Development of a Total Support
of an LNG Carrier
Nono Yamamoto
Senichi Sasaki
*
1.
INTRODUCTION
Judging from energy resources trends, construction of new LNG ships will likely take recent studies on the long-term usage plans of LNG carriers into greater consideration. To ensure the stable use of LNG ships over a long period, it is necessary to control the causes of age-related deterioration both during construction and while in service in order to reduce life
cycle cost and risks. In recognition of these circumstances,
research was carried out over a period of two years from 2007
through 2008 willi the aim of offering an integrated support
service for the entire life cycle of LNG ship to shipowners. as well as shipyards.
The research carried out included development of: (I) corrosion protection technology
(2) technology for safety
management of fatigue strength: and (3) preventive
maintenance technology for machinery and equipment. The
research conducted under each of these themes is listed below.
(1)
Corrosion protection technology
Actual condition survey of coating deterioration in ballast tanks of the ship was carried out. The condition of coating deterioration in ballast tanks was confirmed.
A system for judging the coating deterioration condition in ballast tanks was developed using coating deterioration sensors and potential distribution simulation technology in ballast tanks according to the boundary element method.
(2) Technology for safety management of fatigue strength
(a) Management method during construction was proposed for high risk locations that were bases of fatigue strength management and were selected by risk ranking among potential risk locations. (h) A system for assessing hot-spot stress with high
accuracy was studied.
) The applicability of fatigue condition monitoring
using fatigue sensors was studied as part of fatigue condition monitoring technology for ships after commissioning.
Resaerch Institute. Nippon Kaiji Kyokai (ClassNK)
System for
the Pntire
I if' Cr-1
Deift University of Technology
Ship HydromeChafliCS
laboratory
Library
Mekelweg 2
26282 CD DeIft
Phone: 31 (0)15 2786873
E-mail: p.w.deheer@tUdelft.fll
Preventive maintenance tecnnologv ¡or macninety
and equipment
The change of damage rates over time was summarized by analyzing and assessing damage data from main engine parts.
RBM/RBI system of all machinery/parts including main turbines and engine room auxiliary equipment was prepared.
An overview of the systems for judging the level of coating
deterioration in ballast tank, fatigue strength management procedures during construction, and RBM/RBI system of main engine is introduced here.
(3)
2.
Development of quantitative coating
condition monitoring system
2.1 Concept of monitoring method
Ballast tanks in
ships arc required to be provided with
protective coating. While the paint is effective for insulating tank surfaces from sea water (ballast water), once the coating
deteriorates, local macro-cells form around the deteriorated part
of the coating, and corrosive currents are generated. Usually,
sacrificial anodes are installed to maintain an effective coating
system in the ballast tanks. As a result, galvanic cells are
formed between the deteriorated coating parts and sacrificial
anodes. If the potential changes in the ballast tank
accompanying formation of such
galvanic cells can bemeasured by potential measurement, deterioration of coating
can be detected.
The relationship between the potential Ø [VI in the electrolyte
near the material surface and current density i [A/rn2] can be expressed by polarization characteristics. Using this relationship, the relationship between change in potential and
change in current density can be obtained. The magnitude of
surface resistance is related to the insulation performance; thus,
the effectiveness of coating can be assessed by the surface resistance value. Consequently, if the magnitude of surface resistance can be derived from potential measurements, the condition of the coating can be judged because a correlation
exists between the ratio of reduction rate of surface resistance value and the coating deterioration area.
2.2
Measurement procedures
The tank surface resistance can be assessed by measuring the
differential potential when current is impressed. Fig. I shows
the concept of the newly developed measurement system.
2.3 Verification experiment
To verify the applicability of the developed method, the
potential was measured in a ballast tank of an LNG ship with
an age of 23 years, as shown in Fig. 2. The measurements were performed in one compartment within one transverse space in the side ballast tank shown in Fig. 3. This ballast tank had been painted five years previously. Slight coating deterioration could
be seen on the edge of the
stiffeners,but no coating
deterioration was found in other parts. The condition of the
coating was extremely satisfactory.
