The Single Hybrid Riser (SHR) concept has been used successfully in industry onfloaters such as Floating Production, Storage and Offloading (FPSO) vessels indeepwater applications. This concept is especially effective with floaters inareas where challenging metocean environments result in severe vessel motions. The flexible jumper connecting the vessel to the rigid steel riser effectivelyisolates the dynamic vessel motions from the top-tensioned steel riser section. This results in lower strength and fatigue demand in the steel pipe section ascompared to other riser concepts such as a Steel Catenary Riser (SCR). However, the SHR concept also reaches the design limits of the flexible jumper aspressure, temperature, and sour service operating conditions become moresevere. ExxonMobil has demonstrated the feasibility of using a Steel Catenary Jumper(SCJ) as an alternative to the flexible jumper for extending the operationallimits of the SHR concept. This paper presents the results and designconsiderations for a SHR with a SCJ in 10,000 ft. Water Depth (WD) andpressures up to 10 ksi. The SHR/SCJ configuration was determined iteratively byassessing its strength performance in response to wave and current loading, vessel offset, internal content and pressure. Satisfactory strength and fatigueperformance is achieved under harsh North Atlantic and West Africa environmentswith a predominant fatigue condition. As is the case for SHRs with flexiblejumpers in similar conditions, vessel heading control is required to maintainacceptable response during extreme and long term environmental loading. Installation of the SHR/SCJ concept is determined to be within the presentmarket capability of heavy lift vessels, new generation J-lay vessels and FPSOpull-in facilities. A fabrication and installation procedure for the SHR/SCJconfiguration is presented. Introduction The SCJ was investigated as an alternative to the conventional flexible jumperfor connecting a SHR to a turret moored FPSO vessel. The primary advantage ofusing a SCJ is higher pressure and temperature service limits. Increasedresistance to sour service can also be achieved using Corrosion Resistant Alloy(CRA) lined pipe. The SHR/SCJ configuration was adapted from an existing 9-inID flexible jumper SHR conceptual design sized for a design pressure of 10 ksiand 10,000 ft. WD. Modest increases to the air (buoyancy) can depth, jumper length and distancebetween the FPSO turret and SHR base were made to the flexible jumper SHRconfiguration to arrive at the nominal SHR/SCJ configuration. The SCJ wasconnected to the FPSO vessel and SHR using typical flexible joints with taperedextensions. The SHR comprises a buoyancy can, rigid structural tether, TopRiser Assembly (TRA) with goose neck, dual thickness riser section with taperedsteel stress joints at each end, offtake spool assembly and roto-latchassembly. Strength analyses were performed to North Atlantic extreme storm conditions andfatigue analyses were performed to both North Atlantic and West Africa motionresponses. The harsher North Atlantic environment required an internal turretwhile the milder West Africa environment permitted an external turret. Production, water injection and gas injection riser applications were assessedfor feasibility. The SCJ and SHR were designed to standards API-RP-2RD [1], API-RP-1111[2], and DnV RP-F109 [3].

Design of a steel catenary riser (SCR) requires the use of connection hardware to decouple the large bending moments induced by the host floater at the hang-off location. Reliability of this connection hardware is essential, particularly in applications involving high pressure and high temperature fluids. One option for this connection hardware is the metallic tapered stress joint. Titanium (Ti) Grade 29 has been identified as an attractive material candidate for demanding stress joint applications due to its “high-strength, low weight, superior fatigue performance and innate corrosion resistance”2. Titanium stress joints for deep-water applications are typically not fabricated as a single piece due to titanium ingot volume limitations, thus making an intermediate girth weld necessary to satisfy length requirements. As with steel, the potential effect of hydrogen embrittlement induced by cathodic and galvanic potentials must be assessed to ensure long term weld integrity. This paper describes testing from a joint industry project (JIP) conducted to qualify titanium stress joint (TSJ) welds for ultra-deepwater applications under harsh service and environmental conditions. Corrosion-fatigue crack growth rate (CFCGR) results for Ti Grade 29 1G/PA gas tungsten arc welding (GTAW) specimens in seawater under cathodic potential and sour brine under galvanic potentials are presented and compared to vendor recommended design curves.

