Piggable Wye Technology

Piggable wyes have been used extensively in deepwater oil and gas pipeline operations to allow cleaning and inspection pigs and intelligent pigs access through main lines and the laterals that tie into them. Being able to run these state-of-the-art pigs through the wye improves the operating efficiency and the long-term integrity of the pipeline system. Although wyes improve operations on some levels, there are cases when operators need to run pigs counter to the normal flow direction in the lines. This presents no problem in a pipeline with no piggable wyes, but the internal profile at the wye fitting juncture does not permit reverse flow pigging.

Pigging a single line is a straightforward process, requiring only that the pig be launched from one end and pushed the length of the line using a liquid or gas medium as the propelling force until it is captured in the pig receiver at the end of its journey. However, pigging lateral line tie-in connections requires that the operator pre-plan the installation of a piggable wye fitting in the main line during the construction process. A piggable wye is a Y-shaped fitting that has two inlets, one for each incoming pipeline, and a single outlet that merges the flow of the two converging pipelines. The two lines converge in the Y at an intersecting angle of 30°. This basic wye configuration was originally tested in the 1980s and has proven to be a reliable design.

The internal profile of the wye at the juncture of the two pipelines would likely cause the pig stick in the fitting. Alternatively, if the pig managed to traverse the juncture area of the wye, there would be no way to determine which of the two converging pipelines the pig would flow into.

Bi-directional pigging

A recently developed product now allows pigs to run forward or backward through a wye. The Director Wye has the unique ability to accommodate reverse flow pigging. This new direction in piggable wye technology uses an internal diverter sleeve that is actuated from the exterior of the wye. The diverter can be actuated by an ROV or diver. The internal diverter sleeve rotates within the mainline bore of the wye to direct the pig.

The open position permits normal pigging operations that originate through the main line and/or the lateral line that converges into the single main line downstream of the wye. When the internal sleeve of the Director Wye is rotated to the closed position, the barrel of the sleeve closes the access port within the wye from the lateral line to the main line. Conventional pigging can still be conducted through the main line in the closed position, but the diverter sleeve allows a pig to run in the reverse flow direction through the main line as well. With the lateral line bore closed, the pig cannot jam in the juncture of the wye and cannot inadvertently enter the bore of the lateral line.

The Dual Director Wye

The Director Wye had no more entered the market than an operator asked if the design could be modified to permit bi-directional pigging through both the main line and the lateral legs of the wye. The Dual Director Wye accomplishes this feat through the addition of a mirror-image internal diverter sleeve in the lateral leg of the fitting.

ike its predecessor, the Dual Director can be used as a standard wye with both diverter sleeves in the open position. When the main line diverter is operated to shut off access to the lateral opening, the Dual Director also permits bi-directional pigging through the main line. The differentiating feature of the Dual Director is that if the mainline sleeve closes the main line and the lateral line sleeve is open, bi-directional pigging can be accomplished through the lateral line. The design of the dual internal diverter sleeves and the actuation system is the same for both the Director Wye and Dual Director Wye.

Future applications

There are other potential applications for the Director Wye, many of which offer benefits to shallow-water and onshore operators. Potential applications for the bi-directional piggable wye include:

• Deepwater tie-backs: Standard practice for deepwater subsea tiebacks dictates the installation of dual pipelines to permit roundtrip pigging of the lines. In many cases, the dual lines are several miles long. The installation of a Director Bi-Directional Wye would enable the operator to install a single line and still be able to pig the system. The pig would enter the wye in the reverse flow direction on its way to the wellhead. The internal director sleeve would be rotated to permit the pig to enter the lateral leg of the wye in a standard flow direction and traverse the wye on its return journey to the platform. Eliminating the redundant flowline reduces capital expense. This procedure also reduces the number of marine risers on the platform to a single riser.
• Permanent reverse flow projects: In certain cases a subsea pipeline system is designed to accommodate a future need to use reverse flow to provide feed gas for offshore operations from an onshore processing facility. In this case, the wye functions as a standard wye as long as the offshore structure can produce enough gas for its operational purposes. Once this is no longer the case, the internal sleeve of the Director Wye can be rotated to permit reverse flow of the product and any required pigging operations to one or more offshore structures.
• Coiled tubing access for deepwater risers: A Director Wye could be installed topside in a marine riser to permit more efficient coiled tubing access. The wye would be installed such that pigging the riser and pipeline could follow the conventional flow. The internal director sleeve would be rotated to close off access to the lateral leg of the wye. When coiled tubing access is required, the internal director sleeve would be rotated to open access to the lateral and permit the installation and withdrawal of the coiled tubing. At the conclusion of the operation, the internal sleeve would be rotated again to close off the tubing access lateral opening to permit pigging of the riser once again.
• Onshore applications: The Director Wye would permit the use of intelligent pigs alternatively through either the main line or the lateral without compromising piggability in either line.

