Pipes and tubings are vital elements in upstream oil and gas. From control lines to electrical lines, tubings can be used in diverse applications. In deepwater environments, coiled tubings could run up more than 70,000 feet to produce hydrocarbon.

Here, seamless and welded tube and pipe provider RathGibson, discusses the points to consider when choosing welded, welded and drawn, or seamless tubing or pipe in various applications. The US-based firm recently invited regional players to a forum in Dubai to give an overview of its business, its product lines and its best advice on seamless versus welded. Following the forum, David Manuel talks to experts at the company.

Choosing between different types of tubing or pipe is complex. How are they different from each other? Where do each best fit?

“Welded” can mean longitudinal seam welded tubing manufactured by an autogenous (without filler metal) fusion welding process, as opposed to tubing manufactured by other welding processes, such as solid-state processes.

Welded tubing is made by forming flat products (strip, sheet or plate) into the desired shape, in this case, normally round. Once the desired shape has been achieved, a high energy source is used to melt the metal locally at the weld joint. It is squeezed together and allowed to solidify, forming a weld bead. The high energy source may be an electric arc, a plasma arc, a laser beam, or even an electron beam.  The as-welded weld bead is typically somewhat thicker than the adjacent base metal and needs to be modified to match the base metal thickness, and to correct the undesirable physical, chemical and corrosion resistance attributes of the weld.

Some manufacturers will simply remove the excess material of the weld bead by scarfing the inside portion and either grinding or scarfing the outside portion.  This method of weld bead modification only changes the physical dimension and leaves the undesirable as-welded physical, chemical and corrosion resistance properties as they were.

To properly modify this condition the weld bead is cold worked locally and is given a solution anneal heat treatment.  This results in a microstructure that exhibits the same physical, chemical, mechanical, and corrosion resistance properties as the base metal.

Meanwhile, seamless tubing, sometimes referred to as “drawn tubing”, starts with a solid block or bar of steel that is pierced by extrusion, drilling, oxygen lance, or some other means to create a bore through the length of the starting stock.  This is then called a “hollow”. The hollow is then extruded through a die and mandrel combination to simultaneously reduce the outside diameter and to expand the diameter of the bore. The net result is a reduction in the wall thickness. Before the hollow can be drawn through the die, however, it must be “pointed” – which means one end of the hollow must be tapered to facilitate entry into the die. This tapered section is then cut off and discarded.

Depending on the ductility and malleability of the alloy and the starting and finished sizes this process may need to be repeated several times. Alloys that harden rapidly, like the Hastelloy and Incoloy types, require more cycles than standard austenitic stainless steels like 304 or 316. Because of extreme forces applied in the drawing operations, a very thick high pressure lubricant must be used to preserve both the inside and outside surface integrity.

These lubricants must be removed by cleaning before heat treatment can be performed. The cleaning cycle must use aggressive solvents, and is not always effective on small diameter tubing. The residual lubricants can result corrosion issues in service.

The net result is that all of this handling and additional scrap often results in a more expensive process that has its own unique set of potential defects.

One of the most common problems with seamless tubing is variation of wall thickness around the circumference of the tube at a single point along the length. Because the inside tooling cannot be held in a fixed position and is allowed to float in response to variations in hardness or strength along the hollow, the concentricity of the inside surface relative to the outside surface can become unacceptable.  It is not uncommon for the actual wall thickness of a seamless tube of 2.11 mm minimum, to vary from 2.11 mm to 2.31 mm at a single point. This is one of the reasons that seamless tube is normally ordered as a minimum nominal wall thickness where the tolerance is X.XX mm +20% / -0.  Welded tubing, on the other hand, being made from flat rolled strip material exhibits extremely consistent wall thickness. A 2.11mm nominal wall tube typically shows actual variation of 0.07 mm or less at a point. The variation from production lot to production lot is typically 0.1 mm or less.

The dimensional flaw of welded tubing may be its ovality, or roundness. Seamless tubing has a very round and very consistent diameter as a result of being extruded through a die, with typical measured variations in diameter of +/- 0.025mm for a 25 mm OD size. Roll formed welded tubing on the other hand, typically varies about +/- 0.050 mm to +/- 0.075 mm for the same nominal OD. However, for most applications, good concentricity is more valuable than good ovality. Ovality can be corrected or compensated for during fabrication or installation. A non-concentric (or eccentric) condition cannot.

Welded and drawn tubing is a compromise or combination of the two processes. It combines the positive attributes of each process. It is basically the same as the seamless process, except that the starting hollow is a welded product that has the usual excellent consistency of wall thickness. When the starting wall thickness is consistent, the final wall thickness is consistent.  It also cold works the full cross-section of the metal normally resulting in a very desirable microstructure and its associated properties. Dimensional control is excellent; it is a little bit less expensive than seamless and a bit more expensive than welded.  It can be produced as either finite length sticks or as coil forms in lengths that are only limited by handling and transportation capacities, up to 25,000 meters.

What are the criteria that should be considered?  

