Mud, glorious mud

Meg Chesshyre

June 1, 2016

How to deal with drill cuttings piles as part of the decommissioning process has been a subject of study for some years. Meg Chesshyre examines how North West Hutton may show the way forward.

The day grab used in the 2013 survey. Photo from Gardline. 

Complete seabed recovery of the surface sediments from within 100m of North West (NW) Hutton drill cuttings pile could be possible by 2028, if recovery continues at the current rate, says Marion Collin, senior environmental report writer with survey firm Gardline Environmental.

The NW Hutton platform, in block 211/27 in the East Shetland basin, was decommissioned between 2008 and 2010.

A total of 53 wells had been drilled on NW Hutton, operated by BP, using oil-based drilling fluids (OBM), including diesel OBM until 1984; then lower-toxicity kerosene OBM until 1991. Before 1992, all drill cuttings were disposed of at the seabed, resulting in a cuttings pile about 31,000cu m in volume (200m x 150m x 5.5m).

The day grab used in the 2013 survey. Photo from Gardline. 

BP’s most recent seabed survey was carried out in 2013 from 30 stations using a 0.1sq m day grab, following surveys in 1992, 1997, 1999, and 2002. The objectives of the latest survey included gathering a baseline of the environment post decommissioning activity to fulfill the commitments within the decommissioning plan, to assess the natural recovery of the seabed over time and the reducing footprint of BP’s operation, and to assist regulatory discussions on the close out of BP activity at the site and to determine the future plans for longer term monitoring.

Collin, speaking at the Oceanology International conference in London this spring, said the 2013 survey had a “smart, efficient, intelligent, survey design.” In addition to tracking declining hydrocarbon and barium concentrations over time, she described samples involving two fauna indicator species collected at all stations along the northern transect – Capitella capitata, a polychaete worm, hydrocarbon tolerant, highly opportunistic with high abundances often found in and close to cuttings piles, and Owenia fusiformis, another polychaete worm, which is hydrocarbon intolerant.

A Capitella capitata worm. Image from MESL. 

Capitella capitata, the hydrocarbon-loving worm, was found in very high abundance 500m from the platform in 1992, but by 2013, was only present in quite low numbers only 100m from the platform site. By contrast, Owenia fusiformis, the hydrocarbon intolerant worm, was only visible in fairly low abundance 800m from the platform in 1992, but was in 2013 in much higher abundance from 200m from the platform.

Collin concluded that recovery of seabed surface sediments can happen in decades, not thousands of years, and that this can see a return of macrofaunal communities. Long-term monitoring to aid decommissioning strategies should thus be seen as having value. Also, by leaving parts of the seabed infrastructure and cuttings in place, further disturbance to the environment could be eliminated and allow for faster recovery.

Moving forward, Collin said that collaboration on surveys or survey cruises would save cost/time and make results and data more consistent across operators. It would also permit regional surveys, which would give an idea of the background environment. “If we don’t know what the environment is supposed to be like, how do we know if it is contaminated or if is recovering,” she asked.