Review on Fixed and Floating Offshore Structures. Part I: Types of Platforms with Some Applications

Diverse forms of offshore oil and gas structures are utilized for a wide range of purposes and in varying water depths. They are designed for unique environments and water depths around the world. The applications of these offshore structures require different activities for proper equipment selection, design of platform types, and drilling/production methods. This paper will provide a general overview of these operations as well as the platform classifications. In this paper, a comprehensive review is conducted on different offshore petroleum structures. This study examines the fundamentals of all types of offshore structures (fixed and floating), as well as the applications of these concepts for oil exploration and production. The study also presents various design parameters for state-of-the-art offshore platforms and achievements made in the industry. Finally, suitable types of offshore platforms for various water depths are offered for long-term operations. An extension of this study (Part II) covers sustainable design approaches and project management on these structures; this review helps designers in understanding existing offshore structures, and their uniqueness. Hence, the review also serves as a reference data source for designing new offshore platforms and


Introduction
With the increase in the need for more energy sources, fossil fuel has recently had huge competition as a non-renewable energy source with other renewable energy sources. However, some of these newer platforms have extended technologies that stem from the existing offshore platforms used in oil and gas exploration. Currently, there are advances made in ocean engineering which include a variety of innovative offshore structure designs, ranging from fixed platforms to floating platforms [1][2][3][4][5]. Some of these structures include the deepwater semisubmersible platforms, jack-up rigs, floating offshore wind turbines (FOWTs), FPS (floating production systems) units, floating production storage and offloading (FPSO) units, FSO (floating storage and offloading) units, FSU (floating storage units), FPU (floating production units), FDPSO (floating drilling production storage and offloading), MODU (mobile offshore production unit) and FLNG (floating liquid natural gas vessel) [6][7][8][9][10]. However, there are other applications for offshore platforms, such as dynamic positioning, exploratory activities, drilling/production, navigation, (un)loading ships, fluid transport, and bridge support [11][12][13][14][15][16]. Offshore petroleum structures are utilized for a wide range of purposes and in a wide range of sea depths and environments around the world, hence they need supporting attachments such as drilling marine risers [17][18][19][20][21][22][23], composite production risers [24][25][26][27][28][29][30], marine hoses [31][32][33][34][35][36][37][38][39][40] and mooring lines [41][42][43][44][45][46][47][48][49]. Figure 1 shows different fixed and floating offshore platforms operating in varying water depths (see details in the caption). Offshore platforms could be used as artificial reefs for many years, as they have also been used in a variety of aquatic environments. As a result, their design and upkeep are extremely difficult. Hence, it is pertinent that the design and maintenance of offshore structures are well considered, to prevent early decommissioning, high risks of corrosion, oil spillage, and other irreversible environmental damages. The applications of these offshore structures require different activities for proper equipment selection [50][51][52][53][54][55][56][57], design of platform types [58][59][60][61][62][63][64], engineering management of well bores [65][66][67][68][69][70][71][72][73] and other drilling/production methods [74][75][76][77][78][79][80]. Offshore oil production is one of the most visible of these applications, and it provides a significant task to the product designer or offshore engineer [81][82][83]. The design considerations include environmental loadings [84][85][86][87][88], hydrodynamics [89][90][91][92][93][94][95][96], hydroelasticity [97], corrosion [98], failure analysis [99], ocean wave mechanics [100][101][102][103][104][105][106][107][108], fluid content loadings [109][110][111][112][113][114][115], fatigue limits [116][117][118][119][120], reliability [121][122][123][124][125][126][127][128], etc. Therefore, the designer must ensure that there is safety, stability, high fatigue resistance with a long service life. The design with that is safe, but cost must be considered; hence the designer should make it economical for the client. Generally, these offshore assets must operate safely for at least twenty-five (25) years (depending on the purpose of the offshore structure), because they are exposed to extremely severe marine environments and varying sea depths. Hence the designs are conducted by using peak loads provided during the platform design life by the hurricane wind and waves. Environmental conditions are also important in designing different offshore structures [129][130][131][132][133][134][135]. Also, there are more developments made in oceanography and environmental sciences that reflect in different designs of offshore structures [136][137][138][139][140][141][142]. The fatigue loads caused by waves over the platform's lifetime and platform motion are all critical design issues considered in standards elaboration such as the American Petroleum Institute (API) [136][137][138][139][140][141], and Det Norske Veritas (DNV) [142][143][144][145][146][147][148][149][150][151][152]. Over time, these developed API standards have been revised to include hurricane conditions in the Gulf of Mexico (GoM), adaptable in other seas [153][154][155][156][157]. Strong currents can sometimes hit the platforms, putting strain on the entire system's integrity. Another challenge that oil corporations face is the project scheduling involving the length of time for the design and construction of these offshore assets. Furthermore, the size of these offshore structures is a consideration in designing their stability and hydrodynamics. Figure 2 shows the number of global deep water drilling activities across five (5) continents. Although it reflects a decrease in oil drilling/exploration activities due to the decline in oil price globally in 2016, it is evident that the highest drilling activities were recorded in South America in the time range from 2010 to 2021. Due to the recent COVID19 pandemic in 2020/2021, the exploration also had a decrease in oil well exploration; however, it was seen to pick up