ANCHORAGE, Alaska -- Shell Oil's flotilla of Arctic Ocean vessels is heading for warmer waters.

Royal Dutch Shell PLC announced that it concluded exploratory drilling on Wednesday, its mandatory cutoff date before winter. It completed preliminary drilling at one well at the Burger-A Prospect 70 miles offshore in the Chukchi Sea and one at the Sivulliq Prospect 18 miles offshore in the Beaufort Sea.

"We're looking forward to revisiting these wells as soon as we can next year," Shell Alaska spokesman Curtis Smith said by phone from Prudhoe Bay.

The end of drilling by Shell's two drill ships and about 20 support vessels wraps up a tumultuous season that saw the company penetrate the ocean floor for the first time in more than two decades, finally making progress on an Arctic offshore investment of more than $4.5 billion, including $2.1 billion for Chukchi leases in 2008.

Shell began initial drilling but was prohibited from extending the wells into petroleum reservoirs until it could stage a spill response barge near the exploratory wells. But there were problems getting the spill barge certified by the Coast Guard. Another complication was the continued protest by environmental groups that contend oil companies cannot clean up a spill in waters choked by, or covered by, sea ice.

"This year showed to all of us that even one of the world's biggest companies wasn't ready to move forward in the Arctic," said Mike Levine, an attorney for Oceana, from his office in Juneau.

A Shell drill ship pulled its anchor and nearly ran aground at an Aleutians Island port, Levine said, and the company damaged its new containment dome as it tested its spill response barge off the coast of Washington state.

Less than 24 hours after drilling began Sept. 9 in the Chukchi, Shell moved it drill ship as a precaution because an ice floe 30 miles long and 12 miles wide was heading toward the prospect.

"These problems don't give us confidence that this company is ready to pursue a massive industrial undertaking in one of the remotest places on the planet," Levine said.

The company considers the season a success, Smith said. Moving off a well to dodge ice moving in from a hundred miles away demonstrated the company's Arctic preparedness, he said. Lessons learned from rotating hundreds of workers onto vessels and managing equipment in cold weather will be used to drill up to 10 exploratory wells during the multi-year project, he said.

Shell this year completed two top holes that will be the foundation for a pair of exploratory wells. Top holes consist of mud-line cellars 20 feet in diameter and 40 feet deep that provide protection from ice for a blowout preventer, plus a hole drilled to about 1,500 feet. Drill ships next year will uncap the top holes and drill to oil-bearing rock.

Shell contends that a spill is unlikely in the relatively shallow Arctic waters and that its response gear would quickly address a blowout.

Levine said a response to a blowout like BP PLC's disaster in the Gulf of Mexico would be hindered by the Arctic's storms, wind, ice and darkness. The federal government, he said, is not keeping its commitment for safe drilling.

"We're seeing a government agency that in large part has ignored those realities and bend over backward to allow Shell to undertake these activities," he said.

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  • Niger Delta ExxonMobil Spill, Nigeria - May 2010

    In Nigeria's Akwa Ibom State, an ExxonMobil pipeline ruptured on May 1 and spilled over a million gallons of oil, <a href="http://www.guardian.co.uk/world/2010/may/30/oil-spills-nigeria-niger-delta-shell" target="_hplink">reported the <em>Guardian</em></a>. The leak continued for seven days before it was stopped. <a href="http://www.huffingtonpost.com/omoyele-sowore/the-oil-spill-no-ones-tal_b_649220.html" target="_hplink">HuffPost blogger Omoyele Sowore explained</a> in July 2010 that an oil spill from ExxonMobil operations was nothing new to the country. He wrote that an "environmental catastrophe [had] been going on since December 2009." He described the toll on Nigeria: "There's oil on the surface of the ocean, wildlife coated in crude, fishermen losing their businesses."

