PIPELINE SYSTEMS 

The following pipeline material and technology are readily available:

Material:

Technology:

Tip Click these links below for access to 3D resources: 3D Pictures of Flowlines

There are a number of standard linepipe specifications in use, and for the oil and gas industry the main pipe specifications are:

The standard linepipe specifications define minimum requirements for the supply of linepipe for oil and gas applications and cover the following requirements:

The standard linepipe specifications define the ranges of standard dimensions (diameter and wall thickness) and standard mechanical properties. The diameter for linepipe is standardised on the outside diameter with the inside diameter varying as a function of wall thickness. It is possible to procure pipe to non-standard dimensions, although this could have cost implications. Standard mechanical properties are defined in the linepipe standards. For the pipeline designer, choice of the material grade and defining yield stress (specified minimum yield stress, SMYS) and ultimate tensile strength are of particular interest. The standard material grade designations for carbon-manganese steel, based on API 5L are shown in Table 4.2 below.

API 5L X70 is the highest strength linepipe seen in widespread use although higher grades have and are being used.

Where sour service is specified, additional material requirements are specified to prevent environmental cracking, such as hydrogen induced cracking (HIC) or sulphide stress corrosion cracking (SSCC). In such applications, the additional requirements of NACE standard MR-01- 75 “Sulphide Stress Cracking Resistant Metallic Materials for Oil Field Equipment” are often specified.

The general requirements of this NACE standard are embodied within ISO3183 part 3. Materials shall be selected to be resistant to SSCC if the system pressure is 4.5 bara (65psia) or greater, and if the partial pressure of H2S is greater than 0.0035 bara (0.05psia).

Where [13] is used as the design code, the minimum linepipe requirements specified within the code must be complied with. In addition to the basic specified requirements, there are a number of supplementary material requirements to address specific applications. The supplementary requirements address:

P – Linepipe for plastic Deformation.

U – High utilisation.

D – Enhance Dimensional Requirement for Linepipe.

S – H2S service.

F – Fracture arrest properties.

The following sections will further describe the above material and technology.

Currently, most line pipe steels are made by either basic oxygen or electric-arc furnace steelmaking. Another steelmaking process called open-hearth steelmaking, should be avoided because it doesn’t allow for the production of clean and low-carbon steel. The two former processes can produce line pipe steel of acceptable quality.

4.3.2   Seamless or Seam Welded Pipe

Most line pipe is made as seamless, electric-welded, or furnace butt-welded pipe. Furnace butt-welded pipe is so limited in size and grade that is not a practical method for making pipe for high pressure pipelines. Therefore, the practical choices evolve to seamless and electric-welded pipe. The seams of electric-welded pipe may be electric resistance welded (ERW) or submerged-arc welded (SAW). Other seam welding processes (Gas metal arc welding MIG, combination of gas metal arc welding and submerged arc welding) are recognised by API specification 5L, but they are apparently not widely used.

• Seamless pipe:

  Seamless pipe is made one round at a time from single billets by one of several possible multi-step hot-forming processes. However the general process is practically the same:

  • Heating of cylindrical or parallelepipedic billets up to 1300°C

  • Piercing of billets (1150 – 1250°C) (Figure 4.1, “Billet piercing process”)

  • Lamination at 1000 – 1100°C to obtain the required thickness (Figure 4.2, “Continuous lamination”)

  • Final calibration (800 – 900°C) to fixe the external diameter

  • Cut from the resulting pipe (12m long or more) to the desired length

Figure 4.1 - Billet piercing process

Figure 4.2 - Continuous lamination

As a result it is generally more expensive, than welded pipe. It is made in sizes ranging from 2 3/8-in diameter through 24-in diameter and in grades from Grade A through Grade X65. It may be obtainable in higher grades (X80-X100) but such materials may require special heat treatment. If so the cost will likely be quite high relative to welded pipe.

