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Considering the many complex factors involved in successfully operating a mobile offshore drilling unit, one may ask, “How do I pick the right drilling rig for the job?” The answer is that often there is more than one rig type that technically can do the job. A review of related topics will show many items that must be considered.
Factors affecting MODU selection
Following are some major items that a driller needs to consider when selecting and operating a MODU:
- Mobilization and the drilling site
- Equipment outfitting and capability
- Subsea equipment, stationkeeping, and management
- Well intervention and remotely operated vehicles
- Rig crews and management
- HSE&S (health, safety, environment, & security)
Selecting the right unit for the job
First and foremost, the operator must take the time and effort to be knowledgeable about mobile offshore drilling units (MODUs), drilling contractors, the equipment involved, and the relationship between all the parties (operator, drilling contractor, and third parties). Surprisingly, this does not always occur.
The operator should be aware of and obtain all the permits, and be aware of and set up logistics for:
- Ground transportation
- Office space
- Contracts with third parties
- Bulk mud
- Drill and potable water
- Local supplies
- All related items
The operator must also be aware of any unusual requirements to drill the well, as special requirements may have a major impact on the well plan. Possibilities include:
- Shipping lanes and fairways
- Unusual soil conditions
- Strong currents and/or large ranges of tides
- Strict drill-cutting discharge requirements
- Local government requirements for use of native labor and/or professionals
- Restrictions on the use of harbors and air space
- Military explosive dumping areas
- Non-flyover zones
With the last three points addressed, the operator must take time to engineer the exact type of performance he requires of the drilling equipment before deciding on the type of drilling rig. Sometimes the drilling rig type is obvious, such as an ultradeepwater rig; however, most of the time it is not. A checklist should include:
- Hoisting load and speed
- Mud volume
- Bulk volume
- Sack storage
- Drill-water capacity
- Feedstock capacities for synthetic or oil-based mud and completion fluid
- Metocean conditions in relationship to MODU capabilities
- Deck space
- Well-testing requirements, if applicable (space, deluge for seawater and piping), and many of the items listed in the sections on equipment, outfitting, and capabilities.
- Unfortunately, operators sometime specify a MODU and drilling equipment with not enough capabilities to drill the well with the hope that they will obtain an inexpensive unit. More often, operators specify a unit with complete overkill, eliminating very capable units that could do the job quite nicely at an attractive price. In other words, specify a unit that can do the job comfortably but do not overkill or try to squeak by.
Is the well over a structure, such as a caisson or platform, or at an open location? If it is over a structure, then only a jackup, tender assist drilling (TAD), platform rig, and/or submersible, depending on water depth, should be considered.
If the well is in open water then a jackup, TAD, and/or submersible, depending on water depth, should be considered. A standard moored drillship may also be evaluated if commercial issues are a key consideration.
If the well(s) to be drilled are over a platform, some of the following questions need to be considered:
- On the subject platform, can a cantilever jackup reach the well conductor after jacking up, and does it have enough combined cantilever load rating to drill the well(s)?
- If a platform rig is being considered, is there enough fixed platform space and load-bearing capability? Older platforms sometimes weaken with age and additional production equipment is placed on them, thus reducing the space needed for a platform rig. What is the spacing for the “cap beams,” or the beams the platform rig would skid and rest on? The beams may range from 30 to 62 ft; tension leg platform (TLP), spars and large platforms may even be wider. Standard cap beam spacing usually runs from 35 to 45 ft. A jackup or a TAD should be considered if cap beam spacing, load, or space is a major issue.
- If a platform rig seems to be the best fit, required capabilities are very important when deciding between a standard and a modular unit. As a rule, modular, self-erecting units are less capable overall but offer many advantages over their larger, more expensive cousins, as discussed earlier.
- If there are weak soil conditions that increase the likelihood of a punch through or old spud-can holes that do not fit the available jackups, use of a jackup may be questionable, especially if a capable TAD, preferably a semi, is available.
- If platform space and/or load bearing are critical and the wells to be drilled are extended reach wells (ERWs) or very deep, a high-specification semi TAD will be very attractive because a TAD takes up less space, the drilling equipment set (DES) is much lighter than high-specification platform rigs, the weather effect for loading and unloading consumables is generally not a factor with TADs, and the TAD can store (space and load) a considerable amount of casing, mud, cement, and operator expendables.
- If a TAD appears to be the best solution, weather, space, and VDL should be factored in when considering a monohull vs. a semi TAD.
- For spars and TLPs, modular platform rigs vs. TADs must be explored. Weight is very critical and extremely expensive to accommodate. The TAD, weighing one-fourth to one-fifth as much as a modular rig and requiring about one-third the space, is very attractive. If more than 9 to 12 long ERWs are to be drilled, a TAD spar/TLP instead of a modular platform rig “drilling” spar/TLP may be very attractive. Consumables such as mud (volume and weight), casing (weight and space), supply by boats, and the production and drilling risers will have a key impact on rig efficiency.