  • Trans-Alaska Pipeline Spill - May 2010

    In May 2010, several thousand barrels of oil spilled from the Trans-Alaska pipeline "during a scheduled pipeline shutdown at a pump station near Fort Greely," <a href="http://www.huffingtonpost.com/2010/05/26/alaska-oil-spill-trans-al_n_589974.html" target="_hplink">explained AP</a>. No injuries were reported and officials said the spill was likely "limited to the gravel on top of the containment area's line."

  • Red Butte Creek Spill, Utah - June 2010

    In June 2010, a Chevron pipeline ruptured and spilled oil into a creek near Salt Lake City, Utah. It was first estimated that over 400 to 500 barrels spilled into the creek, which leads into the Great Salt Lake, <a href="http://www.huffingtonpost.com/2010/06/12/utah-oil-spill-500-gallon_n_610232.html#s99698" target="_hplink">reported AP</a>. Around 150 birds were "identified for rehabilitation." The <a href="http://www.huffingtonpost.com/2010/06/14/utah-oil-spill-officials-_n_611014.html" target="_hplink">oil did not reach the Great Salt Lake</a>, however. <a href="http://www.huffingtonpost.com/2010/07/14/chevron-cited-for-oil-spi_n_646340.html" target="_hplink">Chevron was later cited for the spill</a>, which released an estimated 33,000 gallons in total. In March 2012, <a href="http://www.huffingtonpost.com/huff-wires/20120324/us-oil-spill-utah/" target="_hplink">a group of 66 residents of a Salt Lake City neighborhood sued Chevron</a> for damage caused by the Red Butte Creek spill and a smaller spill in December 2011.

  • Kalamazoo River Spill, Michigan - July 2010

    In late July 2010, an Enbridge pipeline in southwestern Michigan sprung a leak and <a href="http://www.huffingtonpost.com/2010/07/27/michigan-oil-spill-among_n_661196.html" target="_hplink">spilled over 800,000 gallons of oil into a creek</a> which flows into the Kalamazoo River. By August, a regional EPA administrator said that significant progress had been made at the site, but "the agency cautioned that it will take months to complete the cleanup," <a href="http://www.huffingtonpost.com/2010/08/02/michigan-oil-spill-epa-of_n_667556.html" target="_hplink">reported AP</a>. By the end of September, <a href="http://www.huffingtonpost.com/2010/09/27/michigan-oil-pipeline-res_n_741233.html" target="_hplink">the pipeline -- which travels from Ontario to Indiana -- was back in operation</a>. The EPA later <a href="http://www.huffingtonpost.com/huff-wires/20111116/us-michigan-river-oil-spill/" target="_hplink">reported that about 1.1 million gallons of oil were recovered</a>, but pipeline operator Enbridge said that it would stick with previous estimates that only about 843,000 gallons were spilled.

  • Xingag Harbor Spill, Dailan, China - July 2010

    In July 2010, China experienced what was reported as the "<a href="http://www.huffingtonpost.com/2010/07/21/china-oil-spill-grows-off_n_653852.html#s120708" target="_hplink">country's largest reported oil spill</a>" after a pipeline rupture near the northeastern port city of Dailan. Several days after the spill, cleanup efforts were underway over a 165 square mile (430 square kilometer) area of the Yellow Sea. The Chinese government reported that about 1,500 tons or 461,790 gallons of oil had spilled, but <a href="http://www.huffingtonpost.com/2010/07/30/china-oil-spill-far-bigge_n_665038.html#s120708" target="_hplink">experts contended that the spill could have been "dozens of times larger,"</a> reported AP.