The seamless pipe is the pipe of choice for deepwater field development due to more and more demanding applications (water depth, HP/HT) and performance (lay tension, plastic Deformations). These set limitations for pipe not only in its mechanical properties (material hardness, limited yield variation, maximum yield/tensile ratio) but also in the area of diameter and pipe wall thickness tolerances and non-destructive testing.

The metallurgical aspects of heavy wall high-strength seamless pipes are important criteria to determine the optimum combination of strength and toughness for deepwater applications.

The optimised chemistry (homogeneous microstructure) and the correct process of quench and temper (Q&T) will provide heavy wall pipe with increase strength without reducing toughness.

The main stage of the process are hot piercing, hot rolling at a retained mandrel mill, and sizing. The subsequent Q&T consists of a first heating up (austenitisation), internal and external quenching, followed by a second pipe heating (tempering).

In this process, optimum combinations of carbon (C), manganese (Mn), chromium (Cr), molybdenum (Mo), nickel (Ni) and microalloying elements (Nb, V, Ti) are to be obtained for optimum strength and toughness seamless pipe.

• Electric resistance welded pipe:

ERW line pipe is made from coiled skelp in a continuous process where the strip is edge trimmed and cold-formed from its initial flat condition into a round shell. As the edges of the strip are brought together electric current is passed between them heating them locally to the point of melting or near melting just as they are forced together by a pair of rollers bearing on the unheated portion of the shell (see Figure 4.3, “Electric Resistance/Induction Welded pipe”). The pressure forces the heated edges together, upsetting excess material to the outside and inside. The excess ‘’flash’’ is trimmed from both surfaces. In most ERW processes the just-completed weld zone is then immediately subjected to post-weld heat treatment. The weld zone is reheated to a temperature which ‘’normalises’’ the microstructure in the area of the bondline minimising or eliminating possibly unfavourable microstructural components created during welding. Pipe segments of the desired length are then cut from the continuous tube. In a few ERW mills the finished tube is subjected to ‘’full-body’’ normalising where the whole tube is heated to the normalising temperature. In either case the pipe is sized by passing it through rolls. ERW line pipe is obtainable in sizes ranging from 2 3/8-in diameter through 24-in diameter and in grades ranging from Grade A through Grade X70. A few manufacturers may offer Grade X80 ERW pipe.

However, ERW pipe is not commonly used in deepwater applications.

Figure 4.3 - Electric Resistance/Induction Welded pipe

• Submerged arc welded pipe

  The third type of line pipe is that made from individual plates with a submerged-arc welded seam. Typically, very large diameter, heavy-walled line pipe materials are made by this process. The process begins with the plates being squared at the ends and edge trimmed and bevelled along the long edges. The plates are then cold-formed into ‘’cans’’ roughly 40-feet in length. In some cases the cans are formed in stages including edge-crimping, U-ing (forming to a U shape) and O-ing (pressing the cans to a round shape). U-O pipe is generally cold-formed (expanded) to its final size after being seam welded by mechanical or hydraulic means (see Figure 4.4, “Submerged Arc Welded pipe fabrication process”). In other cases the cans are formed by means of pyramid rolls and the pipe is sized after being seam welded by means of rolls in much the manner as ERW pipe is sized.

The seam-welding of SAW pipe is done in two passes both in the flat position, one from the ID side and one from the OD side. Usually the ID pass is made first by means of a three-to-five wire welding head. The pipe is then turned 180 degrees, and the OD pass is made with a two or three-wire welder. No post-weld heat treatment is required. Factors which can affect quality are the filler-metal chemistry and flux chemistry and the manner in which the first pass is made. Holding the edges in the proper position and preventing relative movement while the first pass solidifies are essential. For this reason some manufacturers tack weld the edges of the cans prior to seam welding. Others rely solely on mechanical restraint.

Figure 4.4 - Submerged Arc Welded pipe fabrication process

Straight-seam SAW pipe is obtainable in sizes ranging from 16-in OD through 48-in OD (larger sizes may be available from manufacturers in Europe or Japan) and in grades from Grade A through Grade X70. Grade X80 materials may be available through certain manufactures. SAW

pipe is also obtainable with a spirally-oriented seam (see Figure 4.5, “Spirally-oriented seam welded pipe”).

