Pre-installed drilling risers, composites options for 15,000-ft water depths

Developers attacking from many directions

William Furlow
Technology Editor

Aker Marine Contractors, Cameron, and Diamond Offshore have combined their skills to develop a freestanding preinstalled riser and mooring system, called "installation in advance," for deepwater drilling programs [8,978 bytes].
Several recent innovations in the design and deployment of deepwater drilling risers, as well as the materials used in them, should be making their way to the market later this year - just in time for the industry's long awaited assault on ultra-deepwater. It is common knowledge that the drilling riser is one area of technology that has kept drilling under the 10,000-ft water depth mark. Behind closed doors, operators are already looking out to water depths of 15,000 ft. If operations in such depths are to become a reality, designers and manufacturers are going to have to come up with new riser concepts.

A number of ultra-deepwater riser options are under consideration around the globe, and several now have reached the testing stage. Even the riserless drilling concept may soon receive prototype testing of a sort. Mid-water completion is also in the testing stage. Composites and freestanding risers represent the most mainstream of these solutions since they are based on an incremental change rather than a paradigm shift.

Basically, a composite riser is a steel riser that is lighter and has a smaller OD. Other options that currently are cost prohibitive are aluminum or titanium risers. These products would certainly solve the strength-to-weight problem faced in ultra-deep riser scenarios, but are too expensive to be profitably produced.

Affordability is one of the keys to the riser puzzle. There is little doubt that current technology could produce a riser that would hold up to the pressures of ultra-deep drilling, be made up and broken out quickly, and be light enough to be supported by a conventional vessel. In short, the technology is there to drill in the deepest waters of the world.

The problem is cost. The industry needs a novel solution that will pay for itself in rig time saved, lower rig hookloads, and greater reliability. The following is an overview of several options for solving this problem.

Composite advantages

When someone talks about composites they generally mean carbon fibers. These fibers can be woven into a riser tubular that has directional mechanical properties. That means the tubular addresses only those stresses the individual riser joint will experience. Generally, there are three stresses to consider, the pressure of the water column on the outer diameter of the tubular, and the vertical tension of the weight of the mud column, tubulars, and tension between the floating rig and the landed wellhead. These stresses vary from one end of the string to the other. These stresses are calculated into the design of the different tubular joints so there is no wasted material.

With a steel riser, the engineer is given a certain set of material properties to work with and the design is a function of wall thickness. This would also be the case with titanium or aluminum. The problem is that steel is heavy, so the added material needed for ultra-deepwater designs becomes prohibitive.

The composite riser is not only lighter due to the weight of the materials, but also because it is designed for the specific conditions it will encounter in the field. This lighter, smaller riser requires much less buoyancy, which is not only cheaper, but saves space on the critical outside diameter (OD) of the riser joint. This is important because the OD of many conventional riser joints outfitted for use in ultra-deepwater is too high to be accommodated by a conventional 49.5-in. rotary table. In addition, a smaller OD means more joints can be stacked on a rig.

Use 2nd generation rigs

If the goal of the composite riser is not only to lower the variable load requirements of rigs headed to ultra-deepwater, but to open this frontier to a wider range of vessels, then the rotary table limitations and deck storage space are important issues. Most third generation semisubmersibles, for example, have a 49.5-in. rotary table, compared to the new 60-in. rotary table that has become standard on newbuild ultra-deepwater vessels. If all the newbuilds are expected to be drilling in 8,000-15,000 ft water depth in the future, then the question is not only how to get them out there, but how to get existing rigs out to deepwater to fill the gap between conventional depths and this new standard. Smaller OD, lower buoyancy costs, and lighter weight for a drilling riser system could make such a move possible and avoid further high priced upgrades of existing vessels.

There were several limiting factors that have held back the development of a composite riser system. One was the cost of materials. Historically, these fibers are very expensive and had a limited market, mainly in the aerospace industry. As their application has spread, part of the peace dividend in the US, the price has come down. Now, the fibers are used for such things as sporting goods.

Interface joints

Another more urgent limitation to composite risers is the riser flange between the riser joints. It is subject to a variety of exotic stresses that exceed the design capabilities of composite materials. Thus, for the time being, a composite riser system will rely on composite tubulars bonded to steel flanges. Thus, the connection between the flange and the tubular that has been the major triumph of this design. Three firms are involved in using composite materials in risers.

According to Karl Muriby, director of business development with ABB Vetco Gray, this technology was brought to the table by Northrop-Grumman and was developed in the aerospace industry. ABB will not discuss the details of this key technology, but explained that it is critical to prestress the connection during the manufacturing process. If this is done correctly, the connection does not see any additional stresses in the field. The ABB composite riser has been tested through more than 600,000 cycles of loading and unloading without failure.
Kværner is also entering the composite riser game with a product currently in testing. The Kværner system uses a metallic liner rather than one made of thermoplastic. The liner is a critical component because it provides a seal for the inside of the riser, while the composite gives the riser structural integrity.
Another composite option, one being pursued by Stewart and Stevens is composite choke and kill lines, retrofitted on to steel risers. This design is less expensive and can save about 18% of the weight of a typical deepwater riser joint. This means a smaller OD, less buoyancy material, and more joints for a given storage space.

