RATIONALE METHODS
3.5 Economic evaluation
The economic cost of a given item consists of fixed (i.e. investment) and variable (i.e. operation and maintenance) costs. It is difficult to perform accurate cost estimates, as the fixed costs vary depending on the manufacturers and little commercial data is available. Similarly, the variable costs are subject to high uncertainties since the market prices of fuels such as natural gas are highly variable with the time and geographical location.
The total investment costs are calculated, in this work, following these four steps:
(1) the purchased-equipment costs of each item Cpcare estimated by cost correlations, such
as the ones of Turton et al. [79], which have an uncertainty of± 30 %, or by estimation
charts, assuming atmospheric pressure conditions and carbon steel construction:
log10Cpc= k1+ k2log10A+ k3log10A2 (3.21)
where k1, k2and k3are constants and A is the capacity or size parameter specific to the
component under study (e.g. heat transfer area for heat exchangers).
(2) the bare module costs Cbm0 are obtained, adjusting the purchased-equipment costs with
pressure ( fp) and material ( fm) factors:
Cbm0 = Cpcb1+ b2fmfp
(3.22)
where b1and b2are constants. In some cases, these correlations should be adapted to
include design-type and temperature factors to correct these base costs.
(3) the actualised bare module costs Cbmare computed, considering the inflation between
the reference year of the cost data and the date of the estimate with the chemical engineering plant costs indexes (CEPCI):
Cbm= Cbm0 CEPCI CEPCI0 (3.23)
(4) the grassroot costs Cgr, i.e. the total investment costs when installing the equipment
items on a new production site, are deduced from:
Cgr= (1 + α1)
i
Cbm,i+ α2 i
Cbm,i0 (3.24)
where the factorα1(0.18), which depends on the process conditions, accounts for the
contingencies (0.15) and fees (0.03), and the factor α2(0.35), which is independent
3.6 Environmental assessment
Life cycle assessment is a well-established method to evaluate the environmental impacts of the life stages of a product or process, from cradle-to-grave, i.e. from the resource extraction to the final disposal, including all the steps along the production chain. It takes into account all the relevant material and energy flows over the full life cycle of the system under study, which helps considering all the potential environmental impacts, and thus making more informed decisions.
A conventional life cycle analysis consists of four steps:
(1) definition of the goal and scope;
The service delivered, or function, of the studied system is explicitly defined, providing a reference to which all inputs and outputs are scaled linearly. It is quantitatively described by the functional unit (FU), which can be defined in relation to a given input (e.g. 1 kg of petroleum entering the oil and gas platform) or output (e.g. 1 kg of oil exiting the facility). Systems that present the same functions can therefore be compared based on this metric.
The system boundaries (e.g. geographical, life-cycle, technosphere–biosphere) are clearly stated, illustrating the assumptions and limitations of the study, and which materials, energy flows, and processes, are included (Figure 3.5). They are typically defined so that the ones contributing significantly to the analysed product or system are considered, and that the alternative ways to provide the same products or functions can be evaluated consistently.
In the case that the system under study provides multiple products, an issue to address is the partitioning of the several environmental impacts for each individual output, and a relevant allocation method (e.g. division per mass, energy, exergy, area, volume...) should be chosen.
(2) inventory of in- and outflows to the nature (Figure 3.6);
The inputs of raw materials and energy are identified and quantified, as well as the outputs to air, land and water: this accounting is generally performed by developing a flow model of the technical system under study, where all the activities that should be assessed, based on the system boundaries defined earlier, are included. For example, particulate matters are emitted during the production process of oil and gas.
(3) impact assessment;
The environmental impacts that one wants to investigate are selected, each inflow and outflow is assigned to the relevant impact category (e.g. carbon dioxide and methane flow is classified into the global warming potential category), and each flow is quanti-
tatively characterised, using a common equivalence unit (e.g. 1 kg of CH4has a global
(4) interpretation.
The main environmental issues of the product or system under study are identified, and the assessment is completed by sensitivity and consistency analyses, verifying the assumptions and limitations.
Life cycle assessment tools are embedded in the computational framework used in this work,
following the approach of Gerber et al. [200], taking 1 Sm3of oil equivalent exported to the
shore as functional unit, because:
• the function of an offshore platform is to separate and purify the petroleum into its oil and gas phases;
The relative yields of these potential products depend on the initial composition of the reservoir fluid entering the platform, and on the separation efficiency of the plant.
• choosing one unit of oil only as FU may not be suitable for oil and gas facilities where most production consists of gas, which is then exported via pipelines and further sold; Moreover, choosing this FU would unfairly penalise oil and gas platforms where signifi- cant amounts of heat and electricity are used to purify and dehydrate the gas.
• similarly, the choice of one unit of gas only as FU may not be relevant, as several oil plants aim at at maximising the oil production by injecting back the produced gas into the reservoir;
Using this FU would imply that the impacts of storing and transporting oil are allocated to the produced gas, which seems inappropriate as the oil export process is independent of the gas production system.
• taking a unit of oil and gas equivalent presents the advantage of considering both gas and oil as potential products. It ensures that the effects of changes in the process design and in the energy conversion technologies are taken into account and allocated properly.
The inventory of the in- and outflows is based on the data and results obtained from the physical model (e.g. material and energy flows, design, size and operational characteristics of
the equipments). The data from the Ecoinvent®database [201] are used when conducting the
impact assessment phase. All options are compared by considering the following categories: the climate change impact, based on the methods proposed by the Intergovernmental Panel on Climate Change (IPCC), the eutrophication and acidication potentials, the terrestrial and human toxicities, and, finally, the endpoint eco-indicator 99, which lumps all environmental impacts on a single score.
Produced water, Hydrocarbons, Heavy metals, Amines, Nitrates PM,CO2, NOx
Functional unit: 1 Sm3 oil equivalent (o.e)
Exploration
Appraisal
Development
Production
Decommission
Emissions Flare Byproducts
Venting
Materials, Energy, Infrastructure
Refining & TransportStorage
Petroleum
Oil
Gas
Figure 3.5: Conceptual boundary system for the life cycle analysis of the oil and gas platform.
Production manifold Separation Recompression Gas treatment Produced water treatment Oil treatment Well streams Processing system Processing system Water handling + injection Gas Produced water Oil Fuel gas treatment Fuel gas Oil Oil Q– (30 – 100 °C) Q– (30 – 100 °C) Q– (30 – 200 °C) Q+ (30 – 220 °C) Q– (30 – 100 °C) Lift + Injected gas Export gas Export oil Produced water, Hydrocarbons, Heavy metals, Amines, Nitrates
Injected water d
t PM,CO2, NOx, CH4
Seawater
CH3OH, Phosphonates, Polycarboxylic acids, Calcium carbonates, Acetates, Glycol
Q+ (0 – 100 °C)
Functional unit: 1 Sm3 oil equivalent (o.e)
Gas turbines, furnace, waste heat recovery, glycol loop LCI flow Energy flow
Material flow
Utilitytt and heat recoveryrr systemy Utility and heat recovery system
Cooling water