Figure 4.5 - Spirally-oriented seam welded pipe

In this process, the plates are welded end-to-end so that a continuous strip is fed at the proper prearranged angle to a forming stand. The spiralling strip is then continuously welded along the incoming edge into a round tube. Both an ID and an OD pass are made. Pipes of the desired length are then cut from the resulting continuous tube. Finally, if the pipe is to be coated with fusion-bonded epoxy, the pipeline surface condition should be suitable for coating. The main problem to be avoided is surface slivers which, when raised by grit blasting and heating, cause severe coating defect problems.

In deepwater applications, SAW pipes are not commonly used except in bundle configuration where the carrier pipe may be made from SAW pipe as a cost effective alternative to seamless pipe.

There are mainly three (3) flexible pipe manufacturers listed below:

Flexible pipes have been extensively used in Brazil Campos Basin since 1977. The world deepest application (2007) is to be reached on Roncador field at 1800m-1900m.

The structure of a typical flexible pipe (e.g. Technip) is shown in Figure 4.6, “Typical flexible pipe structure for oil production and transportation”:

Figure 4.6 - Typical flexible pipe structure for oil production and transportation

The various layers constituting a typical flexible pipe are detailed hereafter, from the inside part of the pipe to the outside:

Note Fabric tape is placed above the cross-wound tensile armours to bind the under layer of amours during manufacturing before the external plastic sheath is extruded. Antifriction tape (made of polyamide) is applied above the Teta layer to avoid friction and wear between the Teta layer and tensile armours. It is also placed between the two layers of amour. If necessary for extreme high pressure, the interlocked layer is reinforced by a flat steel layer which is not interlocked. For deepwater applications, armour layers made out of fiber glass and carbon have been produced and tested in 1000m WD in Brazil.

The following figure sets the flexible pipe water depth limitations (typical), and further details on flexible pipe technologies and applications are provided in document [77].

Figure 4.7 - Flexible pipe water depth capability (Technip data - 2006)

Note Maximum water depth based on the following basis, Flowline laid empty Max. laying tension of 425T DAF = 1.25

When the use of carbon steel is not possible because:

then stainless steel must be considered. Two of the most widely used methods to produce stainless steel pipe are shown in Figure 4.8, “Different types of stainless steel fabrication process”.

Figure 4.8 - Different types of stainless steel fabrication process

The seamless hot-extruded pipe is produced on the 30,000 ton hydraulic press in the range of 150 – 1200mm Diameter, 10 – 200mm Wall Thickness and 16 metre maximum individual length.

It is desirable that the material should be easily welded. This means, in terms of corrosion, that the weldments must exhibit mechanical properties and a corrosion resistance similar to the base material.

13% chromium steel, widely used for wells to mitigate against CO2 corrosion in the absence of H2S, is much less utilised in surface installations. Austenitic Stainless Steel type 316L (very low carbon content or stabilised to avoid intergranular corrosion) and the various austenitic-ferric steels (Duplex) have a better weldability and above all a better corrosion resistance than 13% Cr steel.

However, they are very sensitive to corrosion Pitting. An intensive welding qualification programme has been set up for the Asgard project to allow the implementation of 13%Cr steel as flowline material.

The use of these materials for flowline is viable even though the cost is greater than carbon steel.

For cryogenic application, the characteristics of Liquefied Natural Gas (LNG), in particular its temperature of -163°C and the large volumes involved, means that traditional materials used for the manufacturing of pipelines for offshore use in the Oil and Gas industry are not acceptable, and that alternative materials must be used as Nickel alloy with 36% of Ni, commercially named INVAR. Because of its very low expansion coefficient and its low Young modulus compared to steel material, the thermal stresses and strains resulting from the temperature difference are maintained low (approximately 10 times lower than steel material).

