Section 3 Extreme Strength Assessment, ESA
Clasification Society 2024 - Version 9.40
Clasifications Register Rules and Regulations - Rules and Regulations for the Classification of Naval Ships, January 2023 - Volume 1 Ship Structures - Part 6 Hull Construction in Steel - Chapter 4 Hull Girder Strength - Section 3 Extreme Strength Assessment, ESA

Section 3 Extreme Strength Assessment, ESA

Parent topic: Chapter 4 Hull Girder Strength

3.1 General

3.1.1 The extreme hull girder strength of the ship is to be adequate to withstand wave loads that have a very low probability of occurring during the ship’s service life at sea.

3.1.2 The Owner may specify that an extreme strength assessment is not required. In this case, the extreme strength ESA1 or ESA2 notation and all other notations which require the extreme strength notation will not be assigned.

3.1.3 The extreme strength capability of the ship to survive severe sea conditions can be assessed using fairly simple or more advanced analysis procedures. The level of complexity and thoroughness of the analysis procedure is reflected in the extreme strength assessment notation level assigned.

3.1.4 Two assessment levels are available for the extreme hull girder strength assessment notation. These are summarised below:

ESA1 – Level 1

  • Extreme hull girder strength considered at a minimum of three longitudinal locations.
  • Capability assessed using elastic analysis, limiting stress criteria and buckling factors of safety.

ESA2 – Level 2

  • Extreme hull girder strength considered at all critical longitudinal locations.
  • Capability assessed using elasto-plastic ultimate strength methods to determine extreme hull girder strength.

3.1.5 For the ESA1 assessment, the extreme hull girder strength is to be verified using elastic theory, based on the actual section moduli and determination of the buckling strength to resist the global hull girder loads. The assessment criteria are given in Vol 1, Pt 6, Ch 4, 3.3 Bending strength – Simplified assessment method ESA1 and Vol 1, Pt 6, Ch 4, 3.4 Shear strength – Simplified assessment method ESA1 Alternatively the strength may be verified using ultimate strength methods.

3.1.6 For the ESA2 assessment, the extreme hull girder strength is to be verified using ultimate strength methods based on non linear stress strain curves which include the stress strain relationship in the post buckling phase. The assessment criteria are given in Vol 1, Pt 6, Ch 4, 3.5 Bending and shear strength – Ultimate strength analysis method ESA2

3.2 Determination of critical sections

3.2.1 A critical section is defined as a transverse cross-section of the hull where the hull girder bending or shear section structural properties are lowest. Typically there will be critical sections in way of machinery spaces, large deck openings and at the ends of superstructure blocks.

3.2.2 The effective geometric properties of critical sections are to be calculated in accordance with Vol 1, Pt 6, Ch 4, 2.2 Bending strength 2.2.1 and Vol 1, Pt 6, Ch 4, 1.4 Calculation of hull section modulus In defining the longitudinal position of each critical section, the shadow areas specified in Vol 1, Pt 6, Ch 4, 1.4 Calculation of hull section modulus 1.4.15 are to be considered together with the proximity of other openings, see Vol 1, Pt 6, Ch 4, 1.4 Calculation of hull section modulus 1.4.17.

3.2.3 For the ESA1 assessment, critical sections are to be considered at approximately the longitudinal positions L R/4, L R/2 and 3L R/4. Other longitudinal positions may also need to be considered depending on the structural arrangement of the ship.

3.2.4 For the ESA2 assessment, critical sections are to be considered at all positions along the length. Typically it is expected that the local critical transverse section between adjacent main watertight bulkheads will be evaluated.

3.3 Bending strength – Simplified assessment method ESA1

3.3.1 If the simplified analysis method is adopted for extreme strength assessment, the longitudinal strength of the ship at each critical section is to satisfy the following criteria for the hogging and sagging conditions:

  1. σBEX < σp

  2. σDEX < σp

where

σp = is the maximum permissible hull vertical bending stress, in N/mm2
= f σ EX σ o
f σ EX = 0,9, limiting hull bending stress coefficient
σDEX = is the extreme hull girder bending stress at strength deck
=
σBEX = is the extreme hull girder bending stress at keel
=
MREX = extreme vertical wave and still water bending moment, in kNm, given in Vol 1, Pt 5, Ch 4, 4.7 Extreme hull girder design loads
Z D = actual section modulus at deck, in m3
Z B = actual section modulus at keel, in m3

f hts and σo are defined in Vol 1, Pt 6, Ch 4, 1.3 Symbols and definitions 1.3.1.

