PSA 2017 Paper Improved Tornado Missile Risk Analysis Using Nonlinear Finite Element Analysis of Nuclear Power Plant Structures.

Transcription

1 Improved Tornado Missile Risk Analysis Using Nonlinear Finite Element Analysis of Nuclear Power Plant Structures PSA 2017 Paper September 25,

2 Improved Tornado Missile Risk Analysis Using Nonlinear Finite Element Analysis of Nuclear Power Plant Structures Authors: Claudia Navarro-Northrup Robert T. Bocchieri, Ph.D. Virginia Phan Jeffrey C. Sciaudone Lawrence A. Twisdale, Ph.D. Applied Research Associates, Inc. 95 1st Street, Suite 100, Los Altos, CA

3 Overview TORMIS computer code Developed to estimate the probability of damage to nuclear power plant structures from debris missile impacts in extreme winds. Relies on calculations for critical damage to a structure. Critical damage to power plant structures Results in loss of function to the plant structure (e.g., crimping of exhaust). Each type of missile causes critical damage at a different impact velocity. Knowing this critical velocity is an important component of performing this risk analysis. Analytical methods (e.g., SDOF) for damage analysis For some target/missile combinations, test data and simple analytical methods exist to predict damage. Can require many conservative assumptions. May result in unnecessarily low critical velocities. Nonlinear dynamic FEA More accurate analysis of impact damage. Higher critical velocities are calculated, leading to lower risk numbers. Now a practical and cost-effective part of TORMIS risk analysis. 3

4 Overview Nonlinear finite element analysis (FEA) can model the complex, dynamic targetmissile interaction to determine accurate critical missile velocities. Soft missiles are hard to assess with simplistic loading methods. These missiles crush significantly during impact, affecting the load applied to the target. Target response affects the missile crushing and trajectory. Important because they can have a high hit frequency. Real targets often have complex boundary conditions (BCs) that affect the target-missile interaction. Ignoring or simplifying the BCs can lead to much lower critical missile velocities. Many target-missile configurations can be analyzed to determine the critical impact location and velocity. Crimping is critical failure mode that can be assessed with FEA. 4

5 Soft Missiles Soft missiles are missiles that are weak compared to the target. Experience significant deformation during impact. Generally higher hit frequencies. Examples include metal siding, steel grating, and wood planks. Hard missiles are comparable to or stiffer/stronger than the target. Hard missiles have little deformation. Examples include a wide flange beam and a channel section beam. Missile Depth (in) Width (in) Length (ft) Weight (lb) Metal Siding Steel Grating Wide Flange Metal Siding (Soft Missile) Steel Grating (Soft Missile) Wide Flange Beam (Hard Missile) 5

6 Exhaust Pipe Crimping Crimping is an important failure mode for exhaust pipes. Exhausts from diesel generators and steam lines. These exhaust pipes have minimum required flow rates to function properly. Target description: 16-in diameter exhaust pipe with a rigid constraint at the bottom. Pipe is in contact with a deformable roof sleeve. The concrete roof penetration is modeled as rigid. 16-in Exhaust Pipe Exhaust Pipe Roof Sleeve Roof Penetration Constraint 6

7 Soft Missile Impact Response Metal siding impact of 16-in exhaust pipe at 540 fps. Pipe shown as semitransparent. 7

8 Soft Missile Impact Response At 8 ms: Max crimping has occurred. Missile has 65% of its original KE. Significant, non-uniform crushing of missile. o The crush shape conforms to the deformed shape of the pipe. Large deformation/buckling of the missile away from the crush zone. At 20 ms: Pipe moves away from the missile. A plastic hinge has formed in the pipe at the roof sleeve interface. o Crimping in the pipe at the roof sleeve interface. t = 8 ms t = 20 ms t = 76 ms At 76 ms: The missile continues to push the exhaust pipe until the missile slides off the pipe. 8

9 Soft vs. Hard Missile Three missiles resulted in very different crimping of the exhaust pipe. Missile configurations are those that resulted in the greatest crimping at the given velocity. Images shown are for the time of maximum crimping. Steel grating: Similar behavior to the metal siding. Significant crushing at the target interface. Plastic deformations/buckling away from missile crush zone. Wide flange beam: Small amount of damage to missile. o Localized bending of the flanges at the impact interface. o Slight bending along the missile length. Metal Siding at 540 fps Steel Grating at 515 fps Wide Flange at 170 fps 9

10 Target Crimping Response by Missile The open area fraction, φ, is: Open Area after Impact φ = Initial Open Area Metal siding: Initially denting was narrow, matching missile profile. As the missile deformed, the load spread out laterally, resulting in a wider crimp zone. Steel grating: The crimp zone remains narrow. The missile deformed significantly vertically but not laterally (the stiff direction). Wide flange beam: The crimp zone remains wide. The missile front end does not deform significantly and the crimp zone does not become localized. Open Area Before Impact φ = 1 Metal Siding φ = 0.30 Steel Grating φ = 0.44 Wide Flange φ =

