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SYNOPSIS A design computer model has been devised which, for given functional requirements and location site criteria, determines the dimensions and structural thickness of the lightest cuboid caisson for a chosen draught and adequate floating stability of the platform during submergence. The deck and towers assumed in the calculations are chosen from a range of standard decks and towers which have previously been designed for adequate performance to suit present design codes. This model permits sensitivity studies to be carried out to test the effect of the parameter of interest on platform weight, and therefore cost. The purpose of this platform weight, and therefore cost. The purpose of this paper is to illustrate how this model can be used to paper is to illustrate how this model can be used to investigate the effect of a given variable on caisson geometry. In this study the variable is the chosen draught dictated by the water depth in the tow-out channel from the construction site. The water depth at location is fixed at 150 m LAT. Minimum weight platforms have been calculated for a range of chosen draughts between 26 and 40 m. The geotechnical characteristics of these platforms have been calculated. The results of the model study arc presented graphically and as a rough test of its validity, the characteristics of a number of fully designed platforms described in available literature are superimposed. The agreement is reasonable which allows a number of general conclusions to be drawn, subject to the stated assumptions, on platform weights and geotechnical characteristics.Below about 25/27 m tow-out water depth, caissons become impractically large. Between 25/27 and about 30 m tow-out water depth, platforms will require the additional lift provided by a buoyancy jacket to be cost competitive. Above about 32 to 34 m tow-out water depth, the geotechnical characteristics of the sea bed site will control caisson size, for the minimum weight caisson (based on floating criteria) will be too narrow for Typical North Sea sites. Above about 37 m tow-out water depth further weight saving is marginal. Introduction As consulting engineers Ove Arup and Partners Scotland are designers of a family of concrete oil production platforms to operate in the North Sea. production platforms to operate in the North Sea. This experience in the design of concrete gravity platform schemes has led to the development of a platform schemes has led to the development of a computer procedure which carries out rapid preliminary platform design for given functional and site criteria. platform design for given functional and site criteria. The procedure gives, within a couple of days, a solution which is likely to be confirmed as feasible by more complete analysis at the derailed design stage. The object is the elimination of costly and timewasting schemes which would eventually be ruled out by detailed analysis. This 'Rapid Design Procedure' (RDP) combines the following analyses:For chosen draught, metacentric height and payload the minimum weight caisson is derived, the most payload the minimum weight caisson is derived, the most suitable deck and tower being chosen from a standard range. For a range of wave periods the most onerous combinations of environmental loads on the structure and foundations are calculated. The geotechnical requirements of the derived platform are calculated for the limiting sliding and platform are calculated for the limiting sliding and overturning stability cases. Assuming the worst tolerable soil conditions based on these stability criteria platform motions are determined under extreme storm conditions and for operating sea states by dynamic analysis. For the installation case skirt lengths are sized on penetration and hydraulic stability criteria and caisson-soil pressures are calculated for given bed topography. Available water depth is a critical factor in the choice of suitable construction sites for concrete platforms, During towing the draught of the structure must be such that, with allowance for tidal variation during the time of passage, the underkeel clearance is always adequate. The passage, the underkeel clearance is always adequate. The available water depth in the tow-out channel will influence the design of the platform, flatter caissons being required for shallower channels. In view of the interest in platform sites throughout the Scottish and continental coast line the RDP was used to determine the influence of chosen draught on platform weight and geotechnical characteristics. Where the available water depth is limited, considerable advantage can be found in certain cases from the introduction of a 'buoyancy jacket'. This is provided in the form of a number of additional concrete cells around the caisson which reduce floating draught, and which are completely flooded - or removed - after initial tow-out. In such a case the minimum weight caisson is determined for an unassisted draught greater than will apply with the use of the 'buoyancy jacket', with significant weight saving. Technical brief The following is the outline technical brief assumed in the RDP for this study:Crude storage facility 1,000,000 barrels Number of legs four
- Europe > United Kingdom > North Sea (0.65)
- Europe > Norway > North Sea (0.65)
- Europe > North Sea (0.65)
- (2 more...)
