Low creep synthetic ropes made from high modulus polyethylene (HMPE) hold the key to overcoming the engineering and installation issues facing naval architects and installation contractors arising from ultra-deepwater, permanent moorings. Lankhorst Ropes’ Sergio Leite and Offspring International’s David Rowley explain why.
The availability of HMPE ropes for permanent moorings is timely for two reasons: deepwater production projects are already planned for water depths around 3000m, beyond the scope of traditional polyester mooring tethers. And, with the current cost of deploying mooring lines already significantly greater than the cost of the lines themselves, the pressure is on to cut mooring systems deployment costs.
Polyester synthetic fibre rope is widely used for permanent deepwater mooring systems, especially in the Gulf of Mexico. Its neutral buoyancy and elasticity, compared with steel wire, enables a softer, more riser friendly, mooring system. The ability of the polyester line to stretch increases its storm survivability. However, the maximum offset experienced by a platform or vessel moored with a polyester line will increase as the mean work loading increases as a result of environmental forces. If the mean load rises to 50% MBL, then the offset will be 70-80m for a 2000m polyester line length. This large horizontal offset will increase the connection tension on the riser, resulting in higher levels of flex fatigue.
HMPE fibers ropes are characterized by their high strength and high modulus, producing lighter and smaller diameter, higher stiffness ropes better suited to the demands of ultra deepwater mooring, and less prone to large horizontal offsets. During station-keeping, wave movements impose cyclic loadings on mooring lines, causing fluctuating fiber elongation, and imposing significant tension-tension fatigue loads. Under these conditions, HMPE fiber ropes have a longer fatigue life than polyester ropes for the same rope construction.
All synthetic ropes are viscoelastic materials and so experience some level of creep. Creep is defined as the permanent elongation of the yarn under load. During mooring system pre-tensioning, the rope lines are pre-loaded to induce permanent elongation in the rope (sometimes called construction stretch or bedding-in) and increase the stiffness of the synthetic rope. Over time there will be some further extension of the rope due partially to creep, dependent on the load induced by wave and current movements. The rate of creep is dependent on fiber type, operating temperature, mean load and loading time.
Until now HMPE fiber has suffered from significantly higher creep rates than those experienced by polyester. A new HMPE yarn introduced by DSM Dyneema, called DM20, has enabled production of a Lankhorst Ropes Gama 98 deepwater rope with less than 0.5% creep elongation over 25 years, which meets the industry requirements for permanent mooring systems.
The speed of reduction in HMPE creep is great news for naval architects, but has highlighted a lack of consensus amongst standards bodies on HMPE test methodologies.
Testing consensus needed
HMPE creep is measured using both predictive modeling and creep monitoring. Based on an extensive range of HMPE fiber test data, DSM Dyneema has developed a predictive model for producing creep rate analyses, creep failure analyses and rope creep tests predictions, thus avoiding the need for full size (sub)rope testing.
Creep monitoring, on the other hand, is used to measure the elongation of HMPE rope in situ and thus the extent of creep over time. Some mooring standards require creep monitoring of the rope in use in the most critical section of the mooring line, usually the top part or top section closest to the water surface because of the higher temperatures involved. A few standards specify a HMPE rope replacement criterion of 10% creep strain of the total length of the HMPE rope. As an aside, the first polyester mooring systems in the Gulf of Mexico also required a short rope insert near the surface. Following a particularly heavy storm, the insert would be removed and a selection of sub ropes extracted for break load testing, fatigue analysis and checks for yarn-on-yarn abrasion, more recent moorings no longer require the insert.
Fiber creep rate tests indicate that, under normal offshore mooring rope loading conditions, the creep rate of DM20 fiber is more than 50 times lower than that of SK78 – an earlier yarn by DSM Dyneema for drill rig mooring. Under the same load levels, DM20 will take significantly longer to reach the start of creep failure than SK78 and with lower elongations. This suggests that the discard criterion of 10% permanent elongation recommended for general HMPE types and SK78 will need to be lowered for DM20 ropes.
