Risk Assessment of Propulsion Systems for LNG Carriers
Senichi Sasaki *
1.
INTRODUCTION
Infiltration of heat from external air sources and ship motions
causes a portion of the cargo carried by low-temperature
liquefied gas carriers (LNG carriers) to be converted into boil off gas (BOG). As this gas can be used to power their boilers, steam turbines have thus become the most common propulsion system used on LNG carriers. In recent years, however, studies have been carried out on a variety of other propulsion systems, including oi!-flred diesel main engines with reliquefaction units,
electric propulsion systems with dual fuel diesel generators,
and dual fuel diesel main engines. Some of these propulsion
systems are already being used on actual ships.
As with general merchant ships. classification society rules do
not include special redundancy requirements for the main
engine and propeller shafting systems of LNG carriers. If the components are determined to be reliable, then a single main
engine is adequate. As a general requirement, a ship is only
expected to be able to maintain a navigable speed (7 knots or half the speed required by the rules for fully-loaded condition) in the event of a single-point failure.
However, despite the lack of special rule requirements, due to the special characteristics of LNG carriers, including:
Regular sailings demanded as part of the LNG supply chain
Requirements for main engine standby at terminals,
main engine redundancy is considered to be a general
requirement for LNG projects. Due to this, redundancy is
generally required
for LNG carrier
propulsion systems,excluding that equipment which is considered to be highly
reliable. (shaft, reduction gear, etc.)
Redundancy has always been established empirically. For
example, the high and low pressure turbines on steam turbine
driven ships can be operated independently of each other.
Further, installing two diesel main engines and two propulsion motors for electric propulsion systems has become common
practice. In this way, equipment and machinery redundancy was ensured merely by installing two of each piece of
machinery. Until now, there have been no real cases where the level of redundancy was analyzed theoretically.
Redundancy is required not only for propulsion systems, but * Research Institute, Nippon Kaiji Kyokai (ClassNK)
ClassNK TECHNICAL BULLETIN 2009
63also for BOG treatment systems as well. When BOG treatment
systems break down. BOG will be inevitably emitted to the atmosphere. In order to deal with such situations, IGC Code
requires that a duplicate BOG system be maintained. For steam
turbine driven ships, the main boiler can also perform BOG
treatment. The surplus steam generated by combustion in the main boiler is treated by the dump condenser, and the surplus heat quantity is released into sea water. Dual fuel diesel engines, reliquefaction units, and gas combustion units may be used as
BOG treatment systems in place of the boiler. Of these,
however. IGC code only requires redundancy for reliquefaction units, and there are no requirements for the other systems.
This report compiles the results from an investigation of the redundancy requirements for propulsion and BOG treatment system using risk assessment as ajudgnient tool.
2.
Systems to be assessed for risk
The basic configurations of the four kinds of propulsion
system (including BOG treatment system) to be assessed for risk are shown in Fig. I.
Conventional steam turbine propulsion system:
This system consists of one low pressure turbine, one high pressure turbine, and two main boilers. With regard to generators, two generators driven by steam turbine and one generator driven by diesel engine were considered.
Oil-fired diesel engine propulsion system:
This is a system that consists usually of two oil-fired diesel main engines. With regard to generators, a system with three generators driven by diesel engine was examined. Reliquefaction unit are generally used as the BOG treatment system tr this propulsion type. (e) Electric propulsion system using DFD generator
engine:
This is an example of a typical system using DFD engine. Four generators driven by DFD engine and two motors for propulsion are provided. Although this may use either a reliquefaction unit, a gas combustion unit (GCU), or an auxiliary boiler as its BOG treatment system, use of a single gas combustion unit was examined in this case. (d) DFD propulsion engine system:
This system theoretically uses a DFD engine for propulsion, however, this type of system has not yet been developed. This system consists of two DFD main
Dump Condenser Reduction Gear Reti L. P. Tu bine H. P. Turbine luction Gear Propulsion Motor
.
2-stroke Main Engine 2-stroke Main Engine Main boueT
e-jf
D/ E D/ E DEI
=
(a) Steam turbine propulsion system
(b) Oil-fired diesel engine propulsion system with re-liquefaction unit
O/E BOG Compressor
D/E
DJE
O/E
DE
engines and three generators driven by diesel engine. Similar to (e), one gas combustion unit is taken as the BOG treatment system.
