Types of Mobile Offshore Drilling Units
See also Drill Down Issue #3 for related information.
Mobile Offshore Drilling Units (MODUs) can be constructed from several designs to support varied operational requirements. In this article, we'll look at the designs associated with the MODUs that are typically utilized on the U.S. Outer Continental Shelf.
Independent-Leg Self-Elevating (Jack-Up)
Independent-Leg Jack-Ups are buoyant shallow water MODUs that can elevate their hulls completely out of the water for a stable platform to conduct drilling operations. With the hull waterborne, they are easily transported (towed) from one site to the next.
Each leg can be jacked down to different depths to be utilized on a seafloor with an uneven contour. The bottom of each leg has a spud can attached (a highly structured tank with a point), which allows for seafloor penetration. The legs will usually be of the truss style, which consists of large steel tubular columns (chords) with smaller tubular pieces crisscrossing and connecting to the chords. This design is strong and lightweight.
Once on site, the jack-up will cantilever the derrick out and over the target area.
There is a level gauge and alarm(s) located at the jacking station to ensure that the MODU stays level during any jacking operation.
Mat-Supported Self-Elevating (Jack-Up)
Mat-Supported Jack-Up MODUs are similar to the independent leg jack-up, except for the leg construction and the mat attached to the bottom of the legs. Mat-supported Jack-Ups also tend to be smaller and rated for less water depth than the Independent Leg Jack-Ups.
This type of jack-up is best utilized on a level bottom due to the mat. The mat is a series of tanks attached to one another to make one large structure (often shaped like an “A”) which attaches to bottom of the legs. Being that the legs are all attached to the same mat, they must be lowered together and cannot operate at different levels. The legs on this type of MODU are typically of the columnar style which can be described as a long hollow steel column.
This type of leg is less stable and doesn’t handle stresses the water applies as well as the truss style. Due to this, this style of jack-up is not utilized in water depths over 250 feet.
Semi-submersible MODUs are units with the main deck attached to the underwater hull (pontoons) by columns or caissons. These pontoons are utilized for ballast and housing machinery. Semi-subs can be of two configurations to maintain location over a drilling site:
1) moored (held in position with multiple anchors) or,
2) dynamically positioned with a dynamic positioning (DP) system.
This hull design has made the ”semi-sub” the most stable of the floating MODUs and can handle harsh weather or site conditions, but they are sensitive to load changes.
Unlike the jack-ups that cantilever out over the drilling site, the “semis” have a large moon pool under the drilling derrick that gives access to the sea. A moon pool is an opening that goes through the deck/hull to allow the drilling derrick to be centered over the drilling site.
Currently, “semis” are utilized in water depths up to 9,500 feet.
Drillship (Surface Unit)
Drillships are ship-shaped MODUs. While early drillships were moored or manually positioned over a drilling site, dynamic positioning (DP) systems are utilized to maintain station on current generation drillships.
These types of MODUs are less stable than the semi-subs, but are not as sensitive to load changes.
Like the semi-sub, they have the drilling derrick located over a moon pool.
Drillships are typically utilized in deep and ultra-deep waters ranging from 2,000 to 10,000 feet.