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Using Flowmaster to Prevent Pressure Surge
for Water Injection Systems in the
Offshore Oil&Gas Industry

water surge

Surge Pressure
Surge pressure, (Water Hammer), is an important design consideration for water injection systems in the Offshore Oil and Gas Industry and the following challenges are frequently encountered:

  • High operating pressures are required to inject large volumes of water into hydrocarbon reservoirs, resulting in high surge pressures, and significant costs are incurred to design a system capable of withstanding a high maximum allowable surge pressure.
  • High fluid velocities are often encountered as operators try to squeeze the maximum performance from their systems, or when an existing system has been expanded considerably beyond its original scope. Higher surge pressures are generated when incompressible fluids at high velocities are suddenly halted.
  • Long Pipelines, which can be several kilometres in length, are often deployed to connect offshore platforms/FPSOs to Subsea water injection equipment. When fluid is halted (e.g. by sudden valve closure) pressure energy is transferred to the pipe wall and a pressure wave is sent, at the speed of sound, from one end of the pipeline to the other. The time it takes for the pressure wave to reach the opposite end of a pipeline is increased with the length of the pipeline; additional fluid flows into the pipeline during this time must be compressed resulting in increased surge pressure.

In this framework, the use of numerical simulation plays a fundamental role; by testing different operating and failure scenarios at an early design stage it allows to ensure safety while avoiding expenses of overlyconservative designs. In particular, the one-dimensional system-level approach provided by Flowmaster is comprehensive and effective from an engineering perspective. Indeed, it allows to study pressure surge in the entire system accounting for complex control systems in a fast and accurate manner.


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Flowmaster screenshot of a Topsides water injection system

Identifying Surge Cases
Surge analysis on water injection systems is performed to determine the maximum pressure surges that can occur as a result of transient events such as rapid valve closure during pipeline operation, or pump trip and restart operations. The first step in any surge analysis is to identify credible cases where surge can arise. For example, closure of a single valve in a complex water injection network is unlikely to be an issue; however, closure of all wells while water injection pumps continue to operate (e.g. due to malfunction of a facility’s shutdown system) is potentially a serious concern.

The role of Flowmaster
After the surge cases have been identified, the next step is to build an accurate simulation model using Flowmaster. Both the Topsides and Subsea systems need to be considered to ensure confidence in any results obtained. Water injection models usually commence at the de-aerator, which, due to the very brief nature of the transient events can be considered to be a constant-head flow source. The water injection pumps and any major fittings are modelled downstream of the de-aerator. It is critical that control systems (e.g. minimum flow controllers), safety devices (e.g. Topsides bursting discs) and shutdown systems are included as they can have a significant impact on the results. For the Subsea system all the pipelines and wells are included. The model usually ends downstream of the injection trees, or immediately upstream of the reservoir. Choke valves are modelled to control the flow rate to each well and the injection wing or master valves are slammed shut to create a surge event. Subsea risers, pipelines and jumpers should be modelled using the elastic pipes option to improve simulation accuracy. When surge analysis is carried out at an early stage in the design of a water injection facility Flowmaster can be used to predict the maximum surge pressure and this can be used to select system design pressure. For surge analysis on an existing system Flowmaster can be used to investigate measures to mitigate surge (e.g. extending valve closure time) to ensure that the system’s maximum allowable surge pressure is not breached. It is important that surge cases considered are realistic otherwise considerable expense may arise due to an overlyconservative design pressure on a new facility, or due to unnecessary modifications to an existing facility. The advantages of using Flowmaster for surge analysis include:

  • The fully elastic behavior of pipelines is accounted for.
  • Small time steps can be used to accurately simulate fast-transients.
  • Simulation times for large models, with many components and nodes, are relatively fast.
  • Complex control systems can be utilized including PID controllers and advanced scripts (e.g. to trip water injection pumps upon detection of high discharge pressure).

flowmaster

Pressure Vs Time for Umbilical Failure Case

Ajmal Zia, Flow Assurance Consultant within the Offshore Oil and Gas industry explains how Flowmaster has been used for surge analysis of water injection systems, ranging from conceptual studies to detailed design. “We recently conducted surge analysis for a Client involving the tie-in of a new drill centre to an existing system. The design pressure of the existing water injection system had to be adhered to, but significant changes to the control system were possible because a new MCS (Master Control Station) was being installed to service the new drill centre.” explains Ajmal Zia. In all cases, the mitigating measures either eliminated surge pressure entirely, or reduced it within acceptable limits.

“Flowmaster was an indispensable tool on this project. The simulations made it possible to quantify the severity of each surge case and to assess the effectiveness of each solution.” concludes Ajmal Zia, Flow Assurance Engineer within the Offshore Oil & Gas industry.



 

Articolo pubblicato sulla Newsletter EnginSoft Anno 9 n°3

Alberto Deponti
EnginSoft

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