Have you ever been curious about the process of constructing offshore oil and gas platforms? These massive structures, or platform rigs (Largest Oil Rigs), are crucial in extracting valuable resources beneath the ocean floor. But what exactly is a platform rig, and how does it work?
Many designs of fixed platforms have been used in different areas throughout the world and developments in platform design are continually taking place. New technologies have enabled operations to move further offshore into ever greater water depths and harsher environments. Self contained multi-well platforms are used for development drilling, if the drainage pattern of the reservoir and the well depth is such as to permit directional drilling of multiple wells from one platform.
They are designed to support one – and occasionally two – complete drilling rigs, the drilling supplies and the quarters for the drilling crews. The number of wells that can be drilled from these platforms may vary from ten to more than forty, with the wells spaced at surface as close as 1.8 to 3.0 metres between well centres. To accomplish this, the drill floor and substructure can be skidded from well to well over the platform skidding beams in two perpendicular directions, similar to the system in use on jack-ups.
Definition of Platform Rig
An offshore platform rig, also known as a drilling rig, is a structure that we utilize for exploring and producing oil and gas in the open sea. Generally, It is a large steel or concrete structure installed in water depths ranging from shallow to deep water, depending on the location and requirements of the project. The purpose of platform rigs is to offer a reliable and secure foundation for drilling equipment and production facilities. With the assistance of compliant towers, we can anchor them to the seabed, while we can use spar platforms or semi-submersibles to place them on the water’s surface. In general, platform rigs enable oil & gas companies to access and extract fossil fuels from the ocean floor, contributing to the energy needs of various industries worldwide.
Types of Platform Rigs
Each platform rig type has its unique characteristics and purposes. Choosing the appropriate rig depends on factors such as water depth, environmental conditions, and the specific requirements of the drilling operation. The significant types of drilling platforms are:
- Fixed Platform
- Steel Jacket Structure
- Gravity structure
- Compliant Platforms
- Compliant tower
- Guyed Tower
- Tension leg platform
Steel Jacket Structure
A steel jacket structure consists of a series of steel tubular legs, ranging from 6 to 10 or more. This structure is reinforced with horizontal and diagonal cross-members to create a rigid structure. (see Figure 1). When positioning the unit, its height is designed to ensure that the bottom of the deck remains clear of any high waves that may occur. The jacket is built on land under carefully controlled quality assurance and testing conditions, with all members being welded together. When built, or partially built, if very heavy, the structure is loaded onto a barge and towed out to its location. Flotation aids enable the jacket to be suspended from the barge and slowly sunk into position on the sea bed.
Once we have positioned the jacket, we proceed to anchor it to the sea bed using piles. These piles can be driven in from either the surface or sub-sea hammers to be around the legs of the jacket to ensure stability. In general, the jacket needs to have a stable foundation to support the addition of a steel deck and various modules for accommodation, housing drilling, and process equipment. These modules can only be installed after the jacket has been placed offshore. These structures are relatively cheap and have been installed in water depths of over 400 meters.
Components of Jacket Platform Rig
- Helideck
- Raised level on a platform to facilitate helicopter landings.
- Solar panels are mounted just below the helideck to facilitate auxiliary power
- Vent/ flare boom.
- Long truss that supports a vent or flare line.
- Topsides (Deck structure)
- The upper part of the platform (generally above the reach of the highest wave height) houses most of the equipment.
- Top-side equipment related to process, mechanical, electrical, piping, and instrumentation.
- Also, houses accommodation and housing like a workshop, battery room, etc.
- Jacket structure
- Part of the platform that supports the topsides and is generally submerged below the water line.
- A jacket is the supporting frame of the platform.
- The wave loads mainly govern its design.
- Crane pedestal
- A sizeable structural tube that supports an offshore crane for lifting purposes.
- Crane pedestals also function as diesel storage tanks since their diameter is large enough to house fuel.
5 Pros:
- Firstly, support large deck loads.
- Secondly, it may be constructed in sections and transported.
