Section 1 Introduction
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 5 Environmental Loads - Chapter 3 Local Design Loads - Section 1 Introduction

Section 1 Introduction

Parent topic: Chapter 3 Local Design Loads

1.1 General

1.1.1 This Chapter contains information regarding the derivation of local design loads that are to be used for the assessment of scantlings to local loads as specified in Vol 1, Pt 6, Ch 2 Design Tools

1.1.2 The formulae for ship motion loads given in this Chapter are suitable for all ships that operate in the displacement mode. Ship motion loads for ships that operate in the fully planing mode will need to be specially considered.

1.1.3  Figure 3.1.1 Overview of derivation of local design loads gives an overview of the contents of this Chapter, the locations of the local load components and the route through the Chapter.

1.2 Environmental conditions

1.2.1 The environmental conditions for the determination of the local loads are to be based on the normal environmental design criteria specified in Vol 1, Pt 5, Ch 2, 2.3 Wave environment unless otherwise stated.

1.2.2 The wave height factor for local loads, f Hs, is dependent on the service area notation and is to be taken as follows:

f Hs = 1,0 for SA1 service area notation, i.e. unrestricted sea-going service
otherwise
f Hs =
=

f Hs is not to be taken as less than 0,5

H dw is given in Vol 1, Pt 5, Ch 2, 2.3 Wave environment 2.3.3.

1.2.3 All other environmental parameters are defined in Vol 1, Pt 5, Ch 2 Environmental Conditions.

1.2.4 The local design loads for assessment of the residual strength notation, RSA, or for special loading conditions where the ship will not experience severe weather or severe sea states may be adjusted for the appropriate environmental conditions. A reduced wave height factor, f Hs, may be applied, see Vol 1, Pt 5, Ch 3, 5.10 Design loads for RSA notation assessmentand Vol 1, Pt 5, Ch 3, 5.10 Design loads for RSA notation assessment 5.10.3.

1.3 Symbols and definitions

1.3.1 The symbols and definitions applicable to this Chapter are defined below or in the appropriate sub-Section. LWL, B, BWL, D, T and Cb are defined in Vol 1, Pt 3, Ch 1, 5.2 Principal particulars.

f Hs = wave height factor for service area, see Vol 1, Pt 5, Ch 3, 1.2 Environmental conditions 1.2.2
V cr = two thirds the cruising speed, in knots
V sp = is to be taken as the greater of the cruising speed or two thirds the sprint speed, in knots. For ships where it is not required to maintain high speeds in severe weather then the value of V sp may be specially considered
T = mean draught at midships, in metres, to the design draught waterline measured from the moulded baseline
Tx = local draught, in metres, measured from the underside of the keel to the design draught waterline at the longitudinal position under consideration, see Figure 3.1.2 Definition of symbols. For the calculation of bottom impact pressures, Tx (TFB) is to be taken as the minimal draught for all operational loading conditions measured from the underside of the keel to the operational waterline at the longitudinal position under consideration, see Vol 1, Pt 5, Ch 3, 4.2 Bottom impact pressure, IPbi 4.2.1.
xwl = longitudinal distance, in metres, measured forwards from the aft end of the L WL to the position or centre of gravity of the item being considered
y = transverse distance, in metres, from the centreline to the centre of gravity of the item being considered. y is positive to port and negative to starboard
z = vertical distance, in metres, from the base line to the position or centre of gravity of the item being considered. z is positive above the baseline
Normally the following definitions are to be applied:
For a longitudinally framed plate panel, z is to be taken at the panel centre
For a transversely framed plate panel, z is to be taken at two thirds of the panel height
For short stiffener members: z is to be taken at the stiffener mid position
For long stiffener members: z is generally to be taken at the stiffener mid position, but may need to be specially considered, especially when there is a significant pressure variation along its length
z k = vertical distance of the underside of the keel above the baseline, in metres, see Figure 3.1.2 Definition of symbols
∆ = moulded displacement of the ship, in tonnes

1.3.2  Froude Number F n. The Froude number is a nondimensional parameter which is the primary constituent part of the wave making resistance and which dictates the maximum displacement speed. It is defined as:

F n =

where

g = is the acceleration due to gravity and is taken to be 9,81 m/s2
V m = is the appropriate ship speed in m/s.

Figure 3.1.1 Overview of derivation of local design loads

Figure 3.1.2 Definition of symbols

1.3.3  Design loading condition. The design loading condition is to be taken as the draught condition with tanks and consumables, etc. in the departure state, see also Vol 1, Pt 5, Ch 4, 2.1 General 2.1.3. Where there is a significant variation in loading conditions or operating modes, then it may be necessary to consider additional loading conditions which represent the extremes of service requirements. For example:

  • a ship which can operate in displacement and non-displacement modes;
  • a ship which is required to operate at a temporary draught deeper than the design draught for off-loading payload.
  • a ship which is required to be involved in humanitarian emergency evacuation incidents or similar situations where short term overloading may be necessary.

1.3.4  LCG. The LCG is the longitudinal centre of gravity of the ship measured in metres from the aft end of the L WL for the design loading condition.

1.3.5  Displacement mode. Displacement mode means the regime, whether at rest or in motion, where the weight of the ship is fully or predominantly supported by hydrostatic forces. Typically this applies to craft with a Taylor Quotient, Γ, less than 3. However, some craft are designed to plane with Γ less than 3 and these should be considered as operating in the non-displacement mode.

1.3.6  Fully planing mode or Non-displacement mode. Non-displacement mode means the normal operational regime of a ship when non-hydrostatic forces substantially or predominantly support the weight of the ship. Typically this applies to craft with a Taylor Quotient, Γ, greater than 3. However, some craft are designed not to plane with Γ greater than 3 and these should be considered as operating in the displacement mode unless they are classified as a high speed craft.

1.3.7 Taylor Quotient Γ. The Taylor Quotient is defined as:

where

1.3.8  Support girth G S . The support girth is the girth distance at the LCG, in metres, and is to be calculated as follows:

The definition aims to define the canoe body. Narrow keels and skegs can be ignored.

Figure 3.1.3 Definition of girth chine location, B C, B WL and G S when chine is below deepest load waterline

Figure 3.1.4 Definition of girth chine location, B C, B WL and G S when chine is above deepest load waterline

Figure 3.1.5 Definition of bilge tangential point, B C, B WL and G S

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