Earthquake Activity in Gulf of Mexico Prompts 2003 Study for MMS
Part 1: Seismic Risk Questioned As Deep Water Development IncreasesThe Gulf of Mexico (GOM) is not known for its earthquake activity; however, a number of earthquakes ranging between 3.0 and 4.9 on the Richter scale caused enough concern for Mineral Management Services (MMS) to engage in a 2003 study of the likely performance of subsea oil production assets during small earthquakes. The Gulf region is classified as Zone 0 for seismic risk. This means that all shallow and deepwater development of crowded subsea structures including oil rigs and pipelines have not been designed or constructed to withstand earthquake activity.
2003 Assessment Ignores Risk Of Major EarthquakesAn Assessment of Seismic Risk for Subsea Production Systems in the Gulf of Mexico was conducted for and submitted to MMS in December 2003. The 165 page study was performed and prepared by the Offshore Technology Research Center of Texas A & M University. Increased earthquake activity in oil producing regions of GOM raised questions about the performance of deepwater subsea systems, which the study hoped to answer.
The study was based on engineer modelling. Their analysis modeled various characteristics of the underwater structures, the Gulf seafloor, and sea currents, to determine if earthquakes would wreck the system. The model only tested for the amount of shaking associated with smaller earthquake magnitudes normally seen in Zone 1 and Zone 2 areas. Therefore, the study offers no guidance as to the amount of destruction that can be expected from earthquakes of higher magnitudes. The fact that the Gulf of Mexico is rated a Zone 0 is not a guarantee that larger earthquakes will not occur in regions now crowded with underwater oil production and pipeline structures.
GOM Oil Development Boom Crowds The Seismic RiskThe number of subsea systems placed into production during the past 2o to 25 years has increased dramatically. The sheer number alone of developments in deeper water heightens the risk. The exposure of these systems involves earthquake shaking, liquefaction potential, and dynamic impact from soil sliding from nearby slope instability. The expansion in recent years has been in deepwater as technology and discovery of new reservoirs have fueled the rush in development. Recent reports of MMS waiving various environmental impact studies underscores the oil industries’ haste and avoidance of caution in pushing these developments. The Western and Central Planning areas as of March 2004 for Gulf of Mexico developments chart the vast regions involved.
The controlling factors in the design of offshore structures are the effect of environmental loads due to wave, current, wind and geologic activity. The American Petroleum Institute requires that earthquake shaking, fault movement, and sea floor instability be accounted for in the design. However, in Zone 0 seismic risk areas, like the GOM, the requirements for earthquake shaking do not apply to existing and future designs and construction. The two maps below demonstrate the massive underwater pipeline system now totaling 44,000 miles and some 4,000 active wells:
Report Considered Effects of Only Smaller Earthquakes
Low seismic risk does not mean that earthquakes are totally absent. We have already mentioned that a number of earthquakes between 3.0 and 4.9 on the magnitude scale prompted the study. Between 1978 and the date of the report in December 2003 the strongest quake was the 4.9 magnitude. This one occurred near the Mississippi Fan region, which includes oil production systems. It is thought that the cause of the earthquake was related to crustal subsidence due to sedimentation loading. (Frohlich 1982) ”Seismic events in other areas of the GOM seem to be associated with the plate boundaries in Mexico, Central America, and the Caribbean.” (Frohlich 1982) The primary source of earthquake recorded information in the GOM is the website for the National Earthquake Information Center. Their records begin back in 1973 for the Gulf. There have been a total of eight earthquakes ranging from 4.0 to 4.9 magnitudes in the Zone 0 seismic risk area and a larger number of smaller quakes.The tables 3.1 and 3.2 list the recorded earthquakes in the GOM, and Fig. 3.2 depicts the location of the epicenters of those seismic events taken from the tables. The earthquakes in table 3.2 occurred in the Bay of Campeche area. From 1974 thru 2003 there were 22 earthquakes, which is considered an infrequent occurrence. This report was completed nearly three years before two of the strongest earthquakes occurred in the GOM in 2006. These tables were published as part of the assessment: