1、 Guide for Spectral-Based Fatigue Analysis for Floating Production, Storage and Offloading (FPSO) Installations GUIDE FOR SPECTRAL-BASED FATIGUE ANALYSIS FOR FLOATING PRODUCTION, STORAGE AND OFFLOADING (FPSO) INSTALLATIONS MAY 2010 (Updated February 2014 see next page) American Bureau of Shipping In
2、corporated by Act of Legislature of the State of New York 1862 Copyright 2010 American Bureau of Shipping ABS Plaza 16855 Northchase Drive Houston, TX 77060 USA Updates February 2014 consolidation includes: February 2013 version plus Corrigenda/Editorials February 2013 consolidation includes: May 20
3、10 version plus Notice No. 1 and Corrigenda/Editorials ABSGUIDE FOR SPECTRAL-BASED FATIGUE ANALYSIS FOR FPSO INSTALLATIONS .2010 iii Foreword Foreword This Guide for Spectral-Based Fatigue Analysis for Floating Storage and Offloading (FPSO) Installations, herein referred to as the “Guide”, provides
4、information on the method to perform spectral fatigue analysis for ship-type “Floating Production Installations”. This type of offshore installation is usually referred to as a “Floating Storage and Offloading (FSO) System”; or “Floating Production, Storage and Offloading (FPSO) System”. FPSO is the
5、 term that will be used in this Guide to denote these ship-type Floating Production Installations. Spectral fatigue analysis performed for FPSOs in accordance with the procedures and criteria in this Guide will be identified in the Record by the notation SFA. The Rules and Guides for Classification
6、for which this Guide is considered to be most relevant are: ABS Rules for Building and Classing Mobile Offshore Drilling Units ABS Rules for Building and Classing Offshore Installations ABS Rules for Building and Classing Floating Production Installations Additionally, the use of the Guide relies on
7、 reference to: ABS Guide for the Fatigue Assessment of Offshore Structures This Guide specifically relates to the latest editions of the above-mentioned Rules and Guides. The use and relevancy of the Guide to other editions of these references, or with other ABS criteria, should be established in co
8、nsultation with ABS. This Guide is based on an earlier publication entitled ABS Guidance Notes on Spectral-Based Fatigue Analysis for Floating Offshore Structures (March 2005). The present document supersedes the earlier one. ABS welcomes comments and suggestions for the improvement of the Guide. Co
9、mments or suggestions can be sent electronically to rsdeagle.org. iv ABSGUIDE FOR SPECTRAL-BASED FATIGUE ANALYSIS FOR FPSO INSTALLATIONS .2010 Table of Contents GUIDE FOR SPECTRAL-BASED FATIGUE ANALYSIS FOR FLOATING PRODUCTION, STORAGE AND OFFLOADING (FPSO) INSTALLATIONS CONTENTS SECTION 1 Introduct
10、ion 1 1 Background and Applicability 1 3 FPSO Areas for Fatigue Assessment . 2 3.1 Hull Structure . 2 3.3 FPSO-Specific Structural Areas 2 5 Tanker Conversion 3 7 General Comments about the Spectral-based Method 3 9 Data to be submitted . 4 FIGURE 1 Schematic Spectral-based Fatigue Analysis Procedur
11、e . 5 SECTION 2 Establishing Fatigue Demand . 6 1 Introduction . 6 3 Stress Transfer Function . 6 5 Basic Loading Conditions . 6 7 Combined Fatigue from Multiple Basic Loading Conditions . 6 9 Transit Cases 7 SECTION 3 Environmental Conditions . 8 1 General . 8 3 Waves . 8 3.1 Wave Spectra (Short-te
12、rm Wave Statistics) 8 3.3 Wave Spectral Formulation . 10 3.5 Wave Scatter Diagram and Rosette (Long-term Wave Statistics) . 11 5 Currents 12 7 Wind 12 FIGURE 1 Definition of Spreading Angles 9 ABSGUIDE FOR SPECTRAL-BASED FATIGUE ANALYSIS FOR FPSO INSTALLATIONS .2010 v SECTION 4 Motion Analysis and W
13、ave-induced Loads 13 1 General . 13 3 Still-water Loads . 13 5 Essential Features of Motion and Wave Load 14 5.1 General Modeling Considerations . 14 5.3 Diffraction-Radiation Methods . 14 5.5 Low Frequency Motions 14 SECTION 5 Wave-induced Load Components 15 1 General . 15 3 External Pressure Compo
14、nent 15 3.1 Total Hydrodynamic Pressures . 15 3.3 Intermittent Wetting . 15 3.5 Pressure Distribution on Finite Element Models 15 5 Internal Tank Pressure Component 16 7 Loads from the Motions of Discrete Masses . 16 SECTION 6 Loading for Global Finite Element Method (FEM) Structural Analysis Model
15、. 18 1 General . 18 3 Number of Load Cases . 18 5 Mooring Loads 18 7 Equilibrium Check . 18 SECTION 7 Structural Modeling and Analysis 19 1 General . 19 3 Areas for Fatigue Strength Evaluations 19 5 3-D Global Analysis Modeling . 19 7 Analyses of Local Structure 20 9 Hot Spot Stress Determination .