Fig. 4 shows the potential distribution measurements at the bottom surface of the compartment (depth 12.8 m) from the ballast water surface (depth 5.2 m) in the compartment. The maximum value of the differential potential was measured at the position where additional anode was installed. Similar
results were also obtained by potential distribution simulation
in the compartment by boundary element method (BEM). In
this study example, the surface resistance value in the
compartment was estimated as
500 (m2).
This valuecorresponds to the result that the mean coating deterioration
area in the compartment is approximately 0.2% (=1/500).
From such actual ship measurements, it was confirmed that measurements of differential potential can be made from the measurement method with system configuration shown in Fig. I. Moreover,
it was also confirmed that the potential
distribution in the
tank can be effectively
estimated bynumerical simulation using the boundary element method. This showed that the coating deterioration state in the ballast tank of a ship can be judged using the proposed method.
Optional
a nod e
Reference
electrode
Figure 1 Potential measurement model in ballast tank
0.018 0.0 16 0.0 14 ? 0.012 C) 0.010 0.008 0.006 0.004 0.002 O
Figure 2 LNG ship in which potential measurements were made
Figure 3 Coating condition ill which the potential measurements were made
-e-
Meas.i---o Meas. 2 -0-- Meas. 3
Analysis
ClassNK TECHNICAL BULLETIN 2009
5 6 7 8 9 10 11 12 13 14
Depth Im]
Figure 4 Measured and evaluated potential distributions when the optional anode was installed at a position 1 .6 m from the
seawater surface
(surface resistance of the tank walls is 500 Çm2)
2.4 Development of practical system
As a pilot test, a trial coating deterioration monitoring system
was designed and installed on an actual LNG ship. The
reference electrodes, additional anodes and connecting cables
were to be installed in the ballast tank for a long period, so the
specifications important to these items were set as waterproof performance. ability to withstand pressure, and durability. Since the cargo areas of an LNG ship are hazardous areas, intrinsically safe circuits are essential for the system to be
installed. An intrinsically safe barrier must be provided at the boundary of the hazardous area and the safe area. Flameproof
cables are required to be used within the hazardous areas. Fig. 5 shows the arrangement of the system assuming its installation on an actual ship.
Safety Area (engine room)
measurement unit intrinsic safety barrier Hazardous flameproofjunction box Area f TTTTiT1 optional anode I ref. electrode optional anode ref. electrode optional anode ref electrode shunt anode ßallast Tank optional anode1 ref. electrode OpL!Onal anode] ref. eledrode I optional anode ref. electrode shunt anode
Figure 5 Block diagram of the system applied to LNG ship
3.
Fatigue management during
construction
3.1 Concept of fatigue management
FatigLie cracks are potentially likely to occur in most welded
joints of the hull structure. The possibility of initiation of a fatigue crack, however, varies depending on the detail of the welded joint type, magnitude of load received, workmanship,
and so on. Also, the degree of hazard for the developed crack varies depending on the position of the member in which it has
occurred and the functions of the member. Accordingly, it is important to assess the overall fatigue risk at the potential fatigue hazard locations in the hull structure, and carefully manage the processes during construction at locations where
high risk has been assessed so as to effectively ensure overall fatigue strength and safety. By clearly recording the conditions at locations with high fatigue risk when the ship is newly built, the records can be effectively used as reference indices during visual inspection after the ship is commissioned.
3.2 Assessment of fatigue risk
For assessing fatigue risk, (i) tile possibility of fatigue crack initiation
(2) the
hazard of initiated crack; and (3)
the possibility of detection of initiated crack during survey, must beassessed from the overall perspective. The assessment of
fatigue risk is to be carried out according to the procedure
below.
