Process simulation and evaluation of options for heat and power generation on offshore oil and gas installations Jonas Brenntrø Master of Energy and Environmental Engineering Submission date: June 2016 Supervisor: Lars Olof Nord, EPT Norwegian University of Science and Technology Department of Energy and Process Engineering Preface This master thesis was written at the Department of Energy and Process Engineering, Faculty of Engineering Science and Technology at the Norwegian University of Science and Technology, NTNU, as a fulfilment of a Master of Science degree in Energy and Environmental Engineering. It was written during the spring of 2016 under the guidance of my supervisor, Associate Professor Lars O. Nord and with the help of PhD Candidate Luca Riboldi. I would like to thank Lars O. Nord for help and expertise during the months it took to write this thesis. Trondheim, 8-June-2016 Jonas Brenntrø V VI Abstract In the efforts of trying to reduce global greenhouse gas emissions, the Norwegian oil and gas industry is looking for ways to improve efficiencies when supplying heat and power offshore. By making a scenario of a platform with set heat and power requirements, this thesis tries to answer the question, “What are good options for heat and power generation offshore and how do they perform in a lifetime analysis?” To answer that question, the modelled platform scenario had varying ambient temperature according to North Sea weather data, and a typical heat and power profile, with a maximum power requirement of 60 MW and a maximum heat requirement of 22 MW. The platform’s lifetime was assumed to be 20 years. 7 different cases were modelled and tested in the process simulation software, Ebsilon Professional, with the VTU gas turbine library. To evaluate the designs, focus was put upon lifetime CO2 emissions and flexibility. A case of two GE LM2500+G4 with WHRUs, the most common power technology used offshore, gave a total lifetime emission of 3.99 mega tonnes CO2. The best alternative for the modelled platform were thought to be a combination of a simple cycle and a combined cycle: One LM2500+G4 giving off heat to a WHRU while another LM2500+G4 providing heat to an OTSG that drives a steam extraction cycle. It had high flexibility and low lifetime emissions of 3.20 mega tonnes CO2. A case of electrifying the platform was also evaluated, with using a gas boiler to provide process heat. It was found that the results were highly dependent on assumed associated emission ratings to onshore electric power. With an assumption of marginal power coming from EU and predicted future emission rates, the electrification case gave off 3.60 mega tonnes CO2. The longer a platform operates or the later it is built; the more favourable electrification becomes due to predicted cleaner electric energy in the future. VII VIII Sammendrag I arbeidet med å prøve å redusere de globale klimagassutslippene, er den norske olje- og gassindustrien på jakt etter måter å forbedre effektiviteten av prosessvarme og strøm offshore. Ved å lage et scenario av en plattform med ett sett varme og strømforbruk, forsøker oppgaven å svare på spørsmålet: "Hva er gode alternativer for varme og kraftproduksjon offshore og hvordan yter de i en livsløpsanalyse?" For å svare på spørsmålet, hadde den modellert plattformen en varierende omgivelsestemperatur i henhold til værdata fra Nordsjøen, og en typisk varme og strøm profil, med maksimalt effektbehov på 60 MW og et maksimalt varmekrav på 22 MW. Plattformens levetid ble antatt å være 20 år. 7 forskjellige ‘cases’ ble modellert og testet i simuleringsprogrammet Ebsilon Professional, med VTU’s gassturbinbibliotek. For å evaluere designene, ble livstid CO2-utslipp og fleksibilitet primært evaluert. En ‘case’ med to GE LM2500+G4 med WHRUs, den vanligste kraftteknologien som brukes offshore, ga en total levetidsutslipp på 3,99 megatonn CO2. Det beste alternativet for den modellerte plattformen ble antatt å være en kombinasjon av en enkel syklus og en kombinert syklus: en LM2500+G4 som avgir varme til en WHRU mens en annen LM2500+G4 avgir varme fram eksosen til en OTSG som driver en dampsyklus med dampekstraksjon. Den hadde høy fleksibilitet og lave levetidsutslipp på 3,20 megatonn CO2. Elektrifisering av plattformen ble også vurdert, med hjelp av en gasskjele for prosessvarme. Resultatene var svært avhengig av antagelser av de tilhørende CO2-utslippene av den landbasert elektriske kraften. Med en forutsetning at den marginale kraften kommer fra EU og at elektrisk energi blir renere i framtiden, ga elektrifiseringscasen ett utslipp på 3,60 megatonn CO2. Jo lengre en plattform er i drift eller hvor senere den er bygget; desto mer gunstig ble elektrifisering på grunn antatt renere elektrisk energi i fremtiden. IX X Table of Contents Nomenclature ......................................................................................................................... XV 1. Introduction ......................................................................................................................... 1 1.1 Background .................................................................................................................. 1 1.2 Objectives .................................................................................................................... 2 1.3 Contributions ............................................................................................................... 3 1.4 Limitations and assumptions ....................................................................................... 3 1.5 Risk assessment ........................................................................................................... 4 2 Heat and power generation offshore ................................................................................... 5 2.1 Greenhouse gas emissions to air .................................................................................. 6 2.2 Heat and power generation offshore ............................................................................ 9 2.3 Electrification ............................................................................................................ 11 3 Theory ............................................................................................................................... 15 3.1 Laws of thermodynamics ........................................................................................... 15 3.1.1 3.1.1 First law of thermodynamics ..................................................................... 15 3.1.2 3.1.2 Second law of thermodynamics ................................................................ 16 3.2 Compression and expansion ...................................................................................... 17 3.3 Heat transfer .............................................................................................................. 20 3.4 Power outputs and efficiencies .................................................................................. 21 3.5 CO2 emissions............................................................................................................ 24 4 Heat and power technologies and components ................................................................. 25 4.1 Gas turbine and waste heat recovery ......................................................................... 25 4.1.1 Brayton cycle ...................................................................................................... 25 4.1.2 Gas turbine ......................................................................................................... 27 4.1.3 Waste heat recovery unit .................................................................................... 31 4.2 Combined cycle heat and power ................................................................................ 31 4.2.1 Rankine cycle ..................................................................................................... 31 4.2.2 Combined cycle .................................................................................................. 33 4.2.3 Steam extraction and back-pressure ................................................................... 34 4.2.4 Once through steam generator ............................................................................ 35 4.2.5 Steam turbine ...................................................................................................... 38 4.2.6 Condenser and deaerator .................................................................................... 41 4.2.7 Pumps ................................................................................................................. 41 4.2.8 Feed water treatment and supply ........................................................................ 42 4.3 Electrification and gas burner for heat supply ........................................................... 42 5 Model description/methodology ....................................................................................... 45 5.1 Weather and temperature profiles .............................................................................. 45 5.2 Heat and power requirements .................................................................................... 47 XI 5.3 Emissions and losses related to electricity from onshore .......................................... 49 5.4 Runtime optimization and GT selection ...................................................................