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NEW SLOSHING ASSESSMENT METHODOLOGY FOR MEMBRANE TANKS IN LNG CARRIERS

Robert E. Sandström Marine Engineering Supervisor

Offshore Division, Upstream Research

William H. Bray Design & Nautical Services Lead

LNG Ships, Development

Dwayne A. Bourgoyne Marine Engineering Specialist

Offshore Division, Upstream Research

ExxonMobil Houston, Texas, USA

ABSTRACT

This paper is for those who are at the leading edge of changes taking place in the delivery of LNG with membrane based insulation systems in LNG carriers. It addresses the relevance of technical developments that support industry’s continued safe application of membrane systems and how these developments can help our industry face more demanding conditions associated with changes underway in LNG carrier designs and operations.

To meet the growing demand for LNG, industry is pursing development of very large LNG membrane ships to reduce the cost of delivery through economies of scale. Industry is also pursuing applications involving offshore cargo transfer operations with membrane carriers. With such changes and the potential for new sloshing load conditions, reliable sloshing assessment methods for LNG tanks are vital to achieving confidence in the integrity of membranes in these carriers.

Leveraging the excellent performance history of LNG carriers, industry’s approach to qualifying new membrane tanks has been to compare predicted sloshing loads for new larger ships to those of conventional ships. For cases where the differences are large or there is not an operating basis for comparison, a comparative approach can be onerous and may lead to design requirements that are potentially impractical and unnecessary for new LNG carrier cargo tanks. A more traditional engineering approach, one that compares load directly to capacity, avoids these shortcomings. ExxonMobil has developed such a methodology, referred to as EMPACT.

This paper provides an overview of our direct methodology (EMPACT) for assessing the effect of sloshing on membrane integrity, along with the feasibility and incentives for the LNG industry to move toward the use of this approach for new membrane carriers and operations.

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INTRODUCTION

Since the mid 1970’s, the size of LNG carriers has remained relatively constant. The largest of these “conventional” carriers transport LNG cargoes ranging from 125,000m3 to 145,000m3. Over this period, the design and cost improvements were relatively incremental. One measure of the success of these carriers is that they have earned worldwide recognition for their excellent history of safe and reliable service.

Recent conditions for LNG market growth are motivating step changes in LNG shipping. These are often the result of opportunities to pursue innovative strategies to improve the efficiency of LNG delivery and access new markets. Two significant changes are the emergence of LNG carriers that have larger tanks with new proportions [1, 2] and the use of carriers that will transfer cargo at new offshore terminals [3]. Many of the new challenges facing LNG delivery are well within the existing engineering capabilities of the marine industry. However, some aspects, such as extending the membrane insulation systems to withstand potential increases in LNG sloshing beyond existing LNG carrier experience, can require engineering capabilities that are not currently necessary for conventional applications.

Over the last five years, industry has significantly increased efforts to address the needs for new engineering capabilities to ensure continued safe performance of membranes in new LNG carrier applications. Industry has utilized comparative methods, which are best suited to incremental changes from existing carriers. For the new generation of LNG transportation projects, a methodology for direct assessment of LNG ship membrane systems can provide a major step forward. In this paper, we discuss the importance for development and application of such a method. Results have already provided ExxonMobil and our co-venturers with a robust basis to support our decisions for new LNG carriers and operations. Within ExxonMobil, we refer to our direct assessment methodology as EMPACT.

In this paper we illustrate that a direct method such as EMPACT is practical and achievable today, and highlight the advantages it can provide, beyond comparative methods, to support safe use of membranes in new and more demanding sloshing service.

INDUSTRY TRENDS WITH POTENTIAL FOR INCREASING SLOSHING LOADS

Relative to the existing fleet of LNG ships that use membrane containment systems, trends from shipyard orders and new ship designs show that new carrier sizes and tank volumes are increasing significantly. Generally, it is reasonable to expect sloshing loads to increase as tank volume grows. Of course, this is only a rough characterization, since other factors will also contribute to the net effects on sloshing. Conclusions about applicability of membranes in any new carrier require further detailed analysis. This trend does demonstrate a motivating factor for the recent increase in efforts across industry on membrane integrity assessment.

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Figure 1. LNG Carriers with Membrane Insulation Systems

Figure 1 shows these trends for LNG carriers (~2/3 existing and ~1/3 on-order) for the most common membrane based technologies. The horizontal axis corresponds to LNG carrier cargo volume, and the vertical axis is tank volume. The tank volumes are approximations and intended only to facilitate groupings by number of tanks per carrier (upper band = 4-tanks, middle band = 5-tanks, and lower band = 6-tanks). The squares represent existing carriers, and the circles represent ships on-order.

The largest carriers in the existing fleet have cargo volumes between 130-152 KCM (1 KCM = 1000 meters3) and 4-tank arrangements. There are ships on order that are larger and tend to have larger tank volumes. The larger of the 4-tank ships on order have tanks with volumes notionally equivalent or greater than those in the 5-tank ships of about 210 KCM. As the size of tanks and resultant loads increase, the industry must assure that the membrane structural capacity is also increased at the appropriate steps and in the appropriate locations. The step from 4 tanks to 5 tanks is also a key break point which reduces tank size but significantly changes tank proportions as well.

Relative to the existing fleet, the large 5-tank carriers on order for LNG export from Qatar employ strengthened membrane systems (i.e. higher capacity) to accommodate predictions for higher LNG sloshing loads. With a direct assessment, such as EMPACT, we were able to uncover the membrane details that controlled system integrity (limit state) and identify practical strengthening strategies for the larger carriers. To check our conclusions, we used EMPACT to develop comparable results for conventional carriers. Those results demonstrated that the strengthened membranes in our large carriers have integrity consistent with the membrane systems in conventional carriers.

Other developments include projects, both underway and in planning stages, that will conduct cargo transfer operations at offshore locations. At these locations, wave conditions have the potential to be greater than those at conventional onshore terminals.

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A basic assumption for design of membrane systems in most conventional LNG carriers is that the vessels will maintain fill levels below 10% or above 70% except at onshore terminals. The very few cases where sloshing has resulted in damage to membranes appear to be associated with a partial fill condition between 10% and 70% full.

Direct methods should prove to be particularly useful for assessment of these partial fill conditions. With the fill level restrictions on the existing fleet, there is insufficient experience to support comparative methods. Furthermore, the types of sloshing events that most likely control partial fill conditions do not exist at currently accepted operating levels.

COMPLEX NATURE OF MEMBRANE RESPONSE TO SLOSHING

The primary role of the membrane system in LNG carriers is to insulate the ship’s steel structure from the embrittling effects of cryogenic temperatures. One of the unique features of membrane systems is that the insulation lines the inside of the ship’s tank. This allows the steel in the tank structure to support the membrane and be part of the hull girder. Being in direct contact with the liquid cargo, membrane systems must have sufficient capacity to withstand all of the loads from liquid sloshing from the motion of LNG carriers in waves. The most common membrane systems in use are non-traditional structures (i.e. insulated plywood boxes or polyurethane foam panels). Considering the long history of successful experience with these systems, minimizing the need for modifying the membrane design certainly has merit.

Changes, such as increasing tank size, varying tank geometry, or introducing new operational conditions, can produce loads that increase the demand, broaden the aerial extent, and vary the location on the membrane system relative to conventional LNG carriers. In such cases, ensuring membrane integrity will require strategies to reduce the demand or to increase the system capacity.

We have successfully used comparative methods [1] to quantify anticipated increases in demand, and to assess relative increases in membrane capacity. This approach works as long as one can obtain general increases in capacity with relatively simple parametric changes (e.g. material thickness, spacing, etc.). However, as demand increases, continuing with simple parametric changes can lead to results that are impractical and in some cases ineffective. This can occur, for example, because the increasing load has features such as loaded area or duration, which only affect a specific detail of the system. Other examples where general scale-up of the system capacity may not be effective are those in which demand increases are due to dissimilar physical events.

