Concrete Gravity Platforms Research Articles

Researching Offshore Facilities and Choosing an Appropriate Platform for Hydrate Extraction in the Bay of Bengal

Offshore buildings are used in various locations and sea depths for several reasons. Different equipment, platforms, and design techniques are needed depending on water depth, climatic conditions, structural arrangement, and new concepts. Offshore constructions usually generate and transport oil, gas, and other commodities. Bangladesh has yet to use the Bay of Bengal's hydrocarbon potential for oil and gas. Bangladesh lags behind India and Myanmar in maritime oil and gas discovery. In 50 years, Bangladesh's sea barriers have yielded no benefits. Bangladesh's economy is hindered by its high crude oil imports. Focus on offshore petroleum exploration to locate "black gold" now. Therefore, an offshore structure is needed. This thesis paper covers the fundamentals of all offshore systems, broadens the research, and recommends appropriate platforms for various sea-water depths, which those platforms are meant to be built for hydrates predicted in the Bay of Bengal. The paper uses the Bay of Bengal sedimentation and estimated water depth to choose offshore constructions. The country's maritime boundaries have 26 oil and gas blocks. There are 11 in shallow water and 15 in deep sea. According to statistics, the blocks are on the continental shelf and deep-water region. Five zones make up the Bay of Bengal continental shelf. They are shores A-B, B-C, C-D, D-E, and E-F after region F continental slope continues till the deep sea area. The continental slope zone C-D has 64-106 m water depth. Therefore, the Bay of Bengal continental shelf is suitable for all permanent offshore buildings except this zone. Only concrete gravity platforms fit this zone. We need the technology to search for oil and gas in water. It takes a lot of work to get foreign companies to work together. Thus, skilled people should work on it and encourage government or private businesses to develop oil and gas production technology.

Analysis, design and construction of gravity offshore structure; state-of-the-art

Abstract: Offshore platforms are divided into many types which are mainly categorized according to waterdepth in the installation location. However, the design differs for each type to accomplish the target of the operation. But for some case of sea waterdepths and an aggressive environment such as the North Sea, steel ones are not suitable, so the heaviest type called gravity platform having enormous mass is used. This type of platform has its special requirements and procedures for construction and needs special types of construction materials in order to resist the climate factors applied due to the aggressive environment. The paper carefully illustrates how the principal Environmental loads (wind and wave), current forces, loads from ice and loads from earth-quake for (earth-quake prone zones) are deployed to archive the design and construction of offshore concrete gravity platforms. Two design methods (Analysis and Design of Shell structures) and the Tangent Modulus Methods of design of Offshore Concrete Gravity platforms are discussed

FLEXURAL BEHAVIOR OF HIGH STRENGTH FIBER REINFORCED CONCRETE BEAMS AS AFFECTED BY RIB GEOMETRY AND GRADE OF MAIN STEEL UNDER STATIC LOADS

The need of high-performance concrete is increasing in recent years for the use in high rise building and the new breed of concrete gravity platforms for offshore structures. The use of high strength concrete needs high percentage of steel reinforcement to get the full capacity of the flexural strength of the member. Hence, the need of using high grade of steel reinforcement and different fibers to concrete mixes is necessary from the economical point of view. So, different ribs are used for the increase of bond strength between steel reinforcement and highperformance concrete. But these materials were brittle and the failure also was brittle. So, fibers are used to enhance their composite properties. There is little information in the available literature about the flexural behavior of high-performance and high-performance fiber-reinforced concrete beams with different rib geometry under partial bond. Therefore, this paper focuses on the application of different types of fibers and steel rib geometry in the technology of high strength R.C. rectangular beams having fc=900kg/cm2 and study their flexural behavior under static loading. Test results showed that, the failure mode, cracks pattern and flexural behavior of high strength and fiber-high strength reinforced concrete beams were clearly affected by rib geometry and quality of the main steel and fibers type. The values of cracking and ultimate load carrying capacity of R.C. tested beams were increased with the increase of the grade of the main steel and its relative rib area (Fy from 3100 to 4750 kg/cm2 & αsb from 0.0 to 0.10) by about 65 & 76% respectively for beams without fibers. Also, they were increased by about 80 & 82% for beams with polypropylene fibers and by about 86 & 83% for beams with harex steel fibers. The addition of fibers to concrete mixes of R.C. beams has greatly improved their ductility by increasing the ultimate strains under maximum compressive stresses and improved the crack propagation patterns for all tested (HPFRC) beams.

Energy Dissipation in Soil-Structure Interaction

Energy dissipation as a means of reducing the seismic response of structures has become a popular topic among researchers and structural engineers who have developed and implemented devices, such as friction dampers, fluid dampers, and isolators, in the design or retrofit of structures. However, a natural source of energy dissipation is the interaction between a structure, its foundation, and the supporting soil medium. To account for this frequency-dependent energy dissipation in dynamic analysis based on modal superposition, relatively simple and practical systems-identification methods are presented to estimate the composite modal damping ratios for the significant modes of vibration. SSI experiments and analysis of simple theoretical models using this method have yielded relatively large modal damping ratios in certain situations for structures such as short to mid-rise buildings, short-span bridges, flat bottom fuel storage tanks, offshore concrete gravity platforms, nuclear power plant containments, and nuclear waste processing plants.

The Ekofisk Protective Barrier

The Protective Barrier is a self-supporting offshore concrete gravity platform which forms a complete ring around the existing Ekofisk Storage Tank. It was installed initially as two half units, and was subsequently connected offshore in order to protect the Tank substructure and the associated production equipment contained thereon from severe environmental action resulting from the effects of subsidence at Ekofisk. The Paper provides a description of the design and engineering of the Protective Barrier, together with an account of the construction and installation work. This experience has led to recommendations for code changes, which would be of benefit in the future design and construction of similar offshore structures.

Concrete gravity platforms—computer aided analysis for safety factors

Concrete gravity platforms have been analysed, designed and constructed. Many attempts have been made to accurately predict wind/wave loads on such structures. Very little information is available on the safety factors of relevant zones of these platforms as and when they are operational. The purpose of this paper is to first develop an ultimate load analysis and then apply it to an existing platform using a standard wave theory. Safety factors are then computed at various levels using these loads. The method will clearly exhibit the strength or weakness of any zone or part of the platform.

The properties of low strength silicate/portland cement grouts

Sodium silicate - sea water - Portland cement grouts at high water/cement ratios (between 2 and 3) have been used for a number of years for void filling in the foundations of offshore concrete gravity platforms. Some of the initial development work on the grouts is briefly reviewed, and the results of an investigation into their long term properties are presented. It is shown that the grouts obey similar strength vs water/cement ratio laws to conventional structural grouts and concretes, but that unconfined swelling may lead to some long term degradation. An examination of the swelling mechanism may be of use in the study of alkali - silica reaction in concrete.

Dynamic analysis of an offshore structure on an elastic-plastic foundation-soil system

The effect of foundation stiffness upon the dynamic response of an offshore structure is investigated. The non-linear deformation behaviour of a stratified seabed is included in the finite element simulation. The seabed is represented by a simplified spring model. In the linear elastic analyses the stiffnesses are derived using elastic half-space theory. For the non-linear problems, elastic-plastic finite element analyses are used to generate equivalent properties. An example of the method is given as applied to a three towered concrete gravity platform. Variation of natural frequency is used to indicate the significance of changes in parameters such as soil shear modulus, structural stiffness and deck mass. The forced response behaviour to random wave excitation is also investigated using a linear elastic analysis. The effect of the interaction of the foundation — soil with the structure on the dynamic response is demonstrated by the results.

Field Measurement of Platform Behavior, Comparison With Design, and Long-Term Monitoring of Responses Stability After Three Years of Service

Field data from structural, geotechnical and wave measurements made on the concrete gravity platform TCP2 of the Norwegian FRIGG field are disclosed in this paper. Data recorded during 3 yr are interpreted and compared to design values. The stability of invariant parameters has been followed and demonstrated in view of monitoring the long-term behavior stability of the platform. The measured behavior differs from the design behavior and exhibits that design assumptions were conservative. Particularly during this 3-yr period the platform experienced very severe storms with waves up to 25.7 m, a value close to the maximum design wave (100-yr wave: 29 m). It is shown from field data how a bridge connection to a neighboring platform may influence the actual behavior compared to the design one and must be taken into account to understand the measured behavior. It is finally shown that supposed invariant parameters like natural frequencies and others are stable on a long-term basis, and how these invariants are intended to be used for the long-term stability monitoring.

Reduction of pore water pressure beneath concrete gravity platforms : Eide, O; Andresen, R; Jonsrud, R; Andenaes, E Norw Geotech Inst PublN143, 1982, 8P

Reduction of pore water pressure beneath concrete gravity platforms : Eide, O; Andresen, R; Jonsrud, R; Andenaes, E Norw Geotech Inst PublN143, 1982, 8P

Inspection of Concrete Offshore Structures

Summary The planning and inspection procedures used on Total Oil Marine Ltd.'s MCP-01 (manifold compression platform) minimize time spent offshore and ensure that the retrieved information accurately describes the structure's condition. Procedures include preparing a detailed field specification; visual, photographic, and nondestructive inspection photographic, and nondestructive inspection techniques; and a report designed for easy access to detailed information. Current research on underwater repair techniques is outlined. Introduction The Offshore Installation (Construction and Survey) Regulations 19741 require that a thorough examination (major survey) of an installation be undertaken to ensure compliance with Schedule 2 of that document before the 5-year certificate of fitness is renewed. In addition, annual surveys are required of a sufficient number of selected parts of the structure to give reasonable evidence of continued compliance with Schedule 2.Schedule 2 of the regulations outlines the factors to be considered for structural integrity of a platform.The first certificate of fitness ensures that the design and construction requirements are satisfied. Therefore, subsequent inspections need only to consider changes in the structure's state. For dead and live loading, these changes must be taken into account in the design limits. Consequently, both annual and major inspections are concerned primarily with material deterioration, with accident primarily with material deterioration, with accident damage included where a separate inspection is not required.The inspection program for Total's MCP-01 satisfies these regulations while minimizing the interference with platform operations and providing detailed, quickly accessible information in annual reports on the structure's condition. This program is considered in two parts:the planning approach to cover the major and annual inspection requirements andvisual inspection and nondestructive test methods.In the event of a deterioration rate that could lead to an unacceptable structural state in the design life or to damage caused by ship impact, dropped objects, fire, or explosion, repairs will be necessary. Although methods of repairing small scale concrete damage underwater exist, there are no techniques for undertaking major repairs.To fill this gap in the offshore industry's capability, a research program aimed at developing such techniques, particularly where reinforcing and prestressing are involved, is underway. prestressing are involved, is underway. Description of MCP-01 The intermediate manifold platform, MCP-01, was positioned as the intermediate station between the positioned as the intermediate station between the Frigg field and the St. Fergus shore-terminal treating facilities in July 1976.MCP-01 (Fig. 1) is a concrete gravity platform, designed by C.G. Doris, comprising a solid raft 101 m in diameter with six tunnels radiating from a hollow central core. The core, 9 m in diameter, extends from the raft through the full height of the structure, which is 147.5 m and houses the 32-in. riser piping with two risers per pipeline. Each radial tunnel contains a seal caisson which provides the watertight sealing joint through the concrete lobed wall for riser entry.At approximately 30-m radius from the core there is a concrete caisson wall (lobed wall) which extends above the waterline. The grillage of concrete deck beams is supported off the core from columns located on top of the lobed wall. JPT P. 2243

Observed foundation behaviour of concrete gravity platforms installed in the North Sea 1973–1978

Available measurements of foundation behaviour of the 13 concrete gravity platforms installed in the North Sea in the years 1973–1978 are presented. The measurements cover both the installation phase and the early part of the operation period. For the installation phase, measurements of dowel and skirt penetration resistance, base contact stresses, tilt, piping and erosion, are presented. For the operation period, the presentation includes measurements of settlements, pore water pressures, base contact stresses, permanent lateral displacements, cyclic displacements, dynamic behaviour and erosion. The measurements are used to back calculate soil properties and to evaluate the adequacy of the design procedures which have been used. The observations give valuable experience with respect to the foundation performance of offshore gravity platforms.

A computerized analysis of offshore concrete gravity platforms

A computerized analysis of offshore concrete gravity platforms

Earthquake Survivability of Concrete Platforms

Watt, B.J., SPE, Brian Watt Assocs. Inc. Boaz, Irvin B., Shell Oil Co. Ruhl, J. A., Shell Oil Co. Shipley, Stanton A., Boeing Engineering and Construction Dowrick, David J., Ove Arup and Partners Ghose, Amitava, Keith, Feibusch Assocs., Engineers Nonlinear dynamic analyses were conducted to assess the survivability of concrete gravity platforms during extreme earthquakes. Structure, foundation, and earthquake characteristics were varied among 15 analysis cases. It was concluded that suitably designed platforms can survive ground-shaking conditions likely to be associated with rare intense earthquakes in the Gulf of Alaska. Introduction This paper describes the results of efforts to quantify the damage that would be sustained by a typical concrete gravity platform during an extreme earthquake. Earthquake-induced loads are rather different from those caused by waves. Among the most significant differences are the greater uncertainty associated with earthquakes and the ratios of the loads at the extreme (very low probability) and design levels for the two phenomena. Consequently, while the safety factors used in design usually will provide adequate protection against collapse from extreme winds and waves, this is not necessarily the case for earthquakes. It is not unreasonable to envisage rare intense earthquakes producing ground-shaking magnitudes much larger than those associated with a design-level event.The structural response to earthquakes is energy-controlled rather than load-controlled; as a consequence, structures can be designed to survive extreme earthquakes by providing them with adequate ductility. Therein lies the key to earthquake-resistant structural solutions that are both reasonably economical and safe. The concept of designing elastically for the design-level earthquake and inelastically for survivability, or rare intense earthquake, is not new. It forms the basis for the more progressive building design codes in seismically active regions and is the kernel for the most recent API RP 2A recommendations for steel-framed structures. Unfortunately, structural ductility is not a straightforward characteristic to define, nor is it easy to set ductility criteria that suit a wide range of structures. Experience, analysis, and experiment have been used to establish ductility requirements for certain classes of structures, such as framed reinforced concrete buildings, with and without shear walls, steel-framed buildings, and offshore jackets and towers.Concrete gravity platforms have not been subjected to the same close scrutiny. The purpose of the investigation described here was to find answers to these questions:Can typical concrete platforms survive the conditions of an extreme or maximum credible earthquake? andWhat would be the extent of the damage? Survivability in the present context means the ability to endure the event without collapse of the structure, foundation, or any primary structural component. It is implied that the system could be unserviceable after the event.Shell Oil Co. has examined several aspects of the seismic design problem in a series of studies, and some preliminary results were published in 1976. This paper describes the development of answers to the above questions and some of the other principal findings from these studies. JPT P. 1090

Soil deformations due to cyclic loads on offshore structures : Andersen, K H; Hansteen, O E; Hoeg, K Norwegian Geotechnical Institute publ N120, 1978, P1–40

The type of offshore structure that may safely and economically be installed at a given site depends to a large extent on the local soil conditions. Fixed offshore structures may be grouped into three classes; pile supported, gravity and anchored. During the last few years large concrete gravity platforms have been used as alternatives to the more conventional piled steel structues at several locations in the North Sea. There are great difficulties encountered in performing high quality soil investigations offshore due to deep waters, restless seas and lack of well suited and calibrated sounding and sampling techniques. The foundation soil behaviour has to be analyzed for large cyclic loads superimposed on the static ones. Experimental results from soil elements subjected to cyclic loads in the laboratory show substantial scatter, which gives rise to large uncertainties in the analytical predictions of field performance. Very recently one has gained experience and recorded data from the crucial installation phase and the later performance of gravity platforms in the North Sea. A systematic evaluation of the reliability of present practice and available engineering analyses has started by comparing before-the-event predictions to field measurements. It is attempted to give an overview of the geotechnical engineering performed for offshore gravity structures and to point out to what extent theories of probability and statistics so far have been used. Specific cases are considered for probabilistic analyses of the platform installation phase and foundation stability during storm loading.

Drilling From Deepwater, Concrete Gravity Platforms

This paper discusses the operation of three different types of gravity platforms in the North Sea. When the Brent, Cormorant, and Dunlin fields platforms in the North Sea. When the Brent, Cormorant, and Dunlin fields were discovered, drilling required the use of a completely untried type of platform. Various design problems and their solutions are described. platform. Various design problems and their solutions are described. Introduction When oil exploration rigs first drilled in the northern North Sea, many companies knew that if oil was found, no established production method existed in such hostile waters. The announcement in Aug. 1972 by Shell U.K. Exploration and Production Ltd. of the discovery of the Brent Field heralded one of the most expensive and innovative oilfield development programs ever seen. In the following year, Shell announced the discovery of two more fields in the far north, Cormorant and Dunlin. For these three fields, six platforms were ordered (summarized in Table 1). Five of these were of a type completely untried for drilling operations: gravity platforms. Unlike a conventional steel jacket, which platforms. Unlike a conventional steel jacket, which relies on piles driven deep into the seabed to keep it secure, a gravity platform relies mainly on its own weight contacting a large area of the seabed to prevent it from moving. This paper discusses Shell's experience in operating three different types of gravity platforms (Fig. 1). (Shell Expro operates in the North Sea on behalf of Shell and Esso in a 50/50 partnership covering all areas where joint acreage is held). Because economic incentives to start exploiting the oil as soon as possible were great, there was no time to think out every detail of platform design before construction began. Simultaneous design and construction became the order of the day. When drilling engineers began to look at the problems of drilling from a gravity structure, it soon was apparent that disturbing the platform foundation (the seabed) by drilling through it would weaken the structure itself. Avoiding undue disturbance of the seabed and near-surface sediments was a major concern. These gravity platforms were larger than any used before. The high derrick-floor elevation along with the great water depth could lead to difficulties in getting mud returns while drilling tophole - the second major concern. A third problem was to design the conductor and subsequent pipe strings to take wind and wave loads, either applied directly or through the platform. Fourth, each individual platform had its own individual problems resulting from differences in general layout and design. Conductor String Design The main criteria for the conductor strings wereto be strong enough to absorb environmental loads,to be strong enough to take vertical loads imposed by platform settlement,to be strong enough to be driven to refusal using the largest diesel hammer that could be used in a derrick (Delmag D-55, 144,000 ft-lbf),if possible, to be driven so deep that mud returns could be obtained at derrick-floor level without fracturing the formation at the conductor shoe when drilling out of the conductor, andto be large enough for a 20-, 13 3/8-, 9 5/8-, and 7-in. casing scheme. P. 926

Loads and moments due to currents and wind on offshore gravity platforms

Abstract The environmental factors that influence the loads imposed on offshore structures are: the ice effects, tidal fluctuations, the wind, the currents and the waves. This paper considers the significance of loads and moments due to wind and currents on offshore gravity platforms. The alogarithm CURLOAD is given to evaluate loads and moments of gravity platforms due to wind and currents alone. This subroutine forms a part of a suite of computer programs F-WAVE which analyse and design gravity platforms under static and dynamic sea load conditions. The programs are tested on Condeep concrete gravity platforms. The output from CURLOAD shows a good agreement with the one predicted by the Condeep designers. In conclusions, the paper gives a comparative study of these results with those produced by the combined effects of wind, currents and waves.

Response of concrete gravity platforms to earthquake excitations : Watt, B J; Boaz, I B; Dowrick, D J J Pet Technol, V30, March 1978, P318–324

Response of concrete gravity platforms to earthquake excitations : Watt, B J; Boaz, I B; Dowrick, D J J Pet Technol, V30, March 1978, P318–324

Response of Concrete Gravity Platforms to Earthquake Excitations

The earthquake response of typical concrete gravity structures in 100 to 200 m of water was evaluated. Response spectrum and fast Fourier-trans-form solution techniques were used. Parametric studies showed that the analysis method markedly affects magnitude and distribution of computed foundation and structural forces and that correct modeling is essential. Introduction One specialty session at the 1975 Seventh Annual Offshore Technology Conference was about offshore earthquake engineering. Few state-of-the-art presentations then addressed the response of concrete gravity presentations then addressed the response of concrete gravity platforms to earthquake influences. This is not surprising platforms to earthquake influences. This is not surprising because the first production platforms of this kind were installed in the North Sea only in 1975 and to the author's knowledge, none have been ordered yet for an offshore region characterized by a high earthquake risk. Among the chief advantages of the concrete gravity platform are its potential for carrying payload to location platform are its potential for carrying payload to location and the limited offshore work required. In a short-installation weather window like that of the North Sea, these advantages only now are appreciated fully. The Gulf of Alaska has weather characteristics similar to the North Sea, but also has a high earthquake risk. This paper presents part of an over-all investigation into the presents part of an over-all investigation into the feasibility of constructing and installing concrete gravity platforms in the Gulf of Alaska. The investigation was platforms in the Gulf of Alaska. The investigation was conducted from Sept. 1974 through April 1975. When the study began, there were sound geological reasons for anticipating suitable foundation conditions at or close to the mud line; however, only limited numerical data existed. Objectives One can assume that the intensive nuclear power development program in the 1960's generated ample means to evaluate earthquake problems of massive concrete structures. While this is undoubtedly true with respect to understanding the basic phenomena and developing methodology, certain factors must be remembered. First, the mass and geometry of a typical gravity production platform are significantly larger than those of a nuclear platform are significantly larger than those of a nuclear reactor. Second, the superstructure of a platform is relatively flexible. Third, the inter-action with the surrounding water represents another complication in a submerged structure. Finally, the emphasis is on the detailed evaluation of a particular site in a typical nuclear-reactor design, whereas this Gulf of Alaska study was geared more to identifying sites that would be satisfactory from all viewpoints. For these reasons, this study was considered only preliminary and emphasis was placed on the following objectives:to assess the importance of foundation compliance, added mass effect from the water, damping behavior, and method of analysis,to quantify typical design forces in the structure and its foundation, andto assess the design implications for the major parts of the structure. These objectives were accomplished within 6 months. JPT P. 318

Deep Sea Construction

Recent operations in the Atlantic Ocean, Gulf of Mexico, Lake Maracaibo, Santa Barbara Channel, Alaska, Canadian Archipelago, and North Sea are presented with evaluation of problems, difficulties, the complex means of overcoming them, costs, and financing. Reference is made to the “Texas Towers” for radar protection planted in the North Atlantic in the l950’s. Innovations to counter ice problems in Alaskan and Arctic Ocean waters, and to combat the particularly deep and stormy waters of the North Sea, are explained in detail. Exploratory drilling facilities; permanent production structures in both steel and concrete, with a detailed account of the rapidly developing prestressed concrete gravity platforms are covered. Concludes with a presentation of buried pipelines to carry products to markets, and of the new extremely heavy and expensive equipment required.