39
1 INTRODUCTION
Marine steam propulsion plants are very rarely
employed nowadays due to higher fuel consumption
of such systems compared with diesel motor engines.
Although the efficiency analysis of its main
components: main and auxiliary turbines [1] and
steam boilers [2] with steam air heaters [3] in the
exploitation of LNG carriers show higher energy
efficiency than diesel engine efficiency, the problem
lies in heat loss during steam condensation in main
condenser. The advantage of steam propulsion plant
compared to diesel engine plant is that marine steam
propulsion plant typically needs less maintenance
due to its simplicity. It is normal for such systems to
open one turbine casing in each dry-dock for internal
inspection. Intermediate inspection of turbines is very
rare and is utilized only in the case of obvious
failures. In order to have better insight to failure
occurrence of marine steam propulsion turbine, LNG
carrier maintenance data had been taken into
consideration for the period of eight years where
reliability of such system is discussed.
2 STEAM PROPULSION PLANT
Observed steam propulsion plant has main turbine
consisting of one high pressure turbine casing and
one low pressure turbine casing with astern turbine
incorporated inside the low pressure turbine casing,
Figure 1. Maximum continues rating (MCR) of
analysed main turbine is 26800 kW at 89 rpm. Normal
continues rating (NCR) of the main propulsion
turbine is 24120 kW at 85.9 rpm
which is 90% of MCR
power [4]. This is the operating point where main
turbine will run for most of its service time.
Rated steam condition for the main turbine is:
steam temperature of 520°C, steam pressure at ahead
stop valve 5.9 MPa and main condenser vacuum of
approximately 40 mmHg at sea water temperature of
27°C. As both turbines are running at high speeds and
main propulsion shaft rotates at significantly lower
speed it is required to employ reduction gear to
satisfy such requirements. Reduction gear is usually
of tandem articulated, double reduction and double
helical gear type [4]. Characteristics of the high
LNG Carrier Main Steam Turbine Reliability in the
Exploatation Period of Time
I. Poljak, J. Orović & V. Knežević
University of Zadar, Zadar, Croatia
V. Mrzljak
University of Rijeka, Rijeka, Croatia
ABSTRACT: In this paper the LNG carrier with steam turbine propulsion plant maintenance records has been
analysed. Actual observed data from the ship, built in 2001, are from ship maintenance history data from
September 2002 until August 2010. During the analysed period, main propulsion turbine had one major failure
and several minor failures. The ship had three dry docks and one was prolonged due to increased requirements
for cargo transport. Total running hours of the main propulsion turbine in the observed period of time were
63204 hours. The list of failures and influence of each mentioned failure of main turbine propulsion machinery
is discussed and analysed in respect to the propulsion autonomy of the vessel.
http://www.transnav.eu
the
International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 14
Number 1
March 2020
DOI:
10.12716/1001.14.01.03
40
pressure and low pressure turbines are given in
Table 1.
Both turbines are fixed above the main condenser.
The low pressure turbine shaft is passing through the
main condenser top and carries astern turbine which
freely rotates when main propulsion turbine is
running in ahead direction. Low pressure turbine
shaft is heavier and more stressed compared to high
pressure turbine shaft. In port, when main turbine is
stopped it should turn with the turning gear with few
revolutions per minute to smoothly pass the cool
down period in order to prevent damage to turbine
stator and rotor blades.
Figure 1. Main propulsion turbine layout
Table 1. Main propulsion turbine characteristics [4]
_______________________________________________
Turbine MCR NCR Critical Number
casing rpm rpm speed rpm of stages
_______________________________________________
High pressure 5888 5683 4320 Curtis wheel +
8 stages
Low pressure 3408 3289 2280 8 stages
_______________________________________________
3 FAILURE EVENTS OF MAIN STEAM TURBINE
Main propulsion steam turbine failures had been
collected for the exploitation period of 8 years. All
data were taken out from the main propulsion turbine
maintenance log [5]. Overview of running hours,
failure dates and failure descriptions are given in
Table 2. The ship was built in 2001 and the first dry
dock was carried out in 2004. The second dry-dock
was in 2006, and the third one in 2008. Accumulated
running hours of the main turbine at the end of
observed period were 63204 running hours.
