FPSO turret features high tolerance bearings coupled with flexible piping

One solution for turret complexity

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When floating production, storage, and offloading vessel (FPSO) projects suffer delays and cost over-runs, complexity of the turret systems is often a key cause. Problems arise in terms of interface and integration with the tanker and also with regard to finalization of design data during system engineering.

In recognition of these problems, Tecnomare in Venice has developed two new concepts designed to improve reliability of the engineering construction and integration process:

  • A turret bearing system based on special rollers which can accommodate large coupling tolerances and relative deformations of load transmitting structures, while ensuring a passive (low friction) rotation and easy maintenance during the FPSO's operating life.
  • A flexible pipeline fluid transfer system which represents an improvement with respect to existing state-of-the-art systems in terms of weight, cost, and operational safety and maintainability.

Compliant rollers

As a base case, this system consists of a lower bearing unit, placed below water level close to the turret hull bottom, and an upper bearing unit placed at tanker deck level. Together, these units assume the horizontal loads and bending moments exchanged between the ship and the turret. The upper bearing unit takes up vertical loads, while the lower bearing unit, because of the overall system arrangement, takes up most of the horizontal loads coming from the mooring system.

Both bearings are made of two concentric races, or rail systems, placed on the turret and on the tanker, between which compliant rollers of a special design are interposed to transfer loads. Each of these rollers consists of three or more concentric and parallel high strength steel rings between which steel/rubber shear pads are mechanically engaged, providing an elastic shear constraint to the displacement of each ring with respect to the contiguous ones.

At the tanker side, the rails are in contact with the roller rings of even order, while at the turret side, the rails are in contact with roller rings of odd order. Loads are thereby transferred elastically in each roller through the elastic shearing of the rubber pads, which permit a maximum relative displacement of several centimeters for the rail system at ship and turret side respectively.

Accepting deformation

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FPSO with the Flag turret system.
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It is thus possible to compensate relatively large race deformations and take up large construction tolerances, currently inconceivable with state-of-the-art steel roller bearing systems.

Upper and lower bearing roller steel contact rings have crowned surfaces to allow small tilting angles and also to comply with radial and axial relative displacements of the turret with respect to the tanker rail systems (induced partly by moonpool deformation under operative loads). Rollers are linked at the two sides by spacer arrays to which they are connected elastically by means of rubber shear pads. These spacer arrays are made in bolted sections to facilitate removal and installation of a single roller unit while keeping the others in operation.

The lower bearing unit is placed underwater, close to the lower end of the turret and to the mooring chain stoppers. In a harsh environment such as the North Sea, it could consist of three horizontal parallel rails welded to the turret cylindrical shell in correspondence to a relevant box-shaped stiffener ring, while two concentric rails could be provided at the tanker moonpool side, in correspondence to a stiffener ring integrated with the moonpool structure.

Compliant rollers, consisting of five high-strength steel contact rings with four interposed rubber shear pads, can be placed with a vertical axis between the above rail systems - the upper, central and lower roller rings being in contact with the turret rails, while the other two are in contact with the moonpool rails. The rollers would be supported close to their contact points by two protruding horizontal ring plates welded at the turret and moonpool sides, on which low friction slide plates (incorporating charged nylon or PTFE) are applied.

In a milder operating environment such as West Africa, the lower bearing system's rollers could be limited to three high-strength steel contact rings with two interposed rubber shear pads.

An annular box-shaped ring structure welded inside the moonpool, below the lower bearing array, features an inner reinforced cylindrical wall plate. This leaves, with respect to a hard rubber fender array fixed to the turret outer reinforced wall, a nominal gap of around 30-40 cm. The arrangement would limit maximum displacement and loading of compliant rollers. When the overall (horizontal) load on the roller bearing exceeds a limit figure - equivalent, say, to a one-year storm - transfer of further load is achieved through the direct contact of this fendering system with the moonpool ring's inner shell.

Inspection

Lower bearing roller units can be inspected, removed, maintained and re-installed through the moonpool gap between tanker and turret, operating from two maintenance stations located 90° from the tanker bow-stern alignment. Maintenance tools could be controlled remotely in order to improve operability in the gap between the turret and moonpool walls. Fendering system units can also be removed and replaced by divers, who can access the tanker bottom through a turret central moonpool.

The upper bearing unit's function is to react to the vertical loads applied to the turret, the mooring and riser system line pull, and the dynamics of the ship. It must also react to the horizontal loads coming from the components of turret weight (because of ship pitch and roll) and to the moments applied to the same by the mooring and riser systems, by wind and by ship dynamics.

To minimize load transference, the bearing configuration is based on an array of inclined rollers moving with their axes on a conical surface, typically with a 60-70° cone angle. It consists of one rail unit, supported by an intermediate annular structure welded below the circular deck of the turret, and two rail units, concentric to the previous one, supported by a ring structure welded on the tanker deck, around the moonpool.

Both rail systems have flat rolling surfaces which are placed on conical surfaces having the same vertex point as the bearing rollers cone angle. Upper bearing rollers can be constructed with three high-strength steel contact rings between which two rubber shear pads are interposed, the central ring being in contact with the turret rail and the external rings in contact with the tanker rails.

Design of this bearing system and the number of rollers required depends on the vertical loads applied by the turret system. No major differences are assumed for the North Sea or West Africa scenarios, therefore a similar basic design may be considered.

