Dynamic Risers: Introduction and Background to the New DNV Offshore Standard (OS-F201)
Authors: Katla E. , Mork K. , Hansen V. 
Published: October 2008

The paper gives an introduction to the acceptance criteria and design procedures in the new DNV Offshore Standard OS F201 for Dynamic Risers. The design format adopted is based on a modern limit state design principles with safety classes linked to consequence of failure. The standard provides a consistent link between global analyses, failure modes, load conditions, characteristic load effects and design cases, which allows for cost effective design. The focus of this paper is an introduction and discussion on recommended global analysis procedures and practical implementation of the design approach in design analyses. The benefit and potential for optimised design solutions is illustrated in a few examples. Further, the performance of the limit states for combined loading versus standard industry practice is discussed.

Intoduction

Background and Motivation

For risers the term optimised may be interpreted as "a fit for purpose design solution in all anticipated scenarios with a minimal life cycle cost." This implies that all possible design conditions must be considered and designed for with an adequate level of safety. For conventional riser concepts, flexible and cost optimal solutions may be obtained by (conservative) standardised wall thickness sizing. For deepwater risers this is not the case since unduly conservatism imply added weight and may render some concepts unattractive or even un-feasible. Compared to shallow-water risers, deepwater risers are characterised by:Increased cost profile.Higher potential for use of metallic compliant configurations.Increased risk for Riser interference and Vortex Induced Vibrations.Increased attention to new potential failure modes such as local buckling (due to external overpressure and bending moment.)

An essential issue in cost optimisation is the ability to control the implicit conservatism in the design via rational design criteria and analyses procedures. Standard industry practice for riser design, e.g. reflected by API RP 2RD, apply the traditional working stress design (WSD) format where structural safety is taken care of by using a single usage (safety) factor. One of the limitations experienced by application of WSD is that a single safety factor leads to a varying safety level strongly dependent on the load conditions. For well-known concepts, this is considered acceptable, but an extension to new concepts and applications is neither optimal nor appropriate. In addition, recent design codes only provide limited guidance on how to establish relevant load effects to be used in the code checks. Hence, a clear need has beenidentified for safe and efficient design criteria and analysis procedures for risers in general and deepwater risers in particular.

The new DNV Offshore Standard OS F201 for Dynamic Risers is considered a contribution towards optimal design. The basis for the standard was partly developed within the recently completed 4 year Joint Industry Project (JIP) "Design Procedures and Acceptance Criteria for Deepwater Risers". The JIP were performed by DNV, SINTEF and SeaFlex and supported by international oil-and manufacturing companies. The standard was issued February 2001. This paper is an update of the Houston paper (M?rk et al. 2000) due tocomment from industry hearing from januar 2000 to october 2000.