Polyester rope lightens loads on Marlim FPSO anchor lines

Weight-load-excursion balance sought

Olivier Jeannin
Single Buoy Moorings
Paulo Roberto Buraque Carneiro
Petrobras

Late in 1996, Petrobras and SBM signed a four-year charter contract for the FPSO II (formerly in service on the West Linapacan Field, The Philippines) to be re-deployed at the Marlim Sul Field. Ten months later in Augsut 1997, first oil was produced from Marlim Sul, where water depths are 1,400-1,700 meters.

The anchoring system for the FPSO II comprises six anchor legs with varying segment characteristics: stud link chain, wire rope, and polyester rope. Several constraints were imposed for the design of the anchoring system:

The excursion of the FPSO should not exceed 8% of the water depth under normal conditions, and should in no case exceed 15% of the water depth, even when one of the six legs is broken. This stipulation was governed by the free-hanging configuration selected for the risers and umbilicals.
The weight suspended to the FPSO's mooring buoy should remain below 600 tons. This was required to insure a minimum freeboard of the buoy, taking into account the suspended weight of the risers and umbilicals for up to two wells.
The system must be sized to resist loads incurred during a storm with a 20-year return period.

Of these constraints, the first dictated a much tauter anchoring system than is normal with catenary moorings. The second dictated a lighter anchor leg than had been employed hitherto for permanent moorings (i.e. chain or steel wire rope), leading to the selection of polyester, which had been tested extensively by Petrobras and by the manufacturers them selves. However, a purely taut leg system incorp orating polyester rope only would have demanded anchors capable of resisting vertical loads.

In this water depth, anchor piles or suction anchor piles were not considered feasible due to anticipated installation problems. Vertical loaded anchors (VLAs) could have been a solution. Experiments on prototypes in the Gulf of Mexico had been witnessed by Petrobras in 1996, which had brought promising results.

However, the Marlim Sul project team preferred to limit the technological breakt hrough to the use of polyester rope for a permanent mooring system, as there appeared insufficient time to address properly all technical challenges related to design, development and installation of VLAs.

The anchor points, therefore, comprise high holding power marine anchors with a length of chain incorporated at the lower part of each leg that is in contact with the seabed. This chain acts like a clump to avoid vertical load on the anchor.

Design of the mooring system was also optimized with regard to the following criteria:

The anchoring system must provide sufficient transverse stability to avoid unstable fish-tailing behavior (the transverse stiffness being the stiffness induced by the anchor transversely to the mean excursion).
The maximum anchor leg tensions must be kept as low as possible in order to minimize the size of the anchor legs' components.
The polyester element must keep a positive tension in all circumstances. This criterion is considered fulfilled as long as the lower end of the first polyester segment remains lifted above the seabed.
No up-lift at the anchor point is permitted in intact conditions, although a limited uplift force is allowed following breakage of a leg.
The anchor leg should be as easy to install as possible.

Methodology

Derivation of the design loads and motions was based on the results of numerical sim ulations, calibrated against the results of model tests for other projects. Extreme loads on the anchoring system are derived from the fol lowing conditions: intact anchoring system and one-leg broken survival conditions (including the transient condition) following maximum loads likely to be experienced in a sea state with a return period of 20 years.

The six legs described were designed using the Adrianc program of Bureau Veritas. Line com position and actual mechanical properties of the various segments, namely un-stretched lengths, linear weights in air and water and axial stiffness, have been properly defined. Based on this model, individual anchor leg static load-excursion curves have been calculated along with static load-excursion curves for the entire anchoring system.

For the polyester segments, special attention has been paid to modeling of axial stiffness - polyester behaves in a stiffer way when exposed to higher loading rates and tension values. This has been modeled by considering four separate loading regimes:

During installation
For static loading due to steady environmental force
For cycling due to slow drift oscillations
For cycling at the primary wave frequencies.

Mean wind, current and wave force and moments acting on the tanker were determined on the basis of coefficients for generic tanker shapes. Also taken into account were the mean loads on the buoy, resulting from the action of the current on the anchor legs and risers. To assess the dynamic amplification factor, use was made of the Flexriser program which can perform fully dynamic 3D analyses of slender structures such as riser lines or anchor legs.

In addition, the lower length of chain in each leg, a length of steel wire rope was added so that the combined length of chain and steel wire rope would exceed the water depth. This has insured that the anchors could be deployed and embedded without using the polyester rope, which could easily have been damaged during that phase.

For each leg, the polyester rope was split into two segments (900 meters and 450 meters), so that hookup of the FPSO buoy could be realized with a tug connecting the lower, longer segment to the upper, shorter one already secured to the buoy.

Finally, a relatively short length of chain was added at the top of each anchor leg to connect into the buoy chain stoppers and to allow for final pre-tension adjustments. Also, several triangular plates were included in each anchor leg assembly to facilitate handling and load transfers during installation.

Petrobras handled installation of the FPSO II, while two anchor handling tugs (Maersk Provider, Maersk Clipper) managed deployment of the anchor lines. Another tug, the Norman Neptune (bollard pull of 220 tons) was used for final pre-tensioning operations, which included a test to develop adequate permanent stretch of the polyester rope.

Although pre-tension adjustments should have been performed directly after anchor leg hook-up, these were in fact made after the risers were hooked up, so as not to delay flexible pipe lay operations. Top chain angle measure ments and calculations were performed to check that the pre-tension achieved so far would be suf ficient to keep excursions of the FPSO mooring buoy within acceptable limits for the risers.

Among the lessons learned from this exercise are that special care must be paid to new prac tical problems created by the use of materials or components not commonly used for the same purpose in the past.