High Speed Engine

High Speed Engine

A 2500kw High speed Main Propulsion engine suffered failure by seperation of the piston Crown.Shown below is the detached crown seized in the liner.

Damage occurred to an extent that the conrod was able to exit the liner space and impact and break several adjacent structures. Shown below is the conrod sitting on an adjacent bearing cap. Damage can be seen to liner, adjacent entablature structure and next unit

The severe action of the conrod caused failure of the bottom end arrangement. Here can be seen that at least one of the cap bolts has failed and that the cap has several hammer blow indentations

This led to failure of both the effect unit and also through loss of lubrication the adjacent unit mounted on the same pin. Shown below are the two bearing shells. In addition damage is shown to the pin in the form of a deep indentation. This alone would have cuased the crankshaft to be scrapped as too great a reduction in diameter would have been required to restore the running surface. There was also a raised section on the opposite side caused by transfer of material either from the shell or con rod.



Several fractures occured as well as material seperation. Here is shown a typical fracture in this case IWO a tie rod

Repair

The replacement of crankshaft and associated other running parts was without question. However, damage to the entablature was so comprehensive that typically replacement would have been the best option. Unfortunately the engine was at an age that locating such was very difficult. In fact a single unit was found for sale, a picture by the vendor showed it sitting unprotected in a field in India. Due to the lack of guarentees on the condition of this casing decision was made to repair. The chosen method was metal stitching. This takes the form of inserting suitable new material attaching by screwed plugs. Shown below is a section of material removed form the entablature that contained a crack

the replacement section

and the plugs used to attach it to the original material and the part remaining once the screw has been inserted


Before the plates can be inserted machining has to occur. Shown are in situ grinding

and in situ boring


Shown below is a part finished and completed repair to entablature stucture


On completion of repairs engine trials are undertaken under the supervision of class. A condition of class will remain on the vessel for some time and regualr inspections are carried out. After a suitable period when class is satisfied that no faults exist with the repair the condition of class is lifted.

Alternator

Alternator

Following failure of original fitwater cooled alternator a similar sized air cooled 2MW unit was fitted.
After approximately 4 months run time the unit failed in service. Prior to this bearing and winding temperatures had been elevated but within limits.
failure took the form of drive end bearing failure. during failure the bearing had rotated on the inner anding surface. Severe brinelling damaged at single element pitch had occured indicating that a significant amount of axial movement had occured. Heating was to deep blue/black and a small fire had occured quickly stinguished by vessels staff.
Repair took the form of disassembly to alow access to the drive end bearing landing to ascertain whether in situ grinding was viable. Measurements taken either side of the landing indicated that a bend of some degre existed in the shaft and the unit was removed for better assesment.
Initial cause of failure was attributed to misalignement. The bedframe had to be substantially modified. FEA was carried out on the new structure and class approved. Close inspection was carried out on the structure of the assembly to ascertain if failure by deformation of fracture had occured. No obvious damage could be seen.
The flexible coupling was inspected and NDT carried out on major components.
Electrical tests were carried out on stator and rotor with no defects found
repair to bearing landing was carried out by machining and sleeving with thermally and galvanically compatible material. Repair by metal spraying was rejected.
After reassembly laser alignement revealed 11mm out of plane misalignement at NDE. The resin chocking was broken and the holes where suitably adjusted. After fitting of tapered dowels new resin chocking was used.
After reassembly the unit was run on test with progressively increasing load. After one hour failure again occured at the drive end bearing.
Investigation revealed that a sprious bearing had been supplied from local distributor. This was of poor quality and failure leading to plastic deformation of the rolling elements and distortion of the cageoccured.

reassembly with correct bearing allowed for proper testing and the unit being placed back in service

Bowthruster

Bowthruster

Inspection of a CP Tunnel thruster was carried out during a docking period due to slight contamination of the gearbox oil by sea water. The intention was to inspect with the head of oil still on the unit to see if it was possible to ascertain if this was due to blade seal leakage, or if it was necessary to remove the output shaft assembly
It was noted that of 8 anodes fitted in this space, 7 had no remaining anodic material. The final anode was severely depleted.

Anodes fitted to the output shaft flange were found absent
The ropeguard was damaged by corrosion and the effects of cavitation and turbulence

This excessive damage was attributed to lack of protection from the anodes exacerbated by cavitation and localised turbulence
The Support tube was pitted to a depth of 20 mm on the blade facing face.

Again this is attributed to corrosion damage exacerbated by localised turbulence.
This damage in particular was carefully examined to ensure that there were no structural strength or stress raiser issues
The adjacent areas to the blades on the tunnel were pitted and evidence of grooving either side of a circumferential weld.

