The offshore oil and gas industry has changed tremendously over the last several years. Moving into deeper waters has created challenges in operations that would be considered commonplace in shallow water projects. Deeper water has not only made tasks more complicated, but also more costly. Becoming more efficient in deep water operations leads to less risk and higher profits.
Traditionally, subsea contractors have looked to wire rope as the strength member for many lowering and lifting applications. Wire rope has a long and successful history of use in winches, cranes, and rigging. Although wire rope has a proven track record, it does have its limitations. When performing deepwater operations, weight can be an issue. Wire rope is very heavy, which means that winch and crane wires will significantly reduce payload capacity of the winch or crane as the depth increases. Often, the winch or crane can only be used at a fraction of its designed load rating, due to the fact that much of load is the wire rope itself.
Other issues with wire rope are its “inflexibility” and maintenance requirements. A wire rope, especially in a large diameter, can be very cumbersome to handle for a diver or ROV. It also can create problems in tight areas. This is true for surface jobs as well. Handling of wire rope causes numerous injuries every year. Maintaining wire ropes can be costly and creates environmental issues that have to be dealt with. Lubricants can be messy, leading to unsafe work areas; this negative environmental impact must be considered.
Synthetic rope technology has evolved in recent years and is replacing wire rope in many applications where weight and/or handling issues prevent wire rope from being used. Synthetic fiber ropes have the same strength as similarly-sized, steel-wire ropes. The largest advantage, however, is the difference in weight. Synthetic ropes weigh 80-85% less, depending on the material used. Some synthetic materials are buoyant, which means they add no weight to the payload regardless of depth.
Fiber ropes require no maintenance, are much more flexible, easier to handle, and are much safer. Most synthetic materials are resistant to common chemicals and have very good ultraviolet resistance. Fiber ropes can also be treated with various coatings to improve certain characteristics.
Two projects demonstrate how synthetic rope technology has replaced steel-wire rope, creating a safer and more efficient operation. Both projects also show how synthetic HMPE (high molecular weight polyethylene) ropes were used to solve design problems.
Perdido Spar
When Shell was designing the Perdido Spar platform’s subsea asset deployment and retrieval system, it faced a problem: at water depths over 8,000ft, a traditional wire-rope drum winch system would be too heavy for the platform on which it was to rest. A new winch/rope system was needed.
Shell chose Samson’s Quantum-12 synthetic rope on a Logan Industries traction winch to solve the problem. It marked the first time Shell had used an HMPE rope system on a spar platform and the first time Samson’s rope had been used in this type of application.
The Perdido Spar, operated by Shell on behalf of partners BP and Chevron, is part of the world’s deepest oil production facility. It is the most remote producing platform in the Gulf of Mexico, almost 220mi. off Texas, and the only development in the region to use subsea separation and boosting equipment. The development features 22 wells and 13 well tie-backs, which must be changed out and serviced at regular intervals.
Rather than deploying offshore service vessels to the remote development at great expense, the spar’s design includes a winch system that allows the platform to service the subsea equipment. Operating in nearly 9,000ft of water, while handling payloads of up to 90,000lb, is a challenge. Compounding the problem, the spar’s design places the winch on a cantilevered deck (a smaller deck extending out from the main platform), imposing significant weight restrictions.
A traditional steel-wire rope of this length would be prohibitively heavy. Weighing 81,750lb, it would require double the lifting capacity of the winch. This would in turn increase the size of the winch and require significant structural modifications to be made to the spar.
Another, and more significant, issue was the winch capacity. When using steel-wire rope, significant winch capacity is consumed by the wire weight. The deeper the job, the longer steel-wire rope is needed, and the less capacity the winch system will have. This means that the winch would have to be larger to accommodate the needed pay load at depth.
A synthetic rope is buoyant; therefore, it adds no weight to the payload, regardless of length. The weight does not change (significantly) regardless of depth, Figure 2.
Resolution
To solve the problem, Shell solicited possible solutions from their winch and cordage suppliers. After reviewing several proposals, Shell decided a synthetic winch line offered many advantages and would require minimal deck space.
Shell turned to Samson for highstrength, lightweight synthetics and to Logan Industries for a simple, tractionstyle winch design. This combination met the weight and footprint restrictions of the cantilevered deck and was capable of performing the subsea operations, Figure 3.
Samson provided 9,200 ft of 2-1/2in. -diameter, Quantum-12 HMPE rope. Weighing 85% less than a wire rope of similar size and length, this greatly reduced the deck weight needed. Samson also worked closely with Logan Industries, assisting the design engineers in creating a smaller, lighter winch system specific to the rope’s characteristics.
The rope provides the same strength as similarly sized wire rope. It is made with Samson’s patented DPX fiber technology for superior abrasion and cut resistance with a higher coefficient of friction (COF) than other HMPE ropes. The higher friction coefficient is an advantage over other HMPE ropes, allowing it to be used on a traction system. Typically, the COF of steel ropes ranges from 0.05 - 0.08. The DPX technology increases this to 0.1 - 0.15, minimizing “slip” and damage. Synthetics also offer superior bend fatigue properties than steel-wire rope at smaller bend ratios. This feature allowed Logan to design the winch system with a smaller diameter traction head, resulting in significant weight reduction.
Since there was no heave compensation needed, cyclic bend fatigue was not a concern. Another advantage that synthetic ropes have is a natural damping effect that helps minimize package motions (vs. steel). Steel-wire rope acts like a spring, transferring vessel motions to the package. A synthetic rope has hysteresis, which provides energy dissipation that minimizes transferred motions. Figure 4 shows the “looping” effect as a synthetic rope is cycled.