Creep failure safety factors are proposed by several industry guidelines; for example a safety factor of 5-8 for long term moorings is recommended by DNV. For HMPE moorings, where it is specified, it is usually different for MODU and permanent moorings, and differs in how it is applied. Moreover, the safety factors are based on either creep failure, creep rupture life or design service life, which, by definition, are not consistent. This said, most standard’s bodies agree that the factor of safety for a monitored rope can be lower than that for an unmonitored mooring rope.
HMPE ultra-deepwater mooring systems create many more opportunities for production at previously inaccessible water depths. And, although HMPE is more expensive than polyester, this can be offset by an overall reduction in mooring system installation costs.
The higher strength DM20 yarn 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 43kg/m, for the same MBL the rope made with DM20 is only 190mm diameter and weighs 16kg/m. The smaller diameter and lighter HMPE rope allows more rope per reel: 900m HMPE vs 600m polyester, and a considerable reduction on the reel dimensions – 4.0m diameter end flanges with polyester are reduced to 3.0m with HMPE, allowing more reels per vessel. The weight of the rope-laden reel is also reduced by over half from 19t with polyester to 7t for HMPE. And, as fewer reels are needed, these can be more readily handled by an anchor handling vessel. Importantly it will permit the installation of more mooring lines and anchors in one trip, a not insignificant cost saving in vessel size and time, when the platform maybe 250 miles offshore.
Another potential installation cost saving is in the area of pre-tensioning. During deepwater mooring system installation, polyester ropes are routinely tensioned using either a specialist, heavy lift, installation vessel or anchor handling vessel to pre-load the rope, increase its stiffness and set the initial bedding-in stretch. For a wholly HMPE mooring system this would be unnecessary; although in practice there may be other reasons to apply a higher load especially with a MODU such as proof testing a vertically loaded anchor (VLA).
Going further offshore can expose the mooring system to greater wave and current movements, however. Just as the elasticity of polyester is a limiting factor at ultradeepwater depths, so too can be the stiffness of HMPE. In high storm and hurricane risk areas, the mooring system needs some stretch to ensure storm survivability, especially in the Gulf of Mexico. One solution is to use hybrid moorings.
Hybrid mooring lines combine HMPE rope segments with polyester rope segments, to give a mooring that is neither too stiff nor too soft. They allow 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 maximum loads during station-keeping in a storm, while ensuring sufficient elasticity to damp peak loads induced by waves.
As the speed of HMPE creep is a function of HMPE type, temperature, mean load and loading time. The preferred hybrid rope configuration is the stiffer HMPE rope in the cooler water close to the seabed, and polyester rope in the warmer waters closer to the vessel. In high storm risk areas, the percentage of HMPE to polyester line lengths used in the hybrid mooring configurations will change as water depth increases, for example, at 1829m water depth: 50:50 HMPE/polyester, at 2286m water depth: 60:40 HMPE/polyester and at 3048m water depth: 75:25 HMPE/polyester.
Of course, as the hybrid mooring system uses polyester this may still need to be pretensioned. However, recent developments in pretensioning practice have suggested that mooring systems designers should only proof test to what they need to ensure the maximum excursion limits are not exceeded and that there is no need to automatically go to 40% MBL which until now has been the ‘norm’ for deepwater moorings.
The ability to use HMPE ropes, made from Dyneema DM20, for permanent deepwater moorings is important. It is a mooring technology grounded in the need to assist naval architects in striking the right balance between the project engineering and economic demands of deploying deepwater moorings. In addition to increasing the scope of synthetic rope for ultra-deepwater moorings, it creates a far wider range of mooring options, and with this greater flexibility in the design and engineering of mooring systems. OE
Sérgio Leite, R&D director at the Lankhorst Ropes Offshore Division based in Portugal, is responsible for the development of the new products and applications especially in the offshore area. Sérgio graduated from Porto University in 1991 with a degree in mechanical engineering and has over 18 years’ experience of the rope industry.
David Rowley is a director of mooring systems services specialist Offspring International, the Lankhorst Ropes Offshore Division’s worldwide agent for single point moorings and deepwater moorings. He has over 30 years’ offshore industry experience.