Wax Control in Subsea Multiphase Pipelines

To achieve the goal of operation conditions optimization for the wax control unit the mathematical model describing the wax control unit is being developed. As the model has to be validated against experimental data, two experiments have been conducted during 2017: by-pass pigging in multi-phase flow using existing flow loop facilities at NTNU, and wax deposition in multi-phase flow using newly made small scale experimental stand. The model will also be validated against already existing data and against results from Olga software for relevant conditions. In parallel to PhD research Subsea 7 is designing an average scale flow loop that could be used for final validation of the model and for qualification of the wax control unit. After the qualification that will take place at the end of 2018, the unit could be designed in full scale and installed for operation in North Sea.

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.