Assessment ofthe possibility offatigue crack
initiation
The fatigue strength assessment of locations in the hull structure where fatigue crack may occur is an essential design requirement. The possibility of fatigue crack initiation is to be
assessed taking the cumulative fatigue damage as ari index. This kind of fatigue strength assessment should preferably be performed in line with the concepts of the "Guidelines for
Fatigue Strength Assessment". However, assessment must be
performed considering the differences in design conditions
specified in the guidelines and those of ships to be assessed (for instance, differences in typical service route which is the basis
of design loads, differences in design life, differences in post
weld treatment, such as grinding and so on).
The judgment on fatigue strength conforming to the guidelines
corresponds to assessment with 2-sigma lower limit curves;
therefore, the possibility of fatigue damage when the allowable
damage is satisfied is
about 2.3%, and the possibility of
damage below this value becomes a pre-condition of
assessment. Based on such a pre-condition, risks can be
categorized into grades such as low degree of hazard with the assessed fatigue life greater than i O times the design life; high
degree of hazard with the assessed fatigue life
less thanI .3 times the design life, and a medium degree of hazard lying in between these two grades.
Assessment of a hazard of initiated fatigue crack Even if the fatigue crack initiated in the hull structure has the
same crack pattern, the degree of hazard differs depending on the importance of the member ill which the crack has initiated.
Moreover, even if the crack is initiated in a member having important structural functions, the effect on the function may
differ depending on the location where the crack initiates. The degree ofhazard must therefore bejudged considering all these points.
The most important function in a hull structural member is watertighiness. From this perspective, cracks with a high degree of hazard may be referred to as cracks that initiate in a member that forms part of a compartment boundary subjected
to liquid pressure. Such a crack initiates at a position close to the boundary of shell plating. watertight bulkhead, stiffeners
attached to shell plating and watertight bulkhead (longitudinals),
and girders attached
to shell plating and tile *atertightbulkhead. Crack with medium degree of hazard refers to a
crack that initiates in a member at the compartment boundary
close to the boundary
of airtight
bulkhead, stiffeners (longitudinals) attached to airtightbulkhead, and girders
attached to airtight bulkhead. A crack with a low degree of
hazard refers to a crack that initiates at a location distant 1mm the compartment boundary.
(3) Assessment of the effects of inspection
Loss of functions and catastrophic failure of structural
members can be prevented beforehand by detecting and
repairing a crack that has initiated in the hull structure during
thc period from its initiation until its growth to the loss of
functions or catastrophic failure. In this case, inspection has important significance, and the effects of inspection can be
assessed by how a crack can be detected before loss of
functions or catastrophic failure occurs with the growth of the
initiated crack. That is, the possibility of detecting a crack
during inspection depends on the ease of inspection (ease and dependence on the inspection environment), and the chances to discover cracks during inspection.
In principle, inspection is performed visually, therefore the
brightness
of the
location to be inspected considerablyinfluences the possibility of discovering cracks. From this
viewpoint, the inspection environment can be classified based
3.3 Enhanced
construction
monitoring
for
fatigue
Fatigue management during construction has the purpose of
adequately ensuring the desired fatigue strength by monitoring the workmanship and work conditions in detailed construction
system, which becomes pre-conditions of fatigue
strength assessment to be considered during design at important fatiguelocations. Conventionally, such monitoring of workmanship
was performed adequately by classification societies, shipbuilders. and ship owner's representatives, but by recording
the monitoring conditions, the important locations can be
clearly identified. Moreover, by properly recording the workmanship condition, the records are anticipated to be used as reference standards for monitoring after the commissioning
of the ship.
In principle, inspection during construction isTable I Classification and management of fatigue risk ranks
on the visibility when performing visual inspection.
Whether the relevant member or location can be approached closely and inspected or not (accessibility) is a key point in performing inspections. If the space within the compartment during dry-docking is not empty, whether inspection can be performed or not can be classified by these two conditions:
possible in practice and not possible.
When considering the crack
propagation,cracks can be
classified by the judgment standards, whether a crack will propagate and lead to major damage during the short period
less than two and a half years between Intermediate Surveys, or
whether the possibility of a crack propagating this way in a
five-year period between Special Surveys is small.