Source:

http://www.offshore-mag.com/articles/print/volume-67/issue-11/drilling-completion/new-direction-in-piggable-wye-technology.html

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Subsea PLEM and PLET

As the oil&gas field development move further away from existing subsea infrastructures, it become advantageous to consider a subsea tie-in of their export system with existing deepwater pipeline system offering spare transport capacity. This necessitates incorporating pipeline end manifolds (PLEM’s) at both pipeline ends to tie in the system. A PLEM is a subsea structure used to connect rigid pipeline with other subsea structure such as a manifold or a tree, through a jumper. It is also called a pipeline end termination (PLET), especially to serve as a support for one pipeline valve and one vertical connector.

Source : 

http://oilandgastechnologies.wordpress.com/2012/08/08/subsea-plem-and-plet/

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Fatigue Free Span Analysis

Nowadays, offshore pipelines have asignificant role in development of oil and gas industry in different parts of the world. This crucial industry is laid on seabed by various methods either embedded in a trench (buried

method) or laid on uneven seabed (unburied method). Construction of unburied pipeline is the most common method for its rapid and economic performance. In this method, however, the pipelines are subjected to various lengths of free spanning throughout the route during its life time, which may threaten the pipelines safety. Free spanning in offshore pipelines mainly occurs as a consequence of uneven seabed and local scouring due to flow turbulence and instability; hence, with no doubt, free spanning occurrences for unburied pipelines are completely inevitable. 

Fredsoe and Sumer (1997) assessed the role of free spans in unburied offshore pipelines. They acknowledged the previous studies and mentioned that resonance is the main problem for offshore pipelines laid on the free spanning. Pipelines resonance happens when the external load frequency as a result of vortex shedding becomes equal to the pipe Natural Frequency. This phenomenon may burst the pipe coating and may lead to develop more fatigue on the pipelines. Different design guidelines, e.g. DNV (1998) and ABS (2001), have accepted a less stringent approach and recommend the free spanning to be reduced to the allowable length to avoid fatigue damage. These guidelines proposed a simple formulation to calculate the first Natural Frequency based on the pipelines specifications and seabed conditions; however, all of the guidelines encourages using modal analysis at the final phase of design.

Choi (2000) studied the effect of axial forces on free spanning of offshore pipelines. The results indicated that the axial force has a significant influence on the first Natural Frequency of the pipe. In this research, the different seabed condition has been broken down into three main types and the general beam equation for the boundary conditions was analytically solved. He also compared his result with Lloyd’s approximate formula which estimates the first Natural Frequency of the beam considering axial load effect. Xu et al. (1999) applied the modal analysis to incorporate the real seabed condition to assess pipelines fatigue and Natural Frequency (NF). Later, Bai (2001) approved Xu et al. (1999) approach and emphasis on applying the modal analysis to determine the allowable length of free span for offshore pipelines.

In practice, a considerable amount of works have been applied to determine the allowable free span length, however, there is still lack of knowledge in assessing the role of all effective parameters in determination of allowable free span length. The objective of this paper is two folds: (i) to assess the role of main effective parameters on Natural Frequency; and (ii) to present a simple formula for allowable free span length with accounting for the seabed condition. To do so, first the approaches of DNV (1998) and ABS guidelines are discussed and then the modal analysis is outlined to have a useful tool to assess the role of all involved parameters. Finally, a case study on the Qeshem pipelines is performed to evaluate the free span allowable length.

DNV and ABS ApporximationFormula

DNV (1998) and ABS (2001) guidelines determine the allowable length of free span with the following equations:

in which  

E = modulus of elasticity;

I = bending moment of inertia of pipeline;

C= coefficient of seabed condition; and

VR= reduced velocity defined according to Fredso and Sumer (1997) by

in which  

U= streamwise flow velocity (normal to the pipe); 

D = outer diameter of pipe;  

me  = effective mass (including structural mass, mass of content and added mass); and 

fn = Natural Frequency of the pipe

free span. In order to solve Eq. (1), fn should be replaced by vortex shedding frequency to avoid resonance. In other words, the pipe Natural Frequency based on these codes is expressed as:

In practice, employing the above-mentioned formula for estimation of pipelines free span length is not very applicable due to the difficulties in determination of the exact seabed conditions: therefore, alternative approaches including modal analysis usually will be adopted.

