Dynamic analysis for normal and inclined tether Tension Leg Platforms

Tension leg platform (TLP) is a type of compliant offshore structure where its
excess buoyancy over the weight produces pretension in the tethers connecting the hull to
the seabed. TLP technology preserves many of the operational advantages of a fixed
platform at the same time reducing the cost of production in deep water. Its production and
maintenance operations are similar to those of fixed platforms. TLP combines the initial
cost-saving benefits associated with floating production system with the operational
benefits attributed to the fixed platforms.
The dynamic responses of TLP such as the motions and the tether tensions when it
is subjected to wave forces are needed for the design and maintenance of the structure. The
amplitudes of motion responses must be within permissible limits to prevent flexural
yielding of the drilling risers which connect the platform to the sea bed completion
template. Limiting these responses leads to better stability and safe drilling operations.
In this study, a MATLAB computer program was developed for determining the
dynamic responses of rectangular, triangular and inclined-tether TLPs subjected to random
waves. The platform was considered as a rigid body and all the six motions were
determined. The hydrodynamic drag and inertia coefficients at each point on the platform
were revised after each time step. Second order wave theory and Modified Morison
equation were used for wave force calculations. Newmark Beta Method of time domain
analysis was used for the dynamic analysis. The response amplitude operators (RAO) for
typical square TLPs were compared with available theoretical results. The inclined-tether
stiffuess matrix was developed, using which the responses of inclined tether square TLP
under regular and random waves were determined. Also, parametric studies were made
varying parameters such as water depth, pretension, wave angle and position of CG. The
inclined-tether square TLP results were compared with the results for the square and
triangular TLPs. The results proved the capability of the developed programme in
predicting the responses. The results also indicated that the square TLPs performed much
better compared to triangular TLPs under random waves. The parametric studies highlighted the remarkable change in platform responses caused by changing the above
mentioned parameters. It was also shown that the inclined-tether square TLP had better
performance compared with vertical-tethered rectangular and triangular TLPs.