in 2021/2022. Another consideration factored in the design is the material density. Most offshore platforms are fabricated in shipyards using massive steel, or in-situ using concrete, as seen in gravity-based structures. These offshore structures-both fixed and floating structures are mostly used for energy generation or oil production. Offshore constructions are meant to be installed thousands of kilometers from shorelines in the open sea, lakes, gulfs, and other bodies of water. Steel, reinforced concrete, or a combination of the two, may be used to construct these buildings. Most oil and gas platforms are produced from a variety of steel grades. These range from mild steel to high-strength steel, despite some earlier structures being made of reinforced concrete called the Concrete Gravity Based Structures (CGBS). Steel platforms come in different sizes and shapes, based on their intended function and, most importantly, the water depth in which they will operate [29][30][31][32][33][34]. However, proper failure analysis and reliability studies have to be carried out on these offshore structures. Offshore platforms are extremely hefty and among the world's tallest man-made structures. Floating structures have been classified, based on water depths, such as shallow water (91-120 m and lesser than 91 m), mid water (121-305 m), deep water (306-1219.50 m) and ultra-deep water (1220.50-2285.69 m and greater than 2285.69 m). These offshore structures are available at different locations, from Offshore West Africa (OWA) to the Baltic Sea, the Persian Sea, the North Sea (NS) and the Gulf of Mexico (GoM). These seas have different oil companies and energy operators involved in offshore operations across different geographical locations. Presently, different oil companies have high impact oil wells as seen in some operators of various offshore platforms. These oil operators range from Exxon, Total, Petronas, CNOOC, Equinor, Qatar Energy, BP, Petrobras, Pemex, Hess, Aker BP, Lukoil and Lundin, as seen in the high impact drilling represented in Figure 3. Part I of the review is conducted on different types of fixed and floating offshore structures. Details of the sustainable design approaches and project management for these offshore structures are given in Part II [5]. In this review, Section 2 provides an overview of sustainable drilling/production operations, the platform classifications and applications. Section 3 presents different types of offshore structures. Section 4 discusses various applications, advantages and disadvantages of various offshore structures. Section 5 presents the conclusions and recommendations for future research. This review helps designers in understanding existing structures and their uniqueness and helps to serve as a reference data source for designing new offshore platforms and other related structures.
Statistically, the number of offshore platforms is not as high as that of land buildings such as high-rise buildings or sky-scrapers. However, some of these offshore platforms are taller than the tallest structures (such as high-rise buildings), although some of their lengths are underneath the sea, as seen in Appendix A. From the image illustrated in Figure A1 of Appendix A, the tallest Truss SPARs (Perdido SPAR in GoM and Aasta Hansteen in Norway) has been compared along the tallest structures in the world such as the Eiffel Tower in Paris, France, the Burj Khalifa in Dubai, United Arab Emirates (U.A.E.) and the One Twin Towers in New York, United States of America (U.S.A.), NICOM House in Lagos, Nigeria, and ONE Shell Plaza in Houston, U.S.A. These structures were found to be tall but not quite as much as the depth of these offshore structures, as most of the structural length of the offshore structures lie under water. However, the illustration in Appendix A also showed that, compared to other offshore structures such as semisubmersibles and the Tension Leg Platforms, the Truss SPARs are very tall. By function characterization, the fixed structures are fixed while the floating structures float [232][233][234][235][236][237][238][239][240][241]. Generally, a platform can be physically anchored to the sea floor in shallow water in some cases which is referred to as a fixed platform setup. The 'legs,' which extend down from the platform and are secured to the bottom with piles, are made of concrete or steel. The weight of the legs and seafloor platform on some concrete constructions is so vast that they do not need to be physically anchored to the seafloor and can just rest on their mass. These fixed, permanent platforms can be designed in a variety of ways. The main advantages of these platforms are their stability and minimal vulnerability to movement due to wind and waves because they are anchored to the sea floor [242][243][244][245][246][247][248][249][250]. However, these platforms cannot be used in ultra-deep water since the cost of construction columns (or legs) that are very lengthy is not economically viable. For ultra-deep waters, specific offshore platforms are designed and deployed in such cases. Although offshore platforms could be fixed or floating structures used, the size of an offshore platform can differ as well as the type of the platform and the water depth where it will be operating [251][252][253][254][255][256][257][258][259][260][261]. Various types of offshore floating platforms operating in varying water depths are illustrated in Figure A2 of Appendix A and Section 3.
Based on platform classification, the selection of offshore platforms for any specific site is determined by the environmental and operational water depth where the oil and gas deposits are discovered. Hence, the following alternatives for the offshore fields were presented by Sadeghi [83], based on the environment and seawater depths: (a) Jack-up rig or Tender rig for extraction of oil/gas, drilling and templates (jackets) in water depths up to 150 m; (b) A semi-submersible drilling rig with a template (jacket) platform for extraction of oil/gas, at sea depths of 150 to 300 m; (c) A semi-submersible drilling rig with guyed-tower platforms for oil/gas extraction at depths of 300 to 400 m; (d) Semi-submersible drilling rig with tension leg platform or semi-submersible oil/gas extraction platform for water depths of 400 m to 1800 m; (e) Drillship rig with tension leg, subsea system, or spar platforms for oil/gas extraction in depths greater than 1800 m; (f) Floating production storage and offloading (FPSO) are found operating in water depths ranging from 200 m to more than 3000 m [260] and depending on the environmental condition, they are maintained in position using either a spread or turret mooring system.

