ASCE Seismic Requirements for Typical Compressor Packages: Do They Apply?

Transcription

1 ASCE Seismic Requirements for Typical Compressor Packages: Do They Apply? By George Weilbacher Compression Analytical Group Manager Exterran Guy Gendron VP Engineering Services BETA Machinery Analysis A Wood Group Company Presented at: Gas Machinery Conference 2015 Austin, TX 1. Introduction Seismic risk is a real hazard in many parts of the world including North America, South America, and Asia. In North America, California, Alaska, some areas in the eastern states, and Canada are prone to significant earthquakes. Compressor Packagers and OEMs are often asked to assess the impact of seismic events on their compressor packages destined for these areas. There are two challenges facing packagers when conducting a seismic analysis: 1. determine which areas of the package would be affected by a seismic event (piping, bottles, skid, etc.); and 2. specify the appropriate engineering/vibration analysis required for the application. A seismic study is typically conducted to the requirements of ASCE Standard 7-10 to determine what loads to apply to a package and to evaluate stresses and displacements. This paper will review the methods proposed in this Standard and evaluate how the code requirements are applicable to compressor packages. The conclusion will evaluate the accuracy of the ASCE Standard and list additional work required for the seismic verification of a compressor package. ASCE Seismic Requirements for Typical Compressor Packages: Do They Apply? Page 1

2 2. A Typical Reciprocating Compressor Package A typical compressor package is shown in Figure 1. Although many different designs can be built, the design shown in Figure 1 gives a good idea of the type of skid-mounted, separable compressor package studied in this paper. Depending on the application, such a package will include suction and discharge bottles (green in Fig. 1), scrubbers (orange), lots of piping, and of course a driver (purple) and a compressor (magenta). A compressor can execute several stages of compression. Between each stage, the gas will usually flow to a cooler (not shown in Figure 1) before it flows back to a scrubber for the next stage of compression. In most cases, the compressor and the driver are mounted on a pedestal whereas the rest of the components are connected directly to skid beams. The skid and the pedestals are isolated in Figure 2. Depending on their construction and the resulting load paths between driver mounting and base skid, pedestals can have varying inherent flexibilities. If that flexibility becomes an issue, packagers will sometimes pour concrete in the pedestals as well as in the skid underneath the pedestals. This has a triple effect of adding mass, stiffness, and damping. Added mass and stiffness tend to shift natural frequencies and lower amplitudes of response, while damping tends to lower amplitudes of response with negligible, if any, effect on the natural frequency of concern. Other components of interest for this project, aside from the pedestal and the skid beams, are isolated in Figure 3. These include suction (light green) and discharge (dark green) bottles as well as scrubbers (orange). It is interesting to note that the scrubbers are skirt-mounted. Scrubber vessels are welded to the skirts, which are in turn either welded or bolted to the skid, sometimes directly to the skid beams, other times through base plates. The use of base plates may occur for either bolted or welded attachments of the scrubber to the skid structure. The selection for mounting compressor packages is often considered within the framework of cost. Primary influencers include compressor horsepower, residual imbalances and dynamic forces, expected life of the field, and soil conditions. The foundation itself for separable compressor packages can vary from compacted earthen gravel pads, concrete mats, concrete blocks, or hybrid pile system with a concrete pile cap consisting of either a concrete mat or block. Today, in the lower 48, there is also a developing trend of mounting separable compressor packages to piles themselves, although this practice has been employed in the far, remote reaches of Canada for some longer period of time. The difference between a concrete block and a concrete mat is distinguished by size and intent of the concrete pour. Blocks are generally sized so they provide a reactive mass of four to five times the weight of a reciprocating machine as suggested by ACI 351.3R [4]. Mats or pads tend to be significantly thinner and are sized without considering reactive mass effects. Yet, they provide a foundation more substantial/positive than the earthen pad. Examples of block, pile with pile cap, and pile mounted separable compressor packages are illustrated in Figures 4 through 6. In each case, anchor bolts are provided between the skid beams and the foundation at locations around the periphery of the skid. In some cases, anchor bolts are also installed underneath the pedestals where vibration forces are typically higher. However, those are better to be avoided, as they are quite difficult to install and add to fabrication and installation costs. Figure 1 Typical Compressor Package ASCE Seismic Requirements for Typical Compressor Packages: Do They Apply? Page 2

