Geological perils of deep water

David Knott
London
[email protected]

A few thousand dollars spent sifting 3D seismic data for information on seabed conditions can save millions on deepwater field development.

So Kerry Campbell, manager of geoscience markets at Fugro-McClelland Marine Geosciences Inc., Houston, told delegates at the recent Deep Offshore Technology conference in The Hague.

Campbell's message reinforced earlier presentations that made clear the value of geological information and skills to planning of field developments in deep water (OGJ, Nov. 10, 1997, p. 41).

Site conditions are not usually a concern early on in offshore field developments. On the Gulf of Mexico continental shelf, said Campbell, platforms are often designed without a formal site investigation.

Campbell says deepwater conditions begin at the end of the continental shelf, typically in 150 m of water or more. On the slope, conditions can get difficult very quickly.

"We looked at 360 blocks in deepwater Gulf of Mexico slope conditions," said Campbell. "Roughly 50% of these had complex topography compared with 7% on the shelf."

Typical hazards

Seabed hazards are the same in all deepwater areas, said Campbell. The difference from region to region is only in frequency of occurrence.

"Rocky sea floor is common in many deepwater areas, " said Campbell. "Outcrops can be as big as office buildings. Landslides are common in deep water but rare on the shelf.

"Many faults are still active today, and the density of faults can change dramatically over a short distance. Added to this, if there is free gas in more than 1,000 m of water, there will be hydrates."

Campbell told delegates of a few projects where lack of seabed knowledge led to problems, although he did not reveal the operator or field names.

One was a pipeline project for which 14 different routes were evaluated. All proved unacceptable because of spanning: "Our client ended up running 10 miles of flexible pipeline that was not budgeted for."

Then there was a floating production system planned without a site survey: "The FPS site location had to be moved several miles, into water 35% deeper than was designed for. The operator farmed the project out."

Cheap insurance

Campbell maintains these problems could have been avoided by utilizing available 3D seismic data to determine site conditions early on.

"When I first became aware of the value of 3D seismic for technical evaluation, I was amazed," said Campbell. "This data can be used in preliminary engineering to create a 'development favorability map'-a field plan highlighting areas of engineering concern."

Typical cost for a predrilling screening survey would be a few thousand dollars for a Gulf of Mexico block. An analysis of 3D data for preliminary engineering would cost a "few tens of thousands of dollars".

"Sometimes companies don't want to drill a $30 million well without knowing site conditions," said Campbell, "in case the well could not be used for later production.

"A site survey for $30,000 would represent cheap insurance. If you ignore deepwater seabed conditions, and if you are unlucky, you could have an expensive tiger by the tail."