Floating Production Systems
Floating production systems are similar to semi-submersible drilling rigs, but they also include petroleum production equipment in addition to drilling equipment. Ships can potentially be utilized as floating manufacturing platforms. Large, heavy anchors or the dynamic positioning mechanism utilized by drillships can be used to keep the platforms in place. With a floating production system, the wellhead is attached to the seafloor rather than the platform once the drilling is completed. The extracted petroleum is delivered by risers from the wellhead to the semi-submersible platform's production facilities. These production devices can work in up to 6000 feet of water.

Fixed Offshore Platform Design
Fixed Offshore Platforms such as the template type platforms made of steel are the most often used offshore platforms in the U.S.A.'s Gulf of Mexico, California shorelines, Niger Delta regions of Nigeria, and the Persian Gulf for oil/gas exploration and production [14,83]. These offshore constructions must be designed and analyzed in compliance with the American Petroleum Institute (API)'s recommendations. There are four different types of fixed offshore platforms, which are conventional fixed platforms, compliant towers, junction platforms and bridged platforms (or complexes, as seen in Figure 4).

Subsea System
Wells on the sea floor, rather than at the surface, are used in subsea production systems. Petroleum is extracted at the seafloor, similar to a floating production system, and then 'tied-back' to an existing production platform. The well can be drilled with a mobile rig, and instead of constructing a production platform for that well, the recovered oil and natural gas can be delivered to a nearby production platform through a riser or even an undersea pipeline. This enables a single strategically located production platform to service a large number of wells across a vast area. Subsea systems can be installed in both shallow waters and deep waters. They are normally utilized at depths of 2100 m (6890 feet) or more, and they can only extract and transfer, not drill. Subsea systems are typically those systems whereby their wells have the wellhead mounted upon the floor of the seabed after drilling operations from the wells, by any of the drilling platforms deployed. Recent advances made in sea systems can be seen in the realization of Statoil's Subsea Factory [232][233][234], as seen in Figure 5. The targeted ambition for such subsea systems is summarized in Table 2.

Types of Offshore Platforms
Different types of offshore oil rigs and platforms are utilized depending on the water depth and location of the offshore oil/gas field. To drill wells and produce oil and gas, rigs are employed, and platforms are set up in the field. To achieve fossil fuels from oil products, drilling and production activities must be carried out using oil rigs and platforms. Drilling can be used for obtaining natural gas and oil offshore, offshore. Most of the oil deposits are far from the closest mainland, which involves a series of obstacles not encountered when drilling onshore. When drilling at sea (i.e., offshore), the sea floor could be many meters below sea level. As a result, while onshore drilling uses the land as a platform, drilling at sea necessitates the construction of an artificial drilling platform. Since there are different types of offshore structures as depicted in Figures 1 and 6, a comparative analysis of offshore structures is necessary.

Moveable Offshore Drilling Platforms
Offshore drilling rigs/platforms are divided into two categories. The first is a mobile offshore drilling rig that can be moved from one location to another, while the second is a stationary offshore drilling rig. Historically, the first submersible mobile drilling equipment to drill offshore in 1954 was called Mr. Charlie [261]. Over the years, newer developments have been made on moveable platforms. Offshore drilling platforms (and drilling rigs) are those platforms that can be moved from one drilling location to another or even higher application in the industry, as seen in recent leases. A mobile offshore drilling unit (MODU) or unit is a ship that can conduct drilling operations to explore for petrochemical minerals or exploit resources such as liquid or gaseous hydrocarbons, sulfur or salt that are present beneath the seabed. MODU can be jack-up, semi-submersible, barge-type or ship-shaped. For offshore oil and gas drilling, rigid platforms are necessary for drilling operations. They can be moved and retained in place by their own azimuth thrusters with dynamic positioning or hauled into place by a tugboat and moored. A recognized design and operational standard for semi-submersible mobile offshore drilling units (MODU) is the IMO's MODU Code.

Drilling Barges
Drilling barges are commonly used for shallow-water inland drilling. Drilling barges are massive floating platforms that require tugs to transport from one location to another. Canals, lakes, rivers, marshes, and other bodies of water are frequent areas for this to occur. Drilling barges are only suitable for still, shallow waterways and cannot survive the water movement found in vast open sea conditions. Figure 7 shows some drilling barges used in earlier oil explorations.

Jackup Drilling Platforms/Rigs
With one difference, jackup rigs are identical to drilling barges. After a jack-up rig is towed to the drilling location, three or four 'legs' are lowered until they land on the seafloor. Unlike a floating barge, the working platform can be raised above the water's surface. Jackup rigs, on the other hand, are only suitable for shallower seas due to the impossibility of extending these legs too far. This rig can only operate in waters up to 500 feet deep. These rigs are usually safer to operate than drilling barges since their working platform is elevated above the sea level [262]. In addition to exploration operations, jack-ups are utilized for drilling operations and wind farms service. Figure 8 shows a Jack-up platform in operation and a labelled projection.

Offshore Wind Turbine Platforms
The global concern about the emission of greenhouse gases has provided a line of research into alternative renewable and clean energy. In this regard, wind power is one of the fastest-growing alternative technologies. However, this technology could help power a clean energy transition only if it can overcome hurdles of cost, design and opposition from fishing. More so, the application of offshore structures in renewable energy has taken more interests on breakwater devices, water energy converters, and wind turbines (such as FOWTs). The scale of wind turbines are larger and more adaptable forms of offshore structures are seen today. Offshore wind turbines may either be fixed-bottom or floating types [263]. The fixed-bottom platform is quite common in off the coast of Denmark, consisting of 91 wind turbines [264]. By design, offshore wind turbines, which are anchored to the seabed with monopile or jacket foundations, can only operate in waters less than 50 m deep. This eliminates sites with the greatest winds and, in many cases, easy access to large markets. However, there are exceptions as the application of Fixed-bottom wind turbines is more economical as it operates in shallow water depths of 50 or 60 m [265]. On the other hand, the floating wind turbines are anchored to the seabed using mooring lines, thus, are suitable for deeper water locations and areas with the soft seabed. Floating wind turbines have been used in water depths of up to 700 m [264,265]. Different concepts have been identified in on FOWTs projects with different platform types. The projects include DeepCWind, HyWind, WindFloat, NRMI, DTU and MARINTEK wind turbines. The ability to install FOWTs in deeper waters has an open huge amount of the oceans for the generation of renewable wind power. Table 3 shows some existing wind turbines with details. Although, the floating wind energy is still in its early stage of utilization, close to 80% of the wind power potential is found in deeper water. However, only about 80 megawatts of a total of about 32 GW (0.25%) of the installed offshore wind capacity is from floating wind turbines [265]. This narrative may change in the near future with the US government under USA's President Joe Biden pledging to build more than 30 GW of offshore wind turbines by the year 2030, worth more than $100 m [265]. Hence, this might bring the assertions of the National Renewable Energy Laboratory (NREL) to reality, which suggests that the floating turbine projects could achieve cost parity with the fixed turbines by the year 2030.
By classification, there are four main types of floating platforms, namely the spar-buoy, the tension leg platform, the semi-submersible and the Pontoon-type (Barge-type) floating wind turbines. However, there are other types and design concepts because these are the most common platform already installed and adapted for various planned projects, such as the Semi-submersible platforms which are expected to be used in about 50% to 75% of projects. Figure 9 shows four (4) types of offshore wind turbines.

