Atlantic Margin conditions reach environmental extremes

Waves, current, cold, thermocline impact floating, subsea installations

Leonard Le Blanc
Editor

Oxygen solubility in seawater is at a maximum near freezing and drops as temperatures warm up. Cold seafloor conditions are ideal for fast corrosion of equipment. [6,939 bytes]
What eastern North Atlantic operators face [209,898 bytes]

Two regions on the globe feature wave heights that top 20 ft over 15% of the time - the South Pacific west of southern Chile and the North Atlantic between Newfoundland and the British Isles. Environmental extremes in these two areas produce conditions that have far-reaching impacts. With an unimpeded distance (fetch) of 1,000-1,500 miles and very deep water, storms in the North Atlantic build very large waves.

These environmental conditions begin with the Northern Hemisphere jet stream that circles the globe at high altitudes. The jet generally sweeps along the New England coastline as it exits North America, and bears northeast into the higher latitudes paralleling the continental shelf off Nova Scotia and Newfoundland. The jet continues climbing through the latitudes until deflected by the thick cold air masses moving down from the Arctic off Greenland and Iceland. From this point, it turns due east and barrels into the British Isles and northern Europe.

This high-altitude jet is the major steering current for weather systems across the North Atlantic much of the year. As the jet exits North America and turns northeast, it races over two massive convergences taking place below:

Ocean currents: The warm Gulf Stream current sweeps northward along the edge of North America and collides with the Labrador Current, pushing down from the Davis Strait, and the Eastern Greenland Current. Both currents have Arctic origins.
Air currents: Above, warm and wet air masses from the equator are rotated northward along the US Atlantic seaboard, and eventually contact southward-bearing Arctic winds somewhere south of Greenland.

Steep gradients

These ocean and air convergences take place along a line that stretches from southern Newfoundland to Iceland. The air convergence typically produces fog banks, but during the fall and winter, the warm and cold air masses move swiftly, producing steep temperature gradients. From these gradient extremes, dangerous North Atlantic storms emerge. Most often, the storms are pulled eastward by the dominant jet stream. Occasionally, the jet stream moves away and eddies may pull the storms west or south. The storms and huge waves aren't the only elements in motion across the North Atlantic, however.

Below the waves, the northwest flowing warm Gulf Stream currents are pulled into the giant North Atlantic Gyre that pushes northeast across the North Atlantic at a one-to-two knot clip. As the warm currents reach the eastern Atlantic, the flow is divided. Part of it bathes the coastline of the British Isles and southern Europe (which also keeps Europe warmer than it would be for that latitude) before turning south and sliding over a denser current exiting the Mediterranean Sea. The remaining Atlantic current is diverted northeast between Scotland and Iceland, where it dissipates in the Norwegian Sea and Barents Sea.

The eastward current flow and storms also impact the North Sea with some of the roughest weather experienced by oil and gas drillers and producers. The North Sea's shallow shelf, coupled with the long fetch provided storms arising to the west and northwest, are why storm wave heights in the North Sea are so impressive. Wave height is exaggerated by current flow on the shelf. Water piling into the North Sea from the west and northwest has to escape somewhere, and tends to flow off the shelf toward the north, feeding at a slight angle into approaching waves.

Atlantic Margin encounter

Air and water convergences in the North Atlantic create major problems as they flow into the Atlantic Margin environment. As the westerly ocean currents enter the corridor between Scotland and Iceland, the seabed begins to slope upward in places, rising to meet the edge of the continental shelf. The currents begin to funnel through two deep passages on either side of a bank that begins with the Rockall upthrust and extends just past the Faeroe Islands.

The currents pick up speed to 2-3 knots as they are funneled along these two passages. Current strength extends through most of the water column.

The force of the shoaling currents against the continent produces other effects:

Eddies: Small, tightly-wound eddies spin out as the currents encounter seafloor ridges upon entering the deepwater corridors west of the British Isles. The largest are the Wyville Thomson Ridge and the Iceland-Faroe Ridge.
Internal currents: Layers of ocean separated by temperature (thermocline) begin intersecting the seabed in the 400-800 meter range, creating internal waves with large changes in current speed and temperature.

Cold bottom currents

But the shoaling currents coming out of the North Atlantic bring something else with them - fingers of near-freezing water from Arctic inflow and North Atlantic upwelling. The cold Arctic water contains much higher levels of oxygen, a concern where equipment will be deployed for a long term.

