The storm magnitudes of Hurricanes Katrina and Rita exceeded the design criteria of most of the jack up rigs caught in their paths through the central Gulf of Mexico, said industry experts at a recent Houston workshop sponsored by the International Association of Drilling Contractors.
Some 12% of the gulf jack up rig fleet was lost or moved off locations as those two hurricanes cut through the central gulf within 24 days of each other in August-September 2005. The primary causes of those rig movements and losses were wind and currents, wave inundation, and foundation or soil movement, officials said. The best news, industry sources agreed, is that there was no loss of life, no threat to safety, and only minimal environmental impact. That validates the “shut in and evacuate” response to hurricanes, said offshore veterans.
Assessments of the three worst hurricanes to hit the oil and gas areas of the Gulf of Mexico in recent years-Katrina and Rita in 2005 and Ivan in 2004-confirm that the magnitude of those storms exceeded the criteria of all jack ups that were destroyed or floated ashore, said Alberto Morandi, executive vice-president of American Global Maritime Inc., Houston. Storm magnitudes also exceeded all of the rigs that survived “in every case but one,” he said.
The lone exception was Rowan Cos. Inc.’s Class 224C cantilevered Super Gorilla jack up “Bob Palmer,” named for Rowan’s Chairman Emeritus C.R. “Bob” Palmer. Outfitted with 713 ft legs, 139 ft more than the company’s late-model Gorilla rigs, and with 30% larger spud cans, the Super Gorilla can work in 550 ft of water in the Gulf of Mexico.
Otherwise, Morandi said, “no one parameter” determined which rigs failed and which survived the three worse hurricanes. “There were rigs in shallow water that failed, rigs in deeper water that survived and rigs that exceeded their vertical reaction and survived,” he said. Among both survivors and wrecked units, there was “a combination of different factors,” he said.
“One MTL 166C rig was subjected to hurricane loading that was 80% over its design load. The combination of large (96 ft) penetration (high fixity) and high soil reserve strength contributed to the rig’s survival without damage,” Morandi reported. On another jack up that was lost, there was “clear evidence” on the sea floor that the spud can had dragged out of the can hole, creating stress that caused the rig to overturn, he said.
Both rig survivals and losses “can be largely explained in terms of a combination of higher or lower reserve strength, favorable or unfavorable soil conditions, and favorable or unfavorable storm headings,” Morandi said. “Leg settlement may have contributed to wave impingement on the deck, further increasing the total load.”
He said, “An air gap of 60-62 ft would have been sufficient to clear the hurricane crests, with some allowance for leg settlement (water depth over 80 ft). This would also be in agreement with an additional number of locations investigated by the LeTourneau Inc. Marine Group. [It is] also in agreement with fixed platform observations.”
Historically, design limits for elevated jack ups are determined via class-approved benchmark storms. The American Bureau of Shipping’s code requires mobile offshore drilling units to be designed to withstand a storm wind velocity of 100 knots for unrestricted offshore service. Other parameters include water depth, wave height and period, air gap, spud can penetration, and current velocity. “The combination of such parameters has to be such that adequate safety factors are met for relevant failure modes,” said Morandi.
In practice, however, the benchmark storm parameters do not reflect site-specific soil conditions or air gap requirements and are not directly related to a return period storm load or a Saffir-Simpson Hurricane Scale. Therefore, Morandi said, “A site-specific assessment is needed, which can be performed at different levels of detail.”
Designing jack ups is “a very complex task with a lot of trade-offs,” Morandi noted. The complexity of industry and regulatory standards is an important factor, especially in today’s “very tight” market for experienced engineers. Such specialized knowledge is more important as standards become more and more complex, he said.
Regulators and drilling contractors are looking harder at installation issues as well. “Jack ups are tremendously affected by soil conditions, which can lead to shallow penetrations and other problems,” Morandi said.
With strong soil structure, a jack up rig can take more of a storm load than it was designed for, he said. However, he said, “Once one leg fails, the other two will go.”
ABS guidelines
The American Bureau of Shipping plans to publish by yearend new guidelines for the inspection of mobile offshore rigs more than 20 years old, said Chris Serratella, ABS director of operational safety and evaluation, at the Oct. 19 workshop.
Current recommendations for new guidelines include more-detailed, close-up survey, nondestructive testing, and thickness gauging of major joints and fatigue-sensitive areas under survey plans that rig owners would create and submit to ABS prior to inspection. These will “ensure that critical areas are surveyed in proper sequence using appropriate techniques,” Serratella said.
More emphasis should be placed on corrosion and coatings maintenance, “especially of internal surfaces.” Critical inspection areas on jack ups include legs, leg well areas, spud cans, cantilever push-up and hold-down structures, crane pedestal connections, mud pits and preload tanks for corrosion.
Moreover, ABS is compiling inspection and wear data on “any rig or rig design where we have enough history that we can go back and pinpoint the critical areas that we need to look at,” Serratella said. “Rather than just look at specific rigs, we would look at the rig design and all the information that ABS gathers on those rigs, then turn around and give that as guidance to the surveyors.”
Conversely, he said, ABS wants to help inspectors “stop wasting time” on close inspection and analysis of areas that “never show any problems” despite frequent inspections and instead focus on those areas with an evident history of problems.