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MODU equipment and capabilities

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We must never lose sight of the fact that a mobile offshore drilling unit (MODU’s) primary goal is to drill and sometimes complete wells. Often, when concentrating on the marine aspects of the offshore drilling business, we forget this fact. Every well and site have their own requirements and demands, but following are some general comments, not always applicable to every situation, on MODU equipment and capabilities that should be considered when selecting a unit or planning a well. This list is far from complete but raises some of the most common considerations:

Variable Deck Load (VDL)

VDL includes any item of weight that is not included in the lightship of the basic vessel.

Lightship is the basic weight of the MODU, including all equipment considered permanent. This includes:

  • Drawworks
  • Mud pumps
  • Rotary
  • Derrick
  • Top drives
  • Power plant
  • All items that cannot be readily lifted off the vessel

VDL includes:

  • The drilling contractor’s drillstring
  • BOPs(Blowout Preventer)
  • Spare parts
  • Vertical tension to hold up a drilling marine riser,
  • Fuel
  • Portable water
  • Anything loose on board.

The remaining VDL is for the operator’s consumables, including:

  • Logging units
  • Casing
  • Bulk and liquid mud
  • Cement
  • Handling tools
  • Subs
  • Anything that he may want to store on the MODU

Hook, rotary, and setback are also considered VDL, and may consist of a large portion of what the MODU is required to safely carry.

The depth of the well and casing program has a big impact on the amount of VDL required, as will water depth. Complicated mud programs requiring changing of mud systems will necessitate more volume and space. It is not uncommon for a development MODU to have three types of mud on board. For floating rigs, storing the entire volume of the marine riser adds significant weight and space requirements to the MODU as water depths increase. Some MODUs report large VDL capacity, but often they do not have the space to store the VDL.

In general, jackups , except for the new premium units, have the least VDL capabilities. They also do not have a lower hull or large tankage like a semi’s lower hull or a drillship’s auxiliary tanks or a submersible’s lower hull tankage. The range of VDL for jackups (Table 1) runs from 1,600 to 2,600 short tons. For semis (Table 2), the VDL ranges from 2,500 to 4,000 short tons for older units and 4,000 to more than 7,000 short tons for newer-generation units.

Stationkeeping Equipment and Marine Riser Tension

For spread-moored MODUs, analysis must be done in relationship to the environment required for it to withstand and hold station in drilling, standby, and survival modes. Metocean data must be obtained and used in an industry standard analysis program like that published by API or other recognized authorities. For Dynamic Positioning (DP) operations, the operating limits of the system must be compared against the metocean and the return periods of major events.

DP stationkeeping, unlike spread-mooring systems, functions so that the unit either maintains location or is steadily forced off location. There is no in-between when reaching the maximum capabilities of the unit. For spread-moored units, as the MODU moves off location because of increasing environmental forces, the mooring system increases in restoring force; however, the offset from the well may be too great to manage the marine-riser system safely.

The mooring and marine riser work hand-in-hand; therefore, a riser analysis in accordance with an industry standard such as API should be done. If the MODU appears to be more than adequate for the proposed location, drilling contractors can usually supply the analysis and guidance. For more challenging locations, a number of competent engineering firms can conduct studies and give guidance as to the acceptability of a specific MODU under consideration.

MODU Classification and Environmental Rating

Every MODU has design ratings approved by:

  • Classification societies
  • Country of registration
  • Regulations by various bodies

A unit may be able to work in the Gulf of Mexico (GOM), but not be rated for the environment or regulatory requirements in the North Sea. Most MODUs can operate in temperate and mild environments, but such areas as the North Sea and west of the Shetlands are restrictive to many units. Some third-world countries do not have any regulatory requirements, and the regulations that do exist are loosely enforced. Pollution and environmental requirements can be major considerations. Some countries, such as Australia and Italy, have very strict rules concerning matters such as:

  • Electrical
  • Mechanical
  • Training
  • Staffing, etc.

This subject is discussed in Offshore HSE in more detail later.

Well-Control and Related Equipment

The anticipated maximum surface pressure in the event of a well-control problem will determine the WP(Working Pressure) rating of the well-control equipment. Most MODUs have a 10,000-psi-WP system. A few have only 5,000-psi-WP systems, but if a 15,000-psi-WP system is required, MODU selection may be restricted. The cost of the MODU and well will increase because of a more restrictive market for high pressure and sometimes high temperature (high bottomhole temperatures). The required capabilities of the MODU can be increased by the need for increased amounts(or heavier) casing or increased amounts(or heavier) muds, both of which increase the VDL requirements.

