Limit State Design in DNV96 Rules for Submarine Pipeline Systems: Background and Project Experience
Authors: Mørk K.J. , Collberg L.J. , Bjømsen T.J. 
Published: October 2008

Abstract
This paper discuss the safety philosophy and limit state design methodology adopted in the new revision of Det Norske Veritas (DNV) "Rules for Submarine Pipeline Systems". The new Rules completed late 1996 (denoted DNV'96 in the following) constitutes a complete revision of the previous Rules from 1981 and provide a significant step forward for pipeline design and operation. DNV'96 adopts the latest research achievements and practical experience gained through the most challenging pipeline projects recently executed. The adopted limit state format allows for a large flexibility in design. It applies explicit design checks for physical limit states rather than implicit design check covering a number of failure modes. The safety philosophy adopted including the novel safety class concept will be discussed. The background for design criteria related to pressure containment are presented and the principles for assessing and accounting for the influence of system effects, material quality, hydro test and pressure safety systems are discussed. The implicit higher allowable utilisation given by the LRFD-format for the pressure containment is illustrated. Design performance, feedback from the industry and experience gained since the issue will be discussed.

Introduction
The pipeline industry has traditionally been rather conservative. The main design requirements in codes for both land based and offshore pipelines have remained more or less as they were at the introduction of the ASME codes several decades ago. Significant technical progress has been achieved in pipeline design, materials, fabrication. installation an operation of offshore pipelines during the last ten to fifteen years without necessarily being implemented in codes. The major changes in the offshore oil and gas industry taken place recently have forced the pipeline industry to focus on cost reduction through innovative design approaches, new materials, standardisation and optimised construction and operational procedures. The industry has also been forced to implement new concepts such as safety and risk. On this basis, there has been a need for update of recognised codes to implement the latest developments and knowledge within the pipeline industry. DNVhas recently completed a major update of the Rules for Submarine Pipeline Systems from 1981 referred to as DNV'96. DNV'96 are based upon the principle of Limit State philosophy, and include the latest research achievements (especially from the SUPERB Joint Industry Project, see Sotberg et. al,. (1996)) and practical experience gained within pipeline technology. The intention is to provide the user with specific guidance in areas with well proven technology and functional requirements otherwise. Even though this may seem restrictive for development of new design concepts, the adopted limit state philosophy gives extensive flexibility for new design scenarios. In this paper the safety philosophy adopted including the novel safety class concept will be discussed. The background for the pressure containment design criteria is presented and the principles for assessing and accounting for the influence of system effects, material quality, hydro-test and pressure safety systems are discussed. The pressure containment limit state formula is a classical allowable stress design formula (ASD). An alternative formula (denoted LRFD-formula) allowing a higher utilisation if the incidental pressure is less than 1.1 times the design pressure will be illustrated in the paper.