The Britpipe

Pipeline Global Buckling

06.23

Global buckling of pipelines may be treated as the buckling of a bar (pipe) in compression. The global buckling may occur either downwards (free span), horizontally (lateral buckling on sea bed) or vertically (as upheaval buckling of buried pipelines or on a crest of exposed pipelines followed by a lateral turn-down). Local buckling is a gross deformation of the pipe cross section. Global buckling is a response to compressive force generated by high temperature and high pressure (HP/HT), which will generally reduce the axial capacity of the pipeline. Pipelines exposed to high temperature and high pressure or pipeline with a low buckling capacity will be governed by global buckling. In DNV’s RP-F110 [19], three global buckling scenarios resulted from HT/HP are introduced: · Exposed pipelines on even seabed. Global buckling occurs in the horizontal plane, post buckling configuration may be allowed. · Exposed pipelines in uneven seabed. Global buckling occurs first in the vertical plane (cause feed-in and uplift) and subsequently in the horizontal plane, or combined scenarios with scenario I, post buckling configuration may be allowed. · Buried/covered pipelines, global buckling in the vertical plane, so called upheaval buckling. Global buckling is a load response, not a failure mode. However, global buckling will imply some failure modes [20] such as: · Local buckling, for pipeline subjected to combined pressure. Longitudinal force and bending, local buckling may occur. The failure mode may be yielding of the cross section or buckling on the compressive side of the pipe. · Fracture, which is caused by tensile strain, generally includes brittle fracture and plastic collapse. · Fatigue, pipeline components such as riser, unsupported free spans, welding should be assessed for fatigue. Potential cyclic loading fatigue damages, which may include vortexinduced-vibrations (VIV), wave induced hydrodynamic loads, cyclic pressure and thermal expansion loads. · Ratcheting, ratcheting generally describes the accumulated plastic deformations under cyclic loads in pipelines that exposed to high temperature and high pressure. · Bursting, it is governed by tensile hoop stress, which may occur in the tensile part of pipeline.

Sumber

(http://www.diva-portal.org/smash/get/diva2:648750/FULLTEXT01.pdf)

Offshore Spiral Pipe

06.20

Spiral welded pipes market, though encountering overcapacity conditions particularly in North America, is expected to witness steady growth in the upcoming years driven by the implementation of new pipeline projects. Investments in oil and gas exploration and production, which are influenced by prevailing crude oil & gas prices, have a considerable impact on the demand for spiral welded pipes and tubes. Resurgent world economy and consequent increase in the demand for industrial natural gas is expected to drive up momentum of the spiral welded pipes market.

Global demand for spiral welded pipes, which are primarily used in the transportation of oil and gas and in water transportation projects, is closely linked to the investments in the energy sector. The energy sector makes use of spiral welded pipes with diameters of up to 60” and up to 80 feet in length. Another factor that is expected to fuel demand for spiral pipes and tubes is new pipeline construction activity due to the shift of population from traditional centers that would necessitate development of infrastructure for delivering oil and natural gas to the new locations. Demand for spiral welded pipes is also expected from the replacement market, as most of the existing pipeline infrastructure, particularly in developed regions, has reached their end of useful life. Structural applications of spiral welded pipes are also gaining momentum, specifically with additional activity occurring in port, offshore loading and infrastructure improvement sectors.

As stated by the new market research report on Spiral Welded Pipes and Tubes, Asia-Pacific represents the largest market worldwide, driven primarily by increased use in transporting natural gas. Besides Asia-Pacific, Latin America ranks among the fastest growing regional markets with compounded annual growth rate ranging between 7.5% and 9.0% over the review period. North American market, on the other hand, is encountering testing times owing to weak demand and overcapacity conditions. Oversupply is the major concern for spiral welded pipes market particularly with regard to large diameter double submerged arc welded or DSAW line pipes, which finds use in transmitting oil, natural gas liquids, and natural gas to consumers from drilling locations.