Semisubmersible Platform
Semisubmersible platforms are offshore oil rigs with floating drill units that incorporates pontoons and columns that, if flooded, will sink to a predetermined depth. The most common type of offshore drilling rig is a semi-submersible rig, which combines the advantages of submersible rigs with the ability to drill in deep water. Historically, the first semi-submersible was BlueWater Rig 1 in 1961 [64]. Semisubmersible rigs are similar to submersible rigs in that the lower hull 'inflates' and 'deflates.' Despite being partly underwater, the rig floats over the drill site. The rig is stabilized while drilling by the lower hull, which is filled with water. Semi-submersible rigs are held in place by massive anchors weighing up to 10 tons each. The platform is sturdy and safe to use in turbulent offshore waters thanks to these anchors and the rig's submerged component.
Semisubmersible drilling rigs are also floating production systems. It is made up of drilling and petroleum production equipment that is simultaneously positioned on the system. This mechanism is properly grounded at the seabed's bottom. In small oil storage facilities, this type of technique is more effective. Based on the system working described, it can be utilized from 1500 to 6000 feet. In general, these systems are less stable when subjected to high wave stress. Submersible rigs, such as jackup rigs, are ideal for shallow water and come into contact with the ocean or lake floor. Platforms with two hulls stacked on top of one another make up these rigs. The living accommodations for the crew, as well as the actual drilling platform, are located in the upper hull. The lower hull functions similarly to a submarine's outer hull: when the platform is moved from one location to another, the lower hull is filled with air, making the entire rig buoyant. The air is released out of the lower hull when the rig is positioned over the drill location, and the rig submerges to the sea or lake floor. This style of rig has the benefit of being mobile in the water, but it can only be used in shallow water.
By classification, there are three types of semisubmersibles: ship-shaped semisubmersibles, column-stabilized semisubmersibles and bottle-type semisubmersibles. These three types of semisubmersibles are classified by the method of rig submergence in water.
While the ship shaped semisubmersibles can be designed as ships, as the name implies, they are also one of the most often used hull systems for the design and construction of offshore deep water drilling and production platforms, followed by the column stabilized semisubmersible platform [56,58,219,252,253]. The bottle-type semisubmersible platform, on the other hand, is made up of bottle-shaped hulls that are positioned beneath the drilling deck and can be submerged by filling them with water. Bottle-type semisubmersibles, the first manifestation of this type of drilling rig, were designed as submersible rigs. The bottles below the rig were totally submerged due to this design consideration for the submersible, which rests on the ocean floor. Furthermore, the rig of the bottle-type semisubmersible provided remarkable drilling stability. It also provides stability for rolling, as well as reducing pitching caused by waves and wind. This type of semisubmersible needs to be studied because of the various environmental conditions. Some drilling sites are always difficult, with turbulent waves and occasional weather concerns such as hurricanes, storms, cyclones, high tides, and strong winds. As a result, it is necessary to dig into deeper and more turbulent seas. Semi-submersibles have recently opened up a new path for exploration and development operations. However, as time went on, naval architects understood that if the bottles were only partially submerged, the rig would keep its stability when drilling in deeper seas. The semisubmersibles are moored using mooring lines, and the anchors are the only connection the rig has with the seafloor. These bottle-type rigs were eventually designed to only be used as semisubmersibles. Bottle-type semisubmersibles' configuration and design have a different impact on their hydrodynamic behavior in rough weather situations, and hence on their use and functionality in ocean engineering. The semisubmersibles can have other classifications based on evolution such as sixth generation semisubmersibles, and by design, such as the Dry-Tree Semisubmersible (DTS). Since the construction of drilling rigs have traditionally taken place during economic booms, different "batches" of drilling rigs have been constructed. Depending on the year of construction and water depth capability, offshore drilling rigs have been roughly categorized into nominal "generations". Table 4 gives different generations for classifying semisubmersibles. Generally, semisubmersibles are multi-legged offshore floating structures consisting of a large deck, with several legs interconnected at the bottom underwater with horizontal buoyant members referred to as pontoons. The semisubmersibles are one of the preferred floating offshore platforms alternatives due to their advantages, including, stability and motion. However, their natural frequencies vary inversely with the draft and length, the appropriate selection of the geometric shape constitutes an essential criterion in the design of semisubmersibles [266]. Semi-Submersible may be stationed using dynamic positioning systems or anchored using mooring systems. For example, in 2002, a semi moored was deployed using spread mooring lines at a water depth 1875 m in offshore Malaysia, while another installation using a dynamic positioning system in 2003 was deployed in Brazil operating at a water depth of 2890 m. In the same year, a semi operating in 2730 m water depth was also positioned in the Gulf of Mexico (GoM) [267]. However, more recent developments have been made. Table 5 presents the list of some semisubmersible platforms used in recent developments. The most recent is the Appomattox semisubmersible platform operated by Shell in GoM, which was installed in 2019, as shown in Figure 10a. Figure 10b illustrates the parts of a semisubmersible.

