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construction sequence of gravity base structure platform

Launch simulation of a self-upending jacket. The structure is built vertically, from the bottom up, in a similar manner to onshore buildings. Following pick-up, the cargo barge is withdrawn and the jacket is upended. Although both top-hung and grouted structures are loosely called templates, some prefer to call the latter a jacket to distinguish the difference in load path. 6—Topsides modules being lift-installed (courtesy of BP). 8 shows an illustration of a concrete gravity-based structure (GBS). This led to the concept of minimum facility platforms (MFPs), which aim to restrict the equipment and facilities on topside structures to the minimum required for production, minimize or eliminate the need for manned installations in light of improved flow-assurance technology, and minimize substructure arrangements without compromising robustness or safety.[3]. The need for heavy hammers to drive large jacket piles has contributed to the development of semisubmersible heavy lift crane barges. Figure 1.10 illustrates the typical construction sequence employed for the North Sea structures. Without exception, the construction and installation of a jacket structure plays a central role in its design. gravity base structure). As the name implies, concrete gravity-based structures rely on their own weight to resist the lateral environmental loads. Copyright © 2021 Elsevier B.V. or its licensors or contributors. Pierre-Claude Aïtcin, Jean-Martin Lessard, in Developments in the Formulation and Reinforcement of Concrete (Second Edition), 2019. The jacket buoyancy has to be designed so as to allow easy access for rigging the upending slings, while the jacket floats horizontally. Figure 17.5. The Troll A platform is a Condeep gravity-based structure offshore natural gas platform in the Troll gas field off the west coast of Norway. The structural design requirements include the categories of material quality, strength, and serviceability. 14.6 shows the tug towing arrangement of one of the early GBS. To avoid any slump loss during mixing due to the dry state of the lightweight aggregates, it was decided to keep the coarse lightweight aggregate continuously saturated by sprinkling water on the stockpiles 24 hours per day. The construction commences in a dry dock adjacent to the sea. Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read, Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro, Concrete gravity based offshore structures. Temporary loading conditions may very well govern the structural design. Once the jacket is set on the seabed, the piles are installed using the same crane vessel and a pile hammer of an adequate size. P.-C. Aïtcin, in Science and Technology of Concrete Admixtures, 2016. Figure 14.5. There are several methods of upending jackets: Two-block upending – upending in air or partially in water using two crane blocks. Cambridge, Massachusetts: MIT. Kiewit's expertise in organizing and executing complex slipforming operations was needed on the construction of this offshore, oil-production platform after the original contractor fell behind schedule and over budget. These temporary cases include: Often, the cellular base walls are not pressurized; consequently, they must be designed to resist the substantial hydrostatic pressure imposed during immersion. That problem did not occur on the inside face of the base because it was not exposed to the same thermal situation as the external surface. In this method, the jacket does not have sufficient buoyancy to float without crane assistance. The grouted pile also provides additional strength to the tubular joints where horizontal and diagonal bracing is welded to the legs. The benchmark design guidelines and standard for fixed steel structures is the American Petroleum Institute (API) RP2A, which was first published in 1969. When the possibility of combined tidal stream and swell forces is considered, a high safety factor will be necessary for a satisfactory confidence level, especially for buoyant submerged devices. If the slipping of the form was done when the concrete of the south face was hard enough, on the north face the concrete would be too weak and collapse because it had not enough time to set. The design of a jacket structure is a matter of: Design forces on jacket structures, shown as arrows in Fig. Shell U.K. Exploration and Production: Design Report for Brigantine BG, report No. Waves, wind and current cause these structures to deflect, but the magnitude of the dynamic loads is greatly reduced. An appropriate wave theory is used to calculate the water-particle velocities and accelerations. For example, the Bullwinkle jacket structure weighed 44,800 tons, and the pile weight was 9,500 tons (i.e., the pile weights are a significant portion of the jacket weight, in this case, nearly a quarter of the structure weight). The installation methods of the following substructures are covered in this section: The jacket is a space frame structure made of tubular steel members. 