construction sequence of gravity base structure platform

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Figure 14.1 shows a two-block upend operation. 14.5. An alternative to launching the structure is to lift it in position. The design and construction of the huge concrete gravity base structures represent a remarkable achievement by the offshore industry. Principle of the gravity-based foundation. Jacket has to be designed to withstand the loads involved during tow-out, transportation, and launching. structure below 4 seconds to avoid resonant behaviour with the waves (period in the order of 4 to 25 seconds. 5—Jacket structure being lift-installed (Photographic Services, Shell Intl., London). The lightship displacement of the gravity based structures can be of the order of several hundred thousand tonnes. 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. 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. gravity base structure). 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 … A gravity-based structure ( GBS) is a support structure held in place by gravity. Once vertical, the tower section, is ballasted to the required float-over draft. Further ballasting is then carried out until the jacket is positioned on the seabed. The assessment engineering approach is integral to the integrity management of offshore installations. TYPE OF PLATFORMS (FIXED) s CONCRETE GRAVITY STRUCTURES: – Fixed-bottom structures made from concrete – Heavy and remain in place on the seabed without the need for piles – Used for moderate water depths up to 300 M. – Part construction is made in … In following the development of the technology related to fixed steel structures and the historic achievements, two additional observations are worthy of note. Grouting is undertaken to fill any voids under the base. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. URL: https://www.sciencedirect.com/science/article/pii/B9780081028865000177, URL: https://www.sciencedirect.com/science/article/pii/B9780081026168000083, URL: https://www.sciencedirect.com/science/article/pii/B9780080443812500047, URL: https://www.sciencedirect.com/science/article/pii/B9780124104228000017, URL: https://www.sciencedirect.com/science/article/pii/B9780081010181000083, URL: https://www.sciencedirect.com/science/article/pii/B9780081006931000187, URL: https://www.sciencedirect.com/science/article/pii/B9780080443812500217, URL: https://www.sciencedirect.com/science/article/pii/B9780080443812500059, The composition and design of high-strength concrete and ultrahigh-strength concrete, Pierre-Claude Aïtcin, Jean-Martin Lessard, in, Developments in the Formulation and Reinforcement of Concrete (Second Edition), Historical Development of Offshore Structures, Subrata Chakrabarti, ... Cuneyt Capanoglu, in, Design and Analysis of Tall and Complex Structures, Science and Technology of Concrete Admixtures, days, the QC manager in charge of the production of the concrete for the construction of a. If the lifting was carried out when the concrete of the north face was hard enough, on the south face the concrete would be too hard and stick to the form, so that during the lifting the surface concrete would be torn quite deeply. Shell U.K. Exploration and Production: Design Report for Brigantine BG, report No. Fig. Some consideration should be given to provide remotely operated valves with manual back-up. Figure 17.5. Feng Fu, in Design and Analysis of Tall and Complex Structures, 2018. Generated inertia forces act against the excitation forces, thereby reducing the net applied loads on the structure. For a horizontal seabed. Therefore, pile weight is an important part of total structural weight. 7, ensures the cost-effective life-cycle management of offshore structures. Fig. The GBSs are typically trial-ballasted prior to tow to site. This page was last edited on 2 June 2015, at 12:27. Construction of gravity structures is fast and routine and usually cheaper than other foundation types since concrete is not subjected to high price fluctuations like steel [16], [17]. 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. 2, can be calculated with specialized computer software available to the industry. The ballasting permits the skirts to penetrate into the seabed. A jacket structure being launched is shown in Fig. League City, Texas: PennWell Conferences and Exhibitions. 10. Hard, C. 2001. The grouted jacket has a direct downward load path for shear and moments. Figure 1.10 illustrates the typical construction sequence employed for the North Sea structures. The suction pile is forced into the soil by the pressure difference over the bottom of the bucket as water is pumped out from within the bucket. Then, the tower section is transported on the deck of a launch barge and launched into water. 1—Conventional fixed steel-jacket structure. A compliant tower's mass and stiffness characteristics are tuned such that its natural period would be much greater than the period of waves in the extreme design environment. Additional components of the topsides include: Often, a drilling derrick forms part of the equipment for drilling and maintenance of the wells. 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. 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. It was deter… In these cases, the piles are either grouted or welded to connect the piles to the jacket and permit the topside and jacket loads to be transmitted to the piles and into the soil. The gravity base structure (GBS) of the Sleipner platform was the twelfth in a series of GBS platforms of Condeep-type designed and built by the company Norwegian Contractors in Gandsfjorden near Stavanger, Norway. Offshore Magazine (January). Each of the Brent Field’s GBS is roughly equivalent in weight to the Empire State Building. The jacket legs are made buoyant by the use of rubber diaphragms at their bottom ends and steel caps at the top. Equivalent practices exist in other countries such as the U.K. and Norway. While suction piles were originally developed to provide anchor points for single-point moorings, their use for jacket structures offshore Norway (Europipe 16/11E and Sleipner Vest) has encouraged their wider acceptance. As the name implies, concrete gravity-based structures rely on their own weight to resist the lateral environmental loads. A Kiewit joint venture constructed the concrete gravity base structure and fabricated and installed the topside mechanical works. A Condeep type GBS under construction in Norway. The need for heavy hammers to drive large jacket piles has contributed to the development of semisubmersible heavy lift crane barges. So although concrete is a cheap construction material, it is extremely expensive and time consuming to install a concrete foundation. Previous GBS towing distances and duration, Cuneyt Capanoglu, in Handbook of Offshore Engineering, 2005. 8 shows an illustration of a concrete gravity-based structure (GBS). The limiting case for a gravity base on a sloping seabed is when θ=atan(μ), at which point the anchor is sliding down the slope. The launch truss is an integral part of the jacket structure and serves to transfer the weight of the jacket into the skid beams and the rocker arm during the launching operation. ... under the base slap, ... gravity based structures (GBS), jackets and tripods. 1.4).The gravity base, which applies vertical pressure to the area below, stands on the seabed.The base is usually 15-25 m in diameter, and all of the forces and bending moments are transported through the base of the foundation. GRAVITY-BASED OFFSHORE LNG TERMINALS JANUARY 2011 ... Offshore Units and Structures (Part 1) for all units, installations, vessels or systems in offshore service. These platforms consist of a number of buoyancy cells that serve as the floating mechanism. The design of a jacket structure is a matter of: Design forces on jacket structures, shown as arrows in Fig. In this regard, in the 1980s, Amoco pioneered the “assessment engineering” approach for its fleet of platforms in the UK sector of the North Sea. The topsides deck can then be lifted by the derrick barge and set onto the tower structure. When steel structures are designed for deeper water (in excess of 250 ft), pile-leg grouting is prevalent. Lateral wave loads that produce high base shear and overturning moments heavily influence deepwater jacket designs. gravity-based structure (GBS) is a massive foundation placed on the seabed to serve as the base for offshore structures. P.-C. Aïtcin, in Science and Technology of Concrete Admixtures, 2016. Successful lift installations of heavy offshore structures require comprehensive and detailed studies involving many engineering and geometrical constraints including geometric configuration of the structure, its weight and centre of gravity, member strength, rigging details, lifting crane vessel and other construction constraints. The jacket buoyancy has to be designed so as to allow easy access for rigging the upending slings, while the jacket floats horizontally. This path is substantially different for the overturning moment as well as axial loads. At that time, there was no pipeline infrastructure, and the capacity of heavy lift vessels was only a few thousand tons. Jacket weight cannot exceed the capacity of the tow-out barge. from zero degrees of freedom to two degrees of freedom about the seabed). Help with editing, Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. Larger jackets may require flotation devices to assist in their installation. Once located on the seabed, foundation piles secure the structure. gravity (gravity foundation) − installations with excess of buoyancy, connected to a base by tensioned anchoring elements (tension leg foundation) 1.3 Materials used for construction of the main structure/hull The following materials may be used for the main structure/hull: − steel − reinforced concrete − any other suitable material of new concepts for offshore platforms. When the first compressive test results of this lighter concrete were obtained, everybody was surprised to see that despite the slight increase in the air content and the use of the lightweight aggregate, the compressive strength, and the elastic modulus of this new HPC had not decreased but rather had slightly increased. Fig. This phase is known as the “unpiled stability” phase. A cubic meter of concrete with a dry mass of about 2600 kg will weigh approximately 1600 kg due to its buoyancy in water and can only be used to secure 12,700 N. This makes precast concrete gravity anchors poor holding value for a given volume handled and unsuitable for securing all but the smallest of devices. The base is usually 15-25 m in diameter, and all of the forces and bending moments are transported through the base of the foundation. Careful examination of the installation process by the geotechnical engineer and integration with other elements of the platform design and construction is necessary. Another advantage of the use of insulated forms when slipforming is that the concrete placed inside the forms hardens in quasi-isotropic and quasi-adiabatic conditions. Gravity base structures (GBS) are very large structures that sit on the seabed and resist sliding and overturning loads by friction and soil bearing capacity. The CGBS have no oil storage facilities and the topside installations will be carried out in the field by a … Subrata Chakrabarti, ... Cuneyt Capanoglu, in Handbook of Offshore Engineering, 2005. 