Petrobras as the operator of the Roncador field located offshore Brazil plans to develop part of this field utilizing a semi-submersible unit located in approximately 1800 meters of water depth. The paper presents key engineering challenges, solutions, and lessons learned from the design and analysis of a large number of steel catenary risers (SCRs) attached to this floating production unit (FPU).
Due to the large number of SCRs (43), rationale selection of SCRs to be analyzed is desirable to reduce analysis time. The function and dimensions of the SCRs, SCR hang off locations, FPU motion effect on SCR strength and fatigue, and feasibility of installation were considered. More than one design cycle was necessary to integrate SCR design with FPU motion response and mooring system design. Fully coupled FPU time trace data were used to generate mean offsets and low frequency motions. The most critical risers and the associated worst load cases have been verified by using time traces.
To reduce the computational efforts, a frequency domain approach was introduced to conduct SCR motion fatigue analysis with time domain verification. Particular attention was paid to the bimodal wave characteristics of fatigue sea-states and second order vessel motions. VIV analyses were conducted considering a damage spreading over the touch down region.
Interference among risers and global arrangement are critical design issues because of the close spacing of risers. SCR routing is another challenge due to congested subsea arrangement and clearance requirements. SCR anchor system and global configuration were designed to resolve these issues.
To demonstrate the robustness of the SCR design, sensitivity studies were performed with respect to governing parameters including hang off angle, soil properties, current data, hang off system, strake coverage, computer modeling, fatigue design data and other design inputs.
The lessons learned from the execution of the SCR design are summarized and discussed. These lessons will have significant relevance to the future SCR projects not only in offshore Brazil but also other geographical locations.
It has been concluded that the Roncador SCR design can safely withstand environmental loads and the effects of the FPU motion and that the system is reliable and robust.
The giant oil field of Roncador, discovered in 1996,is located in the North region of Campos Basin, at 2000m water depth. The development of this field was devised to be done in modules. Among them, the 1A Module (phase 2) of Roncador will be exploited by a large semi-submersible platform, with a displacement of more than 80,000 tons, named Petrobras 52 (P52).
P52 will be moored in a water depth of 1800m. Peak production rates are targeted at 180,000 bpd oil and 3.7 million m3/day of natural gas. The subsea system will be composed of 20 production wells and 10 injection wells, all linked to the platform through 43 risers, 23 umbilicals and 2 gas-lift manifolds and control (used the decrease the number of risers and umbilicals attached).