SparTEC®

Spar Technology

The first Spars were based on the Classic design. This evolved into the Truss Spar by replacing the lower section of the caisson hull with a truss. Spars are often considered along with TLPs for dry tree solutions because they offer small vertical motion. However, Spars are different from both Semis and TLPs in the mechanism of motion control. One of the distinctions of the Spar is that its center of gravity is always lower than the center of buoyancy which guarantees a positive GM. This makes the Spar unconditionally stable. The Spar derives no stability from its mooring system, so it does not list or capsize even when completely disconnected from its mooring.

The deep draft is a favorable attribute for minimal heave motions. Its deep draft and large inertia filter wave frequency motions in all but the larger storms. The natural period in heave and pitch are above the range of wave energy periods. The long response periods for Spars mitigate the mooring and riser dynamic responses, which are common to ship shaped FPSOs and Semis. The deep draft, along with protected centerwell, significantly reduce the current and wave loading on the riser system These loads normally control the tension and fatigue requirements of the production risers on TLP or Semis

One of the principal advantages of the Spar over other floating platforms lies in its reduced heave and pitch motions. Low motions in these degrees of freedom permit the use of dry trees. Dry trees offer direct vertical access to the wells from the deck, which allows the Spar to be configured for full drilling, workover, production processing, or any combination of these activities.

Truss Spar Concept

The Truss Spar is divided into three distinct sections. The cylindrical upper section, called the “hard tank,” provides most of the in-place buoyancy for the Spar. The middle truss section supports the heave plates and provides separation between the keel tank and hard tank. The keel tank, also known as the “soft tank,” contains the fixed ballast and acts as a natural hang-off location for export pipelines and flowlines since the environmental influences from waves and currents and associated responses are less pronounced there than nearer the water line.

In the original designs, the Hard Tank architecture is the same in the Classic and Truss Spar designs  FloaTEC, LLC has recently developed a Ring Spar architecture for the Hard Tank. This architecture offers a more fabrication-friendly option with reduced  fabrication costs. Horizontal decks and radial bulkheads divide the hard tank spaces into tanks and voids. The lower set of tanks is configured to contain seawater ballast, which can be increased, decreased, or moved between tanks to correct for major changes in topsides weight, topsides eccentricity, SCR weight, or hull flooding resulting from damage.  A ring of additional voids is provided at the waterline area. These are used during topsides installation and prior to TTR installation.

The trussed mid-section of the hull is an X-braced space frame constructed of tubular members and flat plates called “heave plates.” The heave plates increase the added mass in the vertical direction and thereby increase the natural heave period of the Spar and bringing it above the range of periods in the wave energy.  In a Truss Spar, they also increase heave damping.

The third section of the hull is the “keel tank,” which is attached to the bottom of the truss at the keel.  It provides the buoyancy while the Spar is wet-towed horizontally to site for installation. The keel tank is flooded to initiate upending and, finally, receives the field-installed, fixed ballast, which is key to the Spar’s unmatched stability.  The porches for the steel catenary export pipeline risers are on the perimeter of the keel tank.
The Spar hull includes two access shafts. These shafts contain the ballast and utility piping and instrumentation. They also allow direct personnel access to the piping and to every void tank without requiring workers to pass through an intermediate compartment. Only one void need be open at a time. Access shafts are painted, lighted, and vented, as required, for entry.

The seawater ballast system has a dedicated centrifugal pump at the bottom of each access shaft for discharging ballast water. Ballast water is supplied to the tanks from the utility seawater manifold. Each seawater ballast discharge pump services the same two ballast tanks served by its access shaft. All ballasting is over the top of the hull, so that ballast tanks have to be intentionally filled by the ballast operator.  This eliminates the possibility of inadvertent flooding, which can occur if a sea-chest system is used.

FloaTEC, LLC’s Innovative Ring™ Spar Concept Improves on the Traditional Truss Spar
Truss spar hull construction methods typically involve fabrication of quadrant sections, often necessitating time consuming and complex construction techniques. In an effort to develop a more construction friendly design, FloaTEC, LLC spent a number of years working with its fabrication yard personnel to come up with a better design. This resulted in FloaTEC, LLC’s innovative Ring™ Spar concept - a robust and construction friendly truss spar design. The design provides for very efficient and cost effective construction. It is a safety conscious design with fewer pieces than the hard tank on a conventional truss spar, and its construction friendly concept enables innovative fabrication methods to be used. Details of this new innovative design can be downloaded by clicking on the links below.

Downloads

The Ring™ Spar Advantage Presentation

Ring™ Spar Delivery Model – Executive Summary

FloaTEC, LLC’s Spar Lineage

FloaTEC, LLC has a strong lineage in spar design and construction as illustrated below.

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