If you have ever seen a Wind Turbine Installation Vessel (WTIV), you may have noticed something unusual: several long “legs” extending through the vessel. During offshore construction, the entire hull may even be raised above the sea.

Why does a ship designed to float need to “stand up”? The answer lies in the design of a jack-up vessel.

From a Floating Vessel to a Stable Platform
A jack-up vessel is equipped with several movable jack-up legs. During transit, the vessel floats normally with its legs raised. Once it reaches the offshore worksite and is correctly positioned, the legs are lowered toward the seabed.
At the bottom of some jack-up legs are enlarged footings known as spud cans. These increase the contact area between the legs and seabed, helping distribute the load and limiting excessive penetration into the soil.
Once sufficient seabed support is established, the vessel’s jacking system raises the hull upward along the legs until it is elevated above the sea surface.

The basic sequence can be summarized as:
Positioning → Lowering the legs → Establishing seabed support → Raising the hull → Offshore operations

Why Raise the Hull Above the Water?
A floating vessel naturally moves with the waves. These motions include heave (up and down), roll (side to side), and pitch (forward and backward).
For normal sailing, such movement is expected. For offshore wind construction, however, it can become a major challenge.
Installing a wind turbine requires large components such as towers, nacelles, and blades to be lifted and precisely positioned high above the sea. Excessive vessel motion can make these heavy-lift operations much more difficult.
By raising the hull clear of the water, the vessel transfers its working loads through the legs to the seabed. This greatly reduces wave-induced hull motions and effectively transforms the vessel into a relatively stable offshore construction platform.

Can a Jack-up Vessel Stand Anywhere?
Not quite. Jack-up operations depend heavily on water depth and seabed conditions. Engineers must understand the seabed soil, terrain, and potential hazards to determine whether the legs and spud cans can safely support the vessel and construction loads.
Before major lifting begins, operations may also include preloading, in which loads are applied to verify that the seabed can provide sufficient support before the hull is elevated to its working position.

So, the next time you see a WTIV standing high above the waves, remember: the vessel is not stranded—it has deliberately transformed itself from a floating ship into a temporary offshore construction platform.


References
 • Guide to an offshore wind farm | An informative resource for offshore wind (Image source)
 • Wind Turbine Installation Vessels (WTIV) | DNV
 • Numerical modeling and analysis of the dynamic motion response of an offshore wind turbine blade during installation by a jack-up crane vessel - ScienceDirect