© Gastech 2005 Advances in Assessment of LNG Sloshing for Large

Advances in Assessment of LNG Sloshing for Large Membrane Ships
A. J. Richardson, W. H. Bray, Qatargas II
R. E. Sandström, R. T. Lokken, ExxonMobil Upstream Research
M. A. Danaczko, ExxonMobil Development Company
GasTech 2005
Bilbao, Spain
March 14-17, 2005
ABSTRACT
Large ships are a strategic element to reducing cost for delivery of liquefied natural gas to worldwide
markets. They reduce delivery cost through economies of scale and they will help meet the demand
for ship volume with the existing construction capacity.
The excellent performance history of LNG ships over the past 40 years is a well-established fact.
However, the introduction of large LNG ships poses new technical challenges for our industry. One of
the challenges in moving to larger ships is the potential to have higher cargo sloshing loads with
larger ship tanks. Since membrane insulation systems are installed inside the tank, they are in direct
contact with the LNG and must be capable of withstanding all loads from sloshing.
Qatargas II and ExxonMobil have led a major technical effort with collaboration with Gaztransport &
Technigaz S.A.s to enable the safe design of a new class of cost effective large LNG membrane ships
for our projects.
This paper highlights our recent advances in sloshing load prediction and membrane capacity
assessment. These advances were essential to establishing requirements for the industry's first
application of LNG ships up to 216,000m3, a significant increase in size over the 155,000 m3 LNG
ships recently ordered by industry, and will enable the safe design of even larger, 250,000m3 LNG
ships.
© Gastech 2005
1
Introduction
The LNG industry will soon see the first application of a new generation of LNG ships from Qatar with
cargo capacities above 200,000m3. These new ships represent the most significant step change in
LNG ship size in nearly 30 years. This step change has been made possible by a major technical
effort led by Qatargas II (QG II) a joint project between Qatar Petroleum (QP), 70% and ExxonMobil
(EM), 30%, and our close collaboration with Gaztransport & Technigaz S.A.s (GTT).
Growth in LNG Ship Size by Year - Figure 1.1
Industry interest in large LNG ships has been the subject of numerous studies over the past 20 years
[Ref 1]. While some of these studies presented designs for large LNG ships, most focused on
compatibility of larger ships with the existing LNG receiving terminal infrastructure, dominated by Far
East markets. Growing demand for LNG in western gas markets is providing new opportunities to
pursue delivery systems that can accommodate larger ships and improve the competitiveness of LNG
gas supplies.
Conventional size LNG ships have established an excellent performance history that spans more than
40 years. In the mid 70’s, the size of LNG ships jumped from 78,000m3 to 126,000m3 [Ref 2]. Since
then conventional LNG ship sizes have grown incrementally to nearly 160,000m3.
The move to significantly larger LNG ships does pose technical challenges for ship design and
construction. One such challenge is to ensure that large LNG tanks have the integrity to withstand
loads due to cargo motion and sloshing. Larger tanks have the potential for higher cargo sloshing
loads. While sloshing is a design consideration for all containment technologies, membrane systems
are unique in that their insulation is located inside the cargo tanks. Being in direct contact with the
LNG, these systems must be capable of withstanding all the loads from liquid movement and impact.
This paper highlights advances in sloshing load prediction and structural capacity assessment that we
developed to establish our confidence in a first application of LNG membrane ships above 200,000m3
(Q-Flex). These advances include improvements in ship motion predictions, physical measurement of
sloshing loads, and characterization of loads and capacity under in-service conditions. Adherence to
underlying fundamental physics has been a key element in the selection, development, and
confirmation of the engineering methods we used in this work.
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Richardson, Bray, Sandström, Lokken, Danazcko 2
Maintaining the excellent safety performance history, as new LNG ship designs emerge, is essential to
our industry. For this reason the membrane ship designs that we evaluated are based on scale-up of
conventional sized ships. This approach enabled us to retain the benefits of existing designs. It also
enabled us to build from the confidence and performance of ships in service and the experience of
GTT. Qatargas II and ExxonMobil are now taking the initiative to share our knowledge on LNG
sloshing to facilitate a consistent understanding of the design integrity and acceptance of new LNG
ships.
2
The LNG Sloshing Design Challenge.
All ships that carry liquids in their tanks are subject to sloshing. Sloshing is a liquid motion within a
tank produced by ship motions at sea. Sloshing becomes more pronounced when the frequencies of
ship motions match frequencies associated with liquid motion in the tanks. Sloshing imposes dynamic
loads on the tank structure. Designers must ensure tank structures have sufficient integrity to
withstand these loads under normal operating and extreme in service conditions.
Pressure [bar]
Rise
Time
0
Ship Motions At Sea
Cargo Sloshing
5
10
15
Time [milliseconds]
20
25
Dynamic Loads On Tanks
Figure 2.1
For many types of ship tanks, designers have relatively straightforward options with internal geometry
such as stiffeners and swash bulkheads to impede liquid motion and reduce sloshing loads to
acceptable levels. However, the unique features of LNG membrane tanks that make them attractive
for LNG ships also require more sophisticated engineering capabilities to ensure tank integrity.
The two most common membrane systems for LNG ships are the Gaz Transport No. 96 (Figure 2.2)
and the Technigaz Mark III (Figure 2.3). The membrane insulation inside the tank protects the ship’s
structure from the effects of cryogenic temperatures. The Gaztransport No. 96 system consists of
two layers of insulating plywood boxes, and the Technigaz Mark III uses two layers of plywood
backed polyurethane foam. These systems are the result of balancing requirements for thermal
protection and strength to withstand cargo loads. Options to impede liquid motion with these systems
can be costly if not impractical. Minimizing changes to proven systems is a key objective of our scaleup to larger LNG ships.
Gaztransport No. 96 Membrane System - Figure 2.2
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Richardson, Bray, Sandström, Lokken, Danazcko 3
Technigaz Mark III Membrane System - Figure 2.3
3
“Comparative” Approach.
Our initial strategy was to compare sloshing on the large LNG ships relative to existing conventional
sized ships. We required peak membrane responses in the large ship to be equal or less than those
in the benchmark ship.
Comparison Between Conventional Benchmark Ship And New Large (Q-Flex) Ships
Figure 3.1
We used a conventional 138,000m3 LNG ship having a 4-tank arrangement as our benchmark for
comparison. It provided reasonably close similarity with our large ship tank proportions. This
geometric similarity helps maintain consistency with the types of liquid motion condition that produce
sloshing conditions. We compared the benchmark and large ships for both the No. 96 and Mark III
membrane systems. Since membrane responses are sensitive to dynamic amplification from sloshing
loads, we based our comparisons on membrane response rather than loads.
Reducing uncertainty in load prediction and membrane response was a significant element of our
work. We undertook a number of investigations to validate key building blocks in our assessment
process. For example, we conducted…
© Gastech 2005
Richardson, Bray, Sandström, Lokken, Danazcko 4
•
multi-scale model tests to ensure safe scaling of results from model test loads to determine
membrane response,
Figure 3.2 – Multi-scale model testing (1/50 – left and 1/20 – right)
•
extensive screening throughout the tank to check sensitivities to fill level, ship heading, and
wave period,
Figure 3.3 – Extensive Screening for Peak Load Locations and Conditions
•
tests with multiple gases, gas densities and gas pressure to ensure adequacy of using water
and air tests
Figure 3.4 – Confirming Adequacy of Air/Water Based Model Tests
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Richardson, Bray, Sandström, Lokken, Danazcko 5
•
measurements of loads at rates up to 50,000 observations per second to establish minimum
sampling rates to avoid underestimating peak loads and adequately define rise times, Tr
(milliseconds),
Rise Time
Examples of Same Time History
at Lower Sampling Rates
Example Sloshing
Pressure Time History
Pressure
20,000 observations per second
(similar to original pulse)
Sampling Rate for original measurement
50,000 observations per second
0
5
10
15
20
Time History (ms)
25
10,000 observations per second
(lower pressure & longer rise time)
30
0
5
10
15
20
Time History (ms)
25
30
Figure 3.5 – Impact of Data Sampling Rates on Measurement of Peak Loads
•
statistical analyses to determine test durations required to achieve statistically repeatable
results, and
10
-3
Exceedence [1/s]
Ppeak pressure defined from
curve fit with 95% confidence
10
3 hour level
-4
Conf. low: 16.9
Conf. hi: 25.8
Pressure [bar]
Figure 3.6 – 30 hr Test Duration to Consistently Define 3 hr Design Loads
•
collaborative effort with GTT on physical testing and analytic modeling to develop dynamic
response for cryogenic in-service conditions
Figure 3.7 – Membrane Models to Evaluate Dynamic Response to Sloshing Loads
The advantage of comparison to existing ships is that it provides a practical basis for establishing
equivalent risk with a well accepted standard design that has a history of outstanding performance.
