This work presents a new methodology for the estimation of the vertical center of gravity (KG) of an offshore semisubmersible oil production unit. There are a lot of stationary semisubmersible (SS) production units that have been working offshore for 20 years or more and some of these units have received a lot of lightweight modifications to comply production improvements, affecting its KG position. Most of them do not have all documentation available and the costs involved in bringing these units onshore to perform a conventional inclining test are so high that makes this option unviable. Some production semisubmersibles still need more equipment and structures to increase their oil production, i.e., deck load capacity that affects the KG which is a limited mandatory variable.
The methodology developed for finding the KG states that it is perfectly possible to identify a unique hydrodynamic behavior around the neutral stability, where the metacentric height (GMT) equals to zero. Under this condition, the KG gets the same value of the transverse metacenter height (KMT), which is a known parameter that only depends on the underwater hull geometry.
A 1:50 scale model of a semisubmersible was built in order to assess the influence of mooring lines and waves on the method's accuracy. The model tests, performed at LabOceano – Ocean Technology Tank of Coppe/UFRJ, revealed very low influence of waves and mooring lines in the methodology accuracy even in waves of different heights and directions. Numerical computations considering hydrostatic properties and hydrodynamic loads in regular seas indicated the same results observed in the model tests. A hydrostatic comparison between the conventional inclining test method and this new neutral stability method has presented very similar results.