Reference:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved

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Horizontal Directional Drilling Process

Knowledge of the directional drilling process by the reader is assumed, but some review may be of value in establishing common terminology. Briefly, the HDD process begins with boring a small, horizontal hole (pilot hole) under the crossing obstacle (e.g. a highway) with a continuous string of steel drill rod. When the bore 

head and rod emerge on the opposite side of the crossing, a special cutter, called a back reamer, is attached and pulled back through the pilot hole. The reamer bores out the pilot hole so that the pipe can be pulled through. The pipe is usually pulled through from the side of the crossing opposite the drill rig.

Pilot Hole

Pilot hole reaming is the key to a successful directional drilling project. It is as important to an HDD pipeline as backfill placement is to an open-cut pipeline. Properly trained crews can make the difference between a successful and an unsuccessful drilling program for a utility. Several institutions provide operator- training programs, one of which is University of Texas at Arlington Center for Underground Infrastructure Research and Education (CUIRE). Drilling the pilot hole establishes the path of the drill rod (“drill-path”) and subsequently the location of the PE pipe. Typically, the bore-head is tracked electronically so as to guide the hole to a pre-designed configuration. One of the key considerations in the design of the drill-path is creating as large a radius of curvature as possible within the limits of the right-of-way, thus minimizing curvature.Curvature induces bending stresses and increases the pullback load due to the capstan effect. The capstan effect is the increase in frictional drag when pulling the pipe around a curve due to a component of the pulling force acting normal to the curvature. Higher tensile stresses reduce the pipe’s collapse resistance. The drill-path normally has curvature along its vertical profile. Curvature requirements are dependent on site geometry (crossing length, required depth to provide safe cover, staging site location, etc.) But, the degree of curvature is limited by the bending radius of the drill rod and the pipe. More often, the permitted bending radius of the drill rod controls the curvature and thus significant bending stresses do not occur in the pipe. The designer should minimize the number of curves and maximize their radii of curvature in the right-of-way by carefully choosing the entry and exit points. 

Pilot Hole Reaming

The REAMING operation consists of using an appropriate tool to open the pilot hole to a slightly larger diameter than the carrier pipeline. The percentage oversize depends on many variables including soil types, soil stability, depth, drilling mud, borehole hydrostatic pressure, etc. Normal over-sizing may be from 1.2 to 1.5 times the diameter of the carrier pipe. While the over-sizing is necessary for insertion, it means that the inserted pipe will have to sustain vertical earth pressures without significant side support from the surrounding soil.

Prior to pullback, a final reaming pass is normally made using the same sized reamer as will be used when the pipe is pulled back (swab pass). The swab pass cleans the borehole, removes remaining fine gravels or clay clumps and can compact the borehole walls.

Drilling Mud

Usually a “drilling mud” such as fluid bentonite clay is injected into the bore during cutting and reaming to stabilize the hole and remove soil cuttings. Drilling mud can be made from clay or polymers. The primary clay for drilling mud is sodium montmorillonite (bentonite). Properly ground and refined bentonite is added to fresh water to produce a “mud.” The mud reduces drilling torque, and gives stability and support to the bored hole. The fluid must have sufficient gel strength to keep cuttings suspended for transport, to form a filter cake on the borehole wall that contains the water within the drilling fluid, and to provide lubrication between the pipe and the borehole on pullback. Drilling fluids are designed to match the soil and cutter. They are monitored throughout the process to make sure the bore stays open, pumps are not overworked, and fluid circulation throughout the borehole is maintained. Loss of circulation could cause a locking up and possibly overstressing of the pipe during pullback.

Drilling muds are thixotropic and thus thicken when left undisturbed after pullback. However, unless cementitious agents are added, the thickened mud is no stiffer than very soft clay. Drilling mud provides little to no soil side-support for the pipe.

Pullback

The pullback operation involves pulling the entire pipeline length in one segment (usually) back through the drilling mud along the reamed-hole pathway. Proper pipe handling, cradling, bending minimization, surface inspection, and fusion welding procedures need to be followed. Axial tension force readings, constant insertion velocity, mud flow circulation/exit rates, and footage length installed should be recorded. The pullback speed ranges usually between 1 to 2 feet per minute. 

