Effect of Mooring Lines Pattern in a Semi-submersible Platform at Surge and Sway Movements

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In the Semi-submersible Platform, for proper operation and preventing damage on systems and equipment of excavation units it required that horizontal motion of structures be limited to less than 1% of water depth ^[23], that is defined in Table 1.

Table 1. Limits of semi-submersible platform movement

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3. Modeling

Intended in simulate, a case study modeled with Flow3D (hydrodynamic software) that gives time domain analysis for investigation of the structure movements.

In this simulation used a platform with specifications of GVA 4000 platform like Iran ALBORZ platform. Table 2 defined the characteristics of this platform.

Table 2. characteristics of Iran ALBORZ platform

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At this research, modeling is done by taking a survival situation, hence, conditions that draught is 16.2 meters.

In this modeling, total weight on the floated body plus body weight is equal to weight of the water displaced by the floating structure. And total weight is applied at the semi-submersible platform body’s center of mass.

Table 3 shows details of semi-submersible platform weight, center of gravity and center of buoyancy.

Table 3. Iran ALBORZ semi-submersible platform weight, center of gravity and center of buoyancy

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Geometry of the flume created for semi-submersible platform in a water depth of 500 m and also meshing of model has been selected as integrated single block. The dimensions of these cells should be considered that the FAVOR (Fractional Area Volume Obstacle Representation) conditions can be satisfied.

In Figure 2 meshed view of semi-submersible platform in flume of the fluid is shown.

Figure 2. Meshing of semi submersible platform with 4 mooring lines

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Validation for grid cell size done with 4 meshing networks with 1000000, 1200000, 1500000 and 2000000 cells was in kind 6 of mooring systems.

And more converged results for the grid cells with 1200000 are valid.

Of course, we used fine mesh in the near range of semi-submersible platform to enhance network quality, and coarse mesh far filed.

In flow3d software both to Cartesian coordinates and cylindrical coordinates there are 6 surfaces to define the boundary conditions.

Boundary conditions are defined in Figure 3 with 6 surfaces.

Figure 3. Boundary conditions with 6 surfaces

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Boundary conditions changes respect to wave direction selection for each of the 0, 45, or 90 for bounds X and Y.

Also, for analyze the variable patterns of mooring lines geometric arrangement from 7 types of layout that those are, 3 kinds with 4 lines in the geometrical arrangement (Figure 4) that in kind 1 mooring lines are perpendicular to the wave direction, in kind 2 mooring lines are parallel to the wave direction and kind 3 that mooring lines are in cross formation (angle of 135 degree between mooring line and structure), and also 4 kinds with 8 lines in the geometrical arrangement that are shown in Figure 5 (Angles of 30, 45, 60 and 90 degree between adjacent mooring lines) and by applying of wave forces direction in angle of 0, 45 and 90 degree, has been studied.

Figure 4. 3 kind of mooring system arrangement with 4 mooring lines

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Figure 5. 4 kind of mooring system arrangement with 8 mooring lines

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To evaluate the impact of environmental conditions on the structural motions, also we use the Caspian Sea environmental information with water depth under 1000 Meters that SADRA Company Published. And apply the final threshold conditions in order to assess maximum possible displacement structures are used. These data are presented in (Table 4).

Table 4. Caspian Sea environmental threshold conditions data

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Also to apply the environmental wave forces on the semi-submersible structure for large members used from Diffraction theory (Equation 3) and for thin members used of Morrison equation (Equation 2) that these equations are as following:

(3)

(4)

Where In equation 3, ρ is fluid density, CI is inertia coefficient, D is pile diameter, ω is angular velocity of fluid particle, H is wave height, k is wave number, y is depth from sea base, h is wave height adjacent pile and ε is angle of arrears.

And In equation 4, Cm is inertia coefficient, ρ is fluid density, D is pile diameter, is horizontal accelerate of fluid particle in pile axis line, Cd is drag coefficient and u is horizontal velocity of fluid particle in pile axis line.

Surge movements analysis of semi-submersible platform specified in Figure 6 for 7 kind mooring systems and 3 incoming wave with angles of 30, 45 and 90 degree.

Figure 6. Semi-submersible platform Surge movement

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Also Sway movements under different angles of the incident wave are defined in Figure 7 for 7 kind mooring systems and 3 incoming wave with angles of 30, 45 and 90 degree.

Figure 7. Semi-submersible platform Sway movement

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