High-modulus polyethylene ropes for deepwater mooring were first trialled 10 years ago, but despite promising early results subsequent problems with excessive creep stalled their application. A decade on, the situation has improved to the point where one operator has elected to deploy the mooring rope in its deepwater MODU operations. Sergio Leite and Chris Johnson from Lankhorst Ropes' offshore division take up the story.
As operators go further offshore, high-modulus polyethylene (HMPE) ropes become an increasingly attractive option for ultra-deepwater moorings, particularly beyond 2000m. Brazil's Petrobras recently placed the first-ever, worldwide order for synthetic mooring rope, made from Dyneema fibre, for deepwater mobile operating drilling unit (MODU) projects with Lankhorst Ropes.
The decision to use HMPE, more specifically Dyneema SK78, rather than conventional deepwater polyester rope reflects the current state of synthetic rope technology and the practicalities of oil and gas field exploration going deeper and further offshore.
Since the mid-1980s, polyester has been the rope yarn of choice for MODU and long-term deepwater moorings beyond 1000-1500m. Deepwater mooring ropes are a balance of rope strength and stiffness. Polyester has excellent elasticity allowing the line to stretch and absorb the dynamic loads experienced by a floating drilling and production unit. It also has excellentfatigue life and very low creep – the rate at which the rope stretches irreversibly over time.
If anchor handling vessels could readily accommodate reels of any size and boat costs were low then polyester's position in deepwater mooring would be unassailable. Unfortunately neither of these are true, added to which beyond 2000m water depth the mooring lines' elasticity becomes a problem as longer mooring lines allow greater horizontal offsets. A 2000m polyester line may have 40m of vertical movement, whereas a 3000m line would allow 60m under the same environmental conditions, creating greater horizontal offsets which may exceed the limits of drilling risers.
To reduce these offset effects, stiffer materials or hybrid mooring systems are required to compensate for the longer line lengths; and provide a more sympathetic riser environment. HMPE is now widely considered to be the most suitable material for these longer line lengths. The yarns are characterised by very high stiffness, combining high strength and modulus, producing lighter and smaller diameter ropes. HMPE fibres are sensitive to long-term static loads, however. Ropes made with HMPE fibre will irreversibly elongate proportionally with time in the steady state creep region. The degree of creep is a function of the operating temperature, mean load and loading time. Recent developments have been aimed at significantly reducing the rate of creep, as well as developing a model to better predict creep rate and creep elongations, as well as to estimate creep lifetime.
The creep performance of HMPE rules out its use for long-term moorings at present, nonetheless the rope is ideal for MODU moorings where the moorings are short term, up to 12 months at a time. After which the ropes are then retrieved and stored between moorings allowing some recovery of rope before reuse at the next drilling site.
Synthetic fibre ropes are manufactured from visco-elastic materials; and so their stiffness characteristics will vary dependent on the size of the load,duration of loading and number of cycles. Both polyester and HMPE will experience some initial elongation caused by the rope ‘bedding in' during the early loading cycles.
During station-keeping the mooring ropes are subject to cyclic loadings due to wave movement. These impose fluctuating fibre elongation and tension-tension fatigue loads on the mooring lines.
Extensive testing in wet conditions at the TUV NEL facility in Scotland shows a longer tensile fatigue life for braided ropes made with Dyneema compared to polyester ropes for the same rope construction. The tests involved a wire-rope construction which is generally accepted as providing the best results in tension fatigue: the non-rotating braided rope (BRC) with Dyneema outperformed the polyester ropes with wire rope construction (PET-WRC).
Although the continuous cycling has the potential to cause fatigue damage in the rope, it is the steel connector components in a mooring system that are more prone to fatigue damage. Inevitably the higher stiffness of HMPE will impose higher fatigue loading on these line components. Early MODU trials with HMPE mooring lines reported some distortion of thimbles; this fatigueeffect has now been reduced through improvements in thimble design and rope torque balancing.
Rope construction
The Petrobras contract will use Dyneema SK78 yarn sub-ropes for the 630t minimum breaking load (MBL) rope, and Lankhorst Ropes' Gama 98 rope construction. The construction is based on high efficiency sub-rope cores laid parallel within an outer braided jacket. Each sub-rope is computer monitored during tether manufacture to ensure all sub-ropes have equal tension and length.
The MODU lines will comprise several sub-ropes, each sub-rope being of a long lay length 8 x 1, 100% torque free rope. As rope twisting during installation can cause axial compression and structural fatigue within the HMPE rope, a continuous red line is applied in the same position along the rope length duringmanufacture using a longitudinal in-line painting systems. The red line allows the mooring line installation engineers to visually monitor the rope deployment for any twisting, enabling corrective action to be taken if necessary.
In total Lankhorst Ropes is supplying 15,000m of Gama 98 rope, manufactured at the company's offshore division factory in Portugal. Given the higher stiffness of HMPE compared with polyester, accurate length measurement is vital to avoid the need for additional and costly top chain during mooring. To ensure length accuracy, the HMPE ropes are manufactured under a machine tension of approximately 1t back tension. A rope length measuring system (LMS) for length measurement under a controlled tension is used. The rope is pre-tensioned to 1% MBL (current limit 20t) for length measurement in 75m (260ft) increments.
The HMPE rope construction also includes a torque balanced braided jacket to protect the rope during handling. Filter elements are used between jacket and sub-rope cores to prevent sand particles getting between the sub-ropes and causing abrasion damage.
Installation benefits
The higher strength of HMPE yarns allows smaller diameter ropes for the same MBL compared with polyester. A polyester deepwater rope with a MBL of 1907t has a diameter of 254mm and weighs in at 43kg/m; for the same MBL the HMPE rope is only 190mm and weighs 16kg/m.
The smaller diameter and lighter HMPE rope has significant operational benefits. It allows more rope per reel: 900m HMPE vs 600m polyester. Therefore fewer reels are needed, and these can be more readily handled by an anchorhandling vessel. As MODU moorings go further offshore, say 150-200 miles, it also permits the installation of MODU mooring lines and anchors offshore to be completed in one trip. Moreover, the lighter rope makes installation easier and quicker, an important factor as the MODU mooring lines may be deployed and recovered some 20 times over a fiveyear period.
The use and experience of HMPE for short-term, deep and ultra-deepwater MODU moorings will be an important stepping stone towards the development of long-term mooring systems for production vessels. At present, neitherwholly HMPE nor polyester lines are suitable and so hybrid mooring lines that combine polyester rope segments and HMPE may offer the best mooring solution from both the perspective of mooring system performance and cost of deployment. OE
About the Authors
Sergio Leite is responsible for the research and development of Lankhorst Ropes' new products and applications, specially for the offshore market. He graduated from Porto University in 1991 with the degree of Mechanical Engineer.
Chris Johnson is sales director at Lankhorst Ropes offshore division.