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Deep water currents are a major hazard in the Gulf of Mexico. Loop current events come through the straits of Yucatan, loop up into the eastern Gulf, then reverse and head down to Florida and the Keys. Here they enter the Gulf Stream travelling across the Atlantic to northern Europe. Every 6 to 12 months an eddy as much as 320 km across drifts north and west into the main area of offshore drilling, the zone of fast moving water extending several hundreds of metres below the surface.
Such currents place tremendous forces on risers and create vortex-induced vibration that can lead to riser fatigue. This is because water flowing around a cylinder creates a steady drag that pushes the structure along with the current. To successfully perform open water drilling operations in a loop current prone area, the shallow part of the hole that requires the largest riser needs to be drilled within a current window. At the same time retrievable riser fairings and other external attachments must be used to streamline the system.
Icebergs present a problem in northern waters. In Canada, off Newfoundland where icebergs are a constant threat, development of the Hibernia giant oil field was delayed by this problem. It finally came onstream in 1997, 14 years after discovery, using a huge gravity base structure. In the same area Terra Nova, discovered in 1994, has been developed with an FPSO with a retractable and disconnectable riser system so that it can be moved out of the way of icebergs. Subsea wellheads are contained in steel glory holes beneath the seabed to prevent damage from grounding ice.
Storms and high seas are an ever-present hazard in the world's northern latitudes, especially the North Sea, Norwegian Sea and the North Atlantic. All offshore structures in these regions are designed to withstand storm force winds and high seas. Hurricanes are an even bigger problem in the tropics, especially the Gulf of Mexico and the South China Sea. Several rigs were damaged during Hurricane Lili in the Gulf of Mexico in 2002 whilst hurricanes Katrina and Rita in 2005 caused massive damage including partially destroying Shell's Mars platform. Gulf of Mexico rigs and platforms during the hurricane season are on constant alert for weather events although the years 2006 and 2007 were relatively benign.
There are other surface accidents that can occur to rigs even in mild weather conditions.In 2002 Schlumberger/Arabian Drilling Company's , sank in the benign waters of the Persian Gulf off Saudi Arabia following an accident that occurred as the rig was being positioned over a Khafji field well. A leg buckled and when the rig collapsed the production tree was sheared, resulting in a blowout that ultimately sank the rig. The operator AOC, an Aramco Joint Venture company, mobilized the AD17 to drill a relief well in an effort to regain control of, or plug the damaged well.
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