3 Figure 2 Skid Beams and Pedestal Figure 3 Main Components of Interest Figure 4 Package Installed on a Concrete Pad ASCE Seismic Requirements for Typical Compressor Packages: Do They Apply? Page 3

4 Figure 5 Package Installed on a Pile Cap Figure 6 Package Installed Directly on Piles 3. Why This Paper? When purchasing separable compressor packages, end users often request a seismic analysis. While the reasons for a seismic analysis are assumed to vary depending on federal, state, and local mandates, other possible considerations include black-start capabilities following an earthquake event, and the ultimate desire to ensure the compressor package does not present a hazard to life in such an event. The latter being especially true in higher population density zones when compressor packages are located near facilities such as homes, subdivisions, schools, etc. or when used in compressing hazardous gases such as H2S where a release of gas can have a significant impact on the range of an evacuation zone. The ultimate goal in all of these cases is the protection of life. As a specialized consulting company in the field of vibration for compressor skids, BETA is often approached to verify that a package can sustain the loads of an earthquake. In most cases, specification requirements are not clear. This leads to some uncertainty as to what must be checked, what are the methods available to assess the integrity and suitability of a package subject to earthquake loadings, and how the verification/analysis is to be performed. Still, one can deduce the primary concern: the threat to life associated with a gas release or the inability of the compressor to provide safe and reliable black-start capability if called upon. ASCE Seismic Requirements for Typical Compressor Packages: Do They Apply? Page 4

5 Having established the primary concern for gas releases 1, the most logical point to start a seismic analysis is the pressure vessel and piping codes from which most compressor packages, at least those produced in North America for use in seismic zones defined in the introduction to this paper, are constructed. The codes of primary consideration are the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC) and its B31.3 Process Piping standard. The code to which most pressure vessels used in compressor packaging are designed and constructed is the ASME Section 8, Division 1 BPVC. Per Section UG-22 (f) of this code, seismic loadings shall be considered in the design of pressure vessels where required. Section UG-23 of this code provides guidance on acceptable stress levels due to seismic loading, however, these are assumed to be static values as the effects of fatigue type loadings are not considered in Section 8 Division 1. Non-mandatory Appendix G provides suggested practices regarding piping reactions and the design of supports and attachments. Section G-5 specifically addresses the skirt reactions of large vertically supported vessels subjected to seismic loadings. What the code is lacking is any specific or recommended method for conducting seismic analyses on pressure vessels. Unlike the pressure vessel code, the ASME Process Piping code, B31.3, provides more explicit information in the conduct of seismic analyses of piping systems in Section In this section, it is stated The effect of earthquake loading shall be taken into account in the design of piping. The analysis considerations and loads may be as described in ASCE 7. Authoritative local seismological data may also be used to define or refine the design of earthquake loads. The American Society of Civil Engineers (ASCE) Standard 7 Minimum Design Loads for Buildings and Other Structures is one of the main authorities on the design of buildings and nonbuilding structures subjected to environmental loads such as wind, rain, snow, and earthquakes. Chapters 11-23, more than 130 pages, of their latest standard, ASCE 7-10, cover the seismic design of buildings and nonbuilding structures. Additional commentary is provided in Appendix C Chapters C.11 through C.19 and C.22. The goal of this paper is to examine ASCE 7-10 and see how its rules and guidelines can be applied to the design of safe compressor assemblies in earthquake prone zones, identify short comings in its recommendations, and make suggestions to be addressed in the future by the compression industry. 1 This is not to negate the need for consideration of other secondary effects such as deformation in the skid and pedestal which could possibly result in misalignment rendering the compressor package inoperable. ASCE Seismic Requirements for Typical Compressor Packages: Do They Apply? Page 5

