Industry News - Offshore Engineer Reports - Composites shape up for risersComposites shape up for risers
from: Offshore Engineer
by: Marshall DeLuca
Friday, December 01, 2000
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 54% for a constant hull diameter of 122ft and by 42% 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'.
Moving to manufacture
One final issue is manufacturing capacity. Salama reckons the technology is ready to go into production, but the ability to meet demand does not exist. 'It all hinges on somebody stepping up and buying it,' he says. 'The technology to fabricate composite risers is there, but the manufacturing capacity to produce them on time and at the proposed mass production cost is not there.
'You want it to be delivered around the same time schedule as steel. Somebody has to step in and put up a plant to make it. You can build a complete plant to make composite risers of different sizes for something like $10 million, so it is not a huge investment. But nobody wants to do this until somebody places an order and nobody wants to order until he sees the plant.'
Wang likens the progress of composite riser development to that of the aerospace industry 20 or 30 years ago. 'In the 1970s it took one test for the Boeing 727 composite tail - the field demonstration that the composite would work - and that opened the door,' he points out. 'After that nobody questioned whether composites would work, just how much they could be used.
'It is the same thing here. One demonstration with very critical elements like the riser or something of that kind, will open up the door. Operators are eager to develop the deepwater and they face challenges where composites can really help.'
A strong move toward acceptance is being made with the help of DNV. The group is in the process of writing proposed design guidelines for composite risers which it hopes will become internationally accepted and possibly ISO certified.
Dr Andreas Echtermeyer, principal engineer in the materials and components section at DNV, says these guidelines will do much to progress industry acceptance. 'As the situation is today, where you have no agreed guidelines or rules to use for these composite risers, we feel it is very difficult to introduce them in the market.'
'Today it is an unproven technology and I think customers are very reluctant to use a composite riser if there are no accepted rules on how it should actually be built,' he says. 'We are using calibrated safety factors to make sure the design is safe and fit-for-purpose and people will be more willing to buy if they buy based on a safe design.'
There are a great deal of advantages, both economic and enabling, that composites can offer for deepwater development. These have been demonstrated on paper, in the laboratory and now, say the experts, it is time to demonstrate them in the field.
Wang adds that the potential for composite applications is almost limitless. 'With composites, you engineer the materials and based on that you engineer the structures, so only your imagination is the limit.'
Riser development update
Research and development of composite risers is one of the more advanced disciplines in the composite realm. Actual development of rigid composite risers began more than 20 years ago in the hands of the Institut Franais du Ptrole (IFP) and Aerospatiale. The two groups began a feasibility study of a 4in, 5000psi working pressure composite tube in 1979.
By 1984 some 40 tubes of glass or carbon fiber had undergone mechanical tests for burst and collapse pressure, ultimate tension and bending and fatigue tests in tension, internal pressure, bending and creep. Further, in 1983 the group had built and tested six 15m long, 4in ID glass and carbon fiber hybrid tubes as choke and kill lines on a drilling riser in three applications in the North Sea.
IFP also participated in two other JIPs in the 1980s. In 1985 the group began proof-of-concept work which constructed eight, 9in ID composite production risers, six of which were tested for burst pressure, ultimate tension, tensile fatigue and bending.
Also in 1988 the group formed a second JIP to extend development work through durability and damage tolerance analysis, damage tolerance residual strength testing and cyclic tensile fatigue testing. A third JIP was proposed for sea-testing of three composite riser joints on a dummy riser on Conoco's Jolliet TLP, but failed due to lack of participation.
An additional JIP was proposed in 1993 by IFP, Coflexip, Aerospatiale and Lincoln Composites aimed at developing and qualifying a composite production riser design in accordance with industry standards to demonstrate suitability and evaluate cost-effectiveness. Again this was shelved due to lack of participation.
