JUSTIN SMITH, Offshore Editor

Transocean’s Polar Pioneer semisubmersible is drilling in Skrugard field in the Barents Sea. Photo by Harald Petersen, courtesy of Statoil.

A critical, though not very glamorous, segment of the deepwater oil and gas sector is riser technology. Risers provide a vital conduit for the circulation of drilling fluids and protect the drillstring in offshore drilling operations. In the production stage of offshore field development, they allow the flow of hydrocarbons between seafloor and surface—both bringing production from the wellhead to the production vessel and, in some cases, returning processed oil and gas to the seafloor for pipeline export. They also transport water and gas to injection wells for secondary recovery. As drilling and production operations have moved into deeper waters, riser technology has faced new challenges, including keeping high subsea pressures at bay and accommodating an increasing array of power, hydraulic and chemical umbilicals.

A number of advances have been made in recent years with regard to deepwater riser applications. For example, traditional steel risers are very heavy, so the addition of buoyancy modules helps to offset that weight. In production operations, another means of dealing with the weight of steel risers is to transfer the load from the production vessel to separate buoys connected to the vessel by flexible surface risers in a hybrid system. Besides being used to conduct fluids, risers have been used to support intervention activities, such as subsea tree installation. In addition to the risers themselves, advances have occurred in the technology used to inspect them, such as a new ROV-based approach.

FREESTANDING HYBRID RISERS

Steel catenary risers (SCRs) have been around for decades and are still an integral part of offshore oil and gas exploration and production. As is the case with all technologies, though, these risers have their share of drawbacks. Later, along came flexible risers, which solved some of the issues from which traditional risers suffer but brought their own set of weaknesses. However, when combined, these two technologies form a kind of chimera called a freestanding hybrid riser (FSHR) that operators are beginning to use in production operations to develop some of their more daunting ultra-deepwater fields.

The first hybrid riser came online in November 2007 at Petrobras’ Roncador field in the Campos basin offshore Brazil as a component of the export system for the company’s P-52 production platform. The export system, which was designed, constructed and installed by Technip, connected the P-52 semisubmersible floating production unit (FPU), operating in 5,900 ft of water, to the PRA-1 fixed platform, an autonomous repumping station installed closer to shore in a water depth of about 300 ft. The FSHR carries P-52 production from the FPU to the seafloor, at which point an 18-in. pipeline carries the oil to PRA-1. A grouted foundation anchors roughly 4,900 ft of vertical SCR in place, a buoyancy can near the surface holds the SCR in the vertical position, and a flexible jumper connects the top of the SCR to the platform.

The Roncador field application demonstrated some of the FSHR concept’s advantages over a simple SCR system. For starters, it allowed for a more compact subsea arrangement beneath the platform, since the base of the FSHR and the export pipeline were installed about 1,000 ft away. Also, the motions of the FPU were transferred to the flexible jumper, not the SCR, which gave the semi the freedom to move around without upsetting the SCR. Plus, since the flexible jumper is suspended between the FPU and the buoyancy can, its load is shared between those two large, uplifting pieces of equipment.

Another advantage is that the installation of the system can be performed by a standard pipelay vessel with the traditional laying methods—J-lay, reel-lay or S-lay—depending on project requirements. In the case of the P-52 system, the J-lay method was used. In addition, since the FPU is not needed to aid in the installation of the FSHR, that process can start long before the floating unit arrives on the field.

Since the initial application offshore Brazil, FSHR systems have been put to use in each of the world’s major deepwater regions, culminating in the ongoing development of Petrobras’ Cascade and Chinook fields in the Walker Ridge area of the US Gulf of Mexico. Engineered by Technip and installed in a water depth of 8,250 ft in 2010, the five Cascade and Chinook FSHRs set several world records, including the greatest water depth for hybrid risers, the first disconnectable hybrid risers, the first hybrid risers installed in a region with severe environmental conditions (e.g., high loop current), the first system designed to withstand 10,000 psi and 230°F, and the first reel-installed FSHR system.

