Design of Steel Risers in Ultra Deep Water-The Influence of Recent Code Requirements on Wall Thickness Design for 10,000ft Water Depth

Abstract

Abstract The feasibility, weight and cost of steel risers in ultradeep water depends critically on the design requirements for resisting functional loads due to internal and external pressure combined with design extremes of tension and bending moment. This paper presents a critical review of the wall thickness design requirements for deepwater risers according to current riser codes and assesses the appropriateness of these requirements as the envelope of application of riser engineering expands towards 10,000ft water depth. The design approach for selecting the wall thickness of steel risers in ultra-deep water has evolved from working stress and limit state codes of practice developed either from pipeline codes or from riser codes applied thus far to moderate water depths. This paper identifies critical differences between these codes, in terms of the weight and cost of the risers designed to them, and evaluates the consistency of current approaches to wall thickness design for ultradeepwater projects. Introduction Several design codes are currently available for designing the wall thickness of a deepwater pipeline or riser. These are divided between specifically developed riser codes, extensions of pipeline codes to address riser design, or codes of practice which have been specifically developed for pipes designed for export service. The most commonly applied codes for design of deepwater risers are:API RP 2RD, "Design of Risers for Floating Production Systems (FPSs) and Tension-Leg Platforms (TLPs)", 1998 [1],API RP 1111, "Design, Construction, Operation, and Maintenance of Offshore Hydrocarbon Pipelines (Limit State Design)", 3rd Edition 1999 [2],ASME B31.4, "Pipeline Transportation Systems for Liquid Hydrocarbons and Other Liquids, Chapter IX - ?Offshore Liquid Pipeline Systems'", 1998 Edition [3],ASME B31.8, "Gas Transmission and Distribution Piping Systems", Chapter VIII - "Offshore Gas Transmission", 1999 Edition [4],Offshore Standard DNV-OS-F201, Dynamic Risers 2001 [5] The authors have undertaken a critical analysis of the wall thickness requirements of several codes of practice, including API RP 1111, API RP 2RD, ASME B31.4 and ASME B31.8. Hoop stress, hydrotest, collapse strength and combined stress requirements have been examined. Overview of Current Code Design Requirements As a precursor to evaluating their effects on the wall thickness design of deepwater pipelines, this section provides a summary of the principal design requirements of different codes of practice. WSD and LSD Design Approaches: Design codes are distinguishable according to two fundamental design approaches:Working Stress Design (WSD) andLimited State Design (LSD) API RP2RD [1], which is a custom-developed riser code, and ASME B31.8/31.4 [3,4] provide design requirements based on WSD, the latter two specifically relating to oil and gas transmission (export pipeline) service. Others, such as API RP1111 [2], which is a pipeline and riser code, specify design requirements based on the LSD approach. WSD is the traditional approach to steel design, which relates allowable stress to the yield stress of the material. LSD is an ultimate strength approach that relates allowable loading to the ultimate strength of the material.