construction sequence of gravity base structure platform
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construction sequence of gravity base structure platform  등록일  2021-01-25

Jackets with a single caisson type leg also exist. The grouted jacket has a direct downward load path for shear and moments. These are also known as monopods. 1982. 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. Compliant towers are made of several rigid steel sections joined together by hinges such that the tower can sway under environmental loads. The base structure would now be safely secured to the seabed and ready to receive the next tower section. 1.4). gravity-based structure (GBS) is a massive foundation placed on the seabed to serve as the base for offshore structures. Regardless of the size or the type of jacket installation, once the jacket is on the seabed, its weight is temporarily supported by mudmats. Waves, wind and current cause these structures to deflect, but the magnitude of the dynamic loads is greatly reduced. The purpose of this consolidation was to emphasize ... construction and surveys for maintenance of class. 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. Insulated slip forms have been used successfully during the construction of three large liquefied gas reservoirs of 180,000 m3 in St. John, New Brunswick, Canada. Find a Structure or Project ... Name Structure Type -- All Structure Types - Barge Caisson Caisson supported on jackets/piles Catenary moored floating platform Compliant tower Floating dyke Floating tension-anchored concrete platform Gravity base structure Pontoon Reclamation ground Ring pontoon base Spar platform Tension leg platform (TLP) Help with editing, Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. Further ballasting is then carried out until the jacket is positioned on the seabed. The gravity base is a body of sufficient mass to adequately resist the vertical loads, and horizontal loads applied to a tidal current energy converter, with an acceptable factor of safety. Jackets typically have three, four, six or eight legs. 1.3.2 Concrete Gravity Platforms Concrete gravity platforms are mostly used in the areas where feasibility of pile installation is remote. The tower is now ready to receive the topsides deck. There are various types of offshore oil and gas platforms 1. By continuing you agree to the use of cookies. Fig. In this method, the jacket is pre-rigged with two sets of four slings. The completed platform is de-ballasted to a minimum draft for towing and is towed using tug boats to its final location and ballasted onto the seabed. 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. The top of the jacket would be connected to a derrick barge and the bottom to its assisting tugs. SPE disclaims any and all liability for your use of such content. Gravity Platforms: Design and Construction Overview. 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. Masui, Naoki / Yoshida, Iwao / Rustad, Kjell (1998): Application of gravity base structure technology to bridge substructures. This phase is known as the “unpiled stability” phase. Jacket weight cannot exceed the capacity of the tow-out barge. League City, Texas: PennWell Conferences and Exhibitions. 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. Because of its different properties in compression and tension, concrete used as a tether block can only hold an embedded bail with a maximum force equivalent to that of half the mass of the block. The tower section is then towed and positioned over the pre-installed jacket base section as shown in Fig. 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 … 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. structure below 4 seconds to avoid resonant behaviour with the waves (period in the order of 4 to 25 seconds. 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. 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. The structure is built vertically, from the bottom up, in a similar manner to onshore buildings. Bader Diab Naji Tahan, in Handbook of Offshore Engineering, 2005. Concrete Gravity Structures 2. The assessment engineering approach is integral to the integrity management of offshore installations. Fig. Fig. Jacket has to be designed to float unassisted in the water following launch. 10—Construction of the Troll platform (Photographic Services, Shell Intl., London). 2000. 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. The installation methods of the following substructures are covered in this section: The jacket is a space frame structure made of tubular steel members. ... under the base slap, ... gravity based structures (GBS), jackets and tripods. 3. 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. This allows economical bottom-founded structures to be designed for water depths, which would not be practical for fixed structures. 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. 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. The foundation aspects are considered individually and collectively as part of the design process for a GBS. Section 14.9.4 lists the typical unpiled stability requirements. 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. The production equipment and facilities are often called the topsides. Drawbacks to this system are the difficulty in ensuring that the grout is adequately placed and of sufficient strength to be counted in the analysis and the additional difficulty in platform removal. Jacket is designed to be uprighted by a crane vessel, then sunk to the seabed with systematic flooding. 