3.3.2 The design extreme stress due to the extreme hull vertical bending moment, σex, for each structural member is given by

where

Z i = actual section modulus at structural element being considered, in m3

3.3.3 It is not necessary to satisfy the plate panel buckling requirements for compressive stresses provided that shear buckling of plate panels and stiffened panels and all buckling modes of failure for longitudinal girders and stiffeners are satisfied. The design factors of safety are given in Vol 1, Pt 6, Ch 5 Structural Design Factors

3.3.4 Consequently, the following sections on buckling control are to be complied with, based on compressive stresses derived in accordance with Vol 1, Pt 6, Ch 4, 3.3 Bending strength – Simplified assessment method ESA1 3.3.2

  1. Secondary stiffening in direction of compression, Vol 1, Pt 6, Ch 2, 3.7 Secondary stiffening in direction of compression.

  2. Secondary stiffening perpendicular to direction of compression, Vol 1, Pt 6, Ch 2, 3.8 Secondary stiffening perpendicular to direction of compression

  3. Buckling of primary members, Vol 1, Pt 6, Ch 2, 3.9 Buckling of primary members

3.4 Shear strength – Simplified assessment method ESA1

3.4.1 If the simplified analysis method is adopted for extreme strength assessment, the shear strength of the ship at each critical section is to satisfy the following criterion:

where

δ 0 is to be taken as the minimum value of δi

τp = maximum permissible mean shear stress, in N/mm2
= fτEX το
fτEX = 0,9, limiting hull shear stress coefficient
QREX = extreme vertical wave and still water shear force, in kN, at the appropriate longitudinal position determined from Vol 1, Pt 5, Ch 4, 4.7 Extreme hull girder design loads

A z, and δ i, are given in Vol 1, Pt 6, Ch 4, 2.3 Shear strength 2.3.6

f hts and τo are defined in Vol 1, Pt 6, Ch 4, 1.3 Symbols and definitions 1.3.1

3.4.2 The design extreme shear stress due to extreme hull vertical shear forces for each structural member, τex, due to hull girder is given by

where

Q REX is given in Vol 1, Pt 6, Ch 4, 3.4 Shear strength – Simplified assessment method ESA1 3.4.1

A z, and δi are given in Vol 1, Pt 6, Ch 4, 3.4 Shear strength – Simplified assessment method ESA1 3.4.1

3.4.3 The following Sections on buckling control are to be complied with based on shear stresses derived in accordance with Vol 1, Pt 6, Ch 4, 3.4 Shear strength – Simplified assessment method ESA1 3.4.2:

  1. Plating subject to pure in-plane shear, Vol 1, Pt 6, Ch 2, 3.3 Plate panel buckling requirements

  2. Shear buckling of stiffened panels, Vol 1, Pt 6, Ch 2, 3.6 Shear buckling of stiffened panels
    The design factors of safety are given in Vol 1, Pt 6, Ch 5 Structural Design Factors.

3.5 Bending and shear strength – Ultimate strength analysis method ESA2

3.5.1 The extreme strength capability of the hull girder may be assessed using a direct calculation ultimate strength analysis method. In this case the longitudinal strength of the ship at each critical section is to satisfy the following criteria for hogging and sagging conditions:

  1. M REX < f UEX M UEX

  2. Q REX , f UEX Q UEX

where

M REX = extreme vertical wave and still water bending moment, in kNm, given in Vol 1, Pt 5, Ch 4, 4.7 Extreme hull girder design loads
M UEX = ultimate bending strength of the critical section, in kNm
Q REX = extreme vertical wave and still water shear force, in kN, at the appropriate longitudinal position determined from Vol 1, Pt 5, Ch 4, 4.7 Extreme hull girder design loads
Q UEX = ultimate shear strength of the critical section, in kN
f UEX = 0,9, limiting ultimate strength coefficient for extreme hull girder strength assessment.

3.5.2 The ultimate strength of each critical section is to be derived by direct calculation using elasto-plastic analysis methods.

3.5.3 If the methods used to derive the ultimate strength do not include allowance for shear loading, then the shear strength requirements of Vol 1, Pt 6, Ch 4, 3.4 Shear strength – Simplified assessment method ESA1 are to be applied.

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