As pipelines are exposed to all types of loading in different situations when they are in operating conditions. To be more specific, the major two types loading the pipelines are exposed are the Static and Dynamic Loading. It is of utmost Importance that those loading are taken into account during the normal operating conditions and they cannot be ignored as such. If they are taken as lightly or ignored, the circumstances we may face are dangerous because due to such loading the amplitude due to vibration may increase in situation if the natural frequency of pipelines met the forced or induced frequency of pipelines due to dynamic loading.
As our focus is on Suspended gas a pipeline, the motion due to vehicles passing by the bridge induces external loading in the vertical direction propagates through the bridge (medium of propagation). This load varies from place to place and is maximum at the point of contact of traffic load and bridge and is extended in all directions. Taking into account the static loading induced in the suspended gas pipelines, the weight of the pipelines plays a vital role but the damage encountered in this case is less dangerous.
Another type of threat which also affects a natural gas pipeline structural integrity is fatigue loading. The pipeline material fatigue can generate from pipeline internal forces, i.e. from internal pressure surges and fluctuations or from external forces such as induced vibration. The latter is a result of traffic on transportation routes in pipeline vicinity and resulting ground-vibration which is generate by vehicle movement over the road surface. Ground-vibration propagates in horizontal direction, in parallel with ground surface, and in vertical or in depth direction as well.. However fatigue life of the engineering material depend on
1. Type of loading
2. Material defect
3. Surface roughness
4. Vehicle speed
5. Vehicle mass
6. Size of pipelines
On many road sections the vehicle mass dominates over the others, especially in sections where other factors have very little impact or are of similar values between different types of vehicles. Heavy load of vehicles imparts more strain in the buried structures and can be more dangerous because it can cause failure of pipelines more convincingly as compared to light load. Since research is regarded as a surface phenomenon. Researchers have proved that in case of axial force during dynamic loading usually the rupture starts at the surface of the pipe. Another important factor is the velocity of the vehicle in dynamic loading. Greater the velocity of the vehicle results in lower impact time, the pipelines will be exposed trivial to fatigue failure. The analytical methods to calculate these loads are discussed in coming chapters.
This paper presents a study of impact of the traffic-induced vibration on a suspended natural gas transmission pipeline. The basic assumption in this study is that the traffic on pipeline-transportation route crossing might have a significant impact on natural gas pipeline structural integrity due to the traffic-induced vibration which propagates from the road surface and excites the suspended natural gas pipeline. The resulting dynamic stress causes pipeline material fatigue loading which consequently may cause pipeline failure with the gas release into the environment exposing the population and the buildings in pipeline surrounding to a significant threat. The experiment on operating buried natural gas pipeline was conducted where measurements were performed on the road surface, the two operating buried natural gas pipelines of external diameter 500 mm and 250 mm and on corresponding casing pipes. The measurement data analysis was performed and the results were used for determination of magnitude of vibrations which has been exposed to traffic-induced vibration. The findings of the study in this paper show that the traffic-induced vibration on given suspended natural gas pipeline is detectable, however this vibration, compared to the other factors which are influencing pipeline’s structural integrity, does not have a significant impact on pipeline lifetime period because of the low magnitude of vibration induced in the suspended gas pipelines.
Vibration which propagates through the road causes an induced vibration of a suspended natural gas pipeline. The consequence of pipeline vibration is fluctuating stress in pipeline wall material. This stress is normally much lower than the value of stress at the ultimate tensile strength of a pipeline material. Additionally, the induced cyclic stress alternates around certain mean stress value under which the pipeline is loaded during normal vibration-free. The integrity of the gas pipeline can be endangered when the vibrational cyclic stress is active for a sufficient time period. In that case the mechanical failure due to fatigue can occur which means that the pipeline wall fractures due to the low magnitude cyclic loading. Fatigue reduces the material tensile strength which may lead to pipe wall fracture and consequently loss of pipeline integrity. The fatigue is a time dependent process, the longer the period of dynamic fatigue loading, the larger the reduction of the material strength. Generally, pipeline strength reduction during the pipeline operation leads to the situation where pipeline safety margin becomes unsatisfactory as to the legislative requirements. In such case the natural gas pipeline and corresponding extra protective measures may become unsuitable for operation at the road crossing area.