Dynamic Positioned Drillships
Drillships are exactly what they sound like: ships that are used to conduct drilling operations. These boats are designed specifically to transport drilling platforms to deep-sea areas. A typical drillship will feature a drilling platform and derrick in the middle of its deck, in addition to all of the other equipment found on a huge ocean ship. Drillships also have a hole called a "moonpool" that runs the length of the ship and down through the hull, allowing the drill string to extend through the boat and into the water. This offshore oil rig is capable of drilling in extremely deep water. 'Dynamic positioning' systems are used by drillships. Drillships have electric motors mounted on the underside of the hull that can move the ship in any direction. These motors are incorporated into the ship's computer system, which employs satellite positioning technology and sensors on the drilling template to guarantee that the ship is always directly over the drill site. Dynamic positioning can also be used to keep semi-submersible rigs in place. Drilling rigs that are Semi-submersible drilling rigs can drill in much deeper water than the rigs mentioned earlier. Deeper depths of up to 6000 feet (1800 m) may now be reached safely and quickly thanks to technological advancements. Figure 11 shows a drillship semisubmersible by Transocean.

SPAR Platforms
Spar platforms are among the most often used offshore platforms. The acronym SPAR stands for Single Point Anchor Reservoir. The SPAR platform is an offshore floating platform with a relatively large draft to diameter ratio (aspect ratio). Its deep draft made the natural periods outside the wave ranges thereby attributing to its wide acceptance for different operational scenarios, especially in deeper waters.
The Spar platform is the world's largest oil extraction platform which can be employed at depths up to 10,000 feet. This platform is mainly comprised of a massive cylinder support system and a standard fixed rig platform. This large cylinder does not stretch all the way to the seabed. It is held together by large steel cables that are attached to the seabed. The extraction devices are mounted above this cylinder and will perform their duties. A big cylinder supports a standard fixed rig platform on these massive platforms. The cylinder, on the other hand, does not reach all the way to the seafloor and is instead held in place by several cables and wires. The big cylinder helps to keep the platform afloat while also allowing for mobility to absorb the energy of any impending hurricanes.
Currently, the Perdido platform, operated by Shell, is the tallest SPAR, and is comparatively one of the tallest structures in the world at 267 m, as depicted in Figure 12  In September of 1996, the first SPAR platform was placed in the Gulf of Mexico (GoM) was commissioned. The platform's cylinder was 770 feet long and 70 feet in diameter, and it functioned at a sea depth of 1930 feet (see details in Table 6). Unlike the semi-submersible, the spar platform consists of a single large diameter cylinder supporting a deck. The hull is normally maintained in position using a taut mooring system consisting of lines ranging from 6-20 [267]. Based on the design, spar platforms are available in three configurations, namely: Truss spar, cylindrical, and cell spar, as illustrated in Figure 13. Table 6 gives a list of some SPARs, while Figure A3 of Appendix A shows a list of different SPAR platforms that have evolved over the years.

Jacket Platforms
Jacket platforms are simply platforms for template (jacket) development. This steelbased fixed platform is commonly found along the shorelines of the Persian Gulf, Gulf of Mexico (GoM) USA, Niger Delta regions of Nigeria, etc. [14,83,87]. Jackets, decks, and heaps are the most common components of template platforms [116,119]. The Template (Jacket) type is used on different petroleum platforms and Seas like the Persian Gulf. Jack-Up platform is shallow water floating offshore structure used for exploration of offshore oil and gas. As the name implies, it has movable legs which can be retracted and extended vertically, that is, once in contact with the seabed the platform begins moving upwards and outside the water surface [268]. Jacket platforms are used for drilling and exploration operations. However, there are other platforms that are also used, based on the requirement of the drilling/production field [269][270][271][272][273][274][275][276][277][278][279][280][281]. Tables 7 and 8 respectively show the list of some jacket platforms constructed, and some wellhead jacket platforms (WHP).

Compliant Towers (Tower Platforms)
Fixed platforms are similar to compliant towers. They are made up of a slender tower that is attached to a seafloor foundation and extends up to the platform. In contrast to the relatively hard legs of a permanent platform, the compliant tower is flexible [241]. Since it can 'absorb' most of the pressure placed on it by the wind and waves, it can function in much deeper water. The compliant tower system is sturdy enough to survive hurricane conditions despite its flexibility. A compliant tower (CT) is a fixed rig structure utilized for offshore oil and gas production. The rig is made up of compliant towers that are flexible, narrow, and made on a pile foundation that supports. This foundation holds the tower, its standard drilling and production deck. Compliant towers are utilized in water depths ranging from 450 to 900 m and are designed to withstand substantial lateral deflections and stresses (1500 to 3000 feet) [282][283][284][285][286]. These structures are self-contained, and freestanding but their media are given supports by water. They exhibit static stability but have a far higher degree of lateral deformation/flexibility (about 2.5%:0.5%) than land-based structures, and are partially supported by buoyancy. In the early 1980s, the commissioning of Exxon's Lena oil platform led to the development of the first compliant tower. The Chevron's Petronius compliant tower, which was 531 m and is now 623 m deep, is currently the deepest, as recorded in Figure 14 and detailed in Table 9.
The compliant towers are designed by considering the natural frequency of the structure. Resonance is minimized and wave forces are de-amplified when flex elements such as axial tubes and flex legs are used. This rig construction can be customized to fit existing fabrication and installation machinery [287][288][289]. Production risers are more traditional than floating systems such as tension-leg platforms and SPARs, and are subjected to less structural loads and bending. However, constructing compliant towers in water depths larger than 1000 m gets uneconomical. Even with the higher cost of anchorage (or moorings) and marine risers, it becomes most appropriate to use of a floating production system [290][291][292][293]. However, one good advantage of the compliant tower system is that it is quite sturdy enough to survive hurricane conditions, despite its flexibility [294][295][296][297][298][299].

