A spar platform is a type of floating offshore structure used to extract oil and gas in deep water, and increasingly to support wind turbines. It gets its name from its shape: a single large vertical cylinder that floats upright in the ocean, anchored to the seafloor by mooring lines. The cylinder sits so deep in the water that wave action near the surface has little effect on it, making it one of the most stable floating platform designs in the offshore industry.
How a Spar Platform Stays Stable
The key to a spar’s stability is simple physics. The bottom of the cylinder is loaded with heavy ballast, typically concrete, which pulls the center of gravity far below the center of buoyancy. This is the same principle that keeps a fishing bobber upright: weight at the bottom, flotation at the top. Because the center of buoyancy sits above the center of gravity, the platform naturally resists tipping. Even in rough seas, it tends to right itself rather than roll over.
This low center of gravity gives spar platforms an advantage over other floating designs like tension leg platforms (TLPs), which rely heavily on their mooring lines to stay upright. A truss spar, one of the more advanced designs, is so inherently stable that it won’t list or capsize even if completely disconnected from its mooring system. The mooring lines are there to keep the platform in position over the wellhead, not to prevent it from toppling.
The depth of the hull also helps. Because most of the cylinder is submerged well below the wave zone, the platform experiences much less wave-driven motion than a vessel sitting on the surface. The structure’s own weight acts as a counterbalance to surface disturbances.
Helical Strakes and Vortex Suppression
If you look at a spar platform, you’ll notice fin-like ridges spiraling around the outside of the hull. These are called helical strakes, and they solve a specific engineering problem. When ocean currents flow past a large cylinder, they create alternating vortices on either side, similar to the way a flag flutters in wind. These vortices can push the platform back and forth in a rhythmic motion called vortex-induced motion (VIM), which stresses the mooring system and the risers that carry oil and gas up from the seafloor.
The helical strakes break up this organized flow pattern. By disrupting the vortices before they can synchronize, the strakes significantly reduce the swaying forces on the hull. Engineers can fine-tune strake placement and geometry for different current conditions, making them one of the most effective passive solutions for keeping the platform steady.
Three Main Spar Designs
Spar platforms come in three primary configurations, each suited to different conditions and budgets.
- Classic spar: A single, uninterrupted cylindrical hull. This is the simplest and oldest design. The entire length of the cylinder provides buoyancy, and ballast tanks at the bottom are filled with heavy material to lower the center of gravity.
- Truss spar: The upper portion is a cylindrical hull that provides buoyancy, but the lower section is an open lattice framework (truss) with a heavy keel at the bottom. This design uses less steel than a full cylinder, reducing cost and weight while maintaining the same deep-draft stability. The truss section allows water to pass through rather than pushing against a solid surface, which further reduces current loads.
- Cell spar: Built from multiple smaller cylinders bundled together rather than one large one. This modular approach can simplify fabrication because smaller cylinders are easier to manufacture and transport.
Spar platforms are generally more economical to build for small and medium-sized developments than tension leg platforms, partly because their stability comes from basic geometry and ballast rather than complex tensioned mooring systems.
How a Spar Gets Installed
One of the more dramatic phases of a spar platform’s life is getting it into position. The hull is typically built onshore or in a sheltered shipyard, then towed horizontally to the installation site, lying on its side like a log floating in a river. Once it arrives, engineers begin a process called upending to rotate the hull from horizontal to vertical.
Upending works by selectively flooding ballast tanks near the bottom (keel end) of the hull. As water fills these tanks, the keel end grows heavier and begins to sink. This shifts the balance of buoyancy forces, and the hull slowly rotates until it’s standing upright in the water. The process subjects the structure to forces very different from anything it will experience during normal operations, so it requires careful planning and monitoring. Once vertical, the mooring lines are connected to anchors on the seafloor, and the topsides (the deck, drilling equipment, and living quarters) are lifted into place by crane vessels.
Spar Platforms for Offshore Wind
The same stability that makes spars effective for oil and gas production has attracted interest from the offshore wind industry. In waters deeper than about 60 meters, fixed-bottom foundations become impractical, and floating platforms are the only viable option. Spar-type foundations are considered one of the most reliable concepts for floating wind turbines because their deep draft and low center of gravity keep the turbine steady in open ocean conditions.
The Hywind project off the coast of Scotland, operated by Equinor, was the world’s first commercial floating wind farm and uses spar-type foundations. However, adapting the design for wind turbines introduces a unique challenge. A wind turbine’s rotor and nacelle sit at the very top of a tall tower, creating a long lever arm. When the platform tilts even slightly, the nacelle at the top swings through a much larger arc than the base, generating high accelerations and large bending forces at the point where the tower meets the platform.
Engineers are working on modifications to address this. One approach adds a vertical channel, called a moonpool, through the center of the spar hull. The column of water trapped inside the moonpool adds mass and inertia to the system, reducing both the horizontal translation and rotational motion of the platform. Testing of this concept has shown meaningful reductions in nacelle acceleration and tower-base stress compared to the original Hywind design.
How Spars Compare to Other Floating Platforms
In deep water, operators generally choose among three floating platform types: spars, tension leg platforms, and semi-submersibles. Each has trade-offs.
Tension leg platforms use vertical tendons pulled taut by the platform’s excess buoyancy, which makes them extremely rigid in the vertical direction. They handle heave (up-and-down motion) very well, but they depend entirely on those tendons for stability. If the tendons fail, the platform loses its ability to stay upright. Spars, by contrast, are self-stable regardless of their mooring.
Semi-submersibles use a wide footprint of pontoons and columns to achieve stability through waterplane area, similar to how a wide raft is harder to tip than a narrow one. They’re versatile and can be towed to different locations relatively easily, but they tend to experience more wave-driven motion than spars because they sit closer to the surface.
Spars excel in very deep water because mooring line costs scale more favorably with depth than TLP tendons do, and their motion characteristics improve as the hull extends deeper below the wave zone. Their main limitation is that the large hull must be fabricated and transported, which requires deepwater port access and specialized heavy-lift vessels for topsides installation.