Tweaks the gas turbine needs for offshore use

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Offshore projects are characterized by requirements that are not encountered in onshore projects. Such issues shall be well considered in the design of gas turbines and their auxiliary systems. Described below are some of those conditions.

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

Motions and Accelerations

Floating offshore projects such as FPSOs, FLNGs and tension-leg platforms (TLP) …etc. are subject to sea motions and accelerations for which vary in level of severity. Gas turbines shall be able to safely operate at 1-year severity sea condition (a storm condition with a probability of occurrence once per year). However, gas turbines shall withstand 100-Year sea condition shut-down and be ready to restart without the necessity of major checks and inspections. For the 10,000-Year condition, gas turbines shall withstand the sea conditions in shut-down state without being damaged, although inspections shall be undertaken before restart. Those various sea conditions shall be applicable of the gas turbine, on-skid auxiliaries and off-skid auxiliaries.

Other considerations shall be considered due to normal sea motions and accelerations such as: Fatigue

Oil and gas projects are usually designed for 25 years of plant life. The repetitive sea motion with wave periods of 12 seconds (or less), the total number of cycles could exceed 100 million. Accordingly, careful attention shall be made to ensure that the items that are subject to those cyclic movements and loads have the endurance to withstand the induced fatigue. For example, expansion joints that are subject to horizontal and transversal movements (due to decks relative movements) and are made of metallic material will have to be verified for the induced fatigue; however, fabric-made expansion joints are less vulnerable to fatigue.

Shaftline Orientation

The orientation of the gas turbine shaftline; longitudinal vs transversal with respect to the major axis of the vessel, is also an important issue that is specific to offshore floating projects. If the shaftline is perpendicular to the axis of the vessel then the vessel movements shall be mapped such that the roll of the vessel is considered as pitch for the gas turbine shaftline. From a dynamics perspective, it is more favourable to have the shaftline parallel to the vessel axis than to have it perpendicular to it. Alignment Due to weight distribution on decks and acceleration forces, the deck on which the gas turbine main skid is installed will be subject to continuous deflections. Such deflections will be transmitted to the gas turbine skid to some extent (partially absorbed by the 3-point mount design) and would induce misalignment. This condition need to be checked. FEA study is usually performed to gas turbine skids to ensure the skid integrity as well as the effects on shaftline misalignment.


Slope for Oil Return Line & Oil Slushing

Lubrication and control oil systems for gas turbines are usually designed to have the oil return from bearings to oil tank by gravity. In case the oil tank is installed on the main skid and below the gas turbine, then oil will flow freely back to the tank regardless of the sea motion (pitch and roll). This might not be the case with driven equipment supplied with oil from the gas turbine lube oil equipment. However, for separate oil console (especially if located on the same deck level with the gas turbine main skid) the oil return line shall be slopped to ensure the flow of oil in the return line even in the worst pitch and/or roll conditions. If this slopping requirement is not satisfied then scavange pumps shall be used.

Another issue is the oil slushing inside the oil tank due to sea motions and accelerations. Oil slushing will create false readings of oil level in the tank and could cause unnecessary trips. Accordingly, the oil tanks shall be designed to prevent oil slushing for the worst scenario of operating sea conditions.

Modular Design Offshore projects are characterized by modular design where several decks are built at yard, then the equipment are installed before having the full module lifted and/ or transported to the vessel or platform. Some of the modular design characteristics and constraints are listed below: Deck Deflections Modules for offshore applications are made of steel which is flexible and subject to deflections due to weight loads. Different deck deflections are anticipated at the installation phase due to erection activities and final removal of temporary supports. Deck deflections are also envisaged during normal operation at site due to sea motions and accelerations. The common practice is to de-couple (or at least minimize) the gas turbine main skid (relatively long skid) from the effects of deck deflections by having a 3-points mount. For the 3-points mount two options are available; either Anti-Vibration mounts (AVMs) or gimbals. The choice between AVMs or gimbals depends on several factors. For instance, in case the gas turbine is required to be isolated from the deck vibrations or the gas turbine main skid is located close to manned areas then AVMs would be a suitable solution. Whereas gimbals are better than AVMs in absorbing higher angular misalignments and thus are recommended for larger skids and higher levels of sea motions and accelerations.

Saline & Humid Environment Saline and humid environment is applicable to floating, and non-floating offshore applications such as for fixed-leg platforms (FLP) and gravity-based structures (GBS). The painting and coating systems shall be suitable for such condition.

Painting, Coating & Material Selection

Painting and coating are critical to prevent corrosion of equipment and piping in offshore applications. It is recommended to paint stainless steel and to have bolting galvanized (or coated with corrosion resistant material) for saline environment. Tubing material shall also be selected carefully to avoid failures due to corrosion problems. De-humidifiers During shut-down periods, gas turbine enclosure will be subject to stagnant and humid air that would induce corrosion problems for the gas turbine components. It is recommended to have space heaters inside the gas turbine enclosure for towing phase and for extended shut down periods at site.