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Exploration & Production Oil & Gas Review - Volume 6 Issue II -
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The principles in the dry-air method of drying pipelines are simple. Low-dewpoint air is blown into the pipeline, and the dry-air stream absorbs moisture since low-dewpoint air has a low moisture vapour pressure. The vapour pressure difference between the air moisture content in the pipeline and the dry-air moisture content is the driving force for drying. The greater the difference, the faster the pipeline will be dried. Since the ground temperature varies and pipelines are in the ground, this temperature becomes the determining factor in the length of the drying period. As the pipe wall will be the same temperature as the ground, the dewpoint of the air inside the pipe will be at the same temperature as the pipe. Therefore, the air inside the pipeline will contain more moisture when the ground temperature is higher, and pipelines can thus be dried more quickly when the ground temperature is higher.
To obtain low-dewpoint air, it is necessary to use both refrigeration and desiccant technologies in the dehumidification process. Dehumidification is defined as the removal of moisture from the air. Since ambient air is used in the dry-air system, the actual moisture content of the air can vary from low in the winter to very high in the summer.
The first part of the dehumidification process uses refrigeration to chill air below its dewpoint. In the summer, ambient air has a high temperature and high moisture content. Dehumidification by mechanical refrigeration is very efficient under these conditions. By passing air through the refrigeration cooling coil, which first cools the air, moisture is then removed by condensation. Normally, the air is cooled to approximately 2ºC; below this temperature, condensed moisture would freeze up on the cooling coil and the dehumidification process would stop. It is important to reduce the temperature to 2°C before the air enters the desiccant phase of the process. This mechanical refrigeration process occurs in a closed, pressurised loop in which a refrigerant such as Freon is circulated through a compressor, condenser, expansion valve and evaporator. The final part of the dehumidification process is achieved by using a desiccant dehumidifier. A desiccant, HPS, is used to absorb moisture from the air stream after it passes through the cooling coil. Since the air stream is cool and saturated, the desiccant is very efficient at removing moisture, thus reaching the final dewpoint of -40°C. Moisture is removed from the air stream in the vapour phase and is not condensed as in the refrigeration process; therefore, the desiccant dehumidification can work at any temperature without freezing up. HPS is impregnated into a honeycomb wheel that rotates at eight to 10 revolutions per hour. The honeycomb wheel rotates continuously between the process or dehumidifying sector and the regeneration sector. The regeneration sector uses electric heaters to dry out the honeycomb wheel. Figure 1 provides a simplified diagram of a rotary honeycomb unit.
Moisture Removal Performance/Drying Time A calculation of the performance of the dry-air system (see Figure 2) and total drying time required was determined, where:
• Vm = moisture removal by dry-air system (kg/hr);
• Wr = dry-air volume (m3/hr);
• W = amount of water in pipeline (kg);
• Xm = moisture content of saturated air in pipeline (g/m3 air);
• Xout = moisture content of dry air outlet (g/m3 air);
• D1 = internal diameter of the pipeline (m);
• D2 = D1 – 2 x thickness of the water film (m); and
• 1,000 = conversion factor from m3 to kg and from g to kg.
Dry-air system (outlet): Vr = 28,000m3/h. Xout = 0.33g/m3 (-30°C dewpoint).
Pipe wall temperature = 15°C.
The air dewpoint inside the pipeline was assumed to be the same as the pipe wall temperature; therefore, the moisture content of the air was: Xm = 12.8g/m3.
The moisture removal capacity of the dry-air system was calculated as: Vm = Vr(Xm – Xout)/1,000, so Vm = 28,000(12.8–0.33)/1,000 = 349kg/h. Remaining water in the pipeline after successful dewatering means that the water film is 0.1mm. For example: 100km DN 1,200. Remaining water: W = 3.14/4(D1*D1 – D2*D2)*L*1,000 = kg water, so W = 3.14/4(1.2*1.2 – 1.1988*1.1998)*100,000*1,000 = 37,700kg water. The total drying time for this project was calculated as: T = W/Vm, so T = 37,700/349 = 108 hours or 4.5 days.
Dry air is blown through the pipeline using a low-pressure system. In this system the air is dried at atmospheric pressure, and the dry air is blown to the pipeline by a roots blower. The maximum pressure for this blower is 1 bar; most pipelines require a pressure between 0.2 and 1 bar to blow and press foam pigs through the pipelines coal reflections in the XXVS and YYVS images is about 5ms two-way traveltime. That agrees very well with the 2.5ms one-way travel-time lag between fast and slow shear waves measured from first arrivals over the same depth interval. Note that, albeit weak, shear-wave splitting in this reservoir is important because it allows us to estimate the predominant fracture orientation around the borehole.
Conclusion
A new virtual source technique allows us to estimate interval shear-wave splitting of deep layers located beneath 3D, complex and anisotropic overburden – a feat that is impossible using other current methods. It requires as input a walk-away or 3D VSP acquired with two identical shear vibrators operating in orthogonal directions and multicomponent receivers in the layer of interest. The same technique can be used for look-ahead imaging with fast and slow shear waves, unaffected by overburden complexities. This opens a whole new field of possibilities for fracture and stress detection at depth and may greatly facilitate the interpretation of multicomponent surface seismic data.
Acknowledgements
We are grateful to Peter Bakker (Shell) for generating the synthetic VSP used in the first example. We thank Shell International E&P Inc. for permission to publish, and RCP at CSM for the Rulison data set.
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Peter Vemmmelund is the General Manager of Pipeline Services & Engineering (PSE)
International Ltd. Between 1984 and 1991 he was a Project Manager for Carl Munters,
working with low-dewpoint equipment and in the field of product development. In
1991, Mr Vemmelund founded Pipetech A/S with Torben and Jan Bilstrup, and since
then he has been involved in designing and constructing many mobile drying units and
drying thousands of kilometres of pipelines worldwide. Most recently, he designed
and constructed the world’s most compact and sophisticated mobile drying unit.
Mr Vemmelund trained as a marine engineer in 1984 and as a construction and
agriculture engineer in 1981.
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