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Earthquake detection and safety system for oil pipelines.

Large pipelines represent huge economic assets. From an ecological point of view, they may be considered as a certain risk. Therefore, pipelines in seismic active zones are designed against earthquakes. However, when an earthquake occurs, the questions are how strong it is compared to the design values and whether the normal operation of the pipeline can continue. These questions are equally important in economic and ecological respects, but the ecological aspect is specifically significant with offshore pipelines.

This article focuses on how an earthquake-detection system would allow early detection by pipeline operators and how pipeline safety can be improved by the application of the Pressure-Relief Before-Break principle.

Earthquake Impact On Pipelines

Earthquakes generate seismic waves. The Trans-Alaska Pipeline System gives an example of the effect of earthquakes on pipelines. Designed for a magnitude of 8.5, it was hit by an earthquake on Nov. 3, 2002 with a magnitude of 7.9. The quake, striking just after 1 p.m., had its epicenter in the Denali National Park, about 120 km south of Fairbanks. It took 40 minutes to come to the decision to shut down the pipeline. Earthquake detection systems give a much faster indication of the actions to be taken on the pipeline after the earthquake.

The inspection of the Trans-Alaska Pipeline after the earthquake revealed that supports were damaged but the pipeline itself had no leaks. Following repairs, full flow was resumed. As shown by this example, the pipeline itself is less endangered whereas discontinuities like supports but also valves and connections to pump stations are vulnerable. However, pipelines are sensitive to secondary seismic effects, mainly the soil liquefaction with lateral spreading, the landslides and the fault movements.

Although only oil pipelines are addressed here, corresponding considerations can be made for gas, water and products pipelines.

Detection For Other Installations

Earthquake detection systems for early warning have been installed in Mexico and Istanbul, Turkey. The purpose of these systems is to alert and direct the government and fire fighting departments. But with regard to immediate actions on pipelines, dedicated systems are needed.

High-speed railways are sometimes equipped with earthquake detection systems. The Shinkansen in northern Japan has had one since 1982. It consists of accelerometers installed at and in the vicinity of the railway. The trains are stopped automatically in case certain threshold values of the acceleration are exceeded. The operation of the system improved after the decision logic was changed from pure acceleration values to a term designated as spectrum intensity.

Civil structures like buildings and bridges can be equipped with Strong Motion Instrumentation. Figure 1 shows an example for high-rise buildings. It consists of accelerometers at several levels of the structure. Such systems have been supplied by several companies in great number.

[FIGURE 1 OMITTED]

Nuclear power plants are also equipped with Strong Motion Instrumentation. The evaluation of the signals is governed by the U.S. Nuclear Regulatory Commission Guide 1.166. It is based on two criteria: Peak Ground Acceleration (PGA); and Cumulative Absolute Velocity, (CAV).

The PGA threshold values are site specific. Typical values of the design earthquake PGA are 0.1 g in Central Europe, 0.25 g in the Caucasus region, and for Turkey and the Far East, 1g=9.81m/[s.sup.2]. For the maximum credible earthquake, about twice these values are taken.

Concerning the CAV, investigations by the U.S. Electric Power Research Institute (EPRI) have shown that a standard value of CAV = 0.16 g x sec can be defined for the design earthquake. Irrespective of the site, below that value damage to the structure can be excluded.

The CAV value is calculated by integration of the absolute acceleration. The calculation method is standardized by EPRI. Figure 2 shows this method. The acceleration record might show a weak wave. Then, the earthquake is supposed to begin. At t=2.1 seconds the acceleration reaches 0.025 g. At that acceleration the CAV calculation is started and the values are integrated continuously. Using the values of Figure 2, the integral amounts to 0.16 g x sec at t=4.2 sec. At that value, the calculation program issues a signal that the CAV threshold value has been reached.

[FIGURE 2 OMITTED]

The PGA design value is supposed to be 0.1 g at that site. In Figure 2, the PGA and CAV threshold values have both been exceeded. Therefore, the earthquake is rated as exceeding the design earthquake. This evaluation of the Strong Motion Instrument signals takes five seconds.

Detection System For Pipelines

The Strong Motion Instrumentation is also suitable for pipelines. Within five seconds, it provides a value of the impact of the earthquake to the pipeline. Figure 3a shows a typical pipeline section. The accelerometers of the Strong Motion Instrumentation are installed in hazardous seismic zones along the pipeline at a distance of 10 to 100 km, depending on the seismic conditions. They are placed in small concrete vaults at zero ground level. With offshore pipelines, the accelerometers are submerged and attached directly to the pipeline.