Despite the prevailing conditions, potential opportunities are expected primarily from the implementation of new pipeline projects in the upcoming years, resurgent growth of the US economy, and increased demand from natural gas exploration operations. Also, overcapacity conditions are expected to fade away in the coming years, as several megaprojects are set to be taken up across the world, particularly in regions such as Southeast Asia, Australia, Middle East, Africa, and West Asia.

Replacement of aging infrastructure offers huge potential for pipe manufacturers. The need to replace old pipelines is particular high in the US and Russia, where pipeline networks were mostly installed during the 60s and 70s. With the average lifespan of oil and gas transportation pipes ranging between 25 and 30 years, opportunities in the replacement market are huge, particularly for HSAW pipes. In the US, replacement demand holds enormous potential as a result of the recent enactment of the legislation that necessitates more inspections to be carried out, which could increase the likelihood of pipeline replacements. The Act is likely to play a critical role in enabling manufacturers of large diameter line pipes to survive the tough economic and overcapacity conditions.

Sumber:

(http://www.prweb.com/releases/spiral_welded_pipes_tubes/DSAW_HSAW_pipes/prweb10402550.htm)

Vortex Induced Vibration

06.18

Corrosion Prevention

06.13

Corrosion can be defined as the destruction or deterioration of a material because of reaction with its environment. Corrosion is a natural occurance and inevitable. Especially in seawater environment, corrosion is a threat for carbon steel pipe (offshore pipeline). Corrosion will damage pipeline and leads to pipe leak in which will be dangerous for the circumstances surround. Petroleum industry spends a million dollars per day to protect its pipelines. And so, there is urgency to protect and prevent pipeline from corrosion.

There are several methods that can be used to prevent and decrease the rate of corrosion on offshore pipeline. These methods are:

MATERIAL SELECTION

This method is just simply selecting the best and appropriate alloy carbon steel to a particular environment. For instance, the use of nickel-based alloy steel allows pipeline to withstand seawater environment without putting additional sacrificial anodes or impressed current, yet it’s far more expensive than having ordinary carbon steel with cathodic protected.

USE OF INHIBITOR

Sometimes corrosion in offshore pipeline attacked from inside (compounds brought by the fluid inside pipe e.g. sulphate). This can be helped by adding inhibitor. Inhibitor is a substance that when added in small concentrations to an environment, decreases the corrosion rate, such as chromate and nitrate.

CATHODIC PROTECTION

Cathodic protection is achieved by supplying electrons to the metal structure to be protected. Basically, cathodic protection has the pipeline become cathode, instead of anode, that way it won’t be corroded. There are two ways to cathodically protect a stucture. Firstly, Impressed Current Cathodic Protection (ICCP) and Sacrificial Anode Cathodic Protection (SACP).

1. Impressed Current Cathodic Protection (ICCP)

ICCP supplies electron by flowing electrical current from a power supply. This method is suitable for large structures regarding cost. For pipelines, anodes are arranged in groundbeds either distributed or in a deep vertical holes depending on several design and field condition factors including current distribution requirements.

2. Sacrificial Anode Cathodic Protection (SACP)

This method is also known as Galvanic Coupling. In the usual application, a galvanic anode, a piece of a more electrochemically “active” metal, is attached to the vulnerable metal surface where it is exposed to the corrosive liquid. Galvanic anodes are designed and selected to have a more “active” voltage (more negative electrochemical potential) than the metal of the target structure.

COATING

Relatively thin coatings of metallic and inorganic materials can provide a satisfactory barrier between metal and its environment. The chief function of such coatings is to provide an effective barrier.

