Fixed platforms, like the name imply are platforms that are fixed to the sea bed. The legs of these platforms are made of concrete or steel or in some cases both concrete and steel. These legs extend all the way to the bottom of the sea. It could either be fixed or weighed down to the sea floor with heavy concrete structures. In the case where it is weighed down by concrete structures, the heavy concrete structures make the legs so heavy that it can just sit on the sea floor without necessarily being fixed. Steel Jackets used for fixed platforms. These platforms could be manned or unmanned, depending on how big it is and its functions.
The manned platforms are usually much bigger and contain a living quarters for the workers, on the other hand, the unmanned ones are usually small and are connected to the manned ones. Unmanned platforms are only visited when there is a problem or work over has to be done. Fixed platforms are usually designed for long term use. WATER DEPTH: Fixed platforms are usually used for shallow water to fairly deep water. The water depth for these platforms ranges from 500ft to 1500ft. This is because it is not economical to build legs long enough for extremely deep or ultra-deep water.
A Fixed platform in the North Sea (Photo Credit: wiseGEEK). PRODUCTION CAPACITY: Fixed platforms depending on the size can handle up to 50, 000 bbl/d. The extracted fluids from the reservoir go up to the platform via risers. STORAGE CAPACITY: In the case where a fixed platform is close to the shore, then the produced oil may be pumped directly to onshore storage facilities through pipes laid on the ocean floor. In cases where the production site is far from the shore, then the platform must include large storage tanks which hold the oil until it can be further transferred to an oil tanker.
These storage tanks are usually below the water line, where they sometimes have floatation capabilities which help the platform resist the force of waves and current. In general, the storage capacity of fixed platforms can go up to 700,000Bbl. COST IMPLICATION: The cost of fixed platforms increases exponentially with depth. Also, the initial and maintenance costs for fixed platforms are high. This brings the cost to a range of US $ 20- 80 million. SAFETY AND RELIABILITY: Fixed platforms are very safe and reliable as it is fixed to the seabed thus it is firm.
This makes fixed platforms safe for long time use ranging from 10-25 years. JACK-UP RIGS DESCRIPTION: The jack up rig is the most common offshore drilling rig. This type of rigs consists of floatable hull with several lattice structure or cylindrical legs which can be raised or lowered to seabed. By continually lowering the legs, the hull containing the drilling facilities can be raised above the water. The rig is a self-contained drilling package with the derrick drill floor generally mounted on a cantilever deck which can be projected out from the hull over the drilling location.
The rig does have independent mobility but the speed is so slow that is generally towed to locations. The hull shape can be triangular, rectangular or pentagonal with up to 8 legs. The legs can be extended up to 300 ft. WATER DEPTH This type of rigs are generally designed for water depths of up to 350 ft. however, when drilling is required in waters that are deeper than the capabilities of a jack-up, semisubmersibles and drill ships become a more logical choice for exploration and development operations. PRODUCTION CAPACITY
Regarding production capacity it is considered the drilling rig Aquamarine Driller is owned by Vantage Drilling. It is a jack-up rig built at Keppel shipyard in 2009. It has maximum production capacity of 20,000 barrels of oil a day. The rig can operate at maximum depth of 375ft under water and can drill to a maximum depth of 30,000ft. It can accommodate 120 people STORAGE CAPACITY The storage capacity of jack up can be estimated as fuel oil= 4000 BBLS, also for drill water approximately 7000 BBLS, portable water is almost 2500 BBLS and finally bulk mud or cement is estimation of 10,800 cu. ft. COST IMPLICATION
The drilling equipment package for jack up is the largest equipment expenditure and typically costs 20 million to 70 million. SAFETY AND RELIABILITY Jack up has the most accidents rate of any of the mobile unit types, averaging 2. 6% of the fleet annually over the 1955-80 period (McClelland et. al. 1982). The significant accidents are leg accidents which include soil foundation failures and also some structure failure. Mr. McClelland stated that about 1/3 of all major accidents occur while rig is being transferred from moving, floating vessel to a fixed, bottom –supported structure. SEMI-SUBMERSIBLE DESCRIPTION:
This kind of rig is not bottom supported but it is designed to float to its other common name called floaters. The structure of this rig is secured by a system of anchors or optionally for deeper water will be held on location by a series of thrusters linked to a dynamic positioning system. This rig has been proven to be one of the most popular options for exploration rig in most offshore specifically for deep water and harsh environment. WATER DEPTH This rig can operate in water depths of up to 3500 ft. however, with current technology development the water depth capability of this rig is increasing.
