PRACtically Speaking

Research Profile        

Sediment Mobility on the Scotian Slope

Paul Hill, Department of Oceanography

Dalhousie University

Paul Hill, Oceanography professor at Dalhousie University, is assessing the mobility of seafloor sediment, a real risk to pipelines used to transport oil and gas.

Around the globe, the search for oil and gas continues to reach into deeper waters at the edge of the continents.  Exploration and production in deep water pose numerous technological challenges, not the least of which is how to get oil and gas back to shore.  Pipelines commonly are used because they can transport large volumes of oil or gas relatively safely.  A major concern with pipelines, however, is the stability of the seafloor underneath them.  If sediment is removed from beneath a pipeline resting on the seafloor, either by the action of waves and currents or by submarine landslides, the pipeline can rupture.  To avoid this risk, pipelines are buried below the depth of active sediment movement.  This solution, while effective, is also costly.  As a result, pipeline engineers need to assess the mobility of sediment extending all of the way from the shore into waters over 1000 m deep.

Figure 1.  The instrumented frame sitting on the dock at Bedford Institute of Oceanography.  The EMCMs are electromagnetic current meters.  The OBSs are optical backscatterance sensors used for estimating sediment concentration in suspension.  The V-camera is a video camera designed to collect images of the seabed.  The PC-ADP is a sensor that measures profiles of flow velocity all of the way to the seabed.  The upward-looking ADCP measures profiles of flow velocity above the instrument frame.  The ABS measures profiles of acoustic backscatter, which is related to sediment concentration in suspension.  The ADVO is an acoustic velocity sensor that measures flow velocity at 52 cm above the bottom.

With the support of PRAC and NSERC, an instrumented frame (Figure 1) was deployed in 280 m of water during the late summer and early fall in 2002 and again in 2003.  The deployment site is on the so-called “Halifax Line” that extends southeastward from Halifax across the continental shelf.  A wide range of sensors for measuring water motion and sediment concentration were affixed to the frame.  These deployments were made possible by close collaboration among scientists in the Department of Oceanography at Dalhousie University, at the Geological Survey of Canada Atlantic and in the department of Fisheries and Oceans Canada. During the 2003 deployment several sensors yielded evidence of active sediment transport.  A camera that takes pictures of the seabed revealed linear ripples (Figure 2).  Ripples like these form when currents are strong enough to erode and transport sediment.  Because the activities of bottom dwelling organisms tend to smooth and erase such ripples, they do not remain on the seafloor for long unless they are actively maintained by nearbed currents.  Their presence therefore indicates transport likely occurred at the site within the week previous to the image.  An acoustic current meter located 52 cm above the seabed measured bottom currents.  These data were used to estimate the stress on the seabed as a function of time in 2003.  The data indicate that the bottom stress exceeded that required to erode sediment at the site several times during the deployment period.  We are now in the process of quantifying the magnitude of sediment transport flux during these periods of elevated stress.

Figure 2.  Image of the seabed collected with the video camera at the beginning of the 2003 deployment. The image is approximately 0.5m across.  The linear features trending from the top left to bottom right are sediment ripples formed by transport of sediment near the seabed.

In summary, on the upper slope of the Scotian margin sediment apparently is re-suspended regularly under typical oceanic conditions.  The mobility of the seafloor in this region therefore may pose a challenge to pipeline engineers.  The data act as a reminder that deep waters remain a relatively under-explored frontier on Earth.  The measurements gathered in this project will help to ensure safe and sustainable use of deepwater resources in the years to come.  As we continue to explore, however, we certainly are in for some surprises.

PRAC News                                

PRAC teams up with CERI to offer industry courses

PRAC has teamed up with the Canadian Energy Research Institute (CERI) to offer four industry courses throughout the region over the next two months.

On September 29^th and 30^th, the first course in this series titled “Introduction to the Canadian Natural Gas Industry: From Wellhead to Burner Tip” will take place in Moncton.

The subject matter of the three remaining courses includes “Electric Industry Fundamentals and Restructuring” to “Natural Gas Market Fundamentals” and “Upstream Petroleum Industry”.

Courses will be offered at various locations throughout the region, including: Moncton, Saint John, Halifax and St. John’s.

Click here for more information.

Trans Ocean Gas Inc. awarded grant for Transfer to Usability (TTU) project

PRAC has recently awarded funding to Trans Ocean Gas Inc. of St John’s, NL for a project that will test the safety and reliability of fiber reinforced plastic (FRP) pressure vessels to be used for the transportation of compressed natural gas (CNG) by ship. 

Model of FRP Vessels

PRAC’s $50,000 contribution toward this project was instrumental in Trans Ocean Gas obtaining additional funds in the amount of $1.4 million.

“Although this technology holds world-wide appeal, it may also facilitate the exploitation of vast stranded natural gas reserves off the East Coast of Canada.  Until now, stranded natural gas fields were too isolated or too small to make use of expensive pipeline or liquefied natural gas (LNG) systems,” says John Melville, director of research planning and industry liaison with PRAC. 

Currently, there are billions of dollars worth of stranded natural gas located offshore in the Atlantic region. 

“Trans Ocean Gas CNG technology has the potential to change this forever, and may allow smaller, more isolated fields to be developed at less cost than would otherwise be the case with older technologies,” says Melville.

The project is due for completion in July 2006.

Eminent speaker grant program continues

PRAC is continuing its program to help offset costs associated with hosting internationally recognized speakers in subject areas relevant to Atlantic Canada’s offshore and onshore petroleum industry. 

The program is available to universities, colleges, not-for-profit organizations and research institutes located in the Atlantic Provinces. 

Grants of up to $2000 per speaker are available on a first come, first served basis and will generally cover 50 per cent of eligible costs. 

To date, three grants have been awarded.

For full details and an application click here.

Comments, questions welcomed!

Please contact communications@pr-ac.ca with any questions or comments about this newsletter and visit the rest of PRAC's website for more information.


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