From the date of their discovery in 1811, gas hydrates were overlooked for nearly a hundred years after as having been only a “chemical curiosity” – until gaining relevance at the beginning of the 20th century as the result of ice-like entities causing pipelines to break down. In the meantime, this problem was met with the injection of various substances – what are referred to as inhibitors. Monoethylene glycol (MEG) is among them. This organic compound has the benefit that it is not only suitable for the prevention of hydrate formation, but also for gas drying. Furthermore, MEG can for the most part be reclaimed from the process, thus being capable of being reused. Not only does this help to preserve the environment, it also lowers the necessity of transporting the chemical to drilling platforms.
Tried-and-tested system in customized design variations
Due to the two MEG application options, it can be utilized on an oil rig for two components. For one, it is intermittently injected into the borehole top between the upper main valve and the throttle valve to prevent the formation of hydrates. For another, an injection into the cooler of the gas export compressor for the purpose of gas drying is possible. The process is usually carried out at low flow rates by means of diaphragm pumps – at large quantities by means of plunger pumps – which, are installed with tanks, piping and instrumentation, all the way to integrated Chemical Injection Packages, exemplified by the globally-active LEWA GmbH. Among other advantages, this has the benefit of being able to take the interactions between individual components into account in the course of an overall optimization effort.
Whereas the process planning is the responsibility of the specific engineering company, it is the manufacturer who needs to undertake the detailed engineering in accordance with the tenders. The difficulty here is to bring the often highly complex, necessary standards – for example, in relation to environmental protection and operating safety – into accord with the directives and laws that are applicable at the location of use, as well as with customer specifications. In most cases, engineering solutions must be found which diverge from customary solutions. For this purpose, LEWA employs engineers from various disciplines such as hydraulic systems engineering, mechanical engineering and electrical engineering, who contribute to the evolution with their expertise, thus being able to react with flexibility to the most diverse regional conditions.
Keeping negative influence on the environment at a minimum
Most importantly, the harsh sea environment makes special designs necessary. The extremely salty atmosphere makes the use of corrosion-protected materials essential. This is why LEWA uses materials such as stainless steels for production, in special cases also using duplex or super duplex stainless steels. Owing to the limited space available on an offshore oil rig, it is mandatory that all components such as the pump and fittings are designed to be as compact as possible while simultaneously being conveniently operable.
Furthermore, the system should have as little of a negative influence on life and work quality, as well as on the environment, as possible. This can be achieved by the reduction of noise emissions, which in some circumstances are carried on through structure-borne sound, as well as the reduction of vibrations. One customer desired an especially quiet package. For this purpose, LEWA carried out a corresponding study and searched for possibilities relating to acoustic decoupling. Finally, vibration mats in combination with noise hoods were utilized. As such, sound emission was lowered from 93 dB to 75 dB. In addition, for the minimization of pressure pulsations, the system was submitted to an evaluation in accordance with API 674 Approach 2.
Prevention of leaks
Since MEG is toxic, the highest requirements are placed on process safety and the absence of faults when handling the substance. For this reason, diaphragm pumps are especially well-suited. They are hermetically tight and do not tend toward leakage. Occasionally, however, plunger pumps may be required if, for various reasons, diaphragm pumps cannot be employed or in the event of large flow rates. These are only able to be dynamically sealed by means of packings, which is why special precautions need to be taken in such cases to prevent leaks. For example, in having built a system in accordance with the EU Pressure Equipment Directive (DGRL) for one customer in particular, LEWA deployed a special leak monitoring system alongside a leakage pan together with outlet drainage in a Closed Drain System: Since commercially available flow meters reach their limits in quantities that are so relatively small, the manufacturer determined it was better to go with measurement using radar.
A reduction in maintenance effort also contributes to an increase in process safety. This is why LEWA skids are fully-automatic and mechanically redundant, designed for very long operational durations while featuring a fully-monitored leakage system. Moreover, high-quality fittings and instruments provide support. Some of these have self-diagnostic systems available. Thus, work that is necessary is reduced to a minimum.
Due to the wide variety of requirements – both those stipulated by the client and those laid down by law – LEWA relies on detailed planning and administration by its project management department, which, for example, takes over administrative tasks such as the coordination of subcontractors, progress supervision as well as documentation for the oil and gas industry.