PRODUCTION CAPACITY Construction of the EAS Shipyard on Brazil’s east coast began in mid-2007 and will be the largest shipyard in the southern hemisphere and have a ship production capacity of 160,000 tones. The P-55 semi-sub production platform being built for Petrobras has a production capacity of 180,000 barrels per day. STORAGE CAPACITY The storage capacity ranges between 500- 3900 m3. The Bennett Offshore semi-submersible structures can be an example. Bennett conducted two independent design reviews of Pride International’s Amethyst Semi-submersibles.
Bennett provided due diligence services for the Amethyst rigs, and Pride employed Bennett to conduct a Gulf of Mexico operability study and a dynamic positioning analysis of the units. COST IMPLICATION When oil fields were first developed in offshore locations, drilling semi-submersibles were converted for use as combined drilling and production platforms. These vessels offered very stable and cost effective platforms. However, the cost of Semi-submersible platform extends more than $600 million. SAFETY AND RELIABILITY During transit an SSCV will be de-ballasted to a draught where only part of the lower hull is submerged.
During lifting operations, the vessel will be ballasted down. This way, the lower hull is well submerged. This reduces the effect of waves and swells. High stability is obtained by placing the columns far apart. The high stability allows them to lift extreme high loads COMPLIANT TOWERS BRIEF DESCRIPTION Compliant towers are much like fixed platforms. They consist of a narrow tower, attached to a foundation on the seafloor and extending up to the platform. This tower is flexible, as opposed to the relatively rigid legs of a fixed platform.
This flexibility allows it to operate in much deeper water, as it can ‘absorb’ much of the pressure exerted on it by the wind and sea. Despite its flexibility, the compliant tower system is strong enough to withstand hurricane conditions. The difference between fixed and compliant platforms is in the way they face environmental (namely wind and wave) lateral actions. Unlike fixed, compliant platforms are designed to move under lateral forces, so that the effects of these forces are mitigated. The trade-off in a compliant platform is between excursion amplitude and restraining force.
Compliant platforms are used in deep water, where the stiffness of a fixed platform decreases while its cost increases, and they are the only technical solution in very deep waters (> 500 m). Two types of compliant platforms will be discussed:- TLP (TENSION LEG PLATFORMS) The tension leg platform is particularly suited for deep water operation. Unlike fixed structures, TLP cost does not dramatically increase with water depth. They are floating structures anchored to the seafloor by a series of vertical tendons (tethers) pre tensioned by extra-buoyancy.
The tethers are made by steel pipes. A TLP is composed by 4 principal parts: the foundation template, the tethers, the hull and the deck. The restoring force is given by extra buoyancy; this is obtained by deballasting the TLP hull once the tethers installed. In addition, TLPs can be re-used. GUYED TOWERS These compliant platforms are composed by a slender jacket, normally pin-joined at its base, whose vertical stable position is ensured by the buoyancy of the structure itself and by a series of mooring centenary lines.
The structure can oscillate under the lateral actions, the restoring force being provided by the buoyancy and the mooring lines. The clump weights provide additional restraining forces in case of storm, when they are lifted off the seafloor. These platforms are used for water depth in the range 200-600 m, and they can be re-used. We will do an analysis of this type of structures based on many factors such as: Water depth, Production capacity, Storage capacity, Cost implication, Safety and reliability. WATER DEPTH:
Compliant towers are designed to sustain significant lateral deflections and forces, and are typically used in water depths ranging from 1,500 and 3,000 feet (450 and 900 m). At present the deepest is the Chevron Petronius tower in waters 623m deep. PRODUCTION CAPACITY: Compliant structures can handle production up to 60,000 B/D. STORAGE CAPACITY: The compliant structures has storage capacity of around 100 MMcf/d COST IMPLICATION: Compliant platforms are used in deep water, where the stiffness of a fixed platform decreases while its cost increases. The deviation cost is around $500,000,000.
SAFETY AND RELIABILITY:- The term compliancy connotes flexibility. In deep waters, bottom founded and floating structures must be designed to be compliant in order to mitigate the impact of hurricane forces of wind, waves, and currents. Besides, the crew must have a large consideration to the marine risers as well as the methods of lifting oil from the seabed. FLOATING PRODUCTION SYSTEM Figure 1 –Floating Production System and Offloading Figure 1 –Floating Production System and Offloading DESCRIPTION Floating production systems are essentially semi-submersible drilling rigs.
They contain petroleum production equipment, as well as drilling equipment. Ships can also be used as floating production systems. The platforms can be kept in place through large, heavy anchors, or through the dynamic positioning system used by drillships. With a floating production system, once the drilling has been completed, the wellhead is actually attached to the seafloor, instead of up on the platform. They can vary by offshore operations? applications: FSU, FPSO. Here FPSO was taken for instance. WATER DEPTH These production systems can operate in water depths from 650- to 6,500 feet.