6 4. Basic Tenets and Discussion of Considerations for Conducting a Seismic Analysis per ASCE 7 One of the basic tenets that govern seismic design according to ASCE is that the life and safety of the public must be preserved in the case of the maximum credible earthquake. However, it is expected that under that maximum credible earthquake, inelastic deformation and damage will occur in the structures. Another important tenet is that the structures are designed in such a way that they will not present any structural damage under a less severe but more frequent earthquake. Other kinds of damage might be occurring, but no structural damage. Thirdly, the reserve strength, the ductility and the redundancy in the lateral force resistance system is taken into account through a response modification factor called R. This factor results in design loads smaller than the loads that would be used to design a structure expected to remain elastic during the design earthquake. R values that vary from 2 to 8 are proposed in ASCE Finally, in most cases, 5% damping is assumed to predict the response of the structure to the design earthquake. These considerations are taken into account to scale either the ground motion or the lateral force that is then used to design the structure. The dilemma presented is that the ASCE code is primarily written to address loading of buildings, tanks, other structures, and piping not necessarily highly mechanically/structurally complex, multi-stage, skid-mounted separable reciprocating compressor packages. In the absence of more specific guidance, however, the ASCE code is the most applicable to the specific topic of compressor packages. 5. ASCE 7-10 vs. ASCE 7-05 Changes in seismic calculations have been introduced in ASCE First, uniform-hazard ground motion has now been replaced by risk-targeted ground motion. ASCE 7-10 now assumes a collapse risk target of 1% in 50 years rather than a 2% in 50-year hazard level as it was the case in previous editions of this standard. Although the specified ground motions have not changed significantly with the introduction of ASCE 7-10, there are a few areas where the new ground motions are different. New Madrid, MO is one example, Charleston, SC is another one. A complete map of the ratio of ASCE 7-10 spectral response at short periods (S S ) values to ASCE 7-05 S S values is provided in [1]. Increases that vary between 10-40% are mainly seen in California, Oregon, and Washington. Increases of more than 40% are for a small area at the border between New Mexico and Colorado. Decreases that vary between 10-20% are noticed on most of the American Midwest and East Coast. Changes in the spectral response at 1 s (S 1 ) values are less severe. Some wording has also been added in ASCE 7-10 to define what a nonbuilding structure is. The wording fits well with the kind of structures considered in this paper. The question that remains is whether nonbuilding structures are considered as similar to buildings or not similar to buildings. To that effect, J.G. Soules in his webinar available on the ASCE website makes an interesting comment. For Soules, to be classified as not being similar to buildings, a nonbuilding structure must present two characteristics: 1) present a lateral force-resisting system that is not similar to a building; 2) present dynamic characteristics (vibration modes) that are not similar to buildings. We will discuss these considerations further when looking at the different components that comprise a skid assembly. ASCE 7-10 also introduced a Risk Category and a Seismic Importance Factor. These are present to recognize the fact that large earthquakes will result in damage to structures. They also take into account the fact that the consequence of damage or failure is not the same for all structures. Serious damages to certain critical facilities will disproportionally impact a community. As a result, the seismic protection requirements are tailored to the relative importance of a structure. Such structures include those that are necessary for response and recovery efforts, present the potential for catastrophic loss in the event of an earthquake, or house a very large number of occupants less able to care for themselves. 6. Methods for Evaluating Seismic Loading According to ASCE In order to define the applied loading necessary to assess structural integrity of compressor packages per the nonbuilding structures and non-structural components requirements of the ASCE 7 code, it is first necessary to select the seismic design category, component importance factor, and risk category. ASCE Seismic Requirements for Typical Compressor Packages: Do They Apply? Page 6

7 In determining the seismic design category, mapped acceleration parameters are selected by considering the ground motion maps provided in Chapter 22 of the ASCE code. An example of these maps is provided in Figure 7. As the maps provide values for Site Class B, the ground motion parameters provided are scaled by the site coefficient values provided in Tables and of the code. Site classes are based on soil properties and determined per Chapter 20. In the absence of soil properties, a Site Class D is chosen. With the above information, the actual seismic design category is determined from the methods of Section 11.6 of the code. Selection of risk categories and importance factors is less cumbersome coming from the information presented in ASCE 7 Table which is presented as Table 1 below and ASCE 7 Table which is presented as Table 2. Figure 7: S S Risk-Targeted Maximum Considered Earthquake (MCE R ) Ground Motion Parameter for the Conterminous US for 0.2s Spectral Response Acceleration (5% of Critical Damping), Site Class B. ASCE ASCE Seismic Requirements for Typical Compressor Packages: Do They Apply? Page 7

