SHALLOW WATER MOORING SYSTEM USING SYNTHETIC MOORING LINES

Abstract

A shallow water mooring system applies to offshore industry for oil and gas exploration, and production. This mooring system employs non-steel synthetic lines to provide the desired flexibility for shallow water mooring system applications. Piled submerged towers at seabed serve as elevated anchors to keep the whole synthetic mooring lines off the seabed under environmental loading conditions. Other buoyant means such as a buoy is also utilized to further help the synthetic mooring lines off the seabed during their service life.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No. 60/595,682, “SHALLOW WATER MOORING SYSTEM USING SYNTHETIC MOORING LINES,” filed Jul. 21, 2005, and hereby incorporated by reference.

BACKGROUND OF INVENTION

1. Field of the Invention

The invention relates generally to a shallow water mooring system.

2. Background Art

Mooring systems utilizing steel wire and steel chains for offshore moored floating vessels have been used for many years. Most vessels are large ones and most of them in moderate or deepwater applications, deeper than 200 feet.

In deepwater mooring system applications, the gravity induced catenary shape of the mooring line provides the basic restoring force as the offsets occur when the vessel under environmental loading such as winds, waves and currents. At the end of the mooring line at seabed, an embedded anchor or a pile is utilized to fix the mooring line to seabed. The catenary shape of the mooring line makes the mooring line lateral stiffness much softer and more flexible comparing to a straight line axial stiffness of steel wires or steel chains.

In a typical chain-wire-chain configuration with an embedded anchor or an installed pile anchor at seabed, a portion of the chains is laid on the seabed in normal conditions. When a large vessel offset occurs, this portion of chains could be lifted off the seabed to add to catenary shape of the mooring line. However, under any design conditions, this catenary mooring line can not be allowed to be a straight line or near a straight line by increasing the line tension, because the line stiffness could be increased significantly under a straight line configuration.

FIG. 1 illustrates one conventional chain-wire-chain mooring line system for an offshore moored floating barge 10. The upper portion of the chains 11 connects to a chain stopper 12 at barge deck 13. The middle portion of mooring line is a steel wire 14. The bottom portion of chains 15 touches seabed 16 and connects to an embedded anchor pile 17.

In a shallow water application such as water depth less than 200 feet, the catenary shape of a conventional chain-wirechain mooring line is very limited and the system becomes ineffective. The line could be easily becomes a straight line as the vessel offset occurs and line tension increases. The sudden increase in line stiffness and associated the sudden increase in line tension could be very dangerous to the mooring system. Conventional method to improve the system is to use a long distance heavy chains on the seabed or to add clump weights to the chains on the seabed.

As the water depth gets shallower and shallower, the catenary mooring systems become less and less effective. The cost of the extended length of the chains laid at seabed and the required clump weights becomes very expensive.

Non-steel synthetic mooring lines, mostly referred to Polyester lines, have been accepted recently by offshore industry. Two deepwater Spar platforms, “Mad Dog” in 4,200 foot water depth and “Red Hawks” in 5,300 foot water depth, have employed Polyester lines as mooring lines and both were installed in Gulf of Mexico in 2004. The chain-Polyester-chain configuration mooring line is much lighter and much more flexible than steel chain-wire-chain configuration. In installed condition, the Polyester line provides its restoring force mostly from its axial stiffness, not from the gravity induced catenary shape like the conventional steel mooring line.

In the application of Polyester mooring lines, there is one restriction to be imposed: the Polyester portion of the line can not laid on the seabed to be exposed to mud or sands for an extended time. The mud or sands could get into between fibers to cause wearing damage to the Polyester lines. For deepwater applications, this restriction can be easily satisfied. The bottom section of the chain could be designed so that the portion of Polyester line will not touch the seabed at all time. In shallow water applications, this restriction is very difficult to be satisfied because of the shallow water depth.

SUMMARY OF INVENTION

In one embodiment, a shallow water mooring system for large offshore floating vessel applications is disclosed. The mooring system utilizes synthetic nonsteel lines and at least one elevation means selected from a submerged tower structure, or at least one buoyant object.

Other aspects and advantages of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a prior art chain-wire-chain mooring system.

FIG. 2 shows a mooring system according to one embodiment of the invention.

FIG. 3 is a side view of a mooring system according to one embodiment of the invention.

DETAILED DESCRIPTION

FIG. 2 illustrates the plan view of one embodiment of a mooring system. The barge 10 is moored by 12 mooring lines in three groups. A submerged tripod anchoring tower 18 is served as an elevated anchor at seabed connected to the bottom portion of chains 15.

FIG. 3 illustrates the side view of the preferred embodiments of the mooring system. The upper portion of the chains 11 connects to a chain stopper 12 at barge deck 13. The middle portion of mooring line is Polyester line 19 in two sections. Between the sections, a buoy 20 is attached with a tri-plate 21 to connect both Polyester sections 19 and the buoy 20. The bottom portion of chains 15 connects to the elevated anchoring tower 18. Three piles 22 are driven into the seabed through the three leg sleeves 23 of the tripod anchoring tower 18. Grouted connections are applied to the connections between the piles 22 and the sleeves 23.

The length of the Polyester line for each mooring line should depend on the required stiffness. The longer of the line, the more flexible of the Polyester line. The size of the Polyester line should depend on the required line tension strength based on dynamic mooring analysis. The height of elevated anchoring tower and the size of the buoy should be based on the mooring analysis results to keep the Polyester line off the seabed.

While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

Claims

1. A shallow water mooring system for a large offshore floating vessel application (>300′ in vessel length) in a water depth range between 20 feet and 200 feet utilizing synthetic nonsteel lines and at least one elevation means selected from (1) the use of a submerged tower structure fixed at seabed to serve as an elevated anchor which will allow the bottom end of a mooring line connection to be at least 5 feet above seabed, and (2) the use of at least one buoyant object disposed at at least one point along the length of the synthetic line.

2. The shallow water mooring system of claim 1 where said synthetic mooring line comprises a Polyester line.

3. The shallow water mooring system of claim 1 wherein said submerged tower structure is a piled structure fixed to the seabed.

4. The shallow water mooring system of claim 1 wherein said submerged tower structure at least partially protrudes above a surface of the water.

5. The shallow water mooring system of claim 1 wherein said submerged tower structure is a gravity-based structure fixed to the seabed.

6. The shallow water mooring system of claim 1 wherein said at least one buoyant object comprises at least one buoy disposed at at least one point along the length of the mooring line.

7. The shallow water mooring system of claim 1 wherein said at least one buoyant object comprises at least one buoyant foam disposed along the length of the mooring line.