Deepwater Flexible Risers and Pipelines. Yong Bai

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СКАЧАТЬ style="font-size:15px;">      Nowadays, even if many researchers are employed to enhance knowledge about flexible pipelines, there are no books available that systematically introduce the design procedures and analysis criteria that are always valid for the wide spectrum of these structures. On the other hand, it is possible to find many reference sources which can streamline the issues.

Moreover, they are widely used also because of their easy and cheap transport and installation, due to the possibility of being prefabricated onshore in long lengths and stored in limited size on reels; in fact, as most relevant structural property, these pipes show very low bending stiffness in comparison to axial tensile stiffness. Besides, the economic benefits, they are considered to be technically proficient due to their easy and fast laying procedure, durability, and recoverability. In the light of the above, the petroleum industry turns into flexible structures, allowing for permanent connection between the subsea system and any facility at the water surface with large motions.

      Being flexible pipes, crucial elements for the right operational oil and gas spill in terms of both performance and pollution, at the same time, a deep finite element model (FEM) is highly recommended in order to verify the reliability of the design. This method called DTA (Design Through Analysis) involves both the abovementioned procedures and the two-step process is used in complementary way in order to reach less conservative outcomes in designing, thus minimizing the project CAPEX (Capital Expenditure) and OPEX (Operating Expenditure). In some cases, it shows that codes and regulations are over conservative, and the real behavior can be captured through a FE simulation when the input parameters are well defined, if not data are statistically modeled in order to produce a reliable distribution for a range of loads and effects.

1.2 Environmental Conditions

      The wide expansion of these structures during the last decades has been made possible thanks to the costs-design optimization. It considers a deep understanding of the environmental conditions in which the pipeline will be installed and operated. Some of the drivers are: water depth and oceanographic data, chemical composition and flow phase of the extracted fluid, quantity of salt in the surrounding water, and operating internal and external temperatures and pressure. In addition, transport and installation circumstances must be considered due to the fact that in some cases, extreme loading conditions are shown during these phases. Most of the time, combination of loads needs to be taken into account. The analysis of the surrounding conditions, for example, temperature and corrosion, cannot be under estimation because of limiting capacity of the structures.

Temperature can affect the correct purpose of the pipe, in some cases, very high or low temperature of the fluids leads to the need of extra thermal insulation design, such as pipe-in-pipe or wet insulation, which considers the different thermal responses of materials. Polymeric materials exhibit lower thermal conductivity and higher thermal expansion coefficients compared to steel, so that the plastic layers govern the temperature profile through the pipe wall. Operating temperature is the foremost principle for selecting the polymer material in order to ensure the correct mechanical behavior of the pipe. In fact, for a given material, as the temperature rises the magnitude of the yield stress and Young modulus decreases, so the ultimate strain increases and vice versa, leading to various mechanical response properties during the service life of the structures. Generally, temperature profile impacts most of the design parameters, it is also influenced by the water depth outwardly and by the reservoir formation internally; thus, it must be carefully considered for both service limit state and ultimate limit state.

      Cross-sectional integrity needs to be mentioned for a remarkable pipeline design, which is of relevant importance especially for deep water. In fact, here hydro-static pressure is high, and the cross-section must be dimensioned against local buckling failure which is extremely influenced by imperfections. Not only, any other steel components provide structural support against axial, bending, and torsional loads, and their integrity is essential. Corrosion and cracks damages are hard to detect, being the structural profile made by different layers. Considering the conservative but actual hypothesis of failure of polymeric materials, steel corrosion is caused by interaction with salt water, air, or internal acid fluids or combination of them, which chemically alter materials. Besides the mechanical properties and price, the main driver for the selection of steel materials is the corrosion resistance to operating environment. The annulus conditions are continuously tested and monitored, and it is common practice to include deterioration protections such as coating, corrosion inhibitors, application of special materials and cathodic protection, in addition to lubrication oil distributed along the pipe during the manufacturing.

During the years, the availability of reservoirs onshore and in shallow water has decreased, and the need of petroleum pushed the industry to open new challenging offshore campaigns. The employment of flexible pipes in subsea brings researchers to focus on the estimation of the structural behavior in deep waters. Here, the environmental conditions are tougher, and reinforcements are necessary. Hydro-static pressure rises as water depth increases, which leads to considerable hoop stresses and buckling issues. For the whole pipe, this problem is managed making use of devices that enhance the strength in this sense, but at the same time, they bring additional structural weight and increasing gravity loads. Now, it can be deduced that as the water column grows, the magnitude of the tensile load rises. Dynamic tension is amplified by existing drag forces if considering the substantial pipe length in deep water, which is directly reflected on the end fitting, with consequently fatigue damages at this point due to constraint effects that may take place.

What being said, the challenge of the engineering is trying to make the best design, keeping the costs as low as possible, moving from local to global analysis. Any structure needs a particular design for the specific field and for precise environmental conditions, at the same time taking advantages from the experiences about other projects. The selection of the most suitable setup for any flexible structure is founded on practice and engineering judgment, in Figure 1.1, some examples of possible riser configurations using flexible pipes for which the choice is based on the abovementioned principles [2–4].

Figure 1.1 Employment of flexible pipelines as risers.

1.3 Flexible Pipeline Geometry

      Even if flexible pipelines have many applications, the common factor is to provide large flexural deformations while they are subjected to other loading forces, strictly necessary during installation and transport. Mostly, this property depends on their geometrical configuration, which is made by many layers. Generally, flexible pipes are compound by many main layers of different materials and functions:

       Internal polymeric sealing layer provides insulation from internal fluids, prevents corrosion and leakage due to extract materials,

       Helical armoring layers provide the required strength against different loading conditions, the number of layers and wires per each layer is variable and depends on the design;

       Outer polymeric layer prevents seawater from interacting with the armor layers.

They can be characterized in two main groups: the metal-based flexible pipes designed to withstand high loads see Figure 1.2 and the composite-based flexible pipes (FCP), which are much simpler and employed for lower functional requirements. These can be further divided in two more groups: bonded and unbonded structures.

      In the bonded structures, all the elements are fused together in the surrounding СКАЧАТЬ