The greatest push towards development for composite risers came in 1995 when two important projects were started focusing on drilling and production risers. The projects - one headed by Lincoln Composites focusing on a composite production riser, and one headed by Westinghouse Electric and ABB Vetco Gray studying a composite drilling riser - were under the auspices of the US Department of Commerce's National Institute of Standards & Technology (NIST) advanced technology program.
These programs have led to the development and testing of several different riser concepts that have just now entered a state of commercial readiness. Work to date (see table page 14) has been done in three main areas: composite production risers, composite drilling risers and composite choke and kill lines.
Production risers
Composite production risers have been evaluated to provide a 60% weight saving over steel risers, reducing the need for tensioners on floating production systems. Besides increasing water depth capability they also reduce deck weight, and the risers have smaller space requirements thereby increasing the number of wells possible for a given vessel. Several major initiatives are presently underway. Some of the major work being done includes:
lNIST JIP (Lincoln Composites): Under this, the companies were to design, develop, manufacture, test, and qualify a TLP composite production riser. In the first phase, they designed a low-cost, lightweight riser for 3000ft to 5000ft water depth, considering both single and dual casing risers. In subsequent phases the group fabricated and tested pre-production prototype specimens and then fabricated and tested full-length joints.
The group chose a 103/4in single/dual casing riser for the prototype. A total of 90 prototypes were fabricated and tested - three 7in subscale joints, 12 design verification test joints, and 75 pre-production prototype joints - and two full-scale joints were fabricated and tested. According to the now complete JIP, the design has been shown to meet cost, weight and performance goals.
lABB Vetco Gray: has developed a 10.05in ID all carbon fiber, top-tension riser for TLPs and Spars. It will have a 175F mean bulk temperature and 6000psi internal operating pressure. The company is presently in the process of static and fatigue testing ten prototypes with full acoustic emissions monitoring and next plans to manufacture five full-scale production risers. They will be tested to static and cyclic loads as well as fatigue tested to failure.
lConoco/Kvaerner: Conoco and Kvaerner are presently qualifying a 95/8in and a 103/4in riser with two options: a steel connector with a titanium liner and a steel connector with an elastomeric liner. The company has performed extensive materials testing and fabricated one joint that is instrumented with optical fibers, which is presently enroute to Oil States in Aberdeen for burst testing. Post burst test, the company will fabricate other joints, also instrumented with optical fibers, for fatigue tests and long term performance studies at DNV. In addition to optical fibers, other innovative inspection techniques, such as acoustic emission, will be qualified as part of this program for potential use for in-service inspection.
Conoco says its goal is to have the first field test of a full riser, not just a single joint. The company has approval from its operational division to perform the test on the Jolliet TLP in the Gulf of Mexico, likely to be carried out sometime next year following qualifications and approval including US Minerals Management Service sanction.
lABB Offshore Systems: is developing a composite catenary riser for production from wet trees. 'The challenges, and hence the associated composite solutions, are very different for top tensioned risers for dry trees and dynamic risers for wet trees,' says Lars Slagsvold, ABB's manager for the development.
'The challenges for the dynamic risers include reeling and high production temperatures,' he says. 'The advantages of dynamic composite risers include that they can be used from vessels with high first order motions, like FPSOs, have even greater weight savings than the top tensioned composite risers and are of comparable cost to steel catenary risers.'
The company is in the process of developing an 8in catenary riser that can be installed by several of the existing reel vessels. Design requirements include internal operational temperature of 320F; a working pressure of 7250psi for water depths of 6500ft; a reel diameter of 36ft; and a design life of 20 years.
The company is presently in the third phase of the project. The first phase consisted of system analysis, material evaluation, process selection and initial pipe design. The second dealt with material and laminate tests, detailed pipe design, small pipe section tests and development of a winding machine.
The company will start fabrication of prototypes next year, and large scale testing the year after. ABB expects to have a production plant for the system in operation by 2003.
