Products: ABAQUS/Standard ABAQUS/Aqua
One problem encountered in offshore pipeline installation is the response of a pipeline lying directly on the seabed and being moved by winching one end or the other toward a point. Since the pipeline lies directly on the ground or seabed, frictional effects are important. Field measurements suggest that, for pipes lying directly on the seabed, the resistance to motion per unit length (the friction coefficient) is higher for motion transverse to the pipe than for motion parallel to the pipe. ABAQUS allows the modeling of this effect through its anisotropic friction option. This is a Coulomb friction model with different friction coefficients for motion transverse and parallel to the pipe. A stiffness method is used in this model; and, by default, ABAQUS will choose an “elastic slip” to occur during sticking. The value of the elastic slip is chosen as a small fraction of the average interface element length in the model. Alternatively, the user has the option of specifying the magnitude of the elastic slip to occur during sticking. This is chosen with the ELASTIC SLIP parameter on the *FRICTION option. A reasonably small elastic slip value should be specified for appropriate frictional interface behavior. Too small a value will result in excessive iteration. A reasonable value to choose for this elastic slip is a typical small model dimension—for example, the diameter of the beam. Friction coefficients are usually taken from field data. We choose to use the ELASTIC SLIP parameter approach in this case since the average interface element length in the model considers only beam element lengths and not their diameters.
A spherical gap element is used to model a weightless cable attached between the end of the pipeline and a fixed point and is used to winch the pipeline into the fixed point. The length of the cable is then specified as a function of time by using the *CONTACT INTERFERENCE option. The cable will only carry tension: if the force in the cable becomes compressive, the cable goes slack and remains slack until the relative positions of the two points is such that the cable will again carry tension. This slack cable concept allows any cable to be made inactive at any time by specifying it to have a very large length.
A pipeline 228.6 m (750 ft) long, with outer diameter 254 mm (10 in) and wall thickness 25.4 mm (1 in) is initially straight and stress-free and is assumed to lie on a hard seafloor. One end of the pipeline is attached through a weightless cable to a fixed point, A, which is offset from the pipeline end as shown in Figure 1.12.1–1. The cable is shortened gradually to simulate a winching process during which the end of the pipeline is pulled toward A. The anisotropic seabed friction capability discussed above is used to model the pipe-seabed interaction; the transverse friction coefficient is 1.0 and the tangential friction coefficient is 0.6. The motion is assumed to be in the plane of the model only. Fifteen hybrid beam elements (type B31H) are used to model the pipe. The hybrid beam elements are specifically formulated for modeling very slender beams and are generally recommended for this type of pipeline modeling.
The contact between the pipeline and the seafloor is modeled with the *CONTACT PAIR option. The seafloor is modeled as an analytical rigid surface with the *RIGID BODY option in conjunction with the *SURFACE option. It is an infinite rigid plane perpendicular to the global z-axis, and the pull-in motion occurs on that plane. The surface of the pipeline is defined by means of the *SURFACE option.
The mechanical interaction between the pipeline and the seabed is assumed to be anisotropic frictional contact, as discussed previously, with “softened” contact (a nonlinear pressure-clearance surface interaction model). Therefore, two suboptions are used with the *SURFACE INTERACTION property option: (i) the *FRICTION, ANISOTROPIC suboption to specify the anisotropic friction coefficients, and (ii) the *SURFACE BEHAVIOR, PRESSURE-OVERCLOSURE=EXPONENTIAL suboption to introduce softened contact between the surfaces. For this class of problem, such a mechanical interaction model is often more realistic than the default assumption of perfectly hard surfaces.
Since the pull-in motion is assumed to take place solely in the 
constant plane, the pipeline is defined to lie a distance of 0.2642 m (0.861 ft) above the seabed and is constrained from motion in the vertical direction. This results in a pressure between the pipe and seabed of 875.63 N/m (60.32 lb/ft). This constraint of the pipe in the direction of the normal to the rigid surface is possible only because we use a softened contact. The default hard contact introduces this constraint automatically whenever a slave node lies on the rigid surface.
The pipeline is constructed of steel, with a Young's modulus of 206.8 GPa (4.32 × 109 lb/ft2) and a shear modulus of 103.4 GPa (2.16 × 109 lb/ft2). The material response is assumed to be elastic, so the *BEAM GENERAL SECTION option is used to specify the pipe section description. This avoids numerical integration of the beam section. The *BEAM SECTION option, with numerical integration, would be needed if material nonlinearity must be considered.
Point A, the point toward which the pipeline is winched, is restrained. The vertical displacement of the pipeline is restrained, as well as the rotations about the x- and y-directions.
The automatic incrementation option is used to obtain the response history. Since the solution involves friction, it is path dependent; hence, a reasonably large number of increments are required to ensure that the solution follows the actual response path closely. For this reason an upper limit on the increment size of 0.1 of the total pull-in is specified.
The configuration of the pipeline at the completion of pull-in is shown in Figure 1.12.1–2. The figure shows that only part of the pipeline is affected by the pull-in. This is a consequence of the values chosen for the coefficients of friction and the flexibility of the pipe. For lower friction coefficients (or a stiffer pipe), more of the pipeline will be moved by the winching process.