Mini-Horizontal Directional Drilling

The Industry distinguishes between mini-HDD and conventional HDD, which is sometimes referred to as maxi-HDD. Mini-HDD rigs can typically handle pipes up to 10” or 12” diameter and are used primarily for utility construction in urban areas, whereas HDD rigs are typically capable of handling pipes as large as 48”diamter. These machines have significantly larger pullback forces ranging up to several hundred thousand pounds.

General Guidelines

The designer will achieve the most efficient design for an application by consulting with an experienced contractor and a qualified engineer. Here are some general considerations that may help particularly in regard to site location for PE pipes:

• Select the crossing route to keep it to the shortest reasonable distance
• Find routes and sites where the pipeline can be constructed in one continuous length; or at least in long multiple segments fused together during insertion
• Although compound curves have been done, try to use as straight a drill path as possible.
• Avoid entry and exit elevation differences in excess of 50 feet; both points should be as close as possible to the same elevation
• Observe and avoid above-ground structures, such as power lines, which might limit the height available for construction equipment.
• The HDD process takes very little working space versus other methods.However, actual site space varies somewhat depending upon the crossing distance, pipe diameter, and soil type.
• Long crossings with large diameter pipe need bigger, more powerful equipment and drill rig.
• As pipe diameter increases, large volumes of drilling fluids must be pumped, requiring more/larger pumps and mud-cleaning and storage equipment.
• Space requirements for maxi-HDD rigs can range from a 100 feet wide by 150 feet long entry plot for a 1000 ft crossing up to 200 feet wide by 300 feet long area for a crossing of 3000 or more feet.
• On the pipe side of the crossing, sufficient temporary space should be rented to allow fusing and joining the PE carrier pipe in a continuous string beginning about 75 feet beyond the exit point with a width of 35 to 50 feet, depending on the pipe diameter. Space requirements for coiled pipe are considerably less. Larger pipe sizes require larger and heavier construction equipment which needs more maneuvering room (though use of PE minimizes this). The initial pipe side “exit” location should be about 50’ W x 100’ L for most crossings, up to 100’ W x 150’ L for equipment needed in large diameter crossings
• Obtain “as-built” drawings based on the final course followed by the reamer and the installed pipeline. The gravity forces may have caused the reamer to go slightly deeper than the pilot hole, and the buoyant pipe may be resting on the crown of the reamed hole. The as-built drawings are essential to know the exact pipeline location and to avoid future third party damage.

Source: 

https://plasticpipe.org/pdf/chapter12.pdf

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Pipeline Inspection

In the United States, millions of miles of pipeline carrying everything from water to crude oil. The pipe is vulnerable to attack by internal and external corrosion, cracking, third party damage and manufacturing flaws. If a pipeline carrying water springs a leak bursts, it can be a problem but it usually doesn't harm the environment. However, if a petroleum or chemical pipeline leaks, it can be a environmental disaster. More information on recent US pipeline accidents can be found at the, National Transportation Safety Board's Internet site. In an attempt to keep pipelines operating safely, periodic inspections are performed to find flaws and damage before they become cause for concern.
When a pipeline is built, inspection personnel may use visual, X-ray, magnetic particle, ultrasonic and other inspection methods to evaluate the welds and ensure that they are of high quality. The image to the left show two NDT technicians setting up equipment to perform an X-ray inspection of a pipe weld. These inspections are performed as the pipeline is being constructed so gaining access the inspection area is not problem. In some areas like Alaska, sections of pipeline are left above ground like shown above, but in most areas they get buried. Once the pipe is buried, it is undesirable to dig it up for any reason.

So, how do you inspect a buried pipeline?

