The recent decision by Brazilian oil and gas producer Petrobras to use high modulus polyethylene (HMPE) mooring lines on deepwater mobile offshore drilling units (MODU) has breathed new life into the technical and practical issues surrounding the deployment of hybrid lines for permanent moorings at water depths beyond approximately 6,500 ft (2,000 m).

Lankhorst Ropes is supplying the synthetic mooring rope with Dyneema to Petrobras. The Gama 98 rope construction, developed by the company specifically for deepwater mooring, is being used on a number of major deepwater projects in the Gulf of Mexico (GoM). This will be the first time the 630-metric-ton minimum breaking load (MBL) rope with Dyneema (comprising 12 sub-ropes of SK78 yarn) has been used for MODU mooring.

HMPE MODU moorings

HMPE for MODU moorings first underwent trials 10 years ago. “Early results were promising,” said Sergio Leite, technical director at Lankhorst Ropes Offshore Division, “However, subsequent problems with creep prevented its use as a deepwater MODU mooring rope.” A decade later things have changed with the development of SK78 by DSM Dyneema.

The Gama 98 rope construction, developed by the company specifically for deepwater mooring, is being used on a number of major deepwater projects in the GoM. (Images courtesy of Lankhorst Ropes Offshore Division)

According to Leite, the rope has a creep rate maximum of 0.5% per year at 61°F (16°C). “This, together with advances in predictive modeling for creep rate and elongation to estimate creep lifetime, are giving HMPE the edge over polyester in ultra-deepwater MODU mooring.”

Beyond 6,500-ft (2,000 m) water depth, the elasticity of the polyester rope becomes a problem. A 6,500-ft (2,000-m) polyester line can have 130 ft (40 m) of vertical movement, while a line of approximately 9,800 ft (3,000 m) would allow approximately 197 ft (60 m) under the same environmental conditions, creating greater horizontal offsets, which could exceed the limits of drilling risers.

HMPE is a stiffer yarn than polyester; so its elasticity is significantly less, making it more riser friendly. In addition, the higher strength of HMPE yarns allows smaller diameter ropes than polyester for the same MBL.

A polyester deepwater rope with a MBL of 1,907 metric tons has a diameter of 254 mm and weighs 43 kg/m. An HMPE rope with the same MBL is only 190 mm in diameter and weighs 16 kg/m. For Petrobras in particular, this has important installation benefits because the MODUs could be up to 124 miles (200 km) offshore. The smaller and lighter HMPE rope allows more rope per reel (approximately 2,950 ft (900 m) HMPE vs. 1,970 ft (600 m) polyester) and a considerable reduction on the reel dimensions – 13-ft (4.0-m) diameter end flanges with polyester are reduced to 10 ft (3.0 m) with HMPE, allowing more reels per vessel. The weight of the rope-laden reel is also reduced from 19 metric tons for polyester to 7 metric tons for HMPE. Since fewer reels are needed, the reels can be handled more readily by an anchor-handling vessel. Fewer lighter-weight reels will permit MODU mooring lines and anchors to be installed in one trip. And the lighter HMPE rope makes installation easier and quicker, which is significant given that the MODU mooring lines could be deployed and recovered as many as 20 times over a 5-yr period.

Hybrid mooring lines

Although HMPE’s creep performance prevents its use for permanent deepwater mooring, Dave Rowley, director at single point and deepwater mooring specialist Offspring International, believes a polyester/HMPE hybrid mooring system offers the best permanent mooring solution beyond 6,500 ft (2,000 m).

“As water depth increases, polyester moorings will not have sufficient stiffness by themselves to maintain a vessel on station and prevent over stressing the risers,” Rowley said. “HMPE has the stiffness but not the creep performance. So hybrid mooring lines that combine polyester rope segments and HMPE to give a hybrid stiffness that is neither too stiff nor too soft, may offer the best solution from both the perspective of mooring system performance and cost of deployment.”

The speed of HMPE creep is a function of temperature, mean load, and loading time. Higher temperatures mean higher rates of creep; so in general, the preferred hybrid rope configuration is to use the stiffer HMPE rope in cooler water close to the seabed and polyester rope in warmer water closer to the vessel. The percentage of HMPE to polyester line lengths used in the hybrid mooring configurations changes as water depth increases. For example:

  • 50%-50% HMPE/polyester at 6,000-ft (~1,800-m) water depth;
  • 60%-40% HMPE/polyester at 7,500-ft (~2,300-m) water depth; and
  • 75%-25% HMPE/polyester at 10,000-ft (~3,050-m) water depth.

Jorn Boesten, segment manager offshore, DSM Dyneema BV, pointed out it is a common misconception that all HMPEs are the same. “This is most definitely not the case,” he said. “Yes, creep rate is dependent upon temperature, load range, and time, but the quality of the feedstock is critical. Different feedstocks can give you different creep performance – hence the importance of developing our predictive modeling tool that allows us to predict creep rate and creep elongations and to estimate creep lifetime for Dyneema. Today it is possible to guarantee the lowest HMPE creep rate over a 15-yr period using SK78, he said.

The preferred hybrid rope configuration is to use the stiffer HMPE rope in cooler water close to the seabed and polyester rope in warmer water closer to the vessel.

Hybrid mooring system

From a rope designer and manufacture perspective, polyester and HMPE rope construction is the same. According to Leite, the only difference is the rope diameter. “The principle benefit of hybrid PE/HMPE ropes is the ability for the mooring system designer to engineer the mooring line’s stiffness and use the lengths of polyester and HMPE segments to provide the stiffness needed to handle peak loads during station-keeping, while ensuring sufficient elasticity to withstand storms.”

Storm survivability rose to the top of the mooring agenda following hurricanes Ivan, Katrina, and Rita, which swept through the GoM casting deepwater MODU moorings with polyester lines adrift.

“Where previously the requirement by the then Minerals Management Service (now BOEMRE) was for MODU moorings to withstand worst case 10-yr storm conditions peak loads, it is now 50-yr storm conditions,” Rowley said. “In permanent deepwater moorings, the requirement is now 1,000-yr storm peak loads compared with 100-yr storm previously.”

Faced with the need to meet these more demanding storm conditions, research by DSM Dyneema BV based on the more severe post-Katrina American Petroleum Institute (API) storm conditions in the GoM showed that a HMPE MODU mooring system can be engineered to have similar performance and survivability as a polyester system in deepwater moorings.

Research results are promising, according to Boesten. “We’ve successfully addressed the issue of storm survivability and station keeping with HMPE and reduced the creep rate to match industry expectations on creep requirements for MODU mooring systems.”