August 2022
Special Focus: Offshore Technology

Defining MODU generation designation

This independent presentation suggests a standard for defining various generations of mobile offshore drilling units (MODUs), centering around the “majority” of rigs. The very latest, most capable MODU generation is not needed to drill every well, but their flexibility attracts operators, at an increased price.
Mark A. Childers / Consultant

The concept of “Generation” designation for Mobile Offshore Drilling Units (MODU) started in the 1990s with semisubmersibles, then dynamically positioned (DP) drillships, followed by jackups. As of this date, submersibles, barges, tender assisted and platform rigs have not been given generation designation. 

Fig. 00. The industry’s first two 7th Generation drillships, Deepwater Atlas (right) and Deepwater Titan (left), are pictured as they neared completion earlier this year. Image: Transocean.
Fig. 00. The industry’s first two 7th Generation drillships, Deepwater Atlas (right) and Deepwater Titan (left), are pictured as they neared completion earlier this year. Image: Transocean.

Historically, the concept has been used as a marketing tool for the drilling contractors rather than a technical designation. However, recently its importance has increased to gain advantage on obtaining drilling contracts, advertising and, more recently and importantly, Wall Street’s valuation of a drilling contractor’s fleet. No one standard has been defined or accepted as to what is included in a generation designation. This has resulted in a lot of self-promotion by rig owners (understandable), Wall Street’s subjective ranking (no standard to go by) and non-qualified individuals’ ranking of MODUs. For better or worse, the generation concept has become an important part of the offshore drilling industry. 

This article is hopefully an unbiased discussion on MODU generation designation. It is very important to understand that the hull supplied by the designer, and then built by the shipyard, may be of one generation, but the drilling equipment and outfitting, commonly referred to as Owner Furnished Equipment (OFE), may ultimately qualify it for another generation. In other words, a hull may be of one generation, but its outfitting of equipment may move it into another generation. This is especially true for later generations. The following centers on what goes into a specific generation designation with the understanding that there are overlaps between generations with numerous exceptions. 

Time period. The generation time period usually spans at least a decade and almost always overlaps adjacent generation time periods. Major upgrades in one period may make the unit jump to the next generation. 

Water depth rating. The time period and water depth rating usually go together, as the industry has evolved into deeper water for all designs of MODUs. 

Major technical advancement. The earlier technical advancements centered on hull designs (size, strength and motion characteristics), but later they centered on outfitting features, such as deck load and volume/area capacities, drilling hoisting loads, subsea equipment, tubular handling, electronics of all types, and power management. 

Significant features: On occasion, a single feature will leap a MODU to a newer generation. Dual Load Line (DL) within a single derrick came out in the late 1990s. This was a step up from “offline makeup/breakout” or “dual activity” on the drillfloor, which had been around for years. DL, prior to running the BOP stack on floating MODUs, allowed for two hoisting activities to occur through two rotaries in a single derrick. Transocean patented, packaged and sold DL on their new DP drillships. Today, every late-generation floating MODU has this feature in some form. 

Acknowledging that there is not complete consensus on the definition of “Generation,” the following is a brief description for Semisubmersibles, Jackups and Drillships, based on years of MODU evolution. For more history and development on MODUs, reference the SPE Petroleum Engineering Handbook, Volume II (Drilling Engineering), Chapter 14. 


Semisubmersibles, especially in the early generations, provided a steady, less-motion drilling platform, especially compared to drillships of that same time period. Semisubmersibles were the preferred floating MODU in the early generations, but in later generations, that role is now filled by the DP drillship, with the exception of very severe environments. 

Generation 1. In 1961, the Bluewater 1, a converted submersible, was the first multi-column hull that drilled from the floating position. The first hull built from the keel up was Ocean Driller, in 1963. This generation period extended into the late 1960s, with a water depth rating of 300 to 600 ft, a one-million-pound hoisting system with two mud pumps, spread mooring, and working mostly in the Gulf of Mexico, North Sea and Australia. They were non-classed hulls, as ABS’ first rules were not published until 1968. Leading contractors include ODECO (Ocean Driller and then Ocean Queen classes) and SEDCO (135 class). Most, if not all, these hulls have been scrapped. 