- Thirdly, support large field, long-term production (supports large no of wells)
- Fourthly, piles result in good stability
- Finally, little effect from seafloor scour
5 Cons:
- Firstly, costs increase exponentially with depth
- Secondly, high initial and maintenance costs
- Not usable
- Steel structure members subject to corrosion
- Finally, corrosion protection measures to be adopted
Topsides
Supported on a deck fixed on jacket structure. It consists of various modules
- Drilling
- Gas turbine
- Production
- Generating sets
- Gas flare stack
- Pumps
- Revolving cranes
- Compressors
- Survival craft
- Helicopter pad
- Hotel & catering facilities
- Living quarters with
Gravity Structure Platform Rig
The construction of the concrete gravity structure involves slip forming, which many do in sheltered water while in a floating mode. It includes a base unit often divided into compartments utilized as storage tanks. They require relatively firm soil conditions for their foundations but can usually easily absorb additional loads after installation. To construct them, it is necessary to have sheltered deep water sites. From the base, several legs, usually three or four, depending on design, take the height of the structure above water level. Therefore, to install the deck and modules, we place them on top of the legs, as shown in Figure 2. The concrete gravity structure is buoyant enough to float even with topside structures, allowing towing it to its final location.
Generally, after reaching the final position, filling the tanks at the base with water, which causes the unit to submerge in the sea bed, is necessary. It maintains its position by its weight and does not rely on piling or other devices. Therefore, we can conduct oil drilling using one or more of the hollow legs or through slots located between the legs. The oil storage will be in base tanks, displacing the water used for ballast, and is ready for export. To keep the unit stable, we usually fill the storage tanks with oil or water to maintain enough weight.
A variation to the concrete gravity structure is the steel gravity structure, which has a steel jacket and underwater storage tanks. It is an amalgam between the two previous types and, like the concrete structure, relies on its weight to maintain its position. Gravity structures have been installed in water depths of over 400 meters.
Types
Most common concrete designs are:
- Condeep (with one, two, three or four columns)
- ANDOC (with four columns)
- Sea Tank (with two or four columns)
- C G Doris
- Ove Arup
Pros:
- Support large deck loads
- Possible reuse
- This is helpful for achieving long-term production goals.
- It is possible to finish construction and testing onshore.
- Support large field
- Supports a large no of wells
- Large storage capacity
- More tolerant to overloading & seawater exposure than steel jacket platforms
Cons
- Subjected to seafloor scour
- Cost increases exponentially with increase in WD
- This particular structure may need a more significant amount of reinforcing steel when compared to a structure that has a complete steel jacket.
- Foundation settlement
Advantages of Gravity over Jacket Platform Rigs
- More excellent safety for people on board and top-side facilities
- Towing to the site with a deck is possible.
- Submerged concrete has lesser problems than steel structure
- Minimizes installation time and cost
- One can reach the seafloor by utilizing the cell compartments situated in the base.
- Low maintenance cost
- Capacity to support larger deck areas
- Adjustable crude oil capacity
- The process of securing the drilling risers involves placing them inside the central shaft.
- Healthy monitoring systems
Compliant Tower
A compliant tower platform offshore rig is a tall, slim structure designed to comply, or sway, slightly with wave action (see Figure 6). The purpose of this structure is to provide a temporary solution that bridges the gap between fixed and floating structures. It is capable of operating in depths of up to 1,000 meters in moderate environments.. The Lena guyed tower, installed in the Gulf of Mexico in 1983 in 300 metres of water, is the only platform application of this concept to date.
Tension Leg Platform Rig
A tension leg platform (TLP), built of either steel or concrete, consists of a hull anchored to the ocean floor with vertical tendons (see Figure 7.1.9). These tendons constrain vertical movement of the platform, eliminating vessel heave. This enables the use of surface wellheads. This type of facility also lends itself to pre-drilling the wells through a sub-sea template. Hence, all wells can be drilled from a semi-submersible rig while the production facility/workover rig is being installed on the TLP before deployment in the field. This unit is easily detachable from the site upon completing the field’s lifespan and potentially refurbished for future use.
Although the TLP concept was first investigated some 20 years ago, it was not fully proven until the construction of Conoco’s TLP platform in the Hutton field in the North Sea in 1984. The TLP, or tension leg platform, is a viable option for operations in deep waters. Especially that the depth of the water has little effect on the weight and cost of a TLP. The Auger TLP owned by Shell Oil was set up in the Gulf of Mexico, at a depth of 872 meters. Engineers believe that TLP technology can be expanded to work in depths of up to 3,000 meters.
References:
- Types of Platform, Onshore | PDF | Offshore Drilling
- Shell Offshore Drilling Manuals
- Offshore Petroleum Drilling & Production Book