Oil Storage facilities (terminals) are usually designed with a pressure rating lower than the rating of the pipeline transporting the fluids. During abnormal operations, terminal piping can be subject to unexpected transient pressure surges that can exceed the allowed values. Mitigations are required a common one is installing a relief system. When a relief valve is installed, it is connected to a tank and the location of this relief tank is critical for the proper operation of the relief system and the overall mitigation of pressure surges. Relief design needs to take into account the length and layout of the piping. Facilities in the northern hemisphere contain pipes installed above ground and prone to experiencing cold temperatures during winter months. If the fluid is stagnant in these pipes, the cold weather increases the viscosity of the fluid. If the relief valve activates, the fluid that has been stagnant in the pipe needs to be pushed out of the pipe and into the tank. This requires a high pressure from the system and is directly affected by the distance of the pipe and the properties of the stagnant fluid. This paper will show how transient pressures change for length of pipe and for varied viscosities of the stagnant fluid. With these findings, engineers can improve their understanding of the effects of temperature and length on surge pressures and they can design safer systems for liquid transportation and storage.

Oil Storage facilities (terminals) that receive fluids from pipelines or inject fluid into them, are usually designed with a lower pressure rating than the actual pipeline between these facilities. This is mostly due to the fact that the pressure expected in the terminal is much lower than the pressure required to transport the oil. However, these terminals are still subject to pressure surges caused by abnormal transient events during normal operations. In cases where the surge pressures exceed the allowed operating pressure of the equipment, a relief system can be installed to mitigate these surges to acceptable levels. When constructing a new terminal or altering an existing one, the hydraulic calculations of these terminals are generally based on design values of the project, such as maximum and minimum flow rates. The hydraulic studies and simulations that are normally done by companies are based on steady state conditions, however, to design intrinsically safe facilities, the system’s entire operating envelope should be considered at the design stage of the project. Once transient analysis results show the need to install a pressure relief device, the proper location of this equipment is critical for the effectiveness of the surge relief system to mitigate overpressures properly. The effect of flow rate, piping configuration, and initial pressure profiles were simulated and compared to determine their impact on pressure surges and on the critical devices along the flow path. Secondly, simulations were done with the relief system installed in different locations along the terminal pipe and the resulting changes in maximum pressure surges. The objective of this paper is to show the importance of a detailed transient analysis based not only on design parameters but also on operational scenarios to mitigate surge overpressures in a more cohesive manner. The secondary objective of the paper is to discuss key parameters that need to be considered for selecting the location of the surge relief valve to ensure critical devices are safe during the upset conditions. The analysis presented in this paper is applicable across a broad configuration of oil facilities.

Transient hydraulic conditions during a shutdown and subsequent start-up of a segment of a pipeline that runs through a mountainous region were simulated using commercially available hydraulic simulation software and a model of the relevant portion of the pipeline facilities. The segment of interest is located in an area where the pipeline is normally operated with vapor present (slack line flow conditions) due to the large change in elevation. Pressure data that was recorded by the pipeline’s data acquisition system indicated a pressure surge occurred when the line was restarted. The suspected cause of this pressure surge was the collapse of the vapor in this pipeline segment. Beginning with an estimate of the flow, pressure and temperature data for the pipeline segment at steady state conditions prior to the shutdown, the simulation was tuned to reasonably match the measured data. The resulting simulated data closely replicated the surge event. Examination of the simulated data provides insights into the hydraulic conditions in the pipeline at locations where pressure data is not measured, as well as during the time intervals between data acquisition scans. It also reveals impact of the timing of the mainline valve opening sequence. Further, since the simulated data does accurately replicate the actual measured data, the model can be used to evaluate how changes to facilities or operating conditions impact the formation and the collapse of vapor in this pipeline segment.

Continuous economic development demands safe and efficient means of transporting large quantities of crude oil and other hydrocarbon products over vast extensions of land. Such transportation provides critical links between organizations and companies, permitting goods to flow between their facilities. Operation safety is paramount in transporting petroleum products in the pipeline industry. Safety can affect the performance and economics of pipeline system. Pipeline design codes also evolve as new technologies become available and management principles and practices improve. While effective operation safety requires well-trained operators, adequate operational procedures and compliance with regulatory requirements, the best way to ensure process safety is to implement safety systems during the design stage of pipeline system. Pressure controls and overpressure protection measures are important components of a modern pipeline system. This system is intended to provide reliable control and prevent catastrophic failure of the transport system due to overpressure conditions that can occur under abnormal operating conditions. This paper discusses common pressure surge events, options of overpressure protection strategies in pipeline design and ideas on transient hydraulic analyses for pipeline systems. Different overpressure protection techniques considered herein are based on pressure relief, pressure control systems, equipment operation characteristics, and integrated system wide approach outlining complete pressure control and overpressure protection architecture for pipeline systems. Although the analyses presented in this paper are applicable across a broad range of operating conditions and different pipeline system designs, it is not possible to cover all situations and different pipeline systems have their own unique solutions. As such, sound engineering judgment and engineering principles should always be applied in any engineering design.

