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For drilling from the floating position, the semi mobile offshore drilling unit (MODU) has become the unit of choice. Sometimes referred to as a “column-stabilized” vessel, the combination of hull mass and its displacement, wave transparency of the hull because of the columns, and its deep draft enable waves to pass through the unit with minimal energy exciting it to excessive roll, pitch, sway, surge, heave, and yaw. With the work deck above the wave crests and the factors listed above, this design is a very capable work platform in severe environments.
Floating units can work in very shallow water depths, less than 100 ft in some cases, to the deepest water depths. The present world-record water depth for a semi is 9,472 ft set by a dynamic positioning (DP) semi in Brazil in 2003 with a surface blowout preventer (BOP). The water depth record for a spread-moored semi is 6,152 ft, set in 2002 offshore Malaysia. A semi in 2003 set the world record for a “taut line” mooring system at 8,950 ft. The same rig also set the record for subsea completions in 7,571 ft in the GOM as the deepest producer.
In shallow water, the concern is that the lower hulls might clash with the BOP stack if the semi moves off location. In other words, the minimal water depth is usually controlled by two main factors:
The distance between the subsea BOP stack when the lower marine-riser package is disconnected The lower hull in the event of a move off location
Also important are the following factors:
- Tidal range
- Slipjoint space-out
- Ability to hold location
Factors that impact semisubmersible units
As with jackups, air gap is critical and is a major design consideration when the unit is rated for environmental conditions. During the design of a semi, hull motion analysis in relation to waves crashing into the upper deck is critical. Under no circumstance should a MODU be designed or rated for environmental conditions in which waves will come in contact with the upper hull. In addition, heave, roll, pitch, sway, yaw, and surge need to be analyzed in terms of the upper limits of motion in which crews and equipment can operate.
For example, significant amounts of heave, if slow (long periods), may be tolerable for most operations; however, short heaves that are very fast (very short periods) are more difficult. From a crew performance standpoint, smooth predictable motions generally do not hinder performance; however, jerky unpredictable motion will have a significant negative impact. Metocean conditions throughout the world result in most semis being operated in less than 8- to 10-second wave or swell periods, so motions below these periods are usually not of concern. A swell period of interest is the “resonance” or natural period in which the hull motion actually exceeds the environmental value (> 1.0 ratio) for motion (i.e., the hull heave is more than the wave height). It is generally agreed for semi designs that the resonance period for heave should be more than 17 to 18 seconds in the Gulf of Mexico (GOM) to prevent resonance. The resonance period varies in other areas.
Table 1 shows the relationship of common semi designs available in today’s market. As seen, the size, mass or displacement, variable deck load (VDL), and water depth ratings vary widely. Generally, the deeper the water depth rating is, the more severe the environmental capability is, and, the bigger the VDL rating is, the larger the semi displacement and dimensional size are. In the 1970s, the average semi displaced 18,000 to 21,000 long tons, whereas some of today’s deepwater units displace more than 50,000 long tons. Larger displacement usually means more VDL and better motion characteristics.
It is common to refer to semis as belonging to a “generation.” This designation is somewhat inexact, but Table 2 gives some guidance for semis. Recently, the newer ultradeep drillships have also adopted this type of designation. Many semis may start out as one generation, but an upgrade may graduate them into another one. This is particularly true of many second-generation units that are upgraded to fourth-generation units.
One of the most unusual conversions and upgrades is Noble Drilling’s EVA-4000 design, which, originally, was a shallow-water submersible. This triangular submersible was a complete redesign and turned into fourth- and fifth-generation semis. VDL and age are poor definition parameters for generation designation because some second-generation units have larger VDLs than some fourth-generation units and because age variations within a generation, especially fourth generation after upgrade, can vary widely. The most defining qualities between generations probably are the following:
- Water depth rating
- The date of new build or upgrade
- The technical capability of the drilling and subsea equipment on board the unit
Fifth-generation units usually have very large VDLs, high marine-riser tension, hook load ratings of 1.5 million lbm, large deck space, high-pressure [7,500-psi working pressure (WP)] mud pumps, and extensive mud-solids control systems. Floating units require the following components:
- Subsea well-control equipment
- A marine-riser system
- Marine-riser tension systems
- Drillstring motion compensation
- Large mooring systems
- Craneage to handle all the tubulars and marine riser
- A guidance system to enter the well and to run the well-control systems
- A sophisticated management system to work all the components together
This equipment and these procedures are discussed in MODU riser and mooring systems
Why would someone want to use a semi? In general, they are the most dependable, motion-free, and capable of all the MODUs. Their cost is generally higher than that of a jackup, but in water depths exceeding that for which jackups are rated, they are the unit of choice.
Noteworthy papers in One Petro
Lim, E.F.H. and Ronalds, B.F. 2000. Evolution of the Production Semisubmersible, Presented at the SPE Annual Technical Conference and Exhibition, 1-4 October. 63036-MS. http://dx.doi.org/10.2118/63036-MS
Chitwood, James E., and McClure, Alan C. 1987. Semisubmersible Drilling Tender Unit, SPE Drilling Engineering 2 (2) 13482-PA. http://dx.doi.org/10.2118/13482-PA