JONATHON MILLER, FLORENCE KOSMALA AND BOB WILDE, INTERMOOR, INC., USA, PROVIDE A HISTORICAL REVIEW OF THE OFFSHORE MOORING INDUSTRY AND LOOK AT WHAT THE FUTURE HOLDS FOR THIS FIELD.
TAKING STOCK
At the start of the 20th century, oil and gas was a fast-paced burgeoning market. Just a few years previous (1896), the fi rst submerged oil wells in salt water were drilled off the coast of Santa Barbara (Summerland), California. Within the next 10 - 15 years, drilling from fi xed structures occurred in the Gulf of Mexico in the tidal zones of Louisiana and Texas. It would be more than 50 years before the fi rst mobile off shore drilling unit called Mr. Charlie (1953), and the fi rst purpose built semi-submersible drilling unit the Ocean Driller (1964), were delivered.
While towing the Ocean Driller to its intended location for Shell Oil Company, the crew noticed that the vessel motions were low when the draft of the semi-submersible was about halfway between the pontoon and the bottom of the deck. T e concept of using a semi-submersible hull to provide a stable drilling platform off shore, and the concomitant need for off shore moorings, was born. Initially, the structures were kept stationary by ballasting down until the hull was on the seafl oor (the submersible). It was not long until drilling in deeper waters made it necessary to moor the rig using wire rope and/or chain. T ese tension members would be replaced by synthetic fi ber ropes made from polyester in the 1990s. Cesar Del Vecchio investigated the mooring of fl oating structures using polyester mooring ropes during his doctoral program at the University of Reading. His work, published in 1992, helped Petrobras know how to be the fi rst to use synthetic ropes for off shore moorings. Station keeping became increasingly important with deeper water depths. Early moored systems used all chain or chain and wire rope in a catenary arrangement, part of which always lies on the seabed. Very few MODU moorings were designed, but were
Photos courtesy of InterMoor, Inc.
instead based on the mooring manual for the vessel; if the manual said they could moor within the depth of the site, no procedures were required or needed. Mooring teams did not engineer a mooring design or perform analysis for every location. By comparison, a very large percentage of the MODU installations are now engineered. T ere are government regulations and API Standards that mandate mooring system design. As deeper waters were attempted, the support vessel capabilities in terms of bollard pull, winch braking and chain locker capacity made it necessary to fi nd a more engineered solution as the weight of chain or wire that was paid out exceeded the holding and towing capacities of the winch brake and vessel bollard pull.
TENSION MEMBERS In the beginning, off shore moorings were dominated by oil rig quality (ORQ) stud-link chain, and wire rope. T e weight of these tension members worked very well in shallow waters because the vessel off sets would make it possible for the change in weight as the mooring leg came off the seabed to act as a restoring force in the horizontal direction. Wire ropes were commonly used (i.e. 6x36 IPS WS IWRC).
WIRE ROPE EXCHANGE MAY-JUNE 2014 25
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