Semi-submersible platforms (SSP) are great for stability because their ballast pontoons are located below water. In areas hit by adverse weather, this enhanced level of stability may be required. Therefore, more excellent stability of the semi-submersible vessel is often recommended for field development.
This article will overview semi-submersible platform history, design, installation, applications, and various SSP benefits and challenges.
A Semi-Submersible Rig, often known as a “Semi-Sub,” “Semi,” or “Floater,” is a drilling rig used to drill wells in water depths unreachable to jack-up rigs with water depths greater than 400 ft. Additionally, Semi-submersible drilling rigs are buoyant, unlike jack-up rigs that float during drilling operations.
Historical Development
1920 – Edward Robert Armstrong may have focused on his notion of “seadrome” landing strips for airplanes, which needed the equivalent use of columns on ballast tanks below the surface and attached to the ocean floor by steel cables.
1961 – In 1961, the first semi-submersible arrived by accident. Blue Water Drilling Company owned and operated the submersible drilling rig Blue Water Rig No.1 for Shell Oil Company in the Gulf of Mexico.
1970 – A new, second-generation semi was developed and built in the early 1970s, with more unique, complex Mooring and subsea equipment. This design was primarily intended for 600 feet or fewer water depths, some reaching more than 1,000 feet.
1990 – The fifth-generation units, such as the Deepwater Nautilus depicted, were more capable in the late 1990s. These machines can work in exceptionally hostile settings and depths of more than 5,000 feet.
Concept and Design
A semi-submersible platform is a type of marine vessel used for offshore purposes such as offshore drilling rigs, safety vessels, oil production platforms, and heavy lift cranes. They have superior ship stability and seakeeping to drillships.
Generally, a semi-submersible rig is a drilling rig that sits on a deck space supported by multiple columns connected to floating pontoons.
Floating Rigs
Semi-submersibles are enormous floating structures with several legs. These legs are joined at the bottom of the ocean by horizontal buoyant elements known as pontoons. Some early semi-submersibles resembled ships, with twin pontoons and a bow and stern.
Pontoons
The pontoons are ballasted, making it easier for large ships to achieve submersion and buoyancy. Also, if the vessel needs to travel from the deeper depths of the offshore areas to the shallow regions, ships can alter the vessel height by pushing out the water from the ballast tanks.
Anchors
Anchors attached to cables and wires in 6 to 12 provide anchoring for the semi-submersible crane vessels. There is no danger of the large ships crashing into the ocean because the functional area of the semi-submersible rigs is placed very high from the submersible pontoons.
Classifications of Semi-Submersible Platform
Semisubmersible work-decks float on top of the water, with the great bulk of its mass retained below the water to help support and keep the platforms in place.
Drilling semi-submersible rigs come in three main types:
Sea Star Platforms
SeaStar is a tiny TLP with a single surface-piercing column. The SeaStar platform is designed to transport a variety of payloads while remaining relatively invisible to environmental factors. Many different SeaStar platform designs have been created and patented. British Borneo created a basic hull design. The following are the structural advantages of the SeaStar platform:
Spar Platforms
Spars are cylindrical structures that sit on a hollow cylinder suspended 700 feet below the sea’s surface. All the tanks are housed in cylindrical construction, which may have permanent ballast at the bottom for stability. In addition, truss construction and heave plates may be used in the cylindrical hull. This stabilizes the platform and allows drilling depths of up to 10,000 feet.
Pontoon Platforms
Pontoon platforms operate like pontoon boats – with large ballasts on two sides of the vessel. These help displace large volumes of water and keep the ship afloat. The larger size also allows for more stability and dynamic positioning systems.
Installation and Construction
Due to the great water depths, semi-submersibles are the preferred drilling method from a floating posture. Semi-submersible (“semi”) rigs float like ships as they are towed into position, where their pontoons can be flooded, partially immersing the rig.
Dynamic Positioning System
This platform has a narrower waterline plane, is less susceptible to wave impacts, has good stability, a long autonomous period, and a significant working depth. It can be anchored or stationed utilizing dynamic positioning. Semi-submersible vessels are less sensitive to wave action than drilling ships because most structures are beneath the sea surface.
Mooring Pattern
Spread Mooring patterns are operated to keep the floating rig in place. It is similar to multiple anchors. Additionally, Mooring is chosen based on the shape of the unit and sea conditions.
