Introduction:The highest payoff in the use of composites is for primary load-bearing elements of deepwater systems. This use is on the verge of testing, as Marshall DeLuca investigates.
Just as the industry moved from wood to steel in a search for durability and strength, so now it is poised for the next step as lightweight composite materials are brought to the fore.
In the coming year, several companies plan the first field demonstrations of a number of advanced composite systems which experts say could move such materials from the research and development phase into mainstream acceptance and use.
Over the past decade reasonable progress has been made towards the use of composite materials offshore. With their high strength-to-weight ratio, fatigue, and corrosion-resistance, design flexibility, thermal insulation and stiffness they have gradually become a replacement for steel in applications such as fiber reinforced pipe and phenolic grating on non-critical elements of topside modules.
While these applications take advantage of the benefits of composites, they are considered secondary structural elements and do not capture the full advantage of composite use.
The future and highest payoff potential for composites, experts say, lies in the primary load-bearing elements of deepwater systems where applications are more weight-sensitive and metal designs become impractical.
Among these primary elements, arguably the most important are drilling and production risers.
One of the greatest limiting factors in exploration and production activities as industry pushes toward greater water depths is the increasing riser load that surface equipment has to support.
As well as lighter weight, the advantages of composite riser systems over steel include better thermal insulation and more fatigue and corrosion resistance.
But, because they are such a critical part of operations, operators hesitate to accept and use them.
While weight is a big factor in overcoming the challenges of deepwater operations, there are still many issues that need to be addressed before operators will accept the technology for use in critical roles.
One of the greatest drawbacks to acceptance is cost. On a direct component-to-component replacement basis composites in general are more expensive than their steel counterparts.
However, when applied to a deepwater floating system, the reduction in weight and improvement in fatigue performance of individual composite components, such as a riser, can cascade in a knock-on effect to provide significant cost and performance benefits at the system level.
This translates into a much lower system cost for newbuilds such as a Spar or TLP.
A prime example was presented by Chevron at last month's Composite Materials for Offshore Operations-3 conference in Houston. In a study of the effects of composite components on Spars, the company found that in a truss Spar where hull diameter is kept constant at 122ft, the substitution of composites for all-steel production riser components allows the total Spar length to be reduced from 710ft to 515ft for 6000ft water depth, and from 855ft to 535ft for 10,000ft water depth.
In addition the study found that under the same circumstances the baseline all-steel spar cost can be decreased by 54percent for a constant hull diameter of 122ft and by 42 percent for a constant hull length of 250ft.
Says Douglas Johnson, director of oilfield products for Lincoln Composites: 'If projects are designed to take advantage of the light weight, system acquisition cost will be lower. If it weren't, the industry rightly would not use the products.'
Beyond cost issues, many technical issues exist which have precluded the use of composites.
One of the leading challenges is the metal-to-composite interface where load is transferred between the composite tube body and the metal end fitting, for example at the joints in a riser string.
Industry has had trouble in the past making a reliable connection between the two components, with most testing failure occurring at this interface.
'This is the technology that has really been holding back composite use in the oil field,' says Gary Galle, manager of composite and floating production systems for ABB Vetco Gray.
'The cost of these risers and the reliability of attaching metal end connectors to the composites are really the commercial and technical stumbling blocks.'
Presently there are three basic ways to establish the interface. The first is bonding in which the composite is bonded to the metal with an adhesive.
This method however, is not often used as the integrity of the interface is directly related to the strength and longevity of the adhesive.
The second method is a trap lock. In this, the filament is wound into grooves in the metal. Therefore the only way to break the interface is if the composite leaves the groove. This is the most common method of application and is the method of choice for the Conoco and Kvaerner projects (see later).
While this method has been proven reliable, it cannot take large torsional load. However, this is not an issue with risers as they are mainly affected by axial loads, bending or pressure.
The third method is the Geometric Trap, developed by ABB Vetco Gray. In this arrangement the outer tailpiece of the system is pre-loaded, moving it relative to the composite and locking it in place due to friction.