To date, the system has performed 250+ operations without any issues. The original rope is still in service.
Jubilee field
The Jubilee field off Ghana, West Africa, is estimated to hold 1.2Tcf of gas and 1.8 billion bbl recoverable crude oil reserves, making it the second largest field in the world. First oil was produced in December 2010, after risers and umbilicals were installed from the seabed at depths of 900-1700m to the FPSO Kwame Nkrumah, which successfully completed the fastest ever, full-scale, deepwater development. Samson’s Turbo-EPX synthetic rope was instrumental in the 11-riser installation, but not before AmSteel-Blue first assisted with mooring the FPSO.
Sofec was in charge of the design and installation of the turret for the Jubilee field development. Included in the turret design was a winch system that would be used for two projects: anchor-chain tensioning and riser pullins, Figure 5.
The company was concerned about limited room on the FPSO deck. The most efficient solution would be to use a small winch for the riser pull-in system. A synthetic rope solution would simplify the winch design and allow a smaller drum diameter to be used. Synthetic rope would also allow a size reduction of the hydraulic power unit and the deck load.
Sofec contacted SWOS, Samson’s master fabricating distributor in Houston, Texas, to discuss a synthetic rope solution for the pull-in lines. Samson synthetic ropes for offshore applications are made with Dyneema fiber, which brings high-performance characteristics such as high strength, light weight, abrasion resistance, and neutral buoyancy to innovative rope constructions and coatings. The light weight and “flexibility” of the synthetic ropes would make handling much easier in a small space. The weight would be 1/7 (14%) that of steel, allowing the ropes to be handled by hand.
Technip was responsible for the risers and had some abrasion concerns as the synthetic ropes were being pulled through the riser tubes, Figure 6. While SOFEC was confident that the ropes would work well in both applications, the riser installation engineers at Technip were not so sure.
Because of the neutral buoyancy of the synthetic ropes, Technip also was concerned that the rope would catch in construction vessel propellers, which would approach very close to the FPSO when the risers were transferred. Samson put application engineers to work developing a solution.
SWOS and Samson engineers worked closely with SOFEC to understand all of the nuances of this project and equipment. The engineers determined that the best combination of equipment would be a split-drum winch with a working load limit of 350 tons, loaded with 350m of 5 5/8in.-diameter Turbo-EPX HMPE rope, which has a minimum breaking strength of 875 tons.
This rope has a jacketed construction with a 12-strand core strength member made with high-strength, low-stretch Dyneema fiber. The jacket is made with polyester that grips the winch and hardware, and is abrasion resistant. To ensure that the rope would sink rapidly enough to avoid catching in tugboat propellers, a segmented lead-line was added to the center of the 12-strand braided core. The addition of the lead allowed the rope to slowly sink out of the way of the propellers. The specific gravity increases from 1.01 to 1.12.
Once the customization and manufacture were complete, the line underwent extreme scrutiny. It was initially break tested, witnessed by the American Bureau of Shipping (ABS). Then Technip, the project managers for the installation, commissioned a study by Bureau Veritas (BV), who found the rope to be more than adequate, with a safety factor of 3:1.
With these certifying agencies’ approval, Samson manufactured three lines for the Jubilee riser pull-in job. Sofec planned to use one of the three lines for the anchor chain pull-in and tensioning line. This left one for the riser installation, and one as a backup.
After further consideration of the anchor-chain pull-in job, Sofec realized that the significant 5 5/8in.-diameter of the rope would cause the line to bear against the cast-steel sidewalls of the chain stopper’s internal cavity. Again, Sofec contacted SWOS, who recommended a smaller diameter rope of AmSteel material to pull-in the mooring chains.
SWOS provided 738ft of 3¼in.-diameter rope with a soft eye on each end. This rope is able to handle a working load limit of 100 metric tons (mt) with a 400mt minimum breaking strength, equaling a 4:1 safety factor.
Results
In a typical FPSO mooring application, both wire and synthetic ropes are used on a split-drum winch; however, the AmSteel-Blue rope worked in place of both. The rope’s easy handling and change-out allowed engineers to simplify the winch-drum design, and provided flexibility for the nine-anchor-chain installation and tensioning. The rope took quite a beating, but finished the job without failure. Once the Kwame Nkrumah was secured in place, the Turbo-EPX rope was reinstalled on the winch and ready to pull in the risers.
Only one of the three Turbo-EPX lines was used to pull the 11 risers. According to one Technip installation engineer, the rope was still in good shape after the job was complete.
These examples demonstrate how synthetic ropes can replace steel-wire ropes, creating more efficient and safer operations. One of the barriers to using synthetics can be initial cost. Synthetic ropes can be 2-4 times more expensive than wire rope. Therefore, it is important to look at total cost of ownership and equipment cost. In most cases, when the total costs are compared, using synthetic ropes can be more cost effective. More importantly, synthetic ropes are safer.
Steel-wire rope has a home in many applications, and is still a tried and true workhorse. Synthetic ropes, however, should be considered when weight and handling become an issue. This is especially true for larger diameter ropes. In many cases, the user will find significant value in using high performance synthetics. OE
Justin Gilmore is technical sales manager for the Offshore Business Unit of Samson and has over 23 years in the synthetic rope industry. He has an extensive background in synthetic rope design, manufacturing processes, quality programs, and application engineering with many field-related publications and patents.
Subscribe
