Flexible Steel Pipe Applications

Transcription

1 Flexible Steel Pipe Applications Dana Fraser 1 REFERENCE: Dana Fraser, Flexible Steel Pipe Applications, Fourth International Conference On Composite Materials For Offshore Operations. Houston, TX, October 4 6, ABSTRACT Onshore flexible steel pipes are increasing replacing stick steel pipes in hydrocarbon gathering and injection applications. More than 100 miles of flexible steel pipe has been successfully deployed in a number of oil and gas gathering and injection applications for more than 30 operators since the product was commercialized in FlexSteel flexible steel pipes have corrosion resistance and flow characteristics superior to rigid steel pipes, and their ease of installation and elimination of field welds result in greater economy on an installed cost basis than rigid steel pipe. One big advantage of flexibles over rigid steel pipe is the relatively narrow rights of way that are feasible with the flexibles. The installations proceed very rapidly, usually 5 to 10 km per day. Flexible steel pipes are ideal for environmentally sensitive applications. Careful equipment selection allows installation with a minimal cleared right of way, and rehabilitation and horizontally directional drilled installations require no preparation along most of the length of the ROW. These pipes were originally developed for use on the North Slope in Alaska, and are well suited to above grade and buried Arctic applications. The exceptional flexibility and durability of the pipe allow above grade installations directly on the tundra, without the requirement for expansion loops or any type of prepared support. Winter operations allow installation using heavy equipment from frozen ground or ice roads down to at least -40 C. This paper summarizes flexible steel pipe properties and presents example installations conducted using a variety of methods. Examples include above ground, trenched, plowed, horizontal directional drilled, and sliplined applications. 1 Wellstream International Limited, Panama City Florida USA 1

2 INTRODUCTION Flexible steel pipes offer performance and cost advantages over traditional welded steel stick pipe. The durable extruded external polymer sheath insolates the steel from the external environment, eliminating requirements for cathodic protection equipment and most coating repairs. Flexible steel pipes have superior flow characteristics because of the low friction factor of the smooth inner liner augmented by the thermal insulating properties of the pipe retaining heat and minimizing viscosity of conveyed hydrocarbons. Together, the inner and outer sheaths give flexible steel pipes excellent corrosion resistance, which is reflected in the long life and high reliability of flexible steel pipe. The total cost of installed flexible steel pipe systems are typically less than those for rigid steel pipe, because installation costs are typically half those of rigid steel pipe. The installation savings result largely from storing and handling long lengths of pipe on a reel, and minimizing the number of field joints with the attendant labor requirements. Operating costs are reduced by the high reliability and corrosion resistance of the flexibles. Periodic inspections and maintenance costs are minimal, and corrosion inhibitors may be eliminated. Traditional flexible steel pipes have been used in demanding conditions offshore for decades. Flexibles are often arranged as dynamic risers tying fixed subsea facilities to floating facilities, while many applications have static flowlines deployed on the seabed or buried in the seabed. Flexible steel pipes have extensive offshore experience, with many thousands of kilometers of installed pipes demonstrating exceptional reliability over decades of use. Traditional offshore flexibles are custom designed for each application. Using the same design tools and techniques, Wellstream has introduced a line of lightweight flexible steel pipes with weights, bending stiffness, and cost optimized for onshore applications. The pipes are true flexible steel pipes, though there are some similarities to reinforced thermoplastic pipes as documented for example, in Reference [1]. FLEXIBLE STEEL PIPE PRODUCT DESIGN The pipe consists of the manufactured pipe body, and the terminations that consist of end fittings with end connectors. The terminations allow attaching flexible steel pipes to standard components and pipes. Pipe Layers and Materials The pipe has four layers, as shown in Figure 1. The innermost layer is an extruded thermoplastic tube that seals in the conveyed fluid. Most liners are of pipe grade polyethylene (PE), but food grade PE or carbon filled PE can be supplied for potable water applications or to meet antistatic requirements. Two contra-wound layers of carbon steel strip reinforcements are spirally wrapped on top of the liner at roughly a 55 lay angle. The steel strips provide strength to resist internal pressure and tensile loads on the pipe, and are of a plain carbon steel strip that combines economy with high strength and good toughness. The tensile layers are not exposed to the bore fluid; instead they are in a considerably milder environment in the annulus between the inner and outer extruded layers. The outermost layer is a thick extruded external shield designed to protect the underlying layers of the pipe from the external environment. This outer shield is of pipe grade PE. The 2