Lifting Operations
The capacity to execute cargo loading and offload is one of the most significant functions of semi-submersible vessels. When doing such duties, the vessel’s main deck must be submerged to a particular draft below sea level by filling the vessel’s ballast tanks. The submerging operation could be dangerous and must be done correctly because any error could result in the vessel being lost. Moreover, semi-submersible crane vessels help transfer hefty weights offshore construction.
Pipelay
Semi-submersibles were created in response to the extensive weather downtime encountered by monohull pipelay barges (especially in the North Sea). These ships have outstanding weather capabilities and can provide a steady platform for pipelaying in Beaufort force eight seas. However, the semi-submersible vessel is frequently unable to operate in rough weather due to the constraints of the anchor handling vessels.
Offshore Production
Offshore production platforms were when we initially established offshore oil fields; drilling semi-submersibles were adapted as combined drilling and production platforms. The industry developed purpose-built production semi-submersible platforms as the technology advanced in deeper water and harsher settings. It also provides highly stable and cost-effective platforms.
Types of Submerged Semi-Submersible Drilling Units (SSDU)
Bottle-Shaped Semi-Submarines
This type is similar to a bottle-shaped hull beneath the drilling deck. It also provides outstanding stability for drilling operations. Initially, the bottles beneath the rig were entirely submerged and resting on the ocean floor. As time passed, the bottles became just partially immersed.
Column-Stabilized Semi-Submarines
These horizontal hulls are connected to the drilling deck above the water via cylindrical or rectangular columns. Smaller diagonal columns support the structure while mooring lines and dynamic positioning assist in keeping the semi-sub in place.
Benefits & Challenges
Benefits
- Semi-submersibles have an excellent (slight) motion response and may thus be positioned more readily over a good template for drilling.
- The most stable floating rig.
- Semi-submersibles have a considerable number of flexible risers.
- It has a vast deck space.
- It is simple to get from one site to another.
- Reusable and adaptable to other support semi-submersible vessels.
Challenges
- High startup and operating costs.
- To export produced oil, pipeline infrastructure or other mechanisms are required.
- Semi-submersible construction timelines are often longer than jack-up rig timelines. Deck load is limited (low reserve buoyancy).
- Fatigue in the structure.
- Moving long distances is expensive.
- There are few dry-docking options available.
- In severe weather, mooring systems and risers are challenging to handle.
Application on Energy Resources
Offshore platforms are essential sources of wind, oil, and gas extraction. Because these enormous constructions are located on the seas, powering the offshore facilities is a significant factor.
Furthermore, many other floater concepts are in the works. These are semi-submersible, spar-type, barge-type, hybrid, and tension-leg designs.
Semi Submersibles for Wind Energy
WindFloat Atlantic
The WindFloat Atlantic project is bound for Viana do Castelo, 20 kilometers off the coast of Portugal. The floating structure – 30 m high and 50 m apart – can accommodate the world’s most commercially available wind turbines on a floating platform, each with an 8.4 MW production capacity.
Semi Submersibles for Oil
Mighty Servant 3
Mighty Servant 3 is a semi-submersible heavy lift ship weighing 27,000 tons. Its deck is 40 by 140 meters (130 by 460 ft). Oshima Shipbuilding Co. built the vessel in 1984. Most Mighty Servant 3’s cargo is oil platforms and drilling sector equipment.
Semi Submersibles for Gas
Q4000
Cal Dive International placed an order for the Q4000 multi-purpose oil field and gas construction and intervention vessel in 1999. It was built for $180 million at the Keppel AmFELS shipyard in Brownsville, Texas. These semi-submersibles are highly suited to offshore support vessel operations due to their high stability, large deck areas, and variable deck load.
Conclusion
Ultimately, the semi-submersible platform industry is likely to grow. Demand for vessels is predicted to increase with the global expansion of the wind, oil, and gas industries. This development is expected to boost oil and natural gas output in these areas, increasing the need for a fleet of semi-submersible drilling platforms.
Given the risks of digging into the unknown, it is impossible to justify such high-priced conversions. The difficulty for the offshore business is drilling safely and economically, which means “economic technology,” with safety, environment, security, and people’s health all playing key roles.
However, technological developments are still expected to innovate the efficiency of these platforms, likely driving new investments soon.