On our way to overall Resistance estimation and, as a part of Effective Wall Thickness determinations, here is some background on stress-inducements that must inform our estimates.
Loads and Forces
Loads and forces, and their resulting stresses, obviously play a large role in failure potential. The design process first considers the loads and forces that are to be resisted.
This discussion is an examination of how the pipeline’s design characteristics impact its ability to resist forces/damages. Certain design concepts are presented to give the evaluator who is not already familiar with pipeline design methods a feel for some of the considerations. This obviously does not replace a design manual or design methodology. Used with the corresponding risk evaluation sections, this section can assist one unfamiliar with design concepts in understanding strength/resistance aspects of the pipeline being examined.
Design of any structure involves examinations of loads and forces. Load is a general term meaning a force applied to a structure. Internal pressure, gravity (or weight), and temperature-induced strains are examples of loads typically experienced by pipelines.
Loads have effects on structures—pipeline components in this case. Those effects include stresses, strains, and deformations. Resistance can be measured in terms of any of these—the ability to withstand a stress, strain, or deformation. Even a pinhole leak from an unpressurized component conceptually falls into this model. The leak will only occur with some driving force, if only gravity or a tiny amount of hydrostatic fluid head. This tiny driving force is no longer resisted if the pinhole has penetrated the entire component wall.
In general, any influence that tries to change the shape of the pipe will cause a stress. Pipe stress can originate from loads that cause or exacerbate:
- Internal pressure
- External pressure
- Longitudinal bending (longitudinal buckling)
- Axial tension
- Axial compression (axial buckling)
- Lateral compression (crushing)
- Thermal expansion/contraction
- Shear
- Cracking (fatigue, etc.)
- And various combinations of these.
Defects in component walls will heavily influence resistance. That will be considered separately from the defect-free analysis.
As a pressure containment system, internal pressure obviously plays a key role in many pipeline strength determinations. While often the dominant load, internal pressure is not the only loading on a typical component. External forces also add stress to the pipe. Loads causing external stresses include the weight of the soil over a buried line, the weight of the pipe itself when it is unsupported, temperature changes, etc. Some of these stresses are additive to the stresses caused by internal pressure. As such, they must be allowed for in the design pressure calculations. Hence, care must be taken to ensure that the pipeline will never be subjected to any combination of internal pressures and external forces that will cause the pipe material to be overstressed.
Tolerable loads are set by maximum stress-carrying capacity. The design phase includes consideration of all loadings to which the pipeline will be subjected. Pipeline loadings typically include internal pressure and physical weights such as soil and traffic over the line. A typical analysis of anticipated basic loads for a buried pipeline would include provisions for:
- static internal pressure
- dynamic internal pressures such as surge pressures
- overburden (Soil loadings, including soil movements).
Additional criteria are considered in detailed design and for special installation circumstances such as drilled crossing and spans. These criteria include provisions for:
- Bending Stresses
- Tensile Loads l Buoyancy.
- Span loadings including gravity and lateral forces
- Traffic loadings
- Strain induced loadings such as from temperature changes.
For each loading combination, all stresses and failure modes must be identified. Failure is often defined as permanent deformation of the material. After permanent deformation, the component may no longer be suitable for the service intended. Permanent deformation occurs through failure modes such as bending, buckling, crushing, rupture, bulging, and tearing. In engineering terms, these relate to stresses of shear, compression, torsion, and tension. These stresses are further defined by the directions in which they act; axial, radial, circumferential, tangential, hoop, and longitudinal are common terms used to refer to stress direction. Some of these stress direction terms are used interchangeably.
As discussed in the previous sections, pipeline component materials have different properties and different abilities to resist loads. Ductility, tensile strength, impact toughness, and a host of other material properties will determine the weakest aspect of the material. If the pipe is considered to be flexible (will deflect at least 2% without excessive stress), the failure mode will likely be different from a rigid pipe. The highest level of stress directed in the pipe material’s weakest direction will normally be the critical failure mode. The exception may be buckling, which is more dependent on the geometry of the pipe and the forces applied.
The critical failure mode for each loading will be the one that fails under the lowest stress level.