Predicting the effects of entrapped gas or vapor formation on surge is very important in design and operation of liquid pipelines. This paper identified the scenarios in which entrapped air and vapor formation need to be considered in pipeline operation and design. Useful modeling methods utilizing common liquid pipeline transient hydraulics software are provided. Validation of the presented methods was completed using experimental data from published literature. Examples are presented in showing the implementation of the provided modeling methods on real pipeline design scenarios. Finally, advantages and limitations of the presented methods was discussed. The methods presented in this paper enable pipeline operators and design engineers to properly estimate the complicated surge issues such as the influence of air bubble venting and column separation and collapse using commonly available single phase hydraulics tools. The operators and engineers will benefit from the provided methods in finding and validating reliable surge mitigation solutions and creating pipeline design with higher integrity level. The paper also presents the limitation of the methods and continuous improvements that can be achieved in the future.

A hydraulic transient occurs when there is a sudden change in the steady state condition of a fluid in a pipeline. The rapid change in velocity of a fluid causes a pressure wave to travel along the pipeline, potentially causing damage to the equipment and piping. Usually, different scenarios are studied depending on whether the fluid is being injected or delivered from the pipeline into a tank. The most commonly studied causes of sudden fluid velocity change in a pipeline are: closing of a fast acting valve (ESD or control valves which close within seconds) and stopping or starting of pumps. When a pipeline is delivering fluid into a tank, the closure of a valve can completely block the flow and create transient surge pressures that exceed acceptable pressure limits, and require mitigation. Although the closure of a fast acting valve is a commonly analyzed scenario, the closure of a motor operated valve (MOV), which is less commonly analyzed, can also create surge pressures which can put the pipeline at risk. There are three characteristics of an MOV that can significantly impact the surge pressures it creates when it is closed. These valve characteristics are: flow coefficient, valve curve and stroke time. Hydraulic simulations were performed to study the effect of these three valve characteristics on transient responses when delivering from pipelines into tanks. Simulation results show that a faster stroke time leads to higher pressure surges, as well as a valve with a quick closing or linear curve. However, the flow coefficient of the valves will have varying effects on transients depending on the piping system being analyzed. The purpose of this paper is to not only highlight the importance of valve characteristics when modeling transient surge events but also to provide key learnings that can be used to design safer delivery terminals.

Fibreglass reinforced plastic pipe (FRP) is the second most common type of pipe in the Canadian oil and gas industry, based on installed length. Industry methods to define risks and prevent failures are difficult because industry is still learning how these types of materials fail. Current industry failure records indicate that the failure rates for some of these materials are higher than steel failure rates. Unique details related to a specific FRP failure will be discussed in this paper. This failure occurred on an 8-inch OD FRP pipeline at the bottom of a riser. The failure resulted in a spill and a fire. The reasons for failure and fire initiation were analysed separately. The failure was a result of a combination of several types of stresses and material degradation. Both static and dynamic stresses contributed to the failure. • Ground settling resulted in high static bending stress of the last section of the pipeline connected to the riser elbow supported by the anchor. • The failure was in the last connection of the pipeline. Static tie-in stresses could have contributed to the failure. • Static stresses were evaluated using Finite Element Analysis (FEA) approach and found to be insufficient for the failure. • Dynamic stresses contributed to the failure. The failure happened soon after a power outage, when numerous wells were restarted, and several fluid surges may have occurred. • Material degradation associated with a specific orientation of glass fibres at the connection pup contributed to the failure. The failure sequence was established and different modes of fire initiation were analysed.

