ABS-104-2005.pdf

GUIDANCE NOTES ON SPECTRAL-BASED FATIGUE ANALYSIS FOR FLOATING OFFSHORE STRUCTURES MARCH 2005 American Bureau of Shipping Incorporated by Act of Legislature of the State of New York 1862 Copyright 2005 American Bureau of Shipping ABS Plaza 16855 Northchase Drive Houston, TX 77060 USA Copyright American Bureau of Shipping Provided by IHS under license with ABS Licensee=Boeing Co/5910770001 Not for Resale, 08/07/2008 19:43:46 MDTNo reproduction or networking permitted without license from IHS -,-,- This Page Intentionally Left Blank Copyright American Bureau of Shipping Provided by IHS under license with ABS Licensee=Boeing Co/5910770001 Not for Resale, 08/07/2008 19:43:46 MDTNo reproduction or networking permitted without license from IHS -,-,- ABS GUIDANCE NOTES ON SPECTRAL-BASED FATIGUE ANALYSIS FOR FLOATING OFFSHORE STRUCTURES . 2005 iii Foreword These Guidance Notes provide additional information and suggestions about particular aspects of the ABS Classification criteria. The present Guidance Notes serve to illustrate an envisioned scope and method to perform spectral fatigue analysis for various types of floating offshore structures. The Rules and Guides for Classification to which these Guidance Notes are considered to be most relevant are: ABS Rules for Building and Classing Mobile Offshore Drilling Units ABS Guide for Building and Classing Offshore Installations ABS Guide for Building and Classing Floating Production Installations Additionally, the use of these Guidance Notes relies on reference to: ABS Guide for the Fatigue Assessment of Offshore Structures These Guidance Notes specifically relate to the latest editions of the above-mentioned Rules and Guides. The use and relevancy of these Guidance Notes to other editions of these references, or with other ABS criteria, should be established in consultation with ABS. These Guidance Notes are based on an earlier publication entitled: ABS Guidance Notes on Spectral-Based Fatigue Analysis for Floating Production, Storage and Offloading (FPSO) Systems (January 2002) The present document supersedes the earlier one. ABS welcomes comments and suggestions for the improvement of these Guidance Notes. Comments or suggestions can be sent electronically to rddeagle.org. Copyright American Bureau of Shipping Provided by IHS under license with ABS Licensee=Boeing Co/5910770001 Not for Resale, 08/07/2008 19:43:46 MDTNo reproduction or networking permitted without license from IHS -,-,- This Page Intentionally Left Blank Copyright American Bureau of Shipping Provided by IHS under license with ABS Licensee=Boeing Co/5910770001 Not for Resale, 08/07/2008 19:43:46 MDTNo reproduction or networking permitted without license from IHS -,-,- ABS GUIDANCE NOTES ON SPECTRAL-BASED FATIGUE ANALYSIS FOR FLOATING OFFSHORE STRUCTURES . 2005 1 GUIDANCE NOTES ON SPECTRAL-BASED FATIGUE ANALYSIS FOR FLOATING OFFSHORE STRUCTURES CONTENTS SECTION 1 Introduction5 1 Background and Applicability.5 3 FPSO Areas for Fatigue Assessment6 3.1 Hull Structure 6 3.3 FPSO-Specific Structural Areas7 5 Tanker Conversion.7 7 General Comments about the Spectral-based Method.7 FIGURE 1 Schematic Spectral-based Fatigue Analysis Procedure.9 SECTION 2 Establishing Fatigue Demand.11 1 Introduction11 3 Stress Transfer Function .11 5 Base Vessel Loading Conditions.11 7 Combined Fatigue from Multiple Base Vessel Loading Conditions12 9 Transit Cases.12 SECTION 3 Environmental Conditions 13 1 General 13 3 Waves13 3.1 Wave Spectra (Short-term Wave Statistics)13 3.3 Wave Scatter Diagram and Rosette (Long-term Wave Statistics) 14 5 Currents .15 7 Wind.15 Copyright American Bureau of Shipping Provided by IHS under license with ABS Licensee=Boeing Co/5910770001 Not for Resale, 08/07/2008 19:43:46 MDTNo reproduction or networking permitted without license from IHS -,-,- 2 ABS GUIDANCE NOTES ON SPECTRAL-BASED FATIGUE ANALYSIS FOR FLOATING OFFSHORE STRUCTURES. 2005 SECTION 4 Motion Analysis and Wave-induced Loads.17 1 General 17 3 Still-water Loads 17 5 Essential Features of Motion and Wave Load.18 5.1 General Modeling Considerations .18 5.3 Diffraction-Radiation Methods.18 5.5 Low Frequency Motions 18 SECTION 5 Wave-induced Load Components21 1 General 21 3 External Pressure Component.21 3.1 Total Hydrodynamic Pressures .21 3.3 Intermittent Wetting .21 5 Internal Tank Pressure Component.22 7 Loads from the Motions of Discrete Masses22 SECTION 6 Loading for Global Finite Element Method (FEM) Structural Analysis Model.23 1 General 23 3 Number of Load Cases23 5 Mooring Loads .23 7 Equilibrium Check24 SECTION 7 Structural Modeling and Analysis25 1 General 25 3 Areas for Fatigue Strength Evaluations.25 5 3-D Global Analysis Modeling25 7 Analyses of Local Structure.26 9 Hot Spot Stress Concentration 26 FIGURE 1 Fine Mesh FEM Model.27 FIGURE 2 Local Structural FE Model: Welds modeled.27 FIGURE 3 Weld Toe Extrapolation Points.29 FIGURE 4 Elements Adjacent to Weld Toe.29 SECTION 8 Fatigue Strength 31 1 General 31 3 S-N Data 31 Copyright American Bureau of Shipping Provided by IHS under license with ABS Licensee=Boeing Co/5910770001 Not for Resale, 08/07/2008 19:43:46 MDTNo reproduction or networking permitted without license from IHS -,-,- ABS GUIDANCE NOTES ON SPECTRAL-BASED FATIGUE ANALYSIS FOR FLOATING OFFSHORE STRUCTURES . 