Types of Mobile Offshore Drilling Units

The quest for oil and gas beneath the ocean floors has led to the evolution of various types of Mobile Offshore Drilling Units (“MODUs”).  These floating marvels of engineering are designed to explore and drill for hydrocarbons in a range of marine environments.  This article examines the most common types of MODUs as recognized by the United States Coast Guard, shedding light on their design, capabilities, and operational environments.

A Jack-Up Rig is a platform that can be towed to a location for exploration and drilling.  After the jack-up is on site, it can then lower legs from the platform to the sea floor beneath it.  Once the jack-up’s legs have been lowered to the sea floor, the platform can then be raised over the surface of the water providing a stable platform for drilling. There are two general subtypes of Jack-Up Rigs: Independent Leg and Mat Supported.  Together, these make up the most common type of MODU.

Independent Leg Jack-Up Rigs have three separate legs that are lowered to the sea floor.  The legs are only connected to each other through the platform.  This allows the Independent Leg subtype to be used in places in which the sea floor is not level terrain, as the legs may be lowered to independent depths necessary to reach the sea floor.  Independent Leg Jack-Up Rigs operate with approximately 60-100 personnel and up to a maximum depth of approximately 650 feet.

Mat Jack-Up Rigs are similar to Independent Leg Jack-Up Rigs except, as the name may suggest, the three legs that are lowered to the sea floor are connected by a large flat “mat”.  It is the mat that sits on the sea floor once the legs are completely lowered.  This sub-type of Jack-Up Rig can only be used in places in which the sea floor is within the limits of generally flat terrain for which the mat was designed. Mat Jack-Up Rigs operate with approximately 60 personnel and up to a maximum depth of approximately 350 feet.

Semi-submersible rigs are floating platforms that are partially submerged in water during operations. They are supported by columns connected to large pontoons submerged below the sea surface. This design offers stability and makes them suitable for use in rougher waters and at greater depths than Jack-Up Rigs.  There are two general subtypes of Semi-Submersible Rigs: Non-Self Propelled and Dynamically Positioned.  Non-Self Propelled Rigs can operate in waters as deep as 6,000 feet and Dynamically Positioned Rigs can operate in waters as deep as 10,000 feet.  A variation of both of these Rigs is a Tension Leg Platform (TLP) in which the Semi-Submersible Rig is moored to the sea floor with tendons meant to provide stability.  TLPs are generally limited to depths of approximately 4,000 feet.  Semi-Submersible Rigs are typically operated by 80-125 personnel.

Drillships are seafaring vessels equipped with a drilling apparatus. These ships are self-propelled, allowing for greater mobility and flexibility in moving from one drilling site to another. Equipped with dynamic positioning systems, drillships can maintain their position over the drilling site in deep waters with high precision. They are capable of operating in ultra-deep waters, often exceeding 12,000 feet and they are operated by up to 200 personnel.

It is clear that MODUs are engineering marvels that are operated in unforgiving environments in order to obtain the much-needed hydrocarbon resources the United States requires.  Personnel working on MODUs perform work in an unforgiving marine environment.  Although the level of risk associated with jobs on MODUs is greater than many other professions, that does not absolve an owner or operator of a MODU from ensuring that risk is mitigated to an appropriate level.  Further, if risk cannot be mitigated, it is ultimately incumbent upon the owner/operator to pause operations until risk mitigation can be performed properly.  Accidents, causing injury or even death, are inevitable if an owner/operator of a MODU fails to monitor and mitigate risk in the offshore environment.

Those impacted by an accident aboard a MODU should know their legal rights and remedies, and act quickly to preserve them. At Spagnoletti Law Firm, our attorneys have extensive experience in maritime cases involving MODUs.  Our experienced lawyers handle maritime personal injury and wrongful death litigation of all kinds and the skills needed to represent the families of loved ones who have lost their lives or those who have been seriously injured as a result of a maritime accident. The lawyers of Spagnoletti Law Firm have handled maritime lawsuits throughout the country.