A text modified from Offshore magazine
Spanish Vicinay Mooring Connectors, one of the world’s longest-established anchor chain groups, has made tremendous strides in modifying the chain grades to keep up with the industry’s widespread shift to deeper water operations.
According to the company’s history, in the late 1980s, Vicinay Mooring Connectors branched into the studless chain. Its first application was theWest Vision semisubmersible in the Norwegian North Sea which is still in operation today. In a conventional stud link chain, the studs have a tendency to work loose over time, putting long-term performance at risk.
However, the result of working with a studless chain is that the incidence of fatigue-induced, premature fractures is low, in turn lessening inspection requirements. Other advantages include lower weight, better elasticity, and only a modest risk of corrosion, compared to stud link.
In the following years, in the mid-1990s, the company next introduced the Variable Geometry and Weight (VGW) chain. Vicinay’s goal was to design the VGW to offer greater flexibility to mooring system designers. The basic idea is to produce tailor-made chains, studded or studless, that allow the moorings to be configured to suit the confines of a particular project. Typical scenarios include:
- Saving weight in the mooring lines – without impacting the chains’ mechanical properties – to permit greater payload capacity in the platform or rig
- Designing the chain links to transit through the customer’s motorized equipment (i.e., winches or pallets) while retaining the same weight and diameter
- Incorporating increased abrasion and corrosion resistance, improved durability or fatigue life without any corresponding increase in the chain’s weight – this might be achieved by increasing the chain pitch, or the diameter of the links
- Designing the chain to achieve a more favourable price.
Vicinay’s specialists in Bilbao, together with some mooring designers, work closely to evaluate options available for each project. This collaboration extends to subsequent manufacturing and certification of the produced chain. Golar-Nor was one of the first clients to accept this approach.
As drilling and production move into increasingly deeper waters, the mooring designer must cater for longer line lengths in numerous ways. These include, among other things, increasing chain link sizes to withstand mechanical stresses, widening the corrosion margin, and extending fatigue life. These demands are liable to pile weight onto mooring systems comprising chains of conventional link sizes or grades. The net result can be an increased payload on the floating rig or platform, and therefore increased construction costs.
Five years ago, Vicinay anticipated these problems and started developing alternative chains graded R4S and R4S+, based on its experience formulating Grade 4. As a starting point, the company experimented with the chemical composition used in Grade 4 steel, working to improve its characteristics without degrading its durability or corrosion resistance.
Recently, in collaboration with the University of Bilbao’s Metallurgical Department -which is one of the highest in the ranking of Shangai- and Spanish steel-maker Sidenor, Vicinay developed R4S steel chain with a mechanical resistance of up to 960 Mpa, which is an 11% increase on the MBL level achieved by previous R4 grades, and R4S+, which offers resistance up to 1,000 Mpa. This latter value represents a 17% increase on the MBL level achieved by previous R4 grades.
Vicinay has also performed full-scale water tests on 149-mm R4S+ chain using in-house test equipment, applying the following parameters:
- Testing frequency: 0.5 Hz
- Loads: max. 2,943 kN; min. 491 kN; average, 1,717 kN
- Wave form: Sine
- Salt-in-water concentration: 35.64 g/liter
- Average saltwater temperature: 19.5° C.
Under these conditions, the predicted chain fatigue life would be 163,592 cycles, according to API RP2SK March 1997, and the figure should be 172,610 cycles, according to DNV OSE301 Position Mooring June 2001. The actual result Vicinay obtained from a test on three R4S+ links last November in Bilbao was 1,575,250 cycles.
Vicinay says that because of these characteristics, R4S and R4S+ can offer reduced chain weight (around 16-17% in R4S+) through decreasing the diameter without impacting resistance, improved tensile homogeneity, and increased abrasion resistance. The company is currently supplying 2,100 tonnes of R4S+ studless chain in 127- and 117-mm diameters for Atlantia Offshore’s Independence Hub semisubmersible platform in Mississippi Canyon block 920 in the Gulf of Mexico.