ACCUMULATED PLASTIC STRAIN EVALUATION OF STEEL TUBE FOR UMBILICAL CABLES

摘要

This paper presents the tests performed for the qualification of the level of accumulated plastic strain (APS) to be respected by manufacturing and installation of SAF 2507 steel tube umbilicals. The manufacturing and installation processes for steel tube umbilicals are carried out through repetitive cycles of coiling and uncoiling the tubes or umbilical during manufacturing.During installation the umbilical is also coiled and uncoiled in installation reels or vessel's baskets (turn tables or carrousels) as well as straightened and bent under tension over sheaves or chutes. These operations impose tensile and compressive strains to tube, which means that the tube is submitted to work hardening and have its mechanical properties modified to a certain extent during its manufacturing and installation processes. Therefore, it becomes necessary to quantify the level of APS expected to occur during these processes and compare it to the maximum level of APS qualified for the tube material being used in order to ensure that the mechanical properties of the steel tube are maintained at a high quality level. The scope of this paper is to present the tensile and fatigue tests performed by Marine Production Systems do Brasil (Brazilian Oceaneering Factory) in strained and unstrained tubes and define a maximum acceptable level of APS. Both strain hardening and fatigue life influence effects are assessed. Based on the results of these tests, the maximum acceptable level of APS to be respected during the manufacturing and installation of umbilicals was defined and associated with this level, the S-N curve determined by the tests must be used for the umbilical service life fatigue analysis.