Using similar processes to evaluate both the benchmark and large ships reduces assessment
uncertainties. Furthermore, the physical similarities between the benchmark and new ships simplify
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the assessment and focus the effort on minimizing differences. These differences are dominated by
load prediction and adjustments for dynamic amplification of membrane response.
The comparative approach only assesses relative integrity and not absolute integrity. The reference
for relative integrity is the benchmark ship. To accommodate higher sloshing loads with the
comparative method, we pursue design changes for the larger ships that will reduce response to
levels predicted in the benchmark ship.
Working with GTT, we were able to identify modest changes to accommodate the sloshing load
increases that we had measured for the Q-Flex ships. Looking ahead to even larger LNG ships,
250,000 m3 (Q-Max), it was not obvious that we could achieve further strengthening without
significant membrane changes. Although the comparative approach provided a sound basis for QFlex requirements, we believe that development of an absolute assessment capability is attainable
and could provide more flexibility for setting Q-Max design requirements. For these reasons the QG
II Project along with ExxonMobil are pursuing developments that will enable us to move toward an
“absolute” practice for assessment of membranes.
4
“Absolute” Approach.
The concept is to assess integrity by demonstrating that membrane capacity is sufficient to withstand
predicted loads. An absolute assessment enables us to establish integrity on the basis of first
principles and traditional engineering methods.
As shown in Figure 4.1, the process starts with definition of the system and design basis. The next
steps involve prediction of sloshing loads and membrane capacity. In developing capacity, we
determine the minimum expected for in-service cryogenic conditions. On the load side, statistical
methods are used to determine the most likely maximum load in the design environment expected.
The final step is to ensure capacity exceeds loads by appropriate safety margins.
Define ship, tanks, and
design criteria
Sloshing
Loads
Structural
Capacity
Predict sloshing
loads from model
tests
Predict dynamic
capacity from
component tests
& in-service
analytic models
Compare loads and capacities
Figure 4.1 – Framework for Traditional Engineering Basis for Sloshing Assessment of
Membranes
Development of an “absolute” assessment methodology poses enormous technical challenges.
Robust capabilities to identify and predict critical sloshing loads and associated in-service dynamic
capacity of membranes are essential.
One of the most significant developments over the past year has come from our identification of the
highly localized nature of sloshing that occurs in tanks with fill levels above 80%. For the Q-Flex tank
geometry, we found during our screening studies that the tank top corners consistently had the
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highest loads. Measurements on the tank top away from the perimeter edges were significantly
lower. Pressure measurements are sensitive to sensor size (diameter). We found significant
differences between sensors as small as 2mm and 5mm (10cm and 25 cm full scale). All combined,
these observations suggested that peak pressures are highly localized and can vary significantly over
areas smaller than individual panels.
Sloshing events at model scale are too fast for visual study. However, high-speed video cameras and
concurrent pressure measurements enable us to gain a much better sense of sloshing during peak
events. The most severe events were those associated with formation of a liquid column running up
the tank wall. The upper surface of this column, even under ideal 2-D test conditions, is rarely
perfectly flat. When the highest point on the column reaches the tank top, initial contact produces
the immediate creation of a jet flow along the tank top that sprays liquid back into the tank.
Recorded pressures decay dramatically after initiation of the jet.
Figure 4.2 shows a typical event during a 2-D test. It shows the point of initial contact with the tank
top (left) and the position of the liquid near the end of the pressure pulse (right). The chart (center)
shows time histories for each pressure sensor on the tank top. The dark objects that appear to
protrude into the tank are actually screws that hold the Plexiglas tank together and are not inside
with the liquid. Other sensors on the tank top see relatively small pressures even during their
subsequent immersion. It should be quite clear from this example that sloshing impacts can be
highly local events.
Figure 4.2 – Anatomy of a Peak Sloshing Load Event
These observations provided a basis for design of a new sensor array that enables measurement of
pressure distributions over a single membrane panel. Figure 4.3 shows a photograph of one of the
new sensor arrays and an example of how the average pressure changes with loaded area. Each
point on the chart represents the expected 3 hour maximum. This level of spatial resolution is
essential to enabling rational absolute assessments.
Interior
(standard panels)
Perimeter
Pressure (bar)
Perimeter
(corner design)
Interior array
Top corner
sensor
250-6
Int block
0.00
0.25
0.50
0.75
Loaded Area (m2)
1.00
Figure 4.3 – Sensor Array to Capture Spatial Distribution of Loads Over Membrane Panels
In the example above, single sensor loads measured at the edge of the tank to assess membrane
capacity would be overly conservative. Although point loads have been sufficient for comparative
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purposes, an absolute approach requires spatial definition of pressures and corresponding membrane
capacities.
