Step changes in wire tensile strength may lead to mooring in ultra-depths

Oct. 1, 2000
Low seawater penetration key to process
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Demand on exploration and production is driving the installation of facilities in ever increasing water depths. This drive for production in deepwater and ultra-deepwater locations has required the development of lighter weight mooring components, while maintaining reliability and endurance essential for 20-year field developments.

The continued use of steel mooring line components is said to remain possible with increasing water depth, maintaining the confidence provided by proven technology. Nevertheless, further improvements in strength-to-weight ratio, providing a lighter cable for a given strength, are being sought not only to extend its useful depth range, but also to offer a more cost-effective solution.

In catenary and semi-taut systems, the floating structure moves laterally in response to environmental loads. The overall compliance, and hence stationkeeping performance, is determined by the water depth, the weight of the line, and the mean tension. In ultra-deepwater locations, the vertical load component can become significant due to a fundamental feature of the catenary mooring system - weight.

Optimizing mooring system weights minimizes vertical loads on the floating structure, resulting in reduced structural cost or increasing the operable water depth before prohibitive loads occur. Through the development of products with a greater strength-to-weight ratio, system loads can be minimized.

Bridon International claims it has been able to reduce the weight of steel mooring cables through the use of materials, manufacturing techniques, and engineering design. Conven-tional steel moorings currently are utilized in depths of 1,465 meters. Through the use of recent improvements in wire strength, the feasibility of these proven components can be extended to 2,000 meters and beyond.

Wire technology

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The strength of a steel cable is governed by the properties of its fundamental building block - the component wire. Three key parameters dictate the finished wire tensile strength: feed material, wire drawing process, and wire finish.

Considering the metallurgical properties required for cable manufacture and considering the expense and availability of exotic materials, the options for feed materials are limited to developments in micro-alloyed, high carbon steel. The challenge lies in the manufacture of high strength wire, which maintains suitable ductility for wire drawing and spinning into finished cable.

Through careful control of the chemical composition these properties can be attained. The inclusion of lead patenting in the manufacturing process ensures a feed material of homogeneous structure from which the final strength can be generated.

The individual wire strength is generated in the cold drawing process. The finished wire UTS attained is dependent upon a combination of the number and sequence of drawing dies, the reduction in diameter achieved by each die, and the speed of the drawing operation. In general, this means the smaller the finished wire diameter, the greater the potential tensile strength. Improve-ments in cable strength-to-weight ratio can be achieved through utilizing a greater number of smaller diameter higher-tensile wires. However, the inclusion of smaller diameter wires is a compromise as corrosion resistance is potentially reduced. Due to the increasing ratio of surface-area-to-wire, load-bearing area as wire diameter decreases, the effect of corrosion on the smaller diameter wire will be a greater percentage loss in strength than for a larger wire subject to the same rate of corrosion progression.

It is the cold working of the wire during the drawing process that generates its ultimate tensile strength. Currently, mooring cables requiring a design life of 10 years or greater use component wires, which are final hot dip galvanized ('A' class). During the galvanizing process, which is completed as the final stage of wire manufacture for 'A' class finish, the wire can lose around 8% of the UTS generated in the cold drawing process.

Drawn galvanized ('Z' class) wire exhibits a greater UTS, as the cold drawing process is completed after galvanizing. Completing the drawing pro-cess after application of galvanizing increases the wire tensile strength, but reduces the galvanizing coat weight, and therefore sacrificial protection, by up to a half.

'Z' class material is commonly used in shorter term applications such as anchor lines for drilling rigs, but is currently not the standard for long term applications on production vessels. Based on current design life recommendations, as dictated by DNV certification note 2.5 (May 1995), there is a compromise between strength and corrosion pro- tection as 'A' class finish is specified for spiral strand for long-term mooring applications.

However, Bridon has completed corrosion progression testing of cables submerged in seawater in excess of 12 years, which demonstrates that the closed construction and heavy blocking compound of the spiral strand prevents water ingress beyond the outer wires. Therefore, it is reasonable to suggest the inner wires could consist of drawn galvanized product.

Taking this a step further by considering that the application of a polyethylene jacket completely seals the strand to water ingress, the potential for corrosion is negligible. There-fore, in principle, the attainable design life would not suffer from being drawn galvanized throughout.

While the strength-to-weight ratio is the ultimate driver for long term moorings in deepwater locations, the selection of finished component wire must also consider the corrosion protection requirements of the long term mooring applications.

Lightweight solution

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Continuous developments have resulted in step changes in the available wire tensile strengths utilized in the manufacture of steel cables for permanent mooring applications.

Historically, long life spiral strand products have used relatively large diameter wire of tensile grade 1,570 N/sq mm, with intermediate improvements up to 1,770 N/sq mm. Through the developments detailed above, wire tensile strength in excess of 1875 N/sq mm is currently available and is close to a 20% increase in wire UTS over original levels. There is potential to further increase wire tensiles to in excess of 2,000 N/sq mm through the use of drawn galvanized wire offering further improvements in strength-to-weight ratio.

Alternatively, we can consider the strength increase in terms of saving in cable weight. To attain the same breaking load 20% less steel is now required, which directly translates into a reduction in cable diameter and weight for a given breaking load.

The cable termination options available compliment the improvement in strand strength-to-weight ratio. The achieved reduction in cable diameter allows the terminations to be optimized around the required strength, rather than dimensions of the cable. Therefore, the termination weight is also minimized. The reduction in weight extends beyond the cable termination to the complete interface between the cable segments and chain segments of a steel mooring system.

The closed socket termination allows direct connection between cable termination and a suitable anchor shackle, offering the minimum total interface weight. Alternatively, an open socket and connection link plate can be utilized for connection to a suitable anchor shackle. While this solution does not minimize mooring system weight, as the connection link is fitted with a high lifting capacity handling padeye, it does allow suitable handling to ameliorate installation loads.

Case in point

The first user to benefit from the strength improvements is Aker Rauma Offshore of Finland for the mooring systems of the two Spar floating production platforms to be used on the Nansen and Boomvang fields, operated by Kerr-McGee Oil & Gas Corporation on behalf of its partners. Both fields are located in the Gulf of Mexico's East Breaks region in water depth of 1,130 meters.

The Nansen and Boomvang platforms will be the first truss design Spars where a tubular truss system replaces the lower part of the cylindrical hull. Existing spar platforms already installed in the Gulf of Mexico comprise fully cylindrical hulls.

The contract involves the supply of 125 mm diameter sheathed spiral strand with a minimum breaking load of 3,420 kips for the two nine-leg mooring systems. In 1997, Bridon supplied sheathed spiral strand mooring cables for a US Gulf Spar at a water depth of 790 meters. The mooring cables for this contract also required a minimum breaking load of 3,420 kips. However, at this time available wire UTS was limited to 1,570 N/sq mm, resulting in a spiral strand of 133 mm diameter. The mooring cable used for the Boomvang and Nansen mooring systems, which provides exactly the same breaking load, exhibits a 12% lower weight.

The table compares the 133 mm diameter mooring cable specification supplied to the project with the 125 mm diameter used for the Boomvang and Nansen spars. Each offers a required minimum breaking load of 3,420 kips.

Worldwide suitability

With increasing production activity planned for the deep water "Golden Triangle" - Gulf of Mexico, West Africa and Brazil - these developments of lighter weight, proven and reliable steel mooring solutions are supporting the needs of the leading operators.

The only question that remains is how much deeper? It is clear that the steel mooring solution is feasible for water depths of 2,000 meters and beyond. The next step in the development of the steel products using the drawn galvanized product will soon be available.