Sidney P. Santos
Copyright 2011, Pipeline Simulation Interest Group and Solar Turbines Incorporated
This paper was prepared for presentation at the PSIG Annual Meeting held in Napa Valley, California , 24 May – 27 May 2011.
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Internal pipe coating technology is available for natural gas pipelines, and can be primarily used to reduce surface roughness, and thus internal friction. This will reduce the pressure drop between compressor stations, and thus allows installing less power and consume less fuel. The potential to lower CAPEX (due to lower compression power requirement) and OPEX (due to lower fuel consumption) are counteracted by the extra cost of the internal coating.
In this paper, a large diameter pipeline case study is used to evaluate the alternatives of (a) coating versus (b) not coating the pipeline, and the results are presented. The impact of coatings on friction factor is based on actual test data. Based on actual cost data from pipeline coating, derived from a large transnational pipeline project, the impact on overall economics is assessed.
The case study will cover a pipeline capacity ramp up curve and the best technical and economical solution with regard to Capital expenditure – CAPEX and Operation expenditure – OPEX and consequently inpact on the transportation cost of service.
With the worldwide demand for gas rising, new pipelines are required to bring gas over longer distances to the market. For long distance pipelines, the transport cost of the gas will make up an increasing portion of the delivery cost to the customer, and can reach 30 to 50% of the total cost at the receiving terminal. This transport cost can be influenced by optimizing the fuel consumption, equipment first cost, equipment operating cost, as well as equipment reliability and availability. The pressure and flow characteristics of pipelines and other factors influence the arrangement of compressors in a station. The question is often about number of units, the spacing of stations, standby requirements or the use of series or parallel arrangements in a station arises, together with type of driver, and type of compressor. When planning a compressor station or, for a new pipeline, a number of stations, considerations include: steady-state and transient capabilities and requirements of the system, growth requirements and capability, availability and total cost of ownership, and delivered cost to shippers and customers. The pipeline hydraulics relate pressure losses to the flow through the pipeline, determine the compressor operating conditions in terms of head and actual flow, and subsequently determine the required power from the driver. Contractual requirements and obligations, such as pressures and volumes at transfer points, have to be met. A key factor in these considerations is the pressure loss in the pipeline for a given flow rate, which, in turn is very much affected by the roughness of pipe.
For a situation where a compressor operates in a system with pipe of the length Lu upstream and a pipe of the length Ld downstream, and further where the pressure at the beginning of the upstream pipe pu and the end of the downstream pipe pe are known and constant, we have a simple model of a compressor station operating in a pipeline system .
The pressure gradient in the pipeline can be described by the Fanning equation which can be integrated. With reasonable simplifications, such as assuming the friction factor f to be constant, and a given, constant flow capacity Qstd , the pipeline will then impose a pressure ps at the suction and pd at the discharge side of the compressor (Eq 1): with a pressure loss coefficient that incorporates the friction losses, and will vary depending on pipe dimensions (length, diameter), temperature profile, pressure profile, surface roughness and flow velocity profile in the respective pipe sections. In particular, is proportional to the length L of the pipeline section (Mohitpour et al.,2000). Obviously, ps and pd are also found as results of more elaborate pipeline hydraulics simulations (Ohanian and Kurz, 2002). Figure 1 outlines the results for pu=pd, , pe=ps and u=e, which is typical for a compressor station in the middle of a pipeline. We find that ‘lightly’ loaded pipelines (for example pipelines with a relatively small distance between stations) tend to have a flatter pipeline characteristic, while pipelines where the stations are farther apart tend to have a steeper characteristic. This is important for compressor selections, because the compressor map has to cover the entire pipline characteristic.This , again, indicates the crucial impact of the loss factor, which depends on peipline roughness, on the effectiveness of the pipeline operation.
The subject of this paper is a newly design pipeline with a length of about 932 miles (1500 km), a pipe diameter of 42 inches (1067 mm) and Maximum Pipeline Operating Pressure (MAOP) of 1423 psig (9.81 MPag) (Figure 2). The attention the authors of this paper were drawn to a discussion where it was claimed that it is more economically feasible to use the pipes without internal coating where as pipeline community’s common knowledge is the opposite. Authors of this paper decided to run number of simulations with the different internal roughness of the pipe and then run economical analysis to find out which solution presents the best economical result.
