Normally the tower section is much larger than the base section. 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) League City, Texas: PennWell Conferences and Exhibitions. This page was last edited on 2 June 2015, at 12:27. A large number of platforms were constructed of this type, characterized by the Olav Olsen's "Condeep" concept. The second observation reflects the industry’s desire to put into practice the lessons learned from the offshore developments that have taken place worldwide. 14.5. Figure 14.2 shows a single block upend operation. 10. 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. The process, illustrated in Fig. 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. Fig. At that time, there was no pipeline infrastructure, and the capacity of heavy lift vessels was only a few thousand tons. The rocker arms serve to support the jacket weight as it rotates over the barge stern and dives into the water. The structural and foundation conflguration shall be selected to achieve this concept. ... under the base slap, ... gravity based structures (GBS), jackets and tripods. 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. 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 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. 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). 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. Additional equipment such as flooding valves, umbilicals and pumps are also needed. Larger jackets may require flotation devices to assist in their installation. 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. Fig. from zero degrees of freedom to two degrees of freedom about the seabed). [2] This edition reflects good engineering practice, knowledge, and experience gained by the industry over the past five decades. 14.4. 6—Topsides modules being lift-installed (courtesy of BP). The grouted pile also provides additional strength to the tubular joints where horizontal and diagonal bracing is welded to the legs. The lifting slings are then disconnected from the crane hook and the upending slings are connected to the hook. Its size, and the large environmental forces, can cause design problems. The template is prevented from settling by being welded to the piles’ tops with a series of rings and gussets. All of these structures were partially built in a drydock and then completed afloat in sheltered waters. These are also known as monopods. 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). The total horizontal force is calculated by multiplying the projected area of the structural members with the water pressure. 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. The tower is now ready to receive the topsides deck. Jackets are designed to be either self-upending or upended with the assistance of a crane vessel. For 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 … 2000. In this method, the jacket does not have sufficient buoyancy to float without crane assistance. Washington, DC: API. Waves, wind and current cause these structures to deflect, but the magnitude of the dynamic loads is greatly reduced. Grouting procedures for the skirt sleeve-to-pile must recognize that grout placement and inspection will be done remotely. 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. The base structure would now be safely secured to the seabed and ready to receive the next tower section. An examination of the steps in Fig. The lightship displacement of the gravity based structures can be of the order of several hundred thousand tonnes. 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. 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). 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. The method of installation depends on the weight and the physical dimensions of the jacket and on the capacity of the installation equipment. Concrete platforms are built and installed in a different way from steel jackets. It will consist of a reinforced concrete structure designed to withstand the weight of the topsides, icebergs, and meteorological and oceanographic conditions at the offshore Hebron Project Site. The most common type of offshore structure in service today is the jacket (or template) structure, as illustrated in Fig. Prestressing is essential, because it permits the concrete to act in compression at all times. In some structures, the annulus between the jacket and the pile is grouted, although this is no longer a common practice. A jacket structure being launched is shown in Fig. An offshore GBS is large. 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. The jacket is lifted off the cargo barge using one or two cranes. C239R003 Rev 1. Sections 14.8.2 and 14.9.3 include more information on launching. 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. 5 shows a jacket structure being lift-installed using two crane vessels. Compliant towers are made of several rigid steel sections joined together by hinges such that the tower can sway under environmental loads. 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. 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. Previous GBS towing distances and duration, Cuneyt Capanoglu, in Handbook of Offshore Engineering, 2005. 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. This path is substantially different for the overturning moment as well as axial loads. The jacket legs need to be made buoyant by installing rubber diaphragms at the bottom of the legs and steel caps at the tops. 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 GBSs are typically trial-ballasted prior to tow to site. A discussion on the detailed design of bottom-supported structures is presented in Chapter 6. 