Analysis of Siting and Evacuation Strategies for Mobile Drilling Units in Hurricanes
Authors: Ying J. , Bea R.G. 
Published: October 2008

Experience with Mobile Offshore Drilling Units (MODUs) in recent hurricanes in the Gulf of Mexico (GOM) has indicated the need to reassess where and how these units are sited. During hurricane Andrew (1992), several of these units moved significant distances (up to ISO km) from their original locations. In several instances there were collisions with platforms and pipelines. Also, in the case of hurricanes weather conditions can deteriorate rapidly, and timely decisions are critical to allow proper securing and evacuation of MODU's. This is particularly crucial in areas with high storm intensity and low predictability. The primary objective of this research was to develop an analytical model to evaluate MODUs' movements in response to the combined load effects of hurricane winds, waves and currents, then use a Monte-Carlo simulation process to evaluate the probability of collisions between the MODU and surrounding large facilities. The computer simulation program was developed, and has been used to investigate alternative siting strategies. The strategies investigated include location of the MODU relative to nearby facilities, capacities of the mooring systems, and incorporation of intentional 'weak links' in the mooring system to cause either the mooring to the anchor connections to break, or the anchors to drag. The results of the parametric studies of alternative siting and mooring strategies are summarized. The second objective of this research was to use probabilistic risk analysis techniques to develop a computer model to help evaluate operation and evacuation systems for MODUs. Results from the evacuation simulation model are intended to assist in developing decision criteria for securing and evacuating MODUs.

Approach

Fig. I summarizes the approach used to develop, verify and implement the simulation models. Based on fundamentals of statistics, hurricane forecasting and modeling, fluid dynamics, mooring strength analysis, and Monte Carlo techniques, the first step was to develop a basic simulation model, to evaluate the movement of MODUs in hurricanes. The program was then used to develop siting strategies for Mobile Offshore Drilling Units. The simulation model incorporates models of hurricane winds, waves, currents, and tracks; storm wind, wave and current forces; mooring capacity characteristics, and finally, a model of movement characteristics that takes into account free floating, intermittent grounding ('skipping'), collision 'holding', and anchor dragging characteristics of MODUs. The program allows the user to specify hurricane characteristics in a probabilistic or deterministic manner. A Monte-Carlo simulation model is utilized to perform probabilistic calculations. The model includes a Markov model to describe the probabilities associated with changes in the tracks of the hurricanes. The model incorporates variable hurricane parameters and their correlation, the storm spatial geometry, and shallow water shoaling effects. The developed program allows one to define the locations and sizes of 'critical facilities' near the MODU location, and then evaluate the probabilities of collisions between the MODU and the critical facilities. The organization and theoretical basis for the program will be detailed later in this paper. Based on the project management and network simulation techniques, a computer simulation model was developed to help evaluate operation and evacuation systems for MODUs in hurricanes. Probabilistic risk analysis and Monte Carlo techniques are used in the model to address the large uncertainties in hurricane forecasts and in the evacuation process.