11 Complex Targets Real targets have complex shapes and boundary conditions. Difficult to represent in simplified analyses without large conservative assumptions. Target constraints (even at a distance) can affect the response at the impact location. Soft missiles take a relatively long time to impart load. Target deforms and moves prior to time of maximum crimping. Ignoring or simplifying soft constraints (e.g., insulation) changes the target-missile interaction. Exhaust Pipe with Angled Exit and Cover Plate. Insulation in the roof penetration modeled with crush response and lock up of the material. Cover plate and angle irons are attached with welds. Bottom of pipe model extends 37.5 ft below the roof line (not shown). Exhaust Pipe and Cover Plate Roof Sleeve Roof Penetration Insulation Roof sleeve and penetration shown as semi-transparent. 11

12 Complex Target Impact Response Metal siding impact of angled exit exhaust pipe with cover plate at 240 fps. Pipe shown as semi-transparent. Missile rebounds at late time. 12

13 Complex Target Impact Response At 6 ms: Missile is engaged with pipe and cover plate. o Localized crushing at the interface with target. Localized deformation of the exhaust pipe and cover plate. Most of the welds are still intact. t = 6 ms t = 20 ms At 20 ms: The front of the metal siding has started to buckle. The exhaust pipe has begun to sway back. Maximum crimping has occurred and the front of the exhaust pipe has impacted the back of the pipe. At 34 ms: Significant buckling of the missile. All welds in the target have failed. At 82 ms: The missile has buckled along its length, well away from the target interface. t = 34 ms t = 82 ms Partial missile shown. 13

14 Complex Target Impact Response Cover plate inhibits deflection of the metal siding away from the exhaust pipe. The buckling of the missile at multiple missile locations has changed loading on the target. Buckled regions reduce load applied and deceleration of missile. Missile buckling, and target deformation, has affected the missile trajectory, allowing the missile to deflect vertically and laterally. t = 82 ms Side View Top View 14

15 Insulation Modeling Rigid Insulation Deformable Insulation No Insulation In previous simulation, a deformable insulation constitutive model was used for the crush response and lock up of the material. Conservatively modeling the insulation as rigid provides a stiff constraint that maximizes crimping. Ignoring the insulation results in less crimping o The air gap allows the pipe to sway out of the way during impact. Impacts to 3 models with a wide flange beam at 112 fps give very different responses. t = 6 ms t = 24 ms t = 34 ms Insulation φ Rigid 0.29 Deformable 0.50 None 0.57 t = 20 ms t = 34 ms t = 82 ms Partial missile shown. The roof penetration, roof sleeve, and insulation are all shown as semi-transparent. 15

16 Complex Target Critical Impact Location Many different target-missile configurations can be analyzed. Critical location for complex targets and soft missiles depends on the target-missile interaction. Not known a-priori. Configuration A: Lowest missile impact location without impacting roof sleeve. Configuration B: Exhaust pipe rotated so the missile impacted the exhaust pipe to the side of the angle irons. Configuration C: The missile was rotated 90. Configuration D: Missile impacts the tallest side (rear) of the angled pipe. Configuration A Configuration B Configuration C Configuration D Partial missile shown. 16

17 Complex Target Critical Impact Location Configuration A Configuration B Configuration C Configuration D Pipe shown as semi-transparent. 17

18 Critical Impact Location Plots of open area fraction (φ) versus velocity highlight the critical configurations. Configuration A: Analyzed at 240 fps and 250 fps with little change in φ. At 240 fps, the missile has pushed the front of the exhaust pipe against the back of the pipe, not allowing for any more crimping. Configuration B: Less crimping than Configuration A. Impacting higher on the pipe on more of the curved exhaust face reduces the amount of crimping at the same velocity. Configuration C: The least effective at pipe crimping. Configuration D: Most effective at crimping. A B C D 18

19 Summary Nonlinear FEA using LS-DYNA was demonstrated to accurately model complex targetmissile interaction Many of the behaviors shown cannot be easily, if at all, modeled with simple analytical methods. Soft missiles crush and deform affecting the target response. Soft missiles have a longer impact duration allowing the target to respond and affecting the overall targetmissile interaction. Critical impact location varies by missile type. Constraints and adjacent structures surrounding a target can also affect the targetmissile interaction. In particular, soft boundary conditions, such as insulation, result in a very different target response. LS-DYNA was used for all the FEA shown here. LS-DYNA is commercially available (developed by Lawrence Livermore National Laboratory). LS-DYNA has an extensive user community that helps validate the code for a variety of applications including crash, blast, and impact. Nonlinear dynamic FEA is now a practical and cost-effective part of TORMIS risk analysis. 19

20 Questions? A category 2 hurricane has maximum sustained winds of 129 mph (189 fps). 20