Two major concepts of concrete foundations for Tension Leg Platforms are discussed, i.e. gravity foundation and gravity-embedded foundation. The latter concept is the preferred solution in a majority of applications. Four types of the gravity-embedded foundation are described. One of these has been successfully installed for SNORRE TLP in 1991. The other three types have resulted from further development and optimization, suitable also for great water depth, INTRODUCTiON Tension Leg Platforms (TLP) represent a viable solution to the demands of today's offshore oil and gas industry, aimed at producing hydrocarbons from reservoirs in deep water. A most vital element in the TLP design is the method chosen for anchoring the tethers to the seabed. During severe storms the load of such a tether system may exceed 50,000 t and after seconds will drop to a small fraction of this. The sucessful application of a TLP concept is literally dependent upon a safe foundation. A concrete foundation concept has been applied for a TLP - the largest ever built - at the Snorre field in the North Sea. The Snorre foundations were successfully installed in the summer of 1991 in a depth of 300 m. This paper focuses on the gravity-embedded concrete foundation. It discusses how the design depends on the static and dynamic tether tension, which variables enter the optimization process, and what characterizes the optimum foundations in various conditions. Some typical foundation designs are described. TYPES OF CONCRETE FOUNDATIONS Two types of foundation are considered:Grmdly iQUndatiQil, installed QO the seabed and designed to resist all tether tension by its own weight.Gravity-embedded foundation, provided with cylindrical elements - skirts - that are penetrated into the seabed, and designed to resist the tether tension through a combination of its own weight and the interaction between the structure and seabed. The gravity foundation is advantageously applied in conditions like very hard or rocky seabed where penetration of skirts is not possible. The latter foundation type, suitable for softer soils, is competitive due to cost and technical advantages such as minimum settlement and precise levelling. The gravity-embedded foundation - the most common of the two - is suitable for the potential TLP sites studied for the Norwegian Continental Shelf and the Gulf of Mexico. Although the principle of this design remains unchanged different configurations have been developed for a range of tether loads and water depth at the installation site. The design aspects are briefly discussed in the sub-section 'Optimization'. PRINCIPLES OF GRAVITY-EMBEDDED FOUNDATION Tether loads on the foundation Tether tension, T, is composed of two main components, I.e. sustained and oscillating (dynamic) tension. The sustained tension, T., calculated by the design engineer to meet requirements of acceptable motion characteristics of the TLP. Ts occurs when the TLP is free from loads by wind, current and waves. The difference between the actual tether tension and the sustained tension is the oscillating (dynamic) component of the tension, T.
The upper bound approach of the theory of perfect plasticity has been applied to develop a method for geotechnical stability analysis of unburied submarine pipelines. The coefficient of? lateral resistance of the soil and the corresponding displacements of the pipe depend on seven geometrical, loading and material parameters. Compared to the comfits based on Coulomb friction, the coefficients obtained by the present method are, in general, higher for pipes on sand and lower for pipes on clay. INTRODUCTION The complex problem of the interaction between water, pipe and soil for an unburied submarine pipeline is traditionally separated into two parts:the determination of wave and current action on the pipe in the form of lift, drag and inertia forces (in this paper termed as hydrokinetic forces); andthe investigation of lateral stability of the pipe exposed to known hydrodynamic forces. Lateral stability is often the critical factor for determining the necessary weight of pipe coating, especially in Shallow waters (?100 - 120 m). Appreciable attention has been devoted to the solution of part (a) of the problem,' including the pertinent gathering of environmental data and experimental results. If this effort is to gain the intended meaning, attention must be paid also to part (b) of the problem - transmission of the hydrodynamic forces into the soil. This paper describes a method for-predicting the lateral soil resistance to a pipe. The method employs a limit analysis by upper bound technique 1, 2,3 afforded by the theory of perfect plasticity. AVAILABLE METHODS OF ANALYSIS The lateral resistance of soil to a submarine pipeline is commonly treated as Coulomb sliding friction, the lateral soil resistance Rl being written as: (Mathematical equation available in full paper) where f = friction coefficient and Wn net Submerged weight of the pipe defined as: (Mathematical equation available in full paper) Here Ws submerged weight of the pipe, and Pv = vertical component of hydrodynamic forces, taken as positive upwards This approach is based on the assumptions that:the support is rigid,the pipe is sliding parallel to the surface of the support, andRl is independent of lateral displacement These assumptions fail for most actual physical problems. The soil is not rigid, the sliding does not develop along the soil surface, and the soil resistance depends on the displacements of the pipe. Thus it should be expected that this approach will fail for the most actual loading - soil - pipe conditions. Reliable results, however, can be obtained for "light" pipelines supported by dense sand using the approach based on Coulomb friction. Lyons7 has concluded that "Coulomb friction is not valid for lateral sliding on soft clay". For this case he has presented some results derived by a finite element procedure. These results alone, however, are not sufficient for determination of lateral resistance of pipes with other geometrical, loading and soil parameters.
Resistance to pipeline motions in cohesive soils is shown to change with time. The following time effects are distinguished: aging of disturbed soil, duration of constant loading on the pipeline (consolidation time) and rate of external loading. Theoretical treatment of the two latter of these effects is compared with experimental results available in the literature. These experiments confirm the increase of the soil resistance with increasing consolidation time as shown by the theory. They are, however, insufficient to demonstrate the effect of loading rate set forth by the theoretical considerations herein.
A philosophy for design of submarine pipelines on the seabed to resist ocean forces is proposed. According to it, the pipeline response to hydrodynamic forces is calculated and the predicted response parameters are compared with the permitted values given by the design criteria. Some guidelines are given to achieve compatibility of individual elements in the design procedure.
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