1、 Guidance Notes on Sloshing and Structural Analysis of LNG Pump Tower GUIDANCE NOTES ON SLOSHING AND STRUCTURAL ANALYSIS OF LNG PUMP TOWER APRIL 2006 (Updated February 2014 see next page) American Bureau of Shipping Incorporated by Act of Legislature of the State of New York 1862 Copyright 2006 Amer
2、ican Bureau of Shipping ABS Plaza 16855 Northchase Drive Houston, TX 77060 USA Updates February 2014 consolidation includes: July 2009 version plus Corrigenda/Editorials July 2009 consolidation includes: April 2006 version plus Corrigenda/Editorials ABSGUIDANCE NOTES ON SLOSHING AND STRUCTURAL ANALY
3、SIS OF LNG PUMP TOWER .2006 iii Foreword Foreword As the demand for Liquefied Natural Gas (LNG) grows worldwide, the size of LNG carriers increase and new tank designs emerge. The LNG industry wishes to apply more advanced and technically sound approaches for the structural assessment of the pump to
4、wer to ensure that the offered designs meet the requirements. This document provides guidance for applying direct calculation procedures for the structural assessment of the pump tower structure. The procedures include ship motion calculation, determination of wave conditions for sloshing simulation
5、, requirements for sloshing analysis, calculation of loads on pump tower structure, finite element analysis procedure and acceptance criteria for the strength assessment. This approach takes advantage of the principles and the experiences gained from the application of the ABS Dynamic Load Approach
6、(DLA), advances in numerical simulation of the sloshing, and experiences from the structural analysis and evaluation of tubular structures. In addition to the strength assessment of pump tower structure, the methodology for fatigue and vibration analysis of pump tower structure is also addressed in
7、this document. The requirements for the strength of hull structure of LNG carriers are addressed in Part 5C, Chapter 12 of the ABS Rules for Building and Classing Steel Vessels. ABS also provides Guidance Notes on Strength Assessment of Membrane-Type LNG Containment Systems under Sloshing Loads. Thi
8、s Page Intentionally Left Blank ABSGUIDANCE NOTES ON SLOSHING AND STRUCTURAL ANALYSIS OF LNG PUMP TOWER .2006 v Table of Contents GUIDANCE NOTES ON SLOSHING AND STRUCTURAL ANALYSIS OF LNG PUMP TOWER CONTENTS SECTION 1 Introduction 1 1 Background1 3 The Concepts of Pump Tower Analysis Procedure.1 5 O
9、verview of Sloshing Analysis and Structural Assessment of Pump Tower.2 5.1 Seakeeping Analysis of the Vessel .2 5.3 Selection of Critical Sloshing Wave Conditions.2 5.5 Sloshing Analysis2 5.7 Load Cases for Structural Analysis .3 5.9 FE Analysis of Pump Tower Structure 3 5.11 Acceptance Criteria.3 5
10、.13 Fatigue Analysis3 5.15 Vibration Analysis3 FIGURE 1 Typical Pump Tower Installed in LNG Tank 4 FIGURE 2 Flowchart for Sloshing Analysis and Structural Assessment of Pump Tower5 SECTION 2 Analysis of Ship Motions 7 1 Overview 7 3 Environmental Condition7 3.1 Wave Scatter Diagram 7 3.3 Wave Spectr
11、um.7 5 Ship Design Considerations 8 5.1 Operational Condition .8 5.3 Filling Levels .8 5.5 Loading Conditions .9 5.7 Tank Location9 7 Spectral Analysis of Motion and Wave Load .10 7.1 General Modeling Considerations.10 7.3 Diffraction-Radiation Methods and Panel Model .11 7.5 Roll Damping Model11 7.
