Fitness for Service Pipeline

Fitness for Service Pipeline

Fitness-for-service assessment is a multi-disciplinary approach to evaluate structural components to determine if they are fit for continued service. Pipelines may contain flaws or other damage, or may be subject to more severe operating conditions than the original design anticipated. Quest Integrity Group’s LifeQuest pipeline assessment solution uses API 579-1/ASME FFS-1 fitness-for-service methodology to deliver an assessment of the pipeline for continued operation at defined maximum allowable operating pressure. An evaluation of remaining life and/or inspection intervals may also be part of such an assessment.

The FFS of any particular material is determined by performing a fitness for service assessment. Performing accurate FFS evaluations is an integral aspect of fixed equipment asset integrity management. On the other hand, failing to perform evaluations can lead to equipment failures which can further result in injury, loss of life, and severe financial and economic consequences.

The reason these examinations are performed is because even if a piece of equipment has a crack or other defect, this doesn’t necessarily mean that it’s unfit for service. Most equipment can continue in service despite small flaws, and to repair or replace equipment that can still be used would be an unnecessary and costly expense. Not only that, but unnecessary weld repairs can actually do more harm than good, as the quality of the new weld can often be less than the original one.

There are several ways to see if a flaw can cause a piece of equipment to be no longer fit for service. For cracks, fracture mechanics provides the mathematical framework for the examination by quantifying combinations of stress, flaw size, and fracture toughness.

While cracks tend to be the most dangerous, they’re not the only flaw that might warrant evaluation. Volumetric flaws such as corrosion pits, porosity, and slag may reduce the load-bearing capacity of a structure. Likewise, structural integrity may also be compromised by locally thinned areas which come grinding out cracks, thus FFS methodologies have been developed to evaluate local thinning. In these cases, acceptance criteria are based on limit load analyses rather than fracture mechanics models. Some examples of these different FFS methodologies are the BS 7910 method, API RP 579-1/ASME FFS-1 method, and the MPC/AP method.

It is important to note though that FFS evaluation can’t provide an absolute delineation between safe and unsafe operating conditions. Uncertainties in input parameters such as stress, flaw size, and toughness often lead to a large uncertainty in the prediction of the critical conditions for failure. In general there are two ways to address this uncertainty. The more traditional approach has been to use conservative input values in a deterministic analysis. The result of such an analysis is a pessimistic prediction of critical flaw size or remaining life.

An alternative approach, one which is becoming more common, entails performing a probabilistic analysis that incorporates the uncertainties in the input data. The latter type of analysis does not result in an absolute yes/no answer as to whether or not a structure is safe for continued operation. Rather, a probabilistic analysis estimates the relative likelihood of failure, given all of the incorporated uncertainties. Probabilistic FFS analysis can be an integral part of a risk-based inspection (RBI) protocol, where inspection is prioritized according to the risk of significant injury or economic loss.

Daftar Pustaka :
http://pipelinesinternational.com/news/fitness-for-service_assessment_of_unpiggable_pipelines/53611
https://inspectioneering.com/tag/fitness+for+service

Engineering Critical Assessment for Offshore Pipeline

 Engineering Critical Assessment for Offshore Pipeline

Pipeline girth welds often contain “imperfections,” which are alternatively termed “flaws” or “defects.”  Traditionally, the tolerable size of those imperfections is set by workmanship-based criteria, such as those in the main body of API Standard 1104.  These criteria are empirically-based and historically proven safe in practice.  In most cases, they are not quantitatively related to the severity of the defects for the safe operation of the pipelines.

ECA in the context of pipeline girth weld refers to the development of weld imperfection acceptance criteria for the purpose of field girth weld inspection and repair (if needed).  The technical basis of ECA is fracture mechanics.  When executed correctly, ECA provides a quantifiable level of safety for the project-specific welds and loading conditions.  ECA is the preferred method for field girth weld inspection and quality control for long distance pipeline projects.

