Decommissioning of SPM buoy

Abstract

Over the following years many of the installations in the North Sea, installed in the 1980s, will have to be decommissioned. Due to rules and regulations most of them will be taken onshore for recycling. In this report a study of the decommissioning process of a SPM buoy in the Statfjord field is perfomed. The column of the buoy is containing 18 compartments and weighs about 7150 tonnes. The topside is weighing about 519 tonnes. The height of the buoy is about 182 m measured from the unijoint. In the decommissioning process the hydrostatics, structural and hydrodynamics should be investigated. In the hydrostatic analysis hand calculations are performed to find equilibrium and ballast condition for given sequences during the operation. In the structural analysis the same sequences are analyzed. The global stress on the buoy is calculated by hand to see of the longitudinal strength of the buoy is satisfying. The hydrodynamic part of the report investigates the difference of linear frequency domain analysis and non- linear time domain analysis, to see if the simpler linear analysis is accurate enough. The hand calculations used in the hydrostatic and structural parts are performed in MathCAD. The calculations for the hydrostatic part are performed by integrating over the geometry to find the hydrostats. The hydrostats was then compared with results obtained in AutoHydro, a program for simulating hydrostatic analysis. To calculate the global stresses the shear force diagram was set up, and later the bending moment and stress diagrams. In the hydrodynamic analysis two programs were used to run the simulations. Wadam was used for the frequency domain analysis and OrcaFlex for the time domain. In both programs Morison theory was used. The hand calculations put up in MathCAD are good enough for rough estimations. The VCB, GMT and LCF are not satisfying for small trim angles. The calculations are satisfying when the buoy is positioned vertically. The bending moment is not zero at bulkhead 19, due to approximation of the lever arm. In the hydrodynamic analysis the damping near the natural period is important. The non- linear Morison theory has only impact near the natural period.