  • Peace River Spill, Alberta, Canada - April 2011

    In late April 2011, a pipeline in northwestern Alberta began leaking, and created the worst spill in the province in 36 years, <a href="http://www.calgaryherald.com/news/alberta/Rainbow+pipeline+leak+largest+years/4720888/story.html" target="_hplink">reported the <em>Calgary Herald</em></a>. About 28,000 barrels of oil were reportedly spilled from the Rainbow pipeline, which is operated by Plains Midstream Canada. The <em>Globe and Mail</em> revealed that the pipeline operators "<a href="http://www.theglobeandmail.com/report-on-business/oil-on-rainbow-line-halted-8-hours-after-problem-detected/article2013335/" target="_hplink">detected a potential problem nearly eight hours before halting the flow of crude</a>." A nearby school in a First Nation community was closed after residents reported "nausea, burning eyes and other symptoms," and several animals were found dead. In late July, Plains Midstream <a href="http://www.huffingtonpost.ca/2011/07/28/alberta-pipeline-owner-as_n_912796.html" target="_hplink">requested to re-open the pipeline</a> and begin to ship oil to Edmonton again.

  • Bohai Bay Spill, China - June 2011

    In June 2011, an oil <a href="http://www.huffingtonpost.com/2011/07/01/china-offshore-oil-spills-bohai-bay_n_888473.html" target="_hplink">spill occurred about 25 miles off the coast of China's Shandong province</a> in Bohai Bay. A second spill followed in July. In late August, <a href="http://www.huffingtonpost.com/2011/08/26/new-seeps-in-china-spill_n_937809.html" target="_hplink">it was reported</a> that ConocoPhillips had discovered more oil seeps in Bohai Bay, although only "1 to 2 liters (a quarter to a half-gallon) of oil and drilling mud were being released each day." The company reported that the 2011 spills released 700 barrels of oil and 2,500 barrels of drilling mud into the bay and that most of it was recovered. In September, <a href="http://www.huffingtonpost.com/2011/09/05/china-oil-spill-conocophillips_n_949745.html" target="_hplink">China's State Oceanic Administration claimed that oil was still seeping</a> underwater. In early 2012, Texas-based ConocoPhillips <a href="http://www.huffingtonpost.com/huff-wires/20120124/as-china-oil-spill/" target="_hplink">reached a settlement deal with the Chinese government</a> for $160 million.

  • Yellowstone River Spill, Montana - July 2011

    In July 2011, a pipeline beneath Montana's Yellowstone River ruptured and sent an oil plume 25 miles downstream, <a href="http://www.huffingtonpost.com/2011/07/02/yellowstone-oil-spill-pro_n_889363.html" target="_hplink">reported AP</a>. Despite reassurances from ExxonMobil that the pipeline was safe, <a href="http://www.huffingtonpost.com/2011/07/06/yellowstone-river-oil-spill-exxon-mobil_n_891246.html" target="_hplink">the July spill released what was originally estimated to be 42,000 gallons of oil</a>. With other 1,000 workers assisting the cleanup, ExxonMobil <a href="http://www.huffingtonpost.com/2011/11/04/yellowstone-river-cleanup-costs_n_1077106.html" target="_hplink">estimated that it would cost $135 million to clean the river</a>. In January 2012, <a href="http://www.huffingtonpost.com/2012/01/19/exxon-yellowstone-oil-spill_n_1216830.html" target="_hplink">it was reported</a> that ExxonMobil had increased its estimate of the spill size by 500 barrels. AP later <a href="http://www.huffingtonpost.com/2012/04/06/yellowstone-river-spill-response-plan_n_1408328.html" target="_hplink">reported the estimated spill size as 63,000 gallons</a>. <em><strong>CORRECTION:</strong> A previous version of this slide stated the estimated spill size as 63,000 barrels instead of gallons.</em>

  • North Sea Spill, United Kingdom - Aug. 2011

    In August 2011, <a href="http://www.huffingtonpost.com/2011/08/15/north-sea-spill-uk-oil-spill_n_927078.html" target="_hplink">an oil rig off the eastern coast of Scotland began leaking</a> oil into the North Sea. Royal Dutch Shell, which operates the Gannet Alpha oil rig, initially reported that 54,600 gallons of oil were spilled. A second leak soon occurred, turning the spill into the worst in the North Sea in a decade, <a href="http://www.huffingtonpost.com/2011/08/16/north-sea-oil-spill-shell_n_927941.html" target="_hplink">reported AP</a>. Several days later, Shell announced that it had "closed a valve from which oil was spilling into the North Sea," <a href="http://www.huffingtonpost.com/2011/08/19/shell-says-it-has-closed-_n_931379.html" target="_hplink">according to AP</a>. The spill released about 1,300 barrels of oil, which spread out over a 2.5 square mile (6.7 square kilometer) area.