BOG Compressor
(e) Electric propulsion system using DFD generator engines
GCU
H. P. BOG Compressor
(d) DFD propulsion engine system
-
Steam Electric'4-
Boil-off Gas Fuel OilFigure 1 Subject propulsion system for risk assessment
Cargo Tank BOG
Cargo Tank Cargo Tank
CIassNK TECHNICAL BULLETIN 2909
3.
Risk assessment
The redundancy of machinery and equipment is determined
after completing the three steps below.
STEP I: Using reliability analysis. find the failure rate and the mean time to repair the entire system.
STEP 2: Using the failure rate and mean time to repair, find the risk and availability.
STEP 3: Using the risk and availability as judgment indices, determine the redundancy of the equipment.
3.1 Reliability analysis
Using reliability software developed by Relex Software Co.,
the failure rate and mean time to repair of the entire system
were determined by Fault Tree Analysis (FTA). The top events were fixed first in FTA, the basic events (failure of equipment)
up to the top event were developed, and failure rate (or mean time between failures) and mean time to repair were assigned
to each basic event so that the failure rate of the top event could
be determined. For instance, Fig. 2 shows an example of an
analysis of the steam turbine propulsion system. Here, the top event is "loss of self- propelling ability" while the intermediate events are 'propeller shafting system failure", "turbine failure",
"power failure", "S/TI failure" and "DIE failure". "Propeller
shafting system failure" is comprised of two basic events
namely "propeller failure" and "shaft failure". By inputting the failLire rate and the mean time to repair for these basic events,
the final failure rate and mean time to repair can be obtained for when the system reaches "loss of self-propelling ability"
level of failure.
3.2 Risk and availability
Risk is defined as failure rate x consequence of failure.
Generally in examinations of machinery systems. the
consequence of failure can be classified into the categories shown in Table I. However, fiar the purpose of the present
research, for instance, the consequence of a failure in the
propulsion system is identified and analyzed as "loss of self-propelling ability". Accordingly, risk is simply expressed asfailure rate.
Table I Classification of tile consequence of failure
Note: Classification according to 1MO FSA Guidelines
In addition to risk, availability is also one of the indices for
judgment in the present research. The availability of equipment A is given by the failure rate A (cases/hour) and the repair rate u (cases/hour) as in the equations below.
A=i/(2+p)
(1)2=l/MTBF
(2)u=l/MTBR
(3)Here, MTBF: Mean time between failures (hours/cases) MTTR: Mean time to repair (hours/cases)
Loss of self-propelling ability
...J Propeller shaft j system failure
Red. Gear failure Turbine failure
Power outage AND
(OR AND S/Ti failure
H
S/T2 failure DIE failure Propeller failure Shaft failure HP turbine failure LP turbine failureFault Tree
Figure 2 Analysis example of a steam turbine propulsion system (Partial)
Category* Explanation
Catastrophic
Total loss of system, loss of life or serious human injuries remaimng as after effects
Severe
Major damage to system resulting in loss of self- propelling ability or serious human inuries
Significant
Damage to system making reduced speed operation inevitable or hunlan Injuries
Minor
Minor damage to system that does not lead to failure of normal navigation and no human injuries
3.3 Estimation of failure rate
Data such as failure rate assigned to basic events has been obtained from the Ship Reliability Investigation Committee database (SRIC database) of the National Maritime Research
institute. However, reliability data does not exist for equipment
with few failures or equipment with practically no operating
records. Data for equipment with few failures was estimated by
the classifying frequency of failures. Further, practically no operating records exist for machinery like DFD engines. For
such machinery, the failure rate distribution of oil-fired diesel engines was determined, and the mean value of parts with high failure rates was found. That is, the DFD engine was treated as
an engine with comparatively high failure rate and the failure
rate was estimated.
3.4 Propulsion systems to be assessed
To study redundancy of machinery or equipment constituting
the propulsion system, risk
assessment was carried
outconsidering the top event of FTA as 'loss of self-propelling
ability". Table 2 shows the system to be assessed. Table 2 Propulsion system to be assessed
Note: DE: Oil-fired diesel engine: DFD: Dual fuel diesel engine: ST Steam turbine
(b)', (c). (d)' in Table 2 indicate systems with one each of
oil-fired diesel main engine, propulsion motor and DFD main
engine respectively. By performing risk assessments of such
systems, the redundancy of engines and propulsion motors can be studied.
3.5 BOG treatment system to be assessed
To study the redundancy of the equipment constituting the reliquefaction unit and gas combustion unit, risk assessment
was carried out taking the top event of FTA as "BOG emission to atmosphere. Table 3 shows the systems to be assessed. The
systems to be assessed are part of the basic system of Fig. 1,
and consist of the BOG treatment system and the power supply units required for powering these systems.