2005 3 SECTION 9 Fatigue Life (Damage) Calculation and Acceptance Criteria.33 1 General 33 3 Combination of Wave-Frequency and Low-Frequency Responses.33 5 Fatigue Damage due to Loading/Offloading of Produced Fluids .34 7 Acceptance Criteria35 APPENDIX 1 Outline of a Closed Form Spectral-based Fatigue Analysis Procedure37 1 General 37 3 Key Steps in Closed Form Damage Calculation .37 5 Closed Form Damage Expression.40 FIGURE 1 Spreading Angles Definition38 Copyright American Bureau of Shipping Provided by IHS under license with ABS Licensee=Boeing Co/5910770001 Not for Resale, 08/07/2008 19:43:46 MDTNo reproduction or networking permitted without license from IHS -,-,- This Page Intentionally Left Blank Copyright American Bureau of Shipping Provided by IHS under license with ABS Licensee=Boeing Co/5910770001 Not for Resale, 08/07/2008 19:43:46 MDTNo reproduction or networking permitted without license from IHS -,-,- ABS GUIDANCE NOTES ON SPECTRAL-BASED FATIGUE ANALYSIS FOR FLOATING OFFSHORE STRUCTURES . 2005 5 S E C T I O N 1 Introduction 1 Background and Applicability These Guidance Notes serve to illustrate an envisioned scope and method to perform spectral fatigue analysis for various types of floating offshore structures. Spectral fatigue analysis relies on the presumed linearity of wave-induced loads with respect to waves. This condition is sufficiently satisfied 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 presentation given herein illustrates the application of the spectral fatigue analysis method to a ship-type hull. For other structural types composed of members with relatively large cross sectional dimensions; such as a Tension Leg Platform, Spar, or Column Stabilized hull, a spectral approach is often employed; but with appropriate modifications to account for the influence of nonlinear drag forces that tend to become more important as cross sectional member dimensions decrease. The presentation given herein assumes that any required modification to linearize the wave-induced load effects has been satisfactorily accomplished. These Guidance Notes employ basic concepts and terminology 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 fatigue 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. Alternatively, 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 especially useful for structural systems that are subjected to nonlinear structural response or nonlinear loading. In these Guidance Notes, the fatigue assessment technique being presented is a direct calculation method based on the spectral analysis method, which can produce a fatigue assessment result in terms 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 source of fatigue damage. However some types of floating offshore structures may also be subjected to significant fatigue damage 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 changes can induce large ranges of hull girder stress and secondary stress (albeit at lower cycles than direct wave loads), and because of changing hull drafts, different locations of the hulls external shell will be exposed to direct wave loads. Significant fatigue damage may also be induced by the operation of equipment associated with the function of the Offshore 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. Copyright American Bureau of Shipping Provided by IHS under license with ABS Licensee=Boeing Co/5910770001 Not for Resale, 08/07/2008 19:43:46 MDTNo reproduction or networking permitted without license from IHS -,-,- Section 1 Introduction 6 ABS GUIDANCE NOTES ON SPECTRAL-BASED FATIGUE ANALYSIS FOR FLOATING OFFSHORE STRUCTURES . 2005 The classification criteria issued for Floating Offshore Installations indicate the structural details for which a fatigue assessment should be performed. Typically, there are a large number of details that are candidates for a fatigue assessment. Because of this, use may be made of “screening” procedures based on a simplified method to sort, and possibly exempt, some details from the full rigor of the spectral-based assessment technique. Good judgment, guided by experience and analytical results, may also be applied to comparable details to decide if the results for one detail can be applied to others. 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 produced 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 this latter type, other kinds of loads, e.g., low-frequency loads or operational dynamic loads, should be included 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 the region from the 1.1 draft to about 1/3 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 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 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 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. 3.1.2(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. Copyright American Bureau of Shipping Provided by IHS under license with ABS Licensee=Boeing Co/5910770001 Not for Resale, 08/07/2008 19:43:46 MDTNo reproduction or networking permitted without license from IHS -,-,- Section 1 Introduction ABS GUIDANCE NOTES ON SPECTRAL-BASED FATIGUE ANALYSIS FOR FLOATING OFFSHORE STRUCTURES . 2005 7 3.1.3 Connections of the Slope Plate to Inner Bottom and Side Longitudinal Bulkhead Plating at the Lower Cargo Tank Corners One selected 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 by 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 spreading moored (4-2/15.1 of the ABS Guide for Building and Classing Floating Production Installations) Turret and its interface with hull, if turret moored (5-4/13 and 5-4/15 of the ABS Guide for Building and Classing Floating Production Installations) Riser porches The details, below and on the deck of the hull, comprising