16、20 FIGURE 1 Fine Mesh FEM Model 21 FIGURE 2 Local Structural FE Model: Welds Modeled 21 FIGURE 3 Weld Toe Extrapolation Points 23 FIGURE 4 Elements Adjacent to Weld Toe 23 SECTION 8 Fatigue Strength . 24 1 General . 24 3 S-N Data . 24 vi ABSGUIDE FOR SPECTRAL-BASED FATIGUE ANALYSIS FOR FPSO INSTALLA
17、TIONS .2010 SECTION 9 Fatigue Life (Damage) Calculation and Acceptance Criteria 26 1 General . 26 3 Combination of Wave-Frequency and Low-Frequency Responses in Wave-induced Fatigue Damage Calculation . 26 5 Low Cycle Fatigue Damage 27 5.1 Low Cycle Fatigue Load 27 5.3 Loading Conditions 27 5.5 Stre
18、ss Range Calculation 27 7 Combined Low Cycle and High Cycle Fatigue Damage . 31 9 Acceptance Criteria . 31 FIGURE 1 Sample Functions of SWand SB. 28 FIGURE 2 A Single Loading/Offloading Cycle . 28 FIGURE 3 keas a Function of SE. 29 FIGURE 4 Low Cycle Fatigue Design Curve 30 APPENDIX 1 Wave Data . 32
19、 TABLE 1 ABS Wave Scatter Diagram for Unrestricted Service Classification . 32 APPENDIX 2 Basic Design S-N Curves . 33 TABLE 1 Parameters For Basic S-N Design Curves . 34 FIGURE 1 S-N Curves 33 APPENDIX 3 Outline of a Closed Form Spectral-based Fatigue Analysis Procedure 35 1 General . 35 3 Key Step
20、s in Closed Form Damage Calculation . 35 5 Closed Form Damage Expression 38 FIGURE 1 Spreading Angles Definition 36 ABSGUIDE FOR SPECTRAL-BASED FATIGUE ANALYSIS FOR FPSO INSTALLATIONS .2010 1 Section 1: Introduction SECTION 1 Introduction 1 Background and Applicability This Guide provides a detailed
21、 description of the method to perform spectral fatigue analysis for ship-type offshore installations in order to obtain the optional classification notation SFA. Spectral fatigue analysis relies on the presumed linearity of wave-induced loads with respect to waves. This condition is sufficiently sat
22、isfied for ship-type offshore installations; such as Floating Production, Storage and Offloading (FPSO) and Floating Storage and Offloading (FSO) systems. For a ship-type hull, linear diffraction forces are the dominant component of wave load. The application of the spectral fatigue analysis method
23、to a ship-type hull is presented in this Guide. For other hull configurations composed of members with relatively large cross sectional dimensions; such as a Tension Leg Platform, Spar, or Column Stabilized units, a spectral approach is often employed; but with appropriate modifications to account f
24、or the influence of nonlinear drag forces that tend to become more important as cross sectional member dimensions decrease. This Guide assumes that any required modification to linearize the wave-induced load effects has been satisfactorily accomplished. This Guide employs basic concepts and termino
25、logy that were defined in the ABS Guide for the Fatigue Assessment of Offshore Structures (2003). In that reference it is stated that: “Fatigue assessment denotes a process where the fatigue demand on a structural element (e.g., a connection detail) is established and compared to the predicted fatig
26、ue strength of that element. One way to categorize a fatigue assessment technique is to say that it is based on a direct calculation of fatigue damage or expected fatigue life. Three important methods of assessment are called the Simplified Method, the Spectral Method and the Deterministic Method. A
27、lternatively, an indirect fatigue assessment may be performed by the Simplified Method, based on limiting a predicted (probabilistically defined) stress range to be at or below a permissible stress range. There are also assessment techniques that are based on Time Domain analysis methods that are es
28、pecially useful for structural systems that are subjected to nonlinear structural response or nonlinear loading.” In this Guide, the fatigue assessment technique that is presented is a direct calculation method based on the spectral analysis method, which can produce a fatigue assessment result in t
29、erms of either expected damage or life. The fatigue strength of structural details is established using the S-N curve approach that is specified in the referenced Guide. It should be borne in mind that for the hull structure of an Offshore Installation, wave-induced loading is usually the dominant s
30、ource of fatigue damage. However some types of floating offshore structures may also be subjected to significant fatigue loading from other loading sources. This can be true for hull types that undergo frequent loading and discharge of produced fluids. For example, in FPSO and FSO systems, such load
31、 changes can induce large ranges of hull girder stress and secondary stress (albeit at lower cycles than direct wave loads). Such load cycle fatigue is also addressed in this Guide. In addition, fatigue loading may also be induced by the operation of equipment associated with the function of the Off
32、shore Installation. The extent to which the spectral-based fatigue method can or will be adapted to take into account these “non-wave” sources of fatigue damage must be further considered by the designer. Section 1 Introduction 2 ABSGUIDE FOR SPECTRAL-BASED FATIGUE ANALYSIS FOR FPSO INSTALLATIONS .2
