Hurricanes Ivan, Katrina, and Rita tracked through a high-density corridor of the oil and gas infrastructures in the Gulf of Mexico. Extreme winds and large waves exceeding the 100-year design criteria of the MODUs during these hurricanes, caused the failure of mooring lines to a number of Mobile Offshore Drilling Units (MODUs) in the Gulf of Mexico. In addition to the damage MODUs undertook during these severe hurricanes, drifting MODUs might impose a great danger to other critical elements of the oil and gas industry. Drifting MODUs may potentially collide with fixed or floating platforms and transportation hubs or rupture pipelines by dragging anchors over the seabed. Therefore, it is desirable to understand the physics of the drift of a MODU under the impact of severe wind, wave, and current and have the capabilities to predict the trajectory of a MODU that is drifting.
In this thesis, a numerical program, named “DRIFT,” is developed for predicting the trajectory of drifting MODUs given met-ocean conditions (wind, current, and wave) and the characteristics of the MODU. To verify “DRIFT,” the predicted drift of two typical MODUs is compared with the corresponding measured trajectory recorded by Global Positioning System (GPS).
To explore the feasibility and accuracy of predicting the trajectory of a drifting MODU based on real-time or hindcast met-ocean conditions and limited knowledge of the condition of the drift, this study employed a simplified equation describing only the horizontal (surge, sway, and yaw) motions of a MODU under the impact of steady wind, current, and wave forces. The simplified hydrodynamic model neglects the first-and second-order oscillatory wave forces, unsteady wind forces, wave drift damping, and the effects of body oscillation on the steady wind and current forces. It was assumed that the net effects of the oscillatory forces on the steady motion are insignificant.
Two types of MODU drift predictions are compared with the corresponding measured trajectories: 1) MODU drift prediction with 30-minute corrections of the trajectory (every 30 minutes the simulation of the drift starts from the measured trajectory), and 2) continuous MODU drift prediction without correction.