Generation 2. With Generation 1 hulls not performing well in the North Sea (excessive motion and structural failures) and demand picking up, this generation was developed primarily around improved motion and structural characteristics from the early 1970s to the early 1980s. Water depth ratings were from 600 to 2,000 ft, with the drilling system the same as the first generation. ODECO (Ocean Victory), SEDCO (700 series), Aker (H-3), Friede Goldman (Pacesetters), Korkut (ERA) and Pentagone were the leading designs. Many, if not all, of these hulls have been scrapped, but some (Ocean Victory class and ERA) have been upgraded, and a few are still available today. This was the largest number of hulls built in a single semisubmersible generation. 

Fig. 1. The Noble Lloyd Noble 4th Generation jackup, designed by GustoMSC and owned by Noble Drilling.  The photo shows the sheer size of this jackup, as it sits next to a North Sea platform with a relatively large DP semisubmersible, the Safe Boreas accommodation unit. Image: GustoMSC.
Fig. 1. The Noble Lloyd Noble 4th Generation jackup, designed by GustoMSC and owned by Noble Drilling. The photo shows the sheer size of this jackup, as it sits next to a North Sea platform with a relatively large DP semisubmersible, the Safe Boreas accommodation unit. Image: GustoMSC.

Generation 3. Only a limited number of hulls were built for this generation. The difference centered on increased variable deck load (VDL), resulting in hull size being over 100% bigger than Generation 2 hulls. They were built in the mid-to-late 1980s, when the offshore drilling business was in a depression. Water depth ratings were around 1,500 to 5,000 ft. Drilling systems were only marginally more capable, most were spread-moored and self-propelled. The ODECO Odyssey (Ocean Ranger class), Aker (H-4) and Enhanced Pacesetter (L-9500) classes were in this generation. 

Generation 4. The significance of this generation is that almost all of them were significantly more capable than second- and third-generation hulls. Many were physical upgrades to existing second- and third-generation hulls, and they were converted during the late 1990s and early 2000s. They feature increased VDL, greater water depth rating (3,500 ft and beyond), much larger deck space for storage, higher-pressure well control systems (15,000 psi WP), significantly more marine riser tension, and very large liquid storage capacities. Some were converted to DP, while most were converted to very large spread-mooring systems. Leading hulls for upgrade were the Ocean Victory class, Korkut and ERA (submersible to semisubmersible). Newbuilds included ENSCO (8500 series), SFXpess (2000) and Reading & Bates (RBS-8, Deepwater Horizon). 

Generation 5. This generation of semisubmersibles features a huge increase in hull size and drilling capability. Most were spread-mooring units, with water depth ratings of 5,000 ft to possibly 8,500 ft. Some were designed for pre-laid mooring assist. More were DP, and the drilling systems were larger and more capable with four mud pumps (7,500 psi WP), hookloads of 1.5 million lbs and greater, larger mud capacities, larger riser tension systems, and hull displacements of over 40,000 metric tons. Generally, they could operate in very harsh environments (more severe than Generation 4). Notable units were the Bingo 9000, ME 5000, and larger conversion of the Ocean Victory class. 

Generation 6. Starting in the early-to-mid-2000s and going to the present, these rigs were all newbuild hulls, centered mainly around hull displacements over 45,000 M tons. They have been rated for very harsh environments, very large VDLs, increased drilling capabilities, and water depth ratings mostly over 7,500 to 10,000 ft. Many are DP and have advanced electronic and computer-driven drilling systems. Some have a “dual activity” or similar tubular handling system. Notable units include the Aker (H-6e), Friede & Goldman (Millennium and ExD), Global Maritime (GM-4000), GustoMSC/KFELs (DSS series), Moss Maritime (CS-50 and CS-60) and GVA (7500). 

Some have argued for a seventh-generation semisubmersible, but the hulls and drilling systems do not meet the four qualifying criteria and/or have not been built to date. 


Fig. 2. The drilling barge, Western Explorer, owned and operated by Chevron in the Santa Barbara Channel in 1956, was the first floating MODU with a subsea drilling system. This generation of floating MODU pioneered the development of floating drilling.
Fig. 2. The drilling barge, Western Explorer, owned and operated by Chevron in the Santa Barbara Channel in 1956, was the first floating MODU with a subsea drilling system. This generation of floating MODU pioneered the development of floating drilling.