Design of a Steel Catenary Riser (SCR) requires the use of connection hardware to accommodate the bending moment that arises from the abrupt change in stiffness at the floater hang-off. Reliability of this connection hardware is of paramount importance in ultra deepwater applications (up to 3000m), especially those involving high pressure and temperature fluids. One type of such connection hardware is a metallic tapered stress joint. Because of its inherent density, strength, and stiffness, steel is not well suited for these applications due to the length and weight constraints. Titanium Gr. 29 (Ti), which is as strong as steel but lighter and more flexible, has been identified as a good material candidate for a tapered stress joint. The required length (∼40ft) and thickness (∼3.5in.) of the Titanium Stress Joint (TSJ) cannot be fabricated as a single piece due to forging size limitations. Thus, an intermediate girth weld becomes necessary. The fracture and fatigue performances in the presence of the external seawater and cathodic protection (CP) and the internal sour production with galvanic effects between the Ti and steel must be assessed to verify the service life of the stress joint. ExxonMobil has developed and initiated a Joint Industry Project to fully address the fracture and fatigue qualification of titanium welds. In particular, the project plans to establish a robust methodology for the future qualification of TSJs that parallels, to the extent possible, the qualification process currently used for SCRs. This paper discusses the primary aspects of the titanium weld qualification: (1) selection of test specimens, (2) load frequency effects on initiation and propagation lives, (3) environmental assisted cracking due to hydride formation under cathodic and galvanic conditions, (4) full-thickness small-scale fatigue, (5) size effect on fatigue, and (6) weld inspection.

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Accurate computation of tensile armor wire stresses remains a major challenge in flexible riser fatigue life predictions and integrity management. Accuracy of the results relies heavily on capturing the kinematics of the flexible’s helically contra-wound tensile armor layers and their interaction with the other metallic and thermo-plastic layers in a dynamic simulation. The standard industry practice to assess the fatigue life of flexibles is to use high fidelity 3D Finite Element Models (FEMs) to capture the complex kinematics and produce accurate stresses. However, direct simulation of flexible riser detailed FEMs is limited to regular wave analyses and computation of wire stress time-histories subjected to irregular waves have been computationally infeasible. This is due to the complexity of the nonlinear FEM and the long simulation time of the irregular wave environment coupled with large number of fatigue sea states. As a result, simplified approaches which do not directly simulate the local model and instead assume that wire stresses can be interpolated based on static stress versus curvature material curves within a pre-defined tension /pressure envelope have been utilized. This paper utilizes Nonlinear Dynamic Substructuring (NDS), a simulation-based approach that that extends the framework of dynamic substructuring to nonlinear problems. NDS enables the efficient nonlinear dynamic simulation of multiple pitch lengths of detailed flexible riser FEM subjected to irregular wave inputs and the computation of wire stress time-histories at any location on the local model. In this paper, a 14-inch diameter flexible riser under consideration by ExxonMobil is subjected to vessel motion and wave load in irregular wave environments and is modeled using a detailed 3D FEM and simulated via NDS. The flexible riser design features four tensile armor layers to mitigate localized lateral buckling of the wires near the touch down point. Tension and curvature time-histories of the riser near the hang-off, calculated from a conventional beam model global analysis, is used to drive a 5.1m long local model. Irregular wave wire stress time-histories extracted at the corners of the tensile armor wires are used to compute the fatigue life of the flexible. To demonstrate the inaccuracies associated with the regular wave approach, fatigue life is computed via the regular wave approach and compared against the irregular wave approach. It is shown that the NDS capability to efficiently compute irregular waves mitigates over- and under-predictions due to environment idealizations leading to a more accurate and reliable flexible riser life prediction and structural integrity assessment.