Strain Limits for High Strength Riser Materials

High strength materials are being used to increase the capability of dynamic risers to withstand deep water and high pressure flows. Design codes generally require that these materials operate well within their ultimate capability in the main body of the riser, but components of the riser such as screwed connectors and flanges often experience high localized or secondary stresses above yield. These stresses are allowable because they are self-limiting or in strain controlled situations. In addition the main body of the riser may also exceed yield in survival conditions when the riser may be allowed to undergo permanent distortion, but not fracture. However, high strength materials often exhibit lower ductility than traditional steels and as a result, there is a need to understand how these materials behave under high multiaxial stress conditions in order to quantify the true margin of safety in the component. The work reported in this paper focuses on titanium Grade 29, which has been used for tapered stress joints on steel catenary risers, and is now being considered for use in the riser touch down zone on several deepwater projects. The work on burst testing was also part of the validation of Dn V-RP-201, “Design of Titanium Risers”. This paper explores two approaches to determining a material model for failure. The first method used the Rice and Tracy void fraction model but this was found to be geometry dependent for this material and not ideal for failure prediction. A method was developed for determining strain limits under triaxial loading conditions using diffuse necking theory. The theory was applied to burst tests of high-pressure cylinders and gave a good prediction of the burst pressure and strain at failure.