Jackup MODUs’ main characteristic is that they drill from a platform raised out of the wave zone, with footings on the seafloor, resulting in a very steady drilling platform. Mobilization is the chief drawback, in that jackups do not tolerate severe towing conditions very well. Independent leg units also may suffer leg soil punch-through, causing serve damage and/or capsizing. Jackups, in general, are for relatively shallow water (400 ft and under) and are the workhorse of the offshore industry. In numbers, they dominate the industry. 

Generation 1. One of the early units, if not the first jackup, was the Offshore Co.’s (now Transocean) No. 51, built in 1953. The Zapata Scorpion was the first Le Tourneau jackup in 1956. The Delong multi-cylinder leg concept used during World War II was a technical contributor. The initial jackup designs and their development were an experiment of ideas, with multiple legs (three to 12), cylinder and latus legs, mat foundations, many different jacking systems, and so on. 

The time period for this generation was from 1953 until the start of ABS classification in 1968. During this period of development, there were many jacking and structural issues, resulting in many accidents. The vast majority were slot-type rigs (no cantilever yet), most were self-contained units (one hull), and oil companies were deeply involved in the design (GUS class by Chevron), with the key designers/shipyards being Bethlehem Shipyard, Le Tourneau and the drilling contractor Offshore Co (now Transocean). 

The Offshore Co. was probably the leader in innovation, with the North Sea-rated structures/jacking systems and self-propulsion on one class (Mercury). They had a relatively simple drilling system, with some having the unique feature of a crown block at the top of the derrick that could skid to up to six well locations on one platform. All these rigs have long ago been lost at sea or scrapped. 

Generation 2. This generation was the start of what is familiar in the industry today. Starting in the late 1960s and continuing through the mid-1980s, these hulls were classed by ABS, etc. A few had slots, but most had cantilever drilling floors (around 40 ft, with only the center position rated for full drilling loads), and water depth generally ranged from 14 ft to 250/300 ft. A few were rated to 350 ft, and most had three legs, with the predominant design being independent leg or large mats. This was the “golden era” for jackups, as hundreds were built, with building turning from the USA to worldwide. 

Design shifted from drilling contractors and operators to independent engineering or shipyard companies, such as Le Tourneau, Bethlehem, Friede Goldman, GustoMSC, KFELs, Baker Marine and one drilling contractor—Offshore Co. (now Transocean). Popular designs include the Baker Marine BMC series, Bethlehem JU, MS and MC series, CFEM, Friede Goldman L-780 and L-780 MOD II, Le Tourneau 82 and 116 (S and C series), Livingston 111 series, DFEM T series, Hitachi series, Mitsui series and ETA Robray series. Many of these units are now out of service. 

Fig. 4. Transocean’s Deepwater Atlas, just prior to delivery. This 7th Generation DP drillship is the first generation to have 20K BOP stacks and more environmentally friendly equipment and systems. Image: Transocean.
Fig. 4. Transocean’s Deepwater Atlas, just prior to delivery. This 7th Generation DP drillship is the first generation to have 20K BOP stacks and more environmentally friendly equipment and systems. Image: Transocean.

Generation 3. This generation (mid-1980s to today) is very similar to Generation 2, except the hull and leg length were much bigger/longer/heavier, with water depth ratings of 350/375 ft or deeper, and a few over 500 ft. Cantilever distances and allowable drilling loads increased significantly, many of the leg structures were designed for harsher environments, and VDL was significantly larger. 

These units were basically much larger and structurally stronger rigs, rivaling Generation 2 semisubmersibles in hull displacement tonnage. Drilling systems were increased significantly, with hookloads over 1.5 million lbs. Dual-activity, offline tubular handling systems were usually included. There were much deeper drilling depth ratings, larger-capacity mud systems at 7,500 psi, and high-end digital drilling systems. These rigs started with the Le Tourneau Gorilla class in the mid-1980s, but they really took hold in the 1990s and into today. Popular designs include the Friede & Goldman L-780 MOD 6, KFELS Mod V “A”, Baker Marine Pacific Class 375, Friede Goldman JU-2000 and Super M2 series, and GustoMSC CJ70-X150A. 

