W. H. "Skipper" Strong
Cal Dive International
Cal-Dive's semisubmersible construction vessel, Uncle John, is shown preparing to install coiled line pipe.
There are two companies who manufacture this well-known product using very similar processes and ship to their respective clientele worldwide. The unique feature is the continuous milling process resulting in a pipe string without butt-welds that is 4000 ft or longer depending on diameter and wall thickness and subsequently coiled onto reels.
The idea of using continuous pipe spooled on a reel is not a new concept. The origin dates back to World War II when Operation Pluto was developed some 50 years ago. Prior to the Allied invasion of Normandy, several 3-in. pipelines were prefabricated in 4,000-ft sections by butt welding 30-ft joints and reeling it onto 40-ft diameter spools. The spools were made buoyant and towed behind cable laying ships. Once the Allies had secured the coast of Normandy, the vessels pulled the large spools unreeling the pipe onto the ocean floor and much needed fuel was made available to the invasion forces.
During the 1960s, small diameter continuous tubing technology was being patented and developed for downhole work. The Pluto project also paved the way for development of current coiled line pipe technology. It was a natural evolution to transfer the application of continuous coiled tubing downhole technology to continuous subsea or sub-terrain pipelines.
Also, during the 1960s the development and application for an improved method of reeling and straightening conventional 40 joints of carbon steel pipe was underway. The obvious disadvantage of this system is the lengthy make up area that is required for spooling the conventional butt-welded pipe on the reel.
The only limitations of continuous coiled pipe at this time are diameters greater than 4.5-in. (6-5/8-in. has been manufactured in a small test run) and single spool pipeline lengths. However, if one butt-welds various spool sections together, actual pipeline lengths are unlimited. The pipeline contractor is merely limited by crane capacity for transferring pipe spools at the job site or by the size of the reel filled with pipe that can be placed on the deck of the vessel.
ManufacturingThe carbon content of the material used in the manufacturing of coiled line pipe is very low. For example, standard grade QTP-52 meets or exceeds API X-52 grade line pipe. The chart below is representative of the chemical properties of X52 and X70 line pipe.
Higher-grade steel material is available should the type of service the pipeline will be expected to serve require premium consideration. The coils of steel from the steel mills are rolled and pressed to specific sizes and tolerances. The individual coils are then split into specific strip widths to conform to the various pipe diameters to be manufactured.
After the strip has been sized for a particular pipeline, the coil will be assembled in the required length using the patented strip, bias-weld process. Once the strip weld has been completed the weld is subjected to, among other methods, X-ray, magnetic particle inspection, and hardness. The accumulated strip is now ready to be formed into a continuous length of coiled line pipe.
Pipe is manufactured from the strips by first setting the forming rolls of the mill for the desired diameter of the pipe. The first series of forming rolls encountered by the strip starts bending the edge of the strip upward gradually forming a "U" shape. The formed strip enters a high frequency induction welder. In this high frequency process, the heat is confined to a narrow band along the edges of the formed strip. A special set of insulated rollers squeezes the edges together while they are at the fusion temperature to produce the weld. Electronic resistance welding (ERW) ensures the metallurgy is as near to original specifications as can be reasonably required. During the welding process, the tube is slightly oversized, then reduced to final specifications in a set of sizing rollers. The final tolerance is +/- 0.01 inch.
The outer and inner flash is removed continuously during the manufacturing process. The longitudinal weld seam is annealed to normalize the microstructure of the steel. Once this is accomplished, the continuous pipe is subjected to full body strip relieving. After this is complete the pipe is subjected to air and final water- cooling. The pipe is continuously coiled onto a storage spool.
Once the pipe is stored on this spool it is subjected to its final quality control inspection and testing. All pipe is drifted and hydrotested prior to issuance of the final certificate. The string is tested to 80% of SMYM or to the client's requirements using treated water. Following this test, a wiper ball is run using air or nitrogen depending on the specific requirements. Each string of pipe is issued a final material certification and will have the string number, heat number, chemistry, physical properties, and hydrotest and bias-weld locations and total footage.
After hydrotesting, the entire string of pipe is coated with a two-layer coal tar mastic and high density polyethylene coating system. A continuous two layer coating system is applied to the coiled pipe by first shot blasting the pipe to the required anchor pattern. A coal tar mastic is applied for corrosion control followed by an extruded high-density polyethylene. The final product is continuously inspected for holidays, then water and air cooled before being accumulated on a shipping spool.
ProjectsCoiled line pipe has been installed in the offshore environment in several locations around the world and in a wide variety of water depths. The project details shown in the chart below include the United States Gulf of Mexico only. Lengths vary from as short as 3,000 ft to over 11 miles. Vessels utilized include barges, large workboats, and monohull dynamically positioned dive support vessels to a dynamically positioned multi-service semi-submersible.
Where necessary for safety requirements or government regulation coiled line pipe can and has been buried using conventional jet sleds. These jet sleds utilize a combination of high-pressure water to loosen the soil and high volume airlifts to excavate the material. This allows the pipe to rest on the bottom in the ditch.
Cathodic protection is worthy of some consideration at this point. Since the pipe is continuous and the two-layer coating is continuous, the opportunity for external corrosive forces is greatly diminished. However, as a general protective measure, 35-lb zinc anodes are attached at specified intervals. These intervals may be as close as 500 ft or as far as 2,000 ft, depending on operating parameters determined by the end user. Generally, a CAD weld is made on a base metal spot attaching the wire lead to the pipe and thus the anode, completing the electrolysis system. The anode is attached to the pipe using the bracelet method. Tape may be applied from the cad weld point to the anode providing protection for the wire.
PricingThere are many factors that must be evaluated in determining the installed cost of coiled line pipe:
- The pipe itself generally is more expensive than conventional stick pipe in 40-ft joints. The major cost factors are material quality and inspection procedures.
- This cost is offset by (a) ease in handling coils versus loose joints, (b) the rapid laying time of unspooling the coils versus making welds every 40 ft on location, (c) fewer number of anodes to be installed because of the continuous coating versus a sleeve every 40 ft, and (d) improved flow characteristics of the continuous pipe versus the internal weld met of conventional pipe, the latter, which causes turbidity in the line.
- Reasonable vessels of opportunity can be utilized, particularly those with adequate deck space, suitable craneage, and proper station keeping characteristics.
- Small crew sizes consisting of experienced personnel keep mobilization costs to a minimum.
- Equipment lay out is simple and therefore completed in a very reasonable period.
- Standard pigging and testing may be employed upon completion of the installation of the pipe burial if required, riser installation and/or tie-ins flanged up securely.
ConclusionThe factors, which make this product effective in offshore field development programs, are:
- The high quality low carbon material and manufacturing process combine proven ingredients for reliability.
- Continuous quality control in all of the stages ensures dependability.
- The installation process can be adapted to a variety of vessels in a wide range of water depths with few geographical limitations.
- Installation time is rapid thereby affording the opportunity to select weather windows of convenience and provide the earliest possible production date.
- The continuous line length improves flow characteristics and reduces external corrosion considerations.
Copyright 1998 Oil & Gas Journal. All Rights Reserved.