scope:

Introduction

Classification Notes shall be considered in connection with DNV Rules for Classification of Ships, Pt.3 Ch.1, Hull Structural Design, Ships with Length 100 metres and above, /1/, and Pt.5 Ch.5, Liquefied Gas Carriers, /2/. The aim of this Classification Note is to describe procedures for strength analysis of gas carriers with IMO Independent Type A cargo tanks.

In general, gas carriers with IMO Independent Type A cargo tanks shall satisfy the strength criteria to main class 1A1 as given in the Rules Pt.3 Ch.1. Additionally, the criteria for classification notation Tanker for Liquefied Gas as given in the Rules Pt.5 Ch.5 shall be complied with for the inner hull, cargo tanks, and cargo tank supports. The requirements of DNV Rules Pt.5 Ch.5 /2/ are considered to meet the requirements of the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk, IGC Code, /3/.

The full scope of structural analyses for type A-tanks as required by the Rules are described in this document. However, some important exceptions can be made based on experience with conventional proven designs, and when the cargo temperature is NOT lower than -55oC (e.g. LPG/NH3 carriers);

- Stationary and /or transient thermal analyses need normally not to be performed.

- Fatigue analysis of cargo tanks and supports is not required, Pt.5 Ch.5 Sec.20 A206. The thermal analyses can be omitted subject to acceptance by the Society based on experience with similar designs. On the other hand, for novel designs, and/or when the cargo temperature is below -55oC;

- Thermal analysis for material selection and thermal stress analysis is to be carried out, Pt.5 Ch.5 Sec.20 A206.

- Fatigue analyses of the cargo tanks and the supports shall be carried out with damage factor Cw ≤ 1.0, as specified in Pt.3 Ch.1 Sec.16 and Classification Note No. 30.7, Fatigue Assessment of Ship Structures /5/ for the hull structure.