8 Table 1: ASCE Table Risk Category of Buildings and Other Structures for Flood, Wind, Snow, Earthquake, and Ice Loads. Table 2: ASCE 7 Table Importance Factors by Risk Category of Buildings and Other Structures for Snow, Ice, and Earthquake Loads. ASCE Chapter 15, Section of ASCE 7-10 states structural analysis procedures for nonbuilding structures similar to buildings shall be selected in accordance with Section It further proposes four possible analysis procedures to assess the effects of an earthquake for nonbuilding structures not similar to buildings. These methods are the Equivalent Lateral Force Procedure per Section 12.8, the Modal Response Spectrum Analysis per Section 12.9, the Seismic Response History Procedure per Section 16.1, and the nonlinear response history procedure in accordance with Section Each procedure presents its own degree of complexity. A fifth alternative allows for prescribed analysis procedures based on specific reference documents, provided the requirements of Section are met. The objective of this section is to briefly present the Equivalent Lateral Force Procedure, the Modal Response Spectrum Analysis and the Seismic Response History Procedure. The interested reader should refer to the ASCE 7-10 standard for details. The first procedure of interest, and the simplest, is called the Equivalent Lateral Force Procedure (ELFP), described in detail in Section 12.8 of ASCE This method is universally used to verify the seismic design of piping systems, vessels, etc. The ELFP method specifies the base shear that should be applied as a static load to a structure to simulate the effects of an earthquake. Once calculated, this base shear is then distributed to each story level of the structure to produce the displacement of the different floors, as well as the forces and moments that should be used to design the beams and columns. The base shear value depends on the weight of the structure and a seismic response coefficient. The seismic response coefficient in turn depends on the location of the installation and how prone this location is to earthquakes, the type of lateral and vertical seismic force-resisting system, as well as the importance of the structure. ASCE Seismic Requirements for Typical Compressor Packages: Do They Apply? Page 8

9 The second procedure is called the Modal Response Spectrum Analysis (MRSA). It is described in detail in Section 12.9 of the Standard. In this procedure, a sufficient number of natural vibration modes of the structure must first be calculated. The response of the structure in each mode is then predicted using a design response spectrum. The spectrum proposed in ASCE 7-10 is shown in Figure 8. This spectrum approximates the response of a 5% damped single degree of freedom system. Its amplitude, specified by S D1 and S DS, depends on the period or frequency of the mode, the location of the structure and the foundation through what is called the Site Class. Essentially and as can be seen in Figure 8, the spectrum first increases for short period modes (high frequency modes), then becomes flat for longer periods, and finally decreases. Also, as a response spectrum, it elicits response from all modes present in the system in which significant spatial coherence is present between the forces and the modes. This leads to question the intent of ASCE 7 which addresses well posed civil type structures versus complex mechanical/structural systems such as skid-mounted, separable compressor packages. Figure 8 Design Response Spectrum The spectrum is used to define an equivalent static force similar to the one calculated with the ELFP procedure, but this time this force is specific to a particular mode of vibration. The response of the structure to which this static load is applied is then calculated. The total response of the structure is finally obtained by combining the responses from each individual mode. Finally, the third method is called Seismic Response History Procedure (SRHP). This method is detailed in Chapter 16 of the standard. It is by far the most complex of the three procedures described here. As its name indicates and contrary to the other two methods presented above, SRHP requires the application of several ground motion accelerations at the base of the structure. The ground motions must be similar to the ground motion recorded during previous earthquakes in the same region. When 3D modeling is used, pairs of ground motions must be applied in two perpendicular directions. Provisions are made to make sure that the response predicted by the SRHP is not significantly lower than the one predicted by the ELFP. The popular code CAESAR which is used for piping systems includes both the ELFP and the MRSA methods. Most structural analysis programs used in the civil industry such as SAP2000 include all three methods. More general FEA programs do not include any of the methods, but procedures that implement those methods can be programmed so that these methods are automated. Finally, the standard includes a table that specifies which methods are permitted. This table is reproduced as Table 3 of this paper. Two parameters must be considered to select the method that should be used: the Seismic Design Category and the Structural Characteristics of the building. The Seismic Design Category depends on the magnitude of the spectral acceleration response (the seismicity of the area where the structure is installed) and the risk category defined in Table 1 of this paper. On the other hand, the characteristics of the building take into account the height of the building and its shape. What is confusing about this table is that all three methods described above are permitted for most structures. It is only for the buildings classified as Seismic Design Category D, E or F (the most critical buildings located in the most active regions from a seismic point of view) and as Other Structures for which the ELFP method is not permitted. This ASCE Seismic Requirements for Typical Compressor Packages: Do They Apply? Page 9