Load Types
A useful listing of load types can be found in [9988] as part of the limit state discussion. Limit states included are ‘ultimate’ (ULS), ‘leakage’ (LLS), and ‘serviceability’ (SLS). These limits may be established based on stress or strain or both. This particular reference categorizes loads based on their potential appearance in the system’s life cycle. It also assigns a time dependency to each combination of loads and limit states, as well as a cross reference to potentially interacting load cases.
When loss of integrity is the focus of the risk assessment, limit states dealing with ruptures and leaks are the focus. Some of the pertinent loads are further discussed below.
Pressure containment
The most commonly used measure of a pipeline’s strength will normally be the documented design pressure—the maximum internal pressure that can be withstood without damage (including permanent deformation). Design pressure is determined from stress calculations, with internal pressure normally causing the largest stresses in the wall of the pipe. Material stress limits are theoretical values, confirmed (or at least evidenced) by testing, that predict the point at which the material will fail when subjected to high stress.
Several key aspects of risk are directly linked to the amount of internal pressure in the line. Pressure levels may vary widely along a pipeline or at a single location over time. The pressure to which a component will be subjected is needed to calculate stress levels and other risk factors in the risk assessment. The assessment may choose any of several commonly cited pressure levels: the maximum tolerable design pressures, the maximum allowable pressure (including safety factors), the maximum working pressure, the normal operating pressures, and others. The terms maximum operating pressure (MOP), maximum allowable operating pressure (MAOP), maximum permissible pressure, and design pressure have specific definitions in some regulatory and industry guidance documents. However, they are often used interchangeably. They all imply an internal pressure level that comports with design intent and certain safety considerations—whether the latter stem from regulatory requirements, industry standards, or a company’s internal policies. In this risk assessment discussion, the term ‘design pressure’ is used for the maximum internal pressure that can be sustained by the component without permanent deformation or other harm to the material.
For purposes of this discussion, design pressure will be used to describe the pressure to which the defect-free component can be subjected without failure (such as yielding). By this definition, design pressure should exclude all safety factors that are mandated by government regulations or chosen by the designer. It should also exclude engineering safety factors that reflect the uncertainty and variability of material strengths and the simplifying assumptions of design formulas since these are technically based limitations on operating pressure. These include safety factors for temperature, joint types, and other considerations. Safety factors that usually allow for errors and omissions, deterioration of facilities, and provide extra ‘cushioning’ between actual conditions and tolerable limits. Such allowances are certainly needed, but can be confusing if they are included in the risk assessment. There is always an actual margin of safety between the maximum stress level caused by the highest pressure and the stress tolerance of the pipeline. Measuring this directly without including the confounding influences of a regulated stress level and stress tolerance, makes the assessment more intuitive and useful, especially when differing regulatory requirements make comparisons more complicated. Regulatory safety factors are therefore omitted from the design pressure calculations for risk assessment purposes.
The design or other ‘maximum allowable’ pressure is appropriate for characterizing the maximum stress levels to which all portions of the pipeline might be subjected, even if the normal operating pressures for most of the pipe are far below this level. This avoids the potential criticism that the assessment is not appropriately conservative.
Although the design pressure could be conservatively used here, this would not differentiate between the upstream sections (often higher pressures) and the downstream sections (usually lower pressures). The alternative of using normal operating pressures, provides a more realistic view of actual stress levels along the pipeline. Pipeline segments immediately downstream of pumps or compressors would routinely see higher pressures, and downstream segments might never see pressures even close to the maximum limits. One approach would be to create a hypothetical pressure profile of the entire line and, from this, identify normal maximum pressures in the section being evaluated.
This approach might be more appropriate for operational risk assessments where actual differences along the pipeline are of most interest. A challenge in using ‘normal’ pressures will be the time period implied: ie, the highest pressure seen in last year? 5 years? The average or median pressure seen in the last 12 months? Etc.
Provisions for surge (water hammer) or other temporary pressures should be independent of design pressure determination. Potential for pressure levels in excess of system tolerances should be considered separately as exposures. Surge potential is discussed in .
