Piping Elbows and Bends are very important pipe fitting which are used very frequently for changing direction in piping system. Piping Elbow and Piping bend are not the same, even though sometimes these two terms are interchangeably used.

A BEND is simply a generic term in piping for an “offset” – a change in direction of the piping. It signifies that there is a “bend” i.e,  a change in direction of the piping (usually for some specific reason) – but it lacks specific, engineering definition as to direction and degree. Bends are usually made by using a bending machine (hot bending and cold bending) on site and suited for a specific need. Use of bends are economic as it reduces number of expensive fittings.

Bend.

An ELBOW, on the other hand, is a specific, standard, engineered bend pre-fabricated as a spool piece  (based on ASME B 16.9) and designed to either be screwed, flanged, or welded to the piping it is associated with. An elbow can be 45 degree or 90 degree. There can also be custom-designed elbows, although most are catagorized as either “short radius” or long radius”.

Elbow.

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.

Whenever the term elbow is used, it must also carry the qualifiers of type (45 or 90 degree) and radius (short or long) – besides the nominal size.

Elbows can change direction to any angle as per requirement. An elbow angle can be defined as the angle by which the flow direction deviates from its original flowing direction (See Fig.1 below).Even though An elbow angle can be anything greater than 0 but less or equal to 90°But still a change in direction greater than 90° at a single point is not desirable. Normally, a 45° and a 90° elbow combinedly used while making piping layouts for such situations.

Elbows or bends are available in various radii for a smooth change in direction which are expressed in terms of pipe nominal size expressed in inches. Elbows or bends are available in three radii,

• Long radius elbows (Radius = 1.5D): used most frequently where there is a need to keep the frictional fluid pressure loss down to a minimum, there is ample space and volume to allow for a wider turn and generate less pressure drop.
• Long radius elbows (Radius > 1.5D): Used sometimes for specific applications for transporting high viscous fluids likes slurry, low polymer etc. For radius more than 1.5D pipe bends are usually used and these can be made to any radius.However, 3D & 5D pipe bends are most commonly used
• Short radius elbows (Radius = 1.0D): to be used only in locations where space does not permit use of long radies elbow and there is a need to reduce the cost of elbows. In jacketed piping the short radius elbow is used for the core pipe.

Here D is nominal pipe size in inches.

There are three major parameters which dictates the radius selection for elbow:

1. Space availability,
2. Cost, and
3. Pressure drop.

Pipe bends are preferred where pressure drop is of a major consideration.

Use of short radius elbows should be avoided as far as possible due to abrupt change in direction causing high pressure drop.

source:

• http://www.whatispiping.com/piping-elbows-and-bends
• http://www.marineinsight.com/marine/marine-news/headline/pipes-and-bends-an-essential-guide-for-second-engineers-part-2/

Pipeline Thermal Insulation And Pipe In Pipe (Pip) As A Solution

Pipeline thermal insulation is a prevention due to temperature difference between pipeline outer surface and inner part of the pipeline. Pipeline thermal insulation is considered as important since its contribution to corrosion as well as wax and hydrate problems in offshore pipeline system.

Pipeline insulation as figured below:

1. Condensation That Lead to Corrosion

Pipes operating at relatively low temperature (with hot fluid flows inside, e.g. offshore pipeline) increases the potential for existing water vapour to condense on pipe surface. And this moisture may lead to corrosion on the pipeline surface.

2. Wax and Hydrate Formation

In oil and gas industry, excessive cooling of the product during transportation can result in drop out of high molecular weight waxes and asphalt. This happens due to the working temperature of the pipeline (deep in the depth, above the seabed where the temperature is relatively low). In wet gas systems, hydrate formation can block pipelines (flow).

INSULATION METHODS

There are few methods that can be used for insulation, such as use of cooling spool, material selection, and pipe in pipe (PIP).

Use of Cooling Spools 

There can be significant cost benefit from cooling the product stream from very high temperature wells to minimize the thermal expansion forces and then maintaining this lower temperature for through efficient pipeline insulation reducing the volume of post lay rock dump or trenching required or enabling more conventional materials and analysis techniques to be employed.

Material Selection

Typically, pipeline insulation must be able to withstand the stresses imposed as a result of the installation methods and strong enough to withstand constant external pressure and function effectively when submerged and saturated.

The dry, load-free environment within the annulus of the pipe allows non-typical insulation materials with much lower thermal conductivity to be applied subsea than has historically been possible e.g. rock-wool systems etc. There are also few materials used for insulation have been around for a few decades and include polypropylene, polyurethanes, epoxies and rubbers.