3 standard shield material is a yellow PE formulated with a package of colorants, stabilizers and antioxidants. It is designed for up to three years of outdoor exposure. A black carbon filled PE and a white PE are optional. The black material has high emissivity that results in significant heating during the day with intense incident sunlight, and cools considerably at night. The white PE outer shield has a low emissivity, is specifically formulated to resist long term exposure, and is preferred for application with high intensity incident UV light. Pipe Design Range Standard FlexSteel pipe designs are indicated in Table 1. Most of the pressure ratings match ANSI classes for compatibility with other components, such as valves and flanges. ANSI class 300 corresponds roughly to 5.15 MPa (750 psi), class 400 to 6.89 MPa (1 000 psi), and class 600 to 10.3 MPa (1 500 psi). Flexible Steel Pipe Standards Flexible steel pipes have decades of offshore experience, which is reflected in recognized industry standards developed by the American Petroleum Institute (API). These include API 17J, 17K, and RP 17B, References [2, 3, 4]. ISO specifications have been derived from the first two of these, as References [5,6]. FlexSteel pipe is designed to API 17J requirement, while the end fitting is designed to the very similar API 17K. API RP 17B controls testing and qualification of flexible steel pipe bought to either API 17J or 17K. FlexSteel pipe has some minor deviations from the requirements of the current API documents. In addition to these standards, many jurisdictions have pipeline codes. These codes generally accommodate older materials like PE pipes and rigid fiberglass reinforced pipes, and newer technologies like flexible steel pipes are typically handled with waivers until the codes are updated. End Fitting Design The end fitting must maintain the integrity of the pipe structure, seal to the inner and outer extruded layers, and provide a fixture to transmit tension and pressure loads to the pipe structure. FlexSteel pipe is terminated with a simple and elegant swaged end fitting of the API 17K type, rather than the large and costly API 17J type end fittings. Swaged end fittings have been proven through many years of experience on hoses. Figure 2 presents a 4-inch end fitting ready for assembly onto the pipe, assembled 3-inch and 4-inch end fittings, and a cutaway showing the basic structure of the end fitting design. The end fitting is fabricated from 316L stainless so that the end fitting as well as the pipe has superior corrosion resistance. The system cost is not greatly affected by selecting stainless steel for the end fitting material, since the cost difference between stainless and carbon steel in this application is not great, and only a few end fittings are used in most pipelines. The end fitting is assembled to the pipe by cutting the pipe to the desired length, and forming the end fitting onto the pipe using a specialized hydraulic press. End fitting installation is simple and quick, provides a uniform tight compression on the pipe wall by the end fitting that prevents sensitivity to ambient temperature or cleanliness, and is easily accomplished under factory or field conditions. The end fitting inside diameter after installation is typically 3 mm (0.12-inch) smaller than the inside diameter of the pipe. The end fittings are normally 3

