State-of-the-art of spread moored systems for deepwater floating production platforms

Richard D'Souza
Kellogg Brown & Root Inc.

All floating production platforms (semisubmersible, Spar, FPSO, and TLP) are positioned by a station-keeping system. The primary function of this system is to constrain horizontal platform offsets to a "watch circle" that enables production and export risers to remain connected for the life of the field.

The mooring system is a critical component of a floating production platform, and its integrity over the field life is of paramount importance. Where the platform has a drilling or workover rig, an active mooring system enables the platform to position itself over the well. The accompanying poster presents permanent mooring systems for spread moored semisubmersibles, Spars, and FPSOs. Station-keeping of TLPs and weathervaning permanent and disconnectable or dynamically positioned FPSOs are not included.

Description and operating principle

Spread moorings consist of multiple legs that are connected to the platform by fairleaders and tensioners and to the seabed by anchors. In deepwater, each leg is typically made up of either steel wire or synthetic rope over most of its length, with a small segment of chain at the top and bottom. The mooring spread could be symmetrically arranged or grouped around the platform. Grouped moorings are generally more efficient when designing for the damaged-line case required by certifying authorities. The mooring legs must resist forces and motions induced by the platform in response to extreme 100-year return period metocean conditions. Design codes specify safety factors for line break strength and fatigue life that are not to be exceeded by loads induced in the mooring legs.

Mooring systems resist steady environmental loads by generating a restoring force created by unbalanced horizontal tensions of the mooring array when the platform offsets from its mean position. The platform offset is controlled by the pretension applied to the lines. Maximum platform offset is a key driver in selecting and designing an optimal mooring configuration and is dictated primarily by the production riser system. Dynamic motions are accommodated by the degree of compliance in the legs by the catenary or elastic stiffness.

History and evolution

The earliest spread moored production platforms in the 1970s and '80s were installed in relatively shallow water (under 500 m) in the North Sea and Brazil. These were mostly all-chain catenary systems, which provided the compliance required to resist wave dynamic response in these depths. Flexible risers permitted large dynamic excursions.

As platforms progressed into water depths beyond 500 m and up to 1,000 m in the '90s, catenary moorings evolved into wire-chain systems, as all-chain systems became increasingly inefficient. In some cases, such as the Placid GC29 semisubmersible (the first deepwater production system in the Gulf of Mexico moored in 500 m) submerged buoys were employed to reduce platform load and line size. Anchoring systems in all cases were either drag embedment or driven piles.

Recognizing the need for low-cost mooring systems in increasing depths, Petrobras introduced lightweight polyester rope in lieu of wire rope for their spread moored and turret moored FPSOs. Petrobras also experimented with vertically loaded plate anchors as an option to pile anchors. Polyester rope permits a taut configuration, as the low elastic modulus can accommodate significant platform dynamic motion. Petrobras' Barracuda and Caratinga FPSOs, scheduled to begin production in 2004, will each be moored with an 18-leg, grouped, taut-leg polyester spread mooring system in about 850-m water depth. Polyester rope moorings are being introduced to GoM production with the first application on BP's Mad Dog Spar in 1,800-m water depth.

Hardware evolution

Mooring system hardware has continuously evolved to provide greater reliability and lower cost. A properly designed system can function for over 20 years without replacement. Mooring line hardware for a typical mooring leg consists of a tensioner and fairlead on the platform, mooring leg with connecting shackles and an anchor. In some cases, submerged buoys may be used. The tables in the poster provide the user with vendor information for each of these mooring components.

Rotary windlasses are commonly used to tension the platform chain. Hydraulic chain jack tensioners are often used for higher capacity systems. Rotary fairleads are preferred to bending shoes as the latter have experienced problems with liner material. Properly designed seven- or nine-pocket fairleads are required to avoid excessive wear or fatigue in the chain.

Submerged buoys are generally not cost-effective in water depths beyond 1,000 m and are only used in steel wire-chain catenary systems. There is a preference for syntactic foam buoys over spherical steel buoys, which experienced failures in early applications. A short length of platform chain is generally used with all systems because it is easier to tension and store on the platform than wire rope. Studless and stud link chain are both employed. The former is lighter weight, while the latter has better fatigue resistance, assuming the studs are properly designed. Strict material and manufacturing processes have eliminated chain failures experienced in early years.

Suspended line length is mainly wire or polyester rope. There are many different types of rope configurations used in moorings, as shown. Of these, the spiral strand rope has proved to be the most reliable and cost-effective. Design life of jacketed galvanized spiral strand exceeds 20 years. The most commonly used polyester rope to date has been the parallel strand and the Superline construction.

Anchoring systems for deepwater platforms are tending toward greater use of suction piles, as they can be installed in water depths to 3,000 m today, whereas driven piles are limited to about 1,200 m by underwater hammer capacity. These anchors can weigh up to 150 tons and are up to 30 m long, so they tend to be costly and difficult to handle. The vertically loaded anchor (VLA) provides the same holding capacity at a fraction of the weight, but may be difficult to install and proof-tension. Innovative VLAs, such as the Sepla, are being qualified for future use.

Design of a robust mooring system requires the prediction of mooring loads in design environments with a high level of confidence. Mooring analysis tools have become increasingly sophisticated, and it is possible to conduct fully coupled time-domain, dynamic response analysis of the platform, moorings and risers in non-collinear wind, waves and currents.

Conclusions

Mooring systems have undergone significant evolution and advancement in the last 15 years. Design and installation of robust spread moorings for FPSOs, semisubmersibles, and Spars in up to 7,000 ft is within the state-of-the-art. Extending these to 10,000 ft is possible with technology available today.

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