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]. 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. 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. Principle of the gravity-based foundation. The lightship displacement of the gravity based structures can be of the order of several hundred thousand tonnes. The base structure would now be safely secured to the seabed and ready to receive the next tower section. 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. They are flat panels that are made of stiffened steel plate or, to reduce weight, from glass reinforced plastics. 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. Through leg piles are connected to the jacket legs at the top using shim plates, known as “crown shims,” that are installed in the annulus between the leg and the pile and are welded to both. 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. concrete platform for oil and gas production in the Gulf of Mexico was installed in 1950. Hard, C. 2001. Table 14.1. Sections 14.8.2 and 14.9.3 include more information on launching. Kurt E. Thomsen, in Offshore Wind (Second Edition), 2014. 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. ... under the base slap, ... gravity based structures (GBS), jackets and tripods. Careful examination of the installation process by the geotechnical engineer and integration with other elements of the platform design and construction is necessary. Copyright © 2021 Elsevier B.V. or its licensors or contributors. 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. The grouted pile also provides additional strength to the tubular joints where horizontal and diagonal bracing is welded to the legs. Norway’s fjords provided the ideal sites to permit construction of these large substructures. Once in water, the jacket base section would be upended by the derrick barge assisted by the jacket buoyancy. 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 piles, after being driven, are cut off above the templates, and the deck is placed on top of the piles. For a horizontal seabed. 2—Typical design forces on conventional-jacket structures (courtesy of MSL Engineering). Small jackets may be lifted in place by a floating crane vessel. API RP2A, Recommended Practice for Planning, Designing, and Constructing Fixed Offshore Platforms, 21st edition. The flotation devices are subsequently flooded to enable the jacket to sink slowly into its final resting place. 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. … 1. Figure 14.2 shows a single block upend operation. The jacket is then ballasted in a controlled manner until it is upended a few meters above the seabed. Figure 14.3. The accommodation/helideck facilities are situated as far from the potentially dangerous hydrocarbon process area as is physically possible. The suction pile essentially comprises a plate, usually circular, and is surrounded and reinforced by a skirt. Since that time, more than 1000 functionally similar concrete structures have been built in that area (Norwegian Con-tractors, 1991a). 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. More information. Compliant towers are made of several rigid steel sections joined together by hinges such that the tower can sway under environmental loads. [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. Installation of the Baldpate piles [De Koeijer, et al 1999]. Spoilt for Choice: How to Classify and Select Minimum Facility Solutions. 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. These are also known as monopods. 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. These types of structures were pioneered by Norway, consistent with their design expertise, construction facilities, and construction skills that leaned heavily toward concrete rather than steel. “Compliant” bottom-founded structures are usually designed so that their lowest natural frequency is below the energy in the waves. Single cranes with two blocks can be used for upending smaller jackets with the jacket length aligned with the plane of the crane boom. Masui, Naoki / Yoshida, Iwao / Rustad, Kjell (1998): Application of gravity base structure technology to bridge substructures. Fig. A typical sequence of steps involved in the installation of a jacket structure by launching is shown in Fig. Once vertical, the jacket will be lowered and manoeuvred into position often with the guidance of a pre-installed docking pile. The top of the jacket would be connected to a derrick barge and the bottom to its assisting tugs. The template is prevented from settling by being welded to the piles’ tops with a series of rings and gussets. Pile driving is addressed in Section 14.4.2 where a more detailed description is provided. 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. A jacket structure being launched is shown in Fig. The lifting slings are then disconnected from the crane hook and the upending slings are connected to the hook. The Condeep base always rests on the sea floor, and the shafts rise to about 30 meters above the sea level. 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 … 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. Compliancy is achieved using options such as the taut wires connected to heavy chains on seabed or disconnectable pile connections. 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. 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 A modern, high-energy hammer for 8-ft-diameter piles typically weighs around 160 tons. SPE disclaims any and all liability for your use of such content. 6—Topsides modules being lift-installed (courtesy of BP). 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. 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. A large number of platforms were constructed of this type, characterized by the Olav Olsen's "Condeep" concept. Required number of tugs, bollard pull and design of the towing attachments. The rocker arms are two beams that are installed at the stern of the barge in line with the skid beams. Steel jacket structures are prefabricated onshore prior to transportation to site by a barge. 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 jacket legs and braces transmit environmental and topsides loads into the piles and subsequently into the seabed. The construction of concrete gravity platforms is altogether different from that employed for template-type structures. 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. Feng Fu, in Design and Analysis of Tall and Complex Structures, 2018. Jackets installed in such a configuration are typically less than 50 m tall. Figure 1.4. Help with editing, Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. At that time, there was no pipeline infrastructure, and the capacity of heavy lift vessels was only a few thousand tons. 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 platform … Figure 14.1. Hydraulic hammers that operate underwater have superseded the early steam-driven hammers. 10. There are a wide variety of bottom-supported structures that have been built and are operational today. Subrata Chakrabarti, ... Cuneyt Capanoglu, in Handbook of Offshore Engineering, 2005. As the name implies, concrete gravity-based structures rely on their own weight to resist the lateral environmental loads. The jacket buoyancy has to be designed so as to allow easy access for rigging the upending slings, while the jacket floats horizontally. Lee, G.C. 9 shows a typical set of construction and installation steps for a GBS. We use cookies to help provide and enhance our service and tailor content and ads. The jacket legs are made buoyant by the use of rubber diaphragms at their bottom ends and steel caps at the top. The benchmark design guidelines and standard for fixed steel structures is the American Petroleum Institute (API) RP2A, which was first published in 1969. Jackets typically have three, four, six or eight legs. 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. Today, this approach is embraced by the industry worldwide and is reflected in recommended practices, design codes, and standards. Bottom-founded structures, with the notable exception of the Gravity Base Structures (GBS e.g. Fig. A Condeep type GBS under construction in Norway. A gravity-based structure ( GBS) is a support structure held in place by gravity. The method of installation depends on the weight and the physical dimensions of the jacket and on the capacity of the installation equipment. Minimal Offshore Facilities of the Future. 1982. Figure 14.4. Fig. Fig. Steel Jacket Fixed Platform 2. 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. An appropriate wave theory is used to calculate the water-particle velocities and accelerations. 4—Jacket structure being launched (photographic Services, Shell Int., London). 3 reveals that a number of critical factors must be considered as part of the jacket design process: Fig. Some are better suited to certain environmental and operational criteria; some are limited by availability of construction sites; and some are chosen simply by subjective preference of an owner/operator. of new concepts for offshore platforms. 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. Launched jackets need to have sufficient reserve buoyancy in order to ensure they float at the end of the launch sequence. Additional components of the topsides include: Often, a drilling derrick forms part of the equipment for drilling and maintenance of the wells. In some structures, the annulus between the jacket and the pile is grouted, although this is no longer a common practice. A launch barge is a flat top cargo barge equipped with skid beams, a rocker arm, launch winches and a suitable ballasting system. 2000. The Sleipner A (SLA-1) platform was used for drilling in the Sleipner gas field in the North Sea. 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. 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 structural and foundation conflguration shall be selected to achieve this concept. A large number of novel and innovative MFP concepts exists at the present time, and sophisticated platform selection tools have been created to permit the most appropriate development options to be considered. 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. An offshore GBS is large. jackets) or under the effect of its weight (e.g. 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. You must log in to edit PetroWiki. 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. Grouting procedures for the skirt sleeve-to-pile must recognize that grout placement and inspection will be done remotely. 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. Two block upending (Marathon East Brae jacket). Once offshore, the jacket is lifted off the deck of the barge and lowered down to the seabed. Bader Diab Naji Tahan, in Handbook of Offshore Engineering, 2005. The foundation piles for this size of jacket structure are typically transported together with the jacket on the same cargo barge. The ballasting permits the skirts to penetrate into the seabed. This reduces their dynamic response to such environment and extends the applicability of fixed platform to deeper water such as 1000 m. A compliant tower structure can be divided into four basic structural components: The tower section(s). In the 1990s in particular, the industry undertook a “self-assessment,” particularly in light of the large number of undeveloped marginal fields in existence worldwide. Jacket- and tower-type fixed platforms, jackups. 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. This adds a new dimension to the installation procedure. 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. Instead, the upending is achieved by hoisting down the block of one of the two cranes while the other is hoisted up. The offshore structure shown in Fig. Once located on the seabed, foundation piles secure the structure. The CGBS have no oil storage facilities and the topside installations will be carried out in the field by a … from zero degrees of freedom to two degrees of freedom about the seabed). Condeep is an abbreviation for concrete deep water structure. This phase is known as the “unpiled stability” phase. 1—Conventional fixed steel-jacket structure. Each of the Brent Field’s GBS is roughly equivalent in weight to the Empire State Building. 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. The weight of the launch truss normally constitutes a significant part of the jacket weight. specific reference to a proposed gravity base structure (GBS) to be located on the sea bed in the Lyell Field of the northern North Sea area, immediately West of the Ninian Field. 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). Jacket structures that are too heavy to be lifted can be launched into the sea off a launch barge. In this method, the jacket is pre-rigged with two sets of four slings. This path is substantially different for the overturning moment as well as axial loads. 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. These compliant structures generate inertia forces due to their motions. An alternative to launching the structure is to lift it in position. 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. Fig. The foundation aspects are considered individually and collectively as part of the design process for a GBS. 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. 1.3.2 Concrete Gravity Platforms Concrete gravity platforms are mostly used in the areas where feasibility of pile installation is remote. 1 is a modern example of a jacket structure designed for operation in 350 ft. of water. 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. This allows economical bottom-founded structures to be designed for water depths, which would not be practical for fixed structures. 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. Historical background on the development of bottom-supported structures is covered in Chapter 1. Presented at: Long-Span and High-Rise Structures , IABSE Symposium, Kobe, 1998 , … The horizontal force from waves consists of drag forces from the kinetic energy of the water and inertia forces from the water-particle accelerations. Paper presented at the 2001 Conference on Minimal Offshore Facilities of the Future, League City, Texas, 9–11 October. 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