Modelling and control of subsea installation

Abstract

The development of subsea processing equipment and the trend to go into deeper waters for untapped oil fields will result in an increased focus on offshore installation tasks and systems. The main purpose of the research in this thesis is to develop advance strategies for the control of subsea installation operations and flexible structures in the marine environment and alleviate some of the challenges.

Splash Zone Transition Control: For the subsea system to be installed on the sea bed, it first has to be lifted off a transportation barge on site using an offshore crane and placed into the water. The transition from air to water is known as splash zone transition and the vertical hydrodynamic loads on the payload can be expressed as a combination of terms from the pressure effects, slamming and viscous forces including the Froude-Kriloff forces, hydrostatic pressure and viscous drag. A simple linear in the parameter (LIP) model that is representative and captures most of the observed hydrodynamic load phenomena is presented. Model based control is designed and neural network (NN) based control is presented for the case where uncertainties exist in the system parameters.

Dynamic Positioning of Payload: When the payload is near the seabed, positioning control in the horizontal plane is investigated for the installation of subsea systems, with thrusters attached, under time-varying irrotational ocean current. Backstepping in combination with adaptive feedback approximation techniques are employed in the design of the control, with the option of High-gain observer for output feedback control. The stability of the design is demonstrated through Lyapunov analysis where semiglobal uniform boundedness of the closed loop signals are guaranteed. The proposed adaptive neural control is able to capture the dominant dynamic behaviors without exact information on the hydrodynamic coefficients of the structure and current measurements.

Subsea Installation Control with Coupled System: Next, the coupled dynamics and control of the vessel, crane, flexible cable and payload under environmental disturbances with attached thrusters for subsea installation operations is investigated. For the practical system with physical constraints, Barrier Lyapunov Functions are employed in the design of positioning control for the flexible crane-cable-payload subsystem to ensure that the constraints are not violated. Uniform stability of the flexible subsystem is shown and asymptotic positioning of the boundaries is achieved. The scenario where nonuniformity of the cable, uncertainties and environmental disturbances exist is considered. Boundary controls are formulated using the nonlinear PDEs of the cable.

Flexible Marine Riser: Finally, active control of flexible marine riser angle and the reduction of forced vibration under a time-varying distributed load are considered using boundary control approach. A marine riser is the connection between a platform on the water surface and the installed subsea system on the sea floor. A torque actuator is introduced in the upper riser package and a boundary control law is designed to generate the required signal for riser angle control and vibration reduction with guaranteed closed-loop stability. Exponential stability can be achieved under the free vibration condition. The proposed control is simple, implementable with actual instrumentation, and is independent of system parameters, thus possessing stability robustness to variations in parameters. The design is based on the PDEs of the system, thus avoiding some drawbacks associated with the traditional truncated-model-based design approaches.