5 A PPLICATION OF P RINCIPLES
5.4 Load-capacity Based Requirements .1 General
5.4.1.1 In general, the Working Stress Design (WSD) method is applied in the requirements, except for the hull girder ultimate strength criteria where the Partial safety Factor (PF) method is applied. The partial safety factor format is applied for this highly critical failure mode to better account for uncertainties related to static loads, dynamic loads and capacity formulations.
5.4.1.2 The identified load scenarios are addressed by the Rules in terms of design loads, design format and acceptance criteria set, as given in Table 2.5.3. The table is schematic and only intended to give an overview.
5.4.1.3 The load scenarios addressed by the rules cover operations such as seagoing conditions, loading and unloading, tank testing conditions, ballast water exchange situations, special operations in harbour (e.g. propeller inspection afloat condition) and accidental flooding.
5.4.1.4 The design load combinations that represent the identified load scenarios are given in Section 7/6 and are denoted by S (static loads), S+D (static+dynamic loads), and A (accidental loads). In addition, the Rules address impact loads and sloshing loads as given in Section 7/4 and fatigue loads as given in Section 7/3.
5.4.1.5 For the strength requirements, the considered loads cover the most severe operational loads that occur, hence the cargo tank finite element analysis and load-capacity based scantling requirements are based on rule loading conditions which simulate the worst possible loading conditions within the operating limits of the vessel.
5.4.1.6 For the fatigue requirements the considered loads cover an expected load history and representative loading conditions covering the ships’ intended service are applied.
5.4.1.7 The acceptance criteria are categorised into three acceptance criteria sets. These are explained below and shown in Tables 2.5.2 and 2.5.3. The specific acceptance criteria set that is applied in the WSD rule requirements is dependent on the probability level of the characteristic combined load.
5.4.1.8 The acceptance criteria set AC1 is applied when the combined characteristic loads are frequently occurring, typically for the static design load combinations, but also applied for the sloshing design loads. This means that the loads occur on a frequent
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or regular basis. The allowable stress for a frequent load is lower than for an extreme load to take into account effects of:
(a) repeated yield
(b) allowance for some dynamics (c) margins for operational mistakes.
5.4.1.9 The acceptance criteria set AC2 is typically applied when the combined characteristic loads are extreme values, e.g. typically for the static+dynamic design load combinations. High utilisation (ηi in Table 2.5.1) of the structural capacity (Ri in Table 2.5.1) is allowed in such cases because the considered loads are extreme loads with a low probability of occurrence.
5.4.1.10 The acceptance criteria set AC3 is typically applied for capacity formulations based on the plastic collapse models such as those that are applied to address bottom slamming and bow impact loads.
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Table 2.5.1
Load Scenarios and Corresponding Rule Requirements
Load Scenarios Rule Requirements
Design Load
(that the vessel is exposed to and is to withstand)
Ref. Static and dynamic loads
in heavy weather 1 S + D
2. γS SG + γD DG ≤ R2/ γR2 AC2 Impact loads in heavy
weather 2 Impact SL + Dimp ≤ η3 Rp AC3
Static and dynamic loads
in heavy weather 5 S + D SG+SL+ DG + DL ≤ η2R1 AC2
Harbour and sheltered operations Loading, in harbour, e.g. propeller inspection afloat or dry-docking loading conditions
8 S SG+ SL ≤ η1 R1 AC1
Accidental condition
for water tight boundaries
1. SL ≤ η2 R1 AC2
9 A for collision bulkhead 2. SL ≤ η1 R1
AC1 Note
1. The symbols defined in this column are defined in the text of 5.4 Where:
cumulative fatigue damage ratio static global load
static local load structural capacity
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5.4.2 Design loads for scantling requirements and strength assessment (FEM)
5.4.2.1 The structural assessment of compartment boundaries, e.g. bulkheads, is based onthe worst possible loading, hence conditions are assessed with a full tank on one side and an empty tank on the other side. The situation with the tank content reversed is also considered. Similarly the shell envelope is assessed for conditions at the deepest draught without internal filling and at the lowest draught with internal filling.
5.4.2.2 The standard loading patterns to be used in the strength assessment (FEM) are given in Appendix B, Tables B.2.3 and B.2.4 for tankers with two oil-tight longitudinal bulkheads and one centreline oil-tight longitudinal bulkhead respectively. The corresponding information for the scantling requirements is given in Section 8.
5.4.2.3 To ensure consistency of approach, standardised rule values for parameters such as GM, Rroll,Tsc and Cb. are applied to calculate the rule load values.
5.4.2.4 The probability level of the dynamic global and local loads (DG, DL and Dimp in Table 2.5.1) is 10-8 and are derived using the long term statistical approach specified in 4.2.6.2.
5.4.2.5 The probability level of the sloshing loads (Dslh in Table 2.5.1) is 10-4 which is a load that occurs frequently.
5.4.2.6 The design load combinations corresponding to the identified load scenarios produce realistic design load sets suitable for the design and verification of the structural capability. Design load sets apply all the applicable simultaneously acting static and dynamic local load components (SL and DL in Table 2.5.1, which are usually pressure load components) and static and dynamic global load components (SG and DG in Table 2.5.1, which is usually hull girder bending moment) for the design of a particular or group of structural members. The relevant design load sets for the scantling requirements are given in Sections 8/2 to 8/5. The design load sets for the Finite Element analysis are referred to as load cases and are given in Appendix B.