The FPSO operating in the deepest waters is the FPSO BW Pioneer, built and operated by BW Offshore on behalf of Petrobras Americas INC. The FPSO is moored at a depth of 2,600 m (which is 6. 56168 feet) in Block 249 Walker Ridge in the US Gulf of Mexico. PRODUCTION CAPACITY The extracted petroleum is transported via risers from this wellhead to the production facilities on the semi-submersible platform. The FSO is designed to handle 800,000 bbl/d (130,000 m3/d) with no allowance for downtime. STORAGE CAPACITY Oil produced from offshore production platforms can be transported to the mainland either by pipeline or by tanker.
When a tanker is chosen to transport the oil, it is necessary to accumulate oil in some form of storage tank such that the oil tanker is not continuously occupied during oil production, and is only needed once sufficient oil has been produced to fill the tanker. At this point the transport tanker connects to the stern of the storage unit and offloads oil. One of the world’s largest FPSO is the Kizomba A, with a storage capacity of 2. 2 million barrels (350,000 m3). It weighs 81,000 tons and is 285 meters long, 63 meters wide, and 32 meters high (935 ft.
by 207 ft. (63 m) by 105 ft. ). COST IMPLICATION The same Kizomba A. FPSO was built at a cost of over US$800 million by Hyundai Heavy Industries in Ulsan, Korea, it is operated by Esso Exploration Angola (ExxonMobil). It is located in 1200 meters (3,940 ft) of water at deepwater block 200 statute miles (320 km) offshore in the Atlantic Ocean from Angola, Central Africa. SAFETY AND RELIABILITY Floating production, storage and offloading vessels are particularly effective in remote or deepwater locations where seabed pipelines are not cost effective.
FPSOs eliminate the need to lay expensive long-distance pipelines from the processing facility to an onshore terminal. This can provide an economically attractive solution for smaller oil fields which can be exhausted in a few years and do not justify the expense of installing a pipeline. Furthermore, once the field is depleted, the FPSO can be moved to a new location. GRAVITY BASE STRUCTURES BRIEF DESCRIPTION Gravity based offshore structure (GBS) is a support structured secured in places by gravity. It can be steel or concrete anchored directly onto the seabed.
These structures are constructed using steel, reinforced concrete which usually has tanks used to control the buoyancy of floatation of the finished gravity based structure. When the construction of the GBS is concluded, it is towed to the intended location and it is sunk. The towing could be wet towing or dry towing. The structure is installed by controlling the ballasting compartments or tanks with sea water. WATER DEPTH Gravity based structures are suitable for water depths greater than 20 m (65ft). It is used for moderate water depths. GBS can go up to 350m (1150ft). PRODUCTION CAPACITY
GBS are usually very large structures hence can handle large fields, long term production and can support a large number of wells. GBS can handle production of up to 200,000 bbl/d. STORAGE CAPACITY Few gravity-based structures are also used for offshore wind power plants. By the end of 2010, 14 of the world’s offshore wind farms were supported by gravity-based structures. The largest gravity based structure (Hibernia) has storage capacity of 1. 3MMBbls. COST IMPLICATION The cost average is in the range of $800 MM – $1 Billion per rig. This is because of the amount of concrete and steel that has to be used. SAFETY AND RELIABILITY
Concrete gravity platforms are used when some particular circumstances are present: 1- Economic factors: in some cases, the construction of a very large concrete structure can be cheaper than the construction of a steel structure; 2- Ecological factors: a concrete platform can be very huge, so as to concentrate onboard some industrial treatments of the crude and to allow a great stocking capacity in the ballast cells; 3- Construction conditions: the pile driving operation for a steel jacket needs usually 5 to 10 days; in the North Sea it is rare to have such a period of fine weather; the installation in the oil field of a concrete gravity platform, complete with its deck, requires a shorter period (1 to 2 days); 4- Decommissioning aspects: concrete gravity platforms can be decommissioned and eventually re-used; Soil conditions: when the soil is made of rock it is impossible to drive piles into it: the gravity solution is then the only one possible; 5- Geographical conditions: the presence of calm and deep waters not far from the oil field is an important factor for the construction phases. The above factors have often been determinant in the choice of this kind of platforms in the North Sea. Nevertheless, rather recently concrete gravity platforms have been commissioned in other parts of the world (East Russia, Philippines and so on). These structures can reach a height of 400 m and weigh more than 800000 t. SPAR PLATFORMS DESCRIPTION
A spar is a type of floating oil platform typically used in very deep waters, and is named for logs used as buoys in shipping that are moored in place vertically. Spar production platforms have been developed as an alternative to conventional platforms. A spar platform consists of a large-diameter, single vertical cylinder supporting a deck. The cylinder is weighted at the bottom by a chamber filled with a material that is denser than water to lower the center of gravity of the platform and provide stability. Spars are anchored to the seabed by way of a spread mooring system with either a chain-wire-chain or chain-polyester-chain composition. There are three primary types of spars; the classic spar, truss spar, and cell spar. The
classic spar consists of the cylindrical hull noted above, with the heavy ballast at the bottom of the cylinder. A truss spar has a shorter cylindrical “hard tank” than a classic spar and has a truss structure connected to the bottom of hard tank. At the bottom of the truss structure, there is a relatively small, square shaped “soft tank” that houses the heavy ballasting material. The majority of spars are of this type. A cell spar has a large central cylinder surrounded by smaller cylinders of alternating lengths. At the bottom of the longer cylinders is the soft tank housing the heavy ballasting material, similar to a truss spar. There is currently only one cell spar in operation.