Have you ever felt the ground move under your feet? If you're standing in New York City, it may be the subway train passing by. However, if you're standing in the middle of a field in Kansas it may be a pig passing under your feet. Huh??? Engineers have developed devices, called pigs, that are sent through the buried pipe to perform inspections and clean the pipe. If you're standing near a pipeline, vibrations can be felt as these pigs move through the pipeline. The pigs are about the same diameter of the pipe so they range in size from small to huge. The pigs are carried through the pipe by the flow of the liquid or gas and can travel and perform inspections over very large distances. They may be put into the pipe line on one end and taken out at the other. The pigs carry a small computer to collect, store and transmit the data for analysis. In 1997, a pig set a world record when it completed a continuous inspection of the Trans Alaska crude oil pipeline, covering a distance of 1,055 km in one run.
Pigs use several nondestructive testing methods to perform the inspections. Most pigs use a magnetic flux leakage method but some also use ultrasound to perform the inspections. The pig shown to the left and below uses magnetic flux leakage. A strong magnetic field is established in the pipe wall using either magnets or by injecting electrical current into the steel. Damaged areas of the pipe can not support as much magnetic flux as undamaged areas so magnetic flux leaks out of the pipe wall at the damaged areas. An array of sensor around the circumference of the pig detects the magnetic flux leakage and notes the area of damage. Pigs that use ultrasound, have an array of transducers that emits a high frequency sound pulse perpendicular to the pipe wall and receives echo signals from the inner surface and the outer surface of the pipe. The tool measures the time interval between the arrival of a reflected echos from inner surface and outer surface to calculate the wall thickness.

Figure 1. Pig and the diagram.

On some pipelines it is easier to use remote visual inspection equipment to assess the condition of the pipe. Robotic crawlers of all shapes and sizes have been developed to navigate the pipe. The video signal is typically fed to a truck where an operator reviews the images and controls the robot.

Figure 2. Pipe crawler.

Source:

http://www.ndt-ed.org/AboutNDT/SelectedApplications/PipelineInspection/

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Pipeline Hydrostatic Test

Hydrostatic testing has long been used to determine and verify pipeline integrity. Several types of information can be obtained through this verification process.

However, it is essential to identify the limits of the test process and obtainable results. There are several types of flaws that can be detected by hydrostatic testing, such as:

• Existing flaws in the material,
• Stress Corrosion Cracking (SCC) and actual mechanical properties of the pipe,
• Active corrosion cells, and
• Localized hard spots that may cause failure in the presence of hydrogen.

There are some other flaws that cannot be detected by hydrostatic testing. For example, the sub-critical material flaws cannot be detected by hydro testing, but the test has profound impact on the post test behavior of these flaws.

Given that the test will play a significant role in the nondestructive evaluation of pipeline, it is important to determine the correct test pressure and then utilize that test pressure judiciously, to get the desired results.

When a pipeline is designed to operate at a certain maximum operating pressure (MOP), it must be tested to ensure that it is structurally sound and can withstand the internal pressure before being put into service. Generally, gas pipelines are hydrotested by filling the test section of pipe with water and pumping the pressure up to a value that is higher than maximum allowable operating pressure (MAOP) and holding the pressure for a period of four to eight hours.

ASME B 31.8 specifies the test pressure factors for pipelines operating at hoop stress of ≥ 30% of SMYS. This code also limits the maximum hoop stress permitted during tests for various class locations if the test medium is air or gas. There are different factors associated with different pipeline class and division locations. For example, the hydrotest pressure for a class 3 or 4 location is 1.4 times the MOP. The magnitude of test pressure for class 1 division 1 gas pipeline transportation is usually limited to 125% of the design pressure, if the design pressure is known. The allowed stress in the pipe material is limited to 72% of SMYS. In some cases it is extended to 80% of SMYS. The position of Pipeline and Hazardous Material Safety Administration (PHMSA) is similar. Thus, a pipeline designed to operate continuously at 1,000 psig will be hydrostatically tested to a minimum pressure of 1,250 psig.

Source:

http://pipelineandgasjournal.com/pipeline-hydro-test-pressure-determination?page=show

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High-Strength Low Alloy Steels: New Generation

High-strength low-alloy (HSLA) steels, or microalloyed steels, are designed to provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbon steels. They are not considered to be alloy steels in the normal sense because they are designed to meet specific mechanical properties rather than a chemical composition; HSLA steels have yield strengths greater than 275 MPa, or 40 ksi.

The chemical composition of a specific HSLA steel may vary for different product thicknesses to meet mechanical property requirements. The HSLA steels in sheet or plate form have low carbon content (C=0.05-0.25 [wt.%]) in order to produce adequate formability and weldability, and they have manganese content up to 2.0%. Small quantities of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium, and zirconium are used in various combinations.

