During the Gulf of Mexico (GoM) hurricane seasons of 2004, 2005 and 2008, numerous moored Mobile Offshore Drilling Units (MODUs) had complete mooring system failures and thus were later found to be adrift. Also, a few units suffered failure of one or more mooring lines but at least one line held to keep the unit from going adrift. As a result of these mooring failures, operators and rig owners expended much effort in recovering the vessels, the mooring components, and then in making the required repairs in order to restart operations. Other major consequences of the mooring failures were: direct damage to the industry's infrastructure such as pipelines and umbilicals, loss of production, and delays in several well programs.

MODUs in the GoM are a critical part of the necessary infrastructure for exploration and development of oil and gas. Industry standards that allow safe and economic operations of MODUs are important to the industry and regulatory authorities. Much work has been done since the events in the 2004 and 2005 GoM hurricane seasons to better understand the causes of moored MODU failures and learn from these failures. This knowledge has resulted in changes to the industry standards regarding mooring of MODUs. As an example, selection of the design return period is no longer prescriptive but based on performing a suitable risk assessment.

Using a Quantitative Risk Assessment (QRA) approach, the risk can be evaluated at every well location planned to be drilled during the GoM hurricane season. These results can be used to make more informed decisions regarding rig deployments during the GoM hurricane season.

In addition, the QRA methodology can be extended to more comprehensive considerations (e.g. all of the GoM) in order to better understand the risk posed by the moored MODU fleet to an operator's entire portfolio of deepwater facilities and infrastructure.

This paper describes a process that can be used to assess the risk associated with moored MODU operations and provides some examples to demonstrate the utility of the process.


As a result of the numerous moored MODU failures during hurricanes Ivan, Katrina and Rita, the industry has reexamined the API guidelines for design of MODU moorings. Changes have been made to the current API guidelines as discussed in Refs. 1 and 2. The cornerstone of API RP 2SK, Appendix K (Ref. 3) is a risk assessment process to provide a consistent technical basis on which to make decisions of suitability of a particular rig at a specific site for a specific period of the GoM hurricane season. Per MMS NTL No. 2009-G09 (Ref. 4), a mooring risk assessment is required with every drilling permit. The QRA approach provides a means to rationally assess the risks, document the results, and compare the results to a company's internal guidelines in order to make better decisions regarding rig deployments. The process also provides a framework for consistently comparing the risk over multiple well locations.

An example site is presented herein that includes nearby surface and subsea infrastructure (at 10 miles or less) to demonstrate how the process works. Results are presented to particulary show the impact of drilling during the peak of the GoM hurricane season (approximately August 15 through October 15). Additionally, a second example shows the conclusions from a QRA that considers a comprehensive portfolio of deepwater infrastructure (surface facilities, pipelines and flowlines) being exposed to the fleet of moored MODUs during GoM (both peak and non-peak) hurricane season is presented. The second example further considers both the pre and post upgraded MODU mooring systems (i.e., relative to their 2004 condition). It is shown that proximity of the MODU to the infrastructure and the lost value placed on that infrastructure greatly influence the outcome of the QRA.

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