Coating can be in the form of, for example, cladding. Cladding involves a surface layer of sheet metal put on by rolling two sheets of metal together. For instance, a nickel and a steel sheet are hot-rolled together to produce a composite sheet with, say, 1/8 inch of nickel and 1 inch of steel. This way the steel are protected with its environment since nickel is layered on the surface. Moreover, in the application for offshore pipeline, high density polyethylene(HDPE) and polypropylene layer can be coated on pipe bare surface. Both HDPE and polypropylene coating have low water permeation which will improve isolation of the pipe from seawater surrounds. Coating for pipeline is illustrated as below:

Sumber:

http://en.wikipedia.org/wiki/Cathodic_protection

http://met-engineering.blogspot.com

HSLA Steel

06.10

Pipeline Corrosion Resistance Alloy Material

06.09

Pipeline Stress Analysis

06.05

It is common practice worldwide that piping designers/layout personnel route pipes with consideration given mainly to space constraints, process and flow constraints (such as pressure drop) and other requirements arising from constructability, operability and reparability. Unfortunately, often pipe stress requirements are not sufficiently considered while routing and supporting piping systems, especially in providing adequate flexibility to absorb expansion/contraction of pipes due to thermal loads. So, when "as designed" piping systems are given to pipe stress engineers for analysis, they soon realize that the layout is "stiff" and suggest routing changes to make the layout more flexible. The piping designers, in turn, make routing changes and send the revised layout to the pipe stress engineers to check compliance again.

Such "back and forth" design iterations between layout and stress departments continue until a suitable layout and support scheme is arrived at, resulting in significant increase in project execution time, which, in turn, increases project costs.

This delay in project execution is further aggravated in recent years as operating pressures and temperatures are increased in operating plants to increase plant output; increased operating pressures increase pipe wall thickness, which, in turn, increase piping stiffness further; increased operating temperatures, applied on such "stiffer" systems, increase pipe thermal stresses and support loads. So, it is all the more important to make the piping layout flexible at the time of routing by piping designers.

The "Design by Color" product "checkSTRESS" from SST (different from CAEPIPE) for AutoCAD, CATIA, Autoplant, PDMS, Cadmatic, etc. can be used to substantially reduce the number of design iterations between the piping layout and stress departments, resulting in huge time savings during design.

Basic Pipe Stress Concepts for Piping Designers

Piping systems experience different loadings, categorized into three basic loading types, namely Sustained, Thermal and Occasional loads.

Sustained Load:

It mainly consists of internal pressure and dead-weight. Dead-weight is from weight of pipes, fittings, components such as valves, operating fluid, test fluid, insulation, cladding, lining etc.

Internal design/operating pressure develops uniform circumferential stresses in the pipe wall, based on which pipe wall thickness is determined during the process/P&ID stage of plant design such that "failure by rupture" is avoided. In addition, internal pressure develops axial stresses in the pipe wall. These axial pressure stresses vary only with pressure, pipe diameter and wall thickness, all three of which are pre-set at the P&ID stage and hence these axial pressure stresses cannot be reduced by changing the piping layout or the support scheme.

On the other hand, dead-weight causes the pipe to bend (generally downward) between supports and nozzles, producing axial stresses in the pipe wall (also called "bending stresses"); these bending stresses linearly vary across the pipe cross-section, being tensile at either the top or bottom surface and compressive at the other surface. If the piping system is not supported in the vertical direction (i.e., in the gravity direction) excepting at equipment nozzles, bending of the pipe due to dead-weight may develop excessive stresses in the pipe and impose large loads on equipment nozzles, thereby increasing the susceptibility to "failure by collapse".

Various international piping codes impose limits, also called "allowable stresses for sustained loads", on these axial stresses generated by dead-weight and pressure in order to avoid "failure by collapse".

For the calculated axial stresses to be below such allowable stresses for sustained loads, it may be necessary to support the piping system vertically. Typical vertical supports to carry dead-weight are:

a. Resting steel supports,

b. Rod hangers,

c. Variable spring hangers, and

d. Constant support hangers.

Both rod hangers and resting steel supports fully restrain downward pipe movement but permit pipe to lift up at such supports. If pipe lifts up at any of the rod hangers / resting supports during operating condition, then that support does not carry any pipe weight and hence will not serve its purpose.

Two examples are presented in this Tutorial to illustrate how piping can be supported by spring hangers and resting steel supports to comply with the code requirements for sustained loads.