Tension Leg Platform and Seastar Platform
The Tension Leg platform is a type of platforms that is held by tendons. The tension leg platform operates on the same principles as the SeaStar platform. Since there is no water chamber to oppose the lateral movement, such a construction is less stable than a SeaStar platform. The Seastar platform is a larger form of the Tension Leg platform. The platform's long, flexible legs are anchored to the seafloor and run up to it. These legs, like the SeaStar platform, allow for a lot of side-to-side movement (up to 20 feet) but very limited vertical mobility. Tension leg platforms are capable of working at depths of up to 7000 feet. SeaStar platforms resemble tension leg platforms in size. The platform is made up of a floating rig, similar to the semi-submersible type (as mentioned in Section 3.5) [299]. When drilling, a lower hull is filled with water, increasing the platform's stability against wind and sea movement. Seastar platforms, in addition to this semi-submersible rig, also include the tension leg system found on larger platforms. Long, hollow tendons that go from the seafloor to the floating platform are known as tension legs. These legs are kept under continual tension and prevent the platform from moving up or down. Their elasticity, on the other hand, allows for side-to-side movement, allowing the platform to endure the power of the ocean and the wind without breaking the legs. When it is not cost-effective to build a larger platform, Seastar platforms are often employed for smaller deep-water reservoirs. They can operate in up to 3500 feet of water.
A floating rig, a lower hull, and tension cables comprise the Seastar platform. A water-filled lower shell boosts the platform's stability against wind and water movement. It also has a tensioned system in addition to the semi-submersible rig. The tension leg, which is made out of high-strength steel cables, is part of the tension system. Tension stress is not a problem with these wires. This construction is vulnerable to high wave and wind pressures, but the water-filled body will mitigate these effects, making the structure more stable. Figure 16 shows an illustration of typical TLP.
The Tension Leg Platform has been used in making notable historical developments in the oil and gas industry. The Heidrun Tension Leg Platform (TLP) was the first platform where a composite riser joint was deployed in 2002. It is also the first platform that composite riser joint was successfully deployed after extensive composite research. The TLP has 56 well slots on the subsea riser template. The Heidrun TLP has a total height of 109 m, a square pontoon having a box cross-section, a pontoon height of 110 m, a pontoon height of 13 m, and eight decks located near each of the four circular columns located at each corner. It is the first and biggest floating TLP with a concrete hull. It is the largest floating structure carrying the largest deck load ever, with a topside weight of 43,000 tons, and a total platform displacement of 288,200 tons. Conoco discovered the Heidrun field in 1985, which lies about 175 km off Norway's coast and north of Kristiansund at a water depth of about 350 m. It produces 65,000 barrels of oil daily, 110,000 barrels of water daily, and 760 m 3 of natural gas. The Heidrun TLP has produced over 944 million oil and gas barrels since October 1995, at 05:37 when the choke valve was opened to become operational. Figure 17 shows the Heidrun Tension Leg Platform (TLP), while Table 10 lists some tension leg platforms constructed with their details.

FPSO
The acronym FPSO stands for floating production storage and offloading. As the name implies, the FPSO is a production system equipped with processing equipment for the separation and treatment of crude oil and gas together with a large storage hull to store the treated oil for export [260,[268][269][270]. With the continuous push of production activities into deeper waters, the FPSOs have over the years dominated the oil and gas industry mainly due to their attributed advantages which include large storage hulls, and their suitability for application in remote offshore areas [269,270]. The International Maritime Association (IMA) and World Energy Reports (WER) reveal a total of 175 FPSO units in operation as of November 2022, which is equivalent to 68% of the overall floating production systems.
Shuttle tankers are also classified as FPSOs used for offshore production activities, such as with loading and discharging fluid products using (un)loading marine hoses [31][32][33][34][35][36][37][38][39]. Depending on the environmental condition, FPSOs are either maintained in position using a spread or turret mooring system as illustrated in Figure 18. A spread of FPSOs located in different geological locations is presented in Table 11 and Figure A4 of Appendix A.

Concrete Gravity-Based Structure (GBS)
A support structure maintained in place by gravity is referred to as a "gravity-based structure," most prominently offshore oil rigs. Due to their protected area and adequate depth, fjords are frequently used to build these structures. The basis of construction for the Concrete Gravity-Based Structure was the application of reinforced concrete. The base's design incorporates vacuum spaces or caissons to provide the structure with natural buoyancy, allowing it to be floated to a field development site. Once on site, the blank spaces on the seabed are flooded, and the topside modules are hauled into place. The vacant holes were then filled with permanent iron ore ballast or utilized as crude oil storage compartments. Due to the sheer massive weight of concrete structures, foundation piles are not required, thus the name gravity base structure [299][300][301][302][303][304][305][306][307]. Figure 19 illustrates a typical GBS, showing Troll A concept.
An example of a concrete gravity-based structure is the Troll A platform (as shown in Figure 20), which exists off the west coast of Norway, in the Troll gas field. This offshore structure was built in 1996, and it is recorded as the largest structure ever moved and dropped into the ocean [301,307]. Table 12 and Figure A5 of Appendix A show different GBS platforms with their details.

Applications of Offshore Platforms
There are a variety of applications for offshore platforms, with the advantages presented in this section.
Although, offshore platforms are subjected to a variety of strong forces (such as ocean waves, wind, and currents), and the materials used to construct them must withstand these forces. Based on platform design, steel and concrete are the most common offshore construction materials, although most concrete based structures are not very popular in recent times due to their limitations. The advantages and disadvantages of different offshore platforms are summarized in Table 13. Figures A6 and A7 of Appendix A show the location of most of the deep water offshore structures in the Gulf of Mexico. unconditional stability because the center of gravity is always lower than the center of buoyancy, resulting in a positive GM; and • derives no stability from the mooring system and hence does not list or capsize even when completely disconnected from its mooring system.
• Topside lift at the installation site • Large derrick barge required for topsides installation • Difficulty of installation because the hull and the topsides can only be combined offshore after the spar hull is upended; • Little storage capacity, which brings along the necessity of a pipeline or an additional FSO; and • Lack of any drilling facilities.

