Synthetic DYNEEMA Ropes for Deepwater MODU

With the increase in deepwater O&G exploration, the demand placed on mooring ropes for MODUs (Mobile Operating Drilling Units) has increased dramatically. Rope manufacturers have been developing products based on new materials and also adapting their manufacturing processes to accommodate new materials and more stringent requirements.

In some deepwater plays mooring ropes will be going down to 2,000 or even 3,000 meters, which places massive stress on the ropes, due not only to its length and weight but also due to the action of subsea currents, ship movement caused by surface waves, currents and wind.

Dutch rope manufacturer Lankhorst Ropes, which has over 200 years of experience in producing maritime ropes, was recently awarded its first worldwide order for Dyneema synthetic mooring ropes by Brazilian national O&G operator Petrobras, these super strong deepwater mooring ropes are to be used on MODUs working the deep and ultra-deepwater pre-salt plays off Brazil. 

The Lankhorst designed deepwater mooring rope GAMA 98 made with Dyneema is manufactured from high efficiency sub-rope cores laid parallel within an outer braided jacket. The sub-cores are high efficiency eight strands braided Dyneema ropes. The external jacket is a 32-carrier twill polyester braid. This construction is torque balanced and is one of the strongest rope constructions currently available in the market. The breaking load is the same whether the rope is wet or dry and considerably higher than nylon or polyester size for size. The characteristics of high strength, low weight, make this rope easy to handle with consequent decrease in operational costs. The polyester jacket confers to the rope an excellent resistance to abrasion.

An interesting aspect of another GAMA 98 rope model, this one made of polyester is the particle filter elements included between the jacket and sub-rope cores. This is done to filter out particles greater than 5 microns, yet still allow free flooding of the rope. Filter systems can also be provided to allow the ropes to be pre-installed on the seabed prior to vessel hook-up.

The fatigue life of polyester rope is supposed to be approximately 80 decades superior to steel wire rope. The fatigue life curve of GAMA 98 rope was established during the durability of polyester in Joint Industry Projects.

The axial stiffness of these ropes is attained by the ratio of rope load to strain variations between the lower and upper stresses imposed on the ropes during testing. According to Lankhorst the GAMA 98 model is probably one of the stiffest ropes currently available for deepwater tether applications, resulting in lower overall elastic stretch. The advantages from this are translated into a lesser platform offset and lower pre-tensioning during installation.

Some of the main O&G projects using Lankhorst manufactured GAMA 98 ropes are the Tahiti spar for Technip and operated by Chevron, Thunder Hawk semi-submersible for SBM Atlantia operated by Murphy and the Cascade & Chinook FPSO for APL/BW Offshore operated by Petrobras Americas. Now we may be seeing a lot of Dyneema ropes in the Brazilian deep also.

Claudio Paschoa

The Thunder Hawk Deep Draft Semi FPU moored in the Gulf of Mexico with Lankhorst Ropes Gama 98 ropes.

Lankhorst Ropes rope braiding machine for production of Gama 98 ropes.