Applications of Pipework
Pipework is extensively used throughout an offshore installation to move fluids and gases from one location to another. It can generally be classified into the following three broad groupings:
Process
Used to convey air, water, etc. to where it is needed for processing, life support and other services or utility functions.
Transportation
Usually large diameter pipelines as used to carry the production products from installation to installation or from the field to the onshore terminal.
Pumps and compressors are used to drive fluids and gases along pipes and valves to route and control the various substances and ensure that they are correctly segregated from each other.
The contents of the pipework are carried at widely varying temperatures, pressures and flow rates and,therefore, different types of pipework and associated equipment are required.
Because of the inherent danger in carrying the oil and gas associated with offshore operations, the design,installation, testing and inspection of certain pipework is ngourously controlled to exacting standards, so that leakage and bursting do not occur.
Design Features
Pipe Materials
Pipes are made in a number of materials, the particular one chosen being dependent upon pressure,temperature, resistance to corrosion, cost etc.
The most commonly used is carbon steel and for process work, this is normally of seamless construction.
It is strong, weldable, ductile, and usually cheaper than pipe made from other materials. It can stand temperatures up to 750ºF and is used whenever it can stand the duty required of it.
Other metals and alloys are sometimes used although they tend to be more expensive. Traditionally, corer and copper alloys were used for instrument lines although they have largely been replaced by stainless steel. They are still used for heat transfer equipment because of their high thermal conductivity.
Pipe can be lined or coated with materials such as vitreous substances, to provide resistance to chemical attack, corrosion, etc.
GRP (Glass Reinforced Plastic) is commonly used offshore on smaller service/potable water lines.
Pipe Sizes
The wall thickness of pipe used is determined by the pipework designer, taking into account the internal pressure, mechanical stresses to which it is subjected (i.e. dead/live loads and expansion stresses), the corrosion allowance and the safety factor to be applied. Wall thickness is determined in the ANSI system by ‘Schedule Number”, Schedule 40 being the most generally used.
Pipe size is determined by the design requirements of flow rate and head loss. Pipe sizes are identified by the Nominal Pipe Size (NPS). It is common practice to refer to Nominal Pipe Sizes 0-12 inches diameter as Nominal Bore (NB) and greater than 12 inches diameter as Outside Diameter (OD).
Methods of Joining Pipe
There are three main methods of joining pipes together and attaching fittings to them. Lines of 2 inch or larger are usually butt-welded, this being the most economic, leak-proof method. Smaller lines are usually joined by socket-welding or screwing.
Where larger diameter piping is required to join up with flanged vessels, valves and other equipment, or where the line has to be opened for periodic cleaning, bolted flange joints are used instead of butt-welding.
These are described more fully later.
Butt-Welded Systems Fittings
Elbows
These are used for making 45º or 90º changes in the direction of the pipe run. Normally used are “long radius”, in which the centre line radius of curvature is equal to 1 1/2 times the nominal pipe size (MPS). Also available are “short radius” in which the centre line radius of curvature is equal to the NI’S.
Reducing Elbow
This makes a change in line size together with a change in direction.
Return Pipe Bend
A return makes a 180 change in direction and is used in the construction of heating coils, etc.
Pipe Bends
Bends are made from straight pipe and common bending radii are 3 and 5 times the NI’S (indicated by 3R and SR respectively).
Reducer
This joins a larger pipe to a smaller one.
Flange
Is a welding-neck flange (the most common type) and a slip-on flange. Flanges are fitted to the ends of pipes, valves, vessels, etc. to enable them to be connected by bolting.
Tee
Socket-Welded and Screwed Systems
Their uses are similar to those described for butt-welded fittings.
Flanged Joints
As described earlier, flanged joints are used whenever the pipes, valves, vessels, fittings etc. require to be connected together by bolting for ease of dismantling and reassembly.
This section describes types of flanged joints, which are commonly encountered.
Flat-Face
Most commonly used for mating with non-steel flanges on the bodies of pumps, valves, etc. The gaskets used (see Gaskets below) have an outside diameter equal to that of the flange itself. This ensures an even pressure distribution across the flange and reduces the risk of cracking of cast-iron or bronze flange on tightening or from plant vibration.
Raised Face
The raised face is the most common type of flange, in which the gasket covers only the raised faces.
Ring-Type Joint (RTJ)
Gaskets
Gaskets are used to make a tight leak-proof seal between two joint surfaces. For pipe flanges, the common types of gaskets are the full-face and ring types which are used for flat-face and raised-face flanges respectively.
Gaskets are made from compressed asbestos, asbestos-filled metal (spiral-wound) and other materials dependent on the conditions to which they are subjected. Spiral-wound gaskets separate cleanly and can often be re-used.
They are useful, therefore, if the joint has to be frequently disconnected. The finish on the joint faces differs according to the type of gasket to be used. A “serrated” face is used with asbestos gaskets and a “smooth” face with spiral-wound ones.
Line Isolation and Blinding
Frequently, a completely leak-proof means of stopping the flow in a line has to be made. This may be because:
• The line, or a piece of equipment in it, has to be isolated to allow maintenance work to be carried out;
• A change in the process requires that the line be closed.
Valves do not offer complete security, as there may always be some degree of leakage and therefore, the line is closed by one of the following methods:
Spectacle Plate and Line Blind: The spectacle plate can be changed over quickly without disturbing the pipework and gives immediate visual evidence of whether the line is open or blinded. it is generally preferable to the simple line blind which is only used where frequent changing is not required.
Line Blind Valve: This allows a line to be quickly and simply blinded by a process operator. There are many types, but a typical one, a spool type line blind.
Removable Spool and Blind Flanges: This method involves removing a complete section of the line between two flanges (the spool) and fitting blind flanges to close the two ends of the line. This gives a very positive visual indication that the line is closed. Blind flanges are used to close any pipe end, vessel entry, etc.
Pipe Supports
Methods of supporting pipework vary greatly, but a selection of some of the more common is covered in this section.
Support: The term “support” refers to any device used to carry the weight of the pipework. Supports are usually made from structural steel.
Hanger: A hanger is a particular type of support by which pipework is suspended from a structure.
Hangers are usually adjustable for height
Anchor: An anchor is a rigid support, which prevents transmission of movement along pipework.
Tie: An arrangement of rods, bars, etc. to restrict movement of pipework.
Dummy Leg: An extension piece of pipe or steel section welded to an elbow.
Guide or Shoe: A means of allowing a pipe to move along its length whilst restricting its lateral movements.
Operation
Checks During Operation
The operation of a piping system is dictated by the operation of the equipment, which it connects. Nevertheless, care must be taken at all times to ensure that
• The piping is not operated beyond its design range of pressure and temperature;
• All joints are checked regularly for leaks and any leaks discovered are reported immediately;
• The piping is correctly isolated and purged, if necessary, before any maintenance work is performed on it;
• Line markings are clearly visible and re-made if not;
• Any abnormal vibration, damage, missing supports, etc are reported immediately.
Maintenance and Inspection
Legislative and other statutory requirements dictate the type and frequency of maintenance and inspection required on piping systems installed on offshore Installations. This maintenance and inspection is necessary to ensure that the Certificate of Fitness of the installation in question remains valid. The responsibility for ensuring that these requirements are met does not lie with the process operator.
However, he will be involved in isolating. purging, etc. at the time the maintenance and inspection are carried out.
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