Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. SPE disclaims any and all liability for your use of such content. More information
Ultradeepwater drilling units
As offshore activities have moved into progressively deeper waters, new types of drilling units have been needed to deal with the conditions found in these locations.
Ultradeepwater drilling units are are the technological forerunners and pioneers in the offshore drilling business. These units are:
- Extremely expensive
- Few in number
- Highly capable
- Huge in size
- Technologically advanced
Table 1 gives some characteristics of these units, most of them drillships of extraordinary size, but some are semis as listed in Table 2. All were built in the late 1990s and early 2000s. Most have some degree of dual-rig activity (i.e., they have two drilling units on one hull). The Transocean Enterprise Class drillships (Fig. 1), for example, have the capability to run two riser and two blowout preventer (BOP) systems with one system drilling and the other completing a well on a subsea template. With this drill-and-complete mode on a multiwell template, companies have claimed efficiency savings of 40% compared with a single-derrick unit. For exploration wells, it is possible to run casing with one derrick set and drill with the other, thus reducing total rig time to complete the operation. Of course, the latter operation is accomplished before running the BOP stack. It is possible to run marine riser and the BOP stack with one derrick set while running and cementing conductor casing with the other. Some have the capability to produce and store crude oil, thus eliminating the need to flare or burn the produced fluid during well testing.
Fig. 1—Ultradeepwater drillship Transocean Discover class Discover Deep Seas that currently holds the world water depth record for drilling (10,011 ft). There are three of these type of units, which are 835 ft long with a power rating of 52,000 hp and displacement of 92,800 tons. Note the dual crown block for dual activity. Courtesy Transocean.
Evoluation and technological development
The ultradeepwater drillships are the outgrowth of the second-generation DP units built in the middle to late 1970s. These units provided technological breakthroughs that led to the newer units shown in Table 3. Such breakthroughs include:
- Re-entry without guidelines
- Power management
- Thruster management
- Priority assignments
The newer units are “D3” rated, in that they have total triple redundancy. In other words, if any component of the system should fail, another one comes online immediately; if another system fails, the third system comes online. This approach is an effort to increase the reliability of the total stationkeeping system and pertains to all system components, including:
- Silicon Control Rectifier (SCR)
- Electrical switch
- Stationkeeping monitoring, etc
The ultradeepwater drillships are the outgrowth of the second-generation DP units built in the middle to late 1970s. These units provided technological breakthroughs in stationkeeping, re-entry without guidelines, power management, thruster management, reliability, priority assignments, and maintenance that led to the newer units shown in Table 3. The newer units are “D3” rated in that they have total triple redundancy from the engines, to SCR, to electrical switch, wiring, fuel, thruster, stationkeeping monitoring, etc. In other words, if any component of the system should fail, another one comes online immediately; if another system fails, the third system comes online. This approach is an effort to increase the reliability of the total stationkeeping system.
The attractiveness of these ultradeepwater units, all of which are fifth-generation units, is their unique ability to drill in up to 7,500 ft—and, in some cases, more than 10,000 ft—of water depth. These units generally cost more than U.S. $400,000,000 to build, with some running more than U.S. $650,000,000. The commercial viability from the contract driller’s viewpoint is still questionable; however, they have proved that the industry has the ability to drill in over 10,000 ft of water depth, a feat not imagined 15 years ago. The current world-record water depth set by the DP drillship Discoverer Deep Seas in 2003 and 2004 is 10,011 ft in the GOM. The current drill and complete for production record is 7,209 ft, also in the GOM and set in 2002 by the sister rig of the Discoverer Deep Seas, the Discoverer Spirit.
Ultradeepwater unit advantage
Why use one of these units? Water depth is the primary reason. Some contract drillers believe that the dual-activity capability makes them competitive with moored units of lesser capability and cost. However, these units are, in general, exploration units with a “niche” development capability for large-numbered multiwell subsea templates in very deep water. They are expensive but very attractive for the right situation. Generally, for exploration wells, the deeper the water depth is and the shorter the well is, the more commercially attractive they become over a standard spread-moored semi. Without them, we could not explore consistently in more than 7,500 ft of water depth.
Noteworthy papers in OnePetro
John S., William D. et al. 2007. More Ultra-Deepwater Drilling Problems, SPE/IADC Drilling Conference, 20-22 February. 105792-MS. http://dx.doi.org/10.2118/105792-MS
2. G. Song, Z. Hu et al. 2006. An Innovative Ultradeepwater Subsea Blowout Preventer (SSBOP) Control System Using Shape Memory Alloy Actuators, IADC/SPE Drilling Conference, 21-23 February. 99041-MS. http://dx.doi.org/10.2118/99041-MS