Poster PO-29

PO-29.1

CONTRIBUTION OF STRUCTURAL ANALYSIS TO LNG PLANT DESIGN

CONTRIBUTION DE L’ANALYSE STRUCTURELLE A LA CONCEPTION D’UNE USINE GPL

Takuya Sato Chief Engineer

Engineering Division JGC Corporation

2-3-1, Minato Mirai, Nishi-ku Yokohama 220-6001, Japan

ABSTRACT

Most of the equipment in LNG plants is designed by conventional design methods. However, for specific equipment, Finite Element Analysis has contributed to achieving optimal design and operational safety of LNG plants. (1) Pressure vessels : The operation mode of the dehydration drier is cyclic and the temperature of the fluid changes rapidly. Transient thermal analysis and thermal stress analysis are performed to evaluate the fatigue life of the drier. (2) Piping : Bowing analysis is performed to ensure that excessive deformation, due to non-uniform temperature distributions over the cross sectional areas of the piping, will not occur while its cooling down. (3) Support components : The insulation around supports of equipment, piping and pumps under very low operating temperatures is designed so as to ensure that freezing of the supports will not occur. (4) Tanks : Several types of accidents are considered, such as impact loads on the roof or walls of an LNG tank. Nonlinear dynamic analysis is performed ensure that accidents such as these will not lead to catastrophic failure of the LNG tank.

RESUME

On fait appel, en effet pour la plupart des équipements, à des méthodes d’étude conventionnelles appropriées en fonction des exigences respectives selon les cas, toujours est-il que pour certains équipements et matériels spécifiques, un moyen d’analyse structurelle qu’est l’analyse des éléments finis a apporté une grande contribution à la réalisation d’une étude optimale ainsi qu’une sécurité opérationnelle dans le domaine de l’industrie GNL. Voici quelques exemples concrets : (1) Récipients à pression : Le mode d’exploitation d’un sécheur ou déshydrateur est cyclique, et la température du fluide varie rapidement, créant un environnement favorisant la fatigue calorifique. Une analyse thermique sur températures transitoires ainsi que sur tension thermique permet d’évaluer la durée de service du sécheur mis dans ces conditions. (2) Tuyauteries : Une analyse sur déformation permet d’éviter qu’une déformation excessive, causée par une transmission thermique irrégulière sur la section de la tuyauterie, ne se produise, lorsque celle-ci se refroidit. (3) Matériels de support : Dans leur conception, les supports pour les équipements, tuyauteries, et pompes dont la températures de service est très basse, sont

Poster PO-29

PO-29.2

thermiquement isolés pour n’être pas gelés. (4) Réservoirs : Plusieurs types d’accidents sont comptés, tels qu’en particulier, qui menace la résistance du toit ou de la virole du réservoir. Une analyse dynamique non linéaire permet d’éviter la production d’un accident catastrophique résultant d’un tel dégât causé au réservoir GNL.

INTRODUCTION

From the viewpoint of structural mechanics, LNG plants generally have the following characteristics:

- As LNG is a highly flammable, cryogenic liquid, extremely high reliability is required in design, manufacturing and construction to maintain the safety of plants.

- Many items of equipment, piping and machinery are required to be designed for very low temperatures.

- Some equipment is operated under cyclic operating conditions. There are many items of expensive equipment, whose sizes are large and whose

required qualities are high.

Most of the pressure equipment in LNG plants is designed based on conventional design codes and standards, which are sometimes known as “Design By Rules (DBR)” or “Design By Formula (DBF).” Standard vessel configurations are sized using a series of design formulae and design charts. This design method is very simple, but tends to be too conservative in some cases. On the other hand, advanced design codes and standards, which are known as “Design By Analysis (DBA) ”, have been developed. “Finite Element Analysis” (FEA), sometimes called, “Finite Element Method” (FEM), is one of the most advanced and strongest design tools for this design method(1).

As mentioned above, the quality required in LNG plant design is very high. Therefore, FEA has contributed to achieving optimal designs and operational safety in LNG plants, even where DBR or DBF is used.

In this paper, several examples of analysis for pressure vessels, piping systems, support components and tanks are provided to show the effectiveness of FEA.