  • Campos Basin Spill, Rio de Janeiro, Brazil - Nov. 2011

    In mid-November 2011, Brazilian authorities began investigating an offshore spill near Rio de Janeiro, <a href="http://www.huffingtonpost.com/2011/11/17/chevron-oil-spill-brazil_n_1100062.html" target="_hplink">reported AP</a>. Chevron initially reported that between 400 and 650 barrels of oil had spilled into the Atlantic, while a nonprofit environmental group using satellite imagery estimated that the spill rate was at least 3,738 barrels per day. Chevron soon claimed full responsibility for the spill. The brazilian division's COO said, Chevron "<a href="http://www.huffingtonpost.com/2011/11/20/brazil-oil-spill-chevron-_n_1104070.html" target="_hplink">takes full responsibility for this incident</a>," and that "any oil on the surface of the ocean is unacceptable to Chevron," reported AP. In December, Brazilian prosecutors announced that they were <a href="http://www.huffingtonpost.com/2011/12/14/brazil-oil-spill-chevron-payment_n_1149554.html"target="_hplink">seeking $10.6 billion in damages</a> from Chevron for the spill that leaked nearly 3,000 barrels of oil. In March 2012, a Brazilian federal judge <a href="http://www.huffingtonpost.com/2012/03/18/brazil-oil-spill-chevron_n_1355923.html" target="_hplink">allowed prosecutors to file criminal charges against Chevron and Transocean</a> and 17 executives from both companies were barred from leaving Brazil.

  • Rena Spill, New Zealand - Oct. 2011

    In October 2011, a Liberian-flagged cargo ship ran aground on a reef in Northern New Zealand and began leaking oil. With oil washing up on shore, a government minister deemed it <a href="http://www.huffingtonpost.com/2011/10/11/new-zealand-oil-spill_n_1004643.html" target="_hplink">the country's largest maritime environmental disaster</a> a week later. Although over 2,000 sea birds were killed by the spill that spilled about 400 tons of fuel oil, <a href="http://www.huffingtonpost.com/2011/11/22/new-zealand-oil-spill-penguins_n_1107289.html" target="_hplink">343 little blue penguins were rescued and cleaned of oil</a>. <em>[<a href="http://www.huffingtonpost.com/2011/12/22/new-zealand-oil-spill-penguins-released_n_1151788.html" target="_hplink">Watch video of the penguins' release into the wild here.</a>]</em> In January, <a href="http://www.huffingtonpost.com/2012/01/10/new-zealand-oil-spill-ship_n_1196568.html" target="_hplink">half of the stricken Rena began sinking</a> into the sea after breaking apart and spilling over 100 cargo containers.

  • Nigeria Oil Spill - Dec. 2011

    The spill, which took place near the coast of Nigeria, was reported as "likely the worst to hit those waters in a decade," <a href="http://www.huffingtonpost.com/2011/12/22/shell-oil-spill-nigeria_n_1164891.html" target="_hplink">according to AP</a>. After two days, the spill had affected 115 miles (185 kilometers) of Nigerian coastline. Several days after the December 20 spill, <a href="http://www.huffingtonpost.com/2011/12/26/shell-nigeria-oil-spill_n_1170198.html" target="_hplink">Shell reported that the leak -- which occurred about 75 miles offshore -- had been contained before it reached the Nigerian coast</a>. The spill, which covered 350 square miles of ocean at its peak, was reported as having released less than "40,000 barrels -- or 1.68 million gallons" of oil.

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