Figure below shows installation of material for pipeline insulation:

PIPE IN PIPE (PIP)

Pipe in Pipe (PIP) is installation of a second pipeline around the product pipeline from the seawater surrounding it and creates a dry chamber around the pipeline that can be engineered to accommodate a range of advanced insulation techniques.

Figure above illustrates Pipe in Pipe technology for offshore pipeline.

source :

• http://www.jpkenny.com/SiteCollectionDocuments/PIPE%20IN%20PIPE%20REV%2001.pdf
• http://en.wikipedia.org/wiki/Pipe_insulation
• http://www.iploca.org/platform/content/element/10744/brederoshaw_pds_thermotiteultra.pdf

Midline Tie-in

Midline Tie-in or Above Water Tie-in (AWTI) is an operation where two laid down pipelines on the seabed are welded together after being lifted above water using vessel davits. 

For AWTI we determine/provide:

• Steps for recovering the pipelines
• Welded Configuration for recovered pipes
• Steps for lowering the completed pipeline
• Weld excavation analysis
• Minimum weld thickness assessment for removal of the welding clamp
• Offshore Procedures to be followed during execution

Static Code checks (pipeline integrity) are performed for every static loadcase. Dynamic Analysis is performed for the respective worst case in Pipe Recovery, Welded configuration and Laydown. DNV buckle checks are used to ascertain pipe integrity during dynamics.

Software Employed: Orcaflex

source : http://www.oesl.nl/expertise/pipelay

Offshore Pipeline Corrosion Protection And Prevention

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:

source :

• Fontana, Mars G. Corrosion Engineering. Singapore. 1987.
• http://en.wikipedia.org/wiki/Cathodic_protection
• http://met-engineering.blogspot.com
• http://www.tenaris.com/en/Products/OffshoreLinePipe/Coating/ExternalAnticorrosion.aspx

Flexible Riser

Conduits to transfer materials from the seafloor to production and drilling facilities atop the water's surface, as well as from the facility to the seafloor, subsea risers are a type of pipeline developed for this type of vertical transportation. Whether serving as production or import/export vehicles, risers are the connection between the subsea field developments and production and drilling facilities.

Multiple Riser ConfigurationsSource: www.atlantia.com

Similar to pipelines or flowlines, risers transport produced hydrocarbons, as well as production materials, such as injection fluids, control fluids and gas lift. Usually insulated to withstand seafloor temperatures, risers can be either rigid or flexible.

Types Of Risers

There are a number of types of risers, including attached risers, pull tube risers, steel catenary risers, top-tensioned risers, riser towers and flexible riser configurations, as well as drilling risers.

The first type of riser to be developed, attached risers are deployed on fixed platforms, compliant towers and concrete gravity structures. Attached risers are clamped to the side of the fixed facilities, connecting the seabed to the production facility above. Usually fabricated in sections, the riser section closest to the seafloor is joined with a flowline or export pipeline, and clamped to the side of the facility. The next sections rise up the side of the facility, until the top riser section is joined with the processing equipment atop the facility.

Also used on fixed structures, pull tube risers are pipelines or flowlines that are threaded up the center of the facility. For pull tube risers, a pull tube with a diameter wider than the riser is preinstalled on the facility. Then, a wire rope is attached to a pipeline or flowline on the seafloor. The line is then pulled through the pull tube to the topsides, bringing the pipe along with it.

Building on the catenary equation that has helped to create bridges across the world, steel catenary risers use this curve theory, as well. Used to connect the seafloor to production facilities above, as well as connect two floating production platforms, steel catenary risers are common on TLPs, FPSOs and spars, as well as fixed structures, compliant towers and gravity structures. While this curved riser can withstand some motion, excessive movement can cause problems.

Top-Tensioned RisersSource: www.atlantia.com

Used on TLPs and spars, top-tensioned risers are a completely vertical riser system that terminates directly below the facility. Although moored, these floating facilities are able to move laterally with the wind and waves. Because the rigid risers are also fixed to the seafloor, vertical displacement occurs between the top of the riser and its connection point on the facility. There are two solutions for this issue. A motion compensator can be included in the top-tensioning riser system that keeps constant tension on the riser by expanding and contracting with the movements of the facility. Also, buoyancy cans, can be deployed around the outside of the riser to keep it afloat. Then the top of the rigid vertical top-tensioned riser is connected to the facility by flexible pipe, which is better able to accommodate the movements of the facility.

First used offshore Angola at Total's Girassol project, riser towers were built to lift the risers the considerable height to reach the FPSO on the water's surface. Ideal for ultra-deepwater environments, this riser design incorporates a steel column tower that reaches almost to the surface of the water, and this tower is topped with a massive buoyancy tank. The risers are located inside the tower, spanning the distance from the seafloor to the top of the tower and the buoyancy tanks. The buoyancy of the tanks keeps the risers tensioned in place. Flexible risers are then connected to the vertical risers and ultimately to the facility above.