12、7 Vessel Motion and Tank Acceleration Response Amplitude Operators .11 vi ABSGUIDANCE NOTES ON SLOSHING AND STRUCTURAL ANALYSIS OF LNG PUMP TOWER .2006 9 Long-Term Response for Ship Motions.12 TABLE 1 Standard Wave Data Recommended by IACS8 FIGURE 1 Definition Sketch of Membrane-Type LNG Tank.9 FIGU
13、RE 2 LNG Cargo Holds and No. 2 Tank Location 10 SECTION 3 Wave Conditions for Sloshing Analysis 13 1 General 13 3 Resonance Sloshing Period 13 5 Sloshing Motion Parameters14 7 Critical Sloshing Wave Condition (CSWC) 14 7.1 Equivalent Wave Amplitude.14 7.3 Critical Sloshing Wave Domain (CSWD).15 7.5
14、Motion Components 15 SECTION 4 Sloshing Analysis 17 1 General 17 3 Sloshing Simulation .17 3.1 Requirement for CFD Tool 17 3.3 Validation of CFD Tool 17 3.5 Modeling of LNG Tanks.17 3.7 Mesh Size and Time Stepping.17 3.9 Duration of Simulation .18 3.11 Numerical Results for Pump Tower Analysis.18 FI
15、GURE 1 Modeling of LNG Tank for Two-Dimensional Sloshing Analysis.18 SECTION 5 Loads on Pump Tower 19 1 General 19 3 Sloshing Load 19 3.1 Morison Formula .19 3.3 Wave Impact Zone 21 3.5 Shielding Effect .22 5 Inertial loads.22 5.1 Inertial Loads in the FEM Structural Model22 7 Thermal Load and Pump
16、Torque Load22 7.1 Thermal Load 22 7.3 Pump Torque.22 9 Load Cases for Pump Tower Structural Analysis 23 9.1 Load Cases 1 and 2: Transverse Force on Pump Tower 23 9.3 Load Case 3: Longitudinal Force on Pump Tower 23 9.5 Load Case 4 and 5: Transverse Force on Pump Tower Base Support.24 9.7 Load Case 6
17、: Longitudinal Force on Pump Tower Base Support24 ABSGUIDANCE NOTES ON SLOSHING AND STRUCTURAL ANALYSIS OF LNG PUMP TOWER .2006 vii FIGURE 1 Definition of Length Coordinate and Normal Vector.20 FIGURE 2 Typical Fluid Motion in Tank at a Partial Filling Condition20 FIGURE 3 Typical Sectional Force Pr
18、ofiles at Partial Filling Conditions 21 FIGURE 4 Simplified Temperature Distribution for Thermal Analysis at Partial Filling Conditions.23 SECTION 6 Structural Analysis of Pump Tower .25 1 General 25 3 Structural Members25 5 3-D Finite element modeling25 5.1 Coordinate System25 5.3 Material Properti
19、es25 5.5 Element Types 26 7 Boundary Conditions27 TABLE 1 Material Properties of Stainless Steel ASTM A312 Gr. 304L26 FIGURE 1 Cargo Tank Configuration28 FIGURE 2 Plan View of Pump Tower28 FIGURE 3 Complete Pump Tower Model .29 FIGURE 4 Stress-Strain Curve for Stainless Steel .30 FIGURE 5 Pump Tower
20、 FE Model-Main Section 30 FIGURE 6 Pump Tower FE Model Liquid Dome Cover .31 FIGURE 7 Pump Tower FE Model Base Plate.31 FIGURE 8 Boundary Conditions32 FIGURE 9 Schematics of the Sliding Joints Mounted on the Top Braces.33 SECTION 7 Acceptance Criteria .35 1 General 35 3 Tubular Members.35 3.1 Summar
21、y of Criteria for Tubular Members 35 3.3 Individual Stresses35 3.5 Members Subjected to Combined Loads 38 3.7 Local Buckling.39 5 Tubular Joints 40 5.1 Joint Types40 5.3 Joint Capacity41 5.5 Strength State Limit.43 7 Liquid Dome Cover and Base Plate.43 7.1 F.E. Model.43 7.3 Strength Criterion43 viii
22、 ABSGUIDANCE NOTES ON SLOSHING AND STRUCTURAL ANALYSIS OF LNG PUMP TOWER .2006 TABLE 1 Strength Factor, Qu.42 FIGURE 1 Examples of Critical Buckling Stress .37 FIGURE 2 Geometry of Tubular Joints40 FIGURE 3 Examples of Tubular Joint Categoriztion .41 APPENDIX 1 References 45 APPENDIX 2 Benchmark Tes
23、ts for Sloshing CFD Tools 47 1 Purpose of Benchmark Tests 47 3 Tank Geometry 47 5 Tank Motion .48 7 Simulation Results .48 7.1 Velocity and Acceleration Time History .48 7.3 Velocity and Acceleration Profile .48 APPENDIX 3 Fatigue Assessment 49 1 General 49 1.1 Load 49 1.3 Stress Range.49 1.5 S-N cu
24、rve 50 1.7 Fatigue Damage50 3 Simplified Fatigue Assessment Method.50 5 Spectral-based Fatigue Assessment Method52 7 Reference 52 APPENDIX 4 Vibration Analysis of Pump Tower. 53 1 General 53 1.1 Sources of Excitation.53 1.3 Loading Conditions53 1.5 Tank Location and Filling Levels .53 3 Free Vibrati