4 electrically continuous with the pipe, and pipe electrical resistance is shown in Table 1. Optionally, end fittings can be fitted without electrically connecting them. The end fitting is terminated to an end connector designed to attach to steel pipe, generally an ANSI raised face flange. A pipe stub end for butt welding to pipes can also be provided. The ANSI flange joint end connector consists of a stainless hub end which is factory welded to the end fitting, and a carbon steel lapped flange. The lapped flange acts as a swivel until the flanges are bolted together, simplifying handling. Flange up is accomplished by installing a standard ANSI raised face gasket, normally a spiral wound metal gasket, and bolting the flanges together with standard studs and nuts. Figure 3 shows end fittings with ANSI flanges attached to a valve, and a pair of mated mid-line end fittings. The steel brackets shown holding the flanges in a fixed location are not used on most installations. Figure 4 shows a butt welded midline connection. Reels Flexible steel pipe is supplied on road portable steel reels as shown in Figure 5. The reel dimensions are generally Ø3.7m (Ø12 ft) x 2.6m (8.5 ft) wide. Roughly 2 km of 2-inch pipe, 1 km of 3-inch or 4-inch pipe, or about 0.5 km of 6-inch pipe fits on a reel. Onshore Flexible Steel Pipe Qualification Wellstream has been qualified to design, analyze, and manufacture flexible steel pipes per API 17B for many years, including meeting API monogram requirements. The qualification is based on reviews of Wellstream processes and test results from many flexible steel pipe designs from various API 17B product families. FlexSteel pipe was designed using API 17J approved methodologies and materials, and the pipe capabilities are well within previously qualified limits of pressure, tension, and collapse. Thus, they can be considered qualified, even without additional qualification testing conducted specifically on FlexSteel pipes. Wellstream has conducted a series of tests per API RP 17B on FlexSteel pipes and end fittings to characterize and further qualify the product. Both the API 17J controlling the pipe design and the API 17K controlling the end fitting design require testing and qualficiation per API RP 17B, and the design methodology and factors are the same for both specifications. The API documents consider the end fittings to be qualified from the burst and axial tension tests conducted on the pipe samples. For conservatism, elevated temperature (end fitting integrity) tests have also been conducted per the ISO Technical Specification for RTP qualification, Reference [7]. FLEXIBLE PIPE APPLICATIONS Wellstream FlexSteel pipe is targeted for use in hydrocarbon production, such as oil and gas gathering lines, water or gas injection lines, and civil or military water and fuel transfer lines. Related uses include mining and agricultural, and utility applications such as gas transmission lines, water distribution lines, and pumped sewage lines where more than a few bar of pressure capability is required. 4

5 Installation Overview Flexible steel pipe is well suited for any of the installation techniques and much of the equipment used with other spoolable products, such as small diameter PE pipe. The installations typically proceed very rapidly, usually 5 to 10 km per day. In most applications, flexibles can be installed using a very narrow right of way compared to those required for rigid steel pipe. Flexible steel pipes are well suited to above ground and buried applications. They have negligible axial strain with internal pressure. Flexible steel pipes easily accommodate thermal strains in above ground applications, and the soil typically fixes buried flexible pipes so they can not move. Thermal strains in buried pipes are readily accommodated by the natural axial compliance of the pipe, with no special measures or derating required. Flexible steel pipe is suited to installation using traditional methods such as surface deployment, trenching, or plowing, or modern trenchless methods such as horizontal directional drilling and sliplining techniques to rehabilitate deteriorating flowlines. Many applications use a combination of methods for best economy. FlexSteel pipe is particularly attractive for rehabilitation because it has its own inherent pressure retaining capability Deployment Flexible pipes are most commonly deployed from a reel being moved along the pipeline right of way. The reel is supported in an installation trailer, or sometime, on a frame set on a flatbed truck. This dynamic deployment is illustrated in Figures [6 8]. Alternately, static deployments can be conducted where the reel is at a fixed location, and the pipe is pulled from the reel by a vehicle for above grade applications, or by HDD equipment or winch for trenchless applications, as shown in Figures [6, 9,10]. Above Ground FlexSteel pipe can be laid directly on the ground without special preparations at any rated temperature. Flexible steel pipe is often installed above ground in semi-arid or desert locations, as shown in Figure 6. Flexible steel pipe has been installed in wildlife refuge areas where regulations prohibit installation of buried pipes. Above ground installations of flexible steel pipe are typically very low cost, and are accomplished simply by deploying the pipe. Above ground flexible steel pipes are ideal for temporary use, and pipelines many km in length can be installed in one or two days on the surface for repairs or early well testing and production. These lines can be later buried or even left above grade for permanent use, or retrieved and reused on other wells. Rigid steel pipes in cold weather applications are often installed above grade on vertical support members. Sufficiently insulated flexibles may be laid directly on the environmentally sensitive tundra soil, eliminating infrastructure such as supports, and without the expansion loops typical of rigid steel pipe. In addition, flexibles can often installed without making permanent roads. The low impact of the flexible steel pipe facilitates removal when the land is restored to its original condition. A drill pad application of a previous generation insulated flexible steel pipe is shown in Figure 7. The jumpers shown were all of identical length and routed with S turns to eliminate the cutting and fitting required for rigid steel pipe. 5