HSLA steels can be divided into six categories:

• Weathering steels, which contain small amounts of alloying elements such as copper and phosphorus for improved atmospheric corrosion resistance and solid-solution strengthening.
• Microalloyed ferrite-pearlite steels, which contain very small (generally, less than 0.10%) additions of strong carbide or carbonitride forming elements such as niobium, vanadium, and/or titanium for precipitation strengthening, grain refinement, and possibly transformation temperature control.
• As-rolled pearlitic steels, which may include carbon-manganese steels but which may also have small additions of other alloying elements to enhance strength, toughness, formability, and weldability.
• Acicular ferrite (low-carbon bainite) steels, which are low-carbon (less than 0.05% C) steels with an excellent combination of high yield strengths (as high as 690 MPa, or 100 ksi) weldability, formability, and good toughness.
• Dual-phase steels, which have a microstructure of martensite dispersed in a ferritic matrix and provide a good combination of ductility and high tensile strength.
• Inclusion-shape-controlled steels, which provide improved ductility and through-thickness toughness by the small additions of calcium, zirconium, or titanium, or perhaps rare earth elements so that the shape of the sulfide inclusions is changed from elongated stringers to small, dispersed, almost spherical globules.

The wide applications of HSLA steels include oil and gas pipelines, heavy-duty highway and off-road vehicles, construction and farm machinery, industrial equipment, storage tanks, mine and railroad cars, barges and dredges, snowmobiles, lawn mowers, and passenger car components. Bridges, offshore structures, power transmission towers, light poles, and building beams and panels are additional uses of these steels.

The choice of a specific high-strength steel depends on a number of application requirements including thickness reduction, corrosion resistance, formability, and weldability. For many applications, the most important factor in the steel selection process is the favorable strength-to-weight ratio of HSLA steels compared with conventional low-carbon steels. This characteristic of HSLA steels has lead to their increased use in automobile components.

The development of high strength steels is shown in Figure 1.

Figure 1: Development of High Strength Steels

As shown in Figure 1, in the seventies, the hot rolling and normalizing was replaced by thermo-mechanical rolling. The latter process enables materials up to X70 to be produced from steels that are microalloyed with niobium and vanadium and have reduced carbon content. By this method, it has become possible to produce higher strength materials like X80, having a further reduced carbon content and excellent field weldability. Additions of molybdenum, copper and nickel enable the strength level to be raised to that of grade X100, when the steel is processed to plate by thermo-mechanical rolling plus modified accelerated cooling.

Natural gas is attracting attention as a source of clean energy because it emits less carbon dioxide than that of petroleum or coal. Furthermore, many long-distance pipelines have been constructed to transport natural gas. In view of these facts, high-strength line pipes X80 up to a strength grade of API 5L X120 are being developed for the purpose of enhancing the transport efficiency of a pipeline by high pressure operation and reducing pipe laying costs by the use of thinner- wall pipes.

In order to arrest running shear fracture and prevent brittle fracture, excellent low-temperature toughness is required of the base metal and the heat affected zone (HAZ) of a welded joint of such a high strength line pipe. In addition, the line pipe is required also to be excellent in weldability in order to improve pipeline construction efficiency. Pipelines constructed in permafrost or seismic regions are subject to a large bending moment caused by ground deformation, and for this reason, large uniform elongation or a low yield ratio has come to be required of a line pipe these years for the purpose of preventing ductile fracture.

The use of X80 line pipe in the construction of the first Ruhrgas X80 pipeline led to a material saving of about 20,000 t, compared with X70 pipes (Fig. 2), through a reduction of the wall thickness from 20.8 mm for X70 to 18.3 mm for X80. This resulted also in a reduction of the pipe laying costs, because of reduced pipe transportation costs and greatly reduced welding costs, as thinner walls meant reduced welding times. The use of materials with still higher strength, such as X100 or X120, could lead to further material savings, as Figure 2 further illustrates.

Figure 2: Possible material savings through use of high-strength materia

The Nippon Steel Corporation has succeeded in developing an innovative technology to improve HAZ toughness called Super-High HAZ Toughness Technology with a Fine Microstructure Imparted by Fine Particles (HTUFF)®. By the developed technology, the coarsening of austenite (γ) grains is prevented, and as a result, the microstructure near a welding fusion line (FL) is made remarkably fine.

For improving uniform elongation and yield ratio, it is effective to form a dual-phase microstructure by applying a thermo-mechanical control process (TMCP). On the basis of these technologies, Nippon Steel has developed a new UOE pipe of an X60 to X80 class having excellent HAZ toughness and large uniform elongation called “Tough-Ace”. This pipe was used for the Sakhalin pipeline project. Two types of X100 line pipes, namely a high HAZ toughness type and a high uniform elongation type, have also been developed with good results of deformability and the mechanical properties.

Source : 

http://www.keytometals.com/page.aspx?ID=CheckArticle&site=kts&NM=209

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