Section 1 On-deck cargo securing arrangements
Clasification Society 2024 - Version 9.40
Clasifications Register Guidance Information - LR’s Guidance Notes for Calculation Procedure of Container Stack Analysis, March 2018 - Chapter 1 On-Deck Cargo Securing Analysis - Section 1 On-deck cargo securing arrangements

Section 1 On-deck cargo securing arrangements

Parent topic: Chapter 1 On-Deck Cargo Securing Analysis

1.1 Introduction

1.1.1 This guidance note is designed to assist Designers, Lashing Companies, Shipyards, Owners and Operators who wish to check their stack configurations using Pt 3, Ch 14 Cargo Securing Arrangements of Lloyd’s Register’s (hereinafter referred to as LR) Rules and Regulations for the Classification of Ships, July 2022.

1.1.2 Stack heights and lashing arrangements have changed rapidly with the development of ultra large containerships. The stack heights have increased and lashing arrangements are secured higher up the stack, changing their behaviour and force distribution. Conventional methods which are based on a shear only model for the stack deformation are not appropriate for high stacks. LR has investigated and identified the key properties governing the stack response and developed a set of high level requirements for the assessment.

1.1.3 Pt 3, Ch 14, 9 Strength of container securing arrangements specifies the requirements for containers and container lashing equipment. The Rules, however, do not specify the calculation methodology used. This was intentionally done to provide flexibility in their application depending on the implementation. This guidance note describes a methodology that complies with the Rule requirements.

1.1.4 All Rule equations are to use units as defined in Pt 3, Ch 14 Cargo Securing Arrangements. Consistent units are to be used throughout all parts of the analysis. Results presentation in kN and mm preferred.

1.2 Analysis of container stack

1.2.1 This guidance note is applicable for containers that are stowed in the fore and aft direction. It is sufficient to calculate the forces in the lashing rods based on assessment of the transverse and vertical loads acting on the containers. The vertical forces in the corner posts are to include the effects of transverse and longitudinal loads acting on the containers.

1.2.2 To assess the forces in the stack using the transverse and vertical loads, the forward and aft end of the stack is assessed independently and assumed decoupled. Only transverse and vertical displacements are considered when establishing the stack forces. Any longitudinal loads are to be converted to vertical forces applied to the container corners based on the reaction moments required to balance the stack.

1.2.3 For containers that are stowed athwartships the principles in this guidance document can be implemented to derive a methodology which complies with Pt 3, Ch 14 Cargo Securing Arrangements.

1.2.4 The application of ship motion and environmental loads will induce forces in the container boxes and lashing devices which result in distortion of the container stack. The key structural items resisting this distortion are as follows:

  • The shear stiffness of the container end and side panels;
  • The axial stiffness of the container corner posts;
  • The stiffness of the lashing devices, including lashing bridge structure, when present;
  • Under certain conditions and lashing arrangements, the twistlock can experience lifting forces which result in increased stack distortion.

All these aspects need to be considered in the stack assessment.

1.2.5 The forces in the containers, twistlocks and lashing devices are calculated from the stack distortion using Hooke’s law. Using the forces applied to the stack in Pt 3, Ch 14, 9.2 Forces applied to each container and the stiffness of the stack, the overall stack deformation is calculated using the standard force–displacement equations. Non-linearity of the system as a consequence of twistlock separation and lashing rods is to be considered in evaluating the deformation. The inclusion of twistlock separation and corner post stiffness can result in an increase in the loads in the lashing devices. The inclusion of a lashing bridge structure significantly affects the support a lashing device can provide for the stack and hence must be considered.

1.2.6 This guidance note calculates the forces and stack distortion using the stiffness method based on Hooke’s law to represent the containers and securing devices. The final distribution of forces in the stack is solved using matrix methods.

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