There is no need of artificial expansion devices (e.g. loops or bellows) due to the thermal or mechanical length variation. Combined with its high mechanical strength, high ductility, toughness and excellent welding properties, the INVAR material simplifies the design and fabrication for the production of cryogenic pipes.

The need for transport unprocessed corrosive multiphase well streams has increased interest in CRA lined or claded flowlines as an alternative to installing inhibited carbon steel.

Handling unprocessed corrosive fluids then raises the issues of reliability, safety, and the costs of possible failures.

Although the initial capital costs of a claded/lined pipeline are quite high, the subsequent operating costs over the life of the project are relatively low. The opposite is true for carbon steel where relatively low initial costs may be coupled with significant operating and repair costs. For this reason, a life cycle cost analysis provides a better measure of the overall costs of clad versus carbon steel flowlines.

4.7.2   Hot Rolling Process:

Figure 4.9, “Hot rolling clad pipe manufacturing process” shows typical hot rolling clad pipe manufacturing process.

Figure 4.9 - Hot rolling clad pipe manufacturing process

In the clad pipe manufacturing process, the clad metal and the backing steel are metallurgically bonded by hot rolling process after slab assembling.

Two types of rolling processes are used:

• One is the application of the thermo-mechanical control process to clad pipe, in which no heat treatment is applied on the clad plate or the welded clad pipe (TMCP plate process)

• The other is conventional rolling process with as-quenching type heat treatment on welded clad pipe by induction heating in a short time (pipe quenching process). The selection from these two processes is determined by taking account of the cladding materials and required properties.

In the application of TMCP to clad plate rolling, it becomes difficult to obtain both high strength and good toughness of backing steel when wall thickness is increased. As-quenching type heat treatment on welded pipes is useful for heavy wall thickness or superior toughness requirement. Quenching from relatively high temperature is also effective to improve corrosion resistance of the clad metal even if the corrosion resistance is deteriorated by hot rolling or longitudinal seam welding.

Longitudinal seam welding procedure consists in first welding backing steel by submerged arc welding (SAW) with one pass in each side, and next performing an overlay welding in two molten pools by tandem Gas Tungsten Arc Welding (GTAW) with the hot wire method.

Finally, inner surfaces of welded clad pipes are polished with wet brush to remove weld scales and to smooth the surfaces, otherwise they deteriorate corrosion resistance of the weld through the crevices created under weld scales.

4.7.3   Thermo-Hydraulic Fit Method

CRA Lined pipe can also be manufactured using the ‘’thermo-hydraulic fit method’’ as shown in Figure 4.10, “Thermo-hydraulic Fit Method For Tightly Fitted Lined Pipe”, which is, in principle, a combination of the ‘’thermal shrink-fit method’’ and ‘’hydraulic expansion method’’. This technique allows producing economically long lined pipe with high fit-in stress. The principle and the procedure of this method is as follows : Into an outer pipe which has been heated and thermally expanded, a liner tube is inserted and expanded by hydraulic pressure. After removing the pressure and heat in the outer pipe, the tight fitted lined pipe (TFP) is obtained. By this method, the accuracy of fitting is achieved by plastic expansion of the liner tube and the desired fit-in stress is achieved by thermal shrinkage of the outer pipe. By the use of this manufacturing procedure, many problem inherent to usual lined pipes, such as implosion, stress corrosion cracking and general corrosion, have been solved.

Figure 4.10 - Thermo-hydraulic Fit Method For Tightly Fitted Lined Pipe

Lined pipelines provide a cost effective alternative to conventional corrosion mitigation methods such as chemical injection, extensive de-oxygenation, use of corrosion resistant alloys (CRA) and the inclusion of sacrificial corrosion allowances in the pipeline wall thickness.

Plastic lined pipelines consist of a carbon steel pipe internally lined with a close fitting polymer pipe. This plastic pipe (known as a liner) acts as a barrier between the transported fluid and the steel pipe. Conventionally the liner material used has been either medium density or high density polyethylene (MDPE or HDPE).