10 leaves a lot of interpretation to the analyst who must then decide whether the structure that they are studying fits in one of the Structural Characteristics categories or not. Depending on that decision, either the simpler and universally ELFP method can be used or not. It is the authors opinion that the ELFP method can be applied to most components of a compressor assembly as will be explained later in this paper. Seismic Design Category Table 3 ASCE 7-10, Table , Permitted Analytical Procedures. Structural Characteristics ELFP MRSA SRHP B,C All structures P P P D,E,F P: Permitted; NP: Not permitted, T S = S D1 /S DS 7. Components That Must Be Studied Risk Category I or II buildings not exceeding two stories above the base P P P Structures of light frame construction P P P Structures with no structural irregularities and not exceeding 160 ft in structural height P P P Structures exceeding 160 ft in structural height with no structural irregularities and with T<3.5T S P P P Structures not exceeding 160 ft in structural height and having only horizontal irregularities of Type 2,3,4 or 5 in Table [ASCE 7-10] or vertical irregularities of Type 4, 5a or 5b in Table [ASCE 7-10] P P P All other structures NP P P In this section, we will consider different types of components and propose a strategy to make sure that equipment is properly designed to withstand an earthquake in accordance with the ASCE 7-10 standard. 7.1 Driver and Compressor on Pedestals Packages include a driver (motor or engine) and a compressor, typically mounted on a pedestal. Such an arrangement is shown in Figure 3. The structure of a typical pedestal is better seen in Figure 2. From a structural point of view, a pedestal can be seen as a fairly flexible structure with respect to lateral excitations. Due to the heavy mass of the driver and the compressor, lower natural frequencies can be obtained from the assembly of the compressor and the driver. The driver and the compressor are stiff structures that can be considered as separate masses with a definite inertia mounted on the flexible structure that constitutes the pedestal. The pedestal is in turn mounted on the stiff structure that constitutes the skid attached to its foundation. ASCE 7-10 already includes some considerations for the case of Vertical Combinations, that is the case where a flexible upper portion is attached to a rigid lower portion. These considerations are presented in article Under certain conditions, a two-stage equivalent lateral force procedure is permitted. That is, the upper portion, the compressor, the driver, the distance pieces, the cylinders, the bottles and the pedestal in our case, can be designed as a separate structure fixed at the base of the pedestal. The rigid lower portion, the skid in our case, can be designed as a separate structure. The reactions from the upper portion shall be as determined during the analysis of that portion. Such twostage strategy could be used for the pedestal verification. ASCE Seismic Requirements for Typical Compressor Packages: Do They Apply? Page 10