5.4.2.7 The simultaneously occurring dynamic loads are specified by applying a dynamic load combination factor to the envelope dynamic load values given in Section 7/3.
The dynamic load combination factors that define the dynamic load cases are given in Section 7/6.4 for the structural strength assessment (FE) and in Section 7/6.5 for the scantling requirements.
5.4.2.8 The dynamic load combination factors have been derived using the equivalent design wave approach to provide realistic simultaneously occurring dynamic loads components suitable for structural assessment.
5.4.2.9 For the determination of design loads for the hull girder ultimate strength requirement given in Section 9/1, the operational loads (i.e. ship loading conditions) and the environmental loads (i.e. hull girder wave bending moments) are maximised for sagging conditions for seagoing conditions. The characteristic value for the still water hull girder sagging bending moments Msw is based on the maximum value from the seagoing conditions specified in Section 8/1. The characteristic value for the wave hull girder sagging bending moments MWV is given in Section 7/3.
5.4.3 Design loads for fatigue requirements
5.4.3.1 For the fatigue requirements given in Section 9/3 and Appendix C, the load assessment is based on the expected load history and an average approach is
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applied. The expected load history for the design life is characterised by the 10-4 probability level of the dynamic load value, the load history for each structural member is represented by Weibull probability distributions of the corresponding stresses.
5.4.3.2 The considered wave-induced loads include:
(a) hull girder loads (i.e., vertical and horizontal bending moments) (b) dynamic wave pressures
(c) dynamic tank pressures.
5.4.3.3 The fatigue analysis is calculated for two representative loading conditions covering the ship’s intended operation. These two conditions are:
(a) full load homogeneous conditions at design draught (b) normal ballast condition.
The proportion of the ship’s sailing life in the full load condition is 50% and in ballast 50%. It is assumed that 15% of the ships’ life is in harbour/sheltered water. It is consequently assumed that the ship will be sailing in open waters in full load condition for 42.5% of the ship’s life and in the ballast condition for 42.5% of the ship’s life.
5.4.3.4 The load values are based on actual parameters corresponding to the applied loading conditions, e.g. GM, Cb, etc., and the applicable draughts at amidships is used. The actual values are taken from specified loading conditions in the loading manual.
5.4.3.5 The simultaneously occurring dynamic loads are accounted for by combination of stresses due to the various dynamic load components. The stress combination procedure is given in Appendix C.
5.4.3.6 Still water loads and static sea and tank pressures from the actual loading conditions are used to determine the mean stress effect.
5.4.4 Structural response analysis
5.4.4.1 In general, the following approaches are applied for determination of the structural response to the applied design load combinations
(a) Beam theory
• used for prescriptive requirements (b) FE analysis
• coarse mesh for cargo hold model
• fine mesh for local models
• very fine mesh for fatigue assessment
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5.4.5 Structural capacity assessment
5.4.5.1 The considered failure modes in the Rules are yield (plastic deformation), buckling, brittle fracture and fatigue. Structural failure due to yield and buckling is primarily controlled by the strength requirements, brittle fracture is primarily controlled by the requirements for material selection and welding, and fatigue failure is primarily controlled by the high cycle fatigue requirements.
5.4.5.2 Generally, the capacity models applied in the prescriptive rules, i.e., the scantling requirements in Section 8, are based on simple beam theory and include elastic yield and plastic capacity models. The buckling capacity is assessed using simplified buckling capacity models or by a more theoretical non-linear analysis procedure.
5.4.5.3 The design verification requirements are based on a linear elastic finite element analysis, a detailed prescriptive fatigue assessment procedure and a simplified ultimate strength assessment procedure. There is also a finite element based fatigue assessment procedure for some structural members, such as the hopper knuckle.
5.4.5.4 The application of the net thickness approach to assess the structural capacity is specified in Section 6/3.3.
5.4.6 Acceptance criteria
5.4.6.1 The acceptance criteria applied in the working stress design requirements are given as acceptance criteria sets shown in Tables 2.5.2 and 2.5.3. There are slight variations within each set depending on the relative contribution of local and global loads, static and dynamic loads and the structural member being considered. The specific acceptance criteria are given in the detailed rule requirements in Section 8 and 9/2.
Table 2.5.2
Principal Acceptance Criteria - Rule Requirements Plate panels and Local
Support Members Primary Support
Members Hull girder members Acceptance
criteria set Yield Buckling Yield Buckling Yield Buckling
AC1:
proportions NA NA
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Table 2.5.3
Principal Acceptance Criteria - Design Verification - FE Analysis
Global cargo tank analysis Local fine mesh analysis Acceptance
criteria set Yield Buckling Yield
AC1: 60-80% of yield stress
Control of stiffness and proportions.
Usage factor typically 0.8
local mesh as 136% of yield stress
averaged stresses as global analysis
AC2: 80-100% of yield stress
Control of stiffness and proportions.
Usage factor typically 1.0
local mesh as 170% of yield stress
averaged stresses as global analysis
5.4.6.2 The purpose of applying different sets is to achieve a consistent and acceptable safety level for all combinations of static and dynamic loads and to account for different capacity models.