The Brent Spar, a platform designed for storage and offloading of crude oil products was installed in the Brent Field in June of 1976. The attempted deep sea disposal of the platform in the 1990s created a huge backlash by Greenpeace. The Spar was eventually dismantled and pieces were used as a foundation for a quay in Norway. The first spar designed for oil and gas production was the Neptune spar, located in the Gulf of Mexico and was installed in September 1996 by Kerr McGee (now Anadarko). The world’s deepest production platform is Perdido, a truss spar in the Gulf of Mexico, with a mean water depth of 2,438 meters. It is operated by Royal Dutch Shell and was built at a cost of $3 billion. WATER DEPTH:
A spar is a type of floating oil platform typically used in very deep waters up to 8000ft. PRODUCTION CAPACITY: Perdido can produce up to 100,000 barrels a day of oil and 20 million cubic feet per day of natural gas. STORAGE CAPACITY: The $3 billion Perdido floats in 8,000 feet (2,438 meters) of water above a thick 300-square-mile ancient layer of rock and salt known as the Lower Tertiary. Geologists estimate the entire trend could hold up to 15 billion barrels of oil. The Aasta Hansteen platform will be the first Spar with storage capacity and will be able to store about 25,000 scm of condensate, and export gas via the Norwegian Sea Gas Infrastructure (NSGI).
Brent “E” was a floating oil storage facility constructed in 1976 and moored approximately 2 km from the Brent “A” oil rig. The storage tank section had a capacity of 50,000 tonnes (300,000 barrels) of crude oil. COST IMPLICATION Statoil – The Aasta Hansteen platform will cost more than $693,621,000 (NOK 4 billion). However, the world deepest spar cost $3 Billion. The US does not have a facility large enough to construct SPAR hulls. Therefore, almost all SPAR hulls have been manufactured overseas, typically in Finland, and then transported to the US, which increases the cost of the project. Truss SPARs the hull of a truss SPAR is smaller, reducing both material cost and the cost of transportation.
Also for some truss SPARs, the actual truss system can be made in the US and then mated with the hard tank when it arrives. Cell spras, because of the reduced size of the cylinders, fabrication of cell SPARs can take place in the US, meaning that there is no transportation cost. Oryx spent $300 million on Neptune, the world’s first production SPAR platform. Neptune was estimated to save Oryx and its 50/50 partner $90 million. SAFETY AND RELIABILITY: Spar platform is a compliant floating structure used for exploration of oil and gas from deep sea. To ensure safe operations, reliability against mooring line failure is a major concern in design.
Furthermore, the mooring lines have high investment costs and are normally not accessible for in-service inspection. The common approach for solving the dynamics of Spar system is to employ a decoupled quasi-static approach which ignores the platform and mooring lines interaction. Coupled analysis, used presently, considers the mooring lines and platform in an integrated single model. Hence, it effectively captures the damping effect due to Spar and mooring lines coupling. Finite element code ABAQUS is used to obtain the response of Spar-mooring system under long crested random sea with current. Limit state function is derived based on failure due to fatigue for probabilistic reliability assessment.
Random variables, participating actively in the limit state function are identified and statistically modeled. The most probable points or the design points are found to be an effective parameter for estimating partial factors of safety for load and resistance variables. First Order Reliability Method (FORM) is used to calculate probability of failure and reliability indices. The results are later checked against Monte Carlo simulation. REFERENCES: http://www. subseaiq. com/data/PrintProject. aspx? project_id=720&AspxAutoDetectCookieSupport=1 Retrieved (12. 06. 2013) http://en. wikipedia. org/wiki/Oil_platform#Spar_platforms Retrieved (12. 06. 2013) https://en. wikipedia. org/wiki/Semi-submersible Retrieved (12. 06. 2013) Ass Prof. Dr. V. J.
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