Epoxy is then injected into the gap between the inner and outer tailpieces so that the preload can never be lost.
This, the company says, takes away all the cyclic stresses in both the composite and the metal.
Galle reckons the Geometric Trap solves the interface problem. 'It has always been a concern and until we solved it we really wouldn't be ready to make a commercial product. But this technology enables us to stand behind it and look for its reliability to be equal to or greater than steel systems.'
Liners:
A further issue is the inner liner. With most work being done on the composite shell of the riser, some say the issue of liner integrity has yet to be seriously addressed.
Under the internal pressures and external loads that occur, the liner acts as a pressure barrier that prevents fluid loss through possible micro-cracking in the outer shell.
However, in drilling operations, equipment sent into the riser could tear or damage the liner. In addition, the high temperature and high pressures of operation can be corrosive to the liner.
Presently companies are using two types of liners: metallic and elastomeric.
'A metallic liner is a complicated design because you have to match the metal to the composite,' says Dr Mamdouh Salama, senior research fellow for production technology at Conoco. 'Your goal is to design the joint so the composite will carry most of the loads.
The composite in general is less stiff than the metal so the metal liner will carry a large portion of the axial load, forcing the designer to make it thicker which means it will carry more load. So you find yourself in a losing battle.
'In addition,' he continues, 'there is the difference in thermal expansion between the two.
It is like applying load to two parallel springs. If the composite is a soft spring then a lot of the load will go into the metal and if it goes into metal it means you have to make it thicker which means the composite will get even less load.
In the case of rubber or elastomer it is not really an issue because it is not carrying any load. It is just to hold the fluid like a bladder.'
While there is no definite solution to this problem, companies are researching. Galle says. ABB Vetco Gray has been testing the reliability of liners to see if they hold up to the rigors of offshore use.
In addition Chevron has an ongoing program evaluating the liner material for the different pressures and temperature which should be completed shortly.
Salama adds: 'If someone could come up with a way to have a reinforced elastomer like a car tire, it would simplify the process quite a bit.'
Inspection:
While the technical issues are being ironed out, long-term reliability is the major issue occupying Dr TM Hsu, senior staff research scientist, floating concepts, for Chevron Petroleum Technology Company.
'That is the part that's missing,' he says. 'Right now we feel very comfortable in the design, in the manufacturing, and the laboratory testing, but we still don't know how it will perform ten to twenty years from now installed in a deepwater environment.'
'What we heard from a meeting with MMS is, how do you know after five years if the joint is bad?' adds Salama. 'Composites are difficult to inspect after you make them. You have different materials, you have liners, and you just can't ultra-sonically scan it.'
Thus, several companies have developed sophisticated inspection methods to ensure the product integrity and build material histories.
One such method underway by Conoco takes advantage of the composite fabrication process by winding fiber optic sensors into the body of the riser.
Salama says the sensors will allow the company to measure load, strain and stresses inside the composite. 'It is a form of in-service inspection and the fiber optics become a part of the structure and is protected.'
ABB Vetco Gray is also using an inspection method called acoustic emission, says Galle. 'We listen to the sound that the composite makes while it is under load. Any change in acoustic signature indicates you are getting a degradation of the composite.'
He adds that the company has created a large materials database testing program whereby composite laminates are put through a series of standardized tests to look at the material properties and material allowables.
'We have completed the database for about 150 samples per material type. This is why we focused on drilling risers first,' he says. 'We wanted to get some field experience and you can retrieve those on a routine basis.'
Dr Su Su Wang, director of Composites Engineering & Applications Center (CEAC) at the University of Houston adds that with accelerated tests, based on firm science foundations, you can run limited and short-term experiments to extrapolate for long-term performance. 'That is the assurance we have that these materials will work into the future.'
But a real field test is still needed. Chevron's Hsu says the company is interested in putting a composite riser on a Spar in either the Gulf of Mexico or West Africa. However, 'without a field trial, our operating company will not entertain the idea'.
by Marshall DeLuca
© 2001 Mena Report (www.menareport.com)