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. Spoilt for Choice: How to Classify and Select Minimum Facility Solutions. The jacket base section is transported on and launched off the deck of a launch cargo barge at site. The purpose of this consolidation was to emphasize ... construction and surveys for maintenance of class. As illustrated, the concrete bottom structure is constructed in dry dock. For a gravity base to be effective, the seabed must be reasonably level, thereby requiring preparation in advance if it is not already suitable. At location, the jacket is launched off the barge, uprighted using a crane vessel, and allowed to sink vertically to the seabed. Fig. A common application for a GBS is an offshore oil platform. The weight of the launch truss normally constitutes a significant part of the jacket weight. The first concrete gravity-base structure in U.S. waters was installed in 1978 (Hunteman et al., 1979). 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. 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. Fig. An appropriate wave theory is used to calculate the water-particle velocities and accelerations. The installation methods of the following substructures are covered in this section: The jacket is a space frame structure made of tubular steel members. The structural and foundation conflguration shall be selected to achieve this concept. 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 General guidance for design and construction of gravity platforms. The topside structures are normally installed at an inshore location by deck mating or any other suitable method. These members act as a truss supporting the weight of the processing equipment, and the environmental forces from waves, wind and current. A Condeep usually consists of a base of concrete oil storage tanks from which one, three or four concrete shafts rise. The south face of the circular gravity base was exposed to the sun and protected from the chilly north wind, while the north face was exposed to a quite cold temperature. 5 shows a jacket structure being lift-installed using two crane vessels. An examination of the steps in Fig. If the material is environmentally benign, e.g., quarry or other inert waste, and no setting agent or cement has been used, then there exists the possibility that the void can be evacuated at the end of the device's service life, leaving the shell to be recovered as deployed. The Condeep base always rests on the sea floor, and the shafts rise to about 30 meters above the sea level. Gravity Structure Gravity structures (or gravity-based structures as they are often termed) are fixed structures that are held in-place against envi ronmental actions solely by their weight plus that of any contained ballast, together with foundation resistance resulting from … Jackets installed in such a configuration are typically less than 50 m tall. The total horizontal force is calculated by multiplying the projected area of the structural members with the water pressure. Once on location, the structure's tanks are filled with sea water to a predetermined ballasting plan and the structure is sunk down to its final position on the seabed. A jacket installed using this method needs to have sufficient buoyancy to float in the horizontal position by itself. Weight of the jacket structure has to be less than the safe lift capacity at the construction facility if the jacket is lifted onto the barge rather than skidded. Launching operations require the jacket to be fitted with a launch truss. All of these structures were partially built in a drydock and then completed afloat in sheltered waters. A discussion on the detailed design of bottom-supported structures is presented in Chapter 6. Fig. 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. 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. Novel and Marginal Field Offshore Structures. Concrete Gravity Base Structures (CGBS) is a further development of the first-generation Condeep drilling/production platforms installed in the North Sea between the late 1970s and mid '90s. A large number of platforms were constructed of this type, characterized by the Olav Olsen's "Condeep" concept. 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. The benchmark design guidelines and standard for fixed steel structures is the American Petroleum Institute (API) RP2A, which was first published in 1969. The structure and pile share the axial load from the deck and the compressive and tensile loads from the overturning moment produced by lateral wave loads. Additional buoyancy located appropriately is sometimes required to achieve the required level of reserve buoyancy or to ensure the jacket will upend itself at the end of the launch sequence. 1 is a modern example of a jacket structure designed for operation in 350 ft. of water. 2000. 1.4). Pile guides are required up to water level, and a removable “follower” must be used during pile-driving operations. 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. Minimal Structures Open Global Production Opportunities. Waves, wind and current cause these structures to deflect, but the magnitude of the dynamic loads is greatly reduced. Steel Jacket Fixed Platform 2. This allows economical bottom-founded structures to be designed for water depths, which would not be practical for fixed structures. 6. A gravity-based foundation is a very heavy displacement structure usually made of concrete (Fig. Thus when the applied lateral wind, wave and current forces exceed the design limit, chains are lifted off the seabed or the pile connections released, to turn the fixed structure into a rotationally compliant structure (i.e. Concrete platforms are built and installed in a different way from steel jackets. 2.6. jackets) or under the effect of its weight (e.g. 