Traditionally, pipeline transmission systems have been designed using steady state simulations. Steady state simulations are sufficient for optimizing a pipeline when supply/demand scenarios are relatively stable. In the case of a gas pipeline, it is also important that flows in and out of storage are not highly variable. In general, steady state simulations will provide the designer with a reasonable level of confidence when the system is not subject to radical changes in mass flow rates or operating conditions. However, situations do arise which require more than a conventional analysis such as large load factors, surges in mass flow rates, the loss of facilities and facility operation (e.g. pigging procedures). In these and other instances, the designer will want to perform a dynamic (or transient) analysis to test the capability of the system, choose the system components and maintain the appropriate level of safety. This paper will illustrate the importance of transient simulations when designing transmission systems subject to aggressive conditions. Example scenarios, taken from current major projects, are used to depict a diverse range of dynamic problems. The examples help identify the need for a transient analysis and exemplify the downfalls in a system when the analysis is not employed during the optimization and design process.

Recycle systems are important components in the operation of centrifugal compressor stations. They are essential during a start-up operation, for surge protection and for emergency shutdown (ESD). These operations are inherently dynamic where interactions between equipment, control and gas flow occur in a complex manner with the associated risk of compressor surge. Of particular importance are the effects of recycle system capacity, the recycle valve characteristics, check valve dynamic behavior, piping geometry and capacitance around the compressor unit, and the performance characteristics of the centrifugal compressor itself. This paper presents numerical results of the effects of some of these parameters on surge control, ESD and unit startup. These parameters are: i) The effects of damping the surge control flow signal in an attempt to suppress the signal noise, on the integrity of the surge control system, ii) The effects of recycle valve characteristics, stroke time and valve capacity on ESD, iii) The effects of recycle line size on ESD and iv) The effects of the recycle valve closing time (or rate) on the startup operation, with the intent of shortening this time to minimum for environmental reasons. Results were obtained from the solution of the pertinent dynamic equations describing the gas and equipment dynamics which has been verified against field and laboratory measurements. The examples presented in this paper were applied to a 24 MW natural gas compressor station on the NOVA Gas Transmission system, and to a scale-down laboratory model. Influence of other parameters from this investigation were published elsewhere and are cited in the reference section.

In 1997 Alberta Natural Gas Co. Ltd (ANG) completed a field testing program of its centrifugal compressor fleet. Field performance and surge testing of centrifugal compressors in pipeline service was done for efficiency evaluation and to re-establish surge line control. By confirming the actual location of the surge line, surge controllers are adjusted to allow a more efficient and greater operating range resulting in fuel savings and operating flexibility. The results of this testing provides an accurate operating window for the compression equipment which is then transferred to a hydraulic analysis computer model used to provide accurate capacity estimates in support of additional gas transmission contractible volumes. As part of the surge testing, suction to impeller eye differential pressure readings (used for surge control) were evaluated to determine strength, stability and repeatability. Finally, baseline data was established to determine current compressor operating efficiencies and will be used to determine future efficiency degradation. ANG is a wholly owned subsidiary of TransCanada PipeLines, one of North America’s leading transporters of natural gas through its energy transmission businesses. ANG owns and operates the British Columbia segment of the Alberta-California pipeline system (ref Figure 1). Compression is provided at three compressor stations with eleven compressors totalling 187,000 installed ISO HP.

A surge control system for a natural gas centrifugal compressor station has been modified in order to reduce shutdowns caused by high discharge temperature and provide a more robust and stable operation. The process consists of a compressor driven by a 14 MW gas turbine and recycle piping, a 16” recycle valve, a PLC based surge control algorithm, a flow measurement element, and a compressor differential pressure transmitter. The control objective is to manipulate the recycle valve to maintain flow through the compressor to a setpoint determined from the differential pressure across the compressor. Field tests were conducted to measure the open loop process dynamics of the valve, piping, compressor and transmitters. From the test data, the relevant process dynamics were determined enabling the development of a first order plus dead time model of the system. The process dynamics are complex due to the gas dynamic effects of the station piping and tend to exhibit inverse and time delayed behavior. Large variations in process gain also create problems with obtaining a consistent flow response under different operating conditions. A stability analysis was completed and the control system was redesigned with several enhancements including derivative control, flow signal filtering, process linearization, and improved controller programming techniques. The results of the modifications are the compressor does not shut down when subjected to transients from other units, the compressor can be started against high head conditions, and the closed loop response time is ten times faster than the previous system. The new system has been in operation since May 1997.