FINITE ELEMENT ANALYSIS (FEA) AS ADVANCED DESIGN TOOL

The concept of DBA is quite flexible and widely applicable in the following cases(2): - As a complement to DBR or DBF for cases not covered there - As a complement for cases requiring superposition of environmental actions, e.g.,

wind, earthquake, etc. - As a complement for fitness-for-purpose cases where manufacturing tolerances

are exceeded - As a complement for cases where detailed investigations are required, e.g., in

major hazard situations - As an alternative to DBR or DBF

Poster PO-29

PO-29.3

In most cases, numerical structural analysis is employed to achieve the design of pressure equipment by the DBA route. FEA has the following advantages and the typical analysis method in this field:

- Applicable to quite complex structures and configurations, i.e., 2D and 3D modeling

- Applicable to quite complex phenomena of structures, i.e., static, dynamic and impact analyses

- Applicable to quite complex material behaviors, i.e., elasticity, plasticity and creep

- Applicable to transient thermal problems - Applicable to interaction analysis of structure and hydrodynamics - Visual presentation of analysis results

PRESSURE VESSELS UNDER CYCLIC OPERATION

FEA is applied to analyze local stresses of structural discontinuities, such as nozzle – shell, head – shell and support – shell intersections, bolted flanges and tube sheets of heat exchangers. These stresses are classified into primary local stress, secondary stress and peak stress to be evaluated based on an applicable design code (DBA), such as ASME Section VIII Division 2(3).

Fatigue analysis of the dehydration drier, whose operation mode is cyclic and in which, the temperature of the fluid changes rapidly, is one of the most interesting problems. Transient thermal analysis and thermal stress analysis are performed to evaluate the fatigue life of the drier, as shown in Figure 1. General-purpose non-linear structural analysis system, ABAQUS(4) was used for the analysis.

The axi-symmetric finite element models of the support – shell intersection for thermal analysis and stress analysis are shown in Figure 2, respectively. The former model includes thermal insulation. The typical operating pressure and temperature are shown in Figure 3. This rapid temperature change may cause fatigue damage at the structural discontinuity.

Transient Heat Transfer Analysis

Stress Analysis

Temperature Distribution

Fatigue Life Evaluation

Material Properties Boundary Conditions

Material Properties Boundary Conditions

Loading Conditions Weight, Pressure etc

S-N Curve Design Fatigue Life

Figure 1 Fatigue analysis flow

Poster PO-29

PO-29.4

The temperature distribution changes gradually with the internal fluid temperature variation. The example of temperature distribution is shown in Figure 4.

The maximum stress occurs at the inside corner of the skirt – shell intersection while the temperature is going up as shown in Figure 5. On the other hand, the minimum stress occurs at the same point while the temperature is going down. This indicates that stresses increase gradually with temperature variation, and shows the maximum value at 45 minutes after the fluid temperature started to change. The fatigue life can be evaluated by comparing this stress range (the difference between the maximum and minimum stresses) with design fatigue curve specified by the applicable design code (DBA). The stress distribution around the skirt – shell intersection at this time is shown in Figure 6.

TE

MP

PRE

SS

TIME (Min)

TIME (Min)

0

0

650

650

1300

1300

20 degree C

320 degree C

57 bar

54 bar

15 degree C / Min

(a) For Heat transfer (b) For stress Figure 2 Finite element models

Figure 3 Typical temperature and pressure history

Figure 4 Temperature distribution Figure 5 Stress history

Time (Hr)

Shear stress

Meridional stress

MPa

Poster PO-29

PO-29.5

Figure 6 Stress distribution

It can be concluded that the maximum stress at the corner is affected by the corner radius, the size of the hot box, the ratio of the shell thickness to the skirt thickness and the rate of temperature change. Especially, the effects of the rate of temperature change and the corner radius are significant.

PIPING SYSTEM SUBJECTED TO TRANSIENT THERMAL LOADING

In general, piping systems are designed for several types of load, such as dead load, pressure, temperature and thermal expansion, seismic load, wind load and anchor movement. In an LNG plant, the piping system shows an interesting behavior, known as “bowing,” during the cooling down process(4).

