A t the start of the 20th 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.

Taking stock

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.

Reprinted from Oilfield Technology January 2014

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 membersIn 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 (11th 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 (1st edition 1987, 3rd 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.

Then came disco and polyester…In the mid-to-late 1970s, there was a flurry of activity in the mooring community as attempts to reach deeper and deeper depths became more financially viable. Synthetic fibre ropes manufactured by a variety of rope companies were being evaluated and assessed. Fibres considered in the initial evaluation included nylon, polyester, and aramid fibres because of the high global production capacity. Other fibres such as polypropylene, high modulus polyethylene (HMPE such as Spectra or Dyneema), and liquid crystal polymer (LCP such as Vectran), were not considered because of performance or production limitations. In the ensuing years, there were many experiences that led to today’s understanding of good mooring practices.

Figure 1. Catenary (left) and Taut Leg (right) moorings.

January 2014 Reprinted from Oilfield Technology

Nylon ropes were among the first man-made fibre ropes used offshore entering in towing applications in the 1950s. Because of their low stiffness and high elongation characteristics, they made a great product for applications where loads would vary widely at very rapid rates. Unfortunately, because of the creep and wet characteristics of the fibre (namely strength loss when wet), they were not accepted for deepwater moorings.

Polyester fibres quickly emerged on the offshore mooring front because of their better strength performance and lower elongation than nylon. Polyester fibres are readily available thanks to the large capacities for production that resulted from the leisure suit fashion of the disco era (actually, airplane tires and other industries pushed the development of polyester fibre manufacturing capacities). Polyester had excellent fatigue properties and after several years of scepticism, has gained wide acceptance and is currently the most commonly used man-made fibre for offshore mooring ropes. During the 1980s many joint industry projects (JIPs) were formed where polyester was more closely examined for offshore moorings. First adoption of the technology for permanent moorings in the Gulf of Mexico happened on the Mad Dog and Red Hawk Spars to extend the water depth. One reason to use polyester is to reduce the risk of damage to subsea infrastructure in a failed line scenario. In 2009, InterMoor installed the first polyester mooring pre-laid on the sea floor for a permanent mooring in the Gulf of Mexico.

Polyester mooring rope design, manufacture, installation and maintenance are guided by API RP 2SM. The original version of the document was issued in 2001, with an addendum issued in 2007. The second edition of the document has been approved, but not yet published.

Promoted for offshore applications almost from its inception, Kevlar fibres (a type of Aramid) were seriously considered for offshore mooring ropes from the early 1970s to the mid 1980s. These types of ropes made aggressive entry into the offshore working ropes market, but had not yet been accepted for application in a pre-set mooring arrangement. After great effort, the fibre was accepted for use in a pre-set mooring for the Ocean Builder 1, which was installing the Lena guyed tower. The moorings were pre-set, a common practice to reduce expenses, and remained in a loose buoyed condition for several weeks prior to vessel hookup. Upon arrival (in 1983) at the field location and subsequent hookup (~ 4 - 6 weeks after presetting the moorings), the ropes failed during the tensioning of the system to set the anchors. The loads when the lines failed were well below the designed breaking strength. This set back the acceptance of synthetic fibre ropes for offshore moorings significantly. The failure was attributed to the poor axial compression fatigue of typical Aramid fibres.

Recently, other fibres for consideration in offshore mooring applications include a new version of HMPE called DM20. This has the same low specific gravity of other HMPEs with far better creep properties. This fibre is just entering the market and it may be many years before it finds wide use in the industry because of the impact of the 1983 failures.

Connector developmentsFor many years, D-Shackles, Kenter links and Baldt connectors have been used to connect tension members. These connectors are still readily accepted for temporary moorings (i.e., MODUs), but for more permanent installations, a variety of new equipment has been developed in the past 25 years. Ball and taper connectors were developed by companies such as the predecessors to First Subsea, BallTec, and Subsea Riser Products (SRP). These connectors make it possible to easily connect two ends of rope or chain using an ROV.

Another type of hardware that was developed, in about 1995, for connecting tension members in permanent mooring systems is the H-Link. Deriving its name from the shape of the connector, the H-Link is produced today by InterMoor and Oceanside. The record for the largest H-link (7 t) and the highest break load H-link (2600 t) is currently held by InterMoor/Oceanside, and was deployed on the Chevron Jack & St. Malo field.

Another connecting device for offshore moorings is the RAR connector (InterOcean Services). Used extensively in Arctic environments during the late 1970s and early 1980s, the RAR connector allows quick disconnection of the moorings when exposed to dangerous ice floes. When the vessel needs to disconnect, an acoustic signal is transmitted through the water to the connector and it is released allowing the vessel to drift out of the path of the advancing ice.

Speciality monitoring devices that report mooring line tensions to the vessel operator include the recently developed Inter-M Pulse connector. This device monitors the line tensions in the system and transmits the information to the vessel periodically, allowing the operator greater confidence in his moorings, especially in the case of heavy storms or hurricanes.

HurricanesIn the aftermath of a number of hurricanes in the 1990s and 2000s, the development of API standards accelerated greatly. After hurricanes Katrina and Rita, ABS formed a JIP to address the mooring failures through major revision of the standards.

Anchor developmentsAnchoring options have progressed dramatically from the early days. Initially, anchors looked much like those you would expect to see on a shipping vessel or tattooed on the forearm of a sailor. With time, the anchors added larger and larger surface areas. The drag embedment anchor has a variety of variations from the Stev- and MK-series anchors, Vertical Load Anchors, and Drag Embedment Near Normal Loading Anchor (DENNLA). Other methods developed in the past 30 years include the free-falling gravity embedment option (i.e., torpedo pile) and suction embedment options (i.e., suction piles, SEPLA).

ConclusionsToday’s mooring systems utilise the best technologies and proven concepts available. Though they are a critical component in the safety of the rig during drilling or production operations, mooring systems are often expected to perform flawlessly without any regularly scheduled maintenance or inspection. ‘Set it and forget it,’ seems to be the mantra. As simple mechanical systems, they require limited but periodic attention to ensure they are still suitably fulfilling their purpose. The industry has changed and continues to change, but the commitment to safety and excellence remains the same.

The industry, has come a long way, and it will be exciting to see how new technologies emerge and gain widespread acceptance in the future. Where will offshore moorings be in the next 50 years?

Figure 2. SEPLA being loaded into follower and ready for deployment.