The Russian natural-gas distribution system includes 125,000 miles of pipe and some 250 major pumping stations equipped with 3,700 turbine-driven compressor units. A majority of these stations were built in the 1970s and 1980s: consequently, their operational efficiency is lower than modern stations equipped with high-efficiency compressor units. In order to improve the efficiency of these compressor stations, over half of the compressor units are being retrofitted with integrated, turbine-and-compressor control systems allowing them to operate closer to pipe pressure, turbine temperature-pressure limits, and compressor surge. It is projected, and results so far confirm, that these new control systems will not only increase compressor-station efficiency and significantly reducing the amount of gas diverted to fuel the downstream pumping stations, but will increase the amount of gas that can be transported. Additionally, several of the new or refined control algorithms, developed to meet customer expectations and acceptance tests, should be applicable in other areas and applications.

When using nitrogen loaded surge relief valves it is believed that the relief valve is in control of the upper pressure limit of the pipeline. In reality, it is the nitrogen system together with the valve in a master and slave relationship that sets the upper threshold. The negative effects of changing temperature on the stability of a nitrogen loaded relief valve’s set point is well known but poorly understood. There have been many attempts to minimize this effect. Burying the plenum and using the soil as an insulator has proven to be ineffective. Manual adjustments are not cost effective. Regulators offer inconsistent performance and are often misapplied. Evolving technology now allows for this process to be automated. Set points can be held with remarkable accuracy and plenums no longer need to be buried.

Pressure transients in piping systems occur whenever there is a change in fluid velocity. If this change is large enough, the pressure wave produced can exceed the Maximum Operating Pressure (MOP) of the system. Canadian and US regulations allow liquid petroleum systems to exceed the MOP under abnormal operating conditions however these surges cannot exceed 110% of MOP even for short periods of time. As part of meeting these regulations, the authors have applied complex computational modeling tools, developed methodologies, and company standards to identify sources of pressure surges, with the ultimate purpose of providing protection solutions useful for mitigating overpressures in oil injection facilities with low rated piping. These computational models and identification methodologies are based on a) abnormal operating conditions recorded in the past, b) potential worst case scenarios of terminal transients, and c) are particularly sensitive to input data such as piping characteristics, fluid types, and the initial states of the operating system. Our paper discusses the above mentioned transient simulation methodologies and their importance in meeting regulations.

Compression systems employed in gas pipeline transmission systems and gas processing plants provide a vital function to the overall operation of both systems, and therefore must be vigilantly monitored in order to ensure a high level of operational reliability. The majority of these compression systems employ centrifugal compressors, driven by either gas turbines or electric motors with/without gearboxes. These systems are designed and operated in a manner to eliminate or minimize the potential for surge, which is a dynamic instability detrimental to the integrity of the unit. Surge can occur when compressors are subjected to rapid transients such as one following an emergency shutdown (ESD) or power failure. To prevent this, compressor stations are designed with recycle systems and special types of recycle valves required to open upon ESD. This paper describes the dynamics surrounding an ESD operation of a compression system composed of two units in series and two independent recycle systems, one for each unit. The conventional sizing of the recycle system applied to a single unit operation needs to be evaluated carefully when there are two units operating in series, and in particular sizing of the recycle system surrounding the higher pressure unit, i.e. the downstream unit. This paper provides an insight into the salient parameters that impact the operation of such system via several dynamic simulations of an ESD event. Different ‘what-if’ design scenarios are analyzed and results presented. The data was obtained from an actual station operation on TransCanada’s Alberta gas transmission system in Canada. This station also employs two large gas volume capacitance aerial coolers which are also involved in the dynamics of the system during ESD.

Terminals are an integral part of a pipeline system. Typically, petroleum products are transported from an initiating terminal to various delivery terminals along the pipeline. Operation safety is paramount in transporting petroleum products in the pipeline industry. Safety can affect the performance and economics of a pipeline system. While effective operation safety requires well-trained operators, operational procedures, and compliance with regulatory requirements, the best way to ensure operation safety is to implement safety systems during the design stage of the pipeline system. A pressure relief system is an important component of an engineered safety system. This system is intended to prevent catastrophic failure of the transport system due to overpressure conditions that can occur under abnormal operating conditions. This paper discusses pressure surge relief as it applies to the design of pipeline terminals. Different pressure surge relief devices such as pressure relief valves and pressure surge vessels are considered and their advantages and disadvantages are discussed. The effects of transport rates, piping configurations, and other equipment operation, such as pumps and valves, on pressure surge relief system, are evaluated. One of the primary objectives of this paper is to discuss pressure surge events, device simulations, and key parameters to consider when selecting a pressure surge relief system for a terminal design to ensure that the piping system and hydraulic components remain safe during abnormal operating conditions. Although the analyses presented in this paper are applicable across a broad range of operating conditions and equipment and devices in terminal system designs, it is not possible to cover all situations. As such, sound engineering principles and engineering judgment should always be applied in an engineering design.