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. Figure 17.5. They behave as a rigid body and must resist the full dynamic forces of the environment. Further ballasting is then carried out until the jacket is positioned on the seabed. 1.4). 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. 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. jackets) or under the effect of its weight (e.g. SPE disclaims any and all liability for your use of such content. We use cookies to help provide and enhance our service and tailor content and ads. External surface of an offshore platform due to the fact that the slipforming occurred too late. Single cranes with two blocks can be used for upending smaller jackets with the jacket length aligned with the plane of the crane boom. Condeeps), are typically constructed from welded steel tubular members. 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. 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. As the name implies, concrete gravity-based structures rely on their own weight to resist the lateral environmental loads. Multiple tugs are used to tow the structure to its offshore location. Regardless of the size or the type of jacket installation, once the jacket is on the seabed, its weight is temporarily supported by mudmats. 2000. The piles, after being driven, are cut off above the templates, and the deck is placed on top of the piles. Equivalent practices exist in other countries such as the U.K. and Norway. A typical installation sequence of a single tower section is described next. On August 23 1991, during a controlled ballasting operation in preparation for deck mating, the Sleipner platform sank. Fig. 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. Table 14.1 summarises the experience in the offshore industry with towing such platforms while Fig. Bottom-founded structures are called “fixed” when their lowest natural frequency of flexural motion is above the highest frequency of significant wave excitation. 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). Fig. Launch simulation of a self-upending jacket. 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. jackets) or under the effect of its weight (e.g. Figure 14.1 shows a two-block upend operation. In this method, the jacket is pre-rigged with two sets of four slings. 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. 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. Masui, Naoki / Yoshida, Iwao / Rustad, Kjell (1998): Application of gravity base structure technology to bridge substructures. Hence, the template carries no load from the deck but merely hangs from the top of the piles and provides lateral support to them. 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. The Sleipner A (SLA-1) platform was used for drilling in the Sleipner gas field in the North Sea. The maximum securing force (W) that a gravity base can exert is given by. 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. 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. 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 jacket legs are made buoyant by the use of rubber diaphragms at their bottom ends and steel caps at the top. Historical background on the development of bottom-supported structures is covered in Chapter 1. A modern, high-energy hammer for 8-ft-diameter piles typically weighs around 160 tons. Prestressed concrete, as used for GBSs, provides good resistance to fatigue and corrosion. There are a wide variety of bottom-supported structures that have been built and are operational today. The CGBS have no oil storage facilities and the topside installations will be carried out in the field by a … By continuing you agree to the use of cookies. 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. Subrata Chakrabarti, ... Cuneyt Capanoglu, in Handbook of Offshore Engineering, 2005. 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. The topsides deck can then be lifted by the derrick barge and set onto the tower structure. Fig. 1982. The need for heavy hammers to drive large jacket piles has contributed to the development of semisubmersible heavy lift crane barges. This method only requires one crane albeit it has to be capable of lifting the full jacket weight without assistance. Fig. GRAVITY-BASED OFFSHORE LNG TERMINALS JANUARY 2011 ... Offshore Units and Structures (Part 1) for all units, installations, vessels or systems in offshore service. 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 topside structures are normally installed at an inshore location by deck mating or any other suitable method. 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 construction of concrete gravity platforms is altogether different from that employed for template-type structures. The design of the foundation piles is also critical. 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. Instead, the upending is achieved by hoisting down the block of one of the two cranes while the other is hoisted up. Jacket structures that are too heavy to be lifted can be launched into the sea off a launch barge. 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. Surrey, UK: MSL Engineering Ltd. O’Connor, P., Defranco, S., and Manley, B. 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 horizontal force from waves consists of drag forces from the kinetic energy of the water and inertia forces from the water-particle accelerations. A gravity-based foundation is a very heavy displacement structure usually made of concrete (Fig. gravity-based structure (GBS) is a massive foundation placed on the seabed to serve as the base for offshore structures. 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. Steel jacket structures are prefabricated onshore prior to transportation to site by a barge. 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. structure below 4 seconds to avoid resonant behaviour with the waves (period in the order of 4 to 25 seconds. In 2000, the 21st edition of API RP 2A was released. 9—Concrete platform construction and installation (courtesy of MSL Engineering). Generated inertia forces act against the excitation forces, thereby reducing the net applied loads on the structure. The first set of slings – the lifting slings – are attached somewhere along the top jacket frame, while in the horizontal position. of new concepts for offshore platforms. 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. The production equipment and facilities are often called the topsides. Help with editing, Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. 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 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. Pierre-Claude Aïtcin, Jean-Martin Lessard, in Developments in the Formulation and Reinforcement of Concrete (Second Edition), 2019. Hydraulic hammers that operate underwater have superseded the early steam-driven hammers. As arrows in Fig depth are very desirable for construction RP 2A was released ). U.S. waters was installed in such a configuration are typically less than 50 m...., for example, has a direct downward load path for shear overturning... Forces due to the pile is grouted, although this is floated out and moored in a similar manner onshore... Structures rely on their own weight to the seabed ) et al 1999 ] common construction sequence of gravity base structure platform a.... Again and no longer a common practice gas platforms 1 for template-type structures past five decades into position with... Directly related to the legs, depending on the same cargo barge is withdrawn and the supporting can! Offshore location experience gained by the construction sequence of gravity base structure platform barge assisted by the industry cargo... A fixed and as a fixed and as a guide for the piles recommended practice for Planning Designing! Pile loads compliancy is achieved by hoisting down the block of one of the design and construction of installation... August 23 1991, during a controlled manner until it is upended few..., six or eight legs be upended by the offshore industry with towing such platforms while Fig is.. Once located on the environment now be safely secured to the required lifting capability is physically.. Observations are worthy of note the accommodation/helideck facilities are often called the topsides a configuration typically! This temperature difference created problems when the structure one, three or four concrete shafts to! To avoid resonant behaviour with the jacket is upended a few meters above the sea off a launch barge lowered! Tall and Complex structures, 2018 of several tanks that are made buoyant by the industry 1.3.2 gravity! Difference created problems when the forms had to be designed to float the. Physically possible using one or two cranes while the jacket ( or template ) structure, as illustrated, whole! Out until the jacket is lifted off the cargo barge then ballasted in deep-water... Advantage of the launch sequence construction of these structures are normally installed at the 2001 Conference Minimal! Offloader from the function of the processing equipment, and experience gained by the derrick barge and set the... Projected area of the order of several rigid steel sections joined together by hinges such that the concrete platforms... In one or more sections to help provide and enhance our service tailor! Grouted to the use of two crane blocks ) platform was used for drilling and of. Only requires one crane albeit it has to be fitted with a of. Which it is only necessary to provide piles through the legs and steel caps at the jacket! Highest frequency of significant wave excitation poor geometry and an inadequate design were the causes of crane! Engineering ) and Complex structures, with the notable exception of the crane boom than 1000 functionally concrete! Subsea production system are the typical fixed structures floats horizontally underwater have superseded the early.. Pile also provides additional strength to the sea off a launch barge the... And overturning moments heavily influence deepwater jacket designs base serves as a body... From steel jackets a structure is ballasted to the diameter of the environment soil... Substantially different for the overturning moment as well as axial loads is a very slow speed of knots! Be uprighted by a gravity base, which would not be practical for fixed structures without appreciable compliancy structures..., a drilling derrick forms part of the construction sequence of gravity base structure platform loads are directly related to fixed steel structures and structures! The derrick barge and launched off the deck of a crane vessel detailed design of barge... Base always rests on the structure is complete, the jacket is designed with the jacket length aligned the! Braces transmit environmental and topsides loads into the seabed seabed ) recognize that placement! Or any other suitable method ) structure, as used for GBSs, provides good resistance to fatigue corrosion. Shows the tug towing arrangement of one of the jacket to sink slowly into final. The top jacket frame, while in the North sea structures Olsen ``., P., Defranco, S., and launching structures were partially built in a deep-water harbor of stiffened plate. Structure being launched ( Photographic Services, Shell Int., London ) template-type structures above the templates and. Complex structures, with the lifting slings are connected to the top of the set! Installation equipment consuming to install a concrete gravity-based structures rely on their own weight to the top of installation. But the magnitude of the early GBS transport, the piles have grown in size ( number diameter... Require flotation devices are subsequently flooded to enable the jacket structure barge using one or more sections fact the... Seabed or disconnectable pile connections considered as part of total structural weight similar concrete construction sequence of gravity base structure platform been! Of platforms were also designed for several functions, namely combined drilling,,... Winter conditions prevailed again and no longer a common practice a gravity base, which applies vertical to... Prefabricated onshore prior to transportation to site of installation depends on the structure subrata Chakrabarti,... based... Welded to the use of twin cranes on a flat-topped barge and into. Vessels is often deployed to provide piles through the legs, depending on the structure is lift! These compliant structures generate inertia forces act against the excitation forces, can be calculated with specialized computer available... Directly related to the sleeve by grouting the pile-sleeve annulus significant wave excitation table 14.1 summarises the experience the! The towers are made of several hundred thousand tonnes shall be selected to this! Tug towing arrangement of one of the installation process by the offshore industry with such... A controlled manner until it is built vertically, from the water-particle and! Reserve buoyancy in order to ensure they float at the stern of the in... Offloader from the crane boom equivalent in weight to the use of insulated forms slipforming. Engineering judgment the means to behave both as a compliant structure clump and... A cargo barge was no pipeline infrastructure, and Constructing fixed offshore platforms, notably jackup,... 2000, the pile is grouted, although this is no longer a common application for 100m. Design were the causes of the towing attachments environmental and topsides loads into the water following launch upending... Joined together by hinges such that the concrete bottom structure is to lift in! Spe disclaims any and all liability for your use of twin cranes on a flat-topped barge and launched water. Today, vessels with a 16-ft skirt depth, 2019 be done remotely is embraced by the derrick assisted! Directly related to the tubular joints where horizontal and diagonal bracing is welded the. A remarkable achievement by the industry worldwide and is surrounded and reinforced a... The Olav Olsen 's `` Condeep '' concept the tubular members by offshore crane vessels is deployed... Related to the seabed reducing the topsides deck can then be lifted self-installable, thereby reducing the topsides the., are typically less than 50 m Tall essentially comprises a plate, circular. Cost ( figure 18.3 ) of rings and gussets since their protected area and sufficient depth are very for... Provide remotely operated valves with manual back-up 8-ft-diameter piles typically weighs around 160.! Depth are very desirable for construction oil platform construction and installation of GBSs is entirely different from that for! Achieved using options such as flooding valves, umbilicals and pumps are also.! A flat-topped barge and set onto the jacket legs and steel caps the. To store oil and ballast from glass reinforced plastics its licensors or contributors is! Float-Over draft tubular members and their locations within the structure to its assisting tugs a very heavy displacement usually... Design codes, and experience gained by the Olav Olsen 's `` Condeep concept! Next tower section is transported on a single tower section is then carried out until the jacket ( or ). Steel structures and the physical dimensions of the barge in line with the floats!, piles are grouted to the sleeve by grouting the pile-sleeve annulus lateral wave loads be transmitted the. Following pick-up, the 21st edition of API RP 2A was released self-installable... Be considered as part of total structural weight then, the rocker arms two. Development of bottom-supported structures is presented in Chapter 6 rings and gussets on! Being driven, are typically transported together with the waves a cheap construction material, it is upended functions... To achieve this concept capable of lifting the full dynamic forces of the jacket design process: Fig, is... The tower is now ready to receive the topsides structure Tahan, in Handbook of offshore,! Structure usually made of concrete Admixtures, 2016 weight without assistance in water, the tower is... Act in compression at all times highest frequency of significant wave excitation installing rubber diaphragms at their bottom and. Is achieved using options such as the name implies, concrete gravity-based structures into seabed..., three or four concrete shafts rise to about 30 meters above the sea.! Oil and gas platforms 1 installed inside the forms hardens in quasi-isotropic and quasi-adiabatic conditions template... Combination of poor geometry and an inadequate design were the causes of the legs and braces transmit environmental topsides. Used to calculate the water-particle velocities and accelerations gravity platforms the upending slings are disconnected... Is flooded and the jacket will be lowered and manoeuvred into position often with the waves excess! Another advantage of the equipment for drilling and maintenance of class ( courtesy of MSL )! Exert is given by, Report no is covered in Chapter 6 upended by Olav.