Cascade-Chinook also marks the first time hybrid risers were designed to accommodate two successive FPU concepts. The system is intended to produce for 30 years, with an early production system using a floating production, storage and offloading vessel (FPSO)—a first for the US Gulf of Mexico—to be followed by a permanent installation at a later time. Unlike the Roncador field system, in which the FSHR was used only for export, the Cascade-Chinook scheme involves four hybrid risers to bring production from the fields to the platform, and one for gas export via pipeline, Fig. 1.

Fig. 1. Petrobras uses four freestanding hybrid risers to gather production from Cascade and Chinook fields to an FPSO, and a fifth to transport gas to the seafloor for pipeline export.

In a paper presented at the 2011 Offshore Technology Conference in Houston, Technip engineers Ruxin Song and Pascal Streit describe how one of the Cascade-Chinook FSHR systems was set up: “The FSHR consists of a vertical rigid pipe anchored to the seabed via a foundation (e.g. suction pile) and tensioned by means of a near-surface buoyancy can that provides the required uplift force. For the single line FSHR, one flexible jumper connects the rigid riser via a gooseneck to the FPU. The connection of the riser to seabed is by means of a mechanical connector (e.g. tie-back connector or roto-latch connector). A riser base jumper, either flexible or rigid, connects the riser offtake spool and [pipeline end termination].”

A disconnectable turret-buoy allows the FPSO to sail away should it need to avoid an approaching tropical storm. Once disconnected, the buoy will submerge to a predetermined depth to protect the mooring lines, risers and umbilicals from the surface environment.

As for the constituent parts of the FSHRs, the flexible jumpers, measuring about 2,400 ft, connect the top of each vertical steel riser to the FPSO’s internal turret. The buoyancy can on top of each rigid riser provides the required uplift to keep the riser in place. The Cascade-Chinook buoyancy cans incorporate a closed-form design that is compartmentalized, with each of the chambers closed using remotely operated vehicle (ROV) plugs and pressurized with nitrogen. The internal pressure of each of the chambers is constant regardless of how much the can may move. Each can is about 107 ft in length and 21 ft in diameter with a net uplift force of about 740 tons.

A gooseneck assembly connects the flexible jumper and the steel riser, and a tether chain system using R5-quality studless chain with a 171-mm diameter connects the buoyancy can to the steel riser. A top riser assembly joins the gooseneck, vertical riser string and buoyancy can together. The assembly is about 32 ft tall and weighs 43 tons.

Both ends of the vertical riser pipe require a tapered stress joint, which is designed to handle large bending moments. These joints are made from F22 low-alloy steel and are welded onshore to a pup joint of riser pipe. Each stress joint is about 40 ft long with a wall thickness of 3.3 in. at the root.

To anchor the riser to the seabed, Technip designed a suction pile based on geotechnical data. The suction pile is about 90 ft in length and 16 ft in diameter, and can withstand the riser bottom tension of roughly 240 tons when operating.

Petrobras’ development of Cascade and Chinook fields has had to clear several hurdles along the way. While the operator received approval from the US federal government to use BW Offshore-owned FPSO BW Pioneer, the nearly yearlong deepwater drilling moratorium in the Gulf delayed drilling of at least one well at the fields. Furthermore, in March one of the buoyancy cans on an FSHR tied back to Chinook field broke loose from the steel riser it was supporting. The riser fell to the seafloor while the buoyancy can floated to the surface and reportedly traveled to within a few miles of Chevron’s Tahiti spar before it was recovered. With these setbacks, Petrobras expects production from the fields to commence in late 2011, a delay from the planned midyear startup.

INTERVENTION SYSTEMS

Traditionally, most operators performing intervention work from a drilling unit have installed and retrieved the production adapter base and tree cap using conventional drill pipe available on rigs. These were simple, open-water operations because they did not require annulus control, pressure containment or fluids circulation. Drill pipe has also been used as a workstring/riser in tubing hanger retrieval operations, with a hydraulic jar deployed inside the pipe to loosen stuck tubing hangers. In these cases, risks arose related to interference between the pipe and the hydraulic umbilical.

As completion and intervention operations in offshore environments have become increasingly complex due to increasing water depths and working pressure requirements, these environmental factors have greatly impacted riser body minimum tensile load capacity and gas-tight connection requirements. As early as 1996, Petrobras worked to develop a new type of completion and intervention system for use at Roncador field. At the time, the company was using a standard dual vertical-bore riser that was too heavy for use in this ultra-deepwater field, exhibited extended tripping times, and had poor gas-sealing capability.

