Belgium, Germany, Spain

Vol. 226 No. 4

A technological challenge in offshore chain fabrication

Irune Inchausti and Javier Vicinay, Vicinay Cadenas S.A.

Over the last decade, the offshore industry has allowed operators to expand their activities into deeper waters and into those fields which have more difficult environmental aspects. These developments led to the customization of offshore components, but not of mooring chains.

In principle, taking into account the present state-of-the-art chain production, the options available for moorings to suit a specific field are restricted to those chains approved by the classification societies, and which meet specific quality and geometry criteria. And the current standards only consider two types of link: 1) Stud link: 6 x D link length, 3.6 x D link width, Standard ISO 170 4-1991, and +/ – 2.5% tolerance; and 2) Studless chain: 6 x D link length, 3.3 x D to 3.6 x D link width, Standard DNV C.N. 2.6 Aug. 1995 Section 8, Appendix C, and +/ – 2.5% tolerance.

With this scenario, and taking into account that the mooring designer has to design for longer lengths – as units will be working in deeper water, or link size has to be increased to meet greater mechanical demands, or the corrosion margin has to be larger, or fatigue life extended – considerable weight will be added to the mooring system using chain of traditional link sizes/ grades. This decreases effective floating unit payload and increases not only mooring cost, but structure of the unit itself.

For this reason, designers now have to consider using alternatives such as combination mooring systems which, sometimes, are unsatisfactory as the catenary effect can be lost when chain is added into the line.

Fatigue machine used to test chain.

Spain’s Vicinay Cadenas S.A. (VC), manufacturers of chains principally used in the offshore industry, considered this situation problematic for mooring design and engineering companies and so, five years ago, took up the challenge to develop a range of alternative products to those already recognized and approved by classification societies. The company now has two customized solutions to the oil industry which will resolve the mooring designers’ problem as to mooring line weight but, at the same time, not prejudice its service life. These solutions are known as:

1. New grades of chain R4S and R4s+ (R5). Note that VC named the new qualities R4S and R4S+ to follow the nomenclature used by IACS, while the classification society ABS indicates that it intends to call the highest grade (R4S+) RQ5 to fit in with its other standards: RQ3, RQ3S, RQ4 and RQ4S.

2. New chain design VGW (Variable Geometry and Weight). This solution optimizes geometrical variables of the links to improve critical aspects of the chain such as: 1) fatigue, corrosion, abrasion; 2) load capacity of the mooring line; and 3) increase or decrease of weight in the catenary.

For R4S and R4S+ (R5/ RQ5) chain, using the knowledge/ experience gained from perfecting Grade 4 chain, VC took as a starting point, the chemical composition used for the steel and worked on its chemical elements to improve its characteristics without detriment to its other strengths such as durability to fatigue, or corrosion resistance. VC’s R&D team, in collaboration with the Metallurgical Department of the University of Bilbao, special steels maker Sidenor S.A. and the management for fabrication process control QWeld and heat treatment DQT, developed these new qualities which have already been used in projects.

Studless chain of 127 mm and 117 mm diameters in Quality R4S+ (RQ5) was specified for Atlantia Offshore Ltd.’s Power Hub Development in the MC 920 Atwater Valley facility, and Studlink 70 mm chain in RQ4S for Noble Drilling’s semi-submersible Theral Martin .Both projects were classed by ABS.

The improvements which these new chain grades give while maintaining the traditional link geometries accepted under the existing standards are summarized in the accompanying table showing weight savings and mechanical properties.

For fatigue life, VC has performed full-scale salt water testing on 149-mm R4S+ chain on its fatigue testing machine with the following parameters:

• Testing frequency: 0.5 Hz
• Loads: maximum, 2,943 kN; minimum, 491 kN; average, 1,717 kN
• Wave form: Sine
• Concentration of salt in water: 35.64 grams/ litre, and
• Average salt water temp.: 19.5°C.

With these test conditions, predicted chain fatigue life according to API RP2SK March 1997 would be 163,592 cycles. Using the prediction in DnV OS-E301 “Position Mooring June 2001,” it would be 172,610 cycles. The actual result obtained in a test on three links in November 2004 was 1,575,250 cycles, more than nine times either prediction.

Considering all the above facts about R4S and R4S+ (R5) designed and developed by Vicinay Cadenas S.A., the following advantages are indicated: 1) reduction of chain diameter to obtain the same resistance but with less weight; 2) better homogeneity in tensile and resilience; 3) increase in resistance to abrasion; and 4) if fatigue durability is considered a risk factor, chain quality can be upgraded.