Biological oxygen demand in the Atlantic Ocean is less than the Pacific Ocean, so the amount of available oxygen is much higher. This is particularly the case, where currents are colder (see accompany article).

Such near-freezing bottom currents, researchers are learning, are quite common in higher latitudes, as well as extreme depths. Petrobras presently is experiencing unexpectedly cold bottom currents on the Marlim Sul development off southern Brazil in 1,700 meters water depth. The north-flowing bottom currents there are believed to have originated in the Antarctic, rather than from upwelling.

Also, Shell found bottom temperatures at its Mensa gas discovery in 1,620-ft depths in the relatively mild US Gulf of Mexico are 1-2 degrees C. The operator is dealing with the prospect of hydrates forming downstream of the wellhead and has built in water drop-out and methanol injection capacity.

Hydrates may be a common problem for all deepwater developments, and the problem is aggravated by the prevalent cold bottom temperatures, along with higher oxygen content in the water than expected.

Wave physics

The large waves generated by storms and high winds crossing the North Atlantic begin to change character as they encounter shoaling depths and ridges off western Europe.

The very long wavelengths (crest to crest) of Atlantic rollers have a diminishing circular wave motion that reach down the water column to nearly half the wavelength distance. For example, a 20-ft wave height, which has a 15% frequency in the North Atlantic, has a 140 ft wavelength (WL = 7WH), which would express a minimal motion about 70 ft below sea level.

Wave heights of 60 ft, which are common in North Atlantic storms, would affect the water column as much as 210 ft below surface (1/2 wavelength). Extremely rare storms with 100-ft wave heights can affect the seabed as much as 350 ft below.

As the shallower ledges and ridges off western Europe begin to interact with the circular motion of individual waves, wave heights increase as frontrunning troughs deepen. To compensate, the wavelengths shorten slightly. As the waves become steeper, the wave tops begin breaking. And, if the wind exceeds 40 knots, which is frequent, the spray becomes airborne, producing a frothy surface punctuated by large rollers breaking at the top.

Gaining a foothold

All of this presents a challenging scenario for oil and gas producers trying to gain a foothold in the prospective Eastern Atlantic frontier. Most of the action has taken place in the Faeroe Islands - Shetland deepwater corridor, but the prospecting is moving south to equally deep areas west of Scotland and Ireland, and north into the Norwegian Sea off northern Norway.

The impact of the weather and current conditions in the eastern North Atlantic and Atlantic Margin is multifold:

Steeper average wave heights: While storm wave heights in the Atlantic Margin are only 15% higher (five meters) than in the Central North Sea, average wave heights tend to be 20-25% greater. Equipment designed for occasional storms must be designed differently to manage large waves much of the time.
High current strength: Strong and somewhat unpredictable currents sharply affect drag force and damping on floating production vessels, mooring components, and risers deployed in the Atlantic Margin. Also, because current strength tends to extend to the seabed, installation of seafloor equipment and use of remotely operated vehicles can be difficult.
Seafloor temperature: Equipment to be installed on the seafloor must be designed for temperatures approximating the freezing point, 10-15 degrees lower than North Sea (much shallower). The near-freezing water in the depths of the Atlantic Margin affects non-metallic mating surfaces, dissimilar metals, and heavy wall equipment designed to combat pressures but not cold temperatures. Contraction forces produce gaps, fissures, and weak points that may not be readily apparent.
Non-concurrent conditions: Unlike the North Sea and most continental shelf areas, and depending on the specific location, extreme conditions - waves, currents, air speed - in the Atlantic Margin do not occur simultaneously very often. This makes oil and gas production system design for safety and mooring system fitness more costly.

While the other conditions may be costly to design for, variable current speed may be the most problematic over the life of the field. In order to install equipment on the seafloor at low-speed current periods, and access it from the surface when required, some amount of predictability is required.

Unlike other deepwater areas in the world, there is a minimum amount of short-time predictability along the Atlantic Margin. The time of the year, counter-current conditions, eddies, thermoclines, and other factors play a role in current speed. Exploration drillers in the area have found that development of a thermocline will presage a current speed change, but most other conditions provide less predictability.

The Atlantic Margin may yet prove to be the most difficult, and probably most costly, deepwater region in the world to develop. Certainly, less is known about metocean conditions in the eastern North Atlantic than the North Sea, US Gulf of Mexico, West Africa, and Brazil.