A dedicated project team was created to develop a drill pipe riser intervention system (DPRIS), which was first delivered in 1999. In addition to subsea tubing hanger and production tree installations, these systems can be used as early production risers. DPRIS use has greatly expanded from the initial deployments offshore Brazil to more than 50 systems presently operating in deepwater offshore regions worldwide.

Since July 2010, a DPRIS designed by VAM Drilling has been in use offshore Nigeria—one of the first such systems to be applied in the West Africa deepwater arena. The system was designed for the Total-operated Usan project offshore Nigeria. Located in Nigeria’s oil mining license (OML) 138 with water depths ranging from 2,150 ft to 2,700 ft, the project consists of an FPSO surrounded by 23 production wells, 10 gas injection wells and nine water injection wells. There is potential for up to 32 production wells, bringing the total well count to 42 bores, each of which will be fitted with a subsea tree.

The drill pipe riser delivered for this project employs a proprietary double shoulder connection and a gas-tight, metal-to-metal seal on the inner shoulder. While the standard offer for high-pressure drill pipe riser is 6⅝-in. diameter, 0.5-in. wall thickness and X95 grade, VAM increased the wall thickness to 0.625 in. for corrosion allowance and upgraded to G-105 for improved tensile strength.

The second-generation metal-to-metal connection seal allows for reduced contact pressure on the sealing surface compared with the first-generation Teflon seal ring. This change provides better stability to the seal contact and reduces the incidence of early galling. The seal’s position prevents gas pressure migration from the internal diameter into the connection.

Other design improvements include a proprietary thread profile, a larger pitch diameter to achieve a better balance between the pin and box critical cross-sections, and a reduction of the threads per inch to speed up assembly.

Since July 2010, Total has used the new DPRIS on Usan field on 20 runs, for the installation of four trees. Twelve runs were performed inside larger-diameter riser while the other eight were in open water. Many more runs are planned for the future, during which 38 more trees are planned for installation.

The four installed trees have had no cleaning issues and no rust debris impacting operations. While, in general, the torque and connection running at the rig have not presented any significant problems, one joint was reported to be damaged due to over-torque at the rig floor. The damaged joint was removed, and operations proceeded without any serious setbacks.

BUOYANCY MODULES

One of the most serious limitations of risers is their considerable cumulative weight. A floating rig or production vessel is capable of supporting only so much weight, restricting the water depth in which these types of units can work. In production operations, as discussed above, freestanding hybrid risers provide a method to remove the riser load from the vessel. In drilling operations, where the riser must be located directly underneath the rig, various methods attempt to reduce the weight of riser strings, including the use of alternative metallurgies (see sidebar) and the use of buoyancy modules.

In April, a Transocean rig set a new water depth drilling record due in large part to riser buoyancy technology from Trelleborg Offshore. Transocean’s ultra-deepwater drillship Dhirubhai Deepwater KG2 drilled a well in 10,194 ft of water off the east coast of India for Reliance Industries. The previous record, 10,011 ft, was set by Chevron using another Transocean drillship, Discoverer Deep Seas, in the US Gulf of Mexico in 2003. Buoyancy modules played a part in that operation as well.

For the well offshore India, Trelleborg designed riser buoyancy modules able to withstand pressures of nearly 5,000 psi, giving them a depth range of 11,000 ft. The company supplied riser buoyancy modules to cover 100 riser joints at the Reliance well. In addition, 11 joints of Trelleborg’s stackable riser guard and riser shims were provided. The riser guard is a free-flooding, self-equalizing, neutrally buoyant unit, offering protection for bare riser joints and external lines during handling, storage and drilling operations.

A low-density composite syntactic foam makes up the core of the buoyancy element. To produce this foam, Trelleborg blends composite minispheres or macrospheres with pure syntactic foam, which itself is a mixture of a base polymer system and even smaller hollow, glass microspheres. Trelleborg describes the minispheres and macrospheres as a “larger fiber-reinforced composite sphere, developed to reduce the pure syntactic foam density to provide additional buoyancy” to the module. The pure syntactic foam bonds these spheres together and supports them when subjected to hydrostatic pressures.