6 Trenched Trenched installation is probably the most common type of buried application. Once the trench has been dug, the pipe is lowered into the trench, and the trench is backfilled. The pipe is normally deployed before the trench has been dug and lowered into the ditch, but it can also be deployed directly into the ditch. Equipment is generally used to position the pipe in the trench, but FlexSteel pipe is light and flexible enough that it can be manually lifted and moved into the trench. Dynamic deployment using a trailer is most common. The pipe can also be pulled of a static reel into position with a winch or tow vehicle for areas with difficult access. Figure 8 shows an open trench installation at the Signalta Chauvin Project. This installation tied production and water injection pipelines for 8 wells to two satellites. It used 7km of 3-inch 1500 psi pipe, and 3 km of 4-inch 1000 psi pipe. Figure 9 depicts a 7km long 3-inch 1500 psi production line installed at the Apache Snipe Lake Project. FlexSteel pipe is durable and impact resistant. In general, no special preparation is required for areas in contact with the pipe. Like plastic and composite pipe, the areas in contact with the pipe should be free of large, sharp rocks or protrusions. In particular, sharp and heavy rocks should not be dropped directly on flexible pipes, especially from a significant height. Crossings under roadways and railways are typically pulled into a carrier pipe, though this is based on code requirements for rigid steel pipe rather than any engineering issue with flexible steel pipe. The bottom of the trench should be prepared with a thin layer of soil or sand in rough or rocky conditions. Once the pipe is laid in the trench, the system is normally flanged up, filled with water, and hydrotested before filling the trench. The pipe can be buried if necessary, as FlexSteel pipe is hydrotested at the factory, and field failures are extremely rare. If movement of the pipe is a concern, some fill can be placed in the trench as needed to restrain the pipe. The pipe should be covered with soil, sand, or fine gravel prior to using rough fill with large rocks. Plowed Plowing is generally the fastest and least costly method to bury a flexible steel pipe. It is best suited to wide open areas, especially in deep soil or sand. It is not well suited for use in rocky areas or in locations with frequent road or pipe crossings. The plow essentially cuts a trench and lays the pipe simultaneously. Plows developed for PE pipe typically works well with FlexSteel pipe, and welded end fittings can be run through the plow. The pipe is normally deployed along the pipe route prior to conducting the plowing operation. Figure 10 shows a typical plowing operation installing 5km of 4-inch 1000 psi production line for the Burlington Viking Project. The plow operates at an average of 1 2 km/hr when operating. This application was installed using a plow with a chute designed for 6-inch plastic pipe, and having a split design that allows the plow cover to open and pass bolted flanges. Sliplining When existing pipes have deteriorated sufficiently to preclude cost effective repair, sliplining the old pipe typically the preferred method of installing flexible steel pipe to repair the line. Since the FlexSteel pipe provides pressure containment, the structural condition of the pipe is unimportant. Rehabilitation applications of FlexSteel pipe typically results in significant cost savings, minimal environmental disruption, and eliminating new pipeline permits. Where 6