2.6. Subrata Chakrabarti, ... Cuneyt Capanoglu, in Handbook of Offshore Engineering, 2005. Many types of offshore structures are in service. Today, this approach is embraced by the industry worldwide and is reflected in recommended practices, design codes, and standards. A Kiewit joint venture constructed the concrete gravity base structure and fabricated and installed the topside mechanical works. Piles made of tubular steel are installed through the legs of the jacket or through the pile sleeves connected to the jacket legs at its base. Feng Fu, in Design and Analysis of Tall and Complex Structures, 2018. Typically, a gravity base is used on semihard, uniform seabed conditions and at shallower water depths, although some projects (e.g., Thornton Bank) have been installed in much deeper water using a gravity-based foundation. To simplify installation and hookup at sea, the equipment and facilities are often placed in modules, which may weigh several hundreds to many thousands of tons. Regardless of the size or the type of jacket installation, once the jacket is on the seabed, its weight is temporarily supported by mudmats. https://petrowiki.spe.org/index.php?title=Fixed_steel_and_concrete_gravity_base_structures&oldid=46915, 4.5 Offshore facilities and subsea systems, Copyright 2012-2021, Society of Petroleum Engineers, Utilities, i.e., electricity, fuel, instrument gas, power gas, water, and sewage, Flare boom for gas flaring (if necessary), Determining the overall dimensions based on water depth and functional requirements, evaluating hydrodynamic loads caused by waves and currents, Sizing of the structure to meet state requirements for strength, fatigue, and serviciability. There are various types of offshore oil and gas platforms 1. 4 and is consistent with Step 3 in Fig. The base structure would now be safely secured to the seabed and ready to receive the next tower section. Once the towers are constructed and topped off, the whole structure is ballasted down to receive the topsides deck and modules. The platform … The design and construction of the huge concrete gravity base structures represent a remarkable achievement by the offshore industry. Piles installed through sleeves on the outside of the leg structure are connected to the sleeve by grouting the pile-sleeve annulus. Figure 1.4. The top-hung template requires that moment from lateral wave loads be transmitted up the structure to be resolved into axial pile loads. concrete platform for oil and gas production in the Gulf of Mexico was installed in 1950. Fig. 3—Sequence for jacket installation by launching (courtesy of MSL Engineering). Bottom-founded structures are called “fixed” when their lowest natural frequency of flexural motion is above the highest frequency of significant wave excitation. There are a wide variety of bottom-supported structures that have been built and are operational today. Some consideration should be given to provide remotely operated valves with manual back-up. The size of the jackets that can be upended with a single crane is limited. The jacket legs and braces transmit environmental and topsides loads into the piles and subsequently into the seabed. … This method only requires one crane albeit it has to be capable of lifting the full jacket weight without assistance. Installation of the Baldpate piles [De Koeijer, et al 1999]. 2000. Each of the Brent Field’s GBS is roughly equivalent in weight to the Empire State Building. The jacket structure is placed horizontally on a flat-topped barge and towed to site. The gravity base, which applies vertical pressure to the area below, stands on the seabed. For a horizontal seabed. The base and the tower sections are lattice space structures fabricated from tubular steel members and thus termed the jacket base and the jacket tower sections. League City, Texas: PennWell Conferences and Exhibitions. Condeep is a make of gravity-based structure for oil platforms developed by engineer Olav Mo in Hoeyer-Ellefsen and fabricated by Norwegian Contractors in Stavanger, Norway. GBS are towed at a large draft and their towing requires very detailed analyses and marine procedures including the following aspects: From: Handbook of Offshore Engineering, 2005, Alan Owen, in Future Energy (Third Edition), 2020. Additional equipment such as flooding valves, umbilicals and pumps are also needed. [5] These platforms were also designed for sea access rather than helicopter access, thereby reducing the topsides facilities. Over the decades, the piles have grown in size (number, diameter, and length) in line with the jacket structures. of new concepts for offshore platforms. Condeep is an abbreviation for concrete deep water structure. Mudmats are added to the bottom of the jacket legs to provide the required bearing area to support the jacket weight and resist environmental loading during installation and until the strength of the piles has sufficiently developed. Additional components of the topsides include: Often, a drilling derrick forms part of the equipment for drilling and maintenance of the wells. 