Trip time

Beyond the question of weight and space is the time it takes to trip and pull a deepwater riser string. Estimates range from six to eight days for running or pulling this deepwater string, which is made up of hundreds of joints, each about 70 ft long, amd each with a series of bolts that must be pretensioned during make up. Each connection is estimated to take between two and three minutes, not counting the time consumed handling these large, bulky, heavy tubulars.

With depressed prices and high day rates, time saved on the critical drilling path quickly translates to cost savings. This need has resurrected a technology first developed in France over 10 years ago. In 1978 Elf, Total, and Esso contracted Institut du Petrole (IFP) and Framatome, a contractor for nuclear power generation, to design a riser system that could be used to drill in 6,000 ft. At that time, this was an unheard-of depth, but the consortium contracted with Sonat, which is now Transocean, to drill with the Discoverer Seven Seas in 6,000 ft of water in the Mediterranean Sea. At the time, the Seven Seas was the world's most advanced drillship. The vessel drilled two wells in 5,615 ft of water.

The CLIPlock riser design was a result of this effort. At the time, Framatome designed and built connectors for the nuclear power industry. France was far ahead in this technology, deriving a majority of its power from nuclear sources. This connector used the incredibly precise tolerances the company refined in its power generation work. The system was designed to handle large loads and be made up and broken out quickly. The riser connection was a success, but after the trial work in the Mediterranean, it was shelved because no one was drilling in these water depths.

In the mid 1980s, National Oilwell negotiated with IFP to be the sole licensee of this technology. The company began showing the product at the annual Offshore Technology Conference, with only a lukewarm reception. With low day rates and high oil prices, there was little demand for the speed this system offered.

It was not until the rush to deepwater two or three years ago that the system began attracting attention. Now with the low-price scenario, Kværner, which bought National in 1994, has sold Pride Petroleum a system capable of drilling in 10,000 ft of water. The original riser system used in the 1970s is now operating offshore Brazil. It seems the industry is ready to cut the make up time of a joint of riser from two minutes to 15 seconds. Wagner said this savings adds up to 12 rig days a year. At around $200,000 a day, that can be a substantial savings. Wagner said the newest version of the riser system can handle as much as 3.5 million lb of tension.

Pre-installation option

Another broad area of promise is the pre-installation of a deep or ultra-deepwater riser system. If the riser can be carried onto location by a separate vessel, installed along with the initial casing strings, and buoyed up before the rig comes on location, then many of the current limitations simply go away. There is no limit to the OD of the joints because they never pass through the rotary table. There is no storage issue, and there is no consideration for the running time.

A Norwegian company, Proffshore has formed an alliance with Statoil to build and test the Atlantis Artificial Buoyant Seabed (ABS). This technology would use a second or third generation rig to drill and cement the conductor casing, install a riser system and then move off station. Anchor handling vessels would then position what amounts to an upside-down bucket to the top of the riser string. This ABS would be moored below the level of sea states, but high enough to allow a conventional subsea wellhead BOP stack and riser system to be installed on top of it. The concept effectively raises the seabed to conventional depths and pre-installs the riser system, except for the joints needed to complete the last 300 ft or so.

This concept illustrates several of the advantages highlighted by all pre-installation designs. There is a substantial time savings running and pulling the riser system. This represents a savings of as much as one week of drilling time. In the Gulf of Mexico, where hurricanes are a common threat, such savings would have a major impact. Current deepwater operations must begin pulling riser as soon as a storm is named in the Atlantic ocean. This downtime is often wasted time, since many storms turn northward in the Atlantic.

Because storm demobilization has to begin so far in advance, the driller has no choice but to react to every storm as if it were an imminent threat. With midwater completion, and other freestanding options, this lead time is cut to as little as eight hours. Not only is the actual time spent pulling the riser reduced, but the number of false alarms is all but eliminated.

Statoil initially planned to test the drilling version of Atlantis later this year, but the well schedule has now been postponed due to low oil prices. Officials expect to drill this North Sea well sometime in 2001.

Aker/Cameron system

More conventional pre-installation solutions have been offered by Aker/Cameron and Hydril. Aker Maritime Riser Business Unit and Aker Marine Contractors has aligned with Cameron and Diamond Offshore to develop a proprietary system named "installation in advance" or IIA. This makes use of Aker Marine Contractor's pre-installed mooring system, which may include the suction embedded plate anchor (SEPLA) and taut-leg mooring system featuring polyester rope combined with Cameron technology for a free-standing, pre-installed drilling riser and the installation of the initial well structure.

