Saeid Mokhatab

Severe slugging in flexible risers : Review of experimental investigations and OLGA predictions

Abstract Flexible risers are key components of floating production systems, particularly in harsh environmental conditions where the riser must deal with substantial vessel motion. Liquid slugging in flexible risers often causes severe operational problems for the downstream production system. To avoid such costly delays, it would be desirable to predict severe slugging behavior in flexible risers using experiments and transient codes. This article details the basic mechanisms and general characteristics of severe slugging in a pipeline-riser system. It also tries to present the research studies that have been carried out on severe slugging in flexible risers in two parts; experiments on different flexible riser configurations, and existing OLGA code predictions.

Experimental Investigations of the Mitigation of Paraffin Wax Deposition in Crude Oil Using Chemical Additives

Abstract Wax deposition from crude oil is a very expensive problem for oil producers around the world. The objective of this study is to understand the characteristics of paraffin wax deposition and to test the effectiveness of solvents in the inhibition of the crystallization and subsequent precipitation of the paraffin wax and to test the most effective concentration of the solvent used. The oil used here is from the Dakota formation from the Fourteen Mile Field in the Big Horn Basin of Wyoming. Two paraffin inhibitors were tested for this crude oil on a horizontal flow system. The inhibitors are mixtures of solvents, pour point depressants, and wax crystal modifiers. These inhibitors were tested at different concentrations and temperatures and the deposition rates were obtained for each. One inhibitor especially designed for this crude oil was relatively successful, reducing the deposition by up to 59% depending on the temperature.

Severe Slugging In a Catenary-shaped Riser: Experimental and Simulation Studies

Abstract: A series of experiments on two-phase, air-water flow in a flowline-riser geometry have been made. The riser has a catenary-shaped form (10.5 m high), and the flowline has a 2-degree downward inclination toward the riser-base (10.16 cm ID). Experimental data were used to characterize the severe slugging and unstable flows in terms of pressure cycling and fluid production characteristics. A model to study dynamic behavior of severe slugging flow in a pipeline-riser system was also developed under the environment of the OLGA2000 code, and experimental test results were compared with the OLGA model predictions. The motive for this work was to identify areas that pose substantial difficultly to the code for the simulation of severe slugging in a catenary-shaped riser. In addition, numerical experiments, exploring some of the reasons for difference between experimental and simulation results, have been presented in this article.

Greening of Petroleum Operations: The Science of Sustainable Energy Production

Petroleum is still the world’s most diverse, efficient, and abundant energy source; however, due to grim climate concerns and high gas prices, there is a growing initiative among global leaders to “go green.” The book is aimed at engineers, scientists, and students to move petroleum operations closer to sustainability. Numerous schemes are being presented as sustainable alternatives—sustainable because the source has been replaced with another source while keeping the process intact. This mode of cognition is very typical of Eurocentric philosophy of the last 900 years or so. This book deconstructs this philosophy and presents truly scientific analysis that involves both the source and the pathway. Focusing on long-term solutions that should “green” all of the petroleum industry’s practices, the book follows the theory of inherent sustainability, showing why current practices are fundamentally flawed and why new proposals to salvage efficiencies offer little hope for remedying the situation. The authors discuss global warming and its apparent relationship with petroleum operations, providing a detailed analysis of greenhouse gas emissions ranging from the Pre-Industrial and Industrial Ages to the golden petroleum era. A newly developed theory is included that shows that carbon dioxide from some sources does not contribute to global warming. Here—for the first time—carbon dioxide is characterized based on various criteria such as the origin, the path it travels, isotope numbers, and age of the fuel source from which it is emitted. Various energy technologies are ranked based on their global efficiency; this book also compares petroleum operations with other energy development technologies, including solar and biofuel energy systems. Written by pioneers in petroleum engineering, Greening of Petroleum Operations offers unique solutions to overcome major energy obstacles by developing genuinely technologies that should green all of the petroleum industry’s practices, from management style to upstream to downstream, and argues that solutions to global warming will come only from knowledge-based technology development. With the proposed solutions, the decline of the economic and environmental conditions not only will be arrested but improved, launching our civilization on an entirely new path.

Letter to the Editor: Why Consider Exploiting Stranded Gas?

Natural gas has remained the fastest growing energy resource in most regions of the world for more than a decade, driven by superior environmental performance as well as attractive conversion efficiency and economics for baseload use in power generation. For almost a century, natural gas has been transported safely, reliably, and on commercially attractive terms via pipeline. Pipelines were ideally suited to the supply and market conditions of the twentieth century, when large reservoirs of gas could be found in accessible locations that provided the stability and long-term supply and offtake security that pipeline projects demand. Now, in the 21st century, the vast majority of the large, easily accessible gas plays have been tapped by the thirsty markets of North America, Europe, and eastern Asia. Attention is shifting to natural gas reservoirs currently stranded in more remote locations. There are several reasons why specific gas reserves can be considered as stranded: remote location from gas markets, lack of transportation infrastructure, lack of access to conversion technologies or the capital to develop them, and geopolitical instability. Exploitation strategies for specific gas reservoirs depend upon size of resource, nature of resource, proximity to market, political environment around resource, and access to infrastructure, capital, and technology by the resource holder and/or operator. Almost 60% of 6,337 trillion cubic feet of proven natural gas reserves in the world (BP Statistical Review of World Energy, 2006) can be categorized as remote or stranded gas. In the past, developers of oil reservoirs with associated natural gas were able to flare off the unmarketable associated gas or, more prudently, take the more expensive option of re-injecting the gas for future exploitation. Flaring gas is no longer acceptable to environmentally conscious producers and many countries are legislating against it. Also, in some cases, re-injection alternatives can be limited by lack of suitable reservoirs to temporarily store produced gas. A number of solutions for commercially exploiting stranded gas reserves are available and are currently being employed, developed and researched (Figure 1). Gas liquefaction (LNG) is an established alternative to pipeline gas which has expanded rapidly and diversified its sources and markets over the past decade. Evolving technologies which convert gas into high-demand liquid petroleum products such as diesel (GTL) are now being developed on meaningful commercial scales in Qatar by several major companies.

