Deepwater Fossil Fuel Extraction and Production
Technologies – a Developing Source of Ocean Noise Pollution
Michael
Stocker - mstocker@OCR.org
Ocean
Conservation Research
P.O.
Box 559 Lagunitas, CA 94938
Popular
version of paper 3aUWa1
Presented
Wednesday morning, November 17, 2010
2nd
Pan-American/Iberian Meeting on Acoustics, Cancun, Mexico
The
era of “easy oil” is rapidly closing and more challenging
reservoirs are being developed “offshore” on outer continental
shelves using a suite of developing technologies. Deepwater (>1000
ft.– 10,000 ft.) development occurs under ambient pressures of 30 to 300
atmospheres (400 to 4000 psi.) and wellhead differential pressures of up to
20,000 psi. Active processing equipment is mounted on the sea floor to diminish
the risks and costs of the multiple pipe runs that would be required for above
sea level processing. The equipment includes separators, multi-phase and multi
stage pumps, injectors, and metering equipment. Given the potential for extreme
pressure differentials and the multi-phase nature of the product (liquids, gas,
and solids) it is likely that some of these processes generate substantial
noise. This paper is a review of the deepwater extraction and production
technologies and an overview of the physical conditions found in deepwater
fossil fuel exploration and production.
Overview
Incidental
noise pollution from human enterprise has been increasing in the ocean since
the mechanization of maritime transportation. As a consequence of the
globalization of trade, shipping noise source has increased exponentially so
that ambient noise in the ocean in certain places is ten times louder than it
was just 50 years ago from shipping noise alone. Additionally, global scale
noise is also increasing by the expansion of offshore fossil fuel development.
From exploration through production, offshore and deepwater fossil fuel
operations are raising the ambient noise levels in the ocean at an alarming
rate.
While
there is some uncertainty about the impacts of the increased ambient noise
levels on marine life, it is clear that the marine acoustic habitat of animals
that depend of acoustic communication is being adversely compromised. The
impacts may include masking of biologically significant signals such as
breeding and community coherence calls of kin, navigation cues provided by
geological or hydrodynamic features in the natural marine soundscape,
and the sounds of predators and prey.
This
concern is not isolated to whales, porpoises, and dolphins, which we know have
complex acoustical perception and communication systems, but also for fish and
perhaps even invertebrates that also rely on acoustical cues for their
survival.
Fossil
fuel exploration to production sequence
Fossil
fuel is the product of once living matter decomposing in geological formations
that due to pressure, heat, and physical structure of the earth store the
hydrocarbons of the decomposed matter in geological deposits. These deposits
are initially identified by way of likely geological features.
Once a likely deposit area is identified the
first stage of exploration begins by exciting the substrate with seismic scale
stimulus. Seismic airgun surveys are the most common
technique used in ocean settings.
When
deposits are found and characterized, exploratory wells are sunk to determine
the production potential of the deposit and the quality of the product. In
deepwater settings exploratory wells are drilled from large stabilized floating
platforms.
At
the beginning of the exploration, caissons are built to mount a blowout
preventer (BOP) – a safety feature used in exploratory settings. Once a
deposit is determined to be suitably productive the BOP will be replaced by a
flow control system to prepare the well for production.
Fossil
fuel deposits are not just oil; rather they contain many other substances in
various concentrations depending on the nature of the deposit. It is not
uncommon for liquids (oil and brine), solids (sand, coal, shale), and gas
(methane, ethane, butane, CO2, nitrogen, etc) to all be part of the product
extracted out of the deposit. So the product coming out of the well is called
“multiphase” containing gas, liquids and solids.
This
multiphase product emerges from the wellhead typically under pressure. The flow
is mediated by a “choke” valve and distributed to a manifold.
Each
of these materials needs to be separated and handled appropriately. Waste water
is often injected back into the well, sand and solids are separated and dumped,
and gasses are either burned off, injected back into the well, or extracted and
utilized – depending on the proximity to gas processing facilities and
the environmental laws of the governing jurisdiction.
In
historic shallow water operations separation was done on platforms above water,
but increasingly these processes are occurring on the seafloor with seafloor
mounted separation and processing equipment.
Control
and monitoring of this equipment is accomplished through instrument packages
mounted on the various pieces of equipment, and by way both tethered (remotely
operated) and autonomous underwater vehicles (ROV’s an AUV’s
respectively). Communication to these instruments and vehicles takes place by
way of wired tethers as well as acoustic modems.