Internal lining of carbon steel pipelines has been successfully used onshore in both hydrocarbon and water injection service and offshore in water injection pipelines in place of existing corrosion protection methods. When compared to carbon steel pipelines, operational costs can be reduced as no corrosion inhibitor is required and carbon steel costs can be decreased, as there is no requirement for an internal corrosion allowance, although cost of lining must be factored in.

The most significant advantage however, is that plastic lined carbon steel pipelines are less expensive than pipelines fabricated from corrosion resistant alloys.

Conventional polymeric liners have been shown to be susceptible to failure when transporting hydrocarbons and other fluids with gaseous components. Permeation of these components through the liner can result in pockets of gas becoming trapped between the liner and the host pipe which, on depressurisation of the bore, may expand and partially or completely collapse the liner.

Traditionally, onshore, this has been achieved by installing vents at regular intervals and evacuating the gases periodically. This venting system has to date been considered impractical for subsea applications. However, Technip has completed a three year development and testing programme based on a grooved liner system which incorporates a reliable venting mechanism, and which is designed to be installed offshore by the reel lay method.

Figure 4.11 - Grooved Liner System

The plastic lined flowline is a competitor to 13% Chromium pipes and Duplex pipes. The liner also is beneficial for applications with sour service fatigue problems such as steel catenary risers (SCRs).

The perforated liner concept is a method of preventing collapse by drilling holes in the liner at regular spacing, thus allowing the pipeline liner annulus to remain pressure balanced with the bore. By careful selection of the diameter and spacing of the holes, the corrosion prevention properties of the liner can be maintained.

The plastic lined pipeline for hydrocarbon transport (PLHT) system is only suitable for reeling applications at present. Further development and economic analysis is required prior to establishing its viability for S or J-lay installation.

The PLHT system is suitable for the following applications:

The benefit of the PLHT system is that it is a cost effective alternative to Duplex or 13% Cr pipe. The liner provides an effective corrosion barrier which negates the need for costly corrosion resistant alloys or expensive chemical inhibition programmes.

The liner also insulates the pipe which in some situations will decrease the external coating required, whilst achieving the same OHTC value.

The PLHT system increases the specific gravity of the system due to the liner’s presence. In some situations this may negate the requirement of the system to be trenched and backfilled.

The presence of the liner between the hydrocarbon and the steel pipe leads to a decrease in the steel temperature. This decrease in temperature reduces axial expansion and in turn may equate in the pipe not requiring mechanical restraint, i.e. trenching and backfilling.

In situations where the liner has reduced external coating requirements there will be situations where vessel trips could be decreased.

The design of a deepwater wet insulated rigid pipeline is based on the following considerations:

From the above, a typical wet insulated system consists of (see Figure 4.12, “Figure 4.9: Composition of wet insulated rigid line”):

Figure 4.12 - Figure 4.9: Composition of wet insulated rigid line

A typical pipe in pipe system for reel lay (see Figure 4.13, “Figure 4.9: Typical pipe in pipe system”) consists of a casing pipe housing an insulated wrap flowline pipe, concentrically positioned in the carrier pipe by means of spacer blocks placed every 2.5m on the production flowline. Insulation material is strapped and attached onto the production flowline pipe using galvanic steel, tape or band wire. The assembly of pipe in pipe consists to pull the production pipe into the outer casing pipe in one continuous length while securing spacer blocks and insulation material on production flowline pipe.

A basic pipe in pipe concept applicable for other methods of installation (S-lay, J-lay, Towing) comprises of an inner product carrying pipe inside an outer sleeve pipe. Variations occur when the specific detail of pipe materials, bulkhead configuration, insulation system, field joints or method of fabrication change.