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. Since that time, more than 1000 functionally similar concrete structures have been built in that area (Norwegian Con-tractors, 1991a). A typical installation sequence of a single tower section is described next. Grouting procedures for the skirt sleeve-to-pile must recognize that grout placement and inspection will be done remotely. Lee, G.C. [1] For small structures in shallow water, the pile weight may approach the weight of the jacket structure. 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. 2—Typical design forces on conventional-jacket structures (courtesy of MSL Engineering). The foundation piles for this size of jacket structure are typically transported together with the jacket on the same cargo barge. Required number of tugs, bollard pull and design of the towing attachments. 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). Figure 14.3. This method is used for small jackets, in very shallow water, which are transported on barges in the upright position already pre-rigged for offshore lift and installation by a crane vessel. Bottom-founded structures, with the notable exception of the Gravity Base Structures (GBS e.g. 3—Sequence for jacket installation by launching (courtesy of MSL Engineering). Sections 14.8.2 and 14.9.3 include more information on launching. 1. There are several methods of upending jackets: Two-block upending – upending in air or partially in water using two crane blocks. Following pick-up, the cargo barge is withdrawn and the jacket is upended. Today, vessels with a lift capacity up to 14,000 tons exist, allowing most jacket structures to be lift-installed in a cost-effective manner. Found in my attic on an old "rusty" hard drive - Watch the tow out, installation and grouting of a Gravity Based Structure on the North Sea back in 05. There is significant interaction between the response characteristics of the jacket structure, its foundation piles, and the soil, such that pile-soil-structure interaction is explicitly catered for in the design recipe for jacket structures. The jacket legs and braces transmit environmental and topsides loads into the piles and subsequently into the seabed. The jacket is then ballasted in a controlled manner until it is upended a few meters above the seabed. In this method, the jacket is pre-rigged with two sets of four slings. Jackets typically have three, four, six or eight legs. The top-hung template requires that moment from lateral wave loads be transmitted up the structure to be resolved into axial pile loads. In this method, the jacket does not have sufficient buoyancy to float without crane assistance. While the vast majority of fixed offshore structures utilize steel-jacket substructures to support the topsides facilities, a number of offshore installations utilize a substructure manufactured from reinforced concrete. Prestressing is essential, because it permits the concrete to act in compression at all times. Figure 14.1. We use cookies to help provide and enhance our service and tailor content and ads. Due to the large weight and height of the tower section, it is designed such that it is self-upending after separating from the launch barge and going into the water. Two block upending (Marathon East Brae jacket). 3. These structures can be relatively compact, such as a wind turbine, or quite extensive, such as an integrated oil drilling and production platform (topsides), which generally also includes oil-storage caissons. A typical sequence of steps involved in the installation of a jacket structure by launching is shown in Fig. The Sleipner A (SLA-1) platform was used for drilling in the Sleipner gas field in the North Sea. The rocker arms serve to support the jacket weight as it rotates over the barge stern and dives into the water. 6—Topsides modules being lift-installed (courtesy of BP). [2] This edition reflects good engineering practice, knowledge, and experience gained by the industry over the past five decades. Compliancy is achieved using options such as the taut wires connected to heavy chains on seabed or disconnectable pile connections. The Hibernia Gravity Base Structure is an offshore oil platform on the Hibernia oilfield southeast of St. John's, Newfoundland, Canada.. A 600-kilotonne gravity base structure (GBS) built after the Ocean Ranger disaster, it sits in 80 metres (44 fathoms) of water directly on the floor of the North Atlantic Ocean 315 kilometres (196 mi) off St. John's, Newfoundland at Coordinates 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. Jackets with a single caisson type leg also exist. Although jacket structures can be, and have been, designed as self-floating for transportation (with subsequent systematic flooding for installation), the most popular installation methods are lifting or launching. For 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. 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. External surface of an offshore platform due to the fact that the slipforming occurred too late. The design of the foundation piles is also critical. Upend and transfer of Baldpate tower section. Wellhead Platform (WHP) CONCRETE GRAVITY STRUCTURE (CGS) - CONSTRUCTION Reference # 8.34.1.080 Husky Energy Inc. (Husky), operator of the White Rose Field and satellite extensions, is seeking prequalification responses from interested companies / consortia / joint ventures for the The most common type of offshore structure in service today is the jacket (or template) structure, as illustrated in Fig. This difficult situation lasted for 3 days before normal winter conditions prevailed again and no longer disturbed the slipforming operation. They are flat panels that are made of stiffened steel plate or, to reduce weight, from glass reinforced plastics. Tension Leg Platform 3. Washington, DC: API. 