4 FAILURE ANALYSIS
According to Table 2 all failures may be divided into
two main groups as critical and non-critical failures.
Critical failure has direct impact on capability of the
system to provide its output [6], i.e. failure which has
direct impact to the main propulsion operation. Non-
critical failure does not cause immediate inability to
produce required function. Non-critical failures
further may be categorized as degraded and incipient
[7].
For the observed turbine, critical failure occurred
on 05.05.2004. At that time, main propulsion turbine
had 20232 running hours. Vessel reported higher
vibration on the main turbine at all running ranges
and company decided to proceed vessel at reduced
speed to dry-dock for inspection. According to the
maintenance log of the main propulsion turbine, high
pressure turbine sixth stage rotor blades and
diaphragm of the sixth stage were exchanged due to
scratches. At the same time, planned dry dock
maintenance of scale deposit removal from the low
pressure turbine first stage was carried out.
Table 2. Main propulsion turbine failure events
_______________________________________________
Date Running Failure description
Hours
_______________________________________________
23.09.2002 7200 Astern turbine safety control oil
valve exchanged.
05.04.2003 12240 Astern turbine steam temperature PT
sensor exchanged.
13.06.2003 13704 Flexible spider clutch for coupling
main turbine control oil pump
No1 and electromotor exchanged.
13.06.2003 13704 Flexible spider clutch for coupling
main turbine control oil pump
No2 and electromotor exchanged.
18.09.2003 15192 Local manoeuvring side telegraph
bell exchanged.
05.05.2004 20232 Sixth stage of high pressure turbine
blades exchanged due to scratches
on the upper halves of diaphragm
casing.
05.05.2004 20232 Sixth stage diaphragm exchanged.
05.05.2004 20232 Forward labyrinth seal springs
renewed.
05.05.2004 20232 Scale deposit at first stage of low
pressure turbine cleaned.
05.05.2004 20232 Steam pressure transmitter at first
stage outlet exchanged.
12.05.2004 20232 High pressure bleed steam shut of
valve solenoid valve exchanged.
29.08.2005 31152 Flexible spider clutch for coupling
main turbine control oil pump
No1 and electromotor exchanged.
11.01.2006 34032 Flexible spider clutch for coupling
main turbine control oil pump
No1 and electromotor exchanged.
12.12.2006 41232 Flexible spider clutch for coupling
main turbine control oil pump
No1 and electromotor exchanged.
18.01.2007 41976 Main propulsion turbine revolution
counter exchanged.
21.05.2007 44856 PT sensor for main turbine steam
temperature at steam chest
exchanged.
11.06.2007 45096 High pressure turbine bleed shut off
valve limit switch exchanged.
18.01.2008 49560 Flexible spider clutch for coupling
main turbine control oil pump
No2 and electromotor exchanged.
26.03.2008 51048 Flexible spider clutch for coupling
main turbine control oil pump
No1 and electromotor exchanged.
26.04.2008 51768 Main turbine lube oil pump No2
electromotor bearing exchanged.
08.06.2008 52608 Main turbine reduction gear dry air
fan exchanged.
21.07.2009 53824 Main turbine manoeuvring log unit
at bridge station exchanged.
10.08.2010 60902 Main propulsion turbine bridge
telegraph CPU unit exchanged.
_______________________________________________
41
High pressure turbine blades scratches are not
regular issues and they have to be treated instantly in
order to avoid serious turbine damages due to metal
particles which may enter to further turbine stages.
For this type of failure it is difficult to say what was
the exact cause but in similar cases this type of failure
is usually connected with water droplets at high
pressure stages which are carried over from the main
boiler or from undrained pipelines. High pressure
turbine casing drain is placed beyond 4
th
turbine stage
and if draining is not appropriate water may enter to
further stages with steam flow through the rotor.
Water droplets increase rotor vibrations and erode
turbine blades. Figure 2 shows high pressure turbine
second stage blade erosion at the same turbine
discovered during the third dry-dock.
Figure 2. High pressure turbine rotor blade erosion
Non critical failures which are listed due to
degradation of the materials are:
− Astern turbine safety control oil valve exchanged.
