New semisubmersible design is a riser-friendly floater

The wet-tree floater market in the Gulf of Mexico (GoM) has been growing, largely due to reservoir properties that favor direct vertical wells, which leads to subsea production; and increased water depth, which makes steel catenary riser (SCR) design easier and top tension riser design more difficult.

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Qi Xu
Jim Ermon
Technip USA

The wet-tree floater market in the Gulf of Mexico (GoM) has been growing, largely due to reservoir properties that favor direct vertical wells, which leads to subsea production; and increased water depth, which makes steel catenary riser (SCR) design easier and top tension riser design more difficult.

In the GoM, wet-tree floater concepts include semisubmersibles, spars, TLPs, and ship-shaped vessels with disconnectable turrets. The semisubmersible has become a favorite choice as a wet-tree floating platform supporting SCRs, mainly because topsides can be integrated at quayside; the deck area is large; it can support a large number of risers; and it is cost-effective.

All semisubmersibles face the challenge of relatively high heave motion. Heave motion increases the loading on the SCR near the riser touchdown region, which is a particular concern for the large-diameter SCRs. As a result of increasing the draft of semisubmersibles, the heave motion is improved.

However, the deeper draft has presented another challenge: high VIM (vortex induced motion) as a result of currents. The high VIM of a deep-draft semisubmersible is due mainly to the increased VIM excitation from the longer columns and reduced damping from the smaller pontoons compared to conventional semis. An SCR's fatigue life at the touchdown point can be compromised by the high VIM. This phenomenon also can make it difficult to meet mooring fatigue design criteria.

Specifically developed as a riser-friendly floater suitable for both dry- and wet-tree applications, the spar is a proven concept with excellent motion performance. Earlier spars were used primarily as dry-tree units, while the recent spars are wet-tree applications.

In the coming years, spars will continue to lead the floating production market. However, in order to meet evolving and increasing market needs, Technip has recently added semisubmersibles and TLPs to its floater product portfolio. The company's semisubmersible is designed to meet the heave and VIM motion challenges that the conventional semisubmersible currently faces. As a result, the new semisubmersible also will be much more riser-friendly. Compared to an equivalent conventional semisubmersible with the same draft, this new design improves the heave motion by about 30% and VIM by about 50%, hence the name HVS: Heave and VIM Suppressed. While the motion performance is improved, HVS does not deviate significantly from a conventional semisubmersible in structural design and fabrication.

Features and performance

The HVS semisubmersible features higher pontoon blisters attached to the base of the columns, and pontoons that are narrower and taller than on a conventional semi. The higher blister is introduced to break the vortex shedding coherence along the column length. The blisters also provide good stability at quayside, and when the hull is ballasted down from towing draft to operating draft. The narrowed pontoon helps reduce heave loading.

Research compared the heave RAO between the HVS and conventional semisubmersible designs for the same payload. The HVS semi was found to reduce heave response by about one-third at the "hump." The draft is 41 m (134.5 ft) in both designs, and they have approximately the same heave natural period.

The pontoon height is raised in order to increase the VIM damping and to provide adequate structural strength. The reduced VIM excitation from the blisters and increased VIM damping from the high pontoons combine to reduce VIM response by 50% or more, as proven by the VIM model test results.

Benefits of HVS semis

As a result of the reduced heave motion, the riser bending stress at the touchdown point is reduced. Riser minimum tension at the touchdown point is increased, and the possibility of riser buckling in extreme and survival conditions is decreased or eliminated.

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The HVS semisubmersible features higher pontoon blisters attached to the base of the columns,
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and pontoons that are narrower and taller than a conventional semi.

The reduced VIM response increases the VIM induced fatigue life by almost an order of magnitude. The HVS semi design also has a lower VIM induced roll motion, which translates into reduced fatigue of the riser stress joint at hang-off.

The mooring chain fatigue due to VIM, which can be dominating for a VIM-prone design, is reduced even more.

In addition to the benefits of enhanced motion performance, the HVS semi introduces several fabrication economies and safety features.

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One of the models used in the VIM model test.

For instance, the increased quayside stability from the blisters makes the hull design less sensitive to topside weight increase, which tends to decouple the hull and topsides design, and enables a more robust project execution. And, the HVS topside to hull weight ratio of approximately 1:1 is still maintained, with little or no penalty for the design improvement – a key commercial advantage.

The HVS semi also is equipped with a caisson-based ballasting system, eliminating the need for pump rooms and sea chests inside the lower hull. Each quadrant has independent fill and discharge lines to eliminate possible cross flooding, resulting in a much safer system. The requirement for entry into the lower hull for maintenance and inspection of the ballasting system is minimized, improving the operational safety and cost.

Finally, the HVS semi can be designed with a deck box similar toThunder Horse and Atlantis, or an open truss type deck similar to Na Kika. A deck box has the benefits of additional stability (a vital measure of last resort in offshore column flooding mishaps), minimized equipment damage from wave run-up during storms, and added structural strength.

In conclusion, compared with conventional semisubmersibles, Technip's new HVS semi design is a riser-friendly floater with improved hull motions, riser fatigue life, and riser strength characteristics. And, equally important, the HVS is designed to be more cost-effective and safer. While the HVS semi incorporates a novel hull shape, it is still inherently a semisubmersible and, therefore, the barriers to market entry/first application are expected to be much lower than for a completely new design.

The authors

Qi Xu is chief technical advisor and Jim Ermon is vice president–HVS Semi, Offshore BU, at Technip USA.

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