Project Deliverable D50 Report on Best Practices ... - ESONET NoE

White Paper for the Panel 3-3 – 20 years plus components, mechanical issues.
Author: I.G. Priede (UNIABDN), J.F. Rolin and R.Person (IFREMER)
Abstract: Although the material choice for long time deployment has not been addressed
since the beginning of the ESONET NetWork of Excellence, it is a key question as can be
seen in the excerpt of the ESONET Concerted Action final (2004) report hereunder:
Materials for subsea observatories
The material choices were a difficult issue for the telecommunication cable industry. Cable
thread protection, polyethylene sheathing, glass epoxy composite repeaters were developed
and now allow high performances. The material choice for long-term underwater deployment
requires the experience of specialized designers and in some cases analysis of experts. A lot
of knowledge was acquired through offshore, academic and military projects. The material
providers are seldom aware of the behaviour of their products in long-term seawater
exposure: the quantity produced for this market is most of the time negligible with respect to
the overall production. Research is active in the field of materials in marine environment. The
processes of corrosion and especially biofouling are requiring tests and theoretical studies.
Some projects such as Neptune Canada are planning R&D activities on materials in parallel to
the subsea observatory design and first deployments. EC funded projects have been devoted
to materials in Marine environment: Composite Housing, BRIE, BRIMON. The hydrothermal
environment is very corrosive. High temperature and high-pressure systems may be
encountered with black or white smokers. Hydrogen sulphide, a vast host of metal-rich
sulphide minerals, carbon dioxide, methane and hydrogen are present. Other places are rich in
chlorides and metals. The monitoring by seafloor observatories will bring very interesting
scientific data on biodiversity, ecological processes, and time related variations of chemical or
physical parameters. Specific tests are required to check the design of the observatory. The
EC FP6 project Exocet/D includes such tests. The deployment of subsea observatories in the
continental margins means a long immersion at pressures of 2000 to 6000 dbar. This is
considered as “ultra-deep” because it exceeds the usual offshore oil industry standards. Under
such pressures, some design parameters and some material behaviours have not been tested
yet. Therefore, extra tests and studies may be required.
Future Observatory Designs
.Use of metals For subsea observatories intended to be deployed during more than 10 years,
the choice of metallic materials for structural design is limited.
Steel with cathodic protection is the standard solution. Rules of the offshore industry may be
applied. For the deep sea, cathodic protection parameters have to be modified: Joint Industry
<strong>Project</strong>s between oil industry companies and research institutes are addressing this matter.
The cathodic protection required must be limited by a preliminary protection by Zn and
painting according to a process guaranteed by the manufacturer. The control and exchange of
anodes will represent a maintenance cost that must be accounted for in the operating costs.
Stainless steel. Common stainless steel is liable of cavernous corrosion and must be
prohibited. Some minor pieces may be built with 316L. Grades designed for seawater
corrosion (904, superduplex…) are not so common and are quite expensive.
Nickel alloys.Several grades of Nickel based alloys are available and constitute safe
solutions: Inconel 625, Hastelloy C22,…
Titanium alloys The Titanium alloys have been one of the technical enhancements allowing
deep underwater intervention. Their extensive use is only limited by the cost. For subsea
observatories, the experience of Geostar housings designed by IFREMER and built by
Tecnomare with a Russian manufacturer is a good example. Unalloyed titanium (T40) is used
<strong>Deliverable</strong> #50 - update October 2010 37