Numerical and experimental procedure for designing sub-sea installation operations

Abstract

Sub-sea equipment installations are very complex operations, requiring pre-installation analysis to define the correct procedure and the weather “window” for a safe operation. This paper addresses the installation of a Mid Water Arch (MWA) intended to provide support to the riser. Connecting the riser to the MWA largely eliminates the dynamic forces that would otherwise cause friction and fatigue. The MWA is composed of riser guides and several buoyancy tanks and is kept in the water with tethers connected to an anchor. The installation procedure involves launching each component of the MWA (anchor, main structure and tethers), during which a tug boat with an A-frame and an assistance vessel are used to keep the buoy away from the tether and the launch cable. The waves induce oscillatory motions throughout the system and may cause large dynamic forces in the cables and tethers. Due to the complexity of the multi-body system, a comprehensive numerical and small-scale experimental analysis is conducted to calculate the proper dimensions for the launch cables and to define the limits of the environmental conditions. Numerical analysis was carried out in the Numerical Offshore Tank–TPN, a multi-processor offshore system simulator that considers the 6 degrees of freedom for each body and all environmental forces acting upon them. The lines are modeled by finite-element analysis. Furthermore, a full set of small-scale experiments were carried out at a towing tank that considered the response of the system when excited by sinusoidal motion at the top and emulated the wave excitation. Comparisons between numerical and experimental results showed good adherence between the calculated values. The validated numerical simulator was then used to analyze the complete complex installation procedure by considering an extensive set of environmental conditions.