Study Finds That Major Hurricanes Produce Seabed Forces as Deep as 90m That Could Threaten Gulf Oil Pipelines
A study by a team at the US Naval Research Laboratory found that strong surface waves and currents generated by major hurricanes can produce extreme forces at the seabed that scour the seafloor and cause massive underwater mudslides. These forces could snap offshore oil pipelines in the Gulf of Mexico and other hurricane-prone areas, the study suggests.
The Gulf of Mexico (GOM) region provides nearly 30% of the United States oil supply and 20% of its natural gas. Hurricanes are major threats to the integrity of offshore operations over the GOM outer continental shelf. Significant damage can occur to underwater pipelines and to other underwater infrastructures such as oil and gas platforms. There are reportedly at least 50,000 km [31,069 miles] of pipeline on the seafloor of the GOM. Damage to pipelines, which often is difficult to detect unless the damage is catastrophic, can be more costly to repair than damage to the superstructures on platforms. Major oil leaks from damaged pipelines could have irreversible impacts to the ocean environment.
—Wijesekera et al.
These pipelines could crack or rupture unless they are buried or their supporting foundations are built to withstand these hurricane-induced currents. The study will be published in the 10 June issue of the AGU journal Geophysical Research Letters.
With the official start of hurricane season approaching on 1 June, news reports about the Deepwater Horizon oil spill that began fouling the Gulf last month have raised questions about how a hurricane might complicate the unfolding disaster.
A hurricane might also create its own spills, the new research indicates. The storms’ powerful winds can raise waves 20 meters (66 feet) or more above the ocean surface. But their effects underwater are little known, although signs of seafloor damage have showed up after some hurricanes.
Based on unique measurements taken directly under the category-4 hurricane Ivan, the new study’s calculations are the first to show that hurricanes propel underwater currents with enough force to damage pipes or other equipment resting on the bottom.
The research team found that strong currents along the sea floor during Ivan pushed and pulled on the seabed, scouring its surface.
Usually you only see this in very shallow water, where waves break on the beach, stirring up sand. In hurricanes, the much bigger waves can stir up the seafloor all the way down to 90 meters [295 feet].
—David Wang, co-author of the study
Ivan’s waves on the surface created powerful currents that dug up the seafloor. Acoustic measurements using sound waves showed that these currents lofted a lot of sediments, which clouded the water up to 25 meters (82 feet) above the seafloor. The team’s seafloor sensors tracking the pressure underwater experienced a big increase, as well. This showed that the ground was washed away beneath the sensors, causing them to sink into a lower, higher-pressure zone.
Using a computer model of wave-induced current stresses, the team estimated how powerful currents would need to be for forces they exert at the sea floor to exceed a “critical force” that triggers sediment suspensions and could lead to underwater mudslides.
According to these estimates, hurricanes considerably weaker than Ivan could still tear up the seafloor. The researchers were also surprised by how long the destructive currents persisted after Hurricane Ivan passed by.
The stress on the sea floor lasted nearly a week. It doesn’t go away, even after the hurricane passes.
—Hemantha Wijesekera, lead author of the study
The researchers say they’re not sure what strengths of forces underwater oil pipelines are built to withstand. However, “hurricane stress is quite large, so the oil industry better pay attention,” Wijesekera says.
The Office of Naval Research funded this study.
Wijesekera et al. (2010) High Sea-Floor Stress Induced by Extreme Hurricane Waves. Geophysical Research Letters, in press. doi: 10.1029/2010GL043124 when published on 10 June