9—Concrete platform construction and installation (courtesy of MSL Engineering). A launch barge is a flat top cargo barge equipped with skid beams, a rocker arm, launch winches and a suitable ballasting system. This may be substantial, e.g., where scouring or settlement allows the gravity anchor to sink below its original installation level, or minimal, e.g., where a square faced gravity anchor is resting on an exposed uneven rock bed. All of these structures were partially built in a drydock and then completed afloat in sheltered waters. The torn surface had to be repaired at a very high cost (Figure 18.3). The first observation relates to the industry’s ability to manage and maintain the structural integrity of existing installations during the service life of the platforms. The tower section is then towed and positioned over the pre-installed jacket base section as shown in Fig. Bader Diab Naji Tahan, in Handbook of Offshore Engineering, 2005. This allows economical bottom-founded structures to be designed for water depths, which would not be practical for fixed structures. Some companies prefer to place packers in the bottom of each template leg and to grout the annular space between the leg and pile from bottom to top. Jacket design is an iterative process because a number of design cycles are gone through before achieving the optimal sizing for a particular member to bear these horizontal forces. Figure 18.3. from zero degrees of freedom to two degrees of freedom about the seabed). Figure 14.1 shows a two-block upend operation. The lightship displacement of the gravity based structures can be of the order of several hundred thousand tonnes. Upend and transfer of Baldpate tower section. Fig. Table 14.1. 2, can be calculated with specialized computer software available to the industry. It is a gravity base structure, built in the fashion of typical Condeep platforms which are reinforced concrete structures floating in water up to 300-m deep. Therefore, it has to be pointed out that the existing standards for gravity base foundations need further development especially for the consideration of cyclic loads. The lifting slings are then disconnected from the crane hook and the upending slings are connected to the hook. 1982. It will consist of a reinforced concrete structure designed to withstand the weight of the topsides, icebergs, and meteorological and oceanographic conditions at the offshore Hebron Project Site. Often it is only necessary to provide piles through the legs, depending on the environment and soil characteristics. The Condeep base always rests on the sea floor, and the shafts rise to about 30 meters above the sea level. Built from reinforced concrete, as of 2014, it was the tallest and heaviest structure that has ever been moved to another position, relative to the surface of the Earth, and is among the largest and most complex engineering projects in history. Jacket- and tower-type fixed platforms, jackups. 4—Jacket structure being launched (photographic Services, Shell Int., London). GBSs are constructed with reinforced concrete and consist of a cellular base surrounding several unbraced columns that extend upward from the base to support the topsides superstructure above the water surface. The structural and foundation conflguration shall be selected to achieve this concept. Minimal Structures Open Global Production Opportunities. The early development of gravity platforms in the 1970s was driven by the generic requirement to store large volumes of oil and support a heavy topsides in deepwater. Use of twin cranes on a single vessel or use of two crane vessels is often deployed to provide the required lifting capability. The construction of concrete gravity platforms is altogether different from that employed for template-type structures. The grouted jacket has a direct downward load path for shear and moments. The second set of slings – the upending slings – are attached to padeyes at the top of the legs when the jacket is in the upright position. From a geotechnical standpoint, the parameters considered in foundation design include the type/extent of contact between platform base and seabed; stability against overturning and sliding; skirt penetration and grouting, to provide lateral sliding resistance and protection from scour; settlement; effects of cyclic loading on the soil; and soil-structure interaction. The topside structures are normally installed at an inshore location by deck mating or any other suitable method. When the structure is complete, the dock is flooded and the structure floats under its own buoyancy. Jacket weight cannot exceed the capacity of the tow-out barge. The ballasting permits the skirts to penetrate into the seabed. Grout is then injected into the gap between the pin and bucket, which provides the structural continuity and the integrity of the entire subsurface structure (base and tower sections). Kurt E. Thomsen, in Offshore Wind (Second Edition), 2014. Sufficient buoyancy and subdivision is also required to ensure that flooding of one compartment does not lead to the jacket sinking or making the installation operation impossible to complete. Coincidentally, the ballasting of the GBS for deck installation prior to towing to site is often regarded as an effective, full-scale, inshore pressure test prior to offshore installation. A gravity base uses the same principle as a deadweight anchor, and its effectiveness can be modeled from a free body diagram [18] (See Fig. A jacket structure being launched is shown in Fig. Spoilt for Choice: How to Classify and Select Minimum Facility Solutions. To achieve this objective, two solutions were implemented: first, an increase of the air content of concrete (1% of additional air decreases the unit weight by 25 kg/m3), and second, the replacement of 50% of the coarse aggregate by an equivalent volume of coarse lightweight aggregate imported from Texas. 