Friday, August 27, 2010

Seismic Risk Questioned As Deep Water Development Increases part 1

Earthquake Activity in Gulf of Mexico Prompts 2003 Study for MMS

Part 1: Seismic Risk Questioned As Deep Water Development Increases

The 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 Earthquakes

An 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.

Note that the Macondo Well of the Deepwater Horizon disaster is located in Mississippi Canyon in Block 252 which places the cursed well within the region of seismic events.  The name Macondo is the same name as the fictitious cursed town in the novel “One Hundred Years of Solitude” by Colombian nobel-prize winning writer Gabriel Garcia Marquez.
It is frightening to envision what could result from a major earthquakc striking the heart of the oil producing region.

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 Risk

The 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:

GOM 4,000 Active Oil & Gas Platforms - NOAA 6-8-10

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:

Significantly, eight of the epicenters occurred in the Mississippi Fan area and four of the epicenters were located further south and east of this area closer to the Florida Scarp.  The strongest being the 4.9 magnitude earthquake in the oil producing Mississippi Fan area.  The earthquakes in the Mississippi Fan area are thought to be associated with the plate boundaries in Mexico, Central America, and the Caribbean. (Frohlich 1982)   However, there is another geologist who has extensive experience in the GOM with a decidedly different theory, which may prove very significant to the future.  This theory will be examined in the next article on the topic. Fig. 4.5 depicts the locations of Deepwater Subsea Systems in relationship to the epicenters of earthquakes in the oil producing areas.

Report Conclusions
The report concluded with a summary that was careful to point to the model criteria used for Zone 1 and Zone 2 seismic loads utilized on the subsea structures.  With those qualifications, the assessment found that the structures should perform acceptably if exposed to those loads.  However, the recommendations were cautious:
“1.  The impact of earthquake shaking on the integrity of a single and multiple casing foundation subsea structures is dependent upon a number of different factors that are specific to site and system requirements.  Seismic risk for proposed projects should be investigated for specific site conditions and equipment constraints.  Specific design information for a subsea system is critical in making a final assessment of the expected performance under site specific ground motions.
“2.  Based on the results from this study, earthquake shaking (within Zones 1 and 2 PGA values) should not dramatically impact the performance of deepwater subsea structures in the GOM.  However, further research should be conducted to determine the impact of sliding soil due to soil instability on the performance of these structures in the GOM.”
This assessment did not involve any inspections of existing subsea structures.  A note in the report suggests that the oil producing companies maintaining proprietary interests in the subsea structures were keeping them confidential.  At any rate, the report and engineering work was strictly done with modeling for the anticipated  forces  as well as structure installations.  The author has not found any further earthquake studies performed by or for MMS following this report despite the fact that two subsequent earthquakes of around the 6.0 magnitude occurred in 2006.



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