The true nature of liquid sloshing in an LNG tank is complex, producing many different types of loads on the membrane. Some are highly focused jet flows with liquid impacts lasting just a few milliseconds in the full-scale system. Other loads cover larger areas and are more gradual, and some are due to entrapped pockets of gas that produce oscillating forces on the membrane. The peak loads can vary by factors of two and more over a single membrane panel having areas as small as one square meter. The types of loads that can develop in a tank also change with fill level. As fill drops into levels considered partial-fill, formation of conditions such as breaking waves becomes possible.

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Each sloshing event or impact has unique load characteristics that affect different details in the membrane system. During the event, there can be many “peak” loads, each associated with imposing specific demands on individual details. From an integrity standpoint, the goal is to ensure that the most challenged details have sufficient capacity. To identify effective strengthening strategies for larger load increases, we found it necessary to use a process that would enable us to relate specific loading characteristics with the most affected membrane details.

PRACTICAL ASSESSMENT OF NEW SLOSHING DEMANDS

Assessment of sloshing and membrane integrity is among the toughest, most technically difficult challenges for LNG ship design. It spans multiple technical disciplines (e.g. fluids, structures, materials, thermodynamics, gas dynamics, statistics) and requires expertise from several fields (membrane systems, ship design, LNG operations). The interaction that exists between physical processes (e.g. fluid motion and structural capacity) is also important. The dimensional scope is equally diverse in both time and space. Time scales range from fractions of a millisecond to exposures lasting hours, and length scales as small as a dinner plate for loads from the motion of liquid in tanks that are larger than most homes. Additionally, peak sloshing loads are the result of a process that is stochastic (i.e. not deterministic), which can be best modeled by physical simulation. This has significant implications for screening operational conditions to identify controlling cases, because numerous permutations can drive sloshing such as tank fill level, sea condition, ship heading, forward speed, and membrane panel locations.

There is no single method available today that can solve this problem exactly. The methods in use today (including EMPACT) address the problem at several different levels. Each has unique limits that users must respect to ensure that the results they produce are appropriate for application.

Figure 2. EMPACT Framework for Membrane Assessment

Assessment Basis Define ship, tanks, and operational conditions

Sloshing Loads Determine design-level

sloshing loads from ship motion predictions and scale-up of sloshing model tests

Structural Capacity Establish membrane capacities

for in-service cryogenic conditions from component tests

and analytical analyses

Integrity Assessment Confirm structural capacity exceeds loads by appropriate margins

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EMPACT is a development that extends our ability to assess sloshing by comparison with existing ships to a new level. It enables us to assess membrane integrity due to sloshing by direct comparison of loads and structural capacity. As shown in Figure 2, EMPACT is based on a traditional engineering framework and it takes into account the fundamental physics of liquid sloshing, materials performance, and dynamic response of the membrane structure.

To account for the fundamental physics of liquid sloshing, materials performance, and dynamic response, EMPACT incorporates numerous advances to support the direct assessment of membrane designs. A summary of these advances is provided below in Table 1, some of which the authors have presented previously [1, 2, and 4]. From our work, we found that many of these elements are necessary for both comparative and absolute assessments.

Table 1. Essential Capabilities of EMPACT

Load Prediction Response and Capacity Prediction

Robust ship motions predictions that properly account for surge motions and forward speed

Physical tests for membranes to establish capacities and validate analytic predictions

Screening strategy to effectively select governing design conditions

Physical tests for different loading conditions, including area dependence

Validated scaling laws to extrapolate model-scale data to prototype for specific sloshing phenomena

Physical tests for different components and details of the membrane system

Pressure sampling rate high enough to capture pressure peaks

Material properties that cover range of test and service conditions: temperature and strain rate

Pre-and post-test dynamic sensor calibration to ensure proper characterization of loads and to eliminate potential sensor bias

Material properties to evaluate capacities for specification and vendor supplied materials

Sufficiently long test records to ensure statistical convergence of maximum sloshing loads

