Offshore Derrick Rig

I. Overview of the Derrick

The derrick is one of the most recognizable structures in the petroleum industry.

It is essentially a tall, mast-like steel framework designed to support various lifting equipment and tools such as cranes, carts, hooks, and other machinery.

Derricks play an essential role in oil and gas drilling operations, providing the height and stability required for tasks like drilling, casing running, and pipe string storage.

A derrick rig, commonly referred to as a drilling rig derrick, is a critical component of a drilling rig, both onshore and offshore.

The derrick supports various drilling equipment such as rotary tables, hoisting mechanisms, and other essential tools for drilling operations.

The key purpose of a derrick is to withstand the substantial weight and force generated during these operations, while also providing ample space for storing drilling pipes, tools, and other materials needed throughout the process.

Whether situated onshore or offshore, the derrick remains one of the most vital parts of the drilling rig.

Offshore derricks, in particular, are designed to withstand harsh marine environments, often towering above the surface of the sea to facilitate the drilling of wells deep beneath the seabed.

II. Types of Drilling Derricks

Derricks are commonly categorized based on their structural characteristics.

These classifications help engineers select the most suitable derrick for the specific drilling operation, considering factors such as environment, load-bearing capacity, and space requirements.

The four primary types of derricks used in the oil and gas industry are:

1. Tower Derrick
2. Type A Derrick
3. Mast Derrick
4. K-Type Derrick (Front-Opening Derrick)

1. Tower Derrick

A tower derrick is typically a space structure with four vertical edges and a truncated cone shape, with a square cross-section.

This type of derrick is often constructed from a single rod assembly, making it time-consuming to install and disassemble.

Despite its higher setup time, the tower derrick offers significant advantages in terms of stability and load-bearing capacity.

The design of the tower derrick features a wide base for support, which provides excellent stability during drilling operations.

The structure is fully enclosed, offering a high degree of rigidity.

This type of derrick is especially useful for offshore drilling rigs, where environmental conditions, such as high winds and waves, demand a robust and stable structure.

2. Type A Derrick

Type A derricks are characterized by a space-frame design that resembles the letter “A.”

The structure comprises two lattice or tubular legs, which are connected by additional rods at the upper part of the derrick, along with a second-floor platform.

This creates an “A”-shaped framework with good stability and bearing capacity.

One of the advantages of Type A derricks is their open-front and open-back design, which gives drillers an unobstructed view of the wellhead, making it easier to monitor drilling operations.

These derricks are typically installed at a lower height, and once installed, they are lifted using a winch mechanism.

They are commonly called “full-view derricks” because of their wide field of vision, improving safety and efficiency during operations.

3. Mast Derrick

Mast derricks are primarily used as part of a rig or workover rig setup.

They are composed of one or more segments of lattice or tubular legs.

Mast derricks are unique in that they are usually inclined at an angle of about 3° to 8° towards the well.

This slight tilt helps to stabilize the derrick during operation and is maintained by a rope or cable system.

Mast derricks are divided into two main types: telescopic and non-telescopic.

Telescopic mast derricks are more commonly used, as they offer the flexibility of extending and retracting to adjust the derrick’s height according to the needs of the operation.

These derricks are typically used on land-based rigs and workover rigs.

4. K-Type Derrick (Front-Opening Derrick)

The K-type derrick is a front-opening derrick, often used for its robustness and flexibility.

The structure is made from a space truss system, with sections that are welded or bolted together.

The design includes a truss rod system at the rear, which provides additional stability and strength.

One of the key benefits of the K-type derrick is its ability to be installed at a low height and then lifted using winches or hydraulic cylinders.

This feature allows for easier transportation and setup, making K-type derricks ideal for various offshore and onshore drilling applications.

Some models of K-type derricks are telescopic, further enhancing their versatility.

The front-opening design also allows for better visibility and accessibility during drilling operations, providing more flexibility for maintenance and operational efficiency.

III. Key Components of a Drilling Derrick (Using K-Type Derrick as an Example)

The derrick is composed of several critical components that work together to support the drilling operations.

Taking the K-type derrick as an example, here are the primary components:

• Derrick Body: The main structure of the derrick is divided into multiple segments, typically 4 to 5 in total. These segments are connected by pin shafts, creating a rigid framework. The upper section of the derrick is usually a closed quadrilateral structure, while the rest of the segments have an open front. The rear part of the derrick is supported by a detachable rigid frame and oblique struts.
• Second-Floor Platform: This platform includes the main deck, an operating platform, finger beams, and safety railings. The second-floor platform provides space for the drilling crew to operate equipment, monitor the progress of drilling, and perform necessary maintenance tasks.
• Cage Ladder: Installed on both sides of the derrick, the cage ladder allows operators to safely ascend and descend between the roof and the second-floor platform. This is essential for both routine inspections and emergency evacuation procedures.
• Lifting Device: The lifting mechanism consists of several components, such as a herringbone lifting system, buffer cylinders, ropes, and pulleys. These components allow the derrick to be raised or lowered as necessary, ensuring that it remains stable during operations.
• Derrick Accessories: These include essential equipment such as riser operating tables, casing centralizers, tongs balancing weights, and more. These accessories help maintain the structural integrity of the derrick and provide the necessary tools for efficient drilling and workover operations.

IV. The Derrick Base

The base of the derrick is another crucial component that plays a key role in supporting the entire drilling operation.

The derrick base must withstand the weight and forces generated by the rig, including the weight of the derrick itself, drilling equipment, and other heavy components.

The derrick base provides a stable foundation for the rig and helps distribute the load evenly to the foundation beneath.

In offshore environments, the base must also be capable of withstanding the additional forces from waves and wind.

There are several types of derrick bases, each with different characteristics:

• Box Stack Base
• Box Block Base (also known as Block Mount Base)
• Rotary Lift Base
• Slingshot Base (also called Double Lift Base)
• Retractable Base

Among these, the last three are classified as jack-up bases, which are commonly used in offshore drilling rigs.

These jack-up bases allow the derrick to be raised above the water surface, ensuring stability even in rough sea conditions.

V. Offshore Derrick: Special Considerations

Offshore derricks are a distinct category of derricks designed specifically for offshore drilling rigs.

These derricks are built to handle the challenges posed by the marine environment, such as high winds, waves, and saltwater corrosion.

Offshore derricks must be exceptionally strong and stable, often featuring reinforced structures and advanced materials to ensure reliability under harsh conditions.

Offshore drilling rigs use various types of derricks, including tower derricks, K-type derricks, and mast derricks, depending on the specific needs of the operation.

The derrick on an offshore rig must be able to lift heavy equipment, drill pipes, and other materials while maintaining stability in the face of challenging environmental factors.

VI. Conclusion

Derricks are indispensable components of the drilling rig, whether onshore or offshore.

They provide the necessary height and stability for the safe and efficient operation of drilling, casing running, and pipe string storage.

By understanding the different types of derricks—tower derricks, Type A derricks, mast derricks, and K-type derricks—engineers can select the appropriate structure based on the requirements of the drilling site and the environmental conditions.

Offshore derricks, in particular, must be designed with additional considerations, such as the need to resist the corrosive effects of saltwater and the ability to withstand strong winds and large waves.

As the oil and gas industry continues to expand into more challenging environments, the importance of robust, versatile derricks and their supporting components will only increase, ensuring that drilling operations can continue safely and effectively in both offshore and onshore environments.