Installing and commissioning a gas turbine in offshore applications

In offshore applications, gas turbine drivers may be mounted on a common 3-point mount skid with the driven equipment. In some cases, the 3-points mounting might create a heavy point load on the structure, mandating a strong structural beam to accommodate the load and precise shimming preparations then need to be done accordingly.

This article is excerpted from the paper, "Gas turbines and associated auxiliary systems in oil and gas applications", by Emmanuel Bustos, Michael Hotho, Mounir Mossolly and France Alfredo Mastropasqua of TechnipFMC at the 2018 Turbomachinery Symposium.

For the 3-points mount installations, shimming under each point should be done based on the reference level of the complete skid. For floating applications, this can only be performed onshore to ensure that the levelling is not impacted by the sea movement. It is important that the skid remains stress-free during the installation sequence. A full welding on each mounting point can be performed (after the skid has been positioned and levelled within minimal tolerances) according to supplier’s requirements.

Flatness & levelness requirements on the interface between the gimbals and structure shall also be respected. Full Non-destructive examination (NDE) will then be applied to control the quality of the weld. For offshore projects where modular design is implemented, gas turbines main skids shall be provided with jacking pads to lift the skid if necessary during the installation adjustments when the use of the crane with slings crossing the decks above is impossible.

Auxiliaries such as ventilation ducts, inlet air filter house inlet air duct and exhaust stack will then have to be erected in a predefined sequence; starting from bottom to top. The expansion joints on all the ducting will allow to have some installation tolerances. Silencers will have to be installed (for example in the exhaust stack) as defined by the supplier (sometimes using special guiding tools). To facilitate the build-up/ erection of the ducting, some parts of the ducts will have to be assembled/ connected horizontally at ground and then tilted and lifted for final installation.

In the pre-commissioning phase, all instruments and electrical auxiliaries shall be verified for proper operability. A complete loop check on all instruments mounted in the gas turbine package (including auxiliaries) must be performed. Additional calibration may be needed to confirm the reliability of the instrument. All motors solo runs should be performed (with pump/fan uncoupled) to verify the cable connection and motor rotation.

As much as possible, all piping networks around the gas turbine and its auxiliaries should be cleaned. For the oil line, a full oil flushing process must be performed. Lube oil pump of the oil console could be used to perform such flushing. Filters with fine mesh according to gas turbine manufacturer requirements should be installed on the return lines to capture solid particles and other oil contaminants. To perform such oil flushing, gas turbine oil lines are disconnected and bearings are bypassed.

Boroscopic inspections for the gas turbine shall also be done at the pre-commissioning phases. A final check shall be done to ensure that the physical equipment is in full compatibility with the engineering documents, such as Piping & Instrumentation Diagram (PID) and hook-up drawings …etc.

Gas turbines are normally commissioned using site fuel gas; unless the gas turbine is dual fuel then commissioning could be done using liquid fuel (ex: diesel). Before starting any test runs of the gas turbine, all the auxiliary systems (including control and power panels) shall be commissioned and proved to be fully functional. Commissioning instructions are always provided by the gas turbine supplier in the commissioning manual, and shall be followed precisely by the commissioning team. Each auxiliary system shall be commissioned by itself.

The fire and gas system (F&G) shall be the first system to be commissioned; including the enclosure systems such as fire dampers …etc., to ensure safety throughout the commissioning process. The commissioning of all permissive-to-start shall be then performed. The sequence of systems that should be commissioned could be: 1. Unit Control and Power Panels; 2. F&G System; 3. Lube Oil Systems (Mineral & Synthetic); 4. Starting System; 5. Enclosure Ventilation System; 6. Fuel Gas System; 7. Inlet Air Filter System; 8. Gas turbine cracking/ motoring system; then; 9. Systems relevant to the driven equipment.

Once the gas turbine is in running mode, the first step is to test the emergency trip system: • Test of the emergency push button installed outside the gas turbine enclosure; • Test of the remote trip signal; Vibration control shall be performed to ensure a well stabilization after some running time. When vibration is stabilized within an acceptable range, then the gas turbine is considered ready for start-up.