The present invention regards arrangements for mooring, loading and unloading of a vessel. The vessel is for example a FPSO-vessel (“Floating Production Storage and Offloading”), but can be a tanker or another type of vessel. The load is preferably comprising hydrocarbons in form of oil or gas, but can be any fluid or for example powder that can be transported through pipes. The invention is in particular relevant for mooring, loading and unloading of a vessel of the FPSO-type at shallow waters.
For mooring, loading and unloading of a vessel at shallow waters, for example a vessel of the FPSO-type, it has proved difficult to arrange anchor chains with sufficient slack such that a suitable resilience is achieved in the mooring system. At too low resilience in the mooring system, sudden vessel movements may result in too large forces. Another crucial factor regarding mooring is to achieve a position restoring force in the mooring, such that the vessel as moored can lay in a stable position during loading and unloading without occurrence of too strong forces.
Regarding conventional single anchoring systems with a floating buoy connected with a chain or a connection arm to a foundation structure fastened to the seabed, it has proved that the position restoring forces are insufficient.
Completely submerged systems for mooring and transfer of load and signals are possible, but significant problems appear in that critical components are submerged in the sea. This shortens the lifetime significantly and results in severe problems with respect to repair and maintenance.
In view of the above, mooring, loading and unloading at shallow waters are preferably undertaken by use of a mooring tower that extends from the seabed and up to above sea level, with a sufficiently resilient anchoring of the vessel to the top of the mooring tower.
Patent publication U.S. Pat. No. 4,516,942 contains description of a mooring tower that extends from the anchoring at the seabed to over the sea level, where the tower is provided with a triaxial rotatable yoke in the top in the mooring tower, for mooring and transfer of fluid and signals. The yoke's fastening in the top of the mooring tower results in that the mooring tower is subject to a very large momentum from the mooring forces, and the dimensioning of the mooring tower is therefore very powerful. Also the vessel that is to be moored to the mooring tower according to U.S. Pat. No. 4,516,942 must have a powerful dimensioning in order to handle the forces involved, in particular since the main structure of the vessel has to take up forces from the yoke from above deck level. The volume of the mooring tower and the yoke above the water line and down to the deepest draught for vessels in the waters is very significant, resulting in risk for collision and heavy impact from ice, current, waves and wind. As apparent from the above a number of disadvantages are associated with the mooring tower according to U.S. Pat. No. 4,516,942.
The objective of the present invention is to provide an arrangement for mooring without the disadvantages associated with the mooring tower according to patent U.S. Pat. No. 4,516,942 and other relevant prior art, as referred to above.
The objective of the present invention is met in that a mooring arrangement is provided, having design and distinguishing features as apparent from claim 1.
With the arrangement for mooring according to the present invention the mooring forces providing momentum on the mooring tower are significantly reduced, with resulting consequences for the dimensioning of the mooring tower and the anchoring to the seabed. The momentum of the mooring forces on the mooring tower can be zero at particularly preferred embodiments involving that the resultant of the mooring forces is in line through the anchoring in the seabed. With the arrangement for mooring according to the present invention also the risk for collision with the vessel that is to be moored is reduced, in particular by placing the yoke lower than the maximum draught of the vessel. The reduced dimensions of the mooring tower are reducing the impact from ice, current, waves and wind.
In the following a particular embodiment is further described with reference to the drawings. In this connection only mooring, loading and unloading with a typical FPSO-vessel is considered.
The arrangement according to the invention is in general most useful for water depths from 20 m to 50 m, but the limits are not fixed. The arrangement is completely making use of known components that all are well proven and have exhibited long service life without need for maintenance. Yet the arrangement and all critical components are designed such that inspection is simple and replacement of critical components is possible. The design life will be chosen to a minimum of 15 years for the illustrated embodiment.
