used for determining the original dimensions and design of the shafting arrangement, but others are due to the combination of engine and propeller choices.
There is some evidence that the combination of a single stern tube bearing and a propeller with more than four blades produces a vibration resonance close to the MCR of the engine and this has resulted in more cases of failure than the older convention of vessels having a four-bladed propeller and a stern tube with both fore and aft bearings. The classification society should ideally require a vibration analysis to be done before approving the arrangement.
Ironically some shaft damage can be the result of routine maintenance or work in drydock when the tailshaft may be required to be withdrawn. Welding in the vicinity of the shaft may result in arc strikes that can be the initiation point for later defects in the shaft.
Sometimes, a potential problem with a shaft can be identified in advance of a complete failure. Usually this will be either because a vibration issue is identified or excessive bearing wear is discovered by lube oil analysis. A survey of the shaft and an alignment check may reveal a bent shaft or a fault in the construction of the ship. If discovered in time, remedial action can be taken that may involve machining of the shaft or cold straightening using high-pressure presses. Where a shaft has been damaged because of a bearing failure. It can sometimes be repaired by machining away any damaged material followed by rebuilding the shaft to its original diameter by way of arc spray coating or laser welding.
In most shaft systems there are one or more intermediate shafts that are collar bolted together. These connections can reduce some of the stresses that would otherwise affect a single shaft length but the flexing of the vessel and the effect on the length of the shaft together with the tightness of the bolts themselves can cause the bolts to shear. Sudden stops such as caused by a propeller hitting an underwater obstruction or a crash stop of the ship will place stress on the collar bolts.
Any movement will cause fretting of the bolts that could develop into fatigue cracking. If that happens even in a single bolt, the loading on the remaining bolts will increase and makes a disconnection of the shafts more likely. If fortunate, the disconnection will not cause damage to the collars and the components can be reconnected but even quite small damage to the faces will create more problems in the long run and repairs or replacement should be considered.
Bearing damage is far more common than a shaft failure although if not rectified in good time a damaged bearing will eventually result in damage to the shaft. Wear of any bearing is quite normal but is normally a slow process and replacement of bearings can be scheduled according to either a periodic or condition-based maintenance system.
Usually bearing wear is monitored by lube oil analysis but vibration and temperature sensors can be permanent fittings or used in a portable monitoring system with measurements taken on a regular basis and analysed by specialists. Lube oil analysis measures wear by the amount of bearing and shaft material in the sample but there have been cases where a bearing has actually disintegrated into large sized pieces that would not show in the sample being too large.
Although in theory, the shaft alignment is fixed and should result in even wear of bearings, in practice the flexing of the ship, bending of the shaft and any unintentional misalignment of components, means there is a high possibility of uneven wear.
In early 2017, DNV GL warned of an increasing trend in bearing damage which it attributed mostly to incomplete propeller immersion. Worryingly, the warning highlighted that in some cases damage occurred in a very short period of time and in several cases this was just a few minutes.
According to DNV GL, typical damages observed involve wiping of white-metal-based bearing material in the aft part of the bearing, indicating a complete or partial loss of the hydrodynamic oil film under the reported operating conditions. Physical contact of the shaft with the bearing material is reflected by an elevated bearing temperature and, in most cases, at an exponential rate of rise. In an isolated case involving extended operation beyond the alarm point (normally 65°C), bearing damage was even observed with a slower rate of temperature rise. The reported damages resulted in expensive and time-consuming repairs.
Bearing failure may lead to the risk of total loss or diminishment of main propulsion function capacity and, in some cases, may be detrimental to the propeller shaft in the event of steel-to-steel contact. In the event of long-term operation with incomplete propeller immersion which does not result in an immediate failure, the risk of fatigue-related bearing failures arising out of excessive cyclic loading and associated shear forces on the bearing will co-exist.
Failure of seals
Bearings need to be lubricated to remove heat and reduce friction. The lubricant can be a mineral oil, an environmentally acceptable lubricant or in some cases sea water. The lubricant needs to be retained in the bearing area and for this purpose there are seals at either end. A variety of different seals are available with some being more effective than others at retaining the lubricant within the bearing.
A failure of the seal will inevitably result in the lubricant leaking from the bearing increasing the risk of damage caused by heat and friction. Even the best seals can be damaged by external factors such as entangled fishing nets or ropes or collision with floating objects. If the aft seal in the stern tube leaks then any lubricant will be released into the environment and if a mineral oil will cause pollution.
Release of mineral oils is prohibited by MARPOL and in many states by local domestic regulations. However, unless the leak from a stern tube is witnessed and reported to the authorities or the Oil Record Book information is found to have been deliberately falsified and is discovered by PSC inspectors, there is little enforcement of pollution caused by seal leakages. In the US, the VGP regulations are more stringent than most other national regulations. Under VGP rules, ships are obliged to use environmentally acceptable lubes unless no such product is compatible with the stern tune bearings and seals. Any leakages of lubricants into the ship will need to be dealt with along with all other machinery waste fluids.
There is a move towards greater use of seawater lubricated bearings in stern tubes although the number of companies producing them is currently quite small compared to more conventional bearing products. A seawater lubricated bearing is clearly not a potential pollution source and if there should be a move to better enforce the rules prohibiting discharge then their popularity will likely increase.