Spar buoys (which look like floating upturned logs) pose huge problems where lifting is concerned. The larger of the two has a hull over 200m in length and 46m in diameter. Spars provide buoyancy to support storage and “topsides” facilities, which, in this case, comprised oil production and processing facilities, a drilling rig and accommodation.
Some 110,000 barrels of oil plus 150 million cubic feet of gas per day are produced from the first spar buoy, while over 100,000 barrels of oil plus 60 million cubic feet of gas is produced each day from the second, which is located in 4,500ft of water.
Both offshore production vessels were made possible by Bardex Corporation, which designed, manufactured and supplied the specialist heavy load handling equipment that pulled in the steel catenary risers (SCRs) – large pipes that connect the subsurface wellheads on the ocean floor with the production facilities on the spar.
In this particular application, Bardex designed its linear chain tensioner to pull-in each SCR with tension loads of up to 350t. The chain jack was also designed to support a load of 750t – this being the worst-case scenario of a “flooded” SCR.
As the catenary riser was pulled into the production spar buoy, its catenary angle was constantly changing so Bardex designed a special A-frame support for its chain jack providing a double-trunnion mounting so that the chain jack could swing in two degrees of freedom to always align with the SCRs.
Two wire line winches were used to provide “steering” of the SCRs so that they could be guided into their permanent location, a receptacle, on the periphery of the spar buoy.
Bardex chain jacks are designed to be fail-safe so that even in the event of a complete hydraulic power failure, the chain pull-in line to the SCR is always held securely no matter in what part of the pull-in or pay-out part of the cycle the failure occurs.
The main lift cylinders were hard-piped with in-line load holding valves, which retain cylinder pressure in the event of any catastrophic sudden loss of pressure from the hydraulic power unit.
SCR pull-in systems are “temporary” load handling systems, which need to be easily moved to different planned locations on the spar buoy, or production vessel, and in many cases they will be entirely removed to shore once all SCRs are connected.
Size and weight are often crucial to effective deployment and removal. Rotary chain windlasses or wire line winches to handle the stall and pull-in loads are often just too large and heavy, typically weighing over 35t whereas the Bardex system specified for this application was less than 15t and can also be broken down into sub-assemblies for ease of handling.
In addition to SCR pull-in systems, Bardex linear chain tensioners are used in the shipyard industry for shiplift systems and in the offshore oil and gas industry for tensioning tendons for mooring systems on “permanently” moored vessels such as spar buoys, FPSOs (floating, production, storage and offshore loading vessels) and production semi-submersibles.
Bridge and specialist module lift applications have also been successfully undertaken. Their relatively small size and low weight make them systems of choice for specialist applications where high reliability and dependability is essential.