The position restoring effect of the mooring arrangement is increased by ballast provided between the outer arms of the yoke. In the illustrated embodiment the ballast is a 25 m long cylinder of diameter 3 m, divided into three compartments. After installation the central compartment of the ballast cylinder is water filled while the two side compartments are filled with heavy slurry to provide suitable pretensioning of the moorings.
Between the top of the outer tower and top of the inner tower a swivel connection is provided to achieve rotatable transfer of load and signals. The swivel deck on
The number of hoses and cables is in principle without any particular limit. When the expression hoses here is used it is considered to mean both stiff pipes and flexible pipes, except from the distance between the top of the outer tower and the vessel where hoses or pipes have to be flexible because they are hanging as catenary lines above the water level.
A typical tower will for example comprise two 12 inch hoses or pipes for loading of crude oil, a 12 inch flexible pipe for water injection, and an additional reserve pipe of 12 inch. The number, the types and dimension of pipes, hoses or cables can of course be varied according to the demand. As illustrated on
The outer tower with all connected equipment and the yoke can weathervane freely with the vessel around the stationary inner tower fixedly anchored to the seabed, under all weather conditions.
The arrangement according to the present invention further comprises equipment over the sea-line or placed dry within the tower, which equipment is easily accessible for inspection and maintenance, and said equipment is preferably of known type for rotatable communication of fluid and signals, and mechanical transfer. The tower structure provides dry placement of said equipment and supports the connections for load transfer and signal transfer, which are hanging as catenary lines about 20 m over the sea level.
With the arrangement according to the present invention the anchoring forces are transferred via the yoke to the seabed via the rotatable fastening, which in the illustrated embodiment is a rotatable disc. The rotatable fastening of the yoke and the outer tower is the only critical component that is not placed dry in the arrangement according to the present invention.
Some further details regarding some of the components are as follows:
The inner tower with mounted rotatable disc over a fast anchoring to the seabed represents the stationary part of the arrangement according to the invention and supports all the equipment above. The inner tower takes up the forces acting on the outer tower because of ice, wind, current, the hanged-up hoses, etc., which forces are transferred from the outer tower via bearings along the length between the towers. The bearings can be designed with sliding surfaces manufactured from Inconel cladded steel. The diameter on the lower part of the inner tower below the rotatable disc is according to the illustrated embodiment 3.7 m, while the diameter of the upper part of the inner tower is 3.1 m.
On the outside of the outer tower, at the water-line level, an ice-breaking device is installed.
Disconnection of the whole mooring arrangement is possible within 48 hours, for example at very severe ice conditions. The arrangement is, however, designed to withstand the worst 100 years conditions of ice.
A footbridge can be installed between the vessel and the tower structure, for use at moderate or better weather conditions.
The rotatable disc has the outer tower mounted with bolts. The rotatable disc has a protrusion to which the yoke is fastened The rotatable disc is preferably of a well known type according to the so called “STL/STP-system”. The bearing means comprises a main radial bearing encompassing the inner tower and an upper axial bearing and a lower axial bearing. Preferably a self lubricating bearing system of the type Oiles 500 is used, with bronze alloy bearings with PTFE-lubrication, chosen for long service life. The bearings are fixed to the rotatable disc. The upper bearing ring supporting the upper axial bearing is locked to the stationary inner tower by use of a segment locking system fastened in a circular groove on the centre shaft. This is a well proven solution used with good results for swivels subsea, according to the SAL-systems.
The fluid transfer system is also illustrated on
Installation and deinstallation is relatively simple because of the suction anchors and large compartments that can be filled with air or ballasted in a controlled way, and by installing the yoke after installation of the tower structure. Then pipes and hoses are installed. It is estimated that 7-8 m water depth is required in order to tow a tower structure as the one illustrated, when it is floating by use of suitable air filling. Preferably a crane vessel, divers and remotely operated vehicles are used in connection with installation and deinstallation.
Number | Date | Country | Kind |
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2002-0265 | Jan 2002 | NO | national |
Filing Document | Filing Date | Country | Kind |
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PCT/NO03/00007 | 1/13/2003 | WO |