TECH_MATTERS_no6_tcm155-236231.pdf

7/22/2019 TECH_MATTERS_no6_tcm155-236231.pdf

1/12

TechnicalMattersCase studies covering technical issues and their solutions

March 2012 Issue 6


2/12

Technical Matters March 2012

2

CASE STUDY 1

Water tank cracking

An investigation was initiated by therepeated cracking of water tanks in theaft body of an LNG carrier due to highlocal vibrations at the aft end.

To identify the source of vibrations andadvise the owner on possible remedial

actions, vibration sensors were placed at anumber of strategic locations. In addition,hull pressures were recorded near thepropeller, and borescope observations were

made. All recorded data was analysed withMATLAB, a technical computing language,using frequency domain representations.

The results showed that the tanks naturalfrequencies coincided with the third,fourth and fifth blade passing frequencies,

resulting in structural resonances withvelocities well above the 30 mm/s peakvalue (the maximum recommended at

those frequencies in Lloyds RegistersShip Vibration and Noise Guidance Notes).

Such higher order blade frequencies donot normally transmit sufficient energyto cause large excitations, so this was an

unusual case.

Borescope observations showed that

significant sheet cavitation was generated

each time a propeller blade passed the topdead centre position. The shed cavitation

interacted with the sheet cavitation on thefollowing blade, resulting in periodic burstsof high energy pressure excitation. This

behaviour indicated a strongly retarded flowinto the propeller plane.

Based on the advice of Lloyds RegistersTechnical Investigation Department (TID),the client installed vortex generators aheadof the propeller. These generators improved

the inflow into the propeller plane andreduced the cavitation significantly. Nofurther cracking of the water tanks has

occurred since then.

LESSON

Strongly retarded inflow in the

propeller plane can result in

dynamic cavitation and higher

order excitations that could

lead to resonances in aft end

structures.

Time histories of vibration and hull pressure

Velocity transducers on panel structure

Frequency analysis of vibration time histories showing principal modes

Vessel

LNG carrier (see front cover)

Issue

Excessive aft end vibration


3/12

3


CASE STUDY 2

Gas dispersion

Natural evaporation of organic chemicalcompounds during the loading ofcrude oil carriers leads to tank pressurebuild-up. When this exceeds the setsafety standard, gaseous compoundsare vented via a riser mast. Becauseinhalation of these compounds poses

a health risk, safety limits are set forpermitted concentration levels.

At the clients request, Lloyds RegistersTechnical Investigation Department (TID)provided a Computational Fluid Dynamics

(CFD) study on the dispersion of theseorganic compounds. This included threedifferent wind speeds, two wind headings

and two riser heights. In all cases theconcentration of hydrogen sulphide (H2S)and hydrocarbons (CxHy) were recorded.

These analyses clearly demonstrated thata threshold wind speed exists, belowwhich concentration levels onboard and in

the vessels direct surroundings exceededsafety limits. Concentrations were particularlyhigh at the aft deck due to re-circulation

behind the wheelhouse. The resultsfurther showed that riser height had littleinfluence on dispersion patterns. Of even

more significance was the velocity of riseremissions as greater velocities increasedispersion volumes.

Given the limited number of case studiesand the likelihood of exceeding safety

levels, further analyses were recommendedto provide a definite answer. For the timebeing, the client was advised not to vent

Hydrogen sulphide on aft deck

Hydrocarbon dispersion cloud

LESSON

CFD analyses can significantly contribute to defining operational

windows for the safe loading of crude oil tankers.

Vessel

Crude oil tanker

Issue

Safe operational loading windows

at low wind speeds. Two possible solutionssuggested to the client were to remove the

heavier hydrocarbons from the emissions orto pre-mix the riser emissions and increasethe outflow velocity.


4/12


4

CASE STUDY 3

Stern tube bearing damage

When the stern tube bearing of a newcontainer ship was damaged duringsea trials, Lloyds Registers TechnicalInvestigation Department (TID) wasasked to investigate the possible causes.

The propeller was removed from the tail

shaft and radial run-out measurementswere taken along with clearances between

the shaft and stern tube bearings. The tailshaft was removed and a visual examinationcarried out on the stern tube bearings and

shaft. Bearing bore and tail shaft diametermeasurements were taken and alignment

checks carried out. Stern bush wall thicknessmeasurements were also taken, andon-board documentation was reviewed.

The failure of the stern tube bearingcould have been caused by any one of,

or combination of, several modes of failure.It was necessary to examine each one inturn and, where justified, discount possiblemodes, to arrive at the most probable

cause of failure.