11 Additionally, it is recommended to verify that the anchor bolts of the driver and the compressor can sustain the loads that will be generated during the design earthquake. The stresses in pedestal plates and members should also be verified with the appropriate load combinations of dead, live, and earthquake loads. It must also be noted that the driver and the compressor are not centered on the skid. This can be observed in Figure 3 where we can see that these components are not centered along the long side of the skid. However, the stiffness of the skid and the stiffness provided by the anchor bolts are fairly uniformly distributed over the entire skid. As a result, some torsional loads will be generated under an earthquake that hits along the long side of the skid. This torsional effect will likely be seen in the distribution of the reaction loads at the anchor bolts. 7.2 Vertical Vessels Most compressor packages will include vertical vessels called scrubbers. Those are shown in orange in Figure 3. A scrubber is typically welded to a skirt that is then welded or bolted to the skid beams. Just like it was the case for the driver and the compressor mounted on a pedestal, a scrubber, its skirt and the skid to which it is attached can be treated as Vertical Combinations. As previously mentioned, such combinations are covered in article of the ASCE 7-10 standard. Under certain conditions, the scrubber can then be analyzed separately and the forces generated can be added to the skid beams to which the scrubber is attached. In some cases, the beams at the base of a scrubber are filled with concrete so as to increase the mass, the stiffness, and the damping of the scrubber base. In this case, treating the skid and scrubber as a vertical combination, as permitted by ASCE 7-10, is even more justified as the base is even stiffer than skid beams only. Some complexity might come from the fact that pipes are connected to scrubbers. Because any significant stiffness will generate larger undesirable nozzle loads during an earthquake, the stiffness of these pipes is typically low compared to the stiffness of the base. In these cases, it is reasonable to assume that during an earthquake, most of the load would be resisted by the base and not transferred to other components by the pipe. This situation needs to be investigated further in order to establish guidelines on the maximum stiffness of the pipes that are connected to a scrubber. Further complicating the analysis of such vessels is the need to consider flexibility of piping attachments per Section Per the ASCE 7 Table Minimum Design Displacements for Piping Attachments, shown below as Table 4, the minimum design displacements for mechanically anchored tanks and vessels are quite severe. These displacements would be a concern for inlet piping to suction scrubbers, the suction spool nozzles, and the cylinder nozzles. It could also be a concern at piping connections at the cooler. Table 4: ASCE 7 Table Minimum Design Displacements for Piping Attachments. In a recent article, Wieschollek et al. [3] have run time history analyses on several types of vessels, including vertical vessels. Although the dimensions of the vessels that they have looked at are much larger than the typical vessels that we find on reciprocating compressor packages, it is interesting to look at their results. The results indicate that the seismic ASCE Seismic Requirements for Typical Compressor Packages: Do They Apply? Page 11

