more closely examined for off shore 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 fi rst polyester mooring pre-laid on the sea fl oor for a permanent mooring in the Gulf of Mexico. Polyester mooring rope design, manufacture, installation and maintenance are guided by API RP 2SM. T e original version of the document was issued in 2001, with an addendum issued in 2007. T e second edition of the document has been approved, but not yet published. Promoted for off shore applications almost from its inception,
Kevlar fi bers (a type of Aramid) were seriously considered for off shore mooring ropes from the early 1970s to the mid 1980s. T ese types of ropes made aggressive entry into the off shore working ropes market, but had not yet been accepted for application in a pre-set mooring arrangement. After great eff ort, the fi ber was accepted for use in a pre-set mooring for the Ocean Builder 1, which was installing the Lena guyed tower. T e 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 fi eld location and subsequent hookup (~ 4 - 6 weeks after presetting the moorings), the ropes failed during the tensioning of the system to set the anchors. T e loads when the lines failed were well below the designed breaking strength. T is set back the acceptance of synthetic fi ber ropes for off shore moorings signifi cantly. T e failure was attributed to the poor axial compression fatigue of typical Aramid fi bers. Recently, other fi bers for consideration in off shore mooring applications include a new version of HMPE called DM20. T is has the same low specifi c gravity of other HMPEs with far better creep properties. T is fi ber is just entering the market and it may be many years before it fi nds wide use in the industry because of the impact of the 1983 failures.
CONNECTOR DEVELOPMENTS For many years, D-Shackles, Kenter links and Baldt connectors have been used to connect tension members. T ese 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). T ese 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. T e 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 fi eld. Another connecting device for off shore 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 fl oes. 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. Specialty monitoring devices that report mooring line tensions to the vessel operator include the recently developed Inter-M Pulse connector. T is device monitors the line tensions in the system and transmits the information to the vessel periodically, allowing the operator greater confi dence in his moorings, especially in the case of heavy storms or hurricanes.
HURRICANES In 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.
SEPLA being loaded into follower and ready for deployment.
ANCHOR DEVELOPMENTS Anchoring 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. T e 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).
CONCLUSIONS Today’s mooring systems utilize the best technologies and proven concepts available. T ough they are a critical component in the safety of the rig during drilling or production operations, mooring systems are often expected to perform fl awlessly 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 fulfi lling their purpose. T e industry has changed and continues to change, but the commitment to safety and excellence remains the same. T e 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 off shore moorings be in the next 50 years? ❙
Article Reprinted with Permission from Oilfi eld Technology WIRE ROPE EXCHANGE MAY-JUNE 2014 27
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