Natural gas is a hydrocarbon gas mixture, which consists primary of methane as well as impurities like water and carbon dioxide. Natural gas is widely used as an important energy source for many applications including heating, electricity generation, industrial power generation and as a fuel for vehicles. Gas is often found in the near vicinity of oil, but also together with oil in deep underground natural rock formations.
In order to use natural gas as a power source, it must be processed to clean the gas and remove the impurities, including water, to meet the requirements of the end user.
Liquefied Natural Gas (LNG)
On account of its low density, it is not straightforward to store natural gas or transport the gas by road. Additionally, transporting natural gas across the oceans is highly impractical. Furthermore, the existing gas pipeline network is already close to capacity such that a significant number of new pipelines are required to fulfill the need for gas in the near future.
Cooling natural gas to about -160 degrees Celsius at normal pressure results in the condensation of the gas into liquid form, known as Liquefied Natural Gas (LNG). LNG is natural gas that has been temporarily converted into a liquid for the ease of storage and transport.
Transport and re-gasification of LNG
LNG production and transportation requires an important infrastructure consisting of one or more LNG trains, each of which is an independent unit for gas liquefaction. Subsequently, the LNG is loaded onto ships and delivered to a re-gasification terminal, where the LNG is reconverted into gas. The re-gasification terminals are usually connected to a storage and pipeline distribution network to distribute the natural gas to the local distribution companies or independent power plants (IPPs).
Gas Compressor Stations
A gas compressor station is a large installation to help the transportation process of natural gas from one location to another. While transporting natural gas through a gas pipeline, the gas needs to be constantly re-pressurized at certain distance intervals.
The location of the compressor station heavily depends on the type of terrain but also on the number of gas wells in the vicinity of the compressor station. A large number of gas wells and frequent elevation changes will require more compressor stations.
Centrifugal and reciprocating compressors
When the natural gas has entered the compressor station, it is compressed by either a turbine, motor, or engine. Turbine compressors gain their energy by using up a small portion of the natural gas that they compress. The turbine itself serves to operate a centrifugal compressor, which contains a type of fan that compresses and pumps the natural gas through the pipeline.
Mechanical Integrity Analysis
Any structure will have a number of mechanical resonance (natural) frequencies. If these frequencies coincide with those of external excitations, for example those due to pumps or the fluid flow within a pipe, then any small pipe deflection caused by the excitation mechanism at these frequencies, could be amplified and result in vibrations in the mechanical structure. These mechanical vibrations, if persistent, could result in problems due to Low Cycle Fatigue or High Cycle Fatigue.
Pulsations and Mechanical Response
Reciprocating compressors produce pulsations in the suction and discharge piping that can be damaging to the piping and to the equipment itself. The pulsations can lead to potential fatigue failure, undesirable vibrations, reduced efficiency or errors in flow measurement results.
Also, pulsations in the piping system might result in cyclic stress levels and fatigue problems. A pulsation analysis is most often performed either in the design phase or as a result of a failure in the field. Field problems are usually inspected by a DRG expert and numerous measurements are taken to help identify the exact nature of the pulsation.
Gas Liquid Separation
As the pipeline enters the compressor station, the natural gas passes through scrubbers, strainers filters or separators. These different types of equipment are all designed to remove any free liquids or dirt particles from the gas before it enters the compressor.
DRG is able to assist you in obtaining full three-dimensional multi-phase flow fields, in order to study and optimize the liquid separation from the gas. For these complex and sensitive systems, it can be necessary to investigate the three-dimensional flow field. This can be obtained by performing a full Computational Fluid Dynamics (CFD) analysis.
LNG Terminals and Floating Platforms
Before or after liquefied natural gas (LNG) is transported over long distances, the LNG is stored in large insulated tanks. Even when very efficient insulation is applied, the LNG cannot be kept cold enough by itself.
LNG terminals involve large installations, often forming purpose-built ports to exclusively export or import LNG, such as the Gate terminal in Rotterdam harbour. Consequently, the design and verification of gas and LNG terminals requires the assessment of the mechanical integrity of a wide variety of supporting equipment and connected transportation lines. The considered systems include but are not limited to:
- Cooling water systems of Glassfiber Reinforced Epoxy (GRE) piping,
- Gas compressor stations, including cooler banks, filters and compressors,
- Firewater systems, which often involves buried GRE piping,
- High and low pressure vessels containing gas or LNG.
FLNG compressor modules
A floating production, storage and off-loading (FPSO) unit is a floating vessel used by the offshore oil and gas industry for the processing of hydrocarbons and the storage of oil. An FPSO vessel is designed to receive hydrocarbons produced from nearby platforms or sub-sea, process them, and store oil until it can be offloaded onto a tanker or, less frequently, transported through a pipeline.
The relatively large bore suction and discharge piping connected to the natural gas compressor on-board a floating LNG platform may be exposed to extreme load conditions. In addition to the normal thermal and pressure design loads also loads due to large wind velocities as a result of storm fields passing by having to be accommodated by the pipework this may result in large compressor nozzle loads. As a result of the sea waves the FLNG compressor module is also subject to rocking motions.
DRG is able to analyse various compressor piping layouts for the feasibility of reconciliation of these conflicting requirements. The target for such an analysis is to keep the nozzle loads under all load conditions within a safety margin which conforms with API 617.