Environmental Impact Assessment
When a subsea spill occurs, environmental consequences can be very severe. Assessing environmental damage is extremely difficult because of the many factors involved in cleanup effects and in estimating the costs for possible civil penalties or fines. Environmental damage is typically assessed based on a dollar-per-barrel estimate for the material and location of release. The consequences of a release from process equipment or pipelines vary depending on such factors as physical properties of the material, its toxicity or flammability, weather conditions, release duration, and mitigation actions. The effects may impact plant personnel or equipment, population in the nearby residences, and the environment. Environmental impact assessment is estimated in four phases:
- Cleanup costs;
- Ecological effects.
Calculate the Volume Released
Sources of hazardous release include pipe and vessel leaks and ruptures, pump seal leaks, and relief valve venting. The mass of material, its release rate, and material and atmospheric conditions at the time of release are key factors in calculating their consequences. Release can be instantaneous, as in the case of a catastrophic vessel rupture, or constant, as in a significant release of material over a limited period of time. The nature of release will also affect the outcome. With appropriate calculations, it is possible to model either of the two release conditions: instantaneous or constant. For a subsea release case, the leak rate and detection times are major factors when determining the volume of the leak:
Vrel = Vleak x tdetect
Vrel: Volume of liquid released from equipment;
Vleak: Leak rate;
tdetect: Detection time.
Estimate Final Liquid Volume
When a vapor or volatile liquid is released, it forms a vapor cloud that may or may not be visible. The vapor cloud is carried downwind as vapor and suspended liquid droplets. The cloud is dispersed through mixing with air until the concentration eventually reaches a safe level or is ignited. Initially, a vapor cloud will expand rapidly because of the internal energy of the material. Expansion occurs until the material pressure reaches that of ambient conditions. For heavy gases, the material spreads along the ground and air is entrained in the vapor cloud, due to the momentum of the release. Turbulence in the cloud assists in mixing. As the concentration drops, atmospheric turbulence becomes the dominant mixing mechanism, and a concentration profile develops across the vapor cloud. This concentration profile is an important feature in determining the effects of a vapor cloud. Several factors determine the phenomena of dispersal:
- Density: The density of the cloud relative to air is a very important factor affecting cloud behavior. If denser than air, the cloud will slump and
spread out under its own weight as soon as the initial momentum of the release starts to dissipate. A cloud of light gas does not slump, but rises above the point of release.
- Release height and direction: Releases from a high elevation, such as a stack, can result in lower ground-level concentrations for both light and heavy gases. Also, upward releases will disperse more quickly than those directed horizontally or downward, because air entrainment is unrestricted
by the ground. • Discharge velocity: For materials that are hazardous only at high concentrations, such as flammable materials, the initial discharge velocity is very important. A flammable high-velocity jet may disperse rapidly due to initial momentum mixing.
- Weather: The rate of atmospheric mixing is highly dependent on weather conditions at the time of release.Weather conditions are defined by three
parameters: wind direction, speed, and stability. When liquid escapes to the environment, the major influence is the environmental pollution,which is determined by the liquid remaining in thewater. Following a spill, a certain fraction of lighter hydrocarbons can evaporate, thus reducing the volume of liquid that needs to be cleaned up. The persistence factor Fliquid is used to quantify the amount of unevaporated liquid. In general, Fliquid is found as follows:
Fliquid = e- kt
t: the time required to complete half the cleanup effort, in hours;
k: evaporation rate constant, in hours–1.
The time, t, includes the time to initiate cleanup, which is estimated as the time required to begin the cleanup effort in earnest, including the time to plan a cleanup strategy and mobilize all necessary equipment.
Determine Cleanup Costs
In general, the cleanup costs for a leak to the environment are estimated using the following expression:
Cost cleanup=Venv x F liquid x C
Venv: Volume released to the environment;
Fliquid: Fraction of liquid remaining;
C: Unit cost of cleanup.
The estimates of the cost to clean up spills of various liquids are the most uncertain variable in an environmental consequence analysis. Every attempt must been made to estimate cleanup costs in a reasonable manner. Based on historical data, cleanup costs for crude oil on open water can vary from $50 to $250 per gallon.
Ecological Impact Assessment
Oil spills on the sea surface can affect a number of marine species. The species most vulnerable to “oiling” are seabirds, marine mammals, and sea turtles that may come into direct contact with the hydrocarbons, although any interaction with the spilled hydrocarbons depends on the time these animals spend on the sea surface. The exact distribution and feeding areas of seabirds, marine mammals, and sea turtles in the offshore environment are unknown. Large swimming animals such as cetaceans and turtles are mobile and could move away from spilled oil and are less likely to be affected. Fishes living beneath the surface can detect and avoid oil in the water and are seldom affected. Fish and Invertebrates
- Atlantic cod
- American lobster
- Atlantic herring
- Snow crab
- Yellowtail flounder
- Sea scallop
- Northern gannet
- Greater shearwater
- Common eider
- Black guillemot
- Harlequin duck
- Greater yellowlegs
- Purple sandpiper
- Piping plover
- River otter
- Fin whale
- Atlantic white-sided dolphin
- Humpback whale
- Harbor seal
- Blue whale
- White-beaked dolphin
Sea Turtle Each of these species is discussed next in terms of their habitats, life stages, and overall vulnerability to oil spills.
Fish and Invertebrates
Near-shore and shallow waters are important for the spawning and early stages of several important fish species, which could be vulnerable to spilled oil depending on the timing of the spill.
Seabirds are the species most vulnerable to the effects of oiling; the species listed above all come into contact with the sea surface and, hence, potentially oil from a spill. Murres and black guillemots are particularly vulnerable because they fly infrequently, spending the majority of their time on the sea surface.
With the exception of seals, otters, and polar bears, marine mammals are not particularly susceptible to the harmful effects of oiling. Harbor seal pups may be susceptible to the effects of oiling. Although not classified as a marine mammal, river otters are included in the list because they spend a great deal of time in the marine environment.
The vulnerability of sea turtles to oiling is uncertain.
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