Deepwater Flexible Risers and Pipelines. Yong Bai
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Название: Deepwater Flexible Risers and Pipelines

Автор: Yong Bai

Издательство: John Wiley & Sons Limited

Жанр: Физика

Серия:

isbn: 9781119322733

isbn:

СКАЧАТЬ process.

      On the other hand, metal-based unbonded flexible pipes are suitable for static and dynamic applications with length of several hundred meters; they are compound by nine layers with different functions. The interlocked metal carcass layer and the pressure armor layer are added in the profile with respect to the unbonded FCP. The first is needed to prevent the collapse due to high hydro static pressure or sudden depressurization of internal fluids and also to avoid erosion from the extracted materials; the second provides strength against high hoop stresses due to internal and external pressure. Also, the tensile strength is improved, substituting strips with wires, which show wider cross-sectional dimensions.

      API Recommended Practice 17B is the guideline to follow in order to have a proficient design. Any pipelines must satisfy code requirements under the actual environmental conditions, beyond which it is not suitable anymore for its purpose. In order to do so, different types of load and failure modes must be considered.

      Failure is not necessary considered as a structural limit, but it can be easily defined as a modification of the flow conditions within the pipe, in fact leakage and reduction of internal cross-section undermine the main achievement of the structure.

      Unbonded composite-based are weaker than metal-based flexible pipes and, in many cases, are not the best choice for deep water. However, a list of all the possible pipe failure modes and mechanisms from the code, valid for both cases, is delivered below:

       Collapse

       Burst

       Tensile failure

       Compressive failure

       Overbending

       Torsional failure

       Fatigue failure

       Erosion

       Corrosion

      All the components are subjected to external loads and they have different reactions and strength, and in general, the effects increase as the water depth grows. Some failure mechanisms for specific elements appear as results of the failure mode due to loads for which they are not designed for. For composite unbonded pipelines, it is common that steel strips, which are designed for withstanding axial loads, may buckle under excessive hydro-static pressure or tensile loads, as well as during the transport phase.

      It needs to be underlined that for weak pipes, such as FCP, the strength provided by internal and external plastic layers cannot be neglected.

      A series of design criteria needs to be satisfied through strength reduction factors. When dealing with oil and gas problems, in particular off-shore, safety factors are very severe. For flexible pipes also, due to the very high uncertainties due to the complexity related to the helical shape of the steel layers, interactions, materials, and residual stresses, besides environmental forces, the safe margin must be considered wide enough. Design criteria express the safety for all the components; they are expressed in terms of allowable quantities which are specified by regulations and manufacturing, for:

       Strain

       Creep

       Stress

       Hydrostatic collapse

       Mechanical collapse

       Torsion

       Crushing collapse and ovalization

       Compression

       Service life factors

      Details of design criterion and analysis methods may be founded in Ref [5–7].

      In order to improve the performance of flexible pipelines, especially for deep water cases, reinforcements must be included in the cross-section design, depending on the environmental conditions.

Schematic illustration of interlocked carcass.

      The main reason of considering the carcass in the design is to provide enough radial resistance against external pressure. Moreover, it delivers the required strength in case of build-up (rapid decompression СКАЧАТЬ