Using Polymers as an Alternative to Metals"
Authored by: Robert Gibson
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The use of metals in the design and manufacture of subsea equipment may not be the best choice of materials for the job. Satisfactory alternatives exist, particularly groups of polymers known as polyurethanes. Polyurethane often offers an advantage over conventional steel materials, and provides a design solution which cannot be matched by these conventional options.
The Problem with Metals
For as long as man has worked in the sea, implements and structures have been produced for use in this unusual environment. Early equipment was produced in iron, and later from steel. However, a problem soon became apparent. Seawater is a highly corrosive medium in which these materials rust very quickly.
Corrosion of metals, and in particular carbon steels, remains a serious problem to this day. Solutions are available. Items can be coated with materials, such as paint, which provide a barrier to attack from the surrounding seawater. However, coatings of this type are easily damaged during handling, so this may not be a solution to long-term exposure.
An alternative method regularly used in conjunction with barrier coatings is the use of cathodic protection. Here, the component in question is fitted with anodes, normally in the form of blocks of metallic alloys. Sacrificial corrosion of the anodes provides a form of electrical protection.
Attempts, with varying degrees of success, have also been made to replace corrosive metals with more highly resistant options. Early attempts focused on the use of alloys such as bronze. More recently, the marine and offshore industries have seen the extensive use of nickel and titanium alloys.
It should also be appreciated, however, that a designer may face difficulties other than corrosion in the use of metals. These materials also tend to be both relatively heavy and rigid. These are attributes which may not be advantageous in every design application.
Alternative Materials - Non-Metals
As the end of the century approaches, engineers now have access to a huge range of non-metallic materials. Many of these alternatives may provide solutions which are superior to the conventional metallic options. The CRP Group is a subsea engineering company which specializes in the design of subsea equipment while making extensive use of a range of polymeric materials.
One family of materials which is particularly favored are polyurethanes (PU). The use of the term PU is even less specific than the term carbon steel, which also covers a large family of materials. At one end of the spectrum, PU can be supplied as a rigid plastic, while at the opposite end, the material can be produced as a flexible elastomer. The features which make PU such an exciting option for designers are that it does not corrode in seawater, is lightweight, is available as a rigid or a flexible material, is cost-effective, and can be molded into large objects several tons in weight.
So what are the disadvantages? The main drawback is that when compared to structural steels, polyurethanes do have a limited mechanical strength. Unlike metals, they also have difficulty in operating at elevated temperatures, and particularly over extended periods.
Despite these drawbacks, PU can be used for a variety of applications. The first is a cable support product known as a bending restrictor. Traditionally, bending restrictors have been manufactured in steel. However, these components tend to be heavy and expensive, and they will corrode. Bending restrictors supplied in polyurethane have largely solved these problems.
Polyurethane is also a good choice for the manufacture of bending stiffeners. These are products which are used to support cables and pipes compliantly and normally in dynamic environments. This demanding application requires the use of a flexible material. The use of conventional metallic materials would be totally unsuitable. Cables and pipes which are used subsea may have critical locations at which they are susceptible to damage due to overbending. For example, the point where a cable exits from a large end connector, or the free span of a flexible pipe behind a connection to a wellhead or manifold.
Bending restrictors are interlocking tubular devices, sometimes referred to as "vertebrae." The presence of the bending restrictor does not hinder the flexure of the cable until a certain critical radius, known as the locking radius, is reached. At this point, the bending restrictor effectively becomes a rigid load-bearing structure. External loads and moments are now carried through the restrictor as opposed to the cable, thus protecting it from damage.
As mentioned previously, if manufactured from steel, the product tends to be heavy, and one must address the problem of corrosion protection. Not an easy task if the restrictor is lengthy and made up from a large number of individual elements.
Polymer bending restrictor elements tend to be larger than their metallic equivalents due to the lower inherent strength of the material. However, they are virtually weightless when submerged in water due to the materials low specific gravity.
The use of a polymer, such as polyurethane, means that the problems associated with corrosion have now been largely overcome. Bending restrictors manufactured from polyurethane are therefore now used extensively around the world and have provided many years of satisfactory service. These components currently support a vast range of pipes and cables ranging from lightweight telecommunication products, to large diameter, flexible export risers.
The last decade has seen the installation of a large number of floating production systems around the world. These vessels are normally linked to the seabed production facilities by flexible risers and umbilicals. At the connection to these seabed facilities, or at the connection to the production vessel, there is a real possibility of damage to the lines as the system moves in response to environmental loads.
To prevent this from occurring, the pipes and cables are fitted with a compliant device known as a bending stiffener at these critical locations. The stiffener normally takes the form of a slender cone which is rigidly fixed to an adjacent structure which serves as its base. In extreme cases, the stiffener, which may be several meters in length, is required to bend through angles up to 90 degrees in a matter of only a few seconds. This is simply not a feasible application for a conventional metallic material with its inherent rigidity.
Polyurethanes, on the other hand, are a very attractive option because they are not subject to corrosion when immersed in seawater. Also, the material is flexible and the degree of flexibility can be adjusted to suit a particular set of design requirements. The material is resilient and is capable of sustaining the millions of loading cycles experienced over the design life. Polyurethanes can be molded into large single piece items weighing several tons.
Bending stiffeners produced from polyurethane elastomer have been installed around the world and in highly fault-critical applications. Their presence and continuing performance assures the continuity of hydrocarbon supplies from an increasing number of important offshore floating production facilities.
Polyurethanes on the ETAP Project
CRP recently supplied a total of 54 thermal insulation flange covers to Coflexip Stena for simulated seabed tests on British Petroleums Eastern Trough Area Project (ETAP) in the North Sea. The ETAP scheme involves the tie-back of a series of high pressure/high temperature wells to processing platforms several kilometers away via a rigid steel pipeline. Production fluids have to be maintained at the temperature they leave the wells (at least 160 degrees C) to prevent wax formations blocking the pipelines. The main pipeline lengths are insulated using a pipe-in-pipe system. However, the flanged connections at several intervals along the length of the pipe give rise to the risk of cool spots.
To handle these high temperatures, complex geometries, high environmental loadings and a requirement for diver installations, CRP developed a unique flanged insulation cover. The new thermal insulation covers are produced from the latest composite materials, including polyurethane foam, polyurethane elastomer, syntactic polyurethane, and syntactic epoxy, chosen for their excellent thermal performances and high strength. The covers are manufactured in one to one-and-a-half meter lengths for ease of handling and are custom made to fit exactly around the pipe with the thickness of the shell taken into account to provide effective insulation. In addition to the flange covers supplied to Coflexip Stena for the ETAP project, these covers have also been installed on the Texaco - Erskine, Shell - Mallard, Britannia, and Lufeng projects. UW
Robert Gibson is Projects Director for CRP Group Ltd. Operating from Skelmersdale, Lancashire, UK, Gibson is responsible for the technical performance of CRPs underwater product range. CRP Marine is a manufacturer of polymers for use in underwater applications.