− Astern turbine steam temperature PT sensor
exchanged.
− Flexible spider clutch for coupling main turbine
control oil pump No1 and electromotor
exchanged.
− Flexible spider clutch for coupling main turbine
control oil pump No2 and electromotor
exchanged.
− Local manoeuvring side telegraph bell exchanged.
− Steam pressure transmitter at first stage outlet
exchanged.
− High pressure bleed steam shut of valve solenoid
valve exchanged.
− Main propulsion turbine revolution counter
exchanged.
− PT sensor for main turbine steam temperature at
steam chest exchanged.
− High pressure turbine bleed shut off valve limit
switch exchanged.
− Main turbine lube oil pump No2 electromotor
bearing exchanged.
− Main turbine reduction gear dry air fan
exchanged.
− Main turbine manoeuvring log unit at bridge
station exchanged.
− Main propulsion turbine bridge telegraph CPU
unit exchanged.
The most frequent and recurring failure is related
to flexible spider clutch for coupling the main turbine
control oil pump No1 and No2 with electromotor. As
this failure is recurring it may be assumed that
hydraulic pump and electromotor are not aligned
properly what causes frequent spider damage. This
type of failure may be corrected once vessel enters
dry-dock.
The second group of recurring failures refer to
monitoring equipment i.e. PT temperature sensors
and pressure sensors that were exchanged three
times. These failures are unavoidable because selected
PT sensors operate at high temperatures and have
limited working temperature range (slightly above
operating temperature). Owner decided to mount
cheap solution in the beginning but frequent failures
do not justify owner’s first choice.
Control equipment failures related to navigation
equipment include: local manoeuvring side telegraph
bell failure, main propulsion turbine revolution
counter failure, main turbine manoeuvring log unit at
bridge station failure and main propulsion turbine
bridge telegraph CPU unit failure. These failures are
not expected in such number. It is a compulsory
requirement that main turbine telegraph order log is
working due to safety requirements and restoration of
manoeuvring in the case of incident.
Miscellaneous failures are related to: astern
turbine safety control oil valve failure, high pressure
bleed steam shut of valve solenoid valve failure, high
pressure turbine bleed shut off valve limit switch
failure, main turbine lube oil pump No2 electromotor
bearing failure and main turbine reduction gear dry
air fan failure. Miscellaneous failures are of low
frequency and they are in expected occurrence range.
Incipient failures/faults are caused due to non-
perfect condition of equipment so that a degraded or
critical failure might occur [7]. In order to prevent
incipient faults, according to Table 2, corrective
actions were taken: forward labyrinth seal springs
were renewed and scale deposit at low pressure
turbine first stage was cleaned.
5 CONCLUSION
In this paper failure events of main propulsion
turbine on LNG carrier were analysed. Analysis
defined three main groups of failures for the main
propulsion turbine in presented time range: critical
failures, non-critical failures due to deterioration of
material and non-critical incipient failures or faults.
The non-critical failures due to material
deterioration are listed as: main propulsion turbine
control system failures related to the flexible spider
clutch with 7 failures, miscellaneous failures with 5
failures, control equipment related to navigation with
4 failures and monitoring equipment with 3 failures.
Incipient failures: related to labyrinth seal and scale
deposit at low pressure turbine with totally 2 events.
Non critical failures may be treated in the port or
during anchorage and they have low impact to
propulsion turbine reliability. The frequency
distribution of failures related to flexible spider clutch
has to be improved in order to avoid possible
dangerous situation with two control oil pumps in
failure. This will cause stoppage of the main
propulsion turbine. In order to avoid such risk, spare
part kit should be on board the vessel for quick repair.
The other weak points were control equipment
42
failures related to navigation equipment where
company should upgrade the system in order to
avoid future failure occurrences.
High reliability of the main propulsion turbine at
sea is required as vessel has restricted contact with
shore services and spare parts supply. It may be
concluded that main propulsion steam turbine had
high reliability in exploitation period.
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analysis of the turbo-generators and steam turbine for
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[2] V Mrzljak, I Poljak, V Medica-Viola; Dual fuel
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[4] Instruction book for marine propulsion unit, Mitsubishi
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[5] Chief engineer file / Main turbine maintenance records,
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