Ropes made with Dyneema®, ultra-high molecular weight polyethylene (UHMWPE), are being successfully used to secure rigs, mobile offshore drilling units (MODUs), buoyant turret mooring (BTM) systems and similar constructions in the offshore oil and gas industry. Users are finding them invaluable when original equipment steel wire rope (SWR) and chains that secure mooring legs break or suffer damage. The back-up lines made with Dyneema are installed as a safety feature until regular maintenance can be carried out.
Dyneema manufacturer DSM Dyneema cites the example of the Chinese floating production, storage and offloading vessel (FPSO), the NanHai KaiTuo. The vessel operated in the Xijiang Oil Field at the mouth of Pearl River about 130km from Hong Kong in the South China Sea. The oil field is operated by ConocoPhillips and the China National Offshore Oil Corporation, CNOOC.
An inspection of the NanHai KaiTuo by a remotely operated vehicle (ROV) found broken wires on two of the SWR mooring legs of its BTM system. SBM Services, a service provider to the offshore industry, was requested by ConocoPhillips to design and supply a back-up system consisting of a special connector to fit a positively buoyant back-up line made with Dyneema. Installation of this back-up system was required in case of a SWR failure in one of the damaged composite anchor legs.
The back-up system is designed to be capable of retaining the FPSO in position and maintaining production, gaining time for ConocoPhillips to mobilise the required vessels and equipment to replace the broken wire leg, in case one of the wires fails due to its reduced strength.
SBM Services verified that the connector and the back-up line in its new configuration could withstand a load of 580t, the maximum likely in the event of a broken leg.
Bexco, a Belgium-based rope manufacturer, designed and manufactured the back-up line with Dyneema. Lines made with Dyneema were selected in preference to SWR and other synthetic materials because of its high strength and low weight. The 1,000t minimum break load (MBL) line consists of parallel laid cores of Dyneema SK78 yarn protected by a polyester cover. It was designed for a minimum of one year under full load. Performance is not affected by sea water, nor does it rot.
For ConocoPhillips, the main reasons for choosing lines with Dyneema were its low weight, neutral buoyancy and high durability and strength compared with alternative synthetic materials. Furthermore, the ability to do the installation job quickly played a role in the decision making process. Installation of the back-up lines was carried out by divers, because an ROV would not have been able to get close enough to the wires to perform the action. The buoyancy of the lines meant the divers could do their job in a fast and safe manner.
ConocoPhillips used the lines for 15 months to its complete satisfaction, before the FPSO NanHai KaiTuo was decommissioned during 2010. The lines are now in storage, ready for further use if necessary.