Verification of the water forces on the stinger of Allseas' Solitaire

Abstract

Allseas Engineering bv ("Allseas") is a major offshore contractor, supplying the oil and gas industry with their expertise in the field of marine pipeline installation. Their biggest ship, the Solitaire, has a horizontal working plane and to prevent the pipe from bending or breaking due to its weight the pipe is guided by an outrigger called the "stinger". The stinger is constructed out of cylindrical elements and will convey the pipeline into the water on its route to the seabed. To calculate the hydrodynamic forces on the stinger the program AQWA is used by Allseas. This program calculates the Morison force for each element in the stinger. In this thesis a verification of the AQWA program is made, which results in more knowledge on the accuracy of the program. The drag and inertia coefficients in the Morison force can be greatly influenced by several effects, which are neglected by AQWA and have to be investigated. In order to determine the accuracy of the program, AQWA is compared with scale model tests done by Maritime Research Institute Netherlands ("MARIN"). From the investigation followed that only the effects of free stream turbulence, angle of attack, interference and vortex shedding can cause a significant difference between AQWA and the reality. MARIN executed scale model test on the Solitaire and his stinger. For a fair comparison between AQWA and the scale test, scale effects are taken into account. The differences in stinger loads between AQWA and MARIN were of limited significance. Where the ship motions coincide, the loads on the stinger were in the same. The trend found in AQWA for the current and regular wave test, was an underestimation by AQWA of the forces in x" and z" direction and the moment around the y" axis. From the comparison it can also be seen that the ship motions have a great influence on the stinger forces and moments. It is advised to execute a test in reality.