Pipe wall damages or suspected weaknesses—anomalies—may impact pipe strength and hence allowable pressures or safety margins. Formal reductions of maximum operating pressure resulting from pipeline anomalies are normally based on approaches described in industry standards[1]. If a new pressure limit is determined based on calculations of remaining strength after a detected weakness, then that should be the new design pressure used in the risk assessment of the component. In this case, it may be hard to determine how much conservatism in the form of extra safety margin has been added in the treatment of some anomalies. If the assessment is able to ascertain the true pressure limit, free from any safety factor, that is the better value to use as design pressure.
The design pressure also plays a role in probability of damage estimates. For instance, in the incorrect operations assessment, there is an important distinction made between a safety-system-protected component and one that is impossible to overpressure due to the absence of adequate pressure production—where it is physically impossible to exceed the design pressure because there is no pressure source (including static head and temperature effects) that can cause an exceedance.
Note also that pressure, from the standpoint of a small leak, can mean the tiny driving force created by hydrostatic head or gravity.
The degree of pressure cycling is another factor to take into account in the evaluation since this can also contribute to failure probability as discussed in .
Load Estimations
Both continuous and intermittent loads are appropriately included in risk assessments.
Normal, continuous loads are addressed in the design phase. Normal, intermittent loads should also be addressed during design, but may not receive the same amount of rigor or they may be compromised over time by changes in system characteristics during its life cycle. Fatigue loadings are an example. Even if considered during design, changes in use over time may change the originally planned number and magnitude of pressure cycles and changes in environment may add new sources of external fatigue cycles.
Intermittent loads, especially when both abnormal and intermittent, require both a categorization of intensity or damage potential and an estimate of frequency. Frequencies may already have been partially captured in exposure estimates for the various time-independent forces—excavator hits, vehicle impacts, landslides, surge pressures, anchor strikes, etc.
Normal loads can often be estimated from design documents, as previously discussed, and can produce a baseline level of resistance.
Special External loadings
Normal external loadings listed in PRMM include the weight of the soil over a buried component, the loadings caused by moving traffic, possible soil movements (settling, faults, etc.), external pressures and buoyancy forces for submerged lines, temperature effects, lateral forces due to water flow and debris impacts, and component weight. See discussion of these in PRMM.
As a special case of ‘failure’, infiltration of a component and subsequent product contamination can occur. For example, groundwater infiltration into a distribution system. This is a form of integrity loss since, for infiltration to occur, the outside pressure exceeds the internal pressure and the components ability to resist. There would presumably also be an integrity loss when groundwater pressures are lower and the component’s internal pressure produces the driving force to create a leak.
Overburden
This is a measure of the weight of soil, objects and anything else over the pipeline. In an offshore environment, this would also include the pressure due to water depth. Uncased pipe under roadways may require additional wall thickness to handle the increased loads from vehicles. The speed and weight of the vehicles, as well as depth of cover, cover type, and other factors will be important determinants of how much stress is transferred to the buried component.
Spans
Similar to the forces of gravity on an onshore spanning component, the stresses from lateral forces of moving waters, debris accumulations, should be considered for offshore susceptible components. Spans are a unique feature in a risk assessment, as discussed in xxx .
Buckling
Pipelines under compressive forces from pressure or thermal forces, can buckle if the axial compression goes beyond a certain level. Buckling can also occur under excessive external force.
Buckling is more common concern with pipelines in deep water. Some offshore designs incorporate controlled lateral buckling as a means to dissipate pressure and thermal expansion induced forces on a long pipeline.
However, buckling as a failure mode can manifest at other, unexpected conditions, far from common external pressure sources. In one operator’s experience, hydrogen permeation through steel repair sleeves caused numerous buckles to the pipe beneath. The source of hydrogen was generated from high CP levels external to the sleeve. An annular space pressure of around 300 psig was sufficient to cause the buckling. [1001]
Accounting for unspecified external loads
Especially for preliminary or screening type risk assessments, it may be appropriate to simply use a factor to account for unknown or unquantified loads. The factor can be set according to the desired level of conservatism in the risk assessment. See also PRMM.