The mooring of rigs in congested, deepwater areas of the Gulf of Mexico is attracting greater attention now that violent storms have become more common.
Preset mooring systems for drilling rigs have been around for almost two decades. However, they have grown in favour over the past couple of years among oil companies operating in the Gulf of Mexico. Many see them as the best way of optimising the operation and movement of these costly assets, especially in congested and ultra deepwater areas.
In this article, we examine some of the factors behind the current trend toward preset systems with the help of Todd Veselis, project manager with InterMoor. For the past three years, Veselis has been deeply involved with the mooring and frequent moving of the Transocean Marianas, a Sedco-700-type semisubmersible drilling rig designed for harsh environments and water depths to 2130 m. Under a contract between Transocean Offshore Deepwater Drilling (Transocean) and BP, the Marianas is involved in a mixture of exploration and development drilling plus well intervention work. BP prefers the rig to be secured with polyester preset mooring lines rather than the rig’s own wire-and-chain mooring system.
BP has moved the Transocean Marianas at least eight times over the past three years between six different locations in the Gulf of Mexico – three times in quick succession during one particularly busy sixmonth period. Each time the rig has been moored with 8 or 12 pres lines, the latter during the hurricane season and as consequence of risk-assessment studies demanded by both the American Petroleum Institute and the Minerals Management Service.
InterMoor leases BP polyester rope, enough to make up as many as 24 mooring lines, and up to 24 suction embedded plate anchors, or SEPLAs, as they have become known. Obviously, this means the rig can be moved from one location to another, even during the hurricane season, without having to transfer the moorings with it.
At first sight it might seem extreme to have two complete sets of moorings in addition to the rig’s own system, so what exactly lies behind this mode of operating the Transocean Marianas?
Well, time has something to do with it, though the thinking is a little more involved, as Veselis explains: “The complete rig move cycle using the preset system is unlikely to be shorter than using the rig’s own wire and chain. It takes several weeks to preset the anchors and lines, and then to pick them up after the rig has moved on. However, the rig can still be working during these periods; essentially, the time needed to preset and pick up the moorings is off the critical path. This gives us some flexibility; we can organise this work to fit with the available boat and crew schedules. Although there is a downside: we can lose the boat at short notice if there is a pressing call from somebody else for its services.”
Veselis also does not claim that the core time to disconnect the rig from one lot of preset moorings, move it, and hook it up to the next set will necessarily always be shorter than the corresponding exercise using the rig’s own mooring system. The weather, the number of available boats, the seabed conditions and the existing infrastructure influence both types of move. However, this is where the preset mooring system starts to look attractive: setting the moorings before the rig arrives enables them to be positioned in the vicinity of any seabed infrastructure relatively easily and generally with less chance of damaging it, when compared with setting the rig’s own system at the time of the move.
“Using the preset system, the move can be planned more thoroughly and with more confidence,” says Veselis. “And because the main mooring components are already installed and the polyester ropes are lighter and easier to handle than chain, the move is more straightforward and simpler to control. Further, there is much greater assurance that the preset system will provide a dependable mooring for the rig because it can be checked beforehand. What it amounts to is that you are faced with fewer potential hazards; the risk of the rig being exposed to the weather for longer than necessary is reduced; and the chances of incurring costly delays are diminished.”
Veselis also points out that the polyester lines and SEPLAs provide a lighter, more reliable mooring, and that by presetting them you can more easily customise the layout to the location, hence providing the optimum restraint for the rig – the best basis for its efficient operation. If any further arguments needed to be made for the preset system, he asserts that presetting 12 mooring lines and attaching them to a rig designed with an eight-point wire-and-chain system is far more straightforward than upgrading the rig and expanding its eight-point system to enable it to set 12 wire-and-chain mooring lines.
It is clear that opting for preset moorings for drilling rigs is not simply about time and equipment costs. As is often the case offshore, risk is the central issue: to schedules, safety and the asset and its smooth operation. When these factors are taken into account, even without a formal analysis, the case for preset moorings for drilling rigs seems compelling.
For InterMoor, what is essentially an ongoing project surrounding the moving of the Transocean Marianas is undoubtedly good business – though hard work. Veselis says there has been hardly a moment over the past three years when his engineering team has not been considering or planning a move; the operations people have not been gearing up to set or pick up moorings; and those responsible for the overall logistics have not had to be on their toes to respond to BP’s operational and schedule needs, which can change at short notice.
“It calls for a close relationship between ourselves and our customer, and a good deal of flexibility,” says Veselis. “It is satisfying to know that our work with the preset mooring system has contributed to the safe and efficient operation of the Transocean Marianas. We have developed a deep sense of responsibility for the rig over the three years we have spent moving it about – you could say we have become strongly attached to it.”
The Transocean Marianas’s move cycle
A Transocean Marianas move begins when InterMoor is given the new rig location by BP. Engineers carry out a site investigation focused on the water depth, the seabed conditions, the metocean data and any existing oil and gas infrastructure in the area. Then, taking into account the design criteria, they conduct a mooring analysis to determine the optimum mooring layout at the site. Detailed procedures are written for presetting the anchors and the polyester lines, and for hooking up the rig to the moorings. These go to Transocean and BP before being submitted to the Minerals Management Service for approval.
InterMoor crews preset the anchors at the new location; each takes about 12–18 hours. The time for the whole operation depends on the weather, the nature of the seabed and, naturally, the number of anchors to be set (8 or 12) and the anchor-handling vessels used. A single boat can typically sail with six to eight SEPLAs. The polyester lines are then preset and left on the seabed; it is normally possible to lay two lines per day. This exercