After, the verticality and orientation of the jacket base are achieved, piles are grouted to the pile sleeves. The modules are completely prefabricated and tested onshore prior to transportation and lifting onto the jacket deck(s) by offshore crane vessels. Figure 14.1. 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 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. Waves, wind and current cause these structures to deflect, but the magnitude of the dynamic loads is greatly reduced. 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. The tower is now ready to receive the topsides deck. These are also known as monopods. Jackets with a single caisson type leg also exist. Fig. Hence, the template carries no load from the deck but merely hangs from the top of the piles and provides lateral support to them. The Sleipner suction pile, for example, has a 45-ft-diameter plate with a 16-ft skirt depth. They are flat panels that are made of stiffened steel plate or, to reduce weight, from glass reinforced plastics. 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. 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. 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. The top of the jacket would be connected to a derrick barge and the bottom to its assisting tugs. The weight of the launch truss normally constitutes a significant part of the jacket weight. Hard, C. 2001. Jackets typically have three, four, six or eight legs. The first set of slings – the lifting slings – are attached somewhere along the top jacket frame, while in the horizontal position. 1.3.2 Concrete Gravity Platforms Concrete gravity platforms are mostly used in the areas where feasibility of pile installation is remote. Selecting a structure type is the first major structural design task after environmental and operational criteria have been defined and will sometimes require preliminary design of several concepts before a choice is made. As the size of a jacket structure increases, it is built and transported on a cargo barge in the horizontal position. The template was derived from the function of the first offshore structures to serve as a guide for the piles. Single-block upending. 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. Therefore, the gravity-based foundation is not the preferred solution. This adds a new dimension to the installation procedure. Once offshore, the jacket is lifted off the deck of the barge and lowered down to the seabed. 3. API RP2A, Recommended Practice for Planning, Designing, and Constructing Fixed Offshore Platforms, 21st edition. 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. The design is targeted to offer least resistance to environmental loads while providing adequate support for the topsides structure. The gravity base, which applies vertical pressure to the area below, stands on the seabed. When the structure is complete, the dock is flooded and the structure floats under its own buoyancy. The method of installation depends on the weight and the physical dimensions of the jacket and on the capacity of the installation equipment. Figure 14.2 shows a single block upend operation. In following the development of the technology related to fixed steel structures and the historic achievements, two additional observations are worthy of note. The benchmark design guidelines and standard for fixed steel structures is the American Petroleum Institute (API) RP2A, which was first published in 1969. Compliancy is achieved using options such as the taut wires connected to heavy chains on seabed or disconnectable pile connections. 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. Minimal Offshore Facilities of the Future. Paper presented at the 2001 Conference on Minimal Offshore Facilities of the Future, League City, Texas, 9–11 October. 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. The offshore structure shown in Fig. 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. A jacket installed using this method needs to have sufficient buoyancy to float in the horizontal position by itself. This path is substantially different for the overturning moment as well as axial loads. 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. The assessment engineering approach is integral to the integrity management of offshore installations. These members act as a truss supporting the weight of the processing equipment, and the environmental forces from waves, wind and current. There are various types of offshore oil and gas platforms 1. 1982. 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). The design of a jacket structure is a matter of: Design forces on jacket structures, shown as arrows in Fig. The CGBS have no oil storage facilities and the topside installations will be carried out in the field by a … Jackets are designed to be either self-upending or upended with the assistance of a crane vessel. A modern, high-energy hammer for 8-ft-diameter piles typically weighs around 160 tons. Copy 1. Today, this approach is embraced by the industry worldwide and is reflected in recommended practices, design codes, and standards. Presented at: Long-Span and High-Rise Structures , IABSE Symposium, Kobe, 1998 , … Once vertical, the tower section, is ballasted to the required float-over draft. Offshore Magazine (January). 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. The design and construction of the huge concrete gravity base structures represent a remarkable achievement by the offshore industry. 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 … Copyright © 2021 Elsevier B.V. or its licensors or contributors. The maximum securing force (W) that a gravity base can exert is given by. The production equipment and facilities are often called the topsides. 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. 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. 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. The accommodation/helideck facilities are situated as far from the potentially dangerous hydrocarbon process area as is physically possible. FPSO – Floating Production, Storage Offloader From the above we shall examine the steel jacket fixed platform in the following sections. Following pick-up, the cargo barge is withdrawn and the jacket is upended. Jacket structures that are too heavy to be lifted can be launched into the sea off a launch barge. The jacket structure is first placed on the seabed, and the foundation piles are driven through the pile sleeves (often called skirt piles) and grouted to form the support system for the structure. The lightship displacement of the gravity based structures can be of the order of several hundred thousand tonnes. 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 2—Typical design forces on conventional-jacket structures (courtesy of MSL Engineering). Help with editing, Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. The jacket base section is transported on and launched off the deck of a launch cargo barge at site. Grouting procedures for the skirt sleeve-to-pile must recognize that grout placement and inspection will be done remotely. 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. 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). For Without exception, the construction and installation of a jacket structure plays a central role in its design. 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. Equivalent practices exist in other countries such as the U.K. and Norway. This temperature difference created problems when the forms had to be lifted. 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). Temporary loading conditions may very well govern the structural design. In Behavior of Offshore Structures. The most common type of offshore structure in service today is the jacket (or template) structure, as illustrated in Fig. 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. 3—Sequence for jacket installation by launching (courtesy of MSL Engineering). The flotation devices are subsequently flooded to enable the jacket to sink slowly into its final resting place. Jacket- and tower-type fixed platforms, jackups. Small jackets may be lifted in place by a floating crane vessel. gravity base structure). [6] The big prize for the industry is to develop MFPs that are self-installable, thereby removing the need for installation vessels. So although concrete is a cheap construction material, it is extremely expensive and time consuming to install a concrete foundation. The foundation piles for this size of jacket structure are typically transported together with the jacket on the same cargo barge. 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. Figure 1.10 illustrates the typical construction sequence employed for the North Sea structures. There are a wide variety of bottom-supported structures that have been built and are operational today. A jacket structure being launched is shown in Fig. 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. 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. 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) 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 Once located on the seabed, foundation piles secure the structure. 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 design of the foundation piles is also critical. Feng Fu, in Design and Analysis of Tall and Complex Structures, 2018. 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. 4—Jacket structure being launched (photographic Services, Shell Int., London). Table 14.1. [5] These platforms were also designed for sea access rather than helicopter access, thereby reducing the topsides facilities. 7—Structural integrity management (SIM) process (courtesy of MSL Engineering). [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 vertical, the jacket will be lowered and manoeuvred into position often with the guidance of a pre-installed docking pile. 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. 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. The first concrete gravity-base structure in U.S. waters was installed in 1978 (Hunteman et al., 1979). where B is the buoyancy force of the anchor, and T is the tensile force in the mooring. 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