Experimental Methodology of the Accumulated Plastic Strains (APS) Effect Evaluation on Steel Tube Umbilical (STU) Performance

Abstract

Abstract Description of the Proposed Paper This paper presents the results of a research project entitled "Experimental Methodology of the Accumulated Plastic Strain Effect Evaluation on Steel Tube Umbilical Performance", performed by Prysmian. The objective is to suggest an increasing of APS level in order to consider it on Steel Tube Umbilical design fatigue life. Experimental methodology through laboratory testings and prototypes in order to evaluate the actual influence of the high steel tube Accumulated Plastic Strains (APS) was used. Usually, APS can be accumulated during the steps of manufacturing and installation and may impose some restrictions for the steel tube umbilical performance and service life, manly when retrievals and post-installations are required. Normally in the market, it is stated that the level around 10% of APS does not down rate the steel tube performance, and the intention of this project is to increase this limit up to 30% of APS. For this purpose it was compared the pratical results from S-N curves between the welded steel tube and the theoretical calculation methodology according to international standard [1]. A more realistc figure regarding to the maximum level of APS will give to the offshore players a more precise idea of the feasibility to an umbilical project or requirement. Methodology For imposing APS on steel tube, the reeling and unrelling methodology was used. It consists of passing the tubes ½??x1.46 mm SAF 2507 from one reel to another and viceversa. So, the APS calculations consider the diameter of the reel and the number of reelings in order to achieve 30%. The samples were already welded in order to be more conservative. The calculations for imposed APS were based on [2]. Results, Observations, and Conclusions From the comparison between experimental test results (30% APS welded samples) and DNV curve, it was observed that there is no significant variation on the steel tube fatigue life, when imposing such level of strain. The respective S-N curves are presented in Figure 1. Two experimental curves are presented. The green curve is the mean SN obtained from the experimental results, while the red one is the curve for a 95% confidence interval. Figure also presents the curve for steel tubes provided in the DNV-RP-C203 (in blue). It was also verified that the mechanical and fatigue properties are not affected. The final conclusion is that the level of 30% of APS can be used by the umbilical manufacturers with a certain safety factor. The petroleum industry is still some uncertain regarding to the maximum level of APS, in face of that, this paper proposes and presents the validation tests for 30% of accumulated plastic strain on steel tube component for use in umbilicals.