IMCA publishes guidance on subsea metrology

Published on 20 February 2012

A new publication ‘Guidance on Subsea Metrology’ (IMCA S019) from the International Marine Contractors Association (IMCA) turns the spotlight on subsea metrology, covers the basics of subsea metrology, engineering requirements, the different methods and technologies, and some of the advantages and limitations of each technique.

“Subsea metrology is the process of acquiring accurate and traceable dimensional measurements for the design of subsea structures, primarily interconnecting pipelines,” explains IMCA’s Chief Executive, Hugh Williams. “These pipeline interconnections are required to join subsea assets to complete the flow of hydrocarbons from the reservoir to processing and storage facilities.

Subsea metrology surveys are conducted to determine accurately the relative horizontal and vertical distance between subsea assets, as well as their relative heading and attitude. This information is then used by pipeline engineers to design connection pieces to join the assets together. Our new document, which has been developed by DOF Subsea under the direction of the IMCA Offshore Survey Division management committee, and reviewed by members of the OGP (International Association of Oil & Gas Producers) Geomatics Committee, explores the most common subsea metrology techniques in use today – long baseline (LBL) acoustics, both diver taut wire and digital taut wire, photogrammetry, and inertial navigation systems (INS).

“The aim of this new guidance is to provide information on the techniques which may be useful to surveyors and surveying organisations; vessel personnel (marine, diving, ROV etc); design engineers, fabricators and client organisations. An example is the accuracy of the metrology and how this impacts fabrication tolerances/fit of the interconnecting pipeline, where lack of fit influences the ability to install and create a pressure-tight joint as well as possibly influencing the working life of the interconnection.”

The new 32-page document can be downloaded free of charge from the IMCA website; printed copies are available at £10 for members and £20 for non-members (zero VAT, plus 20% for delivery outside Europe). Sections include an Executive Summary with short definitions of the five subsea metrology techniques; a glossary; introduction clarifying terminology; subsea metrology requirements; fuller detail on each of the subsea metrology survey methods; subsea metrology deliverables; and references and further reading. Appendices cover dimensional control requirements for metrology; a typical subsea metrology diagram; and a comparison of subsea metrology systems. It does not compare or evaluate different manufacturers’ products or services, or the specific performance of systems, and does not endorse or recommend a specific type, model or make of system.