Battling wear on chain joints in passive deepwater mooring systems
Fairlead design for floaters
Fairlead stopper - hull mounted.
In the next decade, as the offshore industry moves into ultra-deepwater frontiers, the number of floating production vessels will increase substantially. The new deepwater production vessels will demand new innovations in methods and equipment used for stationkeeping.
The mooring system technology used on mobile offshore drilling units (MODUs) will not meet the unique mooring requirements of floating production vessels. In particular, passively moored production vessels have no anticipated requirement to move and may not be equipped with a permanent tensioning system for jacking the mooring chain. The reliability and fatigue of the mooring line will
be more important in these applications because replacing a mooring line during oil production offshore is costly.
Conventional fairlead equipment designs are deficient in providing acceptable long-term chain wear for passive mooring. There have been several wear-related failure studies performed by classification societies and others that identify the high rates of chain and chain wheel wear caused by the underwater fairlead. The BarLatch (TM) fairlead stopper is one design believed to offer a better system for improving long term mooring line integrity and reliability on deepwater floaters.
This fairlead stopper innovation results from placing a chain stopper (to latch the chain) outboard of the fairlead and allowing the stopper to freely rotate about two perpendicular axes. All motion change between the mooring line and vessel occurs on proper bearing surfaces, not between the fairlead and the chain.
Chain link wear
Fairlead stopper - deck mounted.
Presently, on-vessel mooring systems mount the chain stopper at deck level and the fairlead is located underwater, attached to the hull. Choosing this arrangement for passive mooring will always leave the same links bearing on the wildcat or sheave of the underwater fairlead, and will promote the loss of material on these links. The link wear accelerates over time as the floating structure remains on station, while continuously working due to the constant movement of the structure as it responds to the forces of wind, wave, tide, and current. Even with relatively small motions, the contact stresses are extremely high in these localized areas of the chain, producing unacceptable chain wear rates, and fretting corrosion.
These destructive effects are caused by cyclical hinging action as the links immediately outboard of the fairlead rock onto and off of the chain wheel and rotate from changes in catenary angle. There is also considerable wear between the links and the chain wheel shoulders where the links contact. Also contributing to relative motion of the chain is the stretching and contractions of the vertical chain leg leading up to the chain stopper as line tensions vary.
Tension in the vertical chain leg between the fairlead and deck level chain stopper, combined with rotation of the fairlead, promote wear in the links above the fairlead as the chain twists. Fairlead rotation occurs as the platform yaws and weathervanes around its moored location. The same yawing motion rotates and slides the first free link on the outboard side of the fairlead as it overcomes the frictional resistance in the fairlead pivot.
Several years ago, the industry moved to larger diameter chain wheels as it was recognized that the fairlead was the principal element perpetuating chain wear and failures. The currently accepted practice is to use a seven-pocketed chain wheel. While this design approach does not solve the problem, it buys a little more time before the chain needs to be replaced, but this adds size, weight, and cost to the fairlead.
The conventional system uses a deck-mounted chain stopper. The stopper uses valuable deck space and requires extensive deck reinforcement (adding more weight) to take the full break strength of the chain at two locations, one on the hull and one at the deck.
Another method used to extend chain life is the periodic adjustment of the chain to move the affected links away from the critical contact area of the fairlead to reduce the cumulative effect. Such adjustments add operational burdens and some floaters do not have permanently installed tensioning equipment for this purpose. It should be noted that the periodic chain adjustments only spread the wear over more links. The link wear is somewhat reduced, but a single severe storm event can cause sufficient wear damage to adversely limit chain life.
In order to assure reliability of the mooring line, it is necessary to reduce the interlink wear action experienced at the underwater fairlead and its contact surfaces. Four years ago, Bardex design engineers began developing concepts for a swiveling fairlead device incorporating a chain stopper. As the development progressed, experience gained from the company's involvement in providing the Genesis Spar chain jack mooring stations, as well as information from another Gulf of Mexico Spar installation, were incorporated. The new design resulting provides the modern solution to chain link wear on floating production vessels when the mooring line is rarely moved.
The principal components of the fairlead stopper are a horizontally swiveling fairlead and a vertically pivoting chain stopper mounted outboard of the fairlead. The device will rotate freely in both the vertical and horizontal directions in response to environmental changes so the relative motion between the vessel and mooring line occurs at the fairlead stopper. The design configurations can be modified to suit specific applications and installations.
The new fairlead stopper device is designed as a permanently installed unit, which does not require maintenance or replacement. As such, fatigue life and bearing design is based on a 30-year life cycle. The system consists of two principal elements - the chain wheel housing and the latch housing assembly (chain stopper). The chain wheel assembly is essentially a fairlead with a chain wheel and support structure. The housing swivels about the vertical axis and makes use of self-lubricating bronze bearings. The bearings are mounted in the ears, which protrude from the chain wheel housing casting.
The chain wheel need only be a five-pocket design, since the maximum chain tension around the chain wheel is the line pre-tension (about 20% of chain break strength) during mooring line installation. The chain tension over the wheel is removed when the chain stopper is engaged. The chain in contact with the chain wheel surface is not being continuously exposed to the mooring line tension during most of the life of the platform.
The latch housing assembly is a chain stopper suspended from four structural support legs. Additional self-lubricating bronze bushings are used in the connecting ears for attaching the latch housing assembly.
The chain stopper incorporates two articulating latches of essentially identical design as those used in Bardex linear chain jacks. The latch mechanism is designed to be self-locking so the chain tension load forces the latches to close. The latch bars contain machined contoured pockets providing excellent chain support.
The chain mechanically opens the latches when it is pulled through the latches. The latches are spring loaded to close. When hauling in chain, the latches "ratchet" to open and "spring" to close. The latches are designed to be pinned in the open position by a diver or remotely operated vehicle, should chain payout be required. An optional hydraulic latch actuator may be incorporated to achieve remote activation of the latches from the deck, if desired.
A chain guide shoe is bolted onto the lower end of the latch housing below the latches. The guide shoe is a "V" shaped funnel, which contacts the chain links. It is used to lead the chain connector and chain into the latch housing assembly during installation and it maintains the stopper latch alignment. When the chain is hauled in, the chain stopper mounted on the latch housing assembly is free to follow the increasing catenary angle taken by the chain. The fairlead stopper permits passing of wire rope, chain, kenter shackles, and any special connectors that may be required by the mooring line installation scheme. An optional load measurement system can be supplied to measure tension in the anchor chain being held by the latches.
While the fairlead stopper has a 30-year design life, the whole assembly can be detached from the floater's column and retrieved for maintenance/repair. Removal of the system is simplified by an optional pivot pin design, allowing removal without removing the vertical pin. The retrieval involves use of an underwater vehicle and a specially designed retrieval frame built around standard guide wire technology.