Although this damage is typical of tunnel thrusters the degree in comparison to age of the vessel and normal thrusters usage with regard to zero pitch operation was deemed excessive

Repairs

The areas were grit blasted to allow further inspection. This revealed that there was sufficient depth of material remaining to allow repairs to be carried out. Shown is an area on the Tunnel adjacent to the blades after blasting and part coated with repair product

The selected method was build up using a two compound Metallic polymer product supplied by Belzona
After further cleaning and friable material removal Belzona 1111 was applied to the damaged support tube and tunnel areas. An overlap of around 50mm was given to ensure that any edge lifting would not affect the main repair.
The initial application concentrated on ensuring full contact with the base material by forcing small quantities of product into the pits removing any air bubbles. The main layer was then built up on top of this.

After a suitable cure period the areas were overcoated with a smooth layer to reduce turbulence and improve the longevity of the repair.

The product used was Belzona 1341 applied by use of a brush to provide a very smooth tenacious outer layer which also provides extra protection against edge lifting.
Cleaning of tools was made possible by use of a release agent Belzona 9111
Notes were made to specially inspect the damage at next docking. Were edge lifting is evident it is possible to cut back the damaged areas and overlay again with Belzona 1341.
The shaft seal landing face was machined to remove a grove formed by rub of the two parts. The blade palms were inspected and no significant damage noted. All seals were replaced and new anodes and ropeguard fitted.

Cargo Pump

Cargo Pump

The repairs described below are to aid maintenance planning and not a forum for denigrating machinery, companies or personnel

History

The follwing describes a repair to a large twin screw positive displacement pump. The scrolls where synchronised with a pair of timing gears fitted to one end. This is one of four installed all of which had had a history of failure after relatively short run times over a 25 year period (in the region of 6-24 months). Failure normally took the form of bearing failure, in this case the driven shaft non drive end bearing had catastrophically failed. The following briefly describes the repair methods

Investigation

The pump was disassembled with the following damage noted.

The scrolls were in generally good condition with the exception of the bearing and mechanical seal landing surfaces. These had been repaired previously using metal spray

Their was general erosion to the casing to about 0.5%of the diameter. More significant damage was noted to the cutwater beween the two concentric bores. Due to the advance age of the vessel only the damage to the cutwater was addressed

Typically the metal spray coaring had detached from the parent metal under the bearing landings. Thus demonstrating the unsuitablility of this repair under high impact lading conditions, although it is quite satisfactory under normal conditions for bearing landings

Normally metal spray repair is satisfactory for mechanical seal landing. Here is shown a typical poor application of the process. A steel landing surface should have been available for the seal locating grub screws otherwise mechanical damage occurs which then allows further destruction of the coating

It is difficult to show clearly the damage on the drive end bearings but it was not of the form of pitting and did not appear to be due to the permenant set damage known as brinelling. A repeat to a lesser extent was seen on the rolling elements. This was unusual in form and did not fit the authors understanding of rolling element bearing failure and caused some confusion. The author offered that very similar damage is seen in large electrical motors and is caused by electrical discharge through the bearings. It is noted that this pump is driven by a diesel engine which also drives an electrical generator via a clutch arrangement. Similar poor levels of reliability had been seen in the clutch.

The above and below show a before and after picture of the mechanical seal. after crack detection two of the four carbons had to be replaced as well as all rubber seals

Repair

bearing & mechanical seal landings- due to the failure of previous metal spray repair a different approach was taken. The shaft was machined and thin steel half segments were welded on and final machined.It should be noted that the material selected for the shell has to be galvanically and thermally compatible with the parent metal. The possibility of building up the parent metal with weld was discounted due to the poor weldability of the material . Wear in the timing gear, key and shaft meant that specially offset keys had to be fabricated to ensure the scrolls were properly centralised.
The pump was preassembled with steel fars taking the place of the main body. This alowed the scrollsto be accurately centralised using shims behind the bearings.
With regard to the bearing failures the author informed of the existance of ceramic coated bearings and the use of shaft earthing rings

Main Engine Crankshaft

Add caption

The repairs described below are to aid maintenance planning and not a forum for denigrating machinery, companies or personnel

History

The engine is a 280MM bore 9 cylinder medium/high speed engine turning at 1000rpm. The oil mist detection alarm was sounded and the engineer on duty went to investigate. Approaching the engine a heavy mechanical noise was noted and the engine ws stopped by local control of the fuel rack

Investigation

A crancase inspection revealed than both upper and lower bearings were absent from one con rod which had thus been running metal to metal.