Bowing analysis is sometimes performed to ensure that excessive deformation due to non-uniform temperature distributions over the cross sectional areas of the pipe will not occur. This behavior depends on the transient temperature distribution while the piping system is cooling down and is a quite complex behavior. In this paper, a simplified temperature profile over the cross sectional areas is assumed in order to investigate characteristics of bowing behavior using ABAQUS(5). The FEM model is shown in Figure 7, and the temperature distributions are assumed as shown in Figure 8.

The example deformation (Case 2) is shown in Figure 9. It is easily understand that the deformation occurs not only in the horizontal direction but also in the vertical direction and concentrates at the loop portion. The analysis results are summarized in Table 1. Both, the maximum deformation and the maximum stress, occur in Case 2.

Table 1 Effects of liquid level on maximum deformation and stress

Case 1 Case 2 Case 3 Deformation (mm) 9.82 23.55 15.91 Stress (Mpa) 126.6 281.8 238.3

Poster PO-29

PO-29.6

Figure 7 Finite element model (a) Case 1

Figure 8 Temperature distribution assumption

(c) Case 3 (b) Case 2

GasGas

LiquidLiquid

Figure 9 Deformation (Case 2)

(b) Deformation in vertical direction

(a) Isometric view

Poster PO-29

PO-29.7

SUPPORT COMPONENTS OF ROTATING MACHINERY

The insulation around the supports of equipment, piping and rotating machinery under very low operating temperatures is designed so as to ensure that freezing of the supports will not occur. As the support configuration of rotating machinery is usually complex, FEM heat transfer analysis is quite useful to achieve an optimal design of insulation around the supports.

The FEM model of a support (1/8 model considering symmetry), including bolts, nuts, washers, wooden plates, concrete and gravel, is shown in Figure 10. Boundary conditions, such as ambient temperature and wind velocity, affect the temperature distribution quite strongly. In this paper, two kinds of ambient temperature (14 and 21 degree C) are assumed and a zero wind velocity is specified, as conservative conditions. The steady state heat transfer analysis was performed under these conditions using ABAQUS(4).

The temperature distributions of steel parts around rotating machinery are shown in Figure 11. The minimum temperature of foundation is 1.3 degrees C for an ambient temperature of 14 degrees C, and 7.8 degrees C for an ambient temperature of 21 degrees C. These results indicate that freezing of the support will not occur.

Thus, reasonable detailed configurations and sizes of insulation can easily be designed using this type of analysis.

Figure 10 FEM model support

(a) Around rotating machinery

(b) Steel portion

Figure 11 Temperature distribution

Poster PO-29

PO-29.8

LNG STORAGE TANKS SUBJECTED TO IMPACT LOADING

Failure of an LNG storage tank will result in severe damage to the environment. Several types of accidents are considered, such as the failure of the inner tank and the resulting leakage of LNG, which will cause cracking of the concrete outer tank wall (due to both, the dynamic pressure of the LNG jet and the thermal shock) and blast loading or impact loading on the roof or wall of the tank. Nonlinear dynamic analysis is required to ensure that accidents such as these will not lead to catastrophic failure of the LNG storage tank.

Impact loading is classified into an aircraft impacts and tornado missiles in the nuclear field(6)(7). The former is an aircraft crash onto the containment vessel of a nuclear power plant. The latter includes external tornado missiles (steel rods, steel pipes, wooden poles and automobiles) and internal, accident-generated missiles resulting from pipe breaks. Similar situations can be also assumed in LNG plants as the extreme loading conditions.

In this paper, the effect of an external missile (valve: rigid sphere of diameter = 110 mm, mass = 110 kg and velocity = 160 km/hr (44.4 m/sec)), resulting from an explosion in an LNG plant, on the structural integrity of a concrete outer tank (thickness = 600 mm, reinforcement = diameter of 25.4 mm, 150 mm pitch on both, inner and outer side) was investigated. The material properties are summarized in Table 2, and the FEM model is shown in Figure 12. The tendons for pre-stressing are not included in this model because the pitch of the tendons (about 1500 mm) is much larger than the missile size. The valve is modeled as a rigid sphere for simplicity. The dynamic analysis was made using the impact analysis system, ABAQUS-EXPLICIT(8).