The sleeve (or outer) pipe has a multipurpose role in the design of a pipe in pipe system. Keeping the insulating material dry is one of the most basic requirements when designing and installing an insulated subsea pipeline. An obvious solution is to surround the insulation with an external waterproof sleeve (wet insulated system). The use of a sleeve will also protect the insulation material from mechanical damage during installation and service. It reduces the overall thermal loads within the system and can reduce the installation stresses experienced by the system.

Figure 4.13 - Figure 4.9: Typical pipe in pipe system

Load transfer can occur between product carrier pipe and sleeve pipe, which results in reduced thermal expansion. This is typically achieved by use of a welded connection (bulkhead) between inner and outer pipe that also provides a high integrity water stop (see Figure 4.14, “Figure 4.10.2: Bulkhead assembled between 8-in oil line and 14-in sleeve”).

Figure 4.14 - Figure 4.10.2: Bulkhead assembled between 8-in oil line and 14-in sleeve

During the assembly of the pipe in pipe system, the flowline joints are welded together and insulation field joints are applied to fill the gaps at the welds. The sleeve pipes are joined by fillet welding half-shell steel sleeves over the insulation field joints. Preformed half foam shells are top surface fire resistant to prevent from arc burn during welding operation. Preheating may be required for TIG orbital welding process but there is no post weld heating for welded joints of C-Mn steel pipe having nominal wall thickness less than 49mm.

The main drawbacks with the use of bulkheads in the pipe in pipe system are numerous cool points along the flowline at every bulkhead locations.

This configuration is defined as having multiple flowlines, hydraulic control and service lines, and electrical umbilicals encased or integrated in a single carrier pipe or casing (see Figure 4.15, “Figure 4.10.2: Pipeline in bundle configuration”). As required electrical or circulating fluid heating lines could be integrated to reduce the risk of hydrate and wax plug formation when production lines are shut down or to heat up the production lines before start up.

Figure 4.15 - Figure 4.10.2: Pipeline in bundle configuration

In the deepwater pipeline bundle configuration, all flowlines are wrapped in two half shells of moulded low density, open-cell polyurethane foam, approximately 2-in thick. This isolates the warm space near the flowlines from the cool annular space in the casing. Hard polyurethane spacers are used to secure all flowlines and umbilicals. Rollers wheels on the spacers facilitate flowline bundle pull-in into the outer casing. There are no intermediate bulkheads isolating the annular space of the casing. The entire annular space is permanently pressurised with nitrogen, in order to maintain zero differential pressure across the casing wall at the deepwater end. This allows the casing to resist collapse while minimising the required casing wall thickness and bundle diameter. An unusual aspect of the bundle is the high casing diameter to thickness (D/t) ratios, which approach 100 for deepwater pipeline configuration.

The fabrication of the bundle started with the welding of carrier pipe followed by the pull-in of the internal components as the assembling proceeds. The bundle is fabricated onshore using a relatively inexpensive fabrication spread (compared to an offshore lay vessel), and then is towed to location using a construction spread that may consists of two high bollard pull tugs and a survey vessel.

The design of a pipeline bundle to be installed by tow method is considerably more complex than that of a normal submarine pipeline. The design embraces fabrication, installation and operational requirement, and proceeds from the inner pipe outwards to the carrier pipe with iterations of design where necessary.

Typical design considerations are as follows:

For ultra deep water application (i.e. beyond 1500m WD), there are practical difficulties in the design of the carrier pipe for the tow operation due to:

Note On Asgard project, the annulus is filled with inhibited water at ambient pressure after the tow operation.

In this case, the industry is proposing a wet-insulated bundle based on "syntactic foam" with the adequate thickness (e.g. typically in 30" to 32" diameter shell) to provide:

A thinner carrier pipe (e.g. 6mm – 10mm wall thickness) can be retained for the purpose of "wears" during beach-pull (or bottom-tow method), while providing a mechanical containant.

Under external hydrostatic pressure, this carrier pipe would collapse in a non controlled manner onto the syntactic foam which is pressure resistant.

On completion of towing operation and position confirmed by ROV, the bundle will be connected in diverless mode to the others subsea systems and equipments.