[FIGURE 3 OMITTED]

For data transmission of the Strong Motion Instrumentation, the existing SCADA system of the pipeline is used. Some pipelines are equipped with software based SCADA leak detection and a leak caused by an earthquake would also be detected by this. However, the direct and immediate detection of the seismic impact by means of the strong motion instrumentation is preferred.

As in the case of civil structures and nuclear power plants, the processing of the pipeline strong motion instrument signals consists of the following steps:

* Filtration in the range of approximately 0.1 to 10 Hz to eliminate the vibrations caused by other sources;

* Fast Fourier Transformation to determine the cyclic content of the signals; and

* Calculation of PGA and CAV.

For the assessment of the CAV-value, the procedure previously described for nuclear power plants is proposed to be applied. Specifically, the standard threshold value for nuclear power plants of 0.16 g x sec can also be applied to pipelines because, according to investigations by the EPRI, damage to structures can be excluded below that value.

Pipeline Displacement

As described, pipelines are sensitive to the secondary effects, mainly the soil liquefaction with lateral spreading and the landslides. Instruments are available which measure the liquefaction and landslides in the soil. However, in order to detect the influence of these effects on pipelines, the direct measurement of the pipeline displacement is preferred. This can be done with the Distributed Temperature and Strain (DiTeSt) system of the Swiss company Omnisens SA using the fiber-optic technology of Swiss-based SMARTEC SA.

The system is based on an interaction principle designated as stimulated Brillouin scattering. The scattering arises from the interaction between laser light waves and thermally excited acoustic waves. It depends on the strain state of the fiber. Therefore, deformations of a tense fiber can be detected and localized. The scattering depends also on the temperature. Due to the fact that leakages are connected with a temperature modification, leaking points in the pipeline can be detected by the same system.

A recent application of the DiTeSt system was used to monitor a buried, 30-year-old gas pipeline, located near Rimini, Italy, in a landslide area. Three types of sensors were used: tapes, cords and temperature sensing cables. The tape is a sensing optical fiber integrated into a fiber-reinforced composite tape, thickness of 200 m, attached directly to the pipeline. It provides the monitoring of the strain and the deformation of the pipeline.

The cord sensor consists of a sensing optical fiber-integrated into a fiber-reinforced plastic cord of 6 mm diameter. It was installed in the soil below the pipeline in order to monitor the strain changes in the soil. The measurements of the tape and the cord are correlated by the system in order to evaluate the strain transfer from the soil to the pipeline. The strain resolution of the system is 20 micro-strains with a spatial resolution of 1.5 m.

The temperature sensing cable installed at the upper line of the pipeline monitors the temperatures, compensates the strain measurements for temperature and detects leakages. The resolution is 1[degree]C with spatial resolution of 1.5 m.

From the description, it is easy to ascertain that the sensors are attached directly to the pipeline. However, at a new pipeline the applied pipe laying procedure might not allow the time delay required for this. In this case, the fiber-optic sensors are attached to the SCADA tubing running along the pipeline. In case of a displacement of the pipeline by soil liquefaction or by landslides, it is assumed that the SCADA tubing follows the curvature of the pipeline.

The sensor signals are read and evaluated using a single DiTeSt analyzer. It needs approximately 60 seconds to evaluate the signals.

Instrumentation At Fault Zones

Buried pipelines would undergo excessive shear stresses at fault movements. Therefore, the present state of the art is to arrange the pipeline at faults above ground in a zig-zag arrangement so as to allow extended movements. Due to the installation above ground, the deformation can be surveyed by GPS receivers, Figure 3b. For near, real-time evaluation, a fast processing system is to be applied.

Automatic/Manual Interconnection

The interconnection between the earthquake detection system and the pipeline operation might be manual or automatic. In practice, a two-phase procedure is recommended. In the first phase following the installation, manual connection is done. During that time the system is checked for spurious signals and false alarms. Then, in the second phase, the automatic interconnection between the earthquake detection system and the pipeline operation is done.

Earthquake Safety Improvements

The earthquake detection system outlined here provides a numerical basis for decisions to be taken in case of an earthquake. An increased level of safety agaInst the secondary effects of an earthquake, mainly soil liquefaction and landslides, can be achieved by the Pressure-Relief Before-Break Principle described.

Figure 4 shows the behaviour of a pipeline under seismic loads if no earthquake detection system is installed. The normal operational stress in the pipeline by the internal pressure and by other loads is taken as 100%, the fracture stress as 150%. An earthquake is supposed to occur at the time 0. First, the seismic waves hit the pipeline. As described, no damage is expected from that because soil liquefaction and landslides are more important for pipelines.