Water depth of the location has a major impact on MODU selection. Following are some observations that should be kept in mind for bottom-founded units:
- In very shallow water depths (generally < 25 ft, definitely < 14 to 20 ft), submersibles offer many advantages over jackups. The smaller shallow-water jackups usually have limited drilling, deck space, and VDL capability compared with submersibles. Submersibles can operate in 10-ft water depth and generally have relatively attractive drilling capability.
- For independent-leg jackups, which most upscale jackups are, leg penetration may be critical. A 300-ft nominally rated jackup with 100-ft leg penetration becomes a unit that can drill only in ≈ 200 ft of water depth, depending on the required air gap. In addition, it will probably require many preload cycles and thus a long mob and demob period. Pulling legs may also be time consuming.
- For jackups rated for > 300 to 350 ft, a new high-specification, enhanced, premium jackup may be required, along with the additional cost. In other words, the operator should not over specify his requirements. If water depth and/or a 7,500-psi-WP mud system is thought to be required and because there are few of this class of jackup, the operator should expect to pay a premium price to obtain such a MODU.
- Selecting between a mat or an independent jackup should center on soil factors, spud-can holes (although holes can also be a problem for mat rigs if they are around a high-load-bearing area of the mat), economics, and drilling capability. Almost without exception, mat rigs are less capable than equivalent independent-leg units, but they can drill in areas where leg penetration is a major problem and/or leg punch through is of major concern. A relatively new concept for helping to prevent leg punch through, “Swiss Cheese,” is being used on a limited basis. Multiple 26- to 36-in. holes are drilled through the weak load-bearing lens, allowing the spud can to penetrate the weak soil easily through to the stronger soil below the zone in question. However, it is very expensive and not always a sure solution.
- If the well under consideration is in jackup water depth but the soil conditions are very unsuitable, shallow gas flows are likely, and a jackup is not available, a shallow-water semi may be able to drill the well very economically. 
If the water depth exceeds jackup capability, a moored MODU should be considered. Once again, the operator should not generally specify a unit with a lot more capability than required. Following are some observations:
- Semis generally can be grouped into three broad categories of water depth, which usually follows their generation designation: second-generation units work in less than 1,500 to 2,000 ft, third- and fourth-generation units in 2,000 to 5,000 ft, and fifth-generation units in 4,000 to 6,500 ft and beyond. Costs generally increase with water depth, but so do the capabilities of the unit. Again, a sledgehammer is not needed to drive a tack.
- A prelaid taut or semi-taut mooring system can extend the depths of some units, but the prelaid systems are very expensive to purchase, deploy, and maintain. In addition, other requirements, such as VDL, deck space, marine riser tension, and liquid volume capacity may not be adequate.
- A second- or third-generation unit can be “stretched” beyond its normal water depth rating by mooring line inserts, but as pointed out earlier, other requirements may be limited.
- In some limited cases in which day rates for second-generation semis are reasonably competitive with those of deepwater jackups, a semi can drill a well faster and more economically than a jackup. This is usually in water depths of 275 to 300 ft or more and wells of short duration. The reasons are the longer time to preload/pull legs, eliminating all the casing strings that must be run and pulled between the rotary and seafloor, and potential moving delays, all of which the semi does not contend with.
- Generally, a dynamic positioning (DP) MODU will not be commercially competitive with a moored vessel; however, in deep water and short-duration wells, they can be commercially competitive even with much higher day rates.
- Ultradeepwater water depths are generally the domain of DP fifth-generation drillships and a limited number of semis. There usually is no valid substitute for their use other than in some limited cases when slim riser and surface BOP technology and/or a prelaid taut or semi-taut mooring system can be used.
- Environment and metocean have a critical impact on MODU selection. There are three general metocean categories that MODUs fall into. Most can operate any place in the world except the North Sea, in arctic conditions (< 32°F), and in select areas (e.g., the southwest coast of West Australia and New Zealand). The second category of rig can operate in the most severe, hostile, and usually artic conditions. These very-high-end units are very costly. The third category can operate only in the benign to very calm environments of West Africa and the Far East. There are exceptions to these categories, such as some mat jackups, so it is very important to specify the environment, and then compare it with the classification ratings of the unit. Mooring and riser analyses for floating units also need to be performed.
The above points are provided for guidance, but other factors may be the determining ones. Most important is the understanding that many unit types may be able to perform the work. The operators should do their homework and the evaluation in a knowledgeable, methodical manner. Once the technical side has been evaluated further considerations need to be factored into the final selection, including:
- Drilling contractor’s reputation
- Management style
- Drilling contract issues
- Price, etc.
Finally, intangible issues need to be weighed, such as:
- Mutual confidence and respect
- Perception of ability to work out problems with anticipation of an equitable solution
- Political influence with local governments
- Agent’s impact and help
- Sheffield, R. 1980. Floating Drilling: Equipment and Its Use, 2. Houston, Texas: Practical Drilling Technology, Gulf Pub. Co., Book Division.
- Childers, M.A. 1989. Operational Efficiency Comparison Between a Deepwater Jackup and a Semisubmersible in the Gulf of Mexico. Presented at the SPE/IADC Drilling Conference, New Orleans, Louisiana, 28 February-3 March. SPE-18623-MS. http://dx.doi.org/10.2118/18623-MS.