Potential causes were considered and

some discounted. The slope of the sterntube bearing bush housing was satisfactoryand it was probable that the slope of

the original stern tube bearing had been

satisfactory before failure. The originalclearances between the bearing and tail

shaft were within the manufacturersrecommended limits. A small gapexisted between adjacent bearing bush

sections but in this case had not significantlyaffected the bearing load carrying capacity.

Measurements by TID on other vesselshave demonstrated that manoeuvring athigh speed imposes additional loads on

the stern tube after bearing. The additionalloads are particularly severe during turnsto starboard when high pressures and

consequent thin oil films are generated atthe aft edge at the five o clock positionlooking forwards.

The vessel was in the light ballast conditionduring the sea trials. This was only just

sufficient to immerse the propeller andwould have generated an adverse bendingmoment with the centre of thrust belowthe shaft centre line. Records also indicated

that the vessel was undergoing high speed

manoeuvring immediately before the sterntube bearing damage occurred.

This excessive manoeuvring in thelight ballast condition was therefore

considered to be the most probable causeof the damage.

Damage to stern tube aft bearing Damage to propeller tail shaft

Washways

Damaged

areas

Grooving

Black band

LESSONThere should be adequate propeller immersion at all

times and excessive manoeuvres at high speed using

large rudder angles should be avoided.

Vessel

Container ship

Issue

Damage sustained during sea trials


5/12

5


CASE STUDY 4

Propeller optimisation

Design limitations and powerrequirements often force propellerdesigners to accept certain levelsof cavitation. Striking the balancebetween acceptable cavitationand the risk of damage is a delicatematter however.

This is reflected in the significant number

of propeller-induced vibrations cases that

Lloyds Registers Technical InvestigationDepartment (TID) receives each year.

One possible solution is to install finsor vortex generators (VGs) to influencethe flow towards the propeller.

By introducing a fin or VG on a strategiclocation, the inflow in the propeller planecan be improved significantly. In general,the more uniform the propeller inflow

becomes the lower vibration levels arelikely to be.

Aided by Computational FluidDynamics (CFD), the design and locationof fins and VGs can be optimised.

In this particular case, a strong tube

vortex was identified at the end of thewing propellers bossing. At full speed

ahead, the tube vortex interacted with

Before flow improvement After flow improvement

Vessel

Motor yacht

Issue

Improving propeller inflow

LESSON

With the aid of CFD analysis it

is possible to test and optimise

flow improvement devices priorto installation.

the tip vortex, leading to violent

cavitation shedding.

By placing four shaped fins on the shaft

casing, the tube vortexs strength waslargely diminished and its path altered.Instead of going through the tip region,

it now passed through the root sectionof the propeller. As a result of themodification the propeller inflow

improved considerably.


6/12


6

CASE STUDY 5

Environmental impactof wave making

Lloyds Registers Technical InvestigationDepartment (TID) was asked to makea Computational Fluid Dynamics (CFD)parametric study of the wave-makingcharacteristics of a shallow waterarticulated tug/barge (ATB) assembly.

The studys key objective was to estimatethe maximum height of the wavesgenerated by the ATB while operating in a

shallow, 5 metre deep, river and assess itsenvironmental impact.

Using CFD techniques, the project wassplit into two stages. The first (main)

investigation involved 12 parametricruns covering the operation of the ATB

assembly under two loading conditions(load and ballast draughts) and sixspeeds (1, 2, 3, 4, 5 and 6 knots) at zerotrim condition.

The output of each parametric run was thenused to predict values for the maximumwave height at distances of up to 200

metres from the sailing line of the ATB.

A validation study was carried out to

corroborate the CFD predictions. Thestudy modelled the flow around the ATB

at load draught at 10 knots. This studyallowed qualitative comparison withphotographs of a similar ATB travelling atthe same speed, thus increasing confidencein the results of the main investigation.

Comparison between the computedwave patterns, in particular the bow

wave, at 10 knots load draught andthe photograph of a similar ATB at

the same speed indicated a goodqualitative match. The actual maximumpeak-to-trough wave height at a distanceof 25 metres from the sailing line of the

ATB is around 0.3 metres and this wasobserved for the 6 knot load case usingCFD techniques.

It was also predicted that the rateof wave height decay is generallylarger near the ship and decreases

with distance. The CFD techniques alsoconfirmed that wave height, as expected,

decreases with decreasing ship speed.Wave heights at ballast draughtwere predicted to be roughly two-thirdsof those at the same speed at

load draught.