Generation 4. Only a few jackups fall into this class, which started building in the late 2000s, mainly by GustoMSC (CJ-50-X100MC) for deep water (492 ft or deeper) and severe, harsh environments, i.e., North Sea. Built mainly for Maersk Drilling (now Noble Drilling), they operate in the North Sea with very long leg lengths (694 ft or longer), allowing for very large air gaps. They have huge drilling hookloads (2.8 MMlbs-plus), extended cantilever lengths (90 ft) at near full drilling load ratings, significant VDLs (8,820 metric tons), very sophisticated tubular handling systems and extensive digital computer drilling systems. The GustoMSC CJ-70-X150 (Fig. 1) and the KFELS N Class fall into this generation, although the latter is not quite as capable as the GustoMSC unit. 

Even larger jackups, with more capable drilling systems, have been designed, but none have been built to date. 


It should be noted that semisubmersibles have six generations, while jackups have only four, thus illustrating the complexity of development of MODUs. In the past 10 to 15 years or so, DP drillships have become the preferred MODU for floating drilling, especially for ultra-deepwater (7,500-ft-plus water depths). Prior to that, semisubmersibles were more popular, and their numbers overshadowed the drillship total. 

Generation 1. The first floating MODU was the Western Explorer, owned by Chevron, Fig. 2. It operated the world’s first well drilled by a floating unit, using subsea well control equipment, off the California coast in 1956. It was a converted barge, not a ship. The mid-1950s, to the early 1960s, were the development era for floating drilling, with maximum water depth ratings of 200 ft to 600 ft. Most of the hulls were non-propelled barges. Initially, in the 1950s, all the rigs were engineered, built, owned and operated by operators/oil companies. Some of the more notable units were the Humble SM-1, CUSS 1 and Rincon. Most of these units drilled over their sides, rather than have a “moon pool” in their centers. 

Generation 2. The age of the spread-moored drillship was from the early 1960s to the early 1970s. Drilling systems centered around one-million-lb derricks, two styles of BOP stacks (20-in., 2,000 WP and 13 5/8-in., 5,000 WP), two 1,300/1,600-hp mud pumps, and drilling depths of 20,000/25,000 ft. Global Marine (now Transocean), derived from a four-oil company cooperative (CUSS), became the key developer of drillship technology, along with the Offshore Co. (now Transocean). Global Marine developed the Glomar class of self-propelled ships, measuring 268 ft long (four built), followed by the Glomar Grand Isle and Glomar North Sea classes. 

Offshore Co. developed the turret mooring system (four units built) that allowed the unit to turn into the weather, but the concept had severe limitations in the cellar deck and on the drill floor. Some think this design is a separate Generation; however, nothing has ever been built like them since. In the early days, with few MODUs available, mobilization was very important. Thus, drillships had a significant advantage over the few semisubmersibles available. Many drillships of this era were conversions from existing hulls. 

Table 1. Criteria for 4th through 7th Generation DP drillships
Table 1. Criteria for 4th through 7th Generation DP drillships

Generation 3. The next generation of drillships featured the development of dynamic positioning (DP) units, starting in the early 1970s and continuing into the mid-1980s. Few spread-moored units were built in this period. Drilling systems were, more or less, the same as Generation 2, with the exception of most vessels having three mud pumps. 

Among the leaders in this generation were IHC Gusto (Gusto Pelican class, 12 built), SEDCO (400 series, three built) and Offshore Co. (Discover Class, seven built). The Discover Seven Seas, rated at 6,600 ft, was a true pioneer. All these units had about the same size of positioning systems (20,000 to 25,000-hp propulsion) and hull displacement (20,000 to 25,000 ST). Most were rated initially for water depths around 2,000 to 3,000 ft. Even with upgrades, few of these units are still working. 

Generation 4. There was a dry period, from the mid-1980s to the late-1990s, when few drillships were built or converted. That all changed with the introduction of Transocean’s Discoverer Enterprise Class in 1999, featuring significant overall capability. These units had very large hulls (835 × 125 × 62 ft), VDLs of 22,000 tons, over 50,000 thrust hp, a drilling depth of 35,000 ft., accommodation size, and a few units were advertised with water depth ratings as great as 10,000 ft. Four mud pumps at 2,200 hp and 7,500-psi pressure were standard, as were top drives with single motors at 1,000 hp. 

