Page 127 - Pipelines and Risers

100 Chapter 7

Effects of high-pressurehigh-temperature (HP/HT)
High temperatures from the contents of the pipeline causes material expansion of the pipe
steel, this leads to an extension in the pipe length and the pipeline will buckle and seek new
deformation paths to maintain in equilibrium (Figure 7.3). The influence of material
expansion due to variation of temperature may therefore be included in the model.

Material properties such as yield stress, tensile strength and Young’s modulus change with
material temperature, and if necessary may be accounted for.

External hydrostatic pressure is an important factor regarding the strength capacity of deep-
water pipelines. Since the model may include a fully three-dimensional seabed, the external
pressure may be a function of the water depth. Internal pressure can be modeled as constant,
but the possibility to account for the static head of the contents may be included.

Figure 7.3 Top view of the fmite-element model showing buckling due to temperature dependent material
expansion (scaled displacements).

7.2.2 Dynamic Analysis Problem
Wave and current loading
Hydrodynamic forces arise from water particle velocity and acceleration. These forces can be
fluctuating (caused by waves) or constant (caused by steady currents) and will result in a
dynamic load pattern on the pipeline (Figure 7.4). Drag, inertia, and lift forces are of interest
when analyzing the behavior of a submerged pipeline subjected to wave andor current
loading.

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Figure 7.4 Top view of the finite-element model showing horizontal displacement when the pipeline is
subjected to wave and current loading.
Because of the dynamic nature of waves, the pipeline response when subjected to this type of
loading may be investigated in a dynamic analysis. Further, several wave formulations would
be desirable. 2D regular or random long-crested waves and the 3D regular or random short-
crested waves may be included in the finite-element model to supply the wave kinematics in a
dynamic analysis.
Trawl gear pullover response
The trawl gear pullover loads may result in a dynamic plastic response. The calculation of
loads and strength acceptance criteria are discussed in Chapter 11.