Upper shell

damage indicated that the shell had been trapped before exiting. All white metal was absent

lower shell

The impact on the lower shell had been sufficient to fold it completely

The increased clearance allowed slight contact between piston skirt and balance weights. Rub had led to blueing of the con rod

and of the journal

The oil supply was via a main bearing. This was opened and found to be in good condition without damage. Lube oil analysis prior to this indicated no fault. Oil sample taken oafter the event was satisfactory with no indication of fule dilution
During repairs all con rod bearings and main bearing were inspected and no damage was note other than scoring discussed in the conclusions. The engine had 800 hours since all units had been overhauled.
hardness testing of the journal indicated results far in excess of maximums. Spot grinding using a hand grinder to 1.6mm indicated that hardness was still unacceptably high. As this was approaching maximum allowable decision taken to replace crankshaft

Repair

All units were removed. All ancilliaries such as lube oil pumps & filters, jacket cooling water pump, air cooler etc were removed. The block was detached from the bedframe and then lifted using chainblocks
and placed on stand.

The sump pan was then dropped. The crankshaft was supported using chainblock and strops passing though the liners and all main bearing released The new crankshaft was support using extending bolts between the webs to prevent bending and then placed on a slide under the frame and brough into position.

The engine was then lower to meet the crankshaft and the whole assembled. Once the engine is completely assembled it was aligned and chock fasted

Concluding

The level of destruction meant that determination of cause of failure was difficult and was one of removal of possibilities. The vibration damper had been renewed 5 years before therefore torsional vibration problesm were unlikely although question marks had existed over it alignement to the gearbox.
No evidence of lubrication fault could be found and it was felt unlikely that incorrrect assembly would have allowed the unit to operate sucessfully for 800hours.
Scratch marks were noted on several bearings and was probably caused by poor technique when changing lube oil filters. Specifically the filter housing ws not being drained and cleaned before inserion of new filters. These marks lead to a disproportionate loss of bearing loading surface due to the gradual rather than stepped collapse of oil film in the vicinity. Damage to bearing shells means this could not be proved or disproved.
Big end ovality was on manufacturers limits. It was felt that this was the most likely cause of failure.

Repair

Repairs

The repairs described below are to aid maintenance planning and not a forum for denigrating machinery, companies or personnel

History

The vessel was on passage with 60% loading on the CPP driven though the gearbox. A loud noise was heard and smoke issued from the gearbox vent. The drive engine was stopped

Investigation

Damage was noted to the non-controllable PTO shaft.The centre bearing had failed due to unknown cause. The cage had disintegrated and allowing the rollers to move axially. Several rollers had turned through 90' and and begun to slide. Overheating led to a contained explosion of the oil mist.
One off roller exited the damaged bearing and entered the main gear mesh. The roller had fell into the trough of the controllable pinion and then pressed into the metal. The Bull wheel teeth was then damaged by contact with this roller. The damage to the bull wheel teeth caused a deformation and spreading at the root. The reduced backlash then led to interference damage on the teeth of the pinion. Removing the non controllable pinion shaft indicated a badly worn bearing housing.

This was the most liekly source of failures. Oil galleries, flexible couplings were inspected. Recent Lube Oil analysis had not indicated any fault

Repair

The non-controllable PTO shaft was removed and a new shaft supplied. The damage to the bearing keep was such that there was excessive clearance to the bearing.Line boring was not an option with the facilities available at the shipyard. The time to bring in suitable outside contractors would have incurred excessive off hire costs. With manufacturer agreement the shaft was machined to move the bearing location to the good portion of the keep. As the shaft is non-controllable it requires no clutch. Therefore the loading on the centre bearing is reduced and a smaller width bearing used which utilises half the keep landing area.

A spacer was fitted in place of the old bearing to allow reuse of the original circlip groove.
The bull wheel and pinions where had ground using angle grinders and fine sandpaper pads. Blue was used to ensure that there was no contact in the damaged area and sufficent backlash existed.
The entire bull wheel and pinion teeth mesh was magnafluxed. Two samll cracks were detected where the roller had been pressed in. These were ground out completely.

The system was flushed with hydraulic oil to clean and then refilled. The system wsa run at 15% load for one week and then steadily increased to 35% over the next month. Maganaflux was repeated and any asperities indicated by local spalling ground out.
After 6 months condition of class was removed and the gearbox operated normally

Concluding

No vibration monitoring was being carried out on board. It is likely that the extra vibration caused by the damage and increased clearance of the bearing keep could have been picked up at the early stages. This would have allowed proper repair timed to reduce off hire.