Surge analysis on a complex sealine network

Niccolò Sbarigia, Luciana Concilio

Surge phenomenon (often referred as “water hammer”) is a very important topic in the design of long pipelines and complex system like service transport network used in the OIL&GAS.

When fluid in motion is abruptly stopped (i.e. quick valve closure, pump shut-down, check valve closure), a hydraulic surge is created in the system. The kinetic energy, released as pressure, can spike up to the system's Design Pressure - damaging system instrumentation, pumps, pipes, fittings, and valves.
Without any proper suppression device, the shock wave travels pipe’s length back into the system, then reverses again, oscillating back and forth until friction dissipates the pressure spike or a system component fails.

From the above, the study of water hammer effect is essential to guarantee the safety of plant and network.

As part of a detailed design engineering phase, a surge analysis, relevant to MEG (Monoethylene glycol) injection system, has been performed.

The studied system is part of a complex field development where: A and H are offshore fields located in Arabian Gulf in medium-depth waters (40-60 meters). Six production wells for field A and seven production wells for field H are planned, each well is located on a dedicated Wellhead Platform (WHP).

The production fluid from each wellhead platform is conveyed to a gathering Tie-In Platform (TP), where it is commingled for export to shore through the main export pipeline.

MEG is required to be injected continuously on each WHP downstream choke valves to prevent corrosion and the hydrates formation in the system. MEG is pumped from shore to reach the offshore fields through a complex sealine network; water hammer analysis has been done to identify potential criticalities and so avoid any damaging to the system.

The software Flowmaster has been used to simulate this complex network; it is a 1 D software which can solve fluid systems (pipes, vessels, pumps, valves…) linking various components from a library of standard elements.

A lot of possible critical scenarios have been simulated during the design to estimate the maximum surge pressure the system can experience. In this work, the scenario, relevant to effects of a general emergency shut-down, is presented and analysed as the most representative case.