If prospectivity improves to the south and north of the West of Shetlands area now being exploited, oil and gas operators and research institutions will increase spending to clarify and detail earlier atmospheric and oceanic data measurements in the eastern North Atlantic.

References:

Daruvala, J., "West of Shetland: The Metocean Environment," Proceedings, Institute of Marine Engineers Conference, London, April 1997.
Standing, R. et al, "The Development of Response-Based Criteria for the Design of FPSOs in Exposed Locations," Proceedings, Institute of Marine Engineers Conference, London, April 1997.
Patel, M., "Dynamics of Offshore Structures, " 1989, Butterworth's (Reed) ISBN 0-408-01074-6.
Schumacher, M. (editor), "Seawater Corrosion Handbook,", 1979, Noyes Data Corporation, ISBN 0-8155-0736-4.

High oxygen content, current speed enhance deepwater corrosion, erosion

Unlike surface conditions, the water environment in abyssal depths was supposed to be relatively barren and quiet, with low current velocities and oxygen content. Structures deposited on the seafloor would last forever. A different picture emerged, however, as oil and gas exploration and development moved out onto the abyssal slope and ultra-deepwater.

Not only was the rate of corrosion surprisingly fast, but anodes were consumed quickly. Engineers responsible for deepwater development in the US Gulf, Brazil, and in the Atlantic Margin had to take a new look at this previously barren topography.

The rate of corrosion on steel in seawater is determined mostly by chloride content, oxygen availability, current speed, temperature, and pH.

Chloride content: Chloride ions provide high conductivity and are able to penetrate metal surface films and coatings. Some cold deepwater bottom currents are supersaturated with chloride ions.
Oxygen solubility: Oxygen solubility is inversely proportional to temperature, meaning the ability to absorb more oxygen rises as temperature drops. Since seafloor temperatures in deepwater tend to be only 1-2 degrees above freezing, solubility is very high.
Oxygen availability: Photosynthesis produces oxygen, decomposition absorbs it. Originally, researchers expected to find very little oxygen near the deepwater seafloor because photosynthetic organisms (oxygen producers) simply did not operate without sunlight. What they found was that seafloor currents, particularly those originating in the Arctic and Antarctic, were loaded with oxygen.
Reduced pH: High pressures at the seabed level in deepwater push pH toward the acid end of the scale. Calcium carbonate and magnesium hydroxide dissolved in seawater tend to be below saturation levels, and will not precipitate out and protect metal surfaces easily. Conditions at the surface are just the opposite, where seawater is super-saturated with base compounds. Thus, in deepwater, scale formation on metals is depressed, speeding up anode consumption.
Current speed: Another discovery in deepwater was that instead of the 0.5-2-knot currents expected on the seafloor, speeds up to 3-4 knots were not uncommon. Fast currents expose steel surfaces to greater volumes of corrosive oxygen. Also, fast currents destroy the rust barrier on steel equipment.
Suspended sediment: Sediment load in a fast seafloor current acts as a slow-rate sandblasting operation. Not only are rust barriers eroded, but other forms of artificial protection gradually disappear over time. The higher the sediment load, the faster the erosion rate.

Corrosion rates for steel tend to be lower at great depths than at the surface, but there have been some surprises, due to special conditions at deepwater drilling locations.

In the Pacific Ocean, oxygen content in seawater drops from a maximum at the surface to a minimum at about a depth of 2,000 ft, where oxygen demand from decaying organisms also seems to reach a maximum. At greater depths, oxygen content will depend upon current flow and current origin. In the Atlantic Ocean, oxygen content is fairly uniform through all water depths.

Depth (ft)		Oxygen content (ml/l)
	Atlantic		Pacific
0	4.59		5.8
2,000	3.11		0.25
5,000	5.73		1.0

In the Atlantic, oxygen content and current speed in some deepwater areas have led to a projected higher consumption of anodes than would be experienced near the surface. Replacement may be coming sooner than originally planned, in order to prevent metallic erosion.

With costly seafloor equipment as well as surface systems at the mercy of harsh environmental conditions, subsea and mechanical engineers are taking no chances now. Bottom conditions at proposed drilling and development sites are being sampled frequently, not only to determine dominant conditions, but also environmental extremes. Some large subsea expenditures will depend on these measurements.