7 reduced flow from the smaller inside diameter of the pipe is an issue, a flexible steel pipe up to 2-inches larger in diameter than the rigid steel pipe can be installed using pipe bursting or slitting techniques. Field sweeps of up to 90 or more are generally acceptable in rehabilitation lines, elbows are cut out because they have too small a bend radius for flexible pipes. Lengths of pipe that be rehabilitated in a single pull is usually limited by the available equipment, which typically has 1 to 1.3km of wire. With appropriate winches, 2 km or longer lengths of FlexSteel pipe can be installed in a single pull. 3-inch FlexSteel pipe fits in a 4-inch steel pipe, though with little clearance. The 4-inch pipe with welded connection end fittings fits easily in a 6-inch rigid steel pipe. In a pipe pull through applications, the first step is to prepare the existing pipeline for the installation of the flexible. This is usually accomplished by clearing the pipeline, generally with several types of pigs, and cutting out any obstacles such as elbows. Verify the line is clear by passing a gauge pig with aluminum gauge plate sized appropriately or a dummy end fitting with or without an attached short length of pipe. Required tension is typically about 1 ton of tension per km of straight pipe for 3-inch 1500 psi or 4-inch 1000 pipe, and 40% more for 4-inch 1500 psi pipe. Field sweeps can add roughly 40% tension per 90 bend. Once the pipeline is prepared, either a wire rope line is pulled through the pipeline with a pig, or a steel pipe or rod is assembled and passed through the pipeline. This is attached to the flexible steel pipe, and pulled back to draw the flexible pipe through the old pipeline. The pipe can be attached to the fixturing with a breakaway link to limit tension. A lubricant used on the pipe eases its passage. At the Encana Brooks Project a 4-inch 1000 psi flexible steel pipe was installed in 6-inch steel pipe. The 6-inch rigid steel pipe had a number of 90 elbows that were cut out, separating the pipe into 900m section and a 600m section. Elbows near the ends were replaced by curving the flexible to fit. Pulling the flexible steel pipe through the 900m section is shown in Figure 13. Horizontal directional drilling Horizontal directional drilling is used to install pipes at locations where trenching is not feasible, such as road and river crossings. In this technique, a steerable drill head is pushed by a segmented steel rod to make the pilot hole. The head is retrieved in a receiving pit at the end of the run and replaced by a reamer, to which the flexible pipe is attached. As the reamer is pulled back, it increases the diameter of the pilot hole to accommodate the pipe, and pulls the flexible steel pipe behind it. A lubricant can be pumped through the drill pipe to the pulling head to reduce friction. For rigid pipe, oversize reamers are often used to reduce interference and friction from the contact of the pipe with the soil at the many small course deviations in the hole that result from steering the head. The resulting tension load tends to limit rigid steel pipe run length to about 500m. Contractors report much lower tensions with flexible steel pipe, because the pipe easily conforms to the deviations in the boring with minimal increased friction. The Navigo Gift Lake Project used 1 km of 4-inch pipe and 5km of 3-inch production lines rated at 1500 psi. It required 5 total borings at 2 creek crossings of 100m and 250m lengths. Figure 11 shows a typical deployment off a static reel at the receiving pit. Figure 12 shows standard HDD equipment being used to bore a third hole, with 2 flexible lines already pulled into place. 7

