Calculation of VIV Fatigue of Multi-Pipe Systems

In various regions of the world offshore oil and gas production and exploration is met with the challenges of operating in extreme currents. These extreme current loads increase the risk of vortex induced vibration (VIV) of the riser system and the associated fatigue damage induced by these vibrations. Regions of particular concern are the Gulf of Mexico where strong loop currents occur, East Africa where the Benguela current dominates and regions offshore South America where strong eddy currents can occur. In order to ensure robust riser design for these regions it is necessary to perform detailed evaluations of the expected VIV response of the global riser system. The assessment of VIV response of steel riser systems is commonplace in the industry and there are well established industry practices in relation to this. In addition there are a number of commercially available VIV analysis tools. Whilst commercially available VIV analysis tools have proved effective in the analysis of long slender structures such as risers there are also a number of limitations in the level of complexity of the model that can be achieved. The analysis approach utilized by these models is based on simplified geometries and linearization of complex non-linear interactions. As a result of these simplifications these analysis tools are not suited to the analysis of non-linear multi-pipe models. Many offshore riser configurations consist of one or more pipe or casing strings enclosed by an outer pipe which is exposed to the environment. In a scenario where VIV of the outer pipe occurs then there will also be a corresponding deflection and curvature transmitted to the inner pipe(s). The displacements and associated stresses of the inner pipe(s) are largely dictated by the response of the outer pipe. Thus for multi-pipe configurations where the inner pipe is not constrained by cement or centralisers (e.g. landing string inside marine riser) the issue arises as to how to calculate the fatigue of the inner strings due to VIV oscillation of the outer string. At the time of writing no standard industry approach exists to address this issue and therefore this paper will outline and demonstrate a proposed approach to address this issue. The objective of this paper is to outline a novel analysis approach for the calculation of inner string fatigue resulting from VIV of the outer pipe. This proposed methodology combines the benefit of simplified VIV analysis tools with more detailed non-linear global finite element modelling techniques to enable a more comprehensive and accurate assessment of fatigue life of the complete system. The benefits and effectiveness of the proposed method are demonstrated through the analysis of a series of case studies which include landing string fatigue and riser fatigue for deep water applications.