MEMBRANE TANK LNG VESSELS

Transcription

1 Guide for Building and Classing Membrane Tank LNG Vessels GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS (HULL STRUCTURAL DESIGN AND ANALYSIS BASED ON THE ABS SAFEHULL APPROACH) OCTOBER 2002 (Updated March 2013 see next page) American Bureau of Shipping Incorporated by Act of Legislature of the State of New York 1862 Copyright 2002 American Bureau of Shipping ABS Plaza Northchase Drive Houston, TX USA

2 Updates March 2013 consolidation includes: October 2008 version plus Corrigenda/Editorials October 2008 consolidation includes: May 2008 version plus Corrigenda/Editorials May 2008 consolidation includes: February 2006 version plus Notice No. 2 February 2006 consolidation includes: August 2004 version plus Corrigenda/Editorials August 2004 consolidation includes: October 2002 version plus Notice No. 1

3 Foreword Foreword The industry and ABS share a large and successful body of experience with membrane tank LNG vessels. Owners and designers familiar with the benefits of the ABS SafeHull Rule approach in the design and analysis of other vessel types requested that ABS adapt the SafeHull criteria so that it can be used in the Classification of membrane tank LNG vessels. The results of this development are presented in this Guide. This Guide provides criteria that can be applied in the Classification of the hull structure of a membrane tank LNG vessel. The hull strength criteria contained herein are to be considered as an alternative to those for corresponding aspects of Classification as given in Part 5, Chapter 8 of the Steel Vessel Rules (SVR 5C-8). The Owner may select to use either this Guide or SVR 5C-8, however the Classification symbol, SH, (signifying compliance with the SafeHull based criteria in this Guide) will only be granted when the design is based on the criteria of this Guide. In the same design, for aspect of the hull structural design that are covered by both this Guide and SVR 5C-8 it is not valid to switch between the criteria in this Guide and SVR 5C-8. This Guide does not cover the design, fabrication and installation of the LNG containment system. After a certain period for trial use, the criteria contained in this Guide will be incorporated and published in the Steel Vessel Rules. ABS encourages and welcomes at any time the submission of comments on this Guide. The criteria contained in this Guide became effective on 1 MAY Reference Note Reference to a paragraph in the Steel Vessel Rules is made in the format P-C-S/ss.p P is the Part, C is the Chapter, S is the Section, ss is the Subsection and p is the Paragraph. Reference to a paragraph in this Guide is made in the format S/ss.p, S is the Section, ss is the Subsection and p is the Paragraph. Reference to a Figure or Table in this Guide is made, respectively, in the format S/Figure #, or S/Table # S is the Section in which the figure or table is located. ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS iii

4 Table of Contents GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS 150 meters (492 feet) or more in Length CONTENTS SECTION 1 Introduction General Classification Optional Class Notation for Design Fatigue Life Application Internal Members Breaks Variations Loading Guidance Design Vapor Pressure Protection of Structure Containment System... 3 FIGURE SECTION 2 Design Considerations and General Requirements General Requirements General Initial Scantling Requirements Strength Assessment Failure Modes Structural Redundancy and Residual Strength Nominal Design Corrosion Values General... 4 TABLE 1 Nominal Design Corrosion Values... 6 FIGURE 1 Nominal Design Corrosion Values... 5 SECTION 3 Load Criteria General Load Components Static Loads Still-water Bending Moment Wave-Induced Loads... 7 iv ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS. 2002

5 5.1 Wave-Induced Longitudinal Bending Moments and Shear Forces Horizontal Wave Bending Moments and Shear Forces External Pressures Internal Pressures Inertia Forces and Added Pressure Heads Nominal Design Loads General Hull Girder Loads Longitudinal Bending Moments and Shear Forces Local Loads for Design of Supporting Structures Local Pressures for Design of Plating and Longitudinals Combined Load Cases Combined Load Cases for Structural Analysis Combined Load Cases for Failure Assessment Sloshing Loads General Strength Assessment of Tank Boundary Structures Sloshing Pressures Impact Loads Impact Loads on Bow Bottom Slamming Bowflare Slamming TABLE 1A Combined Load Cases for Total Strength and Fatigue Assessment TABLE 1B Additional IGC Load Cases for Initial Scantling Evaluation of Main Supporting Members by FEA TABLE 2 Load Cases for Sloshing TABLE 3 Design Pressure for Local and Supporting Members TABLE 4 Definition of d, C φi and C θi TABLE 5 Values of α TABLE 6 Values of A i and B i FIGURE 1 Loading Pattern... 8 FIGURE 2 Distribution Factor m h FIGURE 3 Distribution Factor f h FIGURE 4 Distribution of h di FIGURE 5 Pressure Distribution Function k o FIGURE 6 Illustration of Determining Total External Pressure FIGURE 7A Tank Shape Parameters FIGURE 7B Definition of Tank Geometry FIGURE 8 Location of Tank for Nominal Pressure Calculation FIGURE 9 Definition of Tank Geometry FIGURE 10 Vertical Distribution of Nominal Slosh Pressure Head, h e FIGURE 11 Vertical Distribution of Nominal Slosh Pressure Head, h i for Low-Filling Resonance ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS v

6 FIGURE 12 FIGURE 13 Distribution of Nominal Slosh Pressure Head, h t on Tank Top Horizontal Distribution of Simultaneous Slosh Pressure Heads, h c (φ s θ s ) or h t (φ s θ s ) FIGURE 14 Loading Patterns for Sloshing Loads Cases FIGURE 15 Definition of Bow Geometry FIGURE 16 FIGURE 17 Distribution of Bottom Slamming Pressure Along the Section Girth Definition of Bowflare Geometry for Bowflare Shape Parameter FIGURE 18 Ship Stem Angle, γ SECTION 4 Initial Scantling Criteria General Strength Requirement Calculation of Load Effects Structural Details Evaluation of Grouped Stiffeners Hull Girder Strength Hull Girder Section Modulus Hull Girder Moment of Inertia Shearing Strength General Net Thickness of Side Shell Plating Net Thickness of Inner Skin Bulkhead Three Dimensional Analysis Double Bottom Structures General Bottom Shell and Inner Bottom Plating Bottom, Bilge and Inner Bottom Longitudinals Double Bottom Girders and Floors Structure in Pipe Duct Space Side Shell and Deck Side Shell Plating Sheerstrake Deck Plating Deck and Side Longitudinals Side Shell and Deck Main Supporting Members General Deck Transverses Deck Girders Side Transverses Side Stringers Web Stiffeners and Tripping Brackets Slots and Lightening Holes Longitudinal and Transverse Bulkheads Longitudinal Bulkhead Plating vi ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS. 2002