Thermal Load (also referred as Expansion Load):

It refers to the "cyclic" thermal expansion/contraction of piping as the system goes from one thermal state to another thermal state (for example, from "shut-down" to "normal operations" and then back to "shut-down"). If the piping system is not restrained in the thermal growth/contraction directions (for example, in the axial direction of a straight pipe), then for such cyclic thermal load, the pipe expands/contracts freely; in this case, no internal forces, moments and resulting stresses and strains are generated in the piping.

On the other hand, if the pipe is "restrained" in the directions it wants to thermally deform (such as at equipment nozzles and pipe supports), such constraint on free thermal deformation generates cyclic thermal stresses and strains throughout the system as the system goes from one thermal state to another. When such calculated thermal stress ranges exceed the "allowable thermal stress range" specified by various international piping codes, then the system is susceptible to "failure by fatigue". So, in order to avoid "fatigue failure" due to cyclic thermal loads, the piping system should be made flexible (and not stiff). This is normally accomplished as follows:

a. Introduce bends/elbows in the layout, as bends/ elbows "ovalize" when bent by end-moments, which increases piping flexibility.

b. Introduce as much "offsets" as possible between equipment nozzles (which are normally modeled as anchors in pipe stress analysis).

For example, if two equipment nozzles (which are to be connected by a pipeline) are in line, then the straight pipe connecting these nozzles is "very stiff". On the other hand, if the two equipment are located with an "offset", then their nozzles will have to be connected by an "L-shaped" pipeline which includes a bend/elbow; such "L-shaped" pipeline is much more flexible than the straight pipeline mentioned above.

c. Introduce expansion loops (with each loop consisting of four bends/elbows) to absorb thermal growth/contraction.

d. Lastly, introduce expansion joints such as bellows, slip joints etc., if warranted.

In addition to generating thermal stress ranges in the piping system, cyclic thermal loads impose loads on static and rotating equipment nozzles. By following one or more of the steps from (a) to (d) above and steps (e) and (f) listed below, such nozzle loads can be reduced.

e. Introduce "axial restraints" (which restrain pipe in its axial direction) at appropriate locations such that thermal growth/contraction is directed away from equipment nozzles, especially critical ones.

f. Introduce "intermediate anchors" (which restrain pipe movement in the three translational and three rotational directions) at appropriate locations such that thermal deformation is absorbed by regions (such as expansion loops) away from equipment nozzles.

A few example layouts are presented below to illustrate how loops/offsets, axial restraints and intermediate anchors are used to reduce thermal stresses in piping (and resulting nozzle loads).

Occasional Load:

This type of load is imposed on piping by occasional events such as earthquake, wind etc. To protect piping from wind (which normally blows in horizontal plane), it is normal practice to attach "lateral supports" to piping systems. During an earthquake, the earth may also move vertically. To protect piping against both horizontal and vertical movement during earthquake, some of the resting supports may be made as "integral 2-way vertical and lateral restraints".

Fortunately, to carry sustained loads, normally vertical supports (as those listed under the Section titled "Sustained Load" above) are required. To withstand static seismic ‘g’ loads, "integral 2-way vertical and lateral restraints" are required. Generally, some of the vertical weight supports can be modified as "integral 2-way vertical and lateral restraints". On the other hand, for thermal loads, zero supports give zero stresses. So, thermal stresses and equipment nozzle loads will normally decrease as the number of supports goes down. Axial restraints and intermediate anchors are recommended only to direct thermal growth away from equipment nozzles.

Sumber:

(http://www.sstusa.com/pipe-stress-analysis-tutorial.php)

The Britpipe

Mengenai Saya

Arsip Blog

Pipeline Global Buckling

Offshore Spiral Pipe

Vortex Induced Vibration

VIV TYPES

Corrosion Prevention

MATERIAL SELECTION

USE OF INHIBITOR

CATHODIC PROTECTION

COATING

HSLA Steel

Pipeline Corrosion Resistance Alloy Material

Pipeline Stress Analysis

Basic Pipe Stress Concepts for Piping Designers

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