Offshore Requirements for Turbine Exhaust System Analysis and Design

Transcription

1 Offshore Requirements for Turbine Exhaust System Analysis and Design Analysis Process and Results for Development of an Optimal Exhaust System from an Internal pressure, Wind, Seismic and Aero-Acoustic Loading Conditions isope-2015 Conference, Hawaii Big Island The 25th International Ocean and Polar Engineering Conference, Big Island, Hawaii, USA, June 21 26, 2015:

2 Bio Gjinolli, E., Agron, P.E. - Universal Acoustic and Emissions Technologies - Subject Matter Expert - Structural Engineering - Topside Turbine Exhaust Systems - Previous: Qualtim Inc., Madison, Wisconsin, USA - Structural Analysis of Complex Nonbuilding Structures - Master of Science, Structural Engineering, University of Wisconsin, CEE, Madison, WI - Analysis of Metal Plate Connected Truss Joints

3 Introduction Offshore platforms require a power generation source Combustion turbines provide the most power in the smallest footprint. Typical loads: Internal pressure, wind, seismic, wave, ice, aero-acoustic, high temperatures and gas flow.

4 Introduction It is important that: Turbine exhaust system components should be designed with minimal weight for ease of installation, and At the same time be adequately robust to resist the extreme environmental loads of the open seas.

5 Structural Analysis and Design Considerations Finite element analysis (FEA) is preferred method for structural analysis. FEA considerations includes modeling for buckling of beam and plate (shell) elements. Stability against localized plate buckling. Second Order Elastic Analysis (SOEA), usually called "P-Δ" analysis. Deflections at Expansion Joints.

6 Structural Analysis and Design Considerations FEA is used for structural failure investigations or studies for special loading conditions. Special loading conditions may include flow induced vibration failures, internal explosions, wind vortex failures, and thermal fatigue in combination with wind, seismic or other loads.

7 Structural Analysis and Design Considerations Main Standards ANSI/AISC and , Specification for Structural Steel Buildings. AISC(2012), Seismic Design Manual, 2 nd Edition. ASCE 7-05 and 7-10, Minimum Design Loads for Buildings and Other Structures. ASME(2013), Pressure Vessel Code, Section VIII, Division 1 and 2. ASME (2011), Steel Stacks STS-1

8 Numerical Example 1 Design Data Two exhaust systems 150 feet above the deck Wind loads V 3s =100 and 120 Mph, Exp. D Topside seismic spectral acceleration (vert. & hor.) of 0.8g and 1.2g respectively. FEA Software: - RISA 3D V12.0 and RISA Section V2.0 - RAM Connection V8.0

9 Numerical Example 1 System Description SL 316L stainless steel - lined and unlined shells. SL EJ SL EJ EJ Unlined SL SL EJ Lined Insulation - between the outside shell and the liner cool outside shell, and for acoustic purposes. Skirt supported silencer connected to the deck through double base ring construction and anchor bolts. SL= Support Lug EJ= Expansion Joint

10 Numerical Example 1 Modeling and Material Properties FEA Modeling, Step 1: the strength and stiffness (elastic) properties of the components at the design temperature. CFD: Calculation of flow velocity, pressure, density, and shell temperature, as functions of the component(s) cross sectional area and time. ANSYS Fluent has been used to perform the CFD.

11 Numerical Example 1 Dead Load Analysis Density is used in the calculation of the member and plate (shell) self-weight dead loads. Modeling with an equivalent mass versus detailing out all the parts. Full model with internals when system is tall and the internal mass is high above the anchorage and rather large. 3D Fabrication Model 3D FEA Model

12 Numerical Example 1 Wind Load Analysis Step 1: Dynamic analysis to obtain mode shapes, frequencies and periods of vibration. The analysis of wind induced vibrations is essential since steel stacks are lightweight and flexible structures with low inherent structural damping.