The foam is encased in a skin of molded, resin-impregnated fiber, which provides a tough outer surface to resist impacts and abrasions. It also makes the module stiffer and more resistant to bending and protects the underlying composite syntactic foam from damage during handling, assembly and installation.

Each buoyancy module consists of elongated cylinders that surround the drilling riser as well as any ancillary lines, Fig. 2. To attach these elements together, either an internal fastener or external straps are used. Raised support pads provide clearance between the outside of the riser and the interior surface of the modules for when the riser flexes during handling or installation, and to accommodate ancillary lines.

Fig. 2. To reduce the weight in water of a drilling riser, Trelleborg’s buoyancy modules are fitted around the riser and any ancillary lines. The company recently designed modules rated to nearly 5,000 psi, allowing Transocean to achieve a new water depth drilling record offshore India.

The length of the riser joints and the limiting dimensions of the drilling equipment, namely the diverter housing and rotary table, determine the overall length and maximum allowable outside diameter of the buoyancy modules. Using the diameters and orientations of the riser and ancillary line, stop collar, clamp band position and sizes, together with the buoyancy requirement, the manufacturer determines what composition of the composite syntactic foam will yield the necessary buoyancy.
To achieve the buoyancy needed to support the riser, several depth ratings or types of buoyancy elements may be required. Trelleborg reports that its clients generally require at least 96%–98% efficiency from the buoyancy modules, representing the proportion of the riser string weight that must be supported by the uplift generated by the buoyancy modules.

INSPECTION

Inspections are a critical part of the maintenance of risers. As an example, the risers that make up the drill pipe riser intervention system discussed above require regular inspections at the rigsite, including a visual inspection of the pipe and threads after each run. Furthermore, when the riser inventory is replaced once a year and sent to shore, all of the internal and external parts of the pipe and threads are given a full inspection.

However, flexible risers are more complex than traditional rigid risers and, therefore, require more sophisticated technologies to inspect. Additionally, operators would like this inspection to be carried out while the risers are still online, creating a drive within the market to develop a method of externally performing the internal inspection of a flexible riser. With that in mind, Fugro Subsea Services and Innospection partnered to develop a flexible riser inspection system that employs ROVs, called MEC-Fit.

Over the course of two years, Innospection developed an external flexible riser inspection system that last year Fugro agreed to interface with a modern work-class ROV, creating a system that the companies will jointly market. Fugro, through its subsea engineering and intervention tooling business, was able to make the inspection tool lighter and more ROV-friendly as well as develop a deployment system.

In addition to being able to quickly make an external scan of a flexible riser that penetrates the various layers of armor, this technology is also able to select specific layers to be inspected. The inspection technique combines direct-current magnetic field lines with eddy-current field lines, which allow for deeper penetration into the ferrite steel material. Innospection has modified the standard eddy current technology to enable the selection and inspection of different layers within the flexible riser pipe, or to allow for the optimization of the inspection for a specific layer that is returning a defect signal.

Localized material defects, such as cracks and corrosion, can be detected beneath the riser coating at the single wires or wire areas. Additionally, the tool has the potential to detect and analyze material fatigue and general wall loss. Unlike traditional inspection methods, this system requires no couplant or annular flooding.

A cage affixed to the ROV allows the scanning device to be attached around the entire outer surface of the riser where it is scanning, Fig. 3. The ROV and cage move along the riser for inspection. The inspection head contains the permanent magnet unit, which can control the strength of its field through the use of hydraulic valves, while the eddy-current sensors are connected to the ROV interface. The data is transmitted in real time via the ROV’s main umbilical back to an inspection computer that is located on the vessel along with the ROV control unit.

Fig. 3. Fugro and Innospection’s riser inspection system involves the use of an ROV with a clamp-on electromagnetic scanner that detects localized material defects, such as cracks and corrosion, as well as material fatigue and general wall loss. This enhanced photo shows the system operating in a test tank.

