Drillships and Semisubs for Dummies

By: Trevor Crone, Rigzone Analyst

Drilling deepwater exploration and development wells can be a difficult concept to grasp for several reasons. The biggest hurdle to get past is the fact that most people have never set foot on an offshore drilling rig and have no idea what equipment is used to get the job done. Another issue is that the majority of operations occur out of sight (from land) so there is no way to visualize the action. In an effort to shed some light on the process, we’ll focus on modern drillships and semisubmersible drilling rigs and their differences.

Drillships are basically what the name describes; a conventional ship-shaped hull with a drilling package installed amidships. The first drillships were built in the late 1950s, and were a little more than converted tankers and cargo ships that were moored on location using an array of anchors off of the bow and stern. Advances in technology and the need to drill in deeper waters guided drillship design to the purpose-built, dynamically positioned (DP) vessels we have today that are capable of drilling 30,000-foot holes in up to 12,000 feet of water.

One attractive feature of modern drillships is their mobility. Using their DP thrusters, they can sail from one location to the next under their own power without the need of hiring expensive ocean-going tug boats. Drillships also provide high variable deck loads and internal storage capacity.

Use of a conventional hull is both an asset and a hindrance to drillships in that higher transit speeds can be achieved but vessel stability in rough seas is sacrificed. Any rig must be capable of operating safely and efficiently in one location for a long period of time which increases the chances of encountering harsh weather. Because of this, drillships are more suited to carrying out exploration drilling programs in relatively benign environments like the U.S. Gulf of Mexico, West Africa and Brazil.

On a basic level, drillship designs haven’t changed much through the years. Drilling equipment, power generation equipment and living quarters must all fit in a ship-shaped hull in roughly the same arrangement to ensure stability and efficiency. Semisubmersibles (semisubs), on the other hand, have been quite varied in terms of size, deck layout and column arrangement.

Semisubs consist of a deck box that contains the drilling package, machinery spaces and living quarters supported by columns that sit atop 2 submerged pontoons that are oriented with the centerline of the rig. Most semisubs built today are designed around 4 columns but there are several older rigs operating based on 3, 6 and 8 columns. The pontoons and columns contain ballast and storage tanks along with some machinery.

A semisub’s method of remaining in one position is largely dependent upon the age of the rig and its rated water depth. These rigs will fall under the category of dynamically positioned (DP) or conventionally moored. DP semisubs, like modern drillships, use steerable thrusters to counteract the effects that currents and wind have on the vessel. Conventionally moored rigs will remain on location using a spread of anchors linked to the corners of the rig by legs of wire rope, chain or a combination of both.

Like a drillship, a semisub’s below water geometry has its plusses and minuses. Concerning mobility, the twin pontoons of a semisub are not as efficient as the single hull of a drillship. A DP rig’s thrusters will use a large amount of power and the vessel will not travel as fast as a drillship. Additionally, in order to mobilize from one location to another, a rig must deballast so that the top of the pontoons are out of the water. This increases stability issues when the rig is on the move as the center of gravity is much higher.

While hindering speed and stability during transit, twin pontoons help make a semisub extremely stable when on location. With only the columns exposed to the water line when ballasted to operational draft (pontoons submerged), the effects of winds, currents and wave-induced heave are minimized, making a semisub inherently stable and suitable to areas of harsh weather such as the North Sea. Operational stability, lots of deck space and easy access to the moonpool make semisubs suitable to both exploratory and development drilling programs in a wide variety of locations around the globe.

Many other factors contribute to an operator’s decision to contract a drillship or semisub such as a rig’s safety and efficiency record, rig availability and current dayrates for each type of vessel. This is only intended to give an overview of the rigs used to provide the world with deepwater oil and gas.

If you have any questions about how the oil and gas industry locates hydrocarbons and pulls them out of the ground, please check out Rigzone’s How It Works page in the training section.

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