Deepwater Floating Production Systems An Overview Introduction In addition to the mono hull, three floating structure designs Tension leg Platform (TLP), Semisubmersible (Semi), and Truss Spar have been successfully developed for use as floating production systems (FPS) over the past several decades. Each of these floater designs has evolved along with subsea and riser technology to meet new field development challenges related to increased water depth, drilling, and reservoir operating pressures and temperatures. Design improvements continue to be made as new developments in deepwater technology are incorporated into the basic designs. The functional requirements for floating structures can be generalized as: Drilling facilities o Number, type and location of drilling rig Production facilities o Weight, area and center of gravity Drilling / production risers o Number and arrangement Well systems o Number of wells, completion and workover methods, minimum well spacing and well bay location Hull compartmentalization o Damage stability considerations Air gap requirements o New metocean criteria As illustrated in the following figure, the relative hull responses of the three designs vary considerably. In all cases the objective is to minimize the response to the environment. Natural Periods of Heave Responses Vertical Motions are Suppressed by Tendons TLPs Floaters Vertical Motions are Controlled Hull Configuration Design Wave Energy 5 10 15 20 25 30 Wave Periods (secs)
Efficient design of floating structures is predicated on functionality and performance it should be capable of supporting all the necessary equipment for production and related tasks while meeting all performance criteria. The structure should provide sufficient space and robustness to fulfill its intended purpose; also, the floating structure should be built at a minimum of cost, which is governed mainly by the hull steel weight. Hull weight estimates are based on global sizing which is determined by the naval architectural and structural design. Global sizing is a key engineering design process in both the concept selection stage and design phase of a floating structure. The sizing of a moored floating structure considers relationships among the payload, the size of the hull, and the mooring system. During the concept selection phase, efforts are concentrated on the main dimensions and weights under the consideration of design standards and performance requirements without a high degree of engineering detail. Finalized dimensions and main properties of the floating structures will be determined in the front-end engineering and design (FEED) stage through various analyses. 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. Truss Spars are often considered along with TLPs for dry tree solutions because they offer favorable vertical motions. However, Truss Spars are different from both Semis and TLPs with regards the mechanism of motion control. One of the distinctions of the Truss Spar is that its center of gravity is always lower than the center of buoyancy which guarantees a positive GM. This makes the Truss Spar unconditionally stable. The Truss 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 Truss 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 Truss 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 Truss Spar to be configured for full drilling, workover operations, production operations, 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 Truss 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. 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 Truss 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 Truss 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 Truss Spar s unmatched stability. The porches for the steel catenary export pipeline risers are on the perimeter of the keel tank.
The Truss 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. TLP Technology The main principal of the TLP is to assure that the vertical forces acting on the platform are in balance, i.e. fixed and variable platform loads plus tendon tension are equal to its displacement. The VCG should be close to the platforms geometrical center. Positive displacement is obtained by locking the platforms draft below the fixed and variable payload displacement draft. This will result in an upward force applied to the tendons, thereby keeping them in constant tension. As a consequence the vertical platform motions (heave) is almost eliminated, except for motions resulting from tendon elasticity and vertical motion as result of environmental introduced lateral platform motions. The tendons do allow a lateral motion of the platform as a result of wind, wave and current. This motion is similar to an inverted pendulum except for the fact that the displacement variation by pulling the hull down is giving a restoring force to the lateral movement. The tendon tension is set within predefined values, or window of operation. If the variable load of the platform exceeds these values by adding risers or drilling loads etc., the tendon pretension is adjusted by re-ballasting of the platform. Consequently the hull is compartmented into void, machinery and ballast spaces. The TLP has a control system monitoring ballast and VCG. Seawater is used for ballast adjustment.
As an evolution of the Classic TLP design, the ETLP (Extended Tension Leg Platform) has been developed. Some of the drivers behind this design development are: Wider tendon base for greater pitch stiffness (stability) Smaller spacing of deck supports for more efficient structure Lower rotational inertia for hull and deck for lower pitch natural period A large moonpool can accommodate conventional top tension risers. De-coupling of tendon porch separation distance from the topsides deck design produces maximum design flexibility. Columns are moved inboard Pontoons extended to tendon porch Conventional TLP ETLP ETLP = Extended Tension Leg Platform Principle Benefits of ETLP Technology: Significant steel weight savings in deck and hull steel Capable of carrying greater payloads into deeper water Number of tendons is reduced Comprised of safe, conventional and well-proven systems ETLPs have been successfully designed, fabricated, and installed at the Kizomba A, Kizomba B Located offshore Angola and Magnolia locations in the Gulf of Mexico. Typical TLP Heave Motion RAO s Heave Motion RAO (m/m) 0.018 0.016 0.014 0.012 0.01 0.008 0.006 0.004 Heave Motion RAO-225 deg Heading High-Tide, No Riser High Tide, Max Riser No Tide, No Riser No Tide, Max Riser 0.002 0 0.00 10.00 20.00 30.00 40.00 Period (sec)
Semisubmersible Technology Semi units offer a number of benefits, including large payload capacity, limited sensitivity to water depth, quayside integration and the ability to relocate after field abandonment. A typical Semi design has four columns connected at the bottom by pontoon with a nominally rectangular cross-section. A truss structure connects the column tops and supports topsides modules. This arrangement provides a high degree of flexibility in fabrication methodology. The Semi is designated as a column stabilized units (USCG, ABS, DnV, etc.). The columns are stability columns and primarily provide floatation stability. Important design variables are column dimensions and spacing, pontoon size and the ratio of pontoon width to pontoon height, draft of the hull, etc. In order to satisfy the stability and motion requirements, ranges for the variables and critical parameters such as GM value, free board value, heave natural period, etc are set as the constraints. Columns are sized to provide adequate water plane area to support all anticipated loading conditions, spaced to support topsides modules, and tuned for a natural period of at least 20 seconds. These columns are supported by two parallel pontoons or a ring pontoon. Pontoons are sized to provide adequate buoyancy to support all weights and vertical loads, and proportioned to maximize heave damping. Typical Semi RAO s Head Sea RAO's 2.5 RAO (ft/ft or deg/ft) 2.0 1.5 1.0 0.5 Surge Sway Heave Roll Pitch Yaw 0.0 3 6 9 12 15 18 21 24 27 30 Wave Period (Sec)