8 Filling and Dewatering Flexible Steel Pipe The pipe is prepared for by installing a polyurethane or foam pig, then pushing it through the pipe with pressurized water. Draining the pipe is normally accomplished using compressed air to drive the pig out, which forces the water out of the pipe. Submerged Installation Submerged installation procedures, in addition to the factors indicated above, should consider the water depth, the bottom conditions and profile, and any contingency plans for abandonment and retrieval. The installation procedure should consider pipe properties and handling concerns, including tensile load limitations. Saddle bag type weights can be applied to provide additional weight if needed. Cold Weather Capability FlexSteel pipes are rated to install and operate to -40 C(-40 F). Numerous tests have been conducted on material samples and on finished pipe to verify the temperature performance. The PE retains good properties to at least -90 C, and the steel to at least -40 C. the coldest temperature at which mechanical tests were conducted. Comparing material properties at ambient temperature and -40 C, the PE gets stiffer and tougher and the steel retains toughness. While the pipe gets roughly 3x stiffer at the cold temperature, handling is still similar to handling at ambient. FlexSteel pipe has been installed down to -20 C (-4 F), as most contractors postpone outdoor work when temperatures drop below this. CONCLUSIONS Flexible steel pipe has been successfully deployed in many of oil and gas gathering and injection applications onshore. Despite being a premium product with especially good corrosion resistance, flow characteristics, and reliability, flexible steel pipe is generally less costly on an installed cost basis than rigid steel pipe when installed by appropriately experienced contractors. In addition, flexible steel pipe has considerably lower operating and maintenance costs than rigid steel pipe. Flexible steel pipes have been used offshore for decades, and have demonstrated superb corrosion resistance and reliability in very demanding service. These pipes are typically bought to accepted American Petroleum Institute standards that have been proven through field experience. Flexible steel pipes are typically custom designed for each application, and designs optimized for onshore use have been developed. These designs share the materials, design techniques, high reliability and ease of installation typical of the offshore pipes, but are have weights, flexibility, and pricing tailored for suitability to the onshore market. The pipes have been rigorously qualified to the offshore standards, as well as to tests adopted from other products to further demonstrate performance. The cost advantages of flexible steel pipe are particularly pronounced when used in rehabilitation projects, when compared to lined steel pipe, and for above grade applications. Temporary flexible lines many miles in length can be installed in one or two days on the surface for repairs or early well testing and production. These lines can be later buried or even left above grade for permanent use, or retrieved and reused on other wells. 8

9 REFERENCES [1] Chapman, et al, Pipe and Coupling Design Methodology for Reinforced Thermoplastic Pipe (RTP) Products, Third International Conference On Composite Materials For Offshore Operations. Houston, TX. Oct. 31 Nov. 2, 2000 [2] API 17J, Specification for Unbonded Flexible Pipe, 2 nd Edition, 2001 [3] API 17K, Specification for Bonded Flexible Pipe, 1st Edition, [4] API RP 17B, Recommended Practice for Flexible Pipe, 3rd Edition, [5] ISO , Petroleum and Natural Gas Industries Design and Operation of Subsea Production Systems Part 2: Flexible Pipe Systems for Subsea and Marine Riser Applications, 1st Edition, [6] ISO 10420, Petroleum and Natural Gas Industries Flexible Pipe Systems for Subsea and Marine Riser Applications, [7] ISO Technical Specification 18226, Reinforced Thermoplastic Piping Systems for Gaseous Fuels, psi; Class psi; Class psi; Class 900 Pipe Design Table 1. Standard FlexSteel Pipe Designs ID [in] US CUSTOMARY UNITS OD [in] Wt [lb/ft] MBR [ft] ID [mm] OD [mm] SI UNITS Wt [kg/m] MBR [m] R [Ω/km] 3 N N N N N N N N N N psi 3 N

10 Outer Sheath Tensile Strips Inner Liner Figure 1.- Flexible steel pipe construction Flange External Jacket Internal Body Figure 2.- Flexible steel pipe end fitting design Figure 3.- Field connections of bolted ANSI type as well as butt welded 10

11 Figure 4.- Field connection using butt weld Figure 5.- Flexible pipe on reel 11

12 Figure 6.- Above ground installation of flexible steel pipe from a static reel Figure 7.- Above ground application of jumpers on the North Slope of Alaska 12

13 Figure 8.- Deployment from installation trailer for open trench installation Figure 9.- Dynamic deployment at the Apache Snipe Lake project trenched installation 13

14 Figure 10.- Plowed installation at the Burlington Viking Project Figure 11.- Flexible steel pipe deploying off a static reel in an HDD application 14

15 Figure 12.- HDD equipment boring for a third flexible line at a creek crossing Wire pulled pipe from static 30m/minute 1 tonne maximum pipe tension per 900m section Figure 13.- Rehabilitation of a 900m long 6-inch line at the Encana Brooks field. 15