Finite element models calibrated with physical tests of membrane details

Definition of sloshing loads as functions of time, loaded area, and location within the tank to match membrane system capacities

Finite element models with detail sufficient to reproduce relevant structural response and assess membrane strengthening strategies

Testing that preserves the density ratio of gas to liquid phases like that of NG/LNG

Finite element models that account for dynamic loading and material properties for in-service strain rates and temperatures

Detection and scaling for sloshing events where gas pockets are trapped

Explicitly accounting for the effect of corrugations on measured sloshing pressures within 3-D tanks

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EMPACT VALIDATION AND ONGOING WORK

The ability to make direct assessments of membrane integrity implies confidence in the prediction of loads and capacity. For this reason, we have subjected EMPACT to extensive validation. Ongoing work related to load prediction, membrane structural capacity, and comparison with existing carriers continues to demonstrate the ability of EMPACT to achieve direct assessment.

One example is our work to establish confidence in prediction of loads. Testing has included models over a range of scales (1:50 to 1:20). We have also compared model scale and full-scale fluid impact tests against available theoretical results (which are scale independent) [1].

During 2006, we successfully demonstrated repeatability of EMPACT load predictions through model testing at two different facilities (Figure 3). The model scale, pressures sensors, data acquisition system for pressure measurement, and the mechanisms to drive the tank motions were different for each facility. Additionally, the comparison also reflects distinctly different physical models built by each facility. The repeatability, which EMPACT achieves, is a necessary condition for direct assessments of sloshing.

Figure 3. Sloshing Predictions with Different Test Rigs and Models

Figure 4 is a representative comparison from the many similar tests conducted at the two facilities. The graph shows predicted full-scale peak sloshing pressure for loaded area over a one-meter square region of the tank. Each peak pressure curve is the result of a statistical analysis of pressures measured during a sloshing model test, as illustrated in Figure 5. The duration of a single run corresponds to the time of exposure to a sloshing condition. Pressures are from sensor arrays at the same location for each tank. The high and low bars in Figure 4 represent variability of peak pressures after multiple test runs to establish statistical convergence. Each line in Figure 4 is the result of processing ~20 gigabytes of data from the array of pressure sensors.

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Figure 4. Repeatability of EMPACT Load Predictions

Figure 5 below, depicts the pressure measurement and processing in the time domain. The test rig produces motions representative of the ship’s response to the wave conditions. Sloshing pressures in the tank model are recorded for multiple 5-hour runs. Each pressure pulse is then analyzed to determine maximum pressure, rise time, and nature of the pulse to apply the appropriate method to scale measurements to full scale. This defines the pressure in relation to time. We apply this process for each of the 100+ sensors to define the pressure distribution in space, for both location and area loading. To identify the most severe loading cases, we repeat this test procedure for various fill levels, vessel headings, and wave periods within the operating conditions for the LNG carrier.

Figure 5. EMPACT Pressure Measurement for Statistical Processing

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Model testing currently is the most practical way to acquire this kind of detailed information. The important load characteristics are simply too complex to be solved directly or numerically modeled in a practical amount of time. The extreme loads in these systems occur only rarely over many hours of sloshing, and can appear and disappear within a few thousandth of a second. Resolving the extreme stochastic loads with these time and length scales is not currently practical. For these reasons, load predictions continue to require physical simulation.

A second example where EMPACT demonstrates robustness is the ability to provide realistic estimates of membrane capacity. We have outlined the EMPACT approach for capacity previously [1, 2]. It starts from three fundamental pillars: material properties; physical component testing; and analytic modeling. With these we first establish the capability to model and analytically predict membrane response at standard test conditions. Such validation is a significant achievement, considering the non-standard unconventional structural characteristics of membrane systems (comprised of plywood, foam, etc.).

While this might be sufficient for membrane details near the tank steel structure, it is not so for details closest to the LNG. We must also account for cryogenic temperatures and impact loading rates as they can significantly affect membrane material properties and the response of membrane details. The weakest details at cryogenic temperature are not always the weakest at room temperature.

