The seismic research vessels ‘Ramform Sovereign’ and ‘Ramform Sterling’ are equipped with towing and spreader ropes made from Dyneema® ultra-high molecular weight polyethylene fibre. These are among the latest in a large fleet operated by Petroleum Geo-Services (PGS), one of the world’s leading offshore services companies. The Ramform Sovereign and Ramform Sterling each deploy up to 20 seismic streamers.

PGS specialises in ‘reflection seismology’ services, using especially equipped vessels fitted with a series of tethered hydrophone arrays that can reach lengths of up to 12km. One of its competitive advantages is its ability to achieve higher resolution data than its competitors, by towing a denser array of sensors. To help reduce the weight of these arrays, PGS has specified UHMWPE fibre of DSM Dyneema for a wide range of towing and other ropes, for the use on vessels throughout its fleet. Applications include super-wide towing ropes, streamer spread ropes, various bridling systems and fabricated multiple splice connections.

“Ropes made with Dyneema are very strong, light-weighted, and resistant to harsh environments,” says Brad Bertsch, mechanical supervisor at PGS. “They can be easily spliced, have very little creep, have very low elasticity and are resistant to chemicals and corrosion. And ropes made with Dyneema are easy on our ships’ equipment, sheaves, and winches.”

Industry reports show that towing lines made with Dyneema fibre require replacement only every two years, whereas steel-wire ropes used in seismic applications typically have a lifetime of just four to five months.

Safety in use provides a further, critical advantage for ropes made with Dyneema. With extremely long lines under very high tension, steel lines always carry the risk of ‘snap-back’ if a wire breaks. Dyneema’ helps to virtually eliminate this threat due its low elongation qualities, a factor that also aids in the precise placement of lines and equipment. Resistance to tension and bending fatigue weigh further in favour of Dyneema.

Dyneema is less dense than water. Bertsch says the buoyancy that this provides is another major advantage over steel cable in seaborne applications. Furthermore, the low weight and low diameter, of the ropes made with Dyneema played an important part in PGS being able to fit greater lengths of rope onto existing winches, and also in controlling costs, such as those associated with fuel usage, that are impacted by towing weight.

“The use of rope made with Dyneema in the Seismic industry with PGS is still growing,” says Bertsch. “There are always new ideas coming out where Dyneema can be used one way or another. PGS is committed to continual technical improvement, and lightweight Dyneema falls clearly into that category.”

“We started adopting this technology in 1994. The superior performance of the ropes, their greater safety in operation and their extended lifetime compared to steel wire make Dyneema fibre an obvious choice for rigging our fleet.”