7 13.3 Transverse Bulkhead Plating Longitudinals and Vertical/Horizontal Stiffeners Transverse Bulkheads Main Supporting Members General Vertical Webs Horizontal Web Stiffeners, Tripping Brackets, Slots and Lightening Holes TABLE 1 Minimum Thickness of Girders and Floors FIGURE 1 Definition of b s and Extent for Calculating A i and A s FIGURE 2 Unsupported Span of Longitudinal FIGURE 3 Effective Breadth of Plating b e FIGURE 4 Definitions of s and b DB FIGURE 5 y for Deck Structures FIGURE 6 Definition of Parameters for Deck and Side Structure FIGURE 7 Effectiveness of Brackets FIGURE 8 Transverse Bulkhead Structures SECTION 5 Total Strength Assessment General Requirements General Loads and Load Cases Stress Components Failure Criteria Yielding General Structural Members and Elements Plating Failure Criteria Buckling and Ultimate Strength General Plate Panels Longitudinals and Stiffeners Stiffened Panels Girders and Webs Hull Girder Ultimate Strength Fatigue Life General Procedures Spectral Analysis Calculation of Structural Responses Methods of Approach and Analysis Procedures D Finite Element Models D Finite Element Models Local Structural Models Load Cases ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS vii

8 FIGURE SECTION 6 Hull Structure Beyond 0.4L Amidships General Requirements General Structures within the Cargo Space Length Forebody Side Shell Structure Side shell Plating Side Frames and Longitudinals Side Transverses and Stringers in Forebody Transition Zone Forebody Strengthening for Slamming Bottom Slamming Bowflare Slamming FIGURE 1 Transverse Distribution of p d FIGURE 2 Definition of Spans APPENDIX 1 Guide for Fatigue Strength Assessment General Note Applicability Loadings Effects of Corrosion Format of the Criteria Connections to be Considered for the Fatigue Strength Assessment General Guidance on Locations Permissible Stress Range Assumptions Criteria Long Term Stress Distribution Parameter, γ Permissible Stress Range Fatigue Inducing Loads and Determination of Total Stress Ranges General Wave-induced Loads Load Components Fatigue Assessment Zones and Controlling Load Combination Primary Stress f d Secondary Stress f d Additional Secondary Stresses f* d2 and Tertiary Stresses f d Resulting Stress Ranges Definitions Determination of Stress Concentration Factors (SCF s) General viii ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS. 2002

9 11.3 Sample Stress Concentration Factors (SCF s) Stress Concentration Factors Determined From Finite Element Analysis Introduction S-N Data S-N Data and SCF s Calculation of Hot Spot Stress for Fatigue Analysis of Ship Structures TABLE 1 Fatigue Classification for Structural Details TABLE 1A Coefficient, C TABLE 2 K s (SCF) Values FIGURE 1 Basic Design S-N Curves FIGURE 2 C n = C n (ψ) FIGURE 3 Cut-outs (Slots) For Longitudinal FIGURE 4 Fatigue Classification for Longitudinals in way of Flat Bar Stiffener FIGURE FIGURE FIGURE 7 Doublers and Non-load Carrying Members on Deck or Shell Plating FIGURE FIGURE FIGURE FIGURE APPENDIX 2 Calculation of Critical Buckling Stresses General Rectangular Plates Longitudinals and Stiffeners Axial Compression Torsional/Flexural Buckling Stiffened Panels Large Stiffened Panels Girders, Webs and Stiffened Brackets Critical Buckling Stresses of Web Plates and Large Brackets Effects of Cut-outs Tripping Stiffness and Proportions Stiffness of Longitudinals Stiffness of Web Stiffeners Stiffness of Supporting Members Proportions of Flanges and Face Plates Proportions of Webs of Longitudinals and Stiffeners ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS ix

10 TABLE 1 Buckling Coefficient, K i FIGURE 1 Net Dimensions and Properties of Stiffeners FIGURE x ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS. 2002

11 Section 1: Introduction SECTION 1 Introduction 1 General 1.1 Classification In accordance with 1-1-3/3 of the ABS Rules for Conditions of Classification (Part 1) and 5C-8-1/1 of the ABS Rules for Building and Classing Steel Vessels (the Rules), the classification notation À A1 Liquefied Gas Carrier, SH, SHCM is to be assigned to vessels designed for the carriage of liquefied gases, and built to the requirements of this Guide and other relevant sections of the Rules. 1.2 Optional Class Notation for Design Fatigue Life Vessels designed and built to the requirements in this Chapter are intended to have a structural fatigue life of not less than 20 years. Where a vessel s design calls for a fatigue life in excess of the minimum design fatigue life of 20 years, the optional class notation FL (year) will be assigned at the request of the applicant. This optional notation is eligible provided the excess design fatigue life is verified to be in compliance with the criteria in Appendix 1 of this Guide, Guide for Fatigue Strength Assessment. Only one design fatigue life value is published for the entire structural system. Where differing design fatigue life values are intended for different structural elements within the vessel, the (year) refers to the least of the varying target lives. The design fatigue life refers to the target value set by the applicant, not the value calculated in the analysis. The notation FL (year) denotes that the design fatigue life assessed according to Appendix 1 of this Guide is greater than the minimum design fatigue life of 20 years. The (year) refers to the fatigue life equal to 25 years or more (in 5-year increments) as specified by the applicant. The fatigue life will be identified in the Record by the notation FL (year); e.g., FL (30) if the minimum design fatigue life assessed is 30 years. 1.3 Application Size and Proportion The requirements contained in this Guide are applicable to membrane tank liquefied gas carriers intended for unrestricted service, having lengths of 150 meters (492 feet) or more, and having parameters within the range as specified in 3-2-1/1 of the Rules Vessel Type These requirements are intended to apply to steel vessels with machinery aft, engaged in the carriage of liquefied gases in membrane type tanks as defined in 5C-8-4/2.2 of the Rules. The technical requirements of the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code) are also to be followed Direct Calculations Direct calculations with respect to the determination of design loads and the establishment of alternative strength criteria based on first principles, will be accepted for consideration, provided that all the supporting data, analysis procedures and calculated results are fully documented and submitted for review. In this regard, due consideration is to be given to the environmental conditions, probability of occurrence, uncertainties in load and response predictions, and reliability of the structure in service. For long term prediction of wave loads, realistic wave spectra covering the North Atlantic Ocean and a probability level of 10-8 are to be employed. ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS

12 Section 1 Introduction SafeHull Construction Monitoring Program A Construction Monitoring Plan for critical areas, prepared in accordance with the requirements of Part 5C, Appendix 1 of the Rules, is to be submitted for approval prior to commencement of fabrication. See Part 5C, Appendix 1 SafeHull Construction Monitoring Program of the Rules. 1.5 Internal Members Section Properties of Structural Members (1 July 2008) The geometric properties of structural members may be calculated directly from the dimensions of the section and the associated effective plating (see 3-1-2/13.3 of the Rules or Section 4, Figure 3, as applicable). For structural members with angle θ (see Section 1, Figure 1) between web and associated plating not less than 75 degrees, the section modulus, web sectional area, and moment of inertia of the standard (θ = 90 degrees) section may be used without modification. Where the angle θ is less than 75 degrees, the sectional properties are to be directly calculated about an axis parallel to the associated plating (see Section 1, Figure 1). FIGURE 1 d w Standard θ = 90 d w θ For longitudinals, frames and stiffeners, the section modulus may be obtained by the following equation: SM = α θ SM 90 α θ = /θ SM 90 = section modulus at θ = 90 degrees 2 ABS GUIDE FOR BUILDING AND CLASSING MEMBRAE TANK LNG VESSELS. 2002

13 Section 1 Introduction The effective section area may be obtained from the following equation: A = A 90 sin θ A 90 = effective shear area at θ = 90 degrees Detailed Design The detail design of internals is to follow the guidance given in 3-1-2/15 of the Rules and in 4/1.5 of this Guide. See also Appendix 1, Guide for Fatigue Strength Assessment. 1.7 Breaks Special care is to be taken to provide structural reinforcements against local stresses at the ends of the cargo tank spaces, superstructures, etc., and throughout the structure in general. The main longitudinal bulkheads are to be suitably tapered at their ends. Where effective longitudinal bulkheads are provided in the poop or deckhouse, they are to be located such as to provide effective continuity between the structure in way of and beyond the main cargo spaces. 1.9 Variations LNG vessels of a special type or design, differing from those described in this Guide, will be specially considered on the basis of equivalent strength Loading Guidance Loading guidance is to be as required by 3-2-1/7 of the Rules except that Subsection 4/5 of this Guide will apply for allowable shear stresses Design Vapor Pressure The design vapor pressure p o as defined in 5C-8-4/2.6 of the Rules should not normally exceed 0.25 bar (0.255 kgf/cm 2, lbf/in 2 ). If, however, the hull scantlings are increased accordingly and consideration is given, appropriate, to the strength of the supporting insulation, p o may be increased to a higher value but less than 0.7 bar (0.714 kgf/cm 2, lbf/in 2 ) Protection of Structure For protection of the structure, see /5 of the Rules as appropriate Containment System The design, fabrication and installation of the LNG containment system is beyond the scope of this Guide. ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS

14 Section 2: Design Considerations and General Requirements SECTION 2 Design Considerations and General Requirements 1 General Requirements 1.1 General The strength requirements specified in this Guide are based on a net ship approach. In determining the required scantlings and performing structural analyses and strength assessments, the nominal design corrosion values given in Section 2, Table 1 are to be deducted Initial Scantling Requirements The initial thickness of plating, the section modulus of longitudinals/stiffeners, and the scantlings of the main supporting structures are to be determined in accordance with Section 4 for the net ship. These net ship values are to be used for further assessment as required in the following paragraph. The relevant nominal design corrosion values given in Section 2, Figure 1 and Section 2, Table 1 are then added to obtain the full scantling requirements. 1.5 Strength Assessment Failure Modes A total assessment of the structures, determined on the basis of the initial strength criteria in Section 4 is to be carried out against the following three failure modes: Material Yielding The calculated stress intensities are not to be greater than the yielding state limit given in 5/3.1 for all load cases specified in Subsection 3/ Buckling and Ultimate Strength For each individual member, plate or stiffened panel, the buckling and ultimate strength is to be in compliance with the requirements specified in Subsection 5/5. In addition, the hull girder ultimate strength is to be in accordance with 5/ Fatigue The fatigue strength of structural details and welded joints in highly stressed regions, is to be analyzed in accordance with Subsection 5/ Structural Redundancy and Residual Strength Consideration should be given to structural redundancy and hull girder residual strength in the early design stages. 3 Nominal Design Corrosion Values 3.1 General As indicated in 2/1.1, the strength criteria specified in this Guide are based on a net ship approach. The net thickness or scantlings correspond to the minimum strength requirements acceptable for classification, regardless of the design service life of the vessel. In addition to the coating protection specified in the Rules for all ballast tanks, nominal design corrosion values for plating and structural members as given in Section 2, Table 1 and Section 2, Figure 1 are to be added. These nominal design corrosion values are being introduced solely for the above purpose, and are not to be construed as renewal standards. 4 ABS GUIDE FOR BUILDING AND CLASSING MEMBRAE TANK LNG VESSELS. 2002

