The spar continues to be a popular drilling and production option for deep water. The spar’s primary attraction is its low motions, which are effected by its deep draft. The motions allow dry-tree completions and result in less damage to the top tensioned and steel catenary risers as well as the mooring system.

Structural fatigue is a main consideration in the design of offshore floating structures. The main sources of fatigue are the floater’s motions in response to the environment and vortex-induced vibrations (VIV) caused by the current acting directly on the structural components.One constituent of the motion-induced damage is the vortex induced motion caused by the current flowing past the structure.

When a hull is exposed to a current, it sheds vortices that cause the hull to oscillate. The amplitude of the oscillation depends on the speed of the current and the size of the structural component (i.e., the hard tank diameter of a spar).

A current flowing past the spar hull induces an oscillating motion because of vortex shedding. Attaching strakes to the hard tank reduces the amplitude of the motion, which reduces the amount of damage sustained by the risers and moorings. (Images courtesy of FloaTEC LLC)

Contending with VIV
Physical structures have natural periods that can be excited by dynamic forces that have harmonic contentclose to these periods. Floating structures in the ocean have natural periods that can be excited by the period at which vortices separate or are “shed.”

The relationship between the current velocity and the natural period of the hull is a function of the hull diameter, natural period of a particular component, and current velocity. Using an empirical coefficient to relate these parameters, the particular current velocity at which the structure will become excited or “lock-on” to the natural period can be predicted. This relationship is referred to as the reduced velocity. Currents in the ocean typically have a range of velocities and therefore cover a range of reduced velocities.

The structure is assumed to oscillate or move back and forth at an amplitude, “a.” This amplitude generally is proportional to some characteristic dimension, typically the diameter of the structure. This motion causes stresses in the structures. The level of stress is related to the size of the amplitude, i.e., large amplitude motions cause large stress ranges.

Continued application of these cyclic stresses induces fatigue damage in the structural components, and although these amplitudes cannot be practically eliminated, reducing the amplitude of the oscillation reduces the stress and thereby reduces the damage.

The conventional means of mitigating the oscillation amplitude is to attach strakes to the spar’s hard tank. Adding strakes affects the amplitude of the vortex-induced motion (VIM) on the spar. A full strake has the maximum affect on reducing VIM amplitude.

Adding strakes affects the amplitude of the VIM on the spar. A full strake has the maximum affect on reducing VIM amplitude.

In general, experimental results have shown that the optimum height of the strake above the wall of the hard tank is approximately 15% of the hard tank diameter. For example, a spar with a 100-ft (30.5-m) diameter hard tank would have a strake height of 15 ft (4.6 m).

Construction considerations
Because of some construction and delivery limitations for spars (such as dockside water depth), it sometimes is necessary to attach clipped strake sections that are less than 15% of the diameter. These abbreviated strakes offer a cer- tain amount mitigation of motion amplitude, but they are not as effective as strakes that are 15% of the diameter of the hard tank. In some cases, the orientation of the installed spar can be optimized to take advantage of the smaller strake height.

Spars generally are constructed in sections on skid rails leading to a waterfront bulkhead. Once the spar is completed, it is offloaded horizontally from the end of the skid rails onto a large transport vessel, which can move it to another location. Alternatively, the spar can be offloaded into the water at the construction site. The skidding weight of the hull typically is in the order of tens of thousands of tons. Because of the large hull weight, it is advantageous to leave the strake off of the underside of the hard tank during skidding operations. Once the hull is in the water, it can be rolled over so the missing strake sections can be attached while the hull is floating dockside.

The strakes also can be installed on land before the skidding operation. If this option is chosen, however, additional support cribbing is needed to elevate the hull above the strake to avoid strake damage during load out. This option is a more practical operation when the hull does not have to be transported in open sea conditions. Transporting a hull in support cribbing over a great distance becomes complex because of the in-sea fastening needed to meet open sea stability requirements. From a safety standpoint, skidding the hull and offloading it into the water dockside is a simpler operation.

The folded strake design provides the required underside clearance gap for horizontal towing.

Often, dockside water depth presents a construction limitation for spars. When strakes are attached to a large-diameter spar hull, dockside water depth generally is notadequate to allow sufficient clearance under the strake for horizontal towing.

One way this can be overcome is to dredge the area next to the dock or create a deep hole near the dock to provide adequate water depth to float the hull off a load-out vessel. Although this approach allows the hull to safely enter the water, it does not accommodate moving the spar away from the dock. Generally, that amount of dredging through a waterway is cost-prohibitive.

Leaving the strake off or clipping the strake for transport are options that have been used in the past, but both choices can compromise the strake’s effectiveness. If the hull is deployed without the full optimum strakes attached, its motions are compromised.

Strakes, VIV
Clearly, the greatest benefit from strakes is derived when they are installed the entire way around the hull at the optimum size. An option to achieve this is being investigated by FloaTEC. The concept is to modify the strake by attaching a hinge to the tip, allowing it to be folded during load-out. The new strake is designed to fold back to provide the required underside clearance gap for horizontal towing. Once the hull is uprighted at the installation site, the folded sections can be opened and secured so the resulting strake is the optimum size.

At present, the folded strake is in the concept stage. The idea is being furthered developed through engineering design and analysis. The objective is to provide strakes that achieve their optimum performance in mitigating VIM on the hull without adding complexity to the load-out operation. If FloaTEC is successful, the end result will be hulls with a greater likelihood of achieving a longer life in the field for risers and moorings.

Offshore