The stresses affecting the integrity of the girth welds may be broadly divided into alternating stresses and static stresses. The alternating stresses are usually induced by the temperature and pressure fluctuations of the pipelines. The static stresses may come from construction and service conditions.

The primary objective of welding procedure qualification is to establish that welds of certain quality can be reliably produced. These welds should have the necessary properties to meet or exceed the requirements set forth by relevant codes, standards, and/or company specifications.

The initial imperfection criteria are often developed following the requirements and specifications of relevant codes and standards. 

Daftar Pustaka :
http://www.cres-americas.com/expertise/engineering-critical-assessment/

Horizontal and Vertical X-mast Tree

Horizontal and Vertical X-mast Tree

Subsea production trees can be segmented into two main types: horizontal trees and vertical trees. Horizontal trees are so called because the primary valves are arranged in a horizontal configuration, and likewise vertical trees have the primary valves arranged in a vertical configuration.

A key requirement of a subsea tree is that access is enabled to the “A” annulus between the production tubing and casing. This is required for a number of reasons, including pressure monitoring and gas lift. As an example, any pressure buildup in the A annulus can be bled to the production flowline via a crossover loop on the tree.

The original designs of subsea vertical trees and tubing hangers were of a dual-bore configuration. Prior to removal of the BOP, it is necessary to set plugs in both the production and annulus bores. Access to both bores requires the use of a dual-bore riser or landing string. The handling and operation of dual-bore systems compared to monobore systems is more complex, and time-consuming and, therefore, more costly.

On a horizontal tree, access to the A annulus is incorporated into the tree design and controlled by gate valves rather than plugs. This enables operations with a mono-bore, less-complex riser or landing string, which can deliver significant advantages, particularly in deep water. It is exactly this logic that led to the introduction of tubing-head spools for use with vertical trees, thereby offering many of the advantages of a horizontal tree.

Daftar Pustaka :

http://www.epmag.com/drivers-influencing-evolution-horizontal-and-vertical-trees-698041#p=2

Pipeline Elbow and Bend

Pipeline Elbow and Bend

here is always a doubt about the terms bends and elbows on ships. They are frequently used as synonyms.   The difference between them is as follows:

1. Bend is a generic term for any offset or change of direction in the piping. It is a vague term that also includes elbows.
2. An elbow is an engineering term and they are classified as 90 deg or 45 deg, short or long radius.
3. Elbows have industrial standards and have limitations to size, bend radius and angle. The angles are usually 45 deg or 90 degrees. All others offsets are classified as pipe bends.
4. Bends are generally made or fabricated as per the need of the piping; however elbows are pre fabricated and standard, and are available off the shelf.
5. Bends are never sharp corners but elbows are. Pipe bending techniques have constraint as to how much material thinning can be allowed to safely contain the pressure of the fluid to be contained.  As elbows are pre fabricated, cast or butt welded, they can be sharp like right angles and return elbows which are 180 degrees.
6. Elbow is a standard fitting but bends are custom fabricated.
7. In bends as the pipe is bent and there is no welding involved, there is less pipe friction and flow is smoother. In elbows, the welding can create some friction.
8. All elbows are bends but all bends are not elbows.
9. Bend has a larger radius then elbows.
10. Generally the most basic difference is the radius of curvature. Elbows generally have radius of curvature between one to twice the diameter of the pipe.  Bends have a radius of curvature more than twice the diameter.

Short Radius and Long Radius
Elbows are again classified as long radius or short radius elbows. The difference between them is the length and curvature. A short radius elbow will be giving the piping a sharper turn than a long radius elbow.

90 degree short radius elbow

Sumber gambar : http://www.piyush-steel.com/img/90-deg-short-redius-buttweld.jpg

 

1. In a long radius elbow the radius of curvature is 1.5 times the nominal diameter. In a standard elbow the radius of curvature is 1.0 times the nominal diameter of the pipe.
2. Long radius elbows give less frictional resistance to the fluid than the short elbows.
3. Long radius elbows create lesser pressure drop than short radius elbows.
4. Short radius is less costly than long radius elbows.
5. The short radius elbows are used where there is scarcity of space.