ABSTRACT Free-standing caissons are used for supporting flare pipes and single well production platforms. However, caissons tend to be flexible and dynamically sensitive, and the static design practice may not be adequate for this type of structure. In order to assess the motion effect on the integrity of the structural system and to quantify the allowable motion for safe operation on board a caisson platform, analytical and experimental studies of the dynamic behavior of a caisson structure have been conducted and are described in this paper. The analytical simulations check well with the motion measurements in a statistical sense. A caisson design procedure considering dynamic effects has been developed. Design considerations include ultimate strength failure, fatigue failure, excessive motion and possible damage during installation. A key feature in an effective caisson design is that the upper part of the caisson should be made as small as possible so that wave loading and the caisson period can be minimized. The fatigue design procedure has been verified with past caisson operational experience. To illustrate the procedure, a flare pipe support caisson in 185 ft of water has been designed and analyzed. INTRODUCTION Free-standing caissons are used for supporting flare pipes or single well production platforms. The attractiveness of a caisson structure lies in the potential economy and the short period required for fabrication and installation. However, a caisson tends to be flexible and dynamic effects may increase the design requirements from both strength and functional standpoints. In order to assess the motion effect on the integrity of the structural system and to quantify the allowable motion level for effective operation on board a caisson platform, analytical and experimental studies of the dynamic behavior of a caisson structure have been conducted and a procedure for designing a caisson considering dynamic effects has been formulated. Observations from the experimental data and computer simulations of the caisson behavior are described in this paper. Verification of the computer simulation and some useful information in developing and using such simulations as well as practical interpretation of the analytical results are also given. Differences between a static design and a dynamic design are illustrated in an example design of a flare support caisson in 185 feet of water. MOTION MEASUREMENT Motion data were taken from a caisson platform offshore Louisiana. General dimensions of the caisson are shown in Figure 1. Motions of the platform were measured by a self-contained, single-channel vibration monitor. The instrument can only take measurements from one direction at a time. Horizontal motions were measured at the heliport, production deck, wellhead deck, and boat landing. The motions were taken in directions both perpendicular to and parallel to the wave crest. Neither a wave staff nor an anemometer was available at the caisson platform and the sea state and wind velocities were obtained by visual estimation. The environmental conditions, maximum platform accelerations and measured platform periods are summarized in Table 1.
- North America > United States > Texas (0.46)
- North America > United States > Louisiana (0.34)
- Research Report > New Finding (0.68)
- Research Report > Experimental Study (0.54)
INTRODUCTION Large gravity based concrete structures have frequently been employed as production platforms in connection with field developments in the North Sea. Examples are Statfjord, Gullfaks, and, most recently, the Sieipner field. In the latter case current plans include at least four additional structures cl.ustered around a central con deep type platform. On this background STATOIL has lately felt an urge to explore wether (OJ) rather how) hydrodynamical interaction effects will lead to amplification of the wave loads on the structures. In order two clarify this point, and resolve the problem, a numerical investigation has been carried out with a panel program, and with the dual objective of:To study the general flow field around a condeep, or condeep like structure, with particular emphasis on wave enhancement effects over the caisson and around the shafts. To look at a specific example involving two condeep type platforms. GENERAL DESCRIPTION OF THE PROBLEM Recognizing that the caisson itself is largely the exclusive factor responsible for the massive disturbances in the wave field around a condeep, and opting to verify this conjecture in.a more general context, a totally submerged, bottom mounted cylinder, with elliptical basis, was chosen to represent an idealized caisson (fig.1). The major and minor axes were oriented transverse and in line with the wave direction, respectively. The ratio b/a of the minor to the major axis varied between 0.25 and 1.0. Equivalently this corresponds to an eccentricity c in the range (equation shown in paper). The actual dimensional size of the major axis varied between 45 and 90 meter, while the corresponding height of the cylinder ranged from 20 to 65 meter. The water depth at the" site" was kept fixed (and constant) at 82.5 meter, matching the projected depth at the Sieipner West Field.
- North America > United States (0.89)
- Europe > Norway > North Sea (0.34)
- Europe > United Kingdom > North Sea (0.24)
- (2 more...)
- Europe > Norway > North Sea > Central North Sea > South Viking Graben > PL 046 > Block 15/9 > Sleipner Field > Draupne Formation (0.99)
- Europe > Norway > North Sea > Central North Sea > South Viking Graben > PL 046 > Block 15/8 > Sleipner Field > Draupne Formation (0.99)
- Europe > Norway > North Sea > Central North Sea > South Viking Graben > PL 046 > Block 15/6 > Sleipner Field > Draupne Formation (0.99)
ABSTRACT The centrifuge testing method is very suitable for investigating new ideas for construction methods in the geotechnical field. The small scale models that are involved can be modified quickly and easily. The tests can be reproduced accurately, so that the effect of small changes in the design can be made visible. Several test programs have been carried out in the geotechnical centrifuge of the University of Delft to examine methods of improving the loading capacity of offshore foundation elements. This paper focuses on improving the loading capacity of circular conical footings (spudcans) and suction caissons. It was found that simple parameters, such as roughness, have a significant influence. Some new ideas for suction caissons were tested. An unexpected observation was that the pullout capacity could be improved by removing parts of the caisson. INTRODUCTION Several types of foundation systems are in use in the offshore industry. Some of them are used to fix floating structures, while others are used to support vertical loads. A typical example of the latter are spudcans, which are large circular conical footings used for bearing mobile platforms (jack-up units). Foundations of this kind can exhibit problems in sliding behavior when they are founded on sand. Several test programs have been carried out in the centrifuge to analyze the sliding behavior, and the experimental results were compared with the governing foundation criterion for a site-specific integrity assessment (Allersma, 1997). However, an interesting question that presents itself is how far the sliding resistance can be improved. A more generally applicable foundation element is what is known as a suction caisson. The main attraction of this system is the convenient installation method. A caisson with a diameter of 9m and a height of 10m can be installed in a few hours using only a pump.
ABSTRACT An unusual design was used in the Gulf of Mexico to develop a small oil reserve in 63-ft of water in ar area of unstable soil conditions. Front end project planning emphasized minimal monetary exposure by staging expenditures and deve1oping specialized procedures to accomplish installation using the upper 1imits of avai1ab1e construction equipment. The result was the installation of a 5 well 8' × 12' diameter satellite production caisson installed adjacent to a discovery well and designed to resist loads induced by a mudslide and a 100-year storm. INTRODUCTION The Main Pass (MP) 74-B caisson project originated in the first quarter of 1984 in response to the need to develop a small oil prospect in an. offshore mudslide-prone area. The prospect is located twenty-five miles east of Venice, Louisiana in Louisiana State Waters and is in the active delta area of the Mississippi River. As in most of the Mississippi de1taregion, sediments in this area have been deposited in large volumes over short periods of time. This results in very soft under consolidated surface soi1s that may be subject to mudslide or downslope mass-movements. Therefore, design of platforms for this area must consider loads induced by soil movement in addition to the typical environmental and functional loads. The existance of Exxon's nearby (3 miles to the north) mp 72-A platform precluded the need for extensive facilities on the MP 74-B structure, and the number of wells required for development was relatively small. Although these functional requirements were minimal, the major challenge of this project was to minimize up-front investments and cost exposure until the results of the prospect confirmation well (B-1) were known. To meet the challenge, a stair step project and expenditure schedule was formulated requiring design and fabrication of the structure to take place while drilling the B-1 well. The resulting satellite structure was a mudslide resistant caisson protecting five internal conductors and supporting one external conductor. A 35'x 35' lower deck was designed to support test facilities and a 50' × 60' upper deck serves as a combination heliport and concentric workover rig platform. An 8-in diameter pipeline was designed to carry two phase production across several mudflow features to the "A" platform. The location of the satellite structure was chosen to minimize water depth and to avoid active mudflow gullies. The MP 74-B project required eight months of planning and engineering and an additional six months for fabrication and installation. Production start up began 14 months after project initiation. DESIGN BASIS Functional Requirements The design basis for the prospect was a mudslide resistant satellite structure with a maximum of six wells. Major design considerations established by the development plan included.
- North America > United States > Louisiana (0.45)
- North America > United States > Gulf of Mexico > Central GOM (0.24)