Articulated tug barge travelling at 10 knots with surrounding wave pattern

CFD prediction of wave pattern at 10 knots which

allows a qualitative comparison

Vessel

Articulated tug barge

Issue

Waves generated in shallow waters

LESSON

Computational Fluid Dynamics

provides a viable alternative

to extensive model testing for

predicting wave heights at

varying distances from the

sailing line.


7/12

7


CASE STUDY 6

Stabiliser fin failure

After six years of active service, oneof a pair of retractable stabilising finsattached to the side of a passenger/ro ro ship was to be found missing.

During examination of the remaining partof the stabiliser stock at the Lloyds Register

Materials and NDE Laboratory it wasfound that a fatigue crack, caused by cyclicunidirectional bending, had formed froma single initiation point at a tear drop

artefact. This artefact was due to a lackof fusion between successive weld beadswithin the intermediate weld between

the parent material and the stainlesssteel cladding.

The purpose of the ferrous intermediate

weld was not clear, although it mayhave been used to reduce the carboncontent of the material to which the

austenitic stainless steel cladding wasto be laid down. However the interfacebetween the carbon steel parent metal

and austenitic stainless steel claddingwould normally be facilitated usinga higher alloy weld material for the

first weld layer onto the carbon steel,

followed by laying down the austeniticstainless steel cladding layer.

The material properties and dimensionsof the parent material and the stainless steelcladding were within design specification.

The distinct bands of slow and fast crackpropagation indicated considerable

variation in service loading due tothe variable sea conditions. The crackpropagation extended almost the entire

diameter of the shaft before the shaftfailed due to overload. This indicated thatthe fin stock shaft had a high margin

against failure for the operationalbending stresses experienced.

It was recommended that the reason

for the intermediate weld should beidentified. Recommendations were

also made that the stabiliser stocksshould be examined both visually andby non-destructive examination (NDE)techniques for the presence of cracks.

Replacement stabiliser stocks should beexamined visually and ultrasonically forcracks after two years in service.

Fracture surface on section of stabiliser stock

Section through initiation point at tear

drop artefact

Fracture initiation zone showing beach marks

indicative of fatigue fracture

LESSON

Poor manufacturing practices can often lead to early failure during

service life. Understanding material properties and the mechanisms of

failure can lead to more robust solutions.

Vessel

Passenger/ro ro ship

Issue

Material failure


8/12


8

CASE STUDY 7

Auxiliary enginecrankshaft problems

After the failure and subsequentoverhaul of an auxiliary diesel engineon a passenger/ro ro ship, a crankshaftbalance weight was dischargedthrough the side of the engine soonafter the ship went back into service.

Lloyds Registers Technical Investigation

Department (TID) was asked to investigatethe cause of the damage and to ascertainwhether the failure was linked to arecent overhaul carried out by the engine

manufacturers personnel.

The engine was examined along with

operational documentation. The causeof the damage was fatigue fracture

of the balance weight which, alongwith the securing studs, was submittedto Lloyds Register Materials and NDELaboratory for further metallurgical

examination.

The fatigue crack had propagated from

a fillet radius in the balance weight.Subsequent rupture caused the impactdamage and shutdown of the engine.

The crack initiation site was within a heat-affected zone associated with the flame-

cutting procedure used in the originalbalance weight manufacturing process.The failure mechanism had previously been

identified by the manufacturers some13 years previously. Finite element analysisdetermined the magnitude of the stress

at the fatigue fracture initiation point.Based on this value, fracture mechanicscalculations indicated a crack depth up to

2 mm might exist before propagation torupture is l ikely.

The fatigue crack had propagated in twophases. The first was during the period ofoperation after the engine was built. The

second, more recent, propagation followedfrom a sudden stoppage of the enginecaused by a dropped valve and coolingwater ingress into a cylinder.

Regular inspections as part of the vesselsplanned maintenance routine failed to

identify any crack. In this regard, theinspection procedures suggested by themanufacturers were considered inadequate.

Following the rupture of the balance weightand subsequent repair of the engine, all theoriginal balance weights were replaced with

those of the new design.

Damaged balance weight after discharge through side of engine

Vessel

Passenger/ro ro ship

Issue

Balance weight failure

LESSON

Poor manufacturing processes

can often lead to early failure.

When such problems are

identified it is essential that

adequate remedial guidance is

provided and appropriatein-service inspections carried out.

Balance weight fracture surface


9/12

9


CASE STUDY 8

Diesel pump failure

After a diesel engine failed in serviceat a water pumping station, aninvestigation was carried out by LloydsRegisters Technical InvestigationDepartment (TID) to pinpoint the cause.

