scope:

Introduction

Objective of the document

This document is intended to give guidance on how to establish the structural resistance by use of non-linear FE methods. It deals with determining the characteristic resistance of a structure or part of a structure in a way that fulfils the requirements to ultimate strength in DNV offshore standards.

Non-linear effects that may be included in the analyses are e.g., material and geometrical non-linearity, contact problems etc.

The characteristic resistance should represent a value that meets the requirement that there is less than 5% probability that the resistance is less than this value. This definition of the characteristic resistance is similar to what is required by many other structural standards using the limit state safety format and these recommendations are expected to be valid for determination of capacities to be used with such standards.

The recommendations are foreseen to be used for cases where the characteristic resistance is not directly covered by codes or standards. The objective is that analyses carried out according to the recommendations given in this document will lead to a structure that meets the requirements to the minimum safety margin in the standard.

This document is not intended to replace formulas for resistance in codes and standards for the cases where they are applicable and accurate, but to present methods that allows for using non-linear FE-methods to determine resistance for cases that is not covered by codes and standards or where accurate recommendations are lacking.

Validity

The document is valid for marine structures made from structural steels meeting requirements to offshore structures with a yield strength of up to 500 MPa.

The recommendations presented herein are adapted to typical offshore steels that fulfil the requirements specified in DNV-OS-C101 /14/ or an equivalent offshore design standard. The specified requirements are made under the assumption that the considered structure is operating under environmental conditions that are within the specifications of the applied offshore standard. If the offshore unit is operating outside these specifications, the failure criterion presented in this Recommended Practice can only be utilized if it can be documented that both the weld and parent material have sufficient toughness in the actual environmental conditions.

This recommended practice is concerned only with failure associated with extreme loads, and failure due to repeated loading from moderate loads (fatigue) needs to be checked separately. See DNV-RP-C203 /17/.