[FIGURE 4 OMITTED]

Following the mechanism of formation these secondary seismic effects have a certain time delay to the earthquake. This may be up to 60 seconds. In our scenario, it is anticipated that a landslide hits the pipeline 40 seconds after the earthquake and, if no actions are taken, the stress in the pipeline starts to increase due to the deformations caused by the landslide.

Shortly afterwards, the stress in the pipeline starts to increase due to the deformations caused by the landslide. A short time later the fracture stress might be reached and the pipeline breaks.

The situation after the installation of the Earthquake Detection & Safety System is presented in Figure 5. Based on the evaluation of the Strong Motion Instrumentation, which requires five seconds, certain control signals are issued. For example, at strong earthquakes the flow rate will be reduced to 50% by partial shutdown of the pipeline pumps. This will reduce the operational pressure and, hence, the stress in the pipeline wall.

[FIGURE 5 OMITTED]

The following considerations are made for a reference point arbitrarily selected at a distance of 25 km downstream of a pump station. The partial stop of pumps creates a pressure surge in the pipeline. According to the Joukowski formula (1)), the pressure surge propagation velocity in a steel pipeline of one meter diameter filled with oil is roughly 1 km/s. Therefore, as demonstrated in Figure 5, the pressure reduction will be felt 25 km downstream of the pumping station within 25 seconds after the flow rate reduction by the partial pump stop.

Including five seconds for the strong motion instrumentation, this is 30 seconds after the earthquake. The operational stress in the pipeline starts to go down at that time. Due to the fact that in our scenario the landslide hits the pipeline somewhat later, the additional stress caused by the landslide can be taken up by the pipeline as shown in Figure 5. This is designated as Pressure-Relief Before-Break Principle.

The above considerations are based on a certain scenario. In a real case, the benefits from the Pressure-Relief Before-Break Principle will have to be assessed by the hydraulic analysis. However, it is evident from Figure 5 that the risk of pipeline breaks can be reduced by this method. In this way, the earthquake safety of pipelines is improved.

The control signals to be issued by the Earthquake Detection & Safety System must be selected based on the seismic design of the pipeline. Usually, the design is based on two levels of earthquakes, the normal design level and the maximum expected level. Based on the strong motion instrumentation, the following control scheme may be envisaged:

* At an earthquake of the normal design level an alarm only shall be issued. Then, inspection of critical components is to be initiated.

* At the maximum expected earthquake, it is proposed to initiate a complete shutdown of the pipeline.

* At an earthquake strength between the normal design level and the maximum expected level, the pumps shall be stopped partially with reduction of the flow to approximately 50%.

In addition to the Strong Motion Instrumentation, the pipeline displacement sensors in soil liquefaction and landslide areas and at seismic faults will be integrated in the control scheme. The respective values must be selected in accordance with the mechanical design of the pipeline.

In conclusion, an earthquake-detection and safety system provides the following advantages:

* Reduced risk of huge investment losses by a pipeline break at extremely strong earthquakes;

* Normal operation at full flow can continue in many cases. Sometimes, a flow rate of 50% can be maintained. A full shut down is initiated if absolutely required.

* Reduced ecological risk of the pipeline.

Joukowski Formula: 1) Pressure surge propagation velocity according to Joukowski: a = [Ef/p/(1+Ef/Ep x D/s x (1-?2)]0.5. Modulus of elasticity: fluid Ef 1.2x109 Pa, pipe material steel Ep 2.1x1011 Pa, Poisson's ratio ? 0.3, fluid density ? 800 kg/m3, pipe diameter D 1m, wall thickness is 10mm (reference values). Therefore a = 1000 m/s.

For additional information contact: Lothar Griesser (Lothar.Griesser@bluewin.ch); Dr. Martin Wieland (Martin.Wieland@ewe.ch, www.ewe.ch); or Roberto Walder (Walder@smartec.ch, www.smartec.ch).

Lothar Griesser, Dr. Martin Wieland, Electrowatt-Ekono Ltd., Zurich, Switzerland and Roberto Walder, Smartec SA, Lugano-Manno, Switzerland
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Title Annotation:Software Benefits
Comment:Earthquake detection and safety system for oil pipelines.(Software Benefits)
Author:Griesser, Lothar; Wieland, Martin; Walder, Roberto
Publication:Pipeline & Gas Journal
Geographic Code:1USA
Date:Dec 1, 2004
Words:2451
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