Float collar

Once the cementation has been completed the rig will wait on cement (V OC), i.e. wait until the cement hardens prior to running in with a new assembly to drill out the plugs, float collar and shoe, all of which are made of easily drillable materials. 

When the bottom wiper plug reaches the float collar there is a brief sharp rise in pump pressure. The pump pressure increase breaks the diaphragm in the lower wiper plug. Once the diaphragm is broken the spacer and the cement slurry flow through the float collar into the final joint or two of the casing... 

The internal volume of the casing string above the float collar is Internal volume = 13,800(0.2086)... 

Therefore, the mud pumps are used to fill the remainder of the casing string at a rate of 300 gal/min. Thus, the additional time T, to get the bottom plug to the float collar is... 

The pressure differential prior to the top plug reaching the float collar is... 

Equation 4-331 is valid for a back-flow valve at the bottom of the casing string such as a float collar. It is also valid for a back-flow valve located at the top of the casing string. 

Float shoe. The float shoe may be placed at the bottom of the liner. This component is a combination of a guide shoe and a float collar. 

In general, the float collar system of a gravity cage is at the water surface and double floating pipes bear the wave-induced loads. For simplicity, the float collar system is simplified to a double-column pipe system, as shown in Fig. 23.6. 

When calculating wave-induced forces on the float collar, the collar can be divided into many mini-segments. The forces acting on the whole collar can be... 

In addition to wave-induced forces, the float collar is also subjected to gravity, buoyancy, and mooring line forces. The gravity acting on the mini-segment can be written as ... 

The three-dimensional motions of the float collar include surge-sway-heave translation and roll-pitch-yaw rotation. In this section, two coordinate systems are adopted, the fixed coordinate system Oxyz and the body coordinate system G123, as shown in Fig. 23.9. The body coordinate system G123 is rigidly attached to the float collar and the coordinate axes 1,2,3 are principal axes with origin at the center of mass G. Initially, axes x, y, and z are parallel to axes 1, 2, and 3. 

Six degrees of freedom are required to describe the motion of a float collar in general spatial motion, resulting in six equations of motion. According to Newton s second law, under fixed coordinates, the three translational equations of motion are given by ... 

The motion of the float collar can be described by three translational displacements of the centroid and three Bryant angles, which can be obtained by solving the simultaneous ordinary differential Eqs. (23.12), (23.13), and (23.16). The equations can be solved using the Runge-Kutta-Verner fifth-order and sixth-order method. During calculation, transformation between the fixed coordinates and body coordinates is carried out by matrix [i ]. It should be noted that the forces and motions of the collar are dependent on the net through the mutual mass points attached to both the net and the collar. 

It is interesting to note that in wave-only conditions the float collar exerts approximately 90% of the total load on the gravity cage, whereas in current-only conditions the float collar accounts for <10% of the total load. Thus, it is very important to select reasonable values for the hydrodynamic coefficients of the float collar in wave conditions, but is less important for current-only conditions. 

The float collar with a diameter of 0.796 m was made of HOPE material. The cage net was made of PE with a mass density of 953 kg/m . The netting was knotless, with a mesh size of 20 mm and a yarn thickness of 1.18 mm. Mounted as diamond meshes, the net then formed an open vertical cylinder with a diameter of 0.796 m and a height of 0.43 m. The full-scale diameter of the net cylinder is assumed to be 16 m. 

When a net cage is subject to wave action, the amplitude of the horizontal motion of tracing points is greater on the lee side than on the wave side of the floating collar. The empirical formula for the maximum amplitude of the horizontal motion of tracing points on the lee side of the floating collar is (correlation coefficient = 0.986) ... 

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