7, ensures the cost-effective life-cycle management of offshore structures. The first set of slings – the lifting slings – are attached somewhere along the top jacket frame, while in the horizontal position. Gravity Base Structures placed on the seafloor and held in place by their weight Troll A gas platform, world's tallest concrete structure in North Sea Offshore Engineering and Technology 2000. In this method, the jacket is pre-rigged with two sets of four slings. Pile guides are required up to water level, and a removable “follower” must be used during pile-driving operations. This path is substantially different for the overturning moment as well as axial loads. To prevent this type of problem, it is only necessary to isolate the concrete placed between the slip-forms with 50 mm of Styrofoam so that the external temperature no longer influences the hardening conditions of the concrete placed between the forms (Lachemi and Elimov, 2007). Launched jackets need to have sufficient reserve buoyancy in order to ensure they float at the end of the launch sequence. The construction and installation of GBSs is entirely different from that employed for jacket structures. Once vertical, the jacket will be lowered and manoeuvred into position often with the guidance of a pre-installed docking pile. During its construction, it became necessary to decrease the weight of the structure to improve its buoyancy because new heavy mechanical equipment had to be added onto the gravity base structure. [4] Shell’s Skiff and Brigantine structures in the southern North Sea, for example, were designed to be installed using a jack-up rig with conductors placed through the legs and exploited as part of the foundation system. Once the jacket is secured with its foundation system, the topsides structure can be installed as separate modules or as a single integrated unit; see Fig. 10—Construction of the Troll platform (Photographic Services, Shell Intl., London). As illustrated, the concrete bottom structure is constructed in dry dock. Because the cost of piling is substantial, an alternative concept that has been developed is the suction pile, or bucket foundation, because its visual appearance is one of an inverted bucket. Minimal Offshore Facilities of the Future. Principle of the gravity-based foundation. Typically, using a range of standards, the structure is designed to meet the criteria laid down for the ultimate progressive collapse, fatigue, and serviceability limit states. When steel structures are designed for deeper water (in excess of 250 ft), pile-leg grouting is prevalent. It can be observed that offshore hookup is minimized because most of the topsides equipment and facilities are commissioned onshore prior to placement on the deck. The topsides structure is similar to that for steel-jacket structures (i.e., it is either an integrated steel-deck configuration or is of modular construction with a module support frame). Jackets with three legs are known as tripods. A jacket installed using this method needs to have sufficient buoyancy to float in the horizontal position by itself. Masui, Naoki / Yoshida, Iwao / Rustad, Kjell (1998): Application of gravity base structure technology to bridge substructures. Thus, the maximum horizontal component that can be applied is inversely proportional to the value of the coefficient of friction (μ), and therefore, in spite of being cheap and easy to make, the applicability of the gravity anchor is restricted to vertical (or near vertical) loads on a flat stable seabed. We use cookies to help provide and enhance our service and tailor content and ads. Jacket has to be designed to float unassisted in the water following launch. Jacket is designed to be uprighted by a crane vessel, then sunk to the seabed with systematic flooding. GBS have been installed in water-depths of up to 300 m. Most gravity based structures are made from concrete although one steel gravity base platform, Maureen, was installed in the North Sea. Offshore Magazine (January). Careful examination of the installation process by the geotechnical engineer and integration with other elements of the platform design and construction is necessary. Pumped slurry can be used to transfer dense negative value material into a void from which the water can drain, leaving the denser material behind. Presented at: Long-Span and High-Rise Structures , IABSE Symposium, Kobe, 1998 , … Available water depth, underkeel and horizontal clearances in the tow route. Piles are transported to the site on cargo barges, lifted off and upended, using the cranes of the derrick barge, lowered, stabbed through the jacket base pile sleeves and driven to target penetration as shown in Fig. Help with editing, Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. Fig. It was found that a combination of poor geometry and an inadequate design were the causes of the platform's failure. 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