Participating mass in colliding risers
SVEIN IVAR SAGATUN1, KJELL HERFJORD1, FINN GUNNAR NIELSEN1, and ERLING HUSE2
1 Norsk Hydro Exploration and Production ASA, Research Center Bergen, Department of Marine Science, N5020 Bergen, Norway
2 MARINTEK AS, Division of Offshore Structures, Trondheim, Norway
Abstract: One of the main challenges in estimating impact energy in collisions between marine risers is the assessment of the riser mass involved in the collision. Evidently the entire riser mass does not contribute to the collision. Hence, the question is: What is the equivalent riser mass which contributes to the impact energy? This article presents three different ways of estimating the riser mass participating in the collision energy. The first method is strictly experimental. The second method uses a numerical experiment together with system identification techniques. The third method is a strictly analytical method, which results in an asymptotically upper bounded estimate of the participating mass. Two risers are examined as case studies. The first riser is a 1:100 model scale riser used in collision experiments carried out at Marintek's towing tank in Trondheim, Norway. The second case uses a real world riser in use on the Troll B oil production platform operating in the North Sea. The proposed methods yield consistent and comparable results.