Since the inspection method is electromagnetic, it is able to measure the integrity of the armored layers of a riser while detecting corrosion, cracking and any other structural changes in the metallic layers. During a demonstration on flexible risers in the North Sea with Chevron, the system was able to penetrate up to three metal layers from the outside of the riser. 

Aluminum risers reduce weight, extend capacity of deepwater rigs JAY GRISSOM, Alcoa Oil and Gas One of the perennial challenges of deepwater drilling is how to maximize the water depth and drilling capacity of existing rigs. Drilling deeper wells, in increasingly deeper water, demands larger, heavier, higher-capacity drilling components. High-volume, high-pressure mud pumps, high-horsepower drawworks and larger-capacity top drives all add to the weight burden of a rig. The requirement for longer risers and drillstrings adds another element to the growing weight load on today’s deepwater rigs. One company, Noble Drilling, began to address the weight issue surrounding deepwater drilling in the mid-1990s. The company’s customers wanted to drill in increasingly deeper water, but its deepwater fleet was limited in its capacity to handle additional weight. As a result, Noble needed to find a way to either increase the weight limits of its existing rigs or reduce the weight of drilling components. The former approach is in many cases economically or technically unfeasible. Noble decided to attempt the latter approach, by replacing conventional steel risers with a lightweight, high-strength aluminum alloy alternative. Development and deployment. Noble Drilling began R&D work on aluminum risers for deepwater applications in 1995. At the time, a resource for extruding large-diameter aluminum alloy tubing had to be located. This need was addressed by a very large facility located in Russia. Originally designed to support Soviet military equipment needs, the facility was capable of continuously extruding the world’s longest large-diameter alloy aluminum tubing, making it particularly applicable to riser fabrication. Another challenge to be addressed was the development of an aluminum alloy formulation that would provide the weight savings required while still delivering the strength needed for deepwater risers. After extensive work, a team of metallurgists eventually developed an alloy formula that best combined lightweight and high-strength characteristics. After significant design and development work, the first 37½-ft-long, 19½-in.-ID, 22-in.-OD tubes were extruded at the Russian plant. The plan was for two of these extruded pieces to be welded to create a standard 75-ft riser tube. Testing in the Gulf of Mexico confirmed the value of aluminum risers in terms of strength-to-weight ratio, but significant corrosion problems were also identified. These corrosion issues were particularly significant at the welds that joined two extruded tubes together. Rather than change the alloy formula, a system of cathodic protection was employed, with anodes strategically located on the risers. In addition, all joints were treated, post-weld, with an aging-critical heat treatment process. After assessment by the American Bureau of Shipping, this new aluminum riser system was certified for use.      Alcoa’s aluminum alloy riser system. In late 2002, two drillships and two semisubmersibles were equipped with the aluminum riser system. The first assignment was a deepwater well for Petrobras, where the system performed as designed. Both the drillships and the semisubmersibles were able to add 2,000–2,500 ft to drilling depth capacity—close to a 50% increase for the drillships and almost doubling the semis’ depth range. Recent developments. In response to the Macondo blowout, the industry is moving toward new blowout preventer designs that will offer greater shear power, primarily by including an additional ram stack. This will make them heavier and add significantly to the load on the rig as they are deployed by the riser string. Potentially, a heavier BOP (combined with the riser system used to land it) may exceed the hanging weight capacity of the rig. To compensate for the increase in BOP weight, operators are considering the use of aluminum risers in lieu of the heavier steel designs. Alcoa’s current aluminum riser system incorporates a number of design elements that reduce weight while still maintaining the strength and integrity demanded by deepwater applications. Uniquely designed tapered wall extrusions are now employed to both reduce the weight and increase the strength of the main riser tube and auxiliary lines. In addition, large, monolithic forgings are used to create the risers’ flanges, which provide them with outstanding strength-to-weight performance. Zinc-copper-based aluminum alloy formulations add strength, while the use of over-aged tempers optimizes both strength and corrosion resistance. In addition to the full aluminum riser system, a new, rapidly deployable hybrid design is in the works, employing aluminum alloy choke and kill lines that can be quickly and easily retrofitted to an existing steel riser system. These changes may be able to extend drilling depth by 20% or more, and the choke and kill lines can be easily retrofitted during routine maintenance cycles.