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Model Summary

General

The platform itself is considered rigid, so it is included for visual effect only, and does not affect the finite element computations. The analysis commences from a point where the flexible is lying flat on the seabed, with the winching cable already positioned within the J-tube. The winching cable is connected to the flexible at one end via a rigid end fitting, passes through the J-tube, and connected to a winch at its upper end. In order to maintain reasonable element lengths in the contact region, the actual winching process is modelled by a single (initially very long) winch element above the platform. As this element contracts over time, the upper end of the flexible is gradually pulled along the J-tube.

Contact between the flexible and J-tube is modelled using the pipe-in-pipe modelling facility. As the degree of relative axial motion between the components is obviously significant, sliding connections are used to model the interaction between the inner and outer sections. No pipe-in-pipe sections are defined, as the J-tube is flooded with seawater, so the external ambient fluid for the flexible remains unchanged as it passes through the tube. A close up of the bellmouth region is shown in the figure below, with the various components clearly visible, as the upper end of the flexible approaches its entry to the J-tube.

Close up of J-Tube Region

Pipe-in-Pipe Modelling

A useful feature in Flexcom is the ability to model pipe-in-pipe configurations. Internal and external sections are modelled separately, and interact with each other by means of special stiffness connections simulating both linear and non-linear resistance to relative motion. Pipe-in-pipe connections can be considered to operate like springs, whether linear or non-linear. However these springs do not directly connect the two nodes (in many cases the nodes are collinear, at least initially, and so cannot be connected by a spring). What happens is that a spring stiffness is added directly into the stiffness matrix at locations corresponding to the appropriate motions of the two nodes. The direction of the spring is computed as being normal to the structure at the outer node.

Sliding connections are similar to standard ones, with the added advantage that the connections are interchangeable. This is appropriate for modelling scenarios where there is significant relative axial motion between the inner and outer pipes. Based on the proximity of the various inner and outer nodes at the beginning of the analysis, the program creates an initial set of (effectively standard) connections between the inner and outer pipes. The program continually monitors the relative axial locations of the inner and outer nodes over the course of the analysis, and the set of connected inner and outer nodes is updated as and when required.

Winch Elements

Winch elements are normal beam-column elements which have the unique property that their lengths can vary during an analysis. In a dynamic analysis, the variation in length is defined in terms of a maximum winch velocity and a winching time sequence. The time sequence consists of (i) a ramp-up time when winch velocity is increased from zero to the maximum value; (ii) a time during which the velocity remains at this maximum, and (iii) a ramp-down time when the velocity returns to zero. The operation in a static analysis is less complex, with an overall change in length being applied linearly from the analysis start time to the end time. Winch elements can be used, for example, in pipelaying applications, for example in simulating the transfer of an SCR from a lay vessel to a TLP or semi-sub.