As offshore fields become more densely occupied with energy infrastructure, it has become necessary to more closely examine the risks and associated consequences of deepwater operations that were previously deemed acceptable. As moored MODU (Mobile Offshore Drilling Unit) risk assessments have evolved, the utilization of DP (Dynamically Positioned) vessels is often perceived to be a "safer" alternative. However, when examining historical statistics for sudden hurricanes with respect to DP vessel T-time, drive offs, and drift offs in a modern drilling scenario, the idea of DP operations being lower risk alternatives is often far from true. This paper will utilize statistical information for failure probabilities and associated consequences for a conventional DP drilling operation in comparison to a DP drilling operation utilizing a passive contingency mooring system. Although the main driver for DP contingency mooring systems should be risk mitigation, considerable savings may be seen by operators through less DP thruster use, reducing fuel costs. A properly designed contingency mooring system could be used to keep station under normal operating conditions, allowing considerable cost savings until larger storms arrive at the drilling location.
The principle function of a contingency mooring system for DP vessels is to mitigate the risks associated with operating events, not extreme weather events, such as hurricanes. Given that purpose, a reliable emergency quick release is discussed within this paper as an integral part of the contingency mooring system. A detailed risk assessment and cost comparison, in conjunction with mooring and hydrodynamic analyses of a proposed hybrid DP and contingency mooring system, will be presented in this paper.