25、on Analysis54 3.1 Local FE Model and Boundary Conditions 54 3.3 Added Mass 54 3.5 Natural Frequency and Mode Shape.54 5 Forced Vibration.55 5.1 Global FE Model and Boundary Conditions.55 5.3 Excitations.55 5.5 Critical Areas.55 7 Acceptance Criteria55 9 References.56 FIGURE 1 First Mode Shape in Lon
26、gitudinal and Transverse Modes 54 ABSGUIDANCE NOTES ON SLOSHING AND STRUCTURAL ANALYSIS OF LNG PUMP TOWER .2006 1 Section 1: Introduction SECTION 1 Introduction 1 Background The design and construction of the hull, superstructure and deckhouses of an ocean-going vessel are to be based on all applica
27、ble requirements of the ABS Rules for Building and Classing Steel Vessels (Steel Vessel Rules). The design criteria for an LNG carrier are located in the Steel Vessel Rules, Part 5C, Chapter 8. Alternative hull strength requirements in compliance with the SafeHull-based criteria are provided in Part
28、 5C, Chapter 12 of the ABS Rules for Building and Classing Steel Vessels. Inside the LNG tank, there are two major structural systems other than the hull: LNG containment system and pump tower structure. These Guidance Notes supplement the Steel Vessel Rules by providing a sloshing and structural an
29、alysis procedure for the structural assessment of the pump tower. The structural assessment of the LNG containment system is addressed in the ABS Guidance Notes on Strength Assessment of Membrane-Type Containment System under Sloshing Load for LNG Carriers. The pump tower system consists of pipes an
30、d pumps to load and discharge the liquid cargo and tubular members to support the structure. The pump tower is located close to the aft bulkhead, hanging over the liquid dome and connected to base support at the tank bottom, as shown in Section 1, Figure 1. The structural integrity of these systems
31、is critical for the safe operation of the LNG carrier. Approval of the structural design of the pump tower systems has typically been made based on the analysis and assessment provided by the manufacturers of the systems. This is a practical and well-founded practice for the review of similar design
32、s when the previous designs have shown good service experience. As the LNG market grows, the size of LNG carriers is increasing and the operational condition is diversifying. New cargo tank designs are also emerging. The LNG industry demands a more advanced and technically sound approach for the str
33、uctural assessment of the pump tower to ensure that the offered designs meet the requirements. This document provides guidance for applying a direct calculation procedure for the structural assessment of the pump tower structure. This approach takes advantage of the basic principles and the experien
34、ces gained from the application of the ABS Dynamic Load Approach (DLA), advances in numerical simulation of the sloshing, and experiences from the structural analysis and evaluation of tubular structures. 3 The Concepts of Pump Tower Analysis Procedure The analysis procedure provided in these Guidan
35、ce Notes is a first-principle-based, direct calculation approach to identify the critical sloshing load on the pump tower during its design life and to evaluate the structural integrity of the pump tower under that load. The procedure adopts an advanced but most practical analysis method. In particu
36、lar, linear seakeeping analysis, two-dimensional sloshing simulation and FE modeling/analysis of the pump tower structure by beam and plate elements are utilized. Section 1 Introduction 2 ABSGUIDANCE NOTES ON SLOSHING AND STRUCTURAL ANALYSIS OF LNG PUMP TOWER .2006 The analysis procedure includes De
37、termination of long-term extreme ship motions arising from design environmental condition Determination of wave conditions that may produce a large sloshing motion Time domain simulation of the sloshing motion of liquid cargo Calculation of sloshing and accompanying loads on the pump tower structure