The diesel engine was the prime mover for

a pump, which pumped floodwater fromthe local drainage system. The engine hadbeen rebuilt two years previously followingtotal immersion in floodwater, and when the

failure occurred only 42 running hours hadbeen accumulated since the rebuild.

The diesel pump had been running for95 minutes when there was a bang, theengine hall filled with smoke and the engine

stopped. None of the engine alarm/tripswere activated at the time of the failure. Avisual examination was made of the engine

components, the records were reviewed and

pumping station personnel interviewed.

The engine was fitted with a turbocharger,with the rotor supported by two oil lubricatedball bearings. The evidence pointed to failure

of the turbocharger turbine end bearing.This would have started a chain of eventsleading to high exhaust gas temperatures,reduced scavenge air flow and higher cylinder

temperatures causing a crank case explosion.

The failed turbocharger bearing, which

had been fitted at the time of rebuild, had

been stationary for prolonged periods. Otherrunning diesel engines could have caused

vibration at the contact points betweenthe stationary bearing rolling elementsand raceway causing impression damage

consistent with false Brinellingand eventual bearing failure.

No routine checks were carried out before

the engine started or while it was running.

End bearing showing evidence of ball race collapse

and overheating

Severe overheating of turbocharger casing and covers

Subject

Pumping station

Issue

Failure in service

This was inappropriate for the age of the

plant and the adequacy of the remotemonitoring systems. While not the primary

cause of the failure, a second watch-keeperand a high exhaust gas temperature alarm/trip may have averted the failure.

Recommendations were maderegarding repairs to the diesel engine,watch-keeping procedures and the level

of remote monitoring.

LESSON

Prolonged periods of diesel

engine idleness should be

avoided. Routine checks

should be carried out prior to

starting and while it is running.

Appropriate levels of remote

monitoring should be provided

that are consistent with the

levels of watchkeeping.


10/12


10

CASE STUDY 9

Propulsion thruster damage

A tug had been experiencingcontamination of the propulsionthruster units. Oil samples were foundto contain high concentrations ofwater and wear products.

The propellers were subsequently removed

and dismantled. A large amount of afine paste as well as ice was found inthe hubs and in the propeller shaft seals.Lloyds Registers Technical Investigation

Department (TID) was asked to investigateand advise.

Each thruster unit was examined alongwith the associated static oil and lubricatingoil systems.

The damage process was likely to haveinvolved a number of contributory factors

relating to the original installation andcommissioning. It is probable that airwas trapped in the hub and this would

have caused poor lubrication of theblade operating mechanism. This mayhave contributed to the initial damage

which then led to water ingress andaccelerated wear.

The height of the oil tank reservoirs andvarious flow restrictions resulted in thetotal head of the static oil system at each

propeller hub being less than the designspecification. Contraction of the air andfluid in the hub after operations, along

with flow restrictions in the propeller shaftseal body, is thought to have produced alow pressure head in the hub resulting inan ingress of water.

Recommendations included repositioningthe static oil system gravity tanks to a

height appropriate for the thruster unitdesign pressure head range. This includedreplacing the interconnecting pipework

Typical galling damage of crank pin ring blade foot axial bearing surface

with pipes of larger diameter and rerouting

with a continuous fall of at least 5 thusreducing the risk of air traps being formed.

Recommendations were also made regardingventing procedures to ensure air did notbecome trapped when filling the static oilsystem. These recommendations included

filling the static oil system up from the hub tothe gravity tank and reconciling the amountof oil added with the volume of the system.

Typical galling damage of blade foot axial bearing surface in hub

Vessel

Tug

Issue

Contamination and damage

LESSON

Care needs to be taken in the

design and installation of podded

propulsors and their associatedstatic oil systems. In particular,

the installed pipework must not

lead to air locks and gravity tanks

should be located at sufficinet

height to maintain adequate

system pressure at all times.


11/12

11


CASE STUDY 10

Overhead crane failure

After an incident in which the trolleyof a power station overhead cranefell from an L-shaped traversing beamduring an overhaul, a team fromLloyds Registers Technical InvestigationDepartment (TID) was asked to makean on-site investigation.

The TID team discovered that duringthe incident the cranes winch

motor windings had been removed,and during the operation the trolleybecame unbalanced and fell from the

traversing beam.

Team members examined the trolley

and the traversing beam, interviewedthe power stations personnel and tookdown and reviewed statements from

Trolley on traversing beam before removal of winch

motor windingsTrolley tilting and sliding on the traversing beam after

removal of winch motor windings

Subject

Power station

Issue

Failure during overhaul

witnesses. Markings on the trolley andtraversing beam also showed where thetrolley wheels had slid over the beams

lower plate before the trolley tipped andfell from the beam.

