Taking stock, Part 1

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.

At the start of the 20^th century, oil and gas was a fast-paced burgeoning market. Just a few years previous (1896), the first submerged oil wells in salt water were drilled off the coast of Santa Barbara (Summerland), California. Within the next 10 - 15 years, drilling from fixed structures occurred in the Gulf of Mexico in the tidal zones of Louisiana and Texas. It would be more than 50 years before the first mobile offshore drilling unit called Mr. Charlie (1953), and the first 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. The concept of using a semi-submersible hull to provide a stable drilling platform offshore, and the concomitant need for offshore moorings, was born.

Initially, the structures were kept stationary by ballasting down until the hull was on the seafloor (the submersible). It was not long until drilling in deeper waters made it necessary to moor the rig using wire rope and/or chain. These tension members would be replaced by synthetic fibre ropes made from polyester in the 1990s. Cesar Del Vecchio investigated the mooring of floating structures using polyester mooring ropes during his doctoral programme at the University of Reading. His work, published in 1992, helped Petrobras know how to be the first to use synthetic ropes for offshore 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 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. There 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 find 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, offshore moorings were dominated by oil rig quality (ORQ) stud-link chain, and wire rope. The weight of these tension members worked very well in shallow waters because the vessel offsets 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 standards issued by API include Spec 9A Specification for Wire Rope (11^th edition as of 2012) and RP 9B Application, Care, and Use of Wire Rope for Oil Field Service (13th edition as of 2012). These standards refer primarily to wire rope use in land based, dry applications. Remarkably, there is not currently an API Standard that addresses the large sizes and constructions of wire rope used for offshore mooring systems. The development of such a standard is within the scope of Resource Group 2 (the body within API that is responsible for development of standards relating to offshore moorings) in the near future.

Over the years, raw materials for wire rope and chain grew in quality. With the development of more accurate and efficient heat treatment processes, chain links and wires could be produced with higher grades and strengths (i.e., R-4 and R-5 Chain: EIPS, EEIPS and EEEIPS wire ropes). Low torque jacketed spiral strand wire was also developed which had better performance long-term than standard 6x36 IWRC wires. All of these higher strength components made it possible for many rigs to meet the newer, stricter, design requirements without a total refurbishment of the mooring fairleads and chain handling equipment.

Chain use in offshore applications is guided by API Spec 2F Specification for Mooring Chain (6th edition in 2010). The first edition of this document came out in 1974. Inspection of chain quickly emerged as a critical process in controlling the quality and safety of offshore mooring installations. API RP 2I Recommended Practice for In-Service Inspection of Mooring Hardware for Floating Drilling Units (1^st edition 1987, 3^rd edition 2008) gave additional guidance on how to inspect the quality of chain, connectors, wires and synthetic ropes in a mooring system to ensure system integrity.

Part 2 of this article can be reached here.

Adapted by David Bizley

Read the article online at: https://www.oilfieldtechnology.com/exploration/23012014/taking_stock_part_1/