Table 3 BOG treatment system to be assessed
Note: DE: Oil-fired diesel engine: DFD: Dual fuel diesel engine: ST:
Steam turbine
The system in (e)' of Table 3 is the case of the system of (e)
having one gas burner, and is installed to study the redundancy of gas burner.
4.
Results of assessment
4.1 Redundancy of propulsion system
Table 4 and Fig. 3 show the calculated results of Table 2. Tab le 4 RedLindancy of propulsion system
The following were the findings with regards to redundancy
of the propulsion system obtained from the calculated results: The basic systems (b), (e) and (d) have smaller failure rates, higher availability, and more adequate redundanc'. compared to the conventional steam turbine propulsion system (a).
Even in case of(c)' having one propulsion motor in the system in (e), the difference in failure rate and
availability in the two systems was not large; this leads to the judgment that one propulsion motor may be used. In the systems (b)' and (d)' having one oil-fired diesel propulsion engine and one DFD propulsion engine, the failure rate was clearly higher and the availability lower than the system in (a). From this, it can be concluded that multiple engines are necessary to obtain a system with reliability equivalent to or greater than then system in (a'i.
System to be assessed Generator
(a) Main boilers x 2 (conventional system)
ST s 2. DE s I
(b) Re-liquefaction unit DE x 3
(e) Gas combustion unit (gas burner x 2) DFD x 4
(c) Gas combustion unit (gas burner x 1) DFD x 4
(d) Gas combustion im it (gas burner s 2) DE x 3
Systems to be assessed X (cases!
1000 hrs) A (a) Steam turbine system
(conventional system)
0.751 0.9982
(b) Oil-fired diesel propelling machinery sysleni (DE x 2)
0.551 0.9987
(b)' Same as above (DE x 1) 1.139 0.9974
(e) Electric propulsion system using DFD generator
(propulsion motor x 2)
0.463 0.9991
(e)' Same as above (propulsion motor x I) 0.483 0.9991 (d) DFD propulsion machinery system (DFD s 2) (1.463 0.9991 (d) Same as above (DFD s 1) 3.000 0.9945
System to be assessed Main engine Generator engine
(a) Steam turbine system (conventional system) LP x 1, HP x 1, boiler x 2 ST s 2, DE s 1 (h) Oil-fired diesel propulsion machinery system DE s 2 DE s 3 (b)' Same as above DE s I DE s 3
(e) Electric propulsion system using DFD generator
Propulsion motor x 2
DFD s 4
(e)' Same as above Propulsion motor s I DFD s 4 (d) DFD propulsion machinery system DFD x 2 DE x 3 (d) Same as above DFD s I DE s 3
4.2 Redundancy of BOG treatment system
The results of the calculation are shown in Table 5. Although
the failure rate was estimated in the analysis
of the
reliquefaction unit and the gas combustion unit, the repair timecould not be estimated, therefore, assessment of the entire
system was made only by failure rate (or risk). Table 5 Failure rate of BOG treatment system
a) (t t) (a u-[1/h r] 102
io
io
1 10.6 io-? 10.8 Acceptable Steam turbineOil-fired diesel main
engine
twin I single
twi n
From the calculated results, the following were the Ondings
related to redundancy of the BOG treatment system:
The systems (b) that included reliquefaction unit and the systems (c) and (d) that included gas combustion unit, were judged to have lower failure rate, more adequate redundancy than the conventional steam turbine propulsion system (a).
Even in the system (c) consisting of the system (e) having one gas burner, the difference in failure rate compared to (e) was small; this suggested that one gas burner may be used.
DFD main engine < 1
Oil-fired diesel main engine x i
Steam turbine
Oil-fired diesel main engine x 2
DFO main engine 2
Electric propulsion
(i) Risk of propulsion system
Elec. propulsion motor twin / single
single twi n
(ii) Availability of propulsion system Figure 3 Risk and availability of propulsion system
NO
Recommended
DFD M/E
twin I single
Systems to be assessed X (cases!
1000 hrs) (a) Steam turbine system
(conventional system)
0.588
(b) Oil-fired diesel propelling machinery system + reliquefaction unit
0.350
(e) Electric propulsion system using DFD generator + gas combustion unit (burner x 2)
0.320
(e)' Sanie as above (burner x I
0.330
(d) DFD propulsion machinery system + gas combustion unit (burner x 2)
0.320
CIassNK TECHNICAL BULLETIN 2009
67(a) (b)
Consequence of
failure
(c) (d)