In an ultra-deepwater scenario, Aker anchor handling vessels would pre-install a tension-leg mooring system, that may use SEPLAs and polyester ropes. The vessels pretension the system, rotating the anchors into an optimal attitude. At this point, a second or third generation semisubmersible is brought on station to install the initial well casing and 8 3/4-in. wellhead. The vessel then moves over about 100 ft on the mooring to drill a parking porch fitted with a dummy wellhead. This will act as a parking place for the freestanding portion of the drilling riser. The top of the free-standing riser section will be located at 300-500 ft below the surface to avoid surface currents, and is supported by a series of concentric air cans, open at the bottom and filled with enough compressed air to support the riser in a vertical attitude.

When the actual drilling rig comes on station, not only does it save the time of running an ultra-deepwater riser system, it also saves the time and weight of installing its own mooring system. This drilling rig then runs the upper portion of the drilling riser, which includes an upper marine riser package (UMRP) and the telescoping joint. The UMRP provides the point for a planned or unplanned disconnect. The subsea BOPs are used to close in the well against any pressure. The drilling fluid in the freestanding portion of the riser can remain or be circulated our, depending on the time available. Reconnection is then made using guidelines with the assistance of a remotely operated vehicle (ROV).

In cases where the lower subsea BOP must be pulled, the rig can disconnect the entire riser string, move over on its mooring lines to the parking place, land the string, pull the upper portion of the riser string then move back over the well and pull the BOP using drill pipe. Once the BOP is again landed, the process is reversed to return the riser to work.

Aker and Cameron envision a program in which there are two freestanding riser and mooring systems dedicated to one drilling program. This allows one system to be pre-installed on the next location while the drilling rig is on a present well location. Once the drilling rig is ready to move to the next well location everything will be preinstalled. Once the drilling rig has completed work on a well location, the riser and mooring systems from that location will be pulled and moved to the next location planned for the drilling rig. In this manner, the systems will leap-frog with the drilling rig, minimizing rig time. The IIA system requires minor modifications of the drilling rig. The mooring system and freestanding riser system will be leased to the operator just as the rig is leased.

Hydril system

Hydril developed its first deepwater riser system, designed for 3,000 meters, to be used by the Ocean Drilling Program in the early 1980s. Unfortunately, soon after that, oil prices fell. Though the riser program, called HST, was put on hold, it offered benefits that the industry is just now catching up to. For one thing the unique connector design is made up and preloaded using a proprietary threading system similar to the way casing is made up. This not only saves time in making up the string, but spreads the weight of the riser string throughout the coupling rather than concentrating this stress on a series of bolts, as is the case with a flange riser. Graeme Reynolds, director of systems development for Hydril, said his company is applying this technology to a new, pre-installed riser system that gives contractors more options for dynamically positioned (DP) exploration drilling in deepwater.

One of the limiting parameters of deepwater DP is drive-off and emergency disconnect. In the case of a power failure or other catastrophic failure, a DP vessel may need to make a rapid emergency disconnect at the mudline. These situations are rare, but if one were to occur, the vessel would not have time to pull the riser string, which takes a matter of days, before drifting off station. To secure the well, the driller must get the mud out of the riser, trip the pipe, then trip the riser. In a power failure scenario, the well would be secured at the mudline, the riser disconnected and basically dragged.

To prevent this, as well as other time-consuming riser trips, Hydril has developed its own pre-installation program. This would reduce the emergency disconnect program from 6-8 days, to 6-8 hours. While power failure or other catastrophes are extremely rare on these vessels, the need to pull riser for storm conditions or other unanticipated problems is more common and equally time consuming.

Using Hydril's near-surface disconnect platform (NSDP), a deepwater DP vessel can disconnect from its riser string at a variable depth of between 300 ft and 2,500 ft of water depth. This variable depth allows the NSDP to be placed below the currents, but close enough to the surface so the rig only has to trip a relatively small number of riser joints. The variable buoyancy of the riser itself ensures there is enough overpull to keep it vertical in a free-stand mode. Reynolds said the formula is one of tension versus offset. There must be enough tension to allow the riser to be freestanding, and enough overpull to keep it vertical.

In addition to supporting the riser, the variable buoyancy reduces the topside tension supported by the vessel. The riser system also provides real-time telemetry on tensions and axial bend, so the buoyancy can be adjusted for optimal performance.

As with the Aker/Cameron IIA system, the Hydril pre-installation would include a set-back porch that allows the well to be closed in at the mudline and at the NSDP, then set back to the porch so the BOP could be pulled for service. The mud is held in the riser during this procedure. Reynolds pointed out that in deepwater operations, this mud is valued between $600,000 and $1.3 million.

This variable buoyancy is at the heart of the Hydril design. Reynolds said the buoyancy is regulated using a series of compartmentalized air cans with a compressor package on the rig.

This design is similar to what Cameron describes when discussing its IIA riser. Reynolds said this system also makes use of a minimum of syntactic foam jackets to support the riser strings. This is important because of the cost involved in applying this technology. Reynolds also points out that the variability in the set depth of this NSDP is a key component of this system, allowing the platform to be set at an "optimal metaocean depth," which is close to the surface but below the loop currents.