A tension-leg platform is a floating offshore structure used to support drilling, production, or research equipment in deep ocean water. Instead of resting on the seafloor, it floats at the surface while long vertical tethers connect it to heavy anchors below. These tethers, called tension legs or tendons, are stretched tight so the platform stays nearly fixed in position.
This design matters because it lets engineers work safely in water too deep for ordinary fixed towers.
The platform floats because buoyancy pushes upward on its hull, but the tension legs pull downward against that upward force. The balance between buoyancy, weight, and tether tension makes the platform stable even when waves, wind, and currents push on it. Tension-leg platforms can move slightly side to side, but they strongly resist vertical motion such as heave.
This makes them useful for deep-water operations where equipment must stay aligned with wells, cables, or instruments on the seafloor.
Key Facts
- Buoyant force is the upward force from displaced water: F_b = rho g V.
- A tension-leg platform is held down by vertical tendons that stay under constant tension.
- Static vertical balance can be written as F_b = W + T_total, where W is platform weight and T_total is total tendon tension.
- Tension legs reduce heave, which is the up-and-down motion caused by waves.
- Horizontal forces from wind, waves, and currents can shift the platform slightly, but the tendons provide restoring forces.
- Tension in each identical tendon is approximately T_each = T_total / n, where n is the number of tendons.
Vocabulary
- Tension-leg platform
- A floating offshore platform held in place by taut vertical tethers attached to anchors on the seafloor.
- Buoyancy
- The upward force exerted by a fluid on an object that displaces that fluid.
- Tendon
- A strong vertical cable, pipe, or tether that connects the platform to the seafloor anchor and remains under tension.
- Heave
- The vertical up-and-down motion of a floating object caused by waves.
- Anchor foundation
- A heavy or deeply embedded seafloor structure that holds the lower end of each tension leg in place.
Common Mistakes to Avoid
- Thinking the platform sits on the seafloor. This is wrong because a tension-leg platform floats at the surface and is connected to the bottom only by long taut tendons.
- Forgetting that the tethers are already stretched before storms occur. The tendons must remain under tension so they can resist vertical motion and keep the platform stable.
- Assuming buoyancy alone holds the platform steady. Buoyancy keeps it afloat, but stability comes from the balance of buoyancy, weight, tendon tension, and restoring forces.
- Treating the platform as completely motionless. It can still move slightly, especially sideways, but the tension legs greatly reduce vertical movement.
Practice Questions
- 1 A platform has an upward buoyant force of 120 MN and a weight of 80 MN. What total downward tension must the tendons provide for vertical balance?
- 2 A tension-leg platform has 8 identical tendons sharing a total tension of 64 MN. What is the tension in each tendon?
- 3 Explain why tension-leg platforms are better than freely floating ships for supporting equipment that must stay aligned with a fixed point on the seafloor.