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. Offshore Magazine (January). A jacket structure being launched is shown in Fig. Fig. The benchmark design guidelines and standard for fixed steel structures is the American Petroleum Institute (API) RP2A, which was first published in 1969. Today, this approach is embraced by the industry worldwide and is reflected in recommended practices, design codes, and standards. The template was derived from the function of the first offshore structures to serve as a guide for the piles. The failure mechanism was then concluded to be a shear failure that split open several walls in one of the platform shafts, which led to leakage of water which induced the final collapse. 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. Novel and Marginal Field Offshore Structures. As the size of a jacket structure increases, it is built and transported on a cargo barge in the horizontal position. An appropriate wave theory is used to calculate the water-particle velocities and accelerations. The jacket legs need to be made buoyant by installing rubber diaphragms at the bottom of the legs and steel caps at the tops. Instead, the upending is achieved by hoisting down the block of one of the two cranes while the other is hoisted up. Two block upending (Marathon East Brae jacket). Bottom-founded structures, with the notable exception of the Gravity Base Structures (GBS e.g. The flotation devices are subsequently flooded to enable the jacket to sink slowly into its final resting place. A gravity-based structure ( GBS) is a support structure held in place by gravity. Figure 14.4. GRAVITY-BASED OFFSHORE LNG TERMINALS JANUARY 2011 ... Offshore Units and Structures (Part 1) for all units, installations, vessels or systems in offshore service. 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. 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. When steel structures are designed for deeper water (in excess of 250 ft), pile-leg grouting is prevalent. The design of the foundation piles is also critical. Table 14.1 summarises the experience in the offshore industry with towing such platforms while Fig. As illustrated, the concrete bottom structure is constructed in dry dock. Jacket- and tower-type fixed platforms, jackups, gravity base structures and the subsea production system are the typical fixed structures without appreciable compliancy. The gravity base, which applies vertical pressure to the area below, stands on the seabed. 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 The Sleipner A (SLA-1) platform was used for drilling in the Sleipner gas field in the North Sea. 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. Copyright © 2021 Elsevier B.V. or its licensors or contributors. The jacket base section is transported on and launched off the deck of a launch cargo barge at site. A fixed substructure is that part of an offshore platform which sits on the seabed and is rigidly connected to it by means of foundation piles (e.g. Once constructed, this is floated out and moored in a deepwater protected harbor. At that time, it was not realized that saturated coarse lightweight aggregates provided some internal curing to HPC, reducing its autogenous shrinkage and increasing the amount of hydrated cement. A discussion on the detailed design of bottom-supported structures is presented in Chapter 6. Larger jackets may require flotation devices to assist in their installation. Tension Leg Platform 3. 7—Structural integrity management (SIM) process (courtesy of MSL Engineering). 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. 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. Shell U.K. Exploration and Production: Design Report for Brigantine BG, report No. When the structure is complete, the dock is flooded and the structure floats under its own buoyancy. 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. For a gravity base to be effective, the seabed must be reasonably level, thereby requiring preparation in advance if it is not already suitable. The need for heavy hammers to drive large jacket piles has contributed to the development of semisubmersible heavy lift crane barges. The process, illustrated in Fig. 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. 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. 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. In this method, the jacket does not have sufficient buoyancy to float without crane assistance. Piles placed through the legs of the jacket are not always sufficient to transfer these loads to the soil; so “skirt piles” are added, normally in clusters around the corner legs. This difficult situation lasted for 3 days before normal winter conditions prevailed again and no longer disturbed the slipforming operation. the gravity base structure. 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). A Kiewit joint venture constructed the concrete gravity base structure and fabricated and installed the topside mechanical works. 1—Conventional fixed steel-jacket structure. At location, the jacket is launched off the barge, uprighted using a crane vessel, and allowed to sink vertically to the seabed. Since that time, more than 1000 functionally similar concrete structures have been built in that area (Norwegian Con-tractors, 1991a). 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. 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. This gravity base structure (GBS), which sits on the ocean floor, is 111 m high and has storage capacity for 1.3 Moilbbl of crude oil in its 85 m high caisson. 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. Grouting is undertaken to fill any voids under the base. Fig. There are a wide variety of bottom-supported structures that have been built and are operational today. Further, these substructures provide the advantageous option of using the gravity base for oil storage. of new concepts for offshore platforms. Once offshore, the jacket is lifted off the deck of the barge and lowered down to the seabed. Fig. The GBSs are typically trial-ballasted prior to tow to site. … [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. Figure 14.3 shows a typical launching sequence of a jacket that was designed to be self-upending. 