Drilling Rig Anchoring Failure Incident Information

A few months ago PSA issued a new journal providing valuable information on some of the most relevant issues and challenges that are being faced by the oil & gas industry in the field of safety. The following information is an extract from PSA’s journal “Dialogue” regarding a mooring incident on the drilling rig Ocean Vanguard in 2004.

2013.11.20 - Drilling Rig Anchoring Failure Incident Information

Ocean Vanguard was lying on the Halten Bank during a storm in 2004 when two mooring lines suddenly lost tension, the riser was torn off and the rig ended up 160 metres from the well. Bad weather was on its way to the Halten Bank on 14 December 2004, and crew on Diamond Offshore’s Ocean Vanguard rig, which was drilling an exploration well for Eni, were preparing to ride out the storm. Winter in the Norwegian Sea demands its precautions.

During the night, the bit was 30 metres above the reservoir. The drill string had been pulled up and hung off in the wellhead, the blowout preventer was closed and mud in the riser replaced
with seawater. Everything was ready for disconnection. The weather steadily worsened. After 22.00, the wind reached 55 knots (27 metres per second), the significant wave height was 10.3 metres and the biggest waves topped 16.9 metres. Although these conditions were felt to the full on board, Ocean Vanguard had been built to tackle them. Eight strong mooring lines kept the heavy rig on station.

At 22.40, a loud bang was suddenly heard from mooring winches 1 and 2. The brakes failed, leaving mooring chain 1 to run out freely. Line 2 followed suit when all tension was thrown onto it.

Emergency disconnection of the riser was initiated. But a little time passes between pressing the button and activating the hydraulic system – by when it was already too late.

The drag from the six remaining mooring lines caused the big rig to list by eight to 12 degrees, and Ocean Vanguard was pulled 160 metres off location in the space of a few minutes.

Nobody was injured, and no pollution was registered. But extensive material damage was nevertheless caused to the wellhead, BOP, riser, riser tensioning systems and mooring winches.
“This incident was very serious,” admits Diamond Offshore executive Egil Kjær. “The well had been secured, but the outcome could have been much worse if the rig was still operational.”

Built in 1982, the semi-submersible unit had been owned by Smedvig Offshore before being taken over by Diamond Offshore in 2002. The Halten Bank well was its first on the Norwegian Continental Shelf (NCS) after a long spell on the UK continental shelf. Diamond Offshore’s Norwegian team comprised Kjær and three colleagues in Stavanger.

“When such an incident occurs, our emergency response team mobilises,” he explains. “We were rapidly reinforced by colleagues at the Aberdeen office and expertise from headquarters in Houston.” A demanding time followed. Specialists came from head office to help establish what had happened and the scale of damage to the rig. Kjær participated in the internal investigation.

The PSA conducted its own investigation, and found barrier failures at both Diamond Offshore and operator Eni. Among the underlying causes identified were

  • Inadequate maintenance of the mooring winches.
  • Ignorance of governing documents.
  • Insufficient maintenance documentation.

The incident resulted to a big financial loss. We were off contract for six months.”

That was a reasonably heavy blow, even at the rates which then prevailed. In addition came the cost of docking and repair, and of cleaning up the seabed on the Halten Bank. “Not a quick job,” Kjær admits. “The work lasted for much of 2005, 2006 and 2007. It was very educational for everyone. And the incident has meant a lot for us as a company. “We had to devote lots of effort to the follow-up, and to identifying other possible problems in our operating routines and the technical condition of the rig. “Extensive gap and risk analyses were conducted along with assessments of compliance with regulations and class rules, and we drew up a number of new procedures. “We revised our maintenance routines, the brake systems were upgraded and changed, and our training systems were reviewed.”

The Ocean Vanguard incident ranks as one of the most serious mooring accidents on the NCS, and overcoming such problems on mobile units has been a PSA priority for many years. A great deal of work has been devoted by the regulator to achieving risk reductions and to cutting the number of serious incidents.

The PSA registered a disturbing increase in incidents related to mooring systems a few years ago, but its initiatives and industry action have had positive results. For the first time in a decade, no cases of mooring lines being lost during operation were reported in 2010. Three such incidents occurred in 2011, including one where two cables broke. The 2012 figure was five incidents, again with one where two lines parted. So industry standards in this area still need to be raised.

Source: PSA

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