33、010 3 FPSO Areas for Fatigue Assessment There are two general categories of FPSO structural details for which fatigue assessments are required. The first type relates to conventional tanker hull details and is indicated in 1/3.1. For some of these details, in addition to wave loads, low cycle produc
34、ed fluid (cargo) loading and offloading induced loads should be considered in the fatigue assessment. The second type of details is specific to an FPSO as indicated in 1/3.3. For some of the latter type, other kinds of loads, (e.g., low-frequency loads or operational dynamic loads) should be include
35、d in the fatigue assessments. 3.1 Hull Structure General guidance on areas of the hull where fatigue assessment should be performed is as follows: 3.1.1 Connections of Longitudinal Stiffeners to Transverse Web/Floor and to Transverse Bulkhead 3.1.1(a) 2 to 3 selected side longitudinal stiffeners in
36、the region from 1.1 maximum draft to about 0.33 maximum draft in the midship region and also in the region between 0.15L and 0.25L from F.P., respectively. 3.1.1(b) 1 to 2 stiffeners selected from each of the following groups: Deck longitudinals, bottom longitudinals, inner bottom longitudinals and
37、longitudinals on side longitudinal bulkheads. One longitudinal on the longitudinal bulkheads within 0.1D from the deck is to be included. For these structural details, the fatigue assessment is to be first focused on the flange of the longitudinal stiffener at the rounded toe welds of attached flat
38、bar stiffeners and brackets. Then, the critical spots on the web plate cut-out, on the lower end of the stiffener, as well as the weld throat, are also to be checked for the selected structural detail. Where the stiffener end bracket arrangements on two sides of a transverse web are different, both
39、configurations are to be checked. 3.1.2 Shell, Bottom, Inner Bottom or Bulkhead Plating at Connections to Webs or Floors (for Fatigue Assessment of Plating) 3.1.2(a) 1 to 2 selected locations of side shell plating near the summer LWL amidships and between 0.15L and 0.25L from F.P. respectively. 3.1.
40、2(b) 1 to 2 selected locations in way of bottom and inner bottom amidships. 3.1.2(c) 1 to 2 selected locations of lower strakes of side longitudinal bulkhead amidships. 3.1.3 Connections of the Slope Plate to Inner Bottom and Side Longitudinal Bulkhead Plating at the Lower Cargo Tank Corners One sel
41、ected location amidships at transverse web and between webs, respectively. 3.1.4 End Bracket Connections for Transverses and Girders 1 to 2 selected locations in the midship region for each type of bracket configuration 3.1.5 Other Regions and Locations Other regions and locations, highly stressed b
42、y fluctuating loads, as identified from structural analysis 3.3 FPSO-Specific Structural Areas The adequacy of the following FPSO-specific areas for fatigue should be suitably demonstrated: Position mooring/hull interface, if spread moored (5A-1-4/3 of the ABS Rules for Building and Classing Floatin
43、g Production Installations (FPI Rules) Turret and its interface with hull, if turret moored (6-2-1/13 and 6-2-1/15 of the FPI Rules) Riser porches Section 1 Introduction ABSGUIDE FOR SPECTRAL-BASED FATIGUE ANALYSIS FOR FPSO INSTALLATIONS .2010 3 The details, below and on the deck of the hull, compri
44、sing the supports of the topside structures. (The interface details between the hull structure and equipment skids and support frames deserve particular attention.) Additional areas, as applicable, including: flare tower foundation, crane pedestals, helideck to deck connections and deck penetrations
45、. 5 Tanker Conversion When an FPSO is converted from a trading tanker, the fatigue damage accumulated during the “trading tanker” phase is to be deducted when establishing the remaining fatigue life for future service as an FPSO. When calculating the fatigue damage for past services, the wave condit
46、ions of specific routes the vessel has experienced in past service can be employed, instead of using the wave condition representing unrestricted service as may have been done for classification as a tanker. When calculating the fatigue damage accumulated during the “trading tanker” phase, the effec
47、ts of vessel speed (encounter frequency) should be included (i.e., in the evaluation of stress RAOs and the number of stress cycles). 7 General Comments about the Spectral-based Method Spectral-based Fatigue Analysis is a complex and numerically intensive technique. As such, there is more than one v
48、ariant of the method that can be validly applied in a particular case. This Guide is not intended to preclude the use of any valid variant of a Spectral-based Fatigue Analysis method by “over specifying” the elements of an approach. However, there is a need to be clear about the basic minimum assump
49、tions that are to be the basis of the method employed and some of the key details that are to be incorporated in the method to produce results that will be acceptable to ABS. For this reason, most of the remainder of this Guide addresses these topics. The main assumptions underlying the Spectral-based Fatigue Analysis method are listed below: i) Ocean waves are the main source of the fatigue-inducing loads acting on the structural system being analyzed. The fatigue damage from other loading sources can be considered separately. ii) In order for the frequency d