(4) Assessment of fatigue risk
Fatigue risk is assessed by estimating the sum of the product of
the degree of risk mentioned earlier, namely (i) possibility of
fatigue risk initiation; (ii) degree of hazard of initiated fatigue crack; (iii) inspection environment; (iv) accessibility to inspection location: and (y) rate of crack propagation. and the weighting factor. Depending on the assessed fatigue risk value, a management method can be decided as shown in Table I.
similar to the conventional inspection procedure, but it is also
necessary to acquire records for confirming the necessary
conditions in each process.
Monitoring method includes preparing the recording forms for
each of the following: (1) summarizing general information
related to monitoring of relevant locations, and (2) summarizing the recommendations for check items and recording items in the inspection at relevant locations; and implementing monitoring according to the assessment category of fatigue risk rank. Such a method can ensure effective monitoring.
The monitoring procedure according to the assessed fatigue
risk rank is as given below.
-I. Perform close-up inspection and record results in each process during construction for locations classified as risk rank A. Record the state of workmanship quality in each
Rank Management method
A location that needs to he monitored carefull. In addition to completion surs ey. inspection during construction is implemented at locations requiring fatigue management during
construction, records of workmanship quality condition in each process maintained, and records of the state of completion also maintained.
B
Although this is a location to be monitored, the risk is judged to be lower than that of rank A. Completion survey is implemented at locations requiring fatigue management during construction, and records of the state of completion are maintained.
C This is a location judged to have no problems in particular. and the usual classification survey is
applicable.
process and also record the state of completion.
-.
Perform close-up inspection and record results in each procs during construction for locations classified as risk rank B. Record the state of completion.The records of state of workmanship quality are meant for
indicating the following states:
State of plate edge in the installed state before welding State that satisfies workmanship quality standards such as gap. misalignment, etc. in the installed state before welding
If specified, the back gouging state External appearance of weld bead
1f specified, the extent of full penetration weld/deep penetration weld
If additional leg length, etc.. is specified, the leg length/throat thickness
If specified. the state of weld treatment Other states deemed necessary.
Recording procedures for recording the state of completion are as below.
(I) Record of final state that shows the position of the hot spots at relevant locations in the structure.
After completion of construction. records that have been divided into several stages that identify the positions of the relevant locations in the hull are to be kept, and finally. records that correspond to the records mentioned above are also to be maintained.
If the records divided into several stages mentioned above are difficult to obtain, then recorded positions
(photographed points) are to be recorded with the photographs.
In case of longitudinals. the names of compartments retrieved from records are to be recorded with the photographs.
4.
RBM/RBI system for main engine
4.1 Background of the development
Although the maintenance and inspection of machinery and equipment in ships is based on time-based maintenance and
inspection with fixed time interval as the basis, condition based
maintenance (CBM) is the nonna! practice for deciding the
period of open-up inspection based on the results of condition
monitoring in case of steam turbine ships. On the other hand.
Risk-Based Maintenance (RBM) /Risk-Based Inspection (RBI)
with which maintenance and inspection arc implemented on a priority basis starting with equipment and parts with high risk
and based on the results of risk assessment, have recently been implemented for in shore-based plants.
4.2 Base ofthe RBM system
ClassNK has developed an RBM system for machinery and equipment to be installed on diesel ships and steam turbine
ships. This system has been developed referring to the "API 581
Risk-Based Inspection
Base Resource Document" of the
American Petroleum Institute (API). Risk is generally expressed as fiuilurc rate x "importance
of damage
(consequence)", but risk is defined as "occurrence probability of failure" x importance of damage (consequence) by API. The
definition of risk in this system also follows the API definition. The definition is as given below.
Occurrence probability of failure = "Standard failure frequency" x "equipment modification factor" x 'management modification factor".
Importance of damage = "Extent of voyage interruption" x
"average repair man-hours".
