World's first taut-leg mooring using VLAs installed for Petrobras - vertical loaded anchors

World Oil, August, 1998

In April 1998, Vryhof Anchors installed 12 of its Stevmanta VLAs (vertical loaded anchors) to allow spread mooring of Petrobras' P-27 semisubmersible floating production unit (FPU) in Voador field, Campos basin, in 1,673 to 1,870-ft (510 to 570-m) water. The system was installed in 12 days.

The installation was the world's first VLA spread mooring, and the first-ever VLA-polyester mooring line system. Two anchor handling vessels (AHVs) were used to handle the system's installation and mooring lines; a third assisted as "back-up" to provide the required 350-mt test load after installation.

Petrobras' application. The short time involved in the anchor installation was important to the operator due to high-level activity in Campos and the resulting high-cost of installation vessels. Every opportunity to get the work done had to be utilized. The installation vessel, Maersk Battler, for example, had just returned from the P-31 installation and a rig move.

The VLAs were designed to be pre-installed to form the taut-leg mooring system for P-27. The spread has a diameter of 4,592 ft (1,400 m); its 12 lines are arranged in four groups of three anchors each, with 10 [degrees] between the center and outside anchors in each group. From the VLA, each line comprises 100 m of 102-mm dia. wire rope, 650 m of 136-mm dia. polyester line and 250 m of 79-ram 12Q3 20% chain.

Required ultimate pull-out capacity is 699 mt, a 1.5 safety factor on maximum damaged load condition. The soil condition at the site is a "very soft clay (mud)," into which the anchors penetrated a measured 69 to 81 ft (21 to 25 m). Installation and proof loads were 252 to 330 mt and 322 to 370 mt (at 33 [degrees] uplift angle), respectively. Each anchor has a surface area of 118 [ft.sup.2] (11 [m.sup.2]).

Installation procedure, the chosen installation method involved a second AHV, as Petrobras did not want to use the polyester mooring line as an installation line. Thus, for each anchor, the poly line handled by Maersk Chieftain had to be transferred to the installation vessel Maersk Battier, for the pulling test. The Far Sailor was used to give Battier extra bollard pull for the testing.

The 11-[m.sup.2] VLAs were configured for a two-line installation. An installation line (comprising wire rope and chain), and the mooring line (comprising polyester and wire rope) were both connected to the anchor's angle adjuster, see accompanying drawing. The design of the angle adjuster and the wire ropes connecting the adjuster to the fluke plate allows the angle of force applied to the plate to be changed, depending on which line is used for pulling. When force is applied to the installation line, the anchor is oriented in the optimal angle for penetration.

After installation load has been reached, the installation line is slacked off and the mooring line is tensioned. Due to the mooring line being tensioned, the fluke plate will rotate in the soil until the resulting force is normal to the plate. The anchor is now in the normal (vertical) loading mode.

A typical installation procedure went like this:

1. Mooring and installation lines were connected to the angle adjuster, and the VLA was lowered to the seabed by keeping tension on both installation and mooring lines, as deployed from the two different AHVs.

2. When the VLA was on the seabed, the mooring line was slacked off, the installation line was slowly tensioned, and the anchor started embedding. A typical drag length was 37 to 54 m.

3. At a predetermined load, the breaking device connecting the installation line to the angle adjuster broke. The installation line was then no longer connected to the VLA and could be recovered and used for the next installation.

4. The mooring line was tensioned to a predetermined load, i.e., the VLA was now in the normal (vertical) loading mode. This load, typically 350 mt, was prescribed by ABS (American Bureau of Shipping) and was equal to the maximum intact design load.

5. The mooring line was then buoyed off, ready for the arrival of P-27.

The first anchor was installed on April 4; 11 days later the full mooring spread had been installed and tested at up to 350 mt.

In conclusion, the installer says the VLA installation, using two lines, required about the same time as a conventional spread. The use of the shear-pin or a breaking device is a "practical and reliable method." And the P-27 job confirmed predictions for drag/penetration for a certain load.

Advantages of the VLA for taut-leg mooring systems has been demonstrated, Vryhof says. And the developer is convinced that applications will follow in the deep U.S. Gulf of Mexico, offshore West Africa, SE Asia and West of Shetlands. Further, they say, use of VLAs for mobile drilling units is also viable.