12 design forces prescribed in the European standard used (equivalent of the ASCE 7-10 standard), result in a design that remains in the elastic range when applied to the vessel. They also indicate a certain amount of strength past the initial elastic behavior before collapse of the vessel occurs. In two of the three cases studied, failure corresponds to the buckling of the skirt. In terms of calculations for vertical vessels, it is recommended that the stress in the vessel and the skirt be checked and verified to be below guideline. The weld at the intersection between the vessel and the skirt as well as the connection between the skirt and the skid beams must also be checked for sufficient strength. The deflection of the scrubber should also be calculated to make sure it is below the guideline of height/100. Of course, there should be enough clearance around the vessel to allow such a motion without hitting other pipes or vessels. 7.3 Pipes and Pipe Supports Pipes and pipe supports should also be checked as they will be excited during an earthquake. ASME B31E covers the seismic design of above-ground piping systems. Table 1 of that standard indicates which analysis methods are permitted. The analysis method must be selected based on the peak spectral acceleration (smaller or larger than 0.3g), the pipe diameter (smaller or larger than 4 in diameter) as well as the criticality of the piping system (must the pipe remain leak-tight or operable during and after the design earthquake?). Three analysis methods are possible. In the case of a 4 in diameter or less noncritical pipe, no additional analysis is required to take into account the loads that will be generated by the design earthquake. In the case of a 4 in or more noncritical pipe located in a zone where the spectral acceleration is larger than 0.3g or for a 4 in or smaller critical pipe located in a zone where the spectral acceleration is less than 0.3g, design by rule is allowed. Finally, for the case of a critical pipe smaller than 4 in in diameter located in a zone where the spectral acceleration is larger than 0.3g as well as for all critical piping of 4 in diameter or more, design by analysis must be performed. In the case of design by analysis, it must be shown that the total stress caused by the pressure in the pipe, the sustained and the seismic loads are smaller than a design allowable depending on the material. In the case of elevated pipes, the clearance from other components to allow the motion of the pipe that will occur during an earthquake must also be checked. 7.4 Horizontal Vessels ASCE 7-10 also includes considerations for saddle supported horizontal vessels. The level of displacements and loads to be sustained by such design are specified. The loads that must be considered depend on whether the weight of the vessel is smaller than 25% of the combined weight of the vessel and the skid or larger. In the case where the weight of the vessel is smaller than 25% of the combined weight, the designer is referred to the section that covers the design of non-structural components. In the second case, when the weight of the vessel is larger than 25% of the combined weight, the requirements depend on the stiffness of the vessel. Stiff vessels can be considered as rigid masses. For more flexible vessels, a combined analysis is recommended. Wieschollek et al. [3] have also analyzed different saddle-supported horizontal tanks. Unfortunately, as it was the case for vertical vessels, the vessels considered are much bigger than what we typically find on compressor skids. Keeping this in mind, it is still interesting to note that when analyzed in detail, the tanks displayed a significant amount of ductility. It is also clear that the design loads prescribed by the European standard result in designs that remain in the elastic range when the typical earthquakes used in this study are applied. Their study has also shown the positive impact of adding reinforcing rings. Reinforcing rings are in fact recommended for large pressure vessels or vessels located in high seismicity areas. 8. Conclusion This paper has described the ASCE requirements for the seismic design of structures and how these requirements can be applied to the design of different components typically found on compressor skids. Although ASCE 7-10 is mainly concerned with seismic requirements as they apply to buildings, it is seen that many components are similar in shape ASCE Seismic Requirements for Typical Compressor Packages: Do They Apply? Page 12

13 and also present a similar dynamic response. For that reason, the ASCE requirements form a good basis to establish the technical requirements for seismic calculations in the case of compressor packages. However, as presented, there are gaps between the application intent of ASCE 7 and the actual application to complex systems such as the modern skid-mounted, separable compressor package. In absence of specific guidance on the application of ASCE 7 or other earthquake criteria to systems such as a compressor packages, it is necessary for purchasers of such equipment to think through their earthquake concerns and ensure these are conveyed to the packager during the contract negotiation period. Likewise, it is important for the packager to specifically define the earthquake requirements and analysis expectations to the third-party specialized engineering firm to ensure end-user concerns are addressed. All parties end-user, packager, and specialized engineering firm need to be cognizant of cost implications as these could place a significant adder to the contracted delivery price of any given unit. Also, the seismic verification of a package will require more work and more discussion between structural, piping, and pressure vessels engineers in order to clearly establish a working procedure that will insure the integrity of the compressor packages. Finally, on top of this coordination effort, additional efforts are required in order to decide on a consistent approach for seismic analysis of reciprocating compressor packages. ASCE assumptions such as tolerated damage during the design earthquake, a 5% damping ratio, response modification factors (R) larger than 1, and even the selection of the method between the Equivalent Lateral Force Procedure, the Modal Response Spectrum Analysis, or the Seismic Response History Procedure should be looked at as part of these additional efforts. 9. References 1. Ghosh, S.K., Significant changes from ASCE 7-05 to ASCE 7-10, part 1: Seismic design provisions, available at: Winter14%20Seismic%20Design%20Precast%20Provisions%20in%20ASCE%207.pdf. 2. Soules, J.G. (Greg), Introduction to the Seismic Design of Nonbuilding Structures to ASCE 7-10, available at ASCE Knowledge & Learning. 3. Wieschollek, M., Diamanti, K., Pinkawa, M., Hoffmeister, B. and Feldmann, M., Guidelines for Seismic Design and Analysis of Pressure Vessels, Proceedings of the ASME 2013 Pressure Vessels and Piping Conference, July 14-18, ACI 351.3R, Foundations for Dynamic Equipment (Reapproved 2011). ASCE Seismic Requirements for Typical Compressor Packages: Do They Apply? Page 13