38、 Structural analysis of the pump tower, using the finite element method Strength assessment of the pump tower structures in accordance with acceptance criteria 5 Overview of Sloshing Analysis and Structural Assessment of Pump Tower 5.1 Seakeeping Analysis of the Vessel Section 1, Figure 2 shows the
39、flowchart of the pump tower analysis procedure that is covered in these Guidance Notes. The procedure starts with the seakeeping analysis of the vessel, which is described in Section 2. The main objective of the seakeeping analysis is to calculate the motion of the vessel and LNG tank for a given op
40、erating and environmental condition, to be used as motion input for the sloshing analysis. Response amplitude operators (RAOs) of ship and tank motions are calculated by linear frequency-domain ship motion analysis, from the hull form and loading conditions provided by the ship designer. Long-term e
41、xtreme responses of ship and tank motion for the design environmental condition are obtained by spectral analysis. 5.3 Selection of Critical Sloshing Wave Conditions The objective of the procedure described in Section 3 is to determine a set of regular wave conditions (wave period, heading and ampli
42、tude), called the Critical Sloshing Wave condition (CSWC), that are likely to produce a large sloshing motion in the cargo tank. Firstly, the Critical Sloshing Wave Domain (CSWD), which is a set of wave conditions (wave period and heading) that may produce a large sloshing motion, is defined based o
43、n tank motion response calculated from the seakeeping analysis and proximity of tank motion period to the sloshing natural period. Among the tank motion parameters, the local values of longitudinal and transverse acceleration at the tank center are used to determine the wave conditions in CSWD. Then
44、, for each CSWD wave condition, equivalent regular wave amplitude is determined from the long-term responses of ship and tank motion and their corresponding RAOs. CSWCs are completely defined by determination of equivalent wave amplitude for each CSWD. Combining the selected wave conditions and tank
45、 motion RAO, the motion input for the sloshing analysis is determined. 5.5 Sloshing Analysis For selected CSWCs, sloshing simulations are performed. Liquid motion inside the tank due to the tank motion is to be simulated in the time domain by a qualified Computational Fluid Dynamics (CFD) tool. The
46、velocity and acceleration profile at the pump tower location are to be utilized for the evaluation of hydrodynamic force on the pump tower. Although the liquid motion inside the LNG tank is three-dimensional, sloshing analysis in three dimensions is not practical at the moment because it necessitate
47、s huge computer time and resources. Two-dimensional sloshing simulation is adopted in the current procedure. The sloshing load on each pump tower structural member is calculated by the Morison formula at each time step of the simulation. Section 4 provides a detailed description of the sloshing anal
48、ysis. Section 1 Introduction ABSGUIDANCE NOTES ON SLOSHING AND STRUCTURAL ANALYSIS OF LNG PUMP TOWER .2006 3 5.7 Load Cases for Structural Analysis A total of five load cases are selected for the FE analysis of the pump tower structure after completion of the sloshing analyses for all selected CSWCs
49、. The selection is based on the total resultant force on the pump tower, which can be obtained by the integrated force from sloshing, inertial and gravitational load. The instantaneous load profile along the pump tower at the moment of maximum total resultant force is selected as input for the FE analysis. Maximum forces in the longitudinal and transverse directions are used as the selection criteria. Also considered is the maximum total resultant force transmitted to the base support to be utilized for the structural assessment of the pump tower base support. After the
