Design Methods in ISO 13628-12 and Implications for Gulf of Mexico SCRs

Abstract

Abstract A new riser code, ISO 13628-12 "Design of Dynamic Risers for Floating Production Installations" has recently been updated as part of an industry JIP and is scheduled for ISO ballot in 2010. Once approved, the new ISO 13628-12 will replace the current edition of API RP 2RD as the Code of Practice for the design of dynamic metallic risers for floating production installations. This paper summarizes the four design methods in draft ISO 13628-12 and investigates their implications on the design of future SCRs. The implication of the new riser design code is illustrated by an example of an SCR design in the Gulf of Mexico. Three of the four design methods allow for certain plastic utilization of the riser cross section, which requires additional considerations for design, analysis, material selection and fabrication. Combined loading code checks with the four different methods are performed and the results are compared. Additional material specific considerations including design stress-strain curve and fabrication requirements are also presented. Introduction The design methods in the updated draft ISO 13628-12 reflect an evolutionary progression from working stress design to limit state design. Limit state design ensures a consistent safety level, based on acceptable failure probabilities, with due considerations to the inherent uncertainty in the design loads and resistance to such loads. Typical limit states for deep water riser design application include:–Accidental limit state (ALS)–Ultimate limit state (ULS)–Serviceability limit State (SLS)–Fatigue limit state (FLS) The ALS is associated with events with an annual probability of less than 10-2 and larger than 10-4, while the ULS corresponds to 100 year return period events or events with 10-2 annual probability. The SLS criteria define the loads during the normal operational conditions and normal temporary events. FLS is an ultimate limit state from accumulated excessive fatigue crack growth or damage under cyclic loading. The performance criteria for the riser design depend on the specific limit state. For example ULS requires that the riser system must survive the design event with no damage, i.e. remain intact and avoid rupture, but not necessarily be able to operate. For ALS the riser system must survive, and may sustain some damage without any rupture i.e. no release of hydrocarbon is allowed. Typical load cases for strength design of SCR are presented in Table 1. The paper presents a summary of the design equations to assess the capacity of the pipe. The performance of an 18 inch oil (X70 grade steel) export SCR suspended from a semi-submersible in 2000 m water in the Gulf of Mexico is assessed for ULS and the ALS conditions using combined loading equations for internal overpressure conditions. Potential for buckling due to compression in the touchdown zone (TDZ) is also investigated.