A recent research project has been working to select and bring to maturity at least one safe riser system concept that could meet the performance demand of high-motion vessels in ultra-deepwater areas (10,000-ft depths). Stress Engineering Services, Inc., has been undertaking this work, under the auspices of the Research Partnership to Secure Energy for America, supported by the US Department of Energy’s National Energy Technology Laboratory.
Additional participants in the project included BP, Shell, Petrobras, ConocoPhillips, ExxonMobil, and Chevron. The project was tasked with performing dynamic analyses and other second-level evaluation of riser concepts that had passed an initial concept evaluation and had included key design parameters.
Host vessels for the riser evaluations were semisubmersibles and disconnectable-turret-moored and captive-turret-moored FPSOs.
Semisubmersible
A semisubmersible hull arrangement with conventional four columns and four pontoons was selected for this study. Catenary-type risers are typically hung-off a semisubmersible from a receptacle structurally affixed to the pontoon. The riser hang-off location can be either on the inside of the pontoon or along the outside of the pontoon.
Motion of the riser hang-off location was a concern. The farther that the riser attachment point is from the center of the vessel, the larger the roll/pitch-induced heave that the riser will experience. Thus, it is advantageous to hang off the riser from the inside of the pontoon. However, if a riser concept is shown to meet design criteria when hung off from the outside of the pontoon, there is a high likelihood that it is a viable design when hung off from the inside of the pontoon. Therefore, for this study it was assumed the riser was hung-off from a point along the outside of the pontoon. The lateral distance from the center of the hull to the riser hang-off location was set to an even 100 ft.