Table 2 Material properties

Young’s Modulus

Poisson’s Ratio

Density Yield Point

Tensile Strength

Fracture Strain

Concrete 28.6 GPa 0.15 2500 kg/m3 49.4 MPa --- 0.24 % Rebar 206 GPa 0.3 7800 kg/m3 345 MPa 400 MPa 50.0 %

As the impact phenomenon is basically an elastic and plastic wave propagation

problem and shows a quite rapid behavior, a small time increment, i.e., about 1.8 to 4.5 micro-seconds, is necessary to obtain a convergent solution. This small time increment results in very large number of time steps, about 14400 time steps, to complete the analysis.

The velocity of the missile decreased gradually and stopped in the concrete wall after about 30 milliseconds, as shown in Figure 13. This figure suggests that the kinetic energy was absorbed rapidly when the missile reached to the steel reinforcement on both, inner and outer side.

Poster PO-29

PO-29.9

The minimum principle stress distribution (compressive stress) and damage of the wall after 0.2, 4.2, 10.2 and 20.2 milliseconds are shown in Figure 14. The compressive stress wave expanded around the impact point and spalling occurred on the front surface of the concrete wall just after impact. The compressive stress wave traveled and reached to the backside surface and reflected as the tensile stress wave. However, scabbing did not occurre on the backside surface in this case.

It can be understood that the damage to the wall due to impact loading is limited to a quite narrow area around the impact point. The diameter of the damaged area is about 500 mm (4.5 times of the size of the missile). In this case, the missile stopped in the wall, and the damage to the LNG storage tank is limited to a small area of the concrete outer tank. This means that catastrophic damage which would result in a leak of LNG and an explosion could be prevented.

An impact phenomenon is quite complicated. Further investigation is expected about several factors, such as modeling of missile, effects of dynamic loading on material properties, fracture model of steel and concrete, composite modeling of steel reinforced concrete, effects of steel reinforcement arrangement and numerical analysis methodology.

This kind of impact analysis can assure the high quality and reliability of LNG storage tanks against extreme loading, such as blast loading, aircraft crash and accident-generated missiles.

Figure 12 Finite element mesh

Figure 13 Velocity of missile vs time

0 5 10 15 20 25 30 35 Time (millisecond)

Vel

ocity

(m/s

econ

d)

40

10

20

30

0

Vel

ocity

(m/s

econ

d)

Poster PO-29

PO-29.10

CONCLUSION

Examples of finite element analysis applied to pressure vessels under cyclic operation, piping systems subjected to transient thermal conditions, support components of rotating machinery operating at low temperatures and the LNG storage tanks subjected to extreme loading were presented. These examples show that finite element analysis has contributed to achieving optimal designs and operational safety of LNG plants. Progress in numerical analysis and computer technology will expand the applicability of finite element analysis in LNG plants. Especially, numerical simulations of potential accidents, such as explosion, blast loading and several kinds of impact loading, will ensure the safety of LNG plants. Further research into, and development of, complicated nonlinear dynamic analysis methods is expected in the near future.

Figure 14 Minimum principle stress distribution and damage of the wall

t = 10.2 milliseconds t =3 0.2 milliseconds

t = 4.2 milliseconds

t = 0.2 milliseconds

t =6.2 milliseconds

t = 0.02 milliseconds

Poster PO-29

PO-29.11

REFERENCES

(1) Y. Uno and S. Hamanaka: “Structural Analysis as an Advanced Solution Tool for Design & Assessment”, The 4th Middle East Refining & Petrochemicals Conference (Petrotech), Bahrain, Sep. 29-Oct.1, 2003.

(2) “The Design-by-Analysis Manual”, European Commission, 1999.

(3) “ASME Boiler and Pressure Vessel Code Section VIII Division 2”, ASME, 2002.

(4) N. Gilbert and Y.M. Chokshi: “Thermal Bowing of Vessels and Pipelines”, 82-PVP-35, ASME, 1982.

(5) “ABAQUS / Standard User’s Manual”, Hibbit, Karsson & Sorensen, Inc.

(6) B. Barbe and J.L. Costaz: “Design and Behavior of French Containments”, Nuclear Engineering and Design, 125, 1991.

(7) R.P. Kennedy: “A Review of Procedure for The Analysis and Design of Concrete Structures to Resist Missile Impact Effects”, Nuclear Engineering and Design, 37, 1976.

(8) “ABAQUS-EXPLICIT User’s Manual”, Hibbit, Karsson & Sorensen, Inc.