The different types of offshore drilling rigs

By Administrator on Jan 26 in Offshore Drilling, Useful Information.


With the advancement and evolution of the hydrocarbon industry, the offshore drilling industry is also constantly upgrading, and different types of marine vessels are used for the drilling of the oil and gas wells in the seas.


We are going to have a look over the three main types of offshore rigs, we will see what is the difference and what is common between them. Also, we will see what are the strong and weak points of each one of them, and what makes them preferred for a certain job.


Jackup Drilling Rig:

So let’s begin with the first and most popular(most commonly used)drilling rig. Generally speaking, this is a platform with legs which are lifting and lowering the platform whenever needed. When the rig needs to be moved, the legs are going up and the platform is going down until the hull reaches sea level and the vessel is ready for sail. When the vessel reaches the drilling site, the jackup needs to make the opposite procedure-lower down the legs to the sea bottom and to rise the hull away from the sea level, at a distance that is high enough so it can avoid harsh sea conditions. According to the type of legs which this type of drilling rigs are using, they are divided into two subtypes

(Open-Truss Leg Jackup drilling rig)Image source:workoffshore.net

COLUMUNAR– Whit this type the legs are made from giant steel pipes. The columnar jackups are cheaper for construction but they are not so stable and this is one of the reasons that they are not so commonly used.

OPEN-TRUSS LEGS– This is the second type which is equipped with legs made from many small pipe sections welded together(similar to the electrical tower’s structure). They are more expensive to build, but thanks to the light structure and to the design that allows the free passing of the wind through the structure of the legs without applying pressure to them, they are preferred and more often used.

Except with the legs shape and structure, the jackup rigs can be distinguished by many other paragraphs, but we are going to discuss this subject some other time.


And so after we found out in basic lines what the jackup drilling rig is let’s see what are their pluses and minuses compared to the other two types of drilling rigs.

Advantages:

  • This type of drilling rig is cheaper for construction than the other two.
  • Their operational cost is lower(compared to the others you don’t need such a large marine team on a drilling jackup)
  • The jackup drilling rigs are extremely stable, which makes them the preferred choice for rough seas.

Disadvantages:

  • Because of the reason that they need to step on the sea bottom when operating, the use of the jackup rigs is restricted only to shallow sea zones.
  • The space on the platform is limited and that’s why often such rigs are supported by few ships loaded with equipment.
  • Jackup rigs are not self- movable, which means, that whenever they need to change the drilling locations, they need the assistance of tugboats or heavy load carriers.

Semi-Submersible:

As shown by the name, this is a type of rig, which is partially submerged. In basic lines, this is a platform that has pontoons attached to the bottom of the hull. When the rig is on a drilling mode, these pontoons are filled up with water and the rig is partially submerged which restricts its movement and makes it more stable. Mooring patterns(Multiple anchors used to stabilize the vessel) are used to keep the vessel in the right position. Additional DP systems can be used to restrict the movement caused by the water currents.


When the rig needs to be relocated the pontoons are empty and the hull is above sea level, which makes the moving of the platform possible.
Like the jackups , here we also have two different subtypes depending on their structure.

(Semisubmersible drilling rig Scarabeo 8)Image source:wikimedia.org

BOTTLE TYPE– This is a kind, in which the legs underneath the hull are connected with pontoons, shaped like bottles.

COLUMN-STABILIZED- Here we have two horizontal hulls, which are connected directly with the platform deck through cylindrical columns above the water.

Now let’s have a look over the pluses and minuses of the Semi-submersible rigs.

Advantages:

  • The design of the platform allows it to operate in extreme water depths.
  • Semi-submersible rigs are relatively stable, considering that, they are floating rigs.
  • These rigs are equipped with large deck space, which allows the storage of a large number of drilling materials.

Disadvantages:

  • Higher operational cost compared to the jackup rigs. Here you need to have a higher amount of marine personnel.
  • With these rigs, you also need the service of tugboats or heavy load carriers when the drilling location needs to be changed.
  • Although they have much more space on the deck, very often before moving(especially in long distances)those rigs need to distribute part of the equipment on support ships, in order to lighten their weight.

Drillship:

As you can already probably noticed from the name, here we are talking about a fully functional ship, specially constructed for the needs of the drilling industry. Not long after the first offshore well is done, the oil and gas industry figured out that, the ships will be a great asset to the business as offshore rigs. But before we see the advantages and disadvantages of the drillship, let’s see what exactly is this vessel.


Some of the marks that distinguish this ship on the outside are the Derek (the high tower which is used for the drilling), and the moonpool (a big hole in the middle of the ship’s hull, from which the drilling equipment is passing).
On the inside, these ships are provided with all necessary equipment for the drilling of oil and gas wells in deep waters.

(Drillship Valaris DS-9)Image source:Valaris

Here are the most common advantages and disadvantages of this type of drilling rigs:

Advantages:

  • Movability is definitely one of the biggest advantages of this type of rig.
  • Storage space. Besides that it is self movable, the drillship has a large amount of space for storing drilling equipment on its board.
  • They are able to operate in extremely deep waters.

Disadvantages:

  • The manufacturing time and the price of this type of drilling rigs are extremely high.
  • As we are talking about a fully functional ship engaged with the drilling, we need a high amount of personnel for the normal operation of the vessel, which leads to a higher operational price.
  • As we mentioned the drillships are perfect for drilling in deep waters, but they are not very stable in high waves and stormy waters.

The offshore wells are a task, which often requires different types of drilling rigs. That’s why we can conclude, that each one of here mentioned types is equally important and needed for the development of the hydrocarbon industry.

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