Coupled Analysis of DP-Tugboat and Onshore Pre-Assembled SCR (Steel Catenary Riser) Transportation System

This paper presents the study of the dynamic behavior of an onshore pre-assembled 8" SCR (Steel Catenary Riser) being transported to the installation place by two tugboats with DP system. The system is composed by two tugboats similar to a Normand Neptun (LOA 80m and displacement of 4200t), one in front of the riser (Lead) and the other at the back of the riser (Trail) controlled by DP system to keep the trajectory for installation location. During the transportation, the system passes through 3 different configurations: Offbottom, BuoyCatenary and Catenary. Offbottom configuration is composed by many buoys (effective buoyancy of 30kN) and chains distributed along the riser to guarantee the relative distance between the riser and the seabed, independently of seabed irregularities. When the sea gets deeper, the riser assume the BuoyCatenary configuration, where the riser forms a catenary with buoys; when it achieves a water depth up to 500 meters, all the system buoy+chain are removed and the riser assume the form of a free catenary until it arrives at the installation place. Two approaches was used to evaluate transport analysis of a onshore pre-assembled SCR. In the first approach (uncoupled analysis), the transportation was simulated using the Anflex software to calculate the dynamic response of the line composed by beam elements, imposing, at the top of the riser, the tugboat motion obtained from spectral analysis. In the second approach (coupled analysis), the transport was simulated using the TPN software to calculate the dynamic motion of the tugboats considering the dynamic positioning system and the Anflex software to calculate the dynamic response of the lines. The main propose of this paper is show the necessity of do coupled analysis for the SCR transport. This study are focus in a parametric analysis of the tugboat viscous damping influence, that were performed (1, 5 and 10% of the critic damping), as well as the damping generated by the riser in the tugboat motion, the relative motion influence in risers tensions and the tugboat’s bollard pull.