- 90 degree stainless long radius elbow
In addition to this classification the elbows are 45 degrees, 90 degrees and 180 degrees also called as a return elbow.

- Mitter bend
Another type of bend is a Miter bend. A Miter bend is a bend which is made by cutting pipe ends at an angle and joining the pipe ends. A true miter bend is a 90 degree bend made by cutting two pipes at 45 degrees and joining them by welding.  Similarly three pipes cut at 22.5 degrees will give a 90 degree miter bend.

 

Mitter bend.

Sumber gambar : http://www.vessel-software.nl/pcc/doc/calculations/asme_b31_3/images/asme_b31_3_miterbend.png

Daftar Pustaka :

http://www.marineinsight.com/tech/pipeing/pipes-and-bends-an-essential-guide-for-second-engineers-part-2/

 

 

Pipeline Stress Analysis

Pipeline Stress Analysis

The analysis of piping under pressure, weight and thermal expansion is complex. This complexity can be understood by knovledge of Principal Axis System.

Stress is considered as the ratio of Force to Area. To find the stress in the small element, say cube of a piece of pipe, construct a three-dimensional, mutually perpendicular principal axis system with each axis perpendicular to the face of the cube it intersects.

Each force, acting on the cube can be resolved into force components, acting along each of the axis. Each force, acting on the face of the cube divided by area of the cube face is called the principal stress.

The principal stress acting along the centerline of the pipe is called Longitudinal principal stress. This stress is caused by longitudinal bending, axial force loading or pressure.

Radial principal stress acts on a line from a radial line from center of pipe through the pipe wall. This stress is compressive stress acting on pipe inside diameter caused by internal pressure or a tensile stress caused by vacuum pressure.

 

Circumferential principal stress, some times called Hoop or tangential stress, acts along the circumference of the pipe. This stress tends to open-up the pipe wall and is caused by internal pressure.

When two or more principal stresses act at a point on a pipe, a shear stress will be generated.

Longitudinal Principal stress, LPS = PD/4T

Circumferential Principal stress, CPS (Hoop) = PD/2T

Radial Principal stress, RPS = P

 

 

 Daftar Pustaka :

 http://www.pipingguide.net/2010/10/stress-analysis-of-piping.html

 

Vortex Induced Vibration on Pipeline

Vortex Induced Vibration on Pipeline

Vortex-induced vibration is a major cause of fatigue failure in submarine oil and gas pipelines and steel catenary risers. One of the serious problems for the structural safety of pipelines is uneven areas in the seafloor as they enhance the formation of free spans. Route selection, therefore, plays an important part in design, Matteelli (1982). However, due to many obstacles it is difficult to find a totally obstruction free route. In such cases the pipeline may have free spans when crossing depressions. Hence, if dynamic loads can occur, the free span may oscillate and time varying stresses may give unacceptable fatigue damage. A major source for dynamic stresses in free span pipelines is vortex induced vibrations (VIV) caused by steady current. This effect is in fact dominating on deep water pipelines since wave induced velocities and accelerations will decay with increasing water depth. The challenge for the industry is then to verify that such spans can sustain the influence from the environment throughout the lifetime of the pipeline.

 The aim of the present project is to improve the understanding of vortex induced vibrations (VIV) of free span pipelines, and thereby improve methods, existing computer programs and guidelines needed for design verification. This will result in more cost effective and reliable offshore pipelines when laid on a very rugged seafloor.To evaluate two different strategies for field development; one based on offshore loading and the other on a pipeline to an onshore gas terminal. A key problem for the last alternative is that the seafloor between these fields and the coast is extremely rugged meaning that a pipeline must have more and longer free spans than what is seen for conventional pipelines.