This case study is based on a pipeline project that goes from a gas supply receipt point to a interconnection 932 miles (1500 km) away delivering 177, 247, 353 and 529 BSCFY (5, 7, 10 and 15 BSCMY) of natural gas on firm contractual basis. Four pipeline internal roughnesses have been evaluated as described below:
- Alternative I: 395 microinches (10 micrometres)
- Alternative II: 590 microinches (15 micrometres)
- Alternative III: 790 microinches (20 micrometres)
- Alternative IV: 1180 microinches (30 micrometres)
|Nomianl Diameter:||42 inches|
|Length:||932 miles (1500 km)|
|Design code:||ANSI B31.8|
|MAOP:||1423 psig (9.81 MPag)|
|End pressure:||1000 psig (6.89 Mpag)|
|Overall heat transfer:||0.29 Btu/h.ft2.F|
|Soil temperature:||51.8 F (11 C)|
|Depth of cover:||3.28 feet (1 meter)|
|Compression ratio:||1.4 ~1.8|
|Suction/Disch. header press. drop:||18.9 psi (130 KPa)|
|After cooler disch. temperature:||86 F (30 C)|
|Site elevation||0 ~492 feet|
|Site Temperature||54.5 F (12.5 C)|
Thermohydraulic simulations were run for all alternatives with capacities and internal roughness as previously defined and the results are presented on table (1).
The simulation was performed using Synergy Gas (REF) steady state anlysis. It solves the steady state, one dimensional mass conservation, momentum conservation and energy conservation equations for each node. The real gas behavior regarding gas density in the hydraulic similutaion is adjusted using a modified Benedict-Webb-Rubin equation, which approximates the Standing-Katz compressibility factor correlation. The friction term, necessary for the closure of the energy and momentum equation is modeled using a friction model, which was, for this study the Colebrook-White friction factor equation with Gas General Flow Equation. The energy equation also allows to consider heat transfer from the pipe to the surrounding environment
Compressor maps for the relationships between flow, head and efficiency, based on actual centrifugal compressor performance, can be implemented into the simulation. The compressor operating point in the head-flow space is determined from pressures and temperatures using the Redlich Kwong equation of state.
The compressor models specifically cover the traditional and active portions of the compressor map from surge to choke. The program has capabilities to recognize surge and choke conditions (recycle flow, generate excess head, etc.) within bounds of available power and speed, but does not include specific performance data beyond surge and choke limits.
This study assumes certain requirements for the compression system. Beyond the quest for higher compressor peak efficiencies, the operating requirements set forth in this study as well as in other references require a compressor capable of operating over a wide operating range at high efficiency.
Wide operating range in a centrifugal compressor can be achieved by a combination of means. Aerodynamic theory suggests a strong relationship between operating range, efficiency and impeller backsweep. However, there is a practical limit to the amount of backsweep. In particular, increasing backsweep reduces the capability of an impeller of given tip speed to make head. However, with the capability to use two impellers in a casing, this perceived disadvantage can be eliminated. The operating range is further increased by the use of a vaneless diffuser.
A practicle approach when comparing project alternatives is to concentrate on what is different between them and compare their results in terms of Net Present Value – NPV. For this case study internal roughness of 10, 15, 20 and 30 micrometres (microinches) where evaluated and their impact quantified on annual capacity demands of 5, 7, 10 and 15 billion standard cubic meter per year – BSCMY ( billion standard cubic feet per year - BSCFY). The alternatives of 10 and 15 micrometres might be associated with internal painting and 20 and 30 micrometres might be associated with pipeline without internal painting.
The side benefit of pipeline internal painting related to atmospheric corrosion protection while in storage prior to assembling and burying has not being quantified since this is close related to the quality of handling and storage of the pipes by Constructors and therefore varies with their expertise and procedures.
Another point of interest while evaluating pipeline internal coating is a commom practice of adopting load factor for pipeline design and simultaneous intall standby compressor units for the compressor station. This practice is not economically optimal. The optimal economic results can be achived by incorporating Monte Carlo simulation and failure analysis in the feasibility study as proposed by Santos (2009).