3. The design of a jacket structure is a matter of: Design forces on jacket structures, shown as arrows in Fig. The size and weight of the foundations (from 1500 to 4500 tons) make transport and installation cumbersome, and it is worth noting that the seabed must be prepared by dredging and backfilling material to install the foundation. 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. So although concrete is a cheap construction material, it is extremely expensive and time consuming to install a concrete foundation. 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. Steel jacket structures are prefabricated onshore prior to transportation to site by a barge. 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). Upend and transfer of Baldpate tower section. Paper presented at the 2001 Conference on Minimal Offshore Facilities of the Future, League City, Texas, 9–11 October. 1999. C239R003 Rev 1. For The design and construction of the huge concrete gravity base structures represent a remarkable achievement by the offshore industry. Norway’s fjords provided the ideal sites to permit construction of these large substructures. 1 is a modern example of a jacket structure designed for operation in 350 ft. of water. Gravity Based Structure Contracting Strategy The Hebron field will be developed using a stand-alone, gravity-based structure (GBS) that sits on the ocean floor. 14.5. 17.5). Then, the tower section is transported on the deck of a launch barge and launched into water. 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. [5] These platforms were also designed for sea access rather than helicopter access, thereby reducing the topsides facilities. Bottom-founded structures are called “fixed” when their lowest natural frequency of flexural motion is above the highest frequency of significant wave excitation. 1. 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. 9 shows a typical set of construction and installation steps for a GBS. Multiple tugs are used to tow the structure to its offshore location. A common application for a GBS is an offshore oil platform. Launched jackets need to have sufficient reserve buoyancy in order to ensure they float at the end of the launch sequence. 6—Topsides modules being lift-installed (courtesy of BP). Most of the horizontal loads are directly related to the diameter of the tubular members and their locations within the structure. These Gravity Base Structures (GBS) are composed of giant concrete “legs“ and oil storage cells at the base that support the topsides above the surface of the sea. 6. A gravity-based foundation is a very heavy displacement structure usually made of concrete (Fig. For a horizontal seabed. Small jackets may be lifted in place by a floating crane vessel. 2000. A typical installation sequence of a single tower section is described next. Jacket- and tower-type fixed platforms, jackups. 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. The horizontal force from waves consists of drag forces from the kinetic energy of the water and inertia forces from the water-particle accelerations. 2—Typical design forces on conventional-jacket structures (courtesy of MSL Engineering). Often it is only necessary to provide piles through the legs, depending on the environment and soil characteristics. The template or jacket structure is a steel space frame that supports, above water, a superstructure comprising one or more decks for production equipment and facilities needed to support and maintain production. Single-block upending. 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. The rocker arms are two beams that are installed at the stern of the barge in line with the skid beams. 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 design is targeted to offer least resistance to environmental loads while providing adequate support for the topsides structure. Therefore, pile weight is an important part of total structural 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. A modern, high-energy hammer for 8-ft-diameter piles typically weighs around 160 tons. Available water depth, underkeel and horizontal clearances in the tow route. Once located on the seabed, foundation piles secure the structure. All of these structures were partially built in a drydock and then completed afloat in sheltered waters. It was deter… These compliant structures generate inertia forces due to their motions. The jacket is lifted off the cargo barge with the lifting slings and lowered into the water until its buoyancy balances its weight. During a cold, sunny, but windy period of 3 days, the QC manager in charge of the production of the concrete for the construction of a gravity base faced a serious problem. These structures are often constructed in fjords since their protected area and sufficient depth are very desirable for construction. 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. Maximum base shear or maximum total force on a structure [ 5 ] these platforms were also designed for water depths, which not! Structural members with the jacket deck ( s ) by offshore crane vessels operation 350. The sea off a launch truss then ballasted in a drydock and then completed afloat in waters. Somewhere along the construction sequence of gravity base structure platform jacket frame, while in the installation of a structure. To its assisting tugs force of the barge in line with the is... Appropriate wave theory is used to store oil and gas platforms 1 manner until it is floated out and in... 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