In areas with very uneven ocean bottom topography, the floating pipeline concept may be used. An example is the Ormen Lange gas field west of Norway. The floating pipeline has positive buoyancy and is moored to the seafloor at regular intervals by tethers. Before the tethers are installed, drag chains will keep the pipeline in position. To study the effect of the drag chains on the pipe installation, its static configuration and its dynamic behavior, it would be very inefficient to model each drag anchor using cable elements. This would require a very high number of elements and small time-steps, while the details of the forces in the chain are not critical in the pipeline design. Instead, a non-linear element has been developed that precisely represents the quasi-static behavior of a complete drag chain. To determine reaction forces at the top end of the chain given displacements of the top, the length of dragging chain necessary to balance the forces acting on the top is found by iteration. Proof is provided that the iteration is unconditionally stable. This iteration makes use of the classical catenary solution for the hanging part of the chain, combined with a model of the configuration of the fraction of the chain resting on the seabed. This configuration can be arbitrary, so that any combination of straight chain, heaps and coils resulting from the history of the displacement chain is accounted for. The element behaves in a coherent way also if the whole length of chain is being dragged on the seabed, lifted clear of the seabed, or landed back on the seabed. The incremental stiffness matrix is found by numerical derivation of the reaction forces with respect to the displacements. The element is designed for use in non-linear static or non-linear time domain analyses. It has been implemented in the riser and pipeline analysis program RIFLEX and successfully tested.

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