Exploratory Application of Offshore Platforms
This study presented various exploratory applications of offshore structures (fixed and floating) used in the oil and gas industry. Due to the orientation of the superstructure, the foundation of this semi-submersible in deeper waters needs high payload integration for minimized motion responses across every degree of freedom (DoF). During production on the platform, the oil and gas are separated and transferred to shore via pipelines or tankers. To achieve these, proper planning must be conducted for the lifting, transportation, installation, design, fabrication, and commissioning of these offshore petroleum platforms. Among the exploratory applications are (un)loading hose applications via shuttle tankers (or FPSO) and single point mooring (SPM) buoys. Other applications are ocean monitoring buoys, breakwater and wave energy devices. However, larger exploratory applications are seen as presented in recent luxury semisubmersibles, semisubmersible crane vessels (SSCV), offshore support vessels (OSV), and rocket launch pads.

Luxury Cruise
Due to the obvious level of stability that semisubmersibles can provide through reconfiguration, deep-draft semisubmersibles are the wave of the future for ocean engineering. Various press publications, ranging from a Cable News Network (CNN) article to an exclusive Forbes article, New York Times, Huffington Post, The Sun News, and Yatch World review made reviewed this Kokomo Ailand [400][401][402][403][404][405][406][407]. These journalists published articles covering the event on a novel kind of luxury cruise that was fashioned after a mini-island in September of 2015 [390]. Migaloo Private Submersible Yachts designed the floating system by using he column stabilized semisubmersible concept to construct the hull of the floating mini island. This hull would have an exceptionally high level of stability in order to operate as a yacht or luxury cruise ship [254]. The type and extent of deck support integration were the main advantages it provided over traditional cruise ships [400][401][402][403][404][405][406][407]. Recent years have seen some rather bizarre ideas in yacht design, from Lego-inspired vessels to futuristic craft that resemble Concorde jets on water [400]. Different rendered views of the super yacht are given in Figure 21. According to Migaloo [401], the Kokomo Ailand is a private floating habitat based on semi-submersible platforms with an overall length of 117 m, a beam of 78 m and a draft between 20.5 m to 9.7 m. However, it is still safe to say that nothing compares to Kokomo Ailand, an 80-m-tall private floating island with two beach clubs, a waterfall, and a shark feeding station. The fact that Kokomo is a real place is arguably the most amazing of all. In reality, the project's designers, Migaloo, presented their ideas at the 'Monaco Yacht Show', and they already had "quite strong" expressions of interest from clients all around the world, at the time of its design [400]. The structure gives a better look at the stunning layout of a super yacht as well as a semisubmersible. The untrained eyes may mistake the ship for an upscale oil rig, despite the fact that it is much more opulent. The futuristic floating island has two elevators, a jacuzzi with a glass bottom, and a penthouse that is 80 m above sea level. However, to transport large, hefty vessels would require time and movement at a speed of eight knots using eight (8) Azipods [402]. It is simply a piece of floating land, yet it is designed like an island that was influenced by nature. It can be supported by specially made support vessels, which are currently popular in the yachting and shipping sectors. Thus, it functions as an offshore primary base or hideaway from which one may travel anyplace. However, the design is also inspired by owners demands and the need to evolve from conventional designs. In a recent article by Migaloo [401], it depicted the evolution of the floating structure (submersible yacht) as a result of the impact of sustaining technological changes with disruptive concepts using the model by Professor Christensen C.M. [408], as seen in Figure 22.

Offshore Rocket Launch and Landing Platform
Another application is an offshore rocket launch and landing platform. Space Exploration Technologies (SpaceX) is investigating the potential use of modified semi-submersible oil drilling rigs for the launch and landing of their new completely reusable rocket Starship on a specific Starship offshore platform [409][410][411][412][413][414][415][416][417]. SpaceX has acquired two old offshore oil drilling rigs as the ENSCO/Valaris 8506 offshore model, which is a specified destination for the Starship spaceship. The two floating spaceports-Phobos and Deimos, were given them the names as the named after the moons of Mars. However, launch pads can also be used in other ways. A semi-submersible drilling rig called Ocean Odyssey has been modified to use as a rocket launcher. The drilling platforms were essentially identical when they were built and when Elon Musk, the owner of SpaceX, purchased them under the names ENSCO/Valaris 8500 and 8501, respectively. As part of a six-month effort, Phobos was relocated from the Port of Galveston to Pascagoula, Mississippi, in January 2021 to start the retrofit of the rig for Starship operations. The majority of the outdated equipment on the rig's deck has been removed as of July 2021. Since one of the platforms was supposed to be substantially operational by the end of 2021 and that Starships would fly out to sea and land on the platform later in 2022 to be carried to the platforms, refitting had also started around January 2021 on Deimos at the Port of Brownsville, USA. At the time of this publication, the Deimos platform was still under development. Figure 23 shows Deimos ocean offshore spaceport which is an offshore launch platform.