15 Section 2 Design Considerations and General Requirements In view of the anticipated higher corrosion rates for structural members in some regions, such as highly stressed areas, additional design margins should be considered for the primary and critical structural members to minimize repairs and maintenance costs. The beneficial effects of these design margins on reduction of stresses and increase of the effective hull girder section modulus can be appropriately accounted for in the design evaluation. FIGURE 1 Nominal Design Corrosion Values (23 August 2004) Longitudinals and Stiffeners: in Tank: Vertical Element: 1.0 mm (2.0 mm for Splash Zone* and within Double Bottom) Non Vertical Element: 2.0 mm in Pipe Duct Space: All Elements: 1.0 mm in Void Space outside Double Bottom: All Elements: 1.0 mm Trunk Deck Plate: 1.5 mm in Void Space Deck Transverse and Deck Girder: 1.0 mm in Void Space Upper Deck Plate: Watertight: 2.0 mm Weathertight: 1.5 mm Nontight: 1.5 mm Side Transverse: 1.5 mm in Tank (2.0 mm for Splash Zone) 1.0 mm in Void Space Side Shell Plate: 1.5 mm Inner Skin Bulkhead Plate: 1.0 mm Inner Deck Plate: 1.0 mm Transverse Bulkhead Plate: 1.0 mm in Cargo Tank Inner Bottom Plate: 1.0 mm (2.0 mm for Tank Top)** Transverse Bulkhead Plate (Wing): 1.5 mm in Ballast Tank 1.0 mm within Void Spaces Side Stringer: in Tank: WT: 2.0 mm NT: 1.5 mm in Void Space: WT: 1.5 mm NT: 1.0 mm Bottom Shell Plate: 1.0 mm Floor, Girders and Transverses: 2.0 mm in Tank 1.5 mm in Pipe Duct or Void Space including watertight bottom girder as duct keel tank boundary Webs on Cargo Transverse Bulkhead: a space between trans. bulkheads is ballast tank: Vertical Web: 1.5 mm (2.0 mm for Splash Zone) Horizontal Web: 2.0 mm a space between trans. bulkheads is void space: Vertical Web: 1.0 mm Horizontal Web: 1.5 mm * Splash Zone is 1.5 m below Tank Top. ** Tank Top is considered in case double bottom has the separate ballast tank with outboard watertight double bottom girders. ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS

16 Section 2 Design Considerations and General Requirements TABLE 1 Nominal Design Corrosion Values (1, 2) (23 August 2004) Nominal Design Corrosion Values in mm (in.) Structural Element/Location in Tank in Void Space Trunk Deck Plating N.A. 1.0 (0.04) Watertight 2.0 (0.08) Upper Deck Plating Weathertight 1.5 (0.06) Nontight 1.5 (0.06) Inner Deck Plating 1.0 (0.04) Side Shell Plating 1.5 (0.06) Bottom Plating 1.0 (0.04) Inner Bottom Plating 1.0 (0.04) (3) 1.0 (0.04) Longitudinal Bulkhead Plating 1.0 (0.04) Transverse Bulkhead Plating in Wing Spaces 1.5 (0.06) 1.0 (0.04) (8) in Cargo Tanks 1.0 (0.04) Deck Transverse and Deck Girder N.A. 1.0 (0.04) (8) Double Bottom Floor and Girder (9) 2.0 (0.08) 1.5 (0.06) (8) Side Transverse 1.5 (0.06) (4) 1.0 (0.04) Side Stringer Watertight 2.0 (0.08) 1.5 (0.06) (8) Nontight 1.5 (0.06) 1.0 (0.04) Webs on Cargo Transverse Bulkhead Vertical Web 1.5 (0.06) (4) 1.0 (0.04) (8) Horizontal Web 2.0 (0.08) 1.5 (0.06) (8) Longitudinals and Stiffeners Vertical Element (5) 1.0 (0.06) (7) 1.0 (0.04) Non Vertical Element (6) 2.0 (0.08) 1.0 (0.04) Longitudinals and Stiffeners within Pipe Duct Space N.A. 1.0 (0.04) Longitudinals and Stiffeners in Void Spaces outside Double Bottom N.A. 1.0 (0.04) Notes 1 It is recognized that corrosion depends on many factors including coating properties, cargo composition and temperature of carriage, and that actual wastage rates observed may be appreciably different from those given here. 2 Pitting and grooving are regarded as localized phenomena and are not covered in this table mm (0.08 in.) for tank top mm (0.08 in.) for Splash Zone (1.5 meters down from tank top). 5 Vertical elements are defined as elements sloped at an angle greater than 25 to the horizontal line. 6 Non vertical elements are defined as elements sloped at an angle less than 25 to the horizontal line mm (0.08 in.) for Splash Zone and within double bottom. 8 When plating forms a boundary between a tank and a void space, the plating NDCV is determined by the tank type. 9 (23 August 2004) 1.5 mm (0.06 in.) for duct keel tank boundary girder. 6 ABS GUIDE FOR BUILDING AND CLASSING MEMBRAE TANK LNG VESSELS. 2002

17 Section 3 : Load Criteria SECTION 3 Load Criteria 1 General 1.1 Load Components In the design of the hull structure, all load components with respect to the hull girder and local structure as specified in this Guide and Section of the Rules are to be taken into account. These include static loads in still water, wave-induced motions and loads, sloshing, slamming, dynamic, thermal and ice loads applicable. 3 Static Loads 3.1 Still-water Bending Moment For still-water bending moment calculations see 3-2-1/3.3 of the Rules. When a direct calculation of wave-induced loads [i.e., longitudinal bending moment and shear forces, hydrodynamic pressures (external) and inertia forces and added pressure heads (internal)] is not submitted, envelope curves of the still-water bending moments (hogging and sagging) and shear forces (positive and negative), are to be provided. Except for special loading cases, the loading patterns shown in Section 3, Figure 1 are to be considered in determining local static loads. 5 Wave-Induced Loads Where a direct calculation of the wave-induced loads is not available, the approximation equations given below may be used to calculate the design loads. When a direct calculation of the wave-induced loads is performed, envelope curves of the combined wave and still-water bending moments and shear forces, covering all the anticipated loading conditions, are to be submitted for review. ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS

18 Section 3 Load Criteria FIGURE 1 Loading Pattern LC 1 & 3 3/4 Full Draft LC 5 3/4 Full Draft LC 7 3/4 Full Draft LC 2 & 4 Full Draft LC 6 3/4 Full Draft LC 8 Full Draft Cargo Loaded Ballast Water Loaded For detailed loading information, see 3/Table Wave-Induced Longitudinal Bending Moments and Shear Forces Vertical Wave Bending Moments The vertical wave bending moments amidships may be obtained from the following equations: M w = k w M ws Wave Sagging Moment kn-m (tf-m, Ltf-ft) M w = k w M wh Wave Hogging Moment kn-m (tf-m, Ltf-ft) k w = 1.0 for the nominal wave bending moment and shear force in determining the hull girder section modulus in 4/3.1.1 and shearing strength in Subsection 4/5. = k s for strength formulation and assessment of local structural elements and members in Sections 4, 5 and 6, and Appendix 1. k s = k o 0.5 at locations aft of (0.35 k o 0.5 )L from the FP = 1.0 at locations forward of 0.35L from the FP = at intermediate locations, k s may be obtained by linear interpolation 8 ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS. 2002

19 Section 3 Load Criteria k o = V 0.47C b V = 0.75 V d, in knots, need not to be taken greater than 24 knots. V d = the design speed, in knots, with the vessel at the summer load waterline and running ahead at the maximum continuous rated shaft rpm. C b = the vessel s block coefficient as defined in 3-1-1/11.3 of the Rules M ws & M wh = wave sagging bending moment and wave hogging bending moment respectively as defined in 3-2-1/3.5.1 of the Rules Vertical Wave Shearing Forces The envelopes of the maximum wave induced shearing forces, F w, expressed in kn (tf, Ltf), may be obtained from the following equations: F w = k w F wp for positive shear force (upward front section) F w = k w F wn for negative shear force (downward front section) k w = as defined in 3/5.1.1 F wp, F wn = envelopes of maximum wave induced shearing forces as defined in 3-2-1/3.5.3 of the Rules in C b is not to be taken less than Horizontal Wave Bending Moments and Shear Forces Horizontal Wave Bending Moments The horizontal wave bending moments, positive (tension port) and negative (tension starboard), may be obtained from the following equation: M H = ± m h k s K 3 C 1 L 2 D (C b ) 10-4 kn-m (tf-m, Ltf-ft) m h = distribution factor obtained from Section 3, Figure 2 k s = as defined in 3/5.1.1 K 3 = 840 (85.656, 7.832) 300 L C 1 = L 300 m = < L 350 m = L 350 L 500 m L C 1 = L 984 ft = < L < 1148 ft = L 1148 L 1640 ft 492 L = scantling length of vessel, in m (ft), as defined in 3-1-1/3.1 of the Rules D = depth of vessel, in m (ft), as defined in 3-1-1/7.1 of the Rules C b = block coefficient, as defined in 3-1-1/11.3 of the Rules, but is not to be taken less than 0.6 ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS

20 Section 3 Load Criteria Horizontal Wave Shear Force The envelope of horizontal wave shearing force, F H, expressed in kn (tf, Ltf), positive (toward port forward) or negative (toward starboard aft), may be obtained from the following equation: F H = ± f h k s k C 1 LD (C b + 0.7) 10-3 kn (tf, Ltf) f h = distribution factor, as given in Section 3, Figure External Pressures k = 360 (36.71, 3.356) k s,c 1, L, D and C b are as defined in 3/5.3.1 above Pressure Distribution The external pressures, p e, (positive toward inboard), imposed on the hull in seaways can be expressed by the following equation at a given location: p e = ρg (h s + k u h de ) 0 N/cm 2 (kgf/cm 2, lbf/in 2 ) ρg = specific weight of sea water = N/cm 2 -m ( kgf/cm 2 -m, lbf/in 2 -ft) h s = hydrostatic pressure head in still water, in m (ft) k u = load factor, and may be taken as unity unless otherwise specified. h de = hydrodynamic pressure head induced by the wave and may be calculated as follows: = k c h di m (ft) k c = correlation factor for a specific combined load case, as given in 3/7.3.1 and Subsection 3/9 h di = hydrodynamic pressure head at location i (i = 1, 2, 3, 4 or 5; see Section 3, Figure 4) = k α i h do m (ft) k = distribution factor along the length of the vessel = 1 + (k o 1) cos µ, k o is as given in Section 3, Figure 5 = 1.0 amidships α i = distribution factor around the girth of vessel at location i. = cos µ for i = 1, at WL, starboard = cos µ for i = 2, at bilge, starboard = sin µ for i = 3, at bottom centerline = 2α 3 α 2 for i = 4, at bilge, port = sin µ for i = 5, at WL, port α i at intermediate locations of i may be obtained by linear interpolation. µ = wave heading angle, to be taken from 0 to 90 (0 for head sea, 90 for beam sea for wave coming from starboard) h do = 1.36 k s k C 1 m (ft) 10 ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS. 2002

21 Section 3 Load Criteria k s = as defined in 3/5.3.1 k = 1 (1, 3.281) C 1 = as defined in 3/5.3.1 The distribution of the total external pressure including static and hydrodynamic pressure is illustrated in Section 3, Figure Extreme Pressures In determining the required scantlings of local structural members, the extreme external pressure, p e, to be used, is as defined in 3/5.5.1 with k u as given in Subsections 3/7 and 3/ Simultaneous Pressures When performing 3D structural analysis, the simultaneous pressure along any portion of the hull girder may be obtained from: p es = ρ g(h s + k f k u h de ) 0 N/cm 2 (kgf/cm 2, lbf/in 2 ) k f = factor denoting the phase relationship between the reference station and adjacent stations considered along the vessel s length, and may be determined as follows: = k fo { 1 [ 1 cos 2π (x xo ) ] cos µ} L k fo = ±1.0, as specified in Section 3, Table 1. x = distance from A.P. to the station considered, in m (ft) x o = distance from A.P. to the reference station *, in m (ft). L = scantling length of vessel, in m (ft), as defined in 3-1-1/3.1 of the Rules µ = wave heading angle, to be taken from 0 to 90 ρ g, h s, k u and h de are as defined in 3/5.5.1 The simultaneous pressure distribution around the girth of the vessel is to be determined based on the wave heading angles specified in Subsections 3/7 and 3/9. * The reference station is the point along the vessel s length the wave trough or crest is located and may be taken as the mid-point of the mid-hold of the three hold model. 5.7 Internal Pressures Inertia Forces and Added Pressure Heads Ship Motions and Accelerations To determine the inertial forces and added pressure heads for a completely filled cargo or ballast tank, the dominating ship motions, pitch and roll, and the resultant accelerations induced by the wave are required. When a direct calculation is not available, the equations given below may be used (a) Pitch. The pitch amplitude (positive bow up): φ = k 1 (V/C b ) 1/4 /L degrees, but need not to be taken greater than 10 The pitch natural period: T p = k 2 C b d i seconds k 1 = 1030 (3380) for L in m (ft) k 2 = 3.5 (1.9323) for d i in m (ft) ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS

22 Section 3 Load Criteria V = 75% of the design speed V d, in knots. V is not to be taken less than 10 knots. V d is defined in 3/5.1.1 d i = draft amidships for the relevant loading conditions L and C b are as defined in 3/ (b) Roll. The roll amplitude (positive starboard down): θ = C R (35 k θ C di /1000) θ = C R (35 k θ C di /1000) ( T r ) θ = C R (35 k θ C di /1000) ( T r ) if T r > 20 seconds if 12.5 T r 20 seconds if T r 12.5 seconds θ in degrees, but need not to be taken greater than 30 k θ = (0.05, 0.051) C R = V C di = 1.06 (d i /d f ) 0.06 V = 75% of the design speed V d, in knots. V is not to be taken less than 10 knots. V d is defined in 3/5.1.1 d i = draft amidships for the relevant loading conditions, m (ft) d f = draft, in m (ft), as defined in 3-1-1/9 of the Rules = k d LBd f C b kn (tf, Ltf) k d = (1.025, ) C b = block coefficient, as defined in 3-1-1/11.3 of the Rules, but is not to be taken less than 0.6 L and B are the length and breadth of the vessel respectively, as defined in Section of the Rules. The roll natural motion period: T r = k 4 k r /GM 0.5 seconds k 4 = 2 (1.104) k r = roll radius of gyration, in m (ft), and may be taken as 0.35B for full load conditions and 0.45B for ballast conditions. GM = metacentric height, in m (ft), to be taken as: GM (full) = = GM (full) for full draft = 1.5 GM (full) for 3/4d f = 2.0 GM (full) for 2/3d f = 0.12B metacentric height for fully loaded condition 12 ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS. 2002

23 Section 3 Load Criteria (c) Accelerations. The vertical, longitudinal and transverse accelerations, a v, a and a t, of tank contents (cargo or ballast) may be obtained from the following formulae: a v = C v k v a o g m/sec 2 (ft/sec 2 ) positive downward a = C k a o g m/sec 2 (ft/sec 2 ) a t = C t k t a o g m/sec 2 (ft/sec 2 ) positive forward positive starboard a o = k o (2.4/L 1/2 + 34/L 600/L 2 ) for L in m = k o (4.347/L 1/ /L 6458/L 2 ) for L in ft k o = V 0.47C b C v = cos µ + ( z/b) (sin µ)/k v µ = wave heading angle in degrees, 0 for head sea, and 90 for beam sea for wave coming from starboard k v = [ (5.3 45/L) 2 (x/l 0.45) 2 ] 1/2 for L in m = [ ( /L) 2 (x/l 0.45) 2 ] 1/2 for L in ft C = (L 200) 10-4 for L in m = (L 656) 10-4 for L in ft k = y/L C t = 1.27[ (x/L 0.45) 2 ] 1/2 k t = y/b x = longitudinal distance from the A.P. to the station considered, in m (ft) y = vertical distance from the waterline to the point considered, in m (ft), positive upward z = transverse distance from the centerline to the point considered, in m (ft), positive starboard g = acceleration of gravity = 9.8 m/sec 2 (32.2 ft/sec 2 ) L and B are the length and breadth of the vessel respectively, as defined in Section of the Rules, in m (ft) Internal Pressures 5.7.2(a) Distribution of Internal Pressures. The internal pressure, p i (positive toward tank boundaries) for a completely filled tank may be obtained from the following formula: p i = ρ g (η + η + k u h d ) + p o 0 N/cm 2 (kgf/cm 2, lbf/in 2 ) p o = (p vp p n ) 0 in cargo tank, N/cm 2 (kgf/cm 2, lbf/in 2 ) = 0 in ballast tank p vp = pressure setting on pressure/vacuum relief valve 6.90 N/cm 2 (0.71 kgf/cm 2, lbf/in 2 ) for integral gravity tanks p n = 2.50 N/cm 2 (0.255 kgf/cm 2, lbf/in 2 ) ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS

24 Section 3 Load Criteria ρg = specific weight of the liquid, not to be taken less than 0.49 N/cm 2 -m (0.05 kgf/cm 2 -m, lbf/in 2 -ft) for the fluid cargoes and not less than N/cm 2 -m ( kgf/cm 2 -m, lbf/in 2 -ft) for ballast water. η = local coordinate in vertical direction for tank boundaries measuring from the top of the tanks, as shown in Section 3, Figure 7b, in m (ft). η = 0 for the cargo tank and the ballast tank whose tank top extends to the upper deck or the trunk deck = a distance equivalent to 2 / 3 of the distance from tank top to the top of the overflow (The exposed height is minimum 760 mm above freeboard deck or 450 mm above superstructure deck.) for the lower tank whose tank top does not extend to the upper deck. k u = load factor and may be taken as unity unless otherwise specified h d = wave-induced internal pressure head, including inertial force and added pressure head. = k c (ηa i /g + h i ), in m (ft) k c = correlation factor and may be taken as unity unless otherwise specified a i = effective resultant acceleration at the point considered, and may be approximated by: C dp is specified in 3/5.7.2(d). a v, a and a t are as given in 3/5.7.1(c). 0.71C dp [w v a v + w ( /h) a + w t (b/h)a t ] m/sec 2 (ft/sec 2 ) w v, w and w t are weighted coefficients, showing directions, as specified in Section 3, Tables 1 and 3. h i = added pressure head due to pitch and roll motions at the point considered, in m (ft), may be calculated as follows: In general, the added head may be calculated based on the vertical distance from the reference point of the tank to the point considered. The reference point is (1) the highest point of the tank boundary after roll and pitch, or (2) the average height of the points, after roll and pitch, which are η above the top of the tank at the overflow, whichever is greater. For prismatic tanks on starboard side, whose tank top extends to the upper deck or the trunk deck, added pressure head may be calculated as follows: i) For bow down and starboard down (φ e < 0, θ e > 0) h i = ξ sin( φ e ) + C ru (ζ e sin θ e cos φ e + η e cos θ e cos φ e η) ζ e = b ζ η e = η ii) For bow up and starboard up (φ e > 0, θ e < 0) h i = ( ξ) sin φ e + C ru (ζ e sin(-θ e ) cos φ e + η e cos θ e cos φ e η) ζ e = ζ δ b η e = η δ h ξ, ζ, η are the local coordinates, in m (ft), for the point considered with respect to the origin in Section 3, Figure 7b. C ru is specified in 3/5.7.2(d). 14 ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS. 2002

25 Section 3 Load Criteria δ b and δ h are the local coordinate adjustments, in m (ft), for the point considered with respect to the origin shown in Section 3, Figure 7b. θ e = 0.71 C θ θ φ e = 0.71 C φ φ = length of the tank, in m (ft) h = depth of the tank, in m (ft) b = breadth of the tank considered, in m (ft) φ and θ are pitch and roll amplitudes, as given in 3/5.7.1(a) and 3/5.7.1(b). C φ and C θ are weighted coefficients, showing directions as given in Section 3, Tables 1 and 3. For prismatic lower tanks on starboard side, whose tank top does not extend to the upper deck or the trunk deck, the added pressure head may be calculated as follows assuming the reference point based on the average height of the overflow. i) For bow down and starboard down (φ e < 0, θ e > 0) h i = (ξ /2) sin (-φ e ) + C ru (ζ e sin θ e cos φ e + η e cos θ e cos φ e η e ) ζ e = b a ζ η e = η + η ii) For bow up and starboard up (φ e > 0, θ e < 0) h i = ( /2 ξ) sin φ e + C ru {ζ e sin(-θ e ) cos φ e + η e cos θ e cos φ e η e } ζ e = ζ b a η e = η + η b a is the transverse distance of over flow from ξ axis. All other parameters are as defined above (b) Extreme Internal Pressure. For assessing local structures at a tank boundary, the extreme internal pressure with k u as specified in Subsection 3/7 is to be considered (c) Simultaneous Internal Pressures. In performing a 3D structural analysis except for L.C 9, 10 and 11, the internal pressures may be calculated in accordance with 3/5.7.2(a) and 3/5.7.2(b) above for tanks in the mid-body. For tanks in the fore or aft body, the pressures should be determined based on linear distributions of accelerations and ship motions along the length of the vessel (d) Definition of Tank Shape and Associated Coefficients i) Rectangular Tank. The following tank is considered as a rectangular tank: b/b or h/h b = extreme breadth of the tank considered b 1 = least breadth of wing tank part of the tank considered h = extreme height of the tank considered h 1 = least height of double bottom part of the tank considered as shown in Section 3, Figure 7a ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS

26 Section 3 Load Criteria The coefficients C dp and C ru of the tank are as follows: C dp = 1.0 C ru = 1.0 ii) J-shaped Tank. A tank having the following configurations is considered as a J-shaped tank. b/b and h/h The coefficients C dp and C ru are as follows: C dp = 0.7 C ru = 1.0 iii) In the case the minimum tank ratio of b/b 1 or h/h 1, whichever is lesser, is greater than 3.0 but less than 5.0, the coefficient C dp of the tank is to be determined by the following interpolation: An intermediate tank between rectangular and J-shaped tank: C dp = (the lesser of b/b 1 or h/h 1-3.0) C ru = ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS. 2002

27 Section 3 Load Criteria FIGURE 2 Distribution Factor m h 1.0 Distribution m h Aft end of L Distance from the aft end of L in terms of L 1.0 Forward end of L FIGURE 3 Distribution Factor f h 1.0 Distribution f h Aft end of L Forward end of L Distance from the aft end of L in terms of L ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS

28 Section 3 Load Criteria FIGURE 4 Distribution of h di h h d5 h d1 W.L. h or h d1 whichever is lesser Port Side h d4 h d3 h d2 Starboard Side FIGURE 5 Pressure Distribution Function k o 2.5 Distribution of k o Aft end of L Forward end of L Distance from the aft end of L in terms of L 18 ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS. 2002

29 Section 3 Load Criteria FIGURE 6 Illustration of Determining Total External Pressure h h d1 h or h d1 whichever is lesser h d h s : Hydrodynamic Pressure Head : Hydrostatic Pressure Head in Still Water : Total External Pressure Head (indicates negative) FIGURE 7A Tank Shape Parameters b 1 h h 1 b ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS

30 Section 3 Load Criteria FIGURE 7B Definition of Tank Geometry F.P. ξ δ b ξ δh ζ O ζ b O h LC B/2 a. Plan View LC B/2 b ζ δ b O δh η b. Elevation η 20 ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS. 2002

31 Section 3 Load Criteria FIGURE 8 Location of Tank for Nominal Pressure Calculation Hold or Tank AP 0.4L FP ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS

32 Section 3 Load Criteria TABLE 1A Combined Load Cases* for Total Strength and Fatigue Assessment L.C. 1 L.C. 2 L.C. 3 L.C. 4 L.C. 5 L.C. 6 L.C. 7 L.C. 8 A. Hull Girder Loads (See Subsection 3/5)** Vertical B.M. Sag ( ) Hog (+) Sag ( ) Hog (+) Sag ( ) Hog (+) Sag ( ) Hog (+) k c Vertical S.F. (+) ( ) (+) ( ) (+) ( ) (+) ( ) k c Horizontal B.M. ( ) (+) ( ) (+) k c Horizontal S.F. (+) ( ) (+) ( ) k c B. External Pressure (See 3/5.5) k c k f C. Internal Tank Pressure (See 3/5.7) k c w v w Fwd Bhd 0.25 Aft Bhd Fwd Bhd Aft Bhd 0.25 Fwd Bhd 0.25 Aft Bhd Fwd Bhd Aft Bhd 0.25 w t Port Bhd Stbd Bhd 0.75 Fwd Bhd 0.2 Aft Bhd -0.2 Port Bhd 0.75 Stbd Bhd Port Bhd -0.4 Stbd Bhd 0.4 Fwd Bhd -0.2 Aft Bhd 0.2 Port Bhd 0.4 Stbd Bhd -0.4 C φ, Pitch C θ, Roll D. Reference Wave Heading and Motion of Vessel Heading Angle Heave Down Up Down Up Down Up Down Up Pitch Bow Down Bow Up Bow Down Bow Up Bow Down Bow Up Roll Stbd Down Stbd Up Stbd Down Stbd Up Notes: * k u = 1.0 for all load components. ** Boundary forces should be applied to produce the above specified hull girder bending moment at the middle of the structural model and the specified hull girder shear force at one end of the middle hold of the model. The sign convention for the shear force corresponds to the forward end of the middle hold. 22 ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS. 2002

33 Section 3 Load Criteria TABLE 1B Additional IGC Load Cases* for Initial Scantling Evaluation of Main Supporting Members by FEA (1 July 2008) L.C. 3 IGC L.C. 5 IGC L.C. 7 IGC A. Hull Girder Loads (See Subsection 3/5)** Vertical B.M. Sag ( ) Sag ( ) Sag ( ) k c Vertical S.F. (+) (+) (+) k c Horizontal B.M. ( ) ( ) k c Horizontal S.F. (+) (+) k c B. External Pressure (See 3/5.5) k c k f C. Internal Tank Pressure (See 5C-8-4/3.2.2 of the Rules) Vertical component of acceleration Longitudinal component of acceleration Transverse component of acceleration D. Reference Wave Heading and Motion of Vessel Heading Angle Heave Down Down Down Pitch Bow Down Bow Down Roll Stbd Down Stbd Down Notes: * k u = 1.0 for all load components. ** Boundary forces should be applied to produce the above specified hull girder bending moment at the middle of the structural model and the specified hull girder shear force at one end of the middle hold of the model. The sign convention for the shear force corresponds to the forward end of the middle hold. Note L.C. 3 IGC, L.C. 5 IGC and L.C. 7 IGC follow the corresponding loading patterns in Section 3, Figure 1 L.C. 3 IGC: maximum internal pressure on inner bottom L.C. 5 IGC: maximum internal pressure on Stbd Bhd L.C. 7 IGC: maximum internal pressure on Stbd Bhd ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS

34 Section 3 Load Criteria V.B.M. [H.B.M. Hull Girder Loads* V.S.F. H.S.F. k u, k u, TABLE 2 Load Cases for Sloshing External Pressures Sloshing Pressures** k c k c ] k u k c k fo k u k c Reference Wave Heading and Motions Heading Angle Heave Pitch Roll LC S - 1 ( ) (+) Down Bow Down [( ) (+) ] Stbd Down LC S - 2 (+) ( ) Up Bow Up Stbd Up [(+) ( ) ] Notes: * For determining the total vertical bending moment for the above two load cases, 70% of the maximum designed still water bending moment may be used at the specified wave vertical bending moment station. : V.B.M. is vertical wave bending moment V.S.F. is vertical wave shear force H.B.M. is horizontal wave bending moment H.S.F. is horizontal wave shear force ** The vertical distribution of the sloshing pressure head is shown in Section 3, Figures 10 and ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS. 2002

35 Section 3 Load Criteria A. Plating & Longitudinals/Stiffeners. Structural Members/ Components 1. Bottom Plating & Long l 2. Inner Bottom Plating & Long l 3. Side Shell Plating & Long l 4. * Deck Plating & Long l (Cargo Tank) 5. Deck Plating & Long l (Ballast Tank) 6. * Inner Skin Long l Bhd. Plating & Long l 7. * Watertight Double Bottom Girder 8. * Watertight Side Stringer 9. * Trans. Bhd. Plating & Stiffener (Cargo Tank) 10. * Trans. Bhd. Plating & Stiffener (Ballast Tank) * See note 4 TABLE 3 Design Pressure for Local and Supporting Members The nominal pressure, p = p i p e, is to be determined from load cases a & b below, whichever is greater, with k u = 1.10 and k c = 1.0 unless otherwise specified in the table Case a - At fwd end of the tank Case b - At mid tank or fwd end of tank Draft/Wave Heading Angle 3/4 design draft/0 3/4 design draft/0 3/4 design draft/60 Design draft/ 0 3/4 design draft/0 3/4 design draft/ 60 3/4 Design draft/ 60 3/4 Design draft/ 0 design draft/ 0 3/4 design draft/0 Location and Loading Pattern Coefficients Draft/Wave Coefficients Heading Location and p i p e Angle Loading Pattern p i p e Full ballast tank A i A e design draft/ 0 Full ballast tank, cargo tanks empty Starboard side of full ballast tank Full cargo tank D i Full ballast tank D i Starboard side of full cargo tank, ballast tank empty Starboard side of full double bottom tank or ballast tank, adjacent space empty Full upper tank, adjacent lower tank empty Fwd. bhd. of full cargo tank Fwd. bhd. of full ballast tank, adjacent tanks empty A i design draft/ 0 B i A e design draft/ 60 B i 3/4 design draft/60 B i A i 3/4 design draft/ 0 A i A i Mid tank of empty ballast tanks Fwd end of full cargo tank, ballast tanks empty Mid tank of empty ballast tanks Fwd end and starboard side of full ballast tank, cargo tank empty Fwd end of Full lower tank, adjacent upper tank empty B e A i B e B i D i ABS GUIDE FOR BUILDING AND CLASSING MEMBRANE TANK LNG VESSELS