Yong Bai - Deepwater Flexible Risers and Pipelines

The modified theoretical model exhibits an acceptable error equal to e = 2.58%, and it obviates to the previous discrepancy simulating the behavior in terms of load in addition to displacements.

Finally, the theoretical prediction of local stresses is precise if considering pipes or rings geometry but for the stainless-steel interlocked carcass, due to the intricate profile of the cross section, while affordable results could be obtained from FEM analysis. The actual stress magnitude is not constant along the structure; in fact, dissimilar values are reached in two different points under analysis. In Figure 2.18, it is reported the relationship between stress and strain for the critical point between the two, at limit value of ovality. It is important to focus the attention on the highest stress value reached which is equal to about 400 MPa, which is far below the proportional limit stress equal to 600 MPa. Thus, it is possible to deliberate on the right assumption done for the theoretical model of considering elastic material properties only.

In this section, the results about the collapse study for the interlocked carcass are compared with the one for a steel strip reinforced thermoplastic pipes, in order to understand until which water depth there is no need of further reinforcement. The SSRTP overall design is made by a variable number of layers of thin steel strips wrapped two by two at opposite lay angles, surrounded by an inner and outer HDPE layer.

Figure 2.17 Results comparison for FEM and modified theoretical model.

Figure 2.18 Stress-strain relationship.

Figure 2.19 HDPE stress-strain relationship.

Steel strips are considered to have an elastic behavior with a modulus of Young equal to 206,000 MPa. While HDPE layers properties account for an elastic-plastic behavior, where the modulus of Young is equal to 930 MPa and the yielding stress equal to 6.52 MPa, as shown in Figure 2.19.

The first case under analysis, named SSRTP-1, presents internal and external HDPE layers and in between four layers of six steel strips each. Geometrical parameters are listed in Table 2.4, where t are the thicknesses of the plasic layers, a is the winding angle between the strip and the pipe axes, and b and h are the strip cross-section dimensions.

The second case proposed, named SSRTP-2, is an improved configuration to withstand higher tensional loads and external pressure. It accounts for two more steel strip layers with higher lay angle in order to provide higher strength in the radial direction. The overall design shows the innermost and outermost HDPE layers, and six steel strip layers, numbered in outward direction, with different thicknesses and cross-section dimensions. In Table 2.5, all the needed geometrical parameters are listed, named as previously done.

Table 2.4 SSRTP-1 parameters.

The solution derived from the already verified theoretical model in Bai, Yong, et al. [3] is applied for both the design cases and results are reported in Table 2.6. SSRTP-2 shows collapse pressure highly improved compared to SSRT-1, while, as it was expected, it is still much lower than the one that considers the interlocked carcass in the design. Being the strips weak elements and mostly suitable for withstanding tensile and torsional loads, it is necessary to underline that the contribution of the plastic layers cannot be negligible, being equal to the 71% and 32% of the overall resistance, for SSRT-1 and SSRT-2, respectively.

Considering that for each meter of water depth, the corresponding external pressure acting is equal to 0.01MPa; it is possible to deliberate that a 6-in inner diameter flexible pipe with interlocked carcass is suitable above 900-m depth. On the other hand, very low strength against external pressure for steel strip reinforced thermoplastic pipes is revealed from the analysis compared to the reinforced cross-section, which means that they are mostly suitable for cases of very shallow water, being the cross-section not massive. In particular, SSRTP-1 and SSRTP-2 are appropriate until 20 and 37 m of water depth. Outcomes do not consider safety factor which must be taken into account considering the environmental conditions at the site.

Table 2.5 SSRTP-2 parameters.

Table 2.6 Collapse pressures.

Throughout this report, the collapse behavior of an interlocked carcass is simulated, and results are investigated theoretically and numerically. A variation of a primary adopted theoretical model is proposed which may be of interest for practical applications. In order to do so, a series of numerical models for the calibration were needed, leading to an empirical formulation. The latter is valid in terms of both pre-buckling and collapse conditions for a defined value of initial ovalzation, so that the critical pressure is computed neglecting the friction between layers for both models. Outcomes are compared with a steel strip reinforced thermoplastic pipes under the same requirements, in order to understand when the reinforcement is required. This study can provide support for factory engineers due to the accurate and reliable results that show very small difference between numerical and theoretical models.

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