Practical engineering is still based on empirical models, while use of computational fluid dynamics (CFD) is considered immature mainly because of the needed computing resources.  CFD models may certainly be linked to a non-linear structural model, but the needed computing time will become overwhelming. Then, one of the main focuses of the present research is investigation about time domain model for analysis of vortex induced vibrations for free span pipelines and the other is about multi free span pipelines where neighbor spans may interact dynamically. The interaction will depend on the length and stiffness of the pipe resting on the sea floor between the spans, and sea floor parameters such as stiffness, damping and friction. Each of them has important issues to investigate for improvement of our VIV knowledge.

 Daftar Pustaka :
http://www.diva-portal.org/smash/get/diva2:217873/FULLTEXT01.pdf

Horizontal Directional Drilling

Horizontal Directional Drilling

 Directional boring, commonly called horizontal directional drilling or HDD, is a steerable trenchless method of installing underground pipe, conduit, or cable in a shallow arc along a prescribed bore path by using a surface-launched drilling rig, with minimal impact on the surrounding area. Directional boring is used when trenching or excavating is not practical. It is suitable for a variety of soil conditions and jobs including road, landscape and river crossings. Installation lengths up to 2000 m have been completed, and diameters up to 1200 mm have been installed in shorter runs. Pipe can be made of materials such as PVC, polyethylene, polypropylene, Ductile iron, and steel as long as it can be pulled through the drilled hole. Directional boring is not practical if there are voids in the rock or incomplete layers of rock. The best material is solid rock or sedimentary material. Soils with cobble stone are not recommended. There are different types of heads used in the pilot-hole process, and they depend on the geological material.

Horizontal directional drilling (HDD) was pioneered in the United States in the early 1970s by an innovative road boring contractor who successfully completed a 183 m (600 ft) river crossing using a modified rod pushing tool with no steering capability (DCCA 1994). By integrating existing technology from the oil well drilling industry and modern surveying and steering techniques, today's directional drilling methods have become the preferred approach for installing utility lines, ranging from large-size pipeline river crossings to small-diameter cable conduits.

    The HDD industry is divided into three major sectors--large-diameter HDD (maxi-HDD), medium-diameter HDD (midi-HDD), and small-diameter HDD (mini-HDD, also called guided boring)--according to their typical application areas. Although there is no significant difference in the operation mechanisms among these systems, the different application ranges often require corresponding modification to the system configuration and capacities, mode of spoil removal, and directional control methods to achieve optimal cost-efficiency. Table 1 compares typical maxi-, midi-, and mini-HDD systems.

Table 1. Comparison of main features of typical maxi-, midi-, and mini-HDD (Iseley and Gokhale 1997)

System Description	Product Pipe Diameter	Depth Range	Drive Length	Torque	Thrust/ Pullback	Machine Weight (including truck)	Typical Application
Maxi-HDD	600-1,200 mm (24-48 in)	< 61 m (200 ft)	< 1,818 m (6,000 ft)	< 108.5 kN-m (80,000 ft-lb)	< 445 kN (100,000 lb)	< 267 kN (30 ton)	River, Highway crossings
Midi-HDD	300-600 mm (12-24 in)	< 23 m (75 ft)	<  274 m (900 ft)	1-9.5 kN-m (900-7,000 ft-lb)	89-445 kN (20,000-100,000 lb)	< 160 kN (18 ton)	Under rivers and roadways
Mini-HDD	50-300 mm (2-12 in)	<  4.5 m (15 ft)	< 182 m (600 ft)	< 1.3 kN-m (950 ft-lb)	<  89 kN (20,000 lb)	< 80 kN  (9 ton)	Telecom and Power cables,

Daftar Pustaka :

https://en.wikipedia.org/wiki/Directional_boring

http://rebar.ecn.purdue.edu/Trenchless/secondpage/Content/HDD.htm