Noise
sources
Airguns:
Heretofore
the main noise concern in the fossil fuel industry was focused on seismic airgun surveys, whereby arrays of airguns
are towed across transects, exploding every ten to twenty seconds, sending a
seismic level impulse down into the water column and into the seafloor below.
While the signals from these arrays are focused
down, noise from the operations can be heard thousands of kilometers from the
source – echoing through entire ocean basins. As the surveys take place
in ever deeper water the reach of this noise will also increase.
At
any given time there are 40 to 50 surveys happening globally.
Illustration courtesy of John Hildebrand
Exploratory
wells:
Once
likely deposits are located, a drilling site is prepared by installing a
wellhead “tree” and blow out preventer (BOP). This is then used to
seal the wellhead and guide the drilling bits and pipe into the earth. The
drill bits and pipe are the fed from the ocean surface from a stabilized
drilling platform such as the recent Deepwater Horizon.
These
platforms can have deck areas the size of a soccer field with displacements in
excess of 30,000 tons. In order to push miles of pipe into a well with accuracy
they are dynamically stabilized using six to eight “thrusters” that
can keep the drilling operation stable within 1 meter on the x, y, and z axes.
Thrusters
are large propellers powered by diesel-electric drives and depending on the sea
conditions during operation can generate significant noise from turbulence and
blade cavitation.
The
actual drilling noise is not substantial because any noise generated by drill
bit will be attenuated by the surrounding earth formation. Even when the pipes
“bang the hole” due to eccentricities in drilling actions, these
noises when monitored by geophones or seismometers will generate at most a
Richter scale value of 1 or less. (By way of comparison, a 40,000 lb. cargo
truck passing by might generate a Richter scale 4 “quake” –
in a logrhithmic scale 1,000 greater energy than a
Richter value of 1.)
Increasingly
though equipment used in seafloor operations are fitted with telemetric
equipment communicating through multi-nodal networks for dynamic positioning
references, equipment identification, equipment condition monitoring, and
AUV/ROV communication.
Illustration: Nautronix
These acoustical modems operate in mid frequency
(1- 10 kHz) and high frequency (10-50 kHz) ranges, at typical source levels of
180dB re: 1 μPa with an operating range of 5-10
km. Noises in these frequency ranges and amplitudes have been correlated with
signals that aggravate beaked whales, porpoises, and dolphins, and may also have
negative impacts on some forage fish such as herring or shad.
Production:
Once
a well becomes productive the exploratory drilling apparatus’ are removed
and production piping and processing equipment are deployed. While this
processing has historically occurred on “Floating Production and Storage
Operations” (FPSO’s), increasingly as fields get out in deeper
water, production processing is being located on the sea floor – called
“subsea processing.”
Wellhead
pressures can be quite high – ½ psi per foot depth in water and an
additional 1 psi per foot depth in rock. (The recent BP Macondo
well was kicking out oil, gas, brine, and solids at an excess of 13,000 psi.)
From
the borehole the first control contact point for the product is the “choke”
– a valve that mediates the flow of product into the distribution tree.
Given the excessive pressures and hostile conditions these chokes are made of
tungsten carbide “tool steel.” Even so they wear out and need
periodic replacement. It is likely that the multiphase product flowing through
the choke and distribution tree will produce loud broad-band noise.
From
the distribution tree the product is piped to separators to separate the
valuable product from brine, sand, and solids. Depending on the composition of
the mix these separators are gravity, centrifugal, or product density driven.
In most cases separators form an expansion point in the product flow,
decreasing the net pressure of the product relative to flow, although if
possible pressures are kept high enough to push the product up to the surface.
In
the cases where the pressure is not adequate to overcome the column weight,
multistage pumps are deployed (driven by electrical motors). Additional pumps
are used to inject the brine and other waste products back into the deposit.
Each one of these pumps may not generate significant noise in and of itself,
but as operations expand across the seafloor cumulative noises from all of
these pumps will increase the noise floor of the marine habitat.
Illustration: FMC
Once the valued product is separated it is piped
to centralized storage and distribution operations. These
“tiebacks” can be 20 km from the wellhead and processing field and
can be floating platforms (FPSO’s) or in some cases located on shore.
Floating platforms will be tended by lighters, tankers, maintenance craft, and
crew transport vessels and helicopters, making these operations an area of high
noise concentrations.
Heretofore
no impact studies have been conducted to determine the effects that these new
noise fields have on the short or long term viability of marine life, but there
is a high probability that the impacts are not negligible.