Back from the brink
  from: Offshore Engineer
  by: Rick von Flatern
  Monday, August 01, 2005

On Friday 8 July, commissioning crews onboard the BP Thunder Horse platform moored in 6050ft of water in the Gulf of Mexico's Mississippi Canyon 778, 150 miles southeast of New Orleans, went through abandonment procedures and departed the installation ahead of the arrival of Hurricane Dennis. By the following Monday morning the world's largest semisubmersible production platform was listing at an estimated 20°, one corner of its topsides beneath the water's surface.

While none of Thunder Horse's wells were in production or had been tied back to the platform at the time of the abandonment, three subsea trees had been installed on live wells located on the seafloor directly beneath the platform.

Within the week BP had turned over responsibility for bringing Thunder Horse right again to Rotterdam-based Smit Salvage. At one point, as many as 700 personnel and 15 vessels including Heerema's massive Balder deepwater construction vessel, were dispatched to the area and another 200 BP personnel were providing engineering and technical support from the company's Houston office.

By 19 July, as OE went to press, BP spokesman Ronnie Chappell confirmed that pumping efforts had restored the vessel to normal trim, freeboard and displacement, and all but residual water had been evacuated from the hull's two flooded columns. BP and Smit were continuing to make the platform seaworthy and storm-safe as a precaution against changes in weather conditions due to Hurricane Emily entering the western Gulf of Mexico.

But even as the good news of Thunder Horse's righting continued to come in, what caused its columns to flood remained a mystery. The hurricane, according to a nearby buoy, created no waves larger than 30ft, far too small to affect the massive structure's stability.

With BP unable to explain the event, speculation in the media and elsewhere seemed to center on improper crew abandonment procedures or automatic ballasting system failure. Analyzing the former, if it is indeed considered, would take considerable time and an answer could be long in coming and likely subject to interpretation.

The latter event seems unlikely as during abandonment the main generator is left shut down with only an auxiliary generator left online to control navigation aids such as horns and lights. The smaller generator cannot provide sufficient power to run the ballasting systems.

During recovery operations, BP reports, all possible sources of water ingress along the hull were covered and no breaches of the hull were found during ROV-based inspections.

At one point, while pumping water from the columns, the company cut one of the 16 mooring lines to determine if that would hasten the vessel's righting. But once done, Chappell said, the company decided to focus on the pumping operations.

With the vessel now at normal trim, the company will attempt to discover the causes of the near-disaster and the extent of the damage. Depending on the outcome of that investigation, BP will decide if the semi must be towed to shore for repairs. Assessment of topside damage, if any, had yet to be begin as OE went to press, engineers at the scene having had their hands full directing recovery operations.

The field was expected to be onstream by the end of the year. The semisubmersible was installed on location in late May and was beginning hookup procedures prior to the evacuation. BP is 75% owner of Thunder Horse with Exxon- Mobil holding the remaining 25% interest. OE

BEFORE AND AFTER: (photos below)
The 59,000t Thunder Horse semisubmersible, the world's largest, was found to be listing approximately 20 degrees on 11 July (pictured top) following evacuation from Hurricane Dennis. By 19 July, after a joint effort spearheaded by BP, the US Coast Guard and the Minerals Management Service and involving more than 900 personnel, the platform had been returned to its proper trim. BP is still investigating the cause of the incident.

Photos courtesy of BP, LEWCO Systems and USCG.

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