Pipeline stress loading from traffic-induced vibration
2.1 Pipeline deformation:
Traffic, in particular large vehicles, with or without trailers, generates road surface vibration, mostly in vertical direction. These vibrations are propagating through the road and induce in suspended gas pipeline.
Pipeline longitudinal vibration is similar to oscillation of a single span full beam which is fixed on both sides. Single span fixed beam can longitudinally oscillate in different
mode shapes n. Mode shape depends on resonant frequencies of a beam fn which are defined by the following equation
Where l is length, E is material modulus of elasticity, I is cross sectional mass moment of inertia, and m is beam’s mass per meter of length.
At higher order modes pipeline shell wall deforms circumferentially (Fig. 2, i ¼ 2 and 3). Such behavior is distinctive at higher frequencies. Approximate values of shell wall natural resonant frequencies at different shape modes fi (i ¼ 1, 2, 3.) can be calculated with the following equation (Antaki, 2003; Price & Smith, 1999, p. 189):
Where R is mean radius of pipeline’s wall, r is mass density of pipeline material, and n is Poisson’s ratio.
2.2 Pipeline cyclic load
Circumferential deformation of pipeline Dy is usually small enough that static load does not exceed such stress levels of sb or sov which would cause a pipeline failure. Traffic related cyclic loads, i.e. amplitude of oscillation Dy, are small as well and displacement causing pipeline failure cannot occur. However, if cyclic loads are reaching significant amplitude levels and simultaneously act for a sufficient period of time, then a pipeline failure due to fatigue
might occur. Cyclic loads can act as alternating stress oscillation (they alternate around mean value s ¼ 0) or they can oscillate around mean value sm. Mean stress value sm is defined as:
The equivalent stress is therefore used for the correction of tensile normal mean stress effect on fatigue strength and can be calculated using the Soderberg empirical model.
Where se is equivalent stress, which is oscillating around the mean stress value sm, sa is alternating stress amplitude of a cycle and symaterial yield stress.
2.3 Material Residual strength:
Fatigue is a process where mechanical properties of material are worsened. The material residual strength will gradually decrease as the object is exposed to cyclic loading and eventually will reach zero strength level where fatigue failure will occur. The residual strength of a natural gas pipeline material is very important information, if available, as it allows us to determine at a given moment which stress level of the cyclic loading the pipeline canstill withstand. The material residual strength is defined with the following equation (Yongyi & Zhixiao, 2002):
Where sn is residual strength of the material after n stress cycles, s0 is the initial strength or endurance limit of the material (at n ¼ 0 stress cycles), and N is the fatigue life under the stress cycle.
2.4 Vibration measurement results
Vibration measurements were performed for over 60 passes of heavy vehicles, two-, three- and four-axle trucks, and also heavier vans and buses. Measured velocities of moving vehicles ranged from 33 km/h to 57 km/h. Measuring with two accelerometers simultaneously enabled the determination of transfer functions between road surface vibration and casing pipe signals and also between casing pipe and natural gas pipeline and additionally the
determination of buried natural gas pipeline vibration mode shape.
2.5 Road surface vs. casing pipe vibration
Typical response of an acceleration sensor while vehicle is passing over the exact location where natural gas pipeline is located is shown in Fig. 6. In case of gas pipeline DN 500 the passing vehicle was a two-axle truck, in case of gas pipeline DN 250 the passing vehicle was a two-axle truck with a three-axle trailer. Comparing the two plotted time series in Fig. 6 (acceleration amplitude over time) leads to the following conclusions: – pipeline vibration increases while vehicle passes by, which can be determined by time series of road and gas pipeline (at the time where road surface vibration amplitude increases, the gas pipeline vibration amplitude increases as well), therefore the moving traffic impacts the induced vibration of buried pipelines;
1) vibration from the source is transferred through each contacting surface with the road e the number of wheels/axles is a significant acceleration amplitude parameter, as shown in Fig. 6;
2) acceleration amplitude depends on vehicle speed and unbalanced rotating parts. Condition of road surface has a significant influence on signal amplitude as well.