13 Numerical Example 1 Wind Load Analysis Wind responses include vortex shedding and ovalling. Vortex shedding is related to the drag force caused by the actual wind flow pattern around the cylinder and the pressure distribution. Flow Velocity V p = -2.5q Unbalanced pressures between the upwind and downwind sides of the cylinder. p = q q = design pressure p = actual pressure

14 Numerical Example 1 Wind Load Analysis ASME STS1-2011: If V cr < V avg for the observed segment, vortex shedding must be considered. Prevention: damping or stiffening methods. Disruption of wind generated vortices: Helical strakes. Wind loading per ASCE 7-05 (10), omnidirectional loading. Strakes => 1.4 x Design V 3s

15 Numerical Example 1 Wind Load Analysis Von Mises stresses (VM) = (Plate Maximum Distortion Energy) Max. the Top Max. VM = 14 ksi, located at the lined duct support lug areas.

16 Numerical Example 1 Seismic Load Analysis ASCE 7-05 (10), Chapter 13 and 15. Determination of the basic seismic parameters for non-building structures (NBS). Calculation of seismic forces on non-building structures supported by other structures. Special considerations for the seismic design of tanks and vessels.

17 Numerical Example 1 Seismic Load Analysis Special considerations for tanks and vessels that need to be emphasized: 1. The importance of anchor rod stretch. 2. The importance of providing seismic freeboard. 3. The importance of providing piping flexibility. 4. Special design requirements for vessel support skirts.

18 Numerical Example 1 Seismic Load Analysis Anchor bolts used for tanks and vessels must stretch under seismic loads to provide the required ductility. Practical: A 325 stretches better than A490 bolts. Section is intended to ensure that anchor connections are designed such that the anchor will yield (stretch) before the failure of the anchor connection to the support / structure. Post Earthquake, Bolt Stretching = Added Washers

19 Numerical Example 1 Seismic Load Analysis Very Important! Skirt supported vessels fail in buckling, which is not a ductile failure mode. Skirt support To prevent collapse, Sect and Table require skirt supported vessels to be designed for seismic loads based on R/I E = 1.0 if the structure is Risk Category IV or if R=3 is used in the design of the vessel.

20 Numerical Example 1 Seismic Load Analysis Response Spectra Analysis (RSA) procedure: Each of the model's modes is considered to be an independent Single Degree of Freedom (SDOF system). VM stresses in the range between 9 and 12 ksi and are located at the lined duct support lug sections, and are lower than the allowable stresses Skirt support

21 Numerical Example 1 Seismic Load Analysis Stress concentrations are mitigated by adding structural repads on the shell. The re-pads help to lower stress concentrations. Shell re-pads help with joint distortions and excessive joint rotations. Re-pad

22 Numerical Example 1 Thermal Load Analysis The boundary conditions for the thermal analysis assumed that sliding can occur at the supports via slotted bolt holes.

23 Numerical Example 2 Vibrations and Resonances Example of FEA used for structural failure investigation of the engine exhaust system, caused by exhaust gas flow induced vibrations. Stack shell cracking due to the combined vibrations caused by the exhaust gas flow, dominant firing frequencies and acoustic modal frequencies. Acoustical resonances of the silencer chamber coupled with the structural resonances of the shell at the dominant engine order frequencies resulted in violent vibration levels, that ultimately caused shell cracking. Typical for low cycle/speed engines.

24 Numerical Example 2 Vibrations and Resonances The measured engine firing frequencies are 45, 90 and 135 Hz. Structural free vibration analysis: Mode 208, Frequency f=137.7hz, PMP=1.101% Field measured vibration Mode Shape due to the sound pressure, f=129/135hz

25 Numerical Example 2 Vibrations and Resonances Long Term Solution Thicker FD Head, Collar and Gussets. Thicker shell Re-pad between the silencer and the inlet pipe. Additional FD Head to de-tune acoustical modes and increase the system stiffness Thicker skirt (first 96 -long shell course) Double base ring construction including Gussets

26 Numerical Example 2 Vibrations and Resonances Comparison of the structural vibration modes between the existing and the new design at approximate F=135 Hz

27 Numerical Example 2 Vibrations and Resonances Modal analysis showed that the new silencer design has acoustic modes and structural resonant frequencies that are not close (not aligned) with the dominant firing frequencies, which was one of the objective s of this study.