Hybrid Riser SystemSource: www.2hoffshore.com

A hybrid that can accommodate a number of different situations, flexible risers can withstand both vertical and horizontal movement, making them ideal for use with floating facilities. This flexible pipe was originally used to connect production equipment aboard a floating facility to production and export risers, but now it is found as a primary riser solution as well. There are a number of configurations for flexible risers, including the steep S and lazy S that utilize anchored buoyancy modules, as well as the steep wave and lazy wave that incorporates buoyancy modules.

While production and import/export risers transfer hydrocarbons and production materials during the production phase of development; drilling risers transfer mud to the surface during drilling activities. Connected to the subsea BOP stack at the bottom and the rig at the top, drilling risers temporarily connect the wellbore to the surface to ensure drilling fluids to not leak into the water.

source : http://www.rigzone.com/training/insight.asp?insight_id=308&c_id=17#sthash.VF8BSyyA.dpuf

Pipe-In-Pipe Technology

Pipe in Pipe (PIP) is installation of a second pipeline around the product pipeline from the seawater surrounding it and creates a dry chamber around the pipeline that can be engineered to accommodate a range of advanced insulation techniques.

Figure above illustrates Pipe in Pipe technology for offshore pipeline.

Pipe-in-Pipe Offshore Construction

1. Fixed

• suitable for majority of lay vessels
• field joint uses bulkheads or swaged connectors, no movement between flowline and carrier pipe
• permits vacuum in annulus
• heat loss from flowline to carrier pipe through metal connectors

2. Sliding

• S-lay & J-lay
• uses a butt weld on carrier pipe and allows the flowline to move freely
• flowline may or may not have alignment spacers
• thermal insulation may be on the outside if flow-through-annulus active heating is used

3. Restrained

• polymer bulkheads to hold insulation material in place
• these bulkheads transfer the load during installation and provide concentric alignment

Flow Assurance

Insulation Capabilities

Pipe-in-Pipe Overview

Deepwater Pipe-in-Pipe

Active Heating Pipe-in-Pipe

source : http://www.deepsea-eng.com/sites/deepsea-eng.com/files/media/DeepSea%20Pipe-in-Pipe%20technical%20presentation.pdf

Subsea Tie-in System

Subsea flowlines are used for the transportation of crude oil and gas from subsea wells, manifolds, off-shore process facilities, loading buoys, S2B (subsea to beach), as well as re-injection of water and gas into the reservoir. Achieving successful tie-in and connection of subsea flowlines is a vital part of a subsea field development.

Subsea fields are developed using a variety of tie-in solutions. Over the past decade, FMC Technologies has developed a complete range of horizontal and vertical tie-in systems and associated connection tools used for the tie-in of flowlines, umbilicals and jumper spools sizes 2” - 36” and for single and multibore application. FMC’s horizontal and vertical tie-in systems have been extensively installed in many of the deepest, highest pressure and largest diameter subsea applications around the world.

Vertical Tie-in System

Vertical connections are installed directly onto the receiving hub in one operation during tie-in. Since the Vertical Connection System does not require a pull-in capability, it simplifies the tool functions, provides a time efficient tie-in operation and reduce the length of Rigid Spools.

Stroking and connection is carried out by the the Connector itself, or by the ROV operated Connector Actuation Tool (CAT) System.

Horizontal Tie-in System

Horizontal Tie-in may be used for both firstend and second-end tie-in of both flowlines, umbilicals and Jumper spools. The termination head is hauled in to the Tie-in point by use of a subsea winch. Horizontal Tie-in may be made up by Clamp Connectors operated from a Tie-in tool, by integrated hydraulic connectors operated through the ROV, or by non-hydraulic collet connectors with assistance from a Connector Actuation Tool (CAT) and ROV. Horizontal connections leave the flowline/umbilical in a straight line, and is easy to protect if overtrawling from fishermen should occur.

source :

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CC0QFjAA&url=http%3A%2F%2Fwww.fmctechnologies.com%2F~%2Fmedia%2FSubsea%2FTechnologies%2FTieInSystems%2FColleteral%2FSubsea%2520Tie%2520In%2520Systems_low%2520res.ashx%3Fforce%3D1%26track%3D1&ei=JhXuUrb6B4mPrQfo5YDoBA&usg=AFQjCNFjkS1ah2m-bzSTWjfoH7Jo02ZvMw&bvm=bv.60444564,d.bmk