TID created a template the same widthas the traversing beams lower flange,placing on it the trolley wheels in the same

position as the trolley had been. The teamdiscovered that by tilting the board, thussimulating the trolleys instability, it waspossible for the trolley to tip over and fall

from the beam.

It was also found that while the lateral

clearance between the trolley wheels

and traversing beams lower flange weresatisfactory when the trolley was resting

normally on the lower flange with itswheels intact, when it was tilted, the lateralclearance became excessive and allowedthe trolley to fall off.

The TID team recommended that the trolley

be modified to reduce the lateral clearancesbetween it and the traversing beamslower flange, and to restrict the angular

movement of the trolley.

It was found that there were shortcomings

in the management system, with nowritten procedures produced by acompetent person. The team advised

that, in future, detailed procedures including provision for adequatelysupporting the balance weight side ofthe trolley prior to removal of the motor

windings be produced before suchwork started.

LESSON

Written procedures should

be in place prior to overhaul.

These should include provision

for adequately supporting the

balance weight side of the

trolley prior to removal of the

motor windings.

Template tilted to simulate the trolley tilting and then

sliding over the traversing beam lower plate


12/12

Technical Matters

is printed on

Core Silk from

the Robert

Horne Group.

The virgin wood

fibre is sourced

from Finland

and Brazil and produced at a

mill accredited with ISO14001

certificate for environmentalmanagement. The pulp is bleached

using an elemental chlorine-free

(ECF) process. Core Silk is an FSC

product group from well-managed

forests and other controlled

sources.

Lloyds Register EMEAT +44 (0)20 7709 9166F +44 (0)20 7423 2057

E [email protected]

71 Fenchurch StreetLondon EC3M 4BSUK

Lloyds Register AsiaT +852 2287 9333F +852 2526 2921

E [email protected]

Suite 3501, China MerchantsTower, Shun Tak Centre168200 Connaught Road CentralHong Kong, SAR of PRC

Lloyds Register Americas, Inc.T +1 (1)281 675 3100F +1 (1)281 675 3139

E [email protected]

1330 Enclave ParkwaySuite 200Houston, Texas 77077USA

www.lr.org

March 2012


Technical MattersEditor:Christopher Browne,

Marine CommunicationsDepartmentT +44 (0)20 7423 2305E [email protected]

Designer: Pipeline DesignT +44 (0)1480 462589E [email protected]

Technical Mattersis produced

by Lloyds Registers Marine

Communications Department

and designed by Pipeline Design.

Care is taken to ensure thatthe information in Technical

Mattersis accurate and up to

date. However, Lloyds Register

accepts no responsibility for

inaccuracies in, or changes to,

such information.

Technical Investigation TeamWe are a large group of highly experienced specialists and support staff. If you have anytechnical queries or issues that need a rapid response, please contact one of the following:

Donald Cameron Manager020 7423 1758 or

[email protected] joined Lloyds Registerin 1989 and has been withTID for most of that time.He is responsible for overallmanagement, recruitment,budgeting, tendering, servicedelivery, and monitoring andassessing performance.

Peter Filcek Technical Manager020 7423 1765 or

[email protected] LR in 1977 and hasbeen with TID since 1979. Peteroversees the technical quality ofTID services and training, and isresponsible for the allocation ofproject managers and projectteams. He represents TID in thediscipline of marine failures.

John Maguire Structural Engineering

Section Manager020 7423 1770 [email protected] joining LR in 1989 andthe TID team in 1994, Johnhas overseen a wide rangeof investigations, particularlythose relating to structuralengineering (marine andnon-marine), includingstructural dynamics, fatigueand fracture.

Peter Davies MachinerySection Manager020 7423 1761 [email protected], who joined LR in 1995 andhas been with TID since then, isresponsible for a wide range ofinvestigations, particularly thoserelating to machinery, propulsionand shafting systems.

Dejan Radosavljevic FluidDynamics Section Manager020 7423 1774 or

[email protected], who has been with LR andTID since 1994, is responsible fora wide range of investigations,particularly those relating to fluiddynamics (marine and non-marine), including computationalfluid dynamics (CFD).

Lloyds RegistersTechnical InvestigationDepartmentWhen the unexpectedoccurs or you require in-depth technical advice, youneed to seek out the bestadvice and support.Enter Lloyds RegistersTechnical InvestigationDepartment who will be

able to assess, evaluate andsolve your problem.

FSCLogo

To drop

in at proof

stage

TECH_MATTERS_no6_tcm155-236231.pdf

Documents

Transcript of TECH_MATTERS_no6_tcm155-236231.pdf