TPG 4140 NTNU Natural Gas Compression Professor Jon Steinar Gudmundsson Department of Petroleum Engineering and Applied Geophysics Norwegian University of Science and Technology Trondheim October 8, 2013

Outline Compression in oil and gas production Types of compressors (piston and centrifugal) Characteristic curves Adiabatic and reversible (=isentropic) compression Real vs. ideal, isentropic head Surge and stonewall Compression in stages (inter-cooling) System curves and parallel/series service

Compression in Oil and Gas Production Recompression in separation train of gas and condensate (same as in oil production). Export compression (from platform to pipeline). Reinjection compression (from platform to subsurface). Various air compressors for control equipment in processing facilities (offshore and onshore). Export compression, from receiving and processing terminal to pipeline. Pressure boosting, onland compression. Gas storage in salt caverns. Subsea compression (now being installed)

A: Recompression, B: Gas Drying, C: Fuel Gas, D: Export Compression, E: Gas Pipeline, F: Injection Compression, G: Injection Well

USA Natural Gas Industry

CO2 Emissions Norway Oil and gas industry 29 %, Cars and trucks 22 %, Industry processes 18 %, Heating 16 %, Boats and ships 9 %, Other sources 6 %

Gas Turbines Offshore Norway 2007

Offshore Gas Turbines The oil and gas industry in 2007 represented 25 % of CO2 emissions in Norway. 70 % is offshore (combustion, diesel and flaring) Requirement for electricity offshore 2007 was 15 TWh (Norway’s total electricity production was then about 125 TWh). In 2007 there were 167 gas turbines on offshore installations amounting to 3000 MW. About 45 % of the power is for electrical equipment. The remaining power is gas turbines for compressors and other equipment. The thermal efficiency of the gas turbines is in the range 30-37 %. Thermal recovery is used in most of the installations, increasing the thermal efficiency to about 40 % Kraft fra land til norsk sokkel (2008), OD o.a.

Simplistic Gas Turbines Working Principles 1-2 Isentropic compression (in a compressor) 2-3 Constant pressure heat addition (in a combustor) 3-4 Isentropic expansion (in a turbine) 4-1 Constant pressure heat rejection

Types of Compressors Reciprocating piston compressors Low flow rate High compression ratio Rotating centrifugal compressors High flow rate Low compression ratio each stage Several stages higher compression ratio

Axial Flow Compressor

Radial Flow Compressor

Centrifugal Compressor

Reciprocating Compressor

Twin Screw Compressor for Subsea

Subsea Compressor

Usage of Compressors

Characteristic Curve

Characteristic Curve

Characteristic Curves Left: Radial, Middel: xxx, Right:Axia A: Centrifugal, B: Axial, C: Piston, D: Screw

Thermodynamic Processes Condition Exponent Adiabatic Isobaric Isothermal Isentropic Polytropic Isometric dq=0 dp=0 dT=0 ds=0 ∑ds=0 dV=0 k=Cp/Cv k=0 k=1 k<n k=∞ Reversible adiabatic process = Isentropic process

Compressor Work A = Suction, B = Compression, C =Delivery, D = Expansion

Piston Compressor

The Carnot Cycle

Adiabatic/Isentropic Process

Adiabatic/Isentropic Compression v [m3/mol], p [Pa=N/m2], W [Nm/mol=J/mol] = Specific work pv=RT, Wm/M [J/mol kg/s mol/kg=J/s=W] = Power

Surge and Stone Wall A: Surge line, B: Stonewall line

Surge Control

Compression in Two Stages Above pressures give minimum power

Intercooling

System Characteristic

Scaling Laws Isentropic Head Example 6000, 7000 and 8000 RPM

Parallel and Series

Hedne (2013)

Summary Recompression, export compression, reinjection compression Reciprocating piston compressors and rotating centrifugal compressors (radial and axial) Characteristic curves P [W] or h [J/kg] vs. q [m3/h] Calculation of compression power (isentropic) Surge (solved by recycle) and stonewall (speed of sound) Compression in stages with intercooling (minimum power when equal power) System curves (the need) and >1 compressor Subsea compression