Transocean led, and anticipated, the industry need for ultra-deepwater drilling and successfully improved the reliability of new DP systems, along with safer tubular handling system. The time period for this Generation runs from 1999 to about mid-2005. Joining this generation were the ship builders and designers, including Daewoo Shipbuilding (DSME), GustoMSC P10000 and Samsung 10000. 

Generation 5. With success and increased demand, the industry, starting in the mid-2000s, improved on the Generation 4 drillships, with many shipyards and contractors jumping into the high-end DP drillship market. This Generation had an explosion of building until the slowdown of late 2014. Thruster horsepower increased to almost 60,000, drilling depths went to 40,000 ft, and hull displacements reached 90,000 to over 100,000 tons. These units featured automated pipe handling systems, including marine riser handling, the ability to store well test crude, and huge VDLs. The cost to build just one unit could be over $600 million. Most have the most advanced tubular handling systems. 

Led by Transocean with its Enhanced Enterprise class, shipyards Samsung Heavy Industries and DSME were the most prolific, offering turnkey equipment packages (OFE), with few if any change orders allowed. Gusto Engineering and Huisman offer some unique designs. Most of this Generation and previous Generations have been retired and/or scrapped, due to technical obsolescence, high reactivation costs and low MODU demand. 

Generation 6. The Gulf of Mexico/Macondo well control incident of 2010 had a significant impact on deepwater MODU capabilities. Almost all the units of this Generation were commissioned after 2010, and they included BOP stacks with six to eight rams. Some units had two BOP stacks. The BOPs had become so heavy and complex, that it took longer to maintain them between wells. In addition, a second unit offers a backup and/or “capping stack,” which is very attractive to some operators and government officials. Like the last Generation, equipment and system advancements drove this Generation. Table 1, entitled “Criteria for 4th through 7th Generation DP Drillships,” outlines the detail differences between the four recent Generations. 

Generation 7. Driven by HPHT deepwater drilling and completions, and more secure well control, especially during completions, 18 ¾-in. 20K WP ram BOPs (Fig. 3) were developed with larger lift and rotating capabilities, as seen in Table 1. Also, the push for more environmentally oriented technology led to this Generation, starting around 2020. Only two rigs, Deepwater Atlas (Fig. 4) and Deepwater Titan, have been built to date. They will start operations late this year and in early 2023, respectively. 

As a result of the overbuilding in the early 2000s, and then the oil and gas demand crash in 2014, many generation rigs have been retired and/or are too expensive to update or bring out of cold storage. Depending on a unit’s stacking condition, when it was delivered, and which OEMs supplied the equipment, it is estimates that it will cost $30 million to $100 million to commission a DP drillship back to work. 

The above discussion is hopefully an independent presentation, suggesting a standard. This discussion is centered around the “majority” of rigs, but there are many exceptions. It should be noted that the very latest and most capable generation of MODUs is not needed to drill every well. The vast, vast majority of offshore wells drilled by MODUs do not need the very latest generation of rig to drill them, but the flexibility of the latest generation is attractive to the operators at an increased price.  

About the Authors
Mark A. Childers
Mark A. Childers has spent the predominance of his career of over 50 years in offshore contract drilling, centered around research, engineering, operations and upper management. He has published over two dozen articles and book chapters, has served on numerous industry committees as a member and/or chairman, holds 14 U.S. patents, and has been a keynote speaker frequently. After semi-retirement in 2007, Mr. Childers spent 14 years as a consultant. He received the SPE Cedrick K. Ferguson Metal in 1969, was an SPE Distinguished Lecturer in 1981, and chaired the IADC International Drilling Conference in 2005. He was named, by his peers, the IADC “Contractor of the Year” in 2006. Mr. Childers has received five Pioneering Technology awards from the Oilfield Energy Center (OEC) and in 2013, he was named, by his peers, to OEC’s “Hall of Fame.” He is a PE in Texas and Louisiana, and now spends most of his retirement time volunteering for the OEC and other industry organizations.
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