Well-control equipment is a subject in itself, and there are a number of good references in the industry. The following text discusses subsea equipment and its relationship with stationkeeping.

Accommodations Capacity

If a simple well is to be drilled and not completed, crew and servicemen capacity requirements are far less than if a complicated well is to be drilled and completed. Most modern MODUs have capacity for, at least, 70 crew, and some of the newer units have capacity for up to 120. The crew includes:

  • The operator’s personnel
  • Service personnel used at various stages
  • The contractor’s crews
  • Catering personnel
  • Any visitors

Room for regulatory personnel is sometimes a requirement, but, as most operators will confirm, there never seems to be enough capacity. This results in a constant shuffling of personnel and crew on and off the MODU to stay within class and lifesaving allowable limits.

Drilling Equipment and Power Plant Requirements

Most drilling engineers and operations personnel will look at a MODU’s drilling equipment and power plant first to see whether the unit is capable of drilling the well under consideration.

Many upgraded units, and even some new builds, may be short in one or more areas. More power and electrical equipment are required if the unit has added, for example:

  • A top drive
  • A third mud pump
  • Enlarged accommodations
  • Centrifuges
  • Solids-control equipment

During upgrades, this additional power is not always added. It generally is advantageous to be able to:

  • Run all mud pumps
  • Lift a heavy load with the drawworks
  • Back ream the hole at high torques
  • Have a maximum utility or “hotel” load simultaneously

Is a high-volume, high-pressure mud system (5,000- vs. 7,500-psi WP) worth the extra money required to hire an upscale MODU? All the aforementioned items should be part of the equipment evaluation. In addition, many more factors should be kept in mind, including:

  • Operating performance
  • Management style of the contractor
  • Safety performance
  • Financial stability
  • Honoring of contracts, etc.

Well Testing

If extensive well testing is anticipated, burning and/or storage of the crude must be considered as part of the MODU selection. If high production rates for a gas test are considered, cooling of the MODU is a major consideration. Piping and safety systems are of paramount importance to ensure that the operation is conducted in a safe and environmentally secure manner.

Crew Capability, Training, Safety, and Overall MODU Performance

Assuming that the “hard” or basic equipment qualifications are met, it is important to determine the capabilities, training, and safety work habits of the crews. Longevity of critical and key members of the crew is an indication of good morale, teamwork among the crew, continuity, and performance. Key crew members include:

  • The offshore installation manager (OIM)
  • Tool pushers
  • Drillers
  • Crane operators
  • Barge engineers
  • Rig mechanics
  • Electricians

The International Association of Drilling Contractors (IADC) has rules and guidelines to measure safety through lost-time incidents (LTIs), non-LTIs, first aid, and near misses. These statistics indicate the MODU and drilling contractor’s commitment to and success in conducting a sound safety program. Overall MODU performance can be measured in downtime, for which every drilling contractor keeps records, and time-vs.-depth curves. Many receive appraisals from their customers on a well-by-well basis. If these forms are not proprietary per the drilling contract, they should be reviewed.

Special Situations and Considerations

If the well is to be drilled in an unusual area or there are atypical circumstances, MODU selection may be restricted. Special equipment may be required in very high-current areas that induce vortex shedding and thus violent vibrations of:

  • Marine risers
  • Jackup legs
  • Mooring lines
  • Guidelines
  • BOP control lines

Severe cold, especially below-freezing temperatures for extended periods of time, requires special winterization, which is not standard equipment. Icebergs and pack ice flows create another unusual situation that must be taken into account when selecting a unit. Extremely large tides, those greater than 20 ft., may eliminate some jackup MODUs because of leg length. Unusual situations and circumstances do not occur often but may have a significant impact on MODU selection when they do occur.

  • The MODU’s maintenance records
  • Age and condition of equipment
  • Type of MODU (some jobs can be done by more than one type of MODU)
  • Day rate and contract conditions
  • Mobilization/demobilization costs and distances
  • Timing and availability of potential MODUs


See also

MODU types

MODU selection

Classification and registration of offshore drilling units


Noteworthy papers in OnePetro

External links