OR REPLACING A TETHER LINE. The present invention relates to a method and a device for maintaining or replacing a tether line anchoring a buoyancy unit to an underwater or seabed anchoring installation as described in the preamble of claims 1 and 6, respectively.

In this respect, reference is made to Japanese patent publication JP-57167887 and US-patent 6.457.908. The upper termination part of a tether line is normally connected to the buoyancy unit by means of an upper interface component, while the lower tether termination part is connected to a lower interface component connected to a sea bed installation such as a sea bed anchor. The two interphase components, or also named coupling units, include well known hinge couplings performing a universal joint function when the buoyancy unit moves in the sea volume.

The area of application for the present invention is illustrated in the enclosed figure 1 . A buoyancy unit 1 1 , or pontoon unit, also named a mid water arch (MWA) is kept in position in the body of water 1 00 at a distance below the sea level/surface 24 by means of two tether lines 22a,22b. The upper part of each tether line is connected to the bottom of the pontoon 1 1 , while the lower tether part is connected to an anchor structure 1 9A, 1 9B fixed to the sea bed surface 23. To save weight the lines in question is preferably made of synthetic fibers instead of steel wire rope.

In the example shown in figure 1 , the buoyancy unit 1 1 is used to support or suspend a number of lines 120 that for example runs partly floating on their own, in smooth patterns from a sea bed installation in a hydrocarbon production plant (not shown) upwards through the body of water to a processing plant platform (not shown).

The object of the invention is to improve the possibility to maintain and replace tether arrangements connecting buoyancy units to a sea bed installation.

An object of the invention is to improve said maintenance and replacing possibility without, or to a very small extent, adding substantially weight load to the buoyancy unit. The method is characterized in that a pull down line is connected to the buoyancy unit and the line is guided through a block fixed to the lower anchoring installation, and then the buoyancy unit is pulled down to slacken the tether line, and the sufficient maintenance or replacement is performed. The preferred embodiments appear in dependent claims 2-7.

The device is characterized by the features given in independent claim 8, and dependent claims 9-13.

The invention is now explained more in detail by reference to the following description and the enclosed drawing figures. Figure 1 shows the area of use for the present invention.

Figure 2 shows the tether system including the present invention .

Figure 3 shows a closer view of the present applied to a tether system.

Figure 4 shows an enlarged view of the upper coupling structure 17 connecting the tether 22,22',22" to the pontoon 1 1 . Figure 5a and 5 b shows the lower coupling structure (an anchor interface

component 14) connecting the lower end of the tether to the sea bed anchor structure 1 9.

As shown in figure 2, the mid water arch (MWA) or pontoon 1 1 or similar buoyancy unit, is normally kept in a position between the seabed 23 and the surface 24 by a tether arrangement 22 connected to the buoyancy unit 1 1 by means of the upper components 1 7 and through the lower anchor coupling unit 14 to the anchor 19 or similar unit on the sea bed, both including trunnion units,. This is the typical and well- known tether arrangement. The tether material could be steel wire rope, chain or synthetic fibre. In a replacement operation, synthetic fibre has an advantage because of the significantly lower weight. As shown the buoyancy unit is floating mid way in the sea water volume between the sea surface 24 and the sea bed 23 by means of said tether lines 22.

When replacement of tether 22 is required, a ROV I submerged from the installation vessel 10 will bring down the hooks of the pull-down bridle 22',22" (or pull-down line if single line) and connect these to the pre-arranged shackles 1 8 on the MWA 1 1 interface components 17a,b. Further, the ROV will bring down the block 21 (or a pulley structure) and connect this to the anchor interface component 14 through the shackle 1 5 which is fixed to the interface component 14. The anchor interface 14 is fixed to the sea bed. A line 1 2 is conducted from the winch or crane 20 down to the anchor interface, through the block or pulley 21 and upwardly to pre-arranged shackles 1 8 on the MWA interface components 17. As shown the line 12 is split in two branches 22',22" forming a Y-shape (on figure 2), each branch is connected to its respective shackle 1 8a, 18b connected to each side of the MWA via the connecting elements 17a, 17b.

The winch or crane 20 on the installation ship will now tighten and pull in the line 12 forcing or pulling the MWA 1 1 further down and deeper into the body of water 100 and by this taking over the tether load. The tether line arrangement 22 is now relieved and slack and may be replaced, or safely maintained, by the use of said ROV 100 (see figure 2). Both the upper and lower interface components,

respectively 17a, 17b and 14, are prepared for ROV operation of the tether connection/disconnection, which means both connecting the block or pulley structure 21 to the shackle 15 on the lower coupling unit including a padeye 16 for connection to the anchor structure 19 or other dedicated structure on the sea bed 23.

The drawing figure 3 show the main structure more in detail.

Figure 4 shows the detailed structure of the upper Y-shaped structure of the tether system 22, and how the pull down line 12 is branched in two branch lines 1 2a, 12b, a bridle line shape. Figure 5a and 5b show in two sections, the lower connecting element 14 fastened to the anchor structure.

Normally, a MWA 1 1 has a limited number of padeyes and load attachment points. An efficient MWA will typically have a minimum of structure to achieve as much net buoyancy as possible. By adding an extra attachment point to the interface component, no additional structure is required for the MWA and only a minimum of additional mass and weight is added to the upper and lower interface components.

In a corresponding way, placement of attachment points on the anchor is critical for the behavior and the balance of the anchor. Use of the main tether connection point on the anchor also for the pull-down operation, will ensure a correct load distribution. An additional pad eye on the anchor may require an increase of the anchor size and additional cost.

In general there is a request to have a as light weight pontoon structure MWA 1 1 as possible, since buoyancy is normally at a high cost. The more connecting points the heavier the buoyancy unit 1 1 will is. Therefore the connecting points for the pull down arrangement of the present invention, are located at the upper 17 and lower 14 trunnion connecting elements.

Legend drawing details.

10 - Installation vessel

1 1 - Mid Water Arch (MWA)

12 - Winch line for pull-down

12a,12b - Bridle-line for pull-down

14 - ROV operable interface component for tether termination to the anchor

15 - Shackle for connection of wire block

16 - Padeye for connection to anchor

17 - ROV operable interface component for tether termination to the MWA

18 - Shackle for connection of pull-down bridle

19 - Seabed anchor

20 - Winch for pull-down line

21 - Wire block, connectable to shackle by ROV

22 - Main tether arrangement