The Toscana became the world’s first permanently moored floating storage and regasification unit earlier this year. Nick Palmer talks more about the unit and its external turret mooring system.
This August saw the final installation and hook up of the floating, storage and regasification unit (FSRU) Toscana.
The Toscana is a 288m-long converted LNG carrier moored using an external turret mooring system 19km (12mi) off Livorno, Italy, in 120m water depth. This is the world’s first permanently moored offshore FSRU. It has a storage capacity of 137,500 cu m of liquefied natural gas (LNG), with capability to supply 3.75 billion cu m per year natural gas to shore, representing about 4% of Italian demand.
The vessel is a steel monohull with four Moss type LNG tanks arranged in its center, the regasification plant at the forward end, and accommodation with central control room and utility machinery in the aft end.
LNG shuttle carriers will berth alongside Toscana so that direct side-by side loading of LNG to the FSRU can be performed. The FSRU then converts the LNG back to gas using the regasification plant. Gas is exported through the turret swivel stack down to the seabed via a flexible subsea riser system and then on shore via subsea pipeline.
Image: A schematic of LMC’s external turret system design for the FSRU Toscana. Image Credit: LMC.
The bow-mounted, column turret system, supplied by UK-based London Marine Consultants (LMC), is one of five systems designed by LMC. It is LMC’s ninth external turret mooring system in service.
This system is an evolution of designs LMC developed for the Farwah FPSO (2003), installed offshore Libya, and for Saipem’s Firenze floating production vessel (2011), installed on ENI’s Aquila field offshore eastern Italy. It is a passive mooring system, allowing the vessel to weathervane around the turret and align itself with the prevalent environmental conditions.
The FSRU and turret mooring system are designed to remain on station for 20 years. The mooring system uses six equally spaced mooring chains (four at 140mm, and two at 103mm diameter) and embedment anchors, in 112m water depth. It is designed for a mooring load of up to 1250-tonne.
The basic design of the column turret differs from traditional external cantilever designs most notably through the turret connection to the vessel, which is provided at two locations at the vessel bow: one at main deck level and one at the level of the bulbous bow.
Structural integration of the turret to the vessel at these locations is through upper and lower cantilever steel constructions, which are joined together by a steel outer column to create a rigid, “portal-frame” structure. LMC was the first designer to include such an outer column.
The part of the turret fixed to the vessel surrounds an inner column connected at its base to the chain table, to which the mooring lines attach via articulated chain stoppers.
The inner column is connected to the vessel-fixed part of the turret with two bearings. A 4m-diameter roller-type slewing bearing is used at the upper connection, similar in type to those employed in LMC’s external cantilever turret designs. At the level of the lower cantilever, a radial plane bearing is used, comprising polymeric bearing pads positioned on the outer column and a machined Inconel rubbing surface on the inner column.
Image: The external turret system after integration at Drydocks World, Dubai. Photo Credit: LMC.
The subsea system consists two 14in. gas export lines and one umbilical, positioned over a mid-water arch. The gas send out-line will go through the turret swivel down to the seabed and, from there, directly to the onshore users’ tie-in points via a single 32in. subsea line.Using two bearings allows for dissociation of the horizontal and vertical loads imparted by the mooring system, with the lower plane bearing carrying the majority of the horizontallyimparted loads, and the upper slewing bearing taking the vertical loads.
This separation of the load components allows for the bending moment on the main slewing bearing—a design driver in the case of many external cantilever turrets—to be hugely reduced, and allows for this critical item to be smaller in diameter and cross section than the equivalent bearing required in an external cantilever turret exposed to the same mooring loads.
The fabrication of a column turret requires a number of additional key elements over and above those of a cantilever turret. Most significantly, these include the control of very fine tolerances required in fabrication of the inner and outer columns (in terms of ovality and concentricity of the circular section component steel rings), and high-precision machining of the supporting surfaces for the slewing and lower plane bearings.
Image: Installation of the external turret system at Drydocks World. Photo Credit: LMC.
Given these fabrication requirements, selection of a column turret is not appropriate in all circumstances. For offshore floating units with a limited number of risers (typically fewer than eight) and exposed to onerous weather conditions, the column turret provides advantages over a cantilever turret, including a reduction in slewing bearing size and cost, and a reduction in overall turret steel weight.
A column turret design is often preferred where incident weather environments and associated mooring loads are high enough for an internal turret to also be considered.
Other advantages include space; no space is taken up within the vessel hull, allowing the space to be used for other items, such as storage or ballast tanks. The swivel stack is maintained at the forward end of the vessel, away from accommodation blocks and other personnel areas, an important consideration, especially on gas production vessels, or with products containing high hydrogen sulfide (H2S).
In addition, a vessel being converted will require less time in dry dock, compared to an FPSO with an internal turret. The external column turret can be fabricated separately to the vessel conversion, and integrated at the end of the conversion work. The turret can, if desired, also be built by a different company to the FPSO conversion contractor.
For the FSRU Toscana, it was particularly impracticable to install an internal turret because the LNG tanks on the existing LNG carrier occupy most of the hull. An external column turret mooring system was selected over an external cantilever due to high environmental loads, including predicted 100-year return storm conditions of significant wave height (greater than 8m), coupled with the very fine lines and light scantlings (steelwork) at the main deck areas in the bow of the LNG carrier. It would have been difficult to stiffen this area to carry the loads from an external cantilever turret.
Image: The FSRU Toscana moored offshore Livorno, Italy. Photo Credit: LMC.
Nick Palmer is a director at London Marine Consultants (LMC). In addition to managing a large number of turret projects, Nick also leads the naval architecture department at LMC. He has experience in mooring and riser design, as well as experience in offshore installation and hookup of turret mooring and riser systems. He is graduate in naval architecture from the University of Southampton.
OLT Offshore LNG Toscana is operator of the FSRU Toscana. Its owners are E.ON Ruhrgas (46.79%) and IREN Mercato (41.71%). The conversion of the LNG tanker Golar Frost into the FSRU Toscana was undertaken at Drydocks World, Dubai, under contract to Saipem, which also carried out installation and commissioning.
As well as installing and integrating the forward external turret, Drydocks World’s work included structural fabrication and installing an aft thruster compartment, crane foundations, bilge keel, lay down area and equipment foundations.
New cryogenic piping work and insulation, were installed, along with loading arms, regasification module, nitrogen module and cargo pumps. The vessel’s rudder, steering system and bow thruster was removed and an azimuth thruster installed, along with a wave monitoring and berthing system, a new fire and gas alarm system and public address and communication system in new compartments. The power generation system was also upgraded with the installation of two turbo generators.
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