Dynamic Analysis of a subsea cable during cable installation

Abstract

The investigation into the dynamic behavior of a cable during installation was done with the aim of improving cable installation for safety, operational and commercial value. In order to increase the operational limit of the vessel during cable installation, the investigation into improvement systems was desired. The workability of a vessel during cable laying operations is governed by the cable integrity design criteria such as maximum tension, touch-down-point tension, side-wall-pressure and the minimum bending radius. In practice during cable-laying the touch-down-point tension and minimum bend radius cannot be measured. Therefore a dynamic analysis of the subsea cable is required to determine the cable motions and tension fluctuations. The main objective of this Master Thesis is to develop and investigate two cable lay improvement systems. Both systems are modeled in OrcaFlex software to analyze its improved effect on the workability during cable laying operations. The cable integrity design conditions are compensated by either active force control at the tensioner systems or by active position control near the departure point on the chute. By controlling one of the two parameters the catenary shape is stabilized and therefore the fluctuating tension and cable motions are controlled. Force control is developed in OrcaFlex by a Tensioner System controlling the tensions in the load cells at a target value, position control is controlling the chute-end location with an Active Heave Compensated Chute System. To actively control the improvement systems, a Proportional-Integral-Derivative (PID)-Controller is used in OrcaFlex as external function where the performance dependents on the defined PID-Parameters. To optimize the improvement systems further, first simplified 1-Degree Of Freedom (DOF) analytical position controlled models are developed to identify the initial guess PID-Parameters. Subsequently, a 6-DOF position (left picture) and force (right picture) controlled models in OrcaFlex, was developed and simulated using the obtained initial guess PID-Parameters. In OrcaFlex the PID-Parameters are further adjusted using regular wave theory to achieve the full performance capacity of the improvement systems based on data obtained from existing systems on the market. Finally a detailed numerical model is developed and analyzed for irregular wave heights to obtain the workability plots. The numerical models are subjected to environmental and hydrodynamic loads during a three-hour simulation. Results show that the force and position controlled systems are able to improve the cable integrity design conditions significantly, but more for the position controlled system. The force controlled system is limited by the pay-out velocity for deep water and by cable compression and the minimum bending radius at shallow water, whereas the position controlled system is limited by the stroke of the cylinder. Also it has been found that the controller performance for the force controlled system must be adjusted for different range of waterdepth, while the position controlled system maintains the same performance. Furthermore, the position controlled system is able to hold the catenary shape nearly still, significantly more that it does for the tension controlled system. In conclusion an increased workability during cable installation can be achieved with one of the improvement systems.