AbstractsEngineering

Installation of deep water Xmas trees for subsea oil production is sometimes done by lowering the tree using one wire. Xmas trees are interfacing with other equip- ment on the seafloor and will therefore need to be positioned, and oriented correct. Today aligning the Xmas tree to existing interfacing structures on the seabed dur- ing installation is done by manual control of an ROV. In this thesis it is proposed to automate this process. The benefits of doing this is to gain a faster and more precise control of the position of the tree, as well as being less prone to human errors. This will make the operation faster, safer and less expensive.This thesis is a feasibility study of this new solution for aligning the Xmas tree in the installation process. In order to investigate the feasibility of the solution a mathematical model of an ROV and a Xmas tree that is hanging in a wire has been developed. Interaction forces between these objects are then defined. A model of the environment and sensor output from the ROV is also developed.A controller algorithm has been defined that uses the position of the Xmas tree to calculate the control forces the ROV need to control the motions of the Xmas tree. This controller algorithm is a nonlinear PID-controller where the output is translated from the body-centre of the Xmas tree to the body centre of the ROV in order to use some of the existing control system in the ROV. In addition to the controller an extended kalman filter has been implemented in order to handle the sensor feedback, and a reference model has been made to generate smooth and feasible trajectories as input to the control system.The mathematical model is used to simulate the behavior of the system, when the control system is connected. The simulations shows very promising results.An experimental setup has been made in order to test the control system in real life. The experimental setup consists of a downscaled Xmas tree model, a docking frame, and a small scale ROV. This setup was used to test the control system in the Trondheim fjord using R/V Gunnerus. Due to an error in the programming the testing was not able to fully verify the simulation results. The results obtained during the test did however seem promising when this error is taken into account, and thus proof of concept was established.