Designing an OCTG finishing floor for welded pipe
Meeting API standards step by step
Although the energy market has seen ups and downs, overall the trend is a growth path. A manufacturer of welded structural and mechanical tube and pipe can get into this market by using its existing mill to manufacture oil country tubular goods (such as line pipe and casing) and adding a finishing floor to its operations. Understanding API requirements is the first step in planning a finishing floor layout.
The current economic climate has caused pipe producers to search for new ways to generate business by expanding their product offerings. One way is to branch out into the oil country tubular goods (OCTG) market. Many companies with experience in producing structural and mechanical tubing are investigating ways to produce electrical resistance welded (ERW) tube and pipe that can be used as OCTG that meet American Petroleum Institute (API) specifications.
OCTG can be produced as seamless or welded tube and pipe. A key difference between welded and seamless is the maximum grades, or yield strengths, that can be produced. The seamless process can achieve higher grades and heavier wall thicknesses than welding can. Typically, welded pipe producers entering the OCTG market concentrate on producing line pipe (API 5L) and in some cases some low grades of API casing (J, K, or N grades of API 5CT).
A key benefit of this strategy is making use of an existing weld mill. A mill that can roll mechanical tubing and structural shapes can roll OCTG line pipe and casing pipe. A second benefit is that these products do not require heat treating, so pipe producers do not need to invest in heat-treating equipment.
Pipe producers can go in one of two directions. Some use ERW to make "green pipe" for OCTG—pipe that is rolled to meet some of the API's requirements, such as the OD and wall thickness. This pipe then is cut to length and shipped. This pipe is not qualified to receive API certification because it has not met the criteria required in the finishing operation.
The other direction is to finish the pipe so it meets API standards. Finishing adds value to the product. Although a finishing operation is a significant capital investment, adding one can help a structural or mechanical tube producer maximize the product's value.
Planning the Space and Layout
The first considerations concern space and layout.
A finishing floor typically requires the same amount of floor space as a weld mill. Finishing processes require a certain flow, and the layout usually can be adapted to fit the available space. Most producers want an inline flow from the mill directly to the finishing floor. However, the flow requires careful planning because a weld mill typically runs faster than a finishing floor. To compensate for the throughput differential, a typical finishing floor needs pipe storage areas, buffer sections, and runout conveyors. One rule of thumb is that the finishing floor should run three shifts to every two shifts that the weld mill runs.
The first finishing process that the pipe undergoes after being cut to length is blowout or flushout. This removes most of the ID weld scarf and cutting chips from the pipe and sends the chips and debris to a central collection point. This protects conveyor lines and other finishing equipment from damage.
Cutting and Testing. The next processes are crop cutting and testing, which are necessary to determine weld seam integrity. The operation must be located close to the end of the weld mill.
API specification requires cutting welded OCTG samples from certain positions on every coil at regular intervals. These samples undergo a series of tests. Crush presses are necessary to comply with the API flattening test requirement. This area also is integrated with laboratory testing processes.
Straightening. The next step is straightening the pipe. One of the most effective and productive means of straightening round tubes is an opposed roll, cross-roll machine. Cross-roll straighteners have rolls mounted at an angle to the line of pass so that the material rotates as it proceeds through the machine. On this type of machine, the rolls are opposite each other, usually in a six- or 10-roll configuration. This machine design allows the pipes to be straightened by deflection as well as crushing forces, providing a rounding-up effect to the pipe.
End Prep. The next process is end facing and beveling. This station consists of two machines, one at each end of the pipe. This system typically has integrated equipment that receives the pipe from the finishing floor's material handling system and automatically positions the product for end facing. After the ends are prepped, the machines perform the final end finishing step. For an API 5L line pipe, this is a weld bevel. Most casing products require threading after finishing, which requires additional equipment.
Upon completion of the end finishing process, the product is returned to the finishing floor's material handling system, where the pipe is conveyed to the next process.
Hydrostatic Testing. All API products require hydrostatic pipe testing. The hydrostatic pipe testing station has its own handling system, separate from that of the finishing floor.
The first stop in the hydrostatic area is the flushout station, which uses high-pressure water to flush the inside of the pipe. This removes cutting chips from the previous end finishing process and any other debris that may have been picked up in the process. The flush station has its own water pit to prevent debris from entering the testing system.
The pipe then moves to an alignment station, which aligns one pipe end before the pipe enters the machine. After the pipe is aligned, it enters the hydrostatic pipe testing bed. API requires the weld seam to be positioned so that the operator can watch for leaks during the test; therefore, a seam location device must be part of the hydrostatic pipe tester design.
The purpose of the hydrostatic pipe tester is to test the pipe under pressure. The pipe is filled with water and pressurized to a point mandated by the API. Once the pipe reaches the mandated pressure, the pressure must be held for a defined time to verify the integrity of the pipe and the weld. If the pipe passes, it moves on to the next station. If the pipe fails, it is removed from the system and inspected to determine the problem's cause.
Because the hydrostatic pipe tester must hold the pipe in process for a set period of time, this station typically is the bottleneck on the finishing floor. However, some hydrostatic pipe testers have several heads and therefore can handle several pipes at once. This requires a more intricate handling system than one that handles one pipe at a time.
The next station is a tilt-drain station. This station drains residual water from the pipe. The station may also incorporate an air-blowout nozzle to dry the ID of the pipe as much as possible. Then the pipe is returned to the finishing floor's material handling system.
Next is the nondestructive testing (NDT) area. Typically, this is a series of tests and inspections, such as a full body lamination test, weld seam test, and end inspection. This area also includes a rework area with integrated conveyors to correct minor imperfections.
Weigh, Measure, Stencil. The pipe is then transferred to the weigh, measure, and stencil (WMS) area of the finishing floor. This station uses a weighbridge and scale system custom-configured and specifically designed for pipe producers. This station also includes a length measuring system designed for rapid measurement and display of the pipe's length. The pipe weight and length information then is transferred to the stencil station where it is inked onto the pipe's surface.
Next is the pipe coater, which applies a coating on the exterior of the pipe to delay the onset of rust. Many types of coaters, coatings, and drying units are available. The simplest process uses an inline contained-box-type coater and air drying.
In most cases, the pipe then is transferred to the shipping area. If the pipe is less than 4 inches in diameter, it may be sent to a bundling station.
According to the Energy Information Administration, international demand for petroleum grew from 63.1 million barrels per day (BPD) to 85.9 million BPD from 1987 to 2007. Although demand fell slightly in 2008, the long-term outlook is for continued growth in the petroleum extraction industry.
Adding an OCTG finishing floor with processes that meet API standards will allow welded pipe producers to diversify their operation and take advantage of this trend. Although welded pipe producers will not be able to compete for high grades of heat-treated seamless products, they may find that they can produce welded line pipe and some casing grades in a cost-effective manner.
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