Production of oil and gas in the Brazilian pre-salt will face several technical challenges. One of them that is a major concern is the fatigue life of top risers end-fittings. The new ultra-deep water fields will require a complete new fatigue assessment methodology with higher accuracy. Besides that, the historical data of failure for this sort of equipment shows that the current methodology is not quite optimized for floating units operating in deep water fields. With this, even worst results are expected in ultra-deep water fields.
During its assembly inside the end-fitting, the tensile armor wires need to be folded, unfolded and set in position using epoxy resin. This assembly process involves localized plastic deformation and as consequence of this produces a complex residual stress field and also introduces an elastic stress to maintain the wires in position. Both stresses are not actually taken into account in the current fatigue assessment methodology.
Therefore, the aim of this work is to develop and evaluate a new fatigue assessment methodology for top risers end-fittings based on remote S-N curves. The proposed methodology takes into account both residual and assembly elastic stresses. The effect of stress concentration on the tensile armor wire at the region with localized plastic deformation is also evaluated. Basically, the objectives of this investigation will be achieved through the construction of remote S-N curves using a test box that contains the deformed wire embedded in resin representing a single wire physical 2D model of an end-fitting, the so called mid-scale testing. For this investigation, a six inches API end-fitting was selected because it is widely used and most available in the market.
The results produced here indicated that the localized plastic deformation imposed during the folding and unfolding process has a very important detrimental effect on the flexible risers end-fittings tensile armor wires fatigue life, which makes mandatory a revision of the current four-point bending fatigue assessment methodology.