Using sensor arrays places heavy demands on data processing and storage with data sampling rates
of 20,000 points per second for each sensor. We measure storage in terms of terabytes (1000 GB or
4 large hard drives). Decisions and quality control during model testing require at least a minimum
level of real time processing of the data. Even with the recent improvements in processor and disk
storage speeds, most of the computer effort is devoted to data capture and storage.
Another very important element of an absolute methodology is the need for accurate measurements
from the pressure sensors. One approach that we are developing draws on experience with wedge
drop tests. They have been the subject of extensive research on ship bow slamming dating back to
the 1930’s. Work by Wagner [Ref 3] and subsequent extensions by others [Ref 4-5] have led to
analytic solutions. These solutions enable design of wedge drop tests to evaluate sensor accuracy by
comparing predicted and measured pressure magnitudes and time histories. These tests provide
reliable data for sensor acceptance, identifying bias and consistency.
Figure 4.4 – Wedge Drop Tests to Confirm Pressure Sensor Accuracy
We are pursuing development of an absolute methodology in steps, starting with bounding estimates
of loads and capacity. Where bounding estimates might lead to a requirement for membrane
modification, it may be productive to first investigate the potential to develop assessment
improvements that could enable us to safely minimize the need for membrane modifications. One
benefit with this approach is that it reduces risks associated with changes to a proven design.
Minimizing membrane modifications preserves elements that yards have built and that have worked
well in service.
5
Closing Remarks
The LNG industry is moving rapidly to expand LNG markets in Europe and the US. Some new
applications will use larger ships. Others will involve operations that may subject ships to partial fill
conditions. Maintaining the LNG industry’s excellent safety and performance through this evolution
will be crucial to the steady growth of new market opportunities.
Experience with design of the QG II large LNG ships has led us to pursue development of an ability to
assess both loads and capacity that can ensure safe / cost effective scale-up of large membrane ships
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Richardson, Bray, Sandström, Lokken, Danazcko 9
beyond Q-Flex. Extrapolation for new tank geometries and partial fill conditions based on load
comparisons alone will not be sufficient.
Advances in comparative assessments like the ones highlighted in this paper have enabled QG II to
establish a sound technical basis for acceptance of large membrane-based LNG ships. New work to
enable a rigorous methodology for absolute assessment of membranes is underway. Some of these
developments are aimed at quantifying the effects from liquid/vapor interaction. Establishing an
absolute methodology will enable evaluation of partial fill operations where the experience basis from
existing ships is insufficient for using a comparative approach.
Findings from our work are providing a basis to resolve many of the different views and assumptions
we encountered from our review of existing literature and discussions with industry experts on load
measurement, load scale-up, sloshing characterization, cushioning, and membrane interaction.
We encourage industry to work toward the unification of an engineering standard that ensures
market confidence in the integrity of new large membrane LNG ships consistent with industry’s
excellent experience with the existing fleet.
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6
Acknowledgements
QGII Project wishes to acknowledge two organizations that have filled important roles in our effort to
develop a confident basis for scale up of membrane based LNG ships. Cooperation and collaboration
with GTT has been essential. Since the beginning of our work, GTT has been working with us on
membrane strength and capacity. GTT is taking steps to incorporate recent advances from our joint
work on load prediction and developing innovative solutions to meet membrane strength
requirements. In addition, collaboration with Marintek on development of experimental methods and
model testing for tank sloshing has been of significant value in this project. Marintek has
demonstrated creativity and dedication through the long testing programs required for this project.
The authors would also like to acknowledge the expertise and commitment of the QGII and
ExxonMobil team that have achieved major advances which are enabling the first application of large
LNG ships.
Figures 2.2, 2.3, 3.7 figures contain GTT graphics with permission from GTT.
Figure 4.4 contains an illustration from Ref 5.
7
References
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2
3
4
5
Ffooks, Roger, NATURAL GAS BY SEA – The Development of a New Technology, Witherby
& Co. Ltd., 1993
SIGGTO, LNG Log 26, 2000
Wagner, H., Über Stoss und Gleitvergänge an der Oberfläche von Flüssigkeiten, Zeitschr.
f. Angewendte Mathematik und Mechanik, Vol 12, pp 192-235, 1932
Zhao, R. & Faltinsen, O., Water entry of two-dimensional bodies, Journal of Fluid
Mechanics, Volume 246, pp 593-612, 1993
Faltinsen, O., Sea Loads on Ships and Offshore Structures, Cambridge University Press,
1990
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Richardson, Bray, Sandström, Lokken, Danazcko 11