construction sequence of gravity base structure platform

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. 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. 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 At location, the jacket is launched off the barge, uprighted using a crane vessel, and allowed to sink vertically to the seabed. A typical sequence of steps involved in the installation of a jacket structure by launching is shown in Fig. Condeep is an abbreviation for concrete deep water structure. 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. Equivalent practices exist in other countries such as the U.K. and Norway. 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. 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. [6] The big prize for the industry is to develop MFPs that are self-installable, thereby removing the need for installation vessels. In following the development of the technology related to fixed steel structures and the historic achievements, two additional observations are worthy of note. 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 foundation aspects are considered individually and collectively as part of the design process for a GBS. The rocker arms are two beams that are installed at the stern of the barge in line with the skid beams. 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]. [1] For small structures in shallow water, the pile weight may approach the weight of the jacket structure. Figure 1.10 illustrates the typical construction sequence employed for the North Sea structures. Small jackets may be lifted in place by a floating crane vessel. The weight of the launch truss normally constitutes a significant part of the jacket weight. 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. … There are several methods of upending jackets: Two-block upending – upending in air or partially in water using two crane blocks. The construction of concrete gravity platforms is altogether different from that employed for template-type structures. gravity-based structure (GBS) is a massive foundation placed on the seabed to serve as the base for offshore structures. 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. Jackets with three legs are known as tripods. 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. In 2000, the 21st edition of API RP 2A was released. Cambridge, Massachusetts: MIT. Each of the Brent Field’s GBS is roughly equivalent in weight to the Empire State Building. Single-block upending. The need for heavy hammers to drive large jacket piles has contributed to the development of semisubmersible heavy lift crane barges. 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. Offshore structures are mainly represented by platforms, notably jackup rigs, steel jacket structures and gravity-based structures. The Sleipner suction pile, for example, has a 45-ft-diameter plate with a 16-ft skirt depth. Most GBSs are designed for several functions, namely combined drilling, production, and oil storage. SPE disclaims any and all liability for your use of such content. Fig. The Hibernia gravity base structure was designed using HPC made with a Type 10 SF cement (Type 1 ASTM cement containing 7%–8% silica fume) and a polynaphthalene superplasticizer, in order to resist the impact of huge icebergs (Hoff & Elimov, 1995; Woodhead, 1993). 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 This phase is known as the “unpiled stability” phase. 5—Jacket structure being lift-installed (Photographic Services, Shell Intl., London). Gravity Platforms: Design and Construction Overview. The gravity base, which applies vertical pressure to the area below, stands on the seabed. Concrete platforms are built and installed in a different way from steel jackets. Fig. Offshore Magazine (January). Bader Diab Naji Tahan, in Handbook of Offshore Engineering, 2005. 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. 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 lightship displacement of the gravity based structures can be of the order of several hundred thousand tonnes. 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. 4—Jacket structure being launched (photographic Services, Shell Int., London). Its size, and the large environmental forces, can cause design problems. Steel jacket structures are prefabricated onshore prior to transportation to site by a barge. The design of a jacket structure is a matter of: Design forces on jacket structures, shown as arrows in Fig. Additional components of the topsides include: Often, a drilling derrick forms part of the equipment for drilling and maintenance of the wells. An appropriate wave theory is used to calculate the water-particle velocities and accelerations. By continuing you agree to the use of cookies. 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. The design is targeted to offer least resistance to environmental loads while providing adequate support for the topsides structure. Figure 14.1. 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. The structure is built vertically, from the bottom up, in a similar manner to onshore buildings. 3 reveals that a number of critical factors must be considered as part of the jacket design process: Fig. concrete platform for oil and gas production in the Gulf of Mexico was installed in 1950. 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. Jackets are designed to be either self-upending or upended with the assistance of a crane vessel. 2001. 1.3.2 Concrete Gravity Platforms Concrete gravity platforms are mostly used in the areas where feasibility of pile installation is remote. The structural and foundation conflguration shall be selected to achieve this concept. Fig. 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.

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