First DP application of SBOP offshore Brazil
With an increasing number of new prospects in deepwater, operators are constantly on the lookout for economic methods for deepwater exploration. Shell, among others, has seen much success in eliminating the conventional subsea BOP and marine riser and the associated weight, in favor of using a surface blowout preventer stack (SBOP) and drilling through a casing riser. The impact of this change has been that Shell is able to contract third generation rigs. By extending their operating capability, the SBOP allows these rigs work in deepwater, currently the domain of higher capability, deepwater rigs.
In a typical 18 3/4-in.deepwater operation a subsea BOP combined with a 21 1/4-in., low-pressure marine riser can weigh in excess of 2,500 tons. This requires a large rig with high tensioning capability and high variable deck load.
By contrast, the SBOP system greatly reduces these weights and allows a smaller rig to operate in deeper water. When one compares the day rate of a fifth generation rig with that of a third-generation unit, the saving can be substantial, depending on market conditions. Graham Brander, Deepwater Well Engineering manager for Shell E&P, said that these savings can make the difference between whether a prospect is drilled or not drilled. At present, there are a large number of operators with discoveries that simply can't be developed with existing deepwater cost structures. The SBOP technique, combined with cheaper third-generation units, offers a potential step change in cost structure to allow more marginal developments to move forward.
The SBOP trend began back in 1996, when Unocal drilled a number of SBOP wells offshore Kalimantan. In 2000, Shell and Unocal worked together in an exploration campaign to jointly drill two wells in the relatively benign metocean environment offshore Indonesia. The technique was further used by Shell in 2001 when five more wells were drilled – three for Brunei Shell and two for Shell Malaysia. For these campaigns, the equipment package did not have the capability to disconnect the rig and riser from the seabed, as the metocean environments associated with each drilling location were essentially benign and the vessels employed were moored. This technique however, proved to be successful on a time, cost, and well-quality basis.
The success of this technique inspired Shell to further develop the technology for use in other parts of the world, particularly in areas such as offshore Brazil, with harsher metocean conditions. The waves, current, and storm conditions in such areas may make it necessary on occasion to move off station. These conditions require equipment to accommodate an emergency disconnect. Brander said the addition of a subsea disconnect system (SDS) allowed for a greater level of confidence in applying this technology to a broader range of metocean environments.
The SBOP module, being installed here, allows a smaller rig to operate in deeper water.
The SDS system developed by Shell, Cameron, and Nautronics for these environments includes two sets of shear rams mimicking the emergency shut-in and disconnect capabilities of a conventional subsea BOP stack. The SBOP, suspended in the moonpool, caters for the well control functions of the subsea BOP. Also, having the BOP at surface eliminates the need for the extremely long high-pressure choke and kill lines and umbilicals used to communicate with and operate a traditional deepwater subsea BOP. In an emergency disconnect situation, the well is secured with the SDS and the riser and associated equipment is disconnected from the well near the mud line.
For the SDS to operate, a novel acoustic telemetry system was developed together with the Australian-based company Nautronics and BOP manufacturer Cameron. Brander said Nautronics had recently offered its commercial customers access to a broadband acoustic system that was originally developed for the military. This new generation acoustic system allows the rig a greater degree of reliably to communicate with the subsea disconnect control system when compared to more conventional systems. For redundancy and increased reliability, Shell added a small diameter electrical control umbilical. This back up communication option will be deployed until sufficient experience and data gathering gives the confidence for its elimination.
To date (four wells), the system has communicated with a better than 99% effectiveness with an interrogation frequency set every 10 sec. The electrical umbilical is strapped to the side of the casing riser as it is run. A third control option for the SDS includes a hook-up for an ROV to activate the system. All of the components are off the shelf, Brander said. This not only offers a cost saving in the design phase, but also adds to the robustness of the component reliability. The BOP components, including the control panel and accumulator system, were designed in modules so they would be portable from one rig to the next.
The non laterally constrained SBOP allows operation on less moderate environments.
In a sense, this design splits the conventional seabed stack into two pieces. The well control is handled at the SBOP, while the disconnect system remains at the mud line. Although the umbilical is strapped to the casing as it is run through the drill floor, Brander said, this additional step doesn't slow the running process. The rig crew in Egypt, for example, was able to run up to 14 doubles of casing riser per hour.
The casing riser itself differs little from a conventional casing string. It is made up just as it would be for below-mud-line purposes. Included in the riser section are two transition joints, one below the SBOP and one above the SDS, to accommodate fatigue and bending effects. When the drilling process is complete, the joints above the mud line are simply broken out and set back.