Converted Offshore Structures
In recent times, the use of converted offshore structures has been seen to have increasing advantages in exploratory, drilling and production activities. For floating offshore units, there are eight (8) distinct hull types to choose from [418]. These converted offshore structures include Mobile offshore drilling units (MODU), Service Offshore Vessels (SOV), and Offshore support vessels (OSV). The ship-shaped monohull is the most prevalent kind of vessel, such as the use of the Floating Storage and Offloading unit (FSO) P-47 which was converted from a former Very Large Crude Cargo (VLCC).
The offshore energy market is subject to frequent fluctuations, which also affect the demand for installation tools, floating structures, and support vessels [419]. The cost of new construction is high, and it frequently takes too long to reap the benefits of an opportunity when it arises. Another option is to upgrade or convert an existing unit. There are different tiers of capability growth when converting or upgrading existing equipment. With each level come higher complexity, hazards, and rebuilding expenses. Options include extending the life and modernising an older ship, temporarily converting it, increasing its capacity, adding features, altering its current function, and finally, completely converting an older merchant cargo ship into a brand-new offshore unit. If a major vessel conversion project is properly planned and managed, it is possible for it to be competitive with newbuilding choices. There are a lot of excellent prospects for upgrades and conversions within the sizable pool of current commercial and offshore vessels, both ageing and new vessels. This new lease on life broadens their operational and financial horizons and assists the offshore industry in making extraordinary strides in the performance of transport, building, and installation [418][419][420][421][422].
The considerations for the selection of offshore platforms are very important also for converting offshore structures from one purpose to another [419]. Purpose-built production semi-submersible platforms were created when the oil industry expanded into harsher regions and deeper waters. 'Deepsea Saga' was converted on Agyll oil field, as Agyll FPF semi-submersible which was later converted to Deepsea Pioneer. The Transworld 58 was the first purpose-built semi-submersible production platform used on the Balmoral field in the UK North Sea, was built in 1986 and later converted. There were various platforms that were converted, though [235,[423][424][425][426][427][428][429][430][431][432]. The Spirit of Columbus drilling rig was converted into Petrobras 36, which sank in 2001. Another offshore vessel that was renovated as part of a joint venture between BP and BHP was the Atlantis PQ. However, the largest converted semi-submersible platform for Production-Drilling-Quarters purpose is Thunder Horse PDQ with GVA40000 design.
Due to their excellent stability, wide deck areas, and variable deck load, semi-submersibles are particularly well suited for some operations which the offshore support vessel can perform [249,[433][434][435][436][437][438][439][440][441][442]. Some vessels have two (2) build dates because the second one represents the date they were rebuilt. However, the latest dates are mostly considered, because it represents the current qualification class and classification for the vessel or offshore structure. These vessels include the offshore multiservice vessel Q4000 which was built in 2002 for Caldive. The Transocean Marianas which is a 1979-built Offshore Safety Support Vessel was later transformed into a drilling vessel. While Sedco/Phillips SS was the first vessel built in accordance with Red Adair's suggestions, Iolair, an offshore safety support vessel, was built for BP in 1982. After being decommissioned, offshore production platforms have also been transformed into other offshore constructions [443][444][445][446][447][448][449][450][451][452][453]. Drilling semi-submersibles were modified for use as integrated drilling and production platforms when oil fields were initially created offshore. The Transworld 58 drilling semi-submersible was converted into the Argyll FPF, the first semi-submersible floating production platform, in 1975 for the Hamilton Brothers North Sea Argyll oil field. These vessels provided incredibly reliable and affordable platforms, such as the recent Norwind Breeze SOV converted by VARD for Norwind Offshore in 2022 [454][455][456]. Table 14 shows the list of some converted vessels and offshore platforms with conversion/delivery date.

Conclusions and Recommendations for Future Research
The manuscript presents a comprehensive review on different offshore structures-fixed and floating offshore platforms to examine some sustainable design approaches. It gives very interesting data and provides a valuable tool in support the general understanding, design and management of these structures, certainly in accordance with the industry design guidelines. The manuscript includes an introduction with a description of the state-of-art of the different types of offshore facilities and their purpose, classification of different types of these applications, with their advantages and disadvantages. Part II of this review [5] presents an in-depth review on considerations of most relevant parameters influencing the design process, a section focused on considerations regarding the management of the offshore facilities and the need for future research.
In this paper, the comprehensive review included the state-of-the-art on various offshore platforms and achievements made in the industry. Suitable types of offshore platforms for various seawater depths are offered for long-term operations, high productivity, high serviceability and sustainability. From this review, it has been identified that these platforms are divided into numerous sorts based on their functionality, and application and the depths of water where they operate. Therefore, it is evident that each offshore platform is different, as this review shows their variabilities and unique applicability. Although these platforms are subjected to somewhat large changes as a result of widespread wave activity. These platforms also remain extremely robust in a severe ocean environment, with a large portion of their structure submerged. An example of such platforms is the semisubmersible, as different oil and gas corporations have taken notice of its adaptable but durable properties.
In general, this type of platform is favored due to its ability to produce oil and gas as well as its cost effectiveness. However, other types of offshore platforms are also utilized based on their respective unique applications. Drilling rigs and Semisubmersible platforms were found to be the most promising design in the review study, and they are a viable choice for offshore exploration and production. These designs are also carried out in a variety of geographic locations and environmental conditions. Fixed jacket platforms have been seen in the North Sea and Persian Sea, as these areas do not require offshore structures with ultra-deep drafts, and the weather are not very extreme as most locations in the Gulf of Mexico. Furthermore, the efficient factors required to thoroughly improve the service life and failure patterns of these offshore structures should be considered. In more recent designs, there are exploratory applications of new concepts, production facilities and related devices such as wave energy and breakwater devices. Hence, new and sustainable design approaches have been applied as these techniques are more adaptable to these devices, and aid faster design of offshore structures. However, adequate validation to verify each design is recommended. In a nutshell, this review presents types of application, benefits and challenges of offshore structures. The solutions from these different technologies can aid in the design and construction of offshore structures by presenting a reference data source. This review also sheds more light towards the understanding of offshore structures to enable designers with more innovative concepts that are more resilient, efficient, durable and sustainable in the industry.

Data Availability Statement:
The data for this study is not shared as it is an aspect of an on-going study on this present research.

Acknowledgments:
The authors also acknowledge the permissions of different publishers and industry firms on the images used in this publication. We acknowledge Christian Gumpold, CEO of Migaloo Private Submersible Yachts and Migaloo Submarines, for permission to use images of Migaloo's Kokomo Ailand. The authors acknowledge the feedback and support of the reviewers.

Conflicts of Interest:
The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. The authors declare no conflict of interest.