Drilling risers
Composite drilling risers, like other initiatives underway today such as riserless or dual gradient drilling, promise industry the capability to drill in deep water using a mid-depth semisubmersible. According to ABB Vetco Gray, such a riser provides a 65% weight saving over steel riser systems. This, the company says, can double the drilling depth of an existing rig or platform. Two major initiatives are presently underway and represent the closest step towards a field trial:
lNIST JIP (ABB Vetco Gray): has the goal of developing cost-effective manufacturing methods with emphasis on a composite drilling riser. ABB Vetco Gray has fabricated a single joint of a 21in prototype all carbon fiber, low pressure drilling riser. The prototype has been static tested to two million lbs tension and fatigue tested at lower loads to 650,000 total cycles at Stress Engineering in Houston.
The company is now in the process of outfitting the riser to go offshore Brazil for a six-month field test early next year with Petrobras. Plans are to insert the single joint into an existing steel string of riser at three different string locations over the six-month testing period. Instrumentation on the riser will be used to gather real-time information and lab testing will continue after field use.
lConoco/Kvaerner: In 1995, while NIST was assembling its JIPs, Conoco and DuPont signed an alliance agreement with Kvaerner to explore and develop composite products for the oil and gas industry. In 1997, following a holistic study identifying opportunities for composites, the group initiated the CompRiser project with the goal of qualifying composite drilling riser technology through testing and field demonstration of a high-pressure drilling riser joint.
The companies developed a 22in riser with a titanium liner and a titanium termination and have performed burst, bending/fatigue testing, and impact testing (a requirement in the North Sea). Prototype joints were subjected to 15,850 psi burst pressure, 50kJ impact loads and bending fatigue loads equivalent to 150 year loading spectrum. Conoco/Kvaerner are now in the process of fabricating a field joint to test on the Heidrun TLP in the North Sea in April of next year. The company plans to install a 50ft joint to substitute for an existing joint at three different positions in the drill string.
Choke and kill lines
According to research, composite choke and kill lines provide about a 60% weight saving over steel, and replacement of steel auxiliary lines on a standard drilling riser string with composite tubulars will result in between 20% and 25% total weight savings. Presently two major projects are underway developing composite choke and kill lines for drilling risers:
lStewart & Stevenson Oiltool Products and Lincoln Composites: Over the past two years the companies have focused on developing a 3in ID, 15,000psi pressure and 0-200F choke and kill line under a five phase testing program. The companies are testing to API16C (Specification for choke and kill systems) but since the specification does not address composite lines, flexible pipe requirements are being used as a guide where applicable.
Presently the companies are in phase three of testing by Stress Engineering. To date burst and fatigue testing have been completed and exposure testing is under way.
For the exposure test, expected to be completed this month, the line is filled with a special test fluid while holding pressure at elevated temperatures for seven days, followed by a 30-day test at working pressure and room temperature.
The companies next plan a flow test. This will involve a short section of the line installed in a choke manifold on a drilling rig, to experience actual kick situations to verify the elastomeric liner holds up to erosive action at high flow rates. An actual field test will then follow.
Once completed, the companies plan to use the choke and kill line on conventional riser strings.
lABB Vetco Gray: As part of the drilling riser program the company has fabricated all-carbon fiber 3in ID choke and kill lines with 15,000psi operating and 22,500psi test pressures for a single joint of the composite production riser. Several separate test programs have been run on the lines including test pressure tests of 22,500psi ten times and operating pressure tests of 15,000psi over 2080 times to simulate 40 years of service.
The company has also performed elevated temperature testing at 175F. All tests have been monitored with acoustic and strain gauge measurements. The company plans to install the choke and kill line on the composite drilling riser for the six-month field test planned to start by March 2001 offshore Brazil with Petrobras.
ABB Vetco Gray has also designed a 41/2in choke and kill line to the same pressure rating as the 3in. The company says both designs are ready for market and it is prepared to complete production orders upon customer request.
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