Wax Control in Subsea Multiphase Pipelines

For å oppnå målet om optimalisering av operasjonen av vokskontrollenheten utvikles det en matematisk modell for enheten. For å kunne validere denne modellen er det i 2017 gjennomført to tester: By-pass pigging i multifaseflow ved bruk av eksisterende flow loop-fasiliteter på NTNU, og test av vokssamling i multifaseflow ved bruk av en nedskalert eksperimentell enhet. Den matematiske modellen vil også valideres mot eksisterende data og resultater fra Olga-software når relevant. I parallell med PhD-forskningen designer Subsea 7 en oppskalert scale flow loop som kan brukes for endelig validering av modellen og for kvalifisering av vokskontrollenheten. Etter planlagt kvalifisering i slutten av 2018 kan enheten designes i fullskala og installeres for operasjon i Nordsjøen.»

The main results of the PhD work are: ? Experimental data on by-pass pigging in multi-phase flow. ? Decision to construct WCS operating in single-phase flow both due to pigging control and wax deposition predictability. ? Simulator program of wax deposition inside the WCS. The program can be used for design of a WCS. ? WCS qualification scale rig testing. According to Norwegian DNV regulations the test showed that the system is ready enough for implementation on an oil field.

Subsea 7 plan to develop a wax control unit (WCU) capable of solving the wax control challenge to enable "cold flow" from remote subsea oil fields. The base concept is that the WCU generates the wax under controlled conditions by cooling the flow till ambient temperatures and then removes wax generated on walls by pigging. This allow for subsequent transport of the wax as a solid component in the multiphase mixtures ("Cold Flow"). Cold flow takes advantage of the fact that without temperature difference between fluid and surroundings, there will be no build up of wax deposition on the pipe wall. The resulting slurry flow will eventually consist of produced multiphase stream with hydrate and wax particles dispersed in the bulk hydrocarbon fluid, that are transported close to the ambient temperature along the flow line to the processing facility. In more detail: Preconditioned fluid is cooled to ambient temperature in a pipeline bundle heat exchanger system applying counter current cooling water. Wax solids are then precipitated in a controlled section of the pipeline inside WCU. Solid wax deposits that form on the walls of the loop will be removed by using a circulating pig. The pig is propelled by the produced fluids, and sweeps the wax deposits off the inner wall of the loop. Solid wax formed in WCU follows in the multi-phase flow along transport pipeline to host. An automated multiple pig launcher is used for local wax removal, as well as for removal of wax in the transport line to host. Literature search shows that the scientific knowledge of wax behaviour in oil has been increased over many years. In Norway, both NTNU, SINTEF and private companies like Statoil are working on this issue. This study shall be built on previous work A PhD is proposed on the challenge of wax management in WCU. The physics of wax systems will be explored with the objectives of arriving at a working model for a WCU.