VEGA Field Single Point Mooring Requalification

Susanna Donà

Studio Tecnico ing. Donà Susanna

Luca Marcadent



The purpose of the analysis presented is to investigate the behavior of the bearings of three joints installed between the FSO (Floating Storage Off-loading) and the seabed, being part of the VEGA Field SPM system, for which a requalification was required.
The three joints were: the U-Joint, installed on the foundation base of the SPM column right above the seabed level, the Triaxial Joint installed at the top of the SPM column and the Hinges installed on the tanker beam of the FSO, connecting the FSO to the Triaxial Joint by means of a YOKE structure. The bearings of these components were the most critical elements of the anchoring system in terms of fatigue, and for each one of them alternation in terms of the maximum principal stress S1 was evaluated and subsequently used for fatigue verification. This calculation was performed based on cumulative fatigue damage theory, specifically following recommended practice DNV-RP-C203 for the fatigue design of offshore steel structures.
The simulations took into account many different wave classes of loading including the presence of the former single-hull FSO VEGA OIL which, due to new European regulations concerning double-hulls, had to be replaced by the new Arfamax type double-hull FSO LEONIS. The calculation exercise for the VEGA FSOs permitted the evaluation of the current condition of the structure and the prediction of the remaining life of the structures.
The results obtained through the virtual model allowed a requalification of the structure, which would not have been achieved through a conventional, conservative hand calculation.