Critical Engineering Aspects for Qualification of the Titanium Pull-In Tube as a New Diverless Technology for Rigid Risers

Abstract This article details the qualification efforts for the Titanium Pull-in Tube (TiPT), to ensure the suitability of this technology against hydrogen embrittlement under fatigue, by avoiding direct exposure to seawater and the platform's cathodic protection system. A qualified external nonmetallic coating, which withstands high shear stresses during pull-in, is thus specified. So, the qualification of wear-resistant external coatings, the pre-design of a specific support-interfacing element including safety barriers and the determination by laboratory tests of the influence of the hydrogenation level on the fatigue performance of forged titanium components are proposed. For the coating qualification, first, a pre-selection of several polymer-candidates for the external coating, based on mechanical properties was made, listing a selection of commercial PU, PEEK and polymeric laminate composites. After that, mechanical tests and FEA were executed to confirm the properties of the batches and to find the parameters of subsequent tribology tests, mainly the contact pressure. Following, two types of wear tests were conducted: small-scale under different severity conditions and mid-scale to determine the best pair of materials for pipe and trumpet. Hydrogen embrittlement tests under cathodic protection considering toughness and fatigue for two titanium alloys are also planned. Considering the contact pressure obtained by FEA, the wear testing results showed no suitable PEEK coating for the high severity conditions, mainly due the inherent small thickness. A high polymer alloy based on epoxy and polyurethane was the best material for pipe protection, while a reinforced medium wave laminate composite was the best counterpart for trumpet liner. The selected tribological pair presented relatively low COF, excellent wear performance, market availability and maturity of the application processes (suitable for production scale of several long sections and field coating application if needed). Even though a wear-resistant coating system is selected, the TiPT development effort described in this paper includes modifications of the interface element from the standard used in flexible risers, to include additional electrical isolation barriers (between the hull and the titanium section) and a measurable criterion to assess eventual hydrogen intake in the remote possibility of serial failures of multiple safety barriers, to assist the component integrity management during service life. This work represents a continuation of another paper published in this same forum in 2023 (OTC-32473-MS. "Titanium Pull-in Tube: A Free SCC-CO2 Solution for Pre-salt Fields"). Both present the efforts to develop, qualify and implement a new, diverless technology with positive practical results in terms of reducing Man-Hours Exposed to Risk in the rigid riser installation phase and precommissioning, also being a definitive solution to the flexible lines' SCC-CO2 problem, for immediate use.