Here, "standard failure frequency" X (%/year) was assigned as
a function of the ships age T. The value of the constant based
on the Society's damage data was determined for each part, and
the initial value of the standard failure frequency was set.
However, the standard failure frequency varies depending on
the state of damage that has occurred during voyage or damage discovered during Periodical Surveys: therefore, in this system. this calculation formula was formulated enabling updates based on the Bayes method.
A=aexp(b/T)
(1)a, b: Constants depending on each part
"Equipment modification factor" is determined by the condition monitoring method used and by the maintenance and inspection method of the equipment. "Management modification factor" is a factor related to ship management.
"Extent of voyage interruption" expresses the consequence of failure for a ship when the relevant machinery or equipment is damaged it is associated with the redundancy of the machinery or equipment. "Average repair man-hours" is expressed as the
product of the number of persons required for repairs and the
hours required for repairs.
4.3 Developed RBM system
Fig. 6 shows ax example of the software screen for specifying
the occurrence probability of failure in the developed system.
while
Fig. 7 shows an example of the
software screen specifying the importance of damage.part of equipment; the next inspection period of the equipment
is determined based on this risk. Fig. 8 shows an example. In this case, the green and yellow colored boxes show that the
relevant equipment can be used in continuation without open-up inspection. The orange and red colored boxes indicate that the period for open-up inspection has been reached. The
maintenance of equipment is planned such that open-up
inspection is performed preferentially starting with high-risk
equipment.
At present, the maintenance and inspection of equipment is
considered to
be time-based maintenance for reasons of
practicality. However maintenance according to CBM or RBM is more rational. In the future, it is anticipated that either CBM
or RBM will be selected and implemented according to
equipment choices. The system developed by ClassNK this time is a prototype of an RBM system that may be developed in the
future, and its detailed image has been offered as an example
system.
Figure 6 Occurrence probability of failure
5.
CONCLUSION
To ensure the stable use of LNG ships over a long period,
management of the causes of deterioration from ageing while in service and during the construction of the ship is important for
reducing life cycle cost and risks. Therefore, research and
development was carried out over a period of two years related
to the following: (1) Corrosion protection technology: (2)
Technology for safety management of fatigue strength: and (3) Preventive maintenance technology for machinery and equipment.
A method based on potential measurements in the ballast tank as a technology that can be practically applied was developed as
a method for judging the state of required coating objectively for corrosion protection. Further, an effective quality control
procedure during construction was developed for fatigue
management according to the risk rank of important locations associated with fatigue. Finally,
a prototype of a system
assigning data for judging maintenance based on assessed risk values was developed for preventive maintenance and management of machinery and equipment.
CIassNK Risk Base Maintenance System
jl;;'-
-Ship Name. #1 NK MARU
Modi
V
End ..1nc r.1ti. Likelihood Equipment Result on. 2 Main EngineV
K
1997 FWD Likelihood Failure Frequency 0.02 1.>
Equip. Modification Factor X, 10. Inspection Effectiveness.Corn po nent #1 CyL Liner.
V
M ana gerne nt+'
Modification Factor
x,
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Condition Monitoring Method. ' Lesell.' : Visual Check O.. Lesell. . Not Equipped
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60
ClassNK TECHNICAL BULLETIN 2009
V
Ship CodeCIassNK Risk Base Maintenance System
Analyzed Result Ship Name Inspection Result Master Data Equipment Result on.-<
Consequence Main Engine.-V
1997 FWD.-V
Voyage InterruptionO.. Rep aiid by Ship.
@. Speed Reduction-c O+ Cylinder Cut
Operation.-O. MiE Stop
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Component 3.Ns
Working Member. #1 Çy. Liner..M
Man Hour.' For Repair. 5WK Hour..
EndCIassNK Risk Base Maintenance System
Figure 7 Importance of damage
BackupL
Restaf
Maintenance Plan Risk Ma elihood Consequence
Figure 8 Output of RBM system
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