During 2006, we used EMPACT analytic models to identify the potential for change in component response due to cryogenic temperatures. For the detail we analyzed, the weak point moved from a location between support points to the support points, with the only change being room temperature versus cryogenic temperature. Consistent with that analysis, our subsequent physical tests of the same detail at both temperatures exhibited similar changes in the location of the weakest point.

CONCLUSION

In this paper, we have presented an overview of ExxonMobil’s EMPACT methodology for direct sloshing assessment and the distinguishing capabilities of a load and capacity-based approach. It extends assessment capabilities beyond those possible with comparative methods. By providing a traditional engineering framework for assessing sloshing loads and membrane performance, EMPACT is enabling users the insights necessary to ensure safe application of membrane systems under more demanding sloshing conditions. Validation, both past and ongoing, continues to demonstrate that a direct assessment approach is feasible and achievable today.

ExxonMobil has already applied the EMPACT methodology successfully in the evaluation of membranes for large LNG carriers. Using EMPACT, we were able to assess the potential for larger sloshing loads and identify effective strategies to strengthen membranes. These vessels capture significant economies of scale not previously available. To achieve these results we had to go beyond the capabilities of traditional comparative methodologies.

More broadly, industry trends indicate that new LNG carrier sizes and tank volumes are increasing. As the industry moves to larger tanks and larger carriers, we must assure

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that the capacity of the membrane structure is increased appropriately. Applications are also emerging with new service conditions for membranes beyond existing experience, such as partial-fill operations at offshore terminals. Our industry has responded to expectations for increasing sloshing loads, and has elevated independent efforts to establish confidence in safe use of membranes in these new applications.

To ensure safe extensions of membrane systems significantly beyond existing experience, we believe the emergence and application of a direct assessment methodology such as EMPACT is both achievable and necessary for the LNG industry. ExxonMobil has undertaken this work to support our projects moving forward with new requirements and specifications. For LNG carriers, we have worked with many of the key industry players in developing these requirements to assure that users can incorporate them easily and within the existing infrastructure and framework. We are not changing or supplanting any of the existing roles and responsibilities in the industry. However, we do expect that the ship owners, shipyards, classification societies, and technology licensors will continue to play their parts in maintaining the excellent safety and reliability record of the LNG transportation industry. ExxonMobil has and is undertaking the technical work to assure that our steps forward also support the industry’s excellent record. Based on what we have learned, we encourage others to move toward direct assessment methods.

With comprehensive and physics-based assessment methods like EMPACT, the LNG delivery industry can confidently pursue new opportunities while maintaining system integrity. The LNG business and the public it serves all benefit from industry efforts to maintain the historically high level of safety and reliability earned by the LNG shipping industry.

ACKNOWLEDGMENTS

We acknowledge that there have been many contributors to the improvements cited in this paper. Specifically the following teams have been most instrumental in these advances; ExxonMobil Sloshing Team; Qatargas II Shipping Team; GTT Development Team.

REFERENCES CITED

1. GasTech 2005 – “Advances in Assessment of LNG Sloshing for Large Membrane Ships”; A. J. Richardson, W. H. Bray, R. E. Sandström, R. T. Lokken, M. A. Danaczko; Bilbao, Spain, March 14-17, 2005.

2. IPTC 2005 – “Large LNG Ships - The New Generation”; M N. Greer, Andy J. Richardson, Robert E. Sandström, IPTC Paper 10703; Doha, Qatar, November 21-23, 2005.

3. LNG 15 – “Design and Construction of Gravity Based Structure and Modularized LNG Tanks for the Adriatic LNG Terminal”; Lisa Waters, Chuck Mueller, Paul Hellen, Gary Hurst; Barcelona, Spain, April 24-27, 2007.

4. SNAME 2005 – “Comments to Authors of Paper: E3(D35) - A Novel LNG Tank Containment Design for Large LNG Carriers”; Robert E. Sandström, Ro T. Lokken, T W. Yung, SNAME Annual Transactions, 2005.