All-Metal Hybrid Riser For Ultradeepwater And Harsh Environments (2005)

Abstract The "All-Metal Hybrid Riser" is a simple and robust engineering concept. There are no complicated interactions and no sensitive components and interfaces. This makes it less susceptible to damage, which makes for sturdiness and low risk-exposure. With a buoyancy tank below the zone of influence of waves it is insensitive to the wave induced motions. The all-metal riser tower may be a single conduit or a multiple-conduit design. Multiple-conduit risers can, by varying the spacing of stiffeners, eliminate the effect of VIV on individual conduits. The response of the complete assembly to vortex shedding is modest for an open array, as these have inherent VIV abating features. Multiple-conduit hybrid riser towers are therefore well suited for use in ultra deep water and harsh environments with high current velocities. These were important features in the work for the Ormen Lange project. For this application it was found that the allmetal hybrid riser contrary to the other alternatives did not face any major engineering hurdles. Avoiding the need for syntactic foam buoyancy makes the cost comparable to that of other alternatives and avoids the need to qualify such elements. Additionally the all metal hybrid riser is suitable for accommodating subsea processing and subsea isolation valves such that they effectively contribute to reducing the risk exposure. Figure 1 - All-metal hybrid riser (Available in full paper) Introduction The all metal riser tower was developed to be more robust and simple than previous designs. This is achieved by using only metal components, and rigid connections to the buoyancy tank as well as the riser base. Tower cross section: A principal feature is the tower cross section made up of aluminium conduit tubes through which the risers run. These conduit tubes have multiple functions:They confine the steel risers such that when they are heated by higher temperature hydrocarbons the resulting expansion is accommodated by the riser deforming into a helix.During tow-out and installation the conduit tubes are gas filled and provide near neutral buoyancy allowing a controlled depth tow.During operation the conduit tubes may be filled with insulating gel to provide the necessary thermal insulation.The array of conduits provides a VIV abating effect.The pipe-in-pipe arrangement also provides structural compliance damping of vortex induced vibrations. Figure 2 - Riser tower arrangement (Available in full paper) As mentioned above, installation of the complete tower is performed as a controlled depth tow. This is performed in one operation and avoids the costly and time consuming operation of mating the tower to a preinstalled riser base. The rigid connection of the individual conduit tubes and risers to the tank and the riser base, make it feasible to tow out and install with the suction and gravity anchor in place and acting as a clump weight. The ballasted buoyancy tank acts as a clump weight at the other end. In order to achieve neutral buoyancy of the tower section the conduit tubes are made in aluminium. For this application aluminium is found to be more cost effective than composites. They are also less sensitive to mechanical damage.