Compression Systems are designed and operated in a manner to eliminate or minimize the potential for surge, which is a dynamic instability that is very detrimental to the integrity of the unit. Compressor surge can occur when compressors are subjected to rapid transients such as one following emergency shutdown (ESD) or power failure. To prevent this from occurring, compressor stations are designed with recycle systems and special types of recycle valves, which are required to open upon ESD. These recycle valves must have specific characteristics to cope with such fast transients and protect the compressor for undergoing surge. This paper describes three methods for determining the effectiveness of any specific recycle system and recycle valve characteristics. These three methods are: i) the concept of the Inertia number, ii) a simplified method based on system impedance, and iii) full dynamic simulation of the compression system. These three methods are applied to a high pressure ratio (up to 3.5) natural gas compressor station involving very high volume of piping and equipment contained within the recycle loop. There is a heat exchanger to utilize the heat of compression to heat the condensate separated downstream in the discharge scrubber, two large aerial coolers to cool the high temperature discharge gas from the compressor, and the discharge scrubber itself. This equipment and associated large volume capacitance contained therein presented a significant challenge and demand on the surge suppression system in all aspects of operation; particularly during ESD. Results are presented from all three methods which show the consistency and complementary nature of these methods.

Enbridge Pipelines Inc. (Enbridge) is a leader in the field of Integrity Management (IM). Enbridge’s primary concern is to ensure the safety of people, equipment, and the environment by building integrity management into company’s standards, procedures and facilities. One example of Enbridge’s commitment to IM includes the attention given to transient modeling within piping systems and the implementation of transient over pressure protection systems such as the Hydraulic Surge Alleviators (HSAs) used at the Edmonton Terminal.

Oil pipelines systems for hydrocarbons transportation are linear projects that can reach great lengths. For this reason, theirs paths may cross different geological formations, soil types, navigable or torrential waters; and they may face geotechnical and hydrological instability problems such as creeping slopes, geological faults, landslides, sudden river surges and differential settling which causes different relative movements between the soil and the pipeline. The OCENSA (Oleoducto Central S.A) 30 ″ and 36 ″ diameter system was built in 1997 to transport crude oil from the eastern foothills of the Andes to the Caribbean Coast along some 830 km of the Eastern Andes mountains range and the spurs of the central Andes mountains range of Colombia: it was a major challenge to secure the integrity of the pipeline in the face of natural events.

Nowadays, anticipating and controlling transient response is a critical design activity for ensuring both safety and integrity of the operational subsea system. Predicting transient effect, commonly known as surge pressure, is of high importance for offshore industry. In order to determine the installation of protection equipments to avoid surge pressure effects, the operational teams have raised concerns, whether the system is adequately designed to protect the subsea system against possible surge pressures during the event of sudden closure of a valve. Researches, referred to transient effects, explain that is necessary to evaluate the system performance under current and desired operating conditions. The main goal of this paper is to predict the surge pressure during unforeseen closure valves at Refrigerated LPG and Gasoline (C 5+ ) pipeline systems. In these systems the valves are located downstream the flowlines. Detailed computer modeling attempts to simulate the complex interactions between flowline and fluid, aiming at providing efficient flowline system integrity. These models are based on Transient Methodology which is defined for a set of nonlinear partial differential equations that relate fundamental variables with pressure head and flow velocity. The solution of differential equations has been carried out by Finite Difference Method that transforms these equations into characteristic equations. These can be accurately solved through high-speed digital computers. Flowmaster, Chicago, USA, was the software used to develop the analysis models. The software offers an advanced graphical interface to build networks and resultant graphics. The results from Flowmaster have been validated through a defined methodology that applies the Characteristics Method based on Wylie and Streeter assumptions. Simulations considering the fluid as gasoline have shown a sudden damping of pressure wave when the valve closure time was 10 seconds, leading to the restoration of the initial flow conditions. The analysis using the Method of Characteristics, however, does not exhibit this sudden damping, although a gradually reduction of fluctuations around the initial pressure are observed. The transient analysis through Flowmaster for Refrigerated LPG leads to a pressure envelope that shows a change of the flow direction triggering a cyclical process until the restoration of the initial operational conditions.