Dynamic simulation of offshore production plants

Abstract: A dynamic model for predicting the effects of severe slugging/unstable flows on handling facilities of the offshore production plants is developed under the environment of the HYSYS.Plant (v.3.2) simulator. An experimental case study shows how the HYSYS model can be used as a useful engineering tool for the reliable simulation of separation facilities during normal transients and more serious upset conditions like severe slugging. In fact, by using this model one can check if the production system handles unstable flows or if the proposed production control system is stressed.

A Review of Current Technologies for Severe Slugging Remediation

Abstract One important problem expeienced in flexible risers is severe slugging phenomenon that typically occurs in the flowline-riser systems found on offshore floating production facilities. The flow and pressure oscillations due to severe slugging have several undesirable effects on the downstream topside facilities unless they are designed to accommodate them. However, designing the topside facilities to accept these transients may require large and expensive slug catchers with compression systems equipped with fast responding control systems. This may not be cost-effective and it may be more prudent to design the system to operate in a stable manner. While lowering production rates (slowing fluid velocity) can minimize severe slugging, operators are investigating alternatives that would allow for maximum production rates without the interruptions caused by slugs. This article references a combination of industrial experience and information from the literature to compile a list of methods of remediating the problems associated with severe slugging in pipeline-riser systems.

Simple Correlation Estimates Critical Properties of Alkanes

Abstract A simple exponential model is proposed to estimate the critical temperature, pressure, and volume of alkanes as a function of normal boiling point and molecular weight. The proposed correlation was used to estimate critical properties of 30 compounds including (C1−C30) of alkanes. The associated parameters were estimated by fitting model to experimental data using nonlinear regression analysis to minimize the sum of squares error employing a trust-region algorithm of MATLAB software and a BFGS algorithm of a solver add-in program in Microsoft Excel. Comparison showed superiority of the proposed model over previous proposed models. The average absolute deviation for estimation of critical temperature, pressure, and volume of the proposed correlation were 0.49%, 1.16%, and 0.97%, respectively.

Letter to the Editor: Is LNG a Competitive Source of Natural Gas?

Natural gas is the cleanest burning fossil fuel. It produces less emissions and pollutants than either coal or oil. Historically, natural gas has been viewed primarily as a regional energy supply source because it was transported to markets via pipelines. But advances in technology are transforming liquefied natural gas (LNG) into an increasingly global energy option, similar to oil. At remote locations, liquefying the natural gas for transport is also increasingly common and it is more economical to transport across large distances. When natural gas is cooled to approximately −259◦F at atmospheric pressure, a condensed liquid forms called liquefied natural gas (LNG). The volume reduction is about 1/600th the volume of natural gas at the burner tip. Natural gas contains primarily methane with low concentrations of other hydrocarbons like ethane, propane and impurities such as nitrogen, carbon dioxide, sulphur compounds and water. During the process known as liquefaction, natural gas is cooled below its boiling point, removing most of these compounds. The remaining natural gas is primarily methane with only small amounts of other hydrocarbons. The LNG process can also be designed to be almost 100% methane. There are many gas fields currently being developed around the globe where monetisation of the project has been possible by employing LNG export schemes. A majority of the world’s LNG supply comes from countries with large natural gas reserves. These countries include Algeria, Australia, Brunei, Indonesia, Libya, Malaysia, Nigeria, Oman, Qatar, and Trinidad and Tobago. Several major field developments are also at the feasibility stage. Examples include the Snohvit field in Norway, North Field in Qatar, South Pars in Iran, Stokman Field in Barents Sea offshore Russia, Nigeria LNG on Bonny Island, Gorgon Project offshore Australia, Egypt LNG, and Peru LNG.

The Effect of Tube Orientation and Pour Point Depressant on Paraffin Wax Deposition

Abstract A flow loop was used to study wax deposition in the laboratory. Data are presented to demonstrate that the tube orientation and pour point depressants have significant effects on the paraffin wax deposition for oil from the Octh Louie, a Cretaceous formation in the Manderson Field, Wyoming. Changing the tube orientation from horizontal to vertical reduced the amount of wax deposited by 28% for a tube wall (Twall) temperature of 42°F. The circulating oil temperature (Tbulk) was held constant at 76 ± 3°F. The amount of wax deposited was found to be a linear function of the driving force (Tbulk – Twall) and the wax appearance temperature (WAT) was estimated by extrapolating this linear relationship to a point of zero wax deposition. The WAT thus obtained (94.5°F) varied considerably with the cloud point (62°F) obtained using the ASTM D2500 method. Change in viscosity with time has also been investigated. Addition of a specific pour point depressant (PPD) at a concentration of 5 mL/gallon reduced wax deposition by 52%; concentrations of 10 mL/gallon of oil reduced the wax by 61%. Changing the flow regime from laminar to turbulent also had a significant effect, reducing the wax deposition by up to 42%. Gas chromatography–mass spectrometry (GC-MS), gas chromatography–flame ionization detection (GC-FID) analyses and other relevant property data pertaining to the oil are also presented.