Two sizes of compressor sets selected according to the power requirement:
- 16000 ISO hp
- 10000 ISO hp
- Fuel consumption based on compressor and driver performance maps.
- One standby compressor unit for each compressor station.
- Construction schedule: 2 years
Compressor units Capex
- (1) x 10000 ISO hp: 14.5 MMUS$
- (1) x 16000 ISO hp: 17.7 MMUS$
- Internal coating: 30 US$/pipeline ton
- O&M C. Sta. (without Fuel): 5% of C.Sta. Capex
- Depreciation: 20 years
- Economic life: 20 years
- Taxes: 40%
- Fuel price: 170 US$/MMSCM; 4.75 US$/MMBTU
- Discount rate: 12% a year
The purpose of the economic analysis is to quantify the influence of the internal roughness of the gas pipeline on the cost of service. This evaluation takes into account the technical and economical assumptions defined previously.
For pipeline operation conditions with low capacity (well under pipeline design capacity) changes in internal roughness did not presented significant operation cost reduction.
The economical benefit is identified for higer pipeline capacity – at or close to optimum design capacity – as shown in Table 1 and Figure 3 and 4. The effect of lowering cost of service is a consequence of lower fuel consumption and/or lower Capex for the pipeline project.
Pipeline internal painting is feasible and economically attractive even if side benefits of atmospheric corrosion protection are not accounted and also adopting conservative values for internal roughness of 10 micrometres (assuming this value after aging of 20 years of operation).
- SANTOS, S. P., “Monte Carlo Simulation – A Key for a Feasible Gas Pipeline Design” In: Pipeline Simulation Interest Group, 2009, Galveston, TX., USA.
- Lubomirsky,M, Kurz,R., Klimov,P., Mokhatab,S., “Station Configuration impacts Availabilty,Fuel Consumption and Pipeline Capacity’, Pipeline and Gas Journal, Jan 2010”.
- Ohanian, S., Kurz, R., Series or Parallel Arrangement in a Two Unit Compressor station, TransASME JGTand Power, Vol. 124, 2002
- Mohitpour,M., Golshan,H., Murray,A., Pipeline Design and Construction, ASME press, New York, 2000.
About the Authors
Matt Lubomirsky, is a Consulting Engineer, Systems Analysis at Solar Turbines Incorporated, in San Diego, California. He is responsible for predicting gas turbines and compressors performance, for conduction of applications studies that involve pipeline and compressor stations modeling. Matt Lubomirsky attended Leningrad Institute of technology in Saint Petersburg, Russia where he received Master Degree in Mechanical Engineering. He has authored numerous publications about turbomachinery and pipeline related topics.
Sidney Pereira dos Santos, is a Senior Consultant at PETROBRAS, holds a BS in Mechanical Engineering, a MBA in Corporate Finance and a Master’s in Logistics. He has 24 years in the oil and gas pipeline design at PETROBRAS. Has developed feasibility studies for gas pipeline projects using Monte Carlo simulation, availability studies and quantitative risk analysis. Has published articles in international magazines and presented technical papers at PSIG, ASME-IPC, IBP (Brazil) and others. Phone: +55 21 8167-0134 e-mail: [email protected]
Rainer Kurz, is the Manager, Systems Analysis at Solar Turbines Incorporated, in San Diego, California. His organization is responsible for predicting compressor and gas turbine performance, for conducting application studies, and for field performance testing. Dr. Kurz attended the Universitaet der Bundeswehr in Hamburg Germany, where he received the degree of a Dr.-Ing. in 1991. He was elected ASME Fellow in 2003 and has authored numerous publications about turbomachinery related topics, with an emphasis on compressor applications, dynamic behavior, and gas turbine operation and degradation.
Zhan Kulzhanov, the author, Deputy Director of Technical Development Department for Intergas Central Asia in Astana, Republic of Kazakhstan. Zhan has graduated Aviation Institute in Kazan, Russia with specialty in Jet Engines. He has Master Degree in Mechanical Engineering. Zhan's responsibilities include economic and technical evaluation of the pipeline projects. He was involved in Trans Asian pipeline project that delivers gas from Central Asian republics to Eastern part of China. Zhan has also previous experience in ICA pipelines Operations and Maintenance Department.