Specifying a manufacturing process rather than specifying measurable results in any product is always a slippery slope.  The type of results and the value of measureable results need to be performance-based and to consider application critical attributes. If working pressure is of concern, then a minimum tensile or yield strength or burst pressure value should be considered, along with dimensional attributes like wall thickness and concentricity.

Wall thickness and concentricity should also be of concern when heat transfer rates are an issue. Tubes that exhibit a non-concentric or eccentric geometry may develop hot spots or weak spots at thin or thick sections around the circumference, as well as along the length. This could significantly affect process parameters in a heat exchanger.

If working temperature, either elevated or cryogenic, is of concern, test methods and data representative of the field conditions should be considered.  Basically, if material selection is properly executed, the product form should be insignificant.

What is the advantage of seamless over welded?

When welded tubing is properly manufactured by a reputable supplier, seamless does not have any advantage over welded.

Which is more cost-effective? Why?

From a tube manufacturing standpoint, typically welded is more cost-effective as a result of the minimised labour input and reduced manufacturing scrap.

From a fabrication standpoint, welded is more cost effective because of the reduced number of field orbital welds needed to join individual lengths of tube together to create the umbilical.

Seamless tubes are typically available in fixed finite lengths such as 6 or 12 metres. Welded tube on the other hand is available in continuous lengths up to 25,000 metres. A single continuous tube from a coil of strip material is typically about 500 metres long. A splice weld is made on the strip material at these 500 metre intervals and is cold worked before roll forming the tube. It is then solution anneal heat treated and X-ray examined. The net result is that strip material is infinitely long and the final length of the tube is then limited only by the size of the spool on which it can be coiled and the associated material shipping and handling capabilities.

Because these splice (orbital) welds are made and processed at the factory in a controlled environment the potential for corrosion is significantly reduced as compared to the field orbital welds which cannot be cold worked and are typically not heat treated.  The microstructure and the physical, chemical and corrosion resistance properties of the factory welds are virtually identical to those of the base metal.

In an oil and gas application, such as control lines, downhole and umbilical applications, what is more advisable between the two product types?

When purchased from a reputable reliable supplier, welded tubing can offer advantages of economy without sacrifice of performance. The economy is realised in both initial purchase costs and in time and labour in fabrication / installation. Coiled welded tubing can be supplied with a splice (orbital) weld that has been cold worked and solution annealed at intervals of about 500 metres. The maximum distance between splice welds in coiled seamless tubes is typically about 30 metres.  The wall thickness dimension control of welded tubing is superior to that of seamless tubing. Both products must meet the same minimum tensile strength and burst strength requirements. Both must meet the same corrosion testing requirements. Both must meet the same chemical composition requirements. The differences lie in the efficiencies of the manufacturing method.

According to industry players, the region’s specifications are inclined towards seamless pipes and tubes. Why is there a preference/bias over its welded counterparts? Is it a manufacturing issue?

One of the biggest roadblocks to the implementation of welded tubing is the perception that the weld itself is a defect and ergo, welded tubing contains one continuous defect along its entire length. It is perceived as a weak spot in an otherwise continuous material. For many years, the industry was not able to provide a suitable quality welded product.  However, since the 1950’s, the industry has advanced significantly and modern day seam welded tubulars, from reputable conscientious manufacturers, perform equally as well as seamless products in field service.  If the seam weld can be identified by the naked eye, some consider it a defect.  This is the attitude that has prevailed in the industry for so long. One needs only to look at the microstructure and physical and corrosion test data to see that this is not necessarily so. A properly processed fusion weld is nearly indistinguishable from the base metal in a metallographic laboratory examination.  A properly processed weld exhibits the same physical and corrosion resistance properties as the base metal.

Do you think seamless are more marketable than welded tubes and pipes? Seamless is marketed on an outdated myth that it does not have flaws but that welded has an inherent flaw throughout the length of each tube, as stated above. Intuitively this concept is easily accepted by purchasers and designers with limited experience with the various products.  Each product form and manufacturing method has its own inherent problems and potential defects.

Consumers must become familiar with what those problems and defects are, and how they might affect particular applications. Then they can make an informed decision based on facts, rather than myth. It is easy to market a manufacturing process and to have it specified in consumer documents rather than market on an ‘in service performance’ basis. Once a process is written into a specification it is normally very difficult to change that perception and requirement.  It is easy to keep the status quo as long as it appears to be working. There is often not sufficient motivation to evaluate alternative materials and manufacturing methods. It is usually a matter of economy that leads to change.

Welded tubing is acceptable as per most pressure vessel codes, and with additional non-destructive examination it can be used in place of seamless for lethal service applications. Why not for energy applications?

Are there specific industrial standards the region has to follow?

The industrial standards required are related to the specific industry.  The American Petroleum Institute standards are the primary standards followed in the Middle East. However, tubing specifications used most widely are the ASTM and ASME standards.  The National Association of Corrosion Engineers (NACE) standards and guidelines are also commonly used.