Fendering

Two fendering systems, similar to the one integrated in the lower bearing unit, covering two sectors typically 60° wide and placed in the bow and stern directions, are provided to limit the turret horizontal displacement in survival conditions. They are supported by dedicated structure sections, mounted on ship deck, and acting on a turret-reinforced wall section. The upper bearing roller units can also be inspected, removed, maintained and re-installed at deck level operating from two maintenance stations located 90° from the tanker bow-stern alignment.

A turret rotation and braking system is provided to control the relative angle between the turret and the tanker. Because of the low friction involved in the bearing system, tanker weathervaning orientation may be assumed passively, driven by environmental loads. Such a system is not required to generate large torque to move the ship around the turrets. It has, nevertheless, the purpose of damping the high frequency (oscillatory) turret rotations, due mainly to wave action, which might overstress, unnecessarily, both the bearings and the fluid transfer system.

This system is based on two symmetrical and independent chain loops engaged in toothed seats situated at the periphery of the turret deck outer shell, with two high torque windlass toothed pulleys, directly coupled to large displacement hydraulic radial motors. The active drive or braking torque is thus transferred from the windlass units to the turret by means of drive chains, which allows a simple, reliable, easily inspected and maintained, redundant, and low cost construction.

Load transfer

To maintain chain loops at their correct tension, chain windlass units are placed on sliding bases loaded by hydraulic cylinders. The braking and drive system may therefore accept a small displacement of the turret, because of the tanker structure deformations, without the danger of shocks due to chain slacks.

The torque windlass units are driven together and are used both for moving the turret to a different accurate heading and for braking the turret oscillations in order to achieve a smooth orientation on the average ship heading. While active displacements require hydraulic energy to be provided by dedicated (redundant) power packs, the braking operating mode is a passive function, not requiring energy depending from on board equipment.

Turret fluid transfer

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Compliant rollers turret with the Flag fluid transfer system.
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Tecnomare's Flag fluid transfer system (FTS) concept employs flexible flowlines to transfer fluids between the turret and tanker. The concept is based on a bundle of flexible flowlines arranged in a flat, vertical array which is deployed horizontally from a fixed structure supported on the tanker deck (or support tower), to a mobile structure (main carriage) which can rotate coaxially with respect to the turret. On this structure, each line - supported and mechanically fastened on auxiliary structures - is bent 180° to form another array which is deployed to a structure connected to the turret (turret connection frame).

Each flexible pipeline is fastened mechanically and is supported on closely spaced saddles in the two upper and lower horizontal sections by means of two pairs of articulated lattice structures. The structures are hinged respectively to the support tower and main carriage in the upper arrangement and to the turret connection frame and main carriage in the lower one.

When the tanker is oriented on a heading corresponding to the prevailing wave, current and wind conditions, the flexible pipeline arrays and the relevant supporting structures are placed along a turret and tanker longitudinal symmetry plane. When the tanker is passively (due to changing sea/weather conditions) or actively (by turret drive system) moved out of this orientation, the main carriage, which is supported on the turret deck via a circular rail and a central rotary bearing, is moved in the same direction at an angle with half the amplitude. The articulated lattice structures are displaced accordingly.

In these conditions, the flexible pipelines in the two parallel arrays are forced to bend according to a geometrical configuration which is symmetrical with respect to a plane passing through the turret axis and pipeline connection points at the main carriage.

In each set of articulated support frames the flexible hoses are housed in a vertical array, lying individually on spaced horizontal bearing beams. They are free to move longitudinally and transversally, sliding on low friction supports. The geometry of the pipeline bent configuration is governed by pipeline elastic stiffness, but may be constrained, in order to limit the minimum assumed bending radius, by providing end stops to the transversal displacements of the lines with respect to the support frames.

Lower and upper terminations of the flexible flowlines are connected to steel piping issuing from the turret riser manifold, interfaced at a turret piping mating rack, and to a steel piping mating rack on the support tower.

Process equipment to be installed on the turret can be limited theoretically to the FTS/riser system interconnection piping with relevant shut-off and deviation valves. The FTS concept also allows the location of pig launchers on tanker process modules.

In heavy weather, when production may have to be shut down, the turret piping may be disconnected from the FTS' flexible lines at the relevant mating rack. The turret connection frame fastening to its rail may also be released together with the main carriage drive line loop fastening to the turret. In extreme conditions, the main carriage and the turret connection frame could be tied to the tanker deck, allowing a free, unconstrained, rotation of the tanker around the turret. The Flag FTS offers the following advantages with respect to multi-line swivel stacks:

  • No dynamic seals are required for high pressure and temperature loops
  • Maintenance of a single line or modifications to the piping arrangement can be undertaken while the other line is kept operating (so complete production shutdown is not mandatory)
  • Allows line pigging and therefore minimizes the amount of equipment to be located on the turret
  • Implemented or modified during the system's operating life with no need for removal to a shipyard for production system upgrading or adaptation to changed reservoir conditions.

Compared with drag chain systems, the concept presents the following main advantages:

  • Size, weight, and cost are lower. The full system weighs less than 200 tons, versus over 500 tons for a drag chain system with the same line specifications, with associated impact on turret sizes and bearing system loading and design.
  • Length and cost of the flexible lines is smaller
  • Minimum dynamic bending radius in the flexible pipelines is reached only when the limit tanker headings occur. Anticipated pipeline fatigue life is therefore considerably longer, as for most operating conditions (headings close to the average prevailing sea state), pipeline bending radii are relatively large.
  • System architecture allows easier access to pipelines for inspection, maintenance or replacement.

The Flag FTS concept may be strongly simplified for scenarios where relatively mild meteo-oceanographic conditions and strongly prevailing headings are encountered (offshore West Africa).

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