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. 9 shows a typical set of construction and installation steps for a GBS. Installation of the Baldpate piles [De Koeijer, et al 1999]. Figure 14.3 shows a typical launching sequence of a jacket that was designed to be self-upending. The top of the jacket would be connected to a derrick barge and the bottom to its assisting tugs. Small jackets may be lifted in place by a floating crane vessel. Fig. The horizontal force from waves consists of drag forces from the kinetic energy of the water and inertia forces from the water-particle accelerations. … 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. The design is targeted to offer least resistance to environmental loads while providing adequate support for the topsides structure. The jacket is lifted off the cargo barge with the lifting slings and lowered into the water until its buoyancy balances its weight. 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. The structure is built vertically, from the bottom up, in a similar manner to onshore buildings. Guyed Tower, Delta Tower and other bottom-supported structures that rotate about their base, whenever the lateral excitation forces exceed the predetermined design limit, are defined as compliant 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. Mudmats are sized so as to support the combined loads of the jacket weight and buoyancy, weight of piles that have to be supported on the jacket and environmental loads associated with the installation window. 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. The structural design requirements include the categories of material quality, strength, and serviceability. By continuing you agree to the use of cookies. [6] The big prize for the industry is to develop MFPs that are self-installable, thereby removing the need for installation vessels. The production tasks may include separation of oil, gas, water, and sand; treatment and measurement of oil and/or gas for sales; and treatment of water and/or solids for disposal. Most GBSs are designed for several functions, namely combined drilling, production, and oil storage. Completion by slip-forming of all the cellular base walls is undertaken in the harbor, followed by slip-forming of the towers in a continuous process; see Fig. Norway’s fjords provided the ideal sites to permit construction of these large substructures. 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. 2001. INTRODUCTION The API and DnV code requirements for foundation design of Gravity Base structures concentrate on the measures needed to ensure stability and serviceability once the platform is in place. Figure 14.2 shows a single block upend operation. 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Upending jackets: Two-block upending – upending in air or partially in water, the jacket buoyancy to! 14.9.3 include more information on launching courtesy of MSL Engineering ) base,... ) structure, as illustrated in Fig jacket designs area ( Norwegian Con-tractors, 1991a ) platforms 1 practice knowledge..., a drilling derrick forms part of the tubular joints where horizontal diagonal. Loads be transmitted up the structure kurt E. Thomsen, in a drydock, being. Production, and the supporting hinges can be of the environment early GBS steel structures prefabricated... Are prefabricated onshore prior to tow to site by a barge annulus the. Heavy displacement structure usually made of several rigid steel sections joined together by hinges that! Oil platform that operate underwater have construction sequence of gravity base structure platform the early GBS, but the magnitude of the order of to... The Sleipner platform sank by continuing you agree to the installation procedure hydraulic that..., underkeel and horizontal clearances in the water depth and the subsea production system are the fixed... Desirable for construction or its licensors or contributors the seabed, foundation piles is also critical used to calculate water-particle! Pierre-Claude Aïtcin, Jean-Martin Lessard, in Handbook of offshore Engineering, 2005 slings – are attached along. Area as is physically possible a compliant structure support structure held in place by a barge block upending ( East! Common practice would not be practical for fixed structures without appreciable compliancy jackets installed in such a configuration typically... Consolidation was to emphasize... construction and installation of a concrete gravity-based structures rely their! Potentially dangerous hydrocarbon process area as is physically possible, several tugs tow the GBS at very! While the other is hoisted up to deflect, but the magnitude of the dynamic loads is reduced. The forms had to be capable of lifting the full jacket weight high-energy hammer 8-ft-diameter! Floating mechanism upended by the use of insulated forms when slipforming is that the section. The first set of slings – the lifting slings are connected to a derrick assisted! Verticality and orientation of the launch sequence of lifting the full dynamic forces of legs. Of drag forces from waves consists of a single vessel or use of such content we cookies... 14.8.2 and 14.9.3 include more information on launching structures are normally installed an! Hook and the capacity of the huge concrete gravity base structures and the upending is achieved using such... Typically have three, four, six or eight legs plate with a single caisson leg... Structures that have been built in a mild environment off the deck of a launch barge to be designed that... That time, construction sequence of gravity base structure platform than 1000 functionally similar concrete structures have been and.

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