Development of Standards for Offshore Mooring
Authors: Vishnubhotla Srinivas , Ahjem Vidar , Hogsaet Ingrid Skutle , Paulsen Tone Julseth 
Published: May 2011

This paper presents an overview of available standards and recommended practices concerning station keeping with synthetic fiber ropes. The coherence between documents is presented, and an overview is given for where to find the specific information that may be needed throughout a project from the perspective of owner/operator, contractor or equipment manufacturer. An overview is given of recent document changes, and what will be published in 2011. The paper explains how findings from joint industry projects and other research efforts are incorporated.

Det Norske Veritas (DNV) has recently updated its offshore codes in the form of rules, standards and recommended practices related to offshore mooring. The recent updates have different implications for drilling units moored for periods of less than 5 years, and for floating production units that keep position permanently for a duration that can extend anywhere between 5 - 30 years.

Fiber ropes behave very differently to steel ropes or chain when subjected to mean and dynamic loads due to varying stiffness characteristics and time-dependent behavior. The documents now reflect that failure modes and mechanisms differ from those of mooring-equipment made from steel. Continuous development of standards and recommended practices is considered a key contribution to the available tools and methods for design, testing and analysis of offshore mooring fiber ropes.

Since late 1990s comprehensive work has been undertaken in the area of offshore mooring systems including fiber rope material. Ever since synthetic fiber ropes have been found as a viable option for offshore mooring applications, there has been a considerable interest in understanding the behavior of fiber ropes to meet the challenges of the future. As the number of fiber-rope based mooring system continually increases, the need for standards and practices will continue to develop.

Introduction

Working with components for position mooring systems under the auspices of Det Norske Veritas (DNV), there are several documents available. The types of documents are organized according to information category such that the information will be easy to find. This paper is presented in order to provide a convenient overview, and to highlight some particulars of the DNV documents that might differ from other standards and guidelines. New elements in the most recent issues are also discussed, together with foreseen development of the documents.

The arrangement of the DNV documents

For each global topic, the documents are divided into the following main types:Offshore Service Specification (OSS)Offshore Standard (OS)Recommended Practices (RP)

The Service Specifications, Standards and Recommended Practices issued by DNV can be obtained free-of-charge on the following link: http://www.dnv.com/resources/rules_standards. A brief explanation to each of the following document categories is presented in the following. Figure 1 illustrates how the document categories cohere through serving an explicit purpose for each information category.

The Offshore Service Specifications contain procedural requirements for obtaining and retaining class certificates and other conformity statements for offshore vessels.