Brander said there was originally some concern about casing wear during drilling, due to the smaller inside diameter of the casing riser and the thinner wall of the tubulars. On inspection, it was found that very little wear occurs because the casing riser is kept taut by the tensioners and does not sag, as is the tendency of a conventional riser with the rig offset. A conventional 21 1/4-in. riser with a subsea BOP stack at its base tends to bow when the drilling vessel moves off station. The casing riser is held in tight tension so there are none of the angles that would allow for wear. Thanks to the favorable wear characteristics, the casing riser is reusable from one well to the next for a designated period before being taken out of service.
In terms of vortex induced vibrations (VIV), the casing riser can be equipped with VIV suppression strakes; however, due to the much-reduced diameter of the casing over the conventional riser and the tension that can be applied, the chances of VIV occurring are greatly reduced. This alone offers a considerable cost savings over a conventional riser package, which has to be designed to counter VIV conditions.
There are other advantages to the SBOP system. The smaller volume of the high-pressure riser means it is easier to maintain effective hole cleaning than with a conventional marine riser. Many deepwater prospects are relatively shallow below the mud line. This means that, in a well-control event, the escaping hydrocarbons may only have a short distance to travel before reaching the subsea BOP and can extend into the riser prior to the subsea BOP being closed. By moving the BOP to surface (several thousands of feet above the mud line), the driller has much more reaction time to control the influx. In addition, it takes less time, in an emergency shut-in, to activate the surface BOP.
SBOP from a DP rig
As Shell's confidence in the SBOP design grew, an opportunity arose on BMC block 10, offshore Brazil in the Campos Basin to drill in water depths between 9,000 and 10,300 ft. The company already had a dynamically positioned rig, theStena Tay, rated for 8,050-ft water depth, on long-term contract off Brazil, but the well in question was beyond the operating capability of the vessel. The use of surface BOPs presented a cost-effective means of extending the depth capability of the rig (the alternatives being major structural upgrades to the existing rig or mobilizing an alternative deepwater vessel, both of which were cost prohibitive). Shell and partners Petrobras, Wintershall, and Statoil felt confident that the SBOP system was the solution for this challenging water depth and went on to successfully drill and evaluate the well.
The project confirmed that this system is not subject to the same high bending loads that affect a conventional riser string, which is a distinct advantage in harsh environments. The system used offshore Brazil had a conventional 18 3/4-in. subsea wellhead. Above this was landed the subsea disconnect system. The casing riser then ran all the way to the surface, with transition joints above the SDS and below the SBOP. A tension ring adaptor was fabricated to hang off the riser in the rig tensioners, with a spacer above to ensure no interference of the SBOP with the in-line tensioners. Above the SBOP a three-part slip joint connected to the rig's diverter housing completed the SBOP system.
Some teething problems were experienced initially when a rupture disc separating the control system and the high-pressure accumulator system blew. This required that the system be retrieved for remedial action. Another problem related to one of the rams in the SDS closing when functioning the other set of rams. This was traced to backpressure in the lines taking the relief fluid back to the respective balance pods. Both of these problems were rectified. Some weather delays were also experienced during the campaigns in Brazil and Egypt.
The running of the casing riser, which included strapping on the umbilical, continued to improve each time it was used, Brander said. The project in Brazil was considered a success, and several water depth records were achieved at 9,474 ft. The records include the deepest water depth drilled in Brazil, a Shell water-depth record, and the first ever SBOP operation from a DP rig. Since then, the system has been used successfully on three further wells offshore Egypt.
This side-by-side comparison shows the difference in size between the SBOP and a conventional subsea stack.
Shell is extremely encouraged by these results and Brander said they hope to improve the system so that the technique will be applied to deepwater development wells. Right now, the limiting factors are the running tools and control device that would allow them to land tubing hangers within the narrow ID of the casing riser.
In the meantime, industry guidelines are being developed for SBOP operations by the IADC, so that other operators can benefit from improvements in the design and the evolving best practices. This is being done with an eye towards applying this system to fields in the Gulf of Mexico. Brander said the IADC "Surface BOP Best Practices Guidelines for a MODU" should be ready for publication late this fall.
While there will always be a role for fifth-generation rigs in deepwater, there are many prospects and field developments that can benefit greatly from these new technologies, Brander said. This is both good news for the many third- and fourth-generation rig operators currently unable to work in the deepwater hotspots around the globe and for oil companies who will have access to more rigs at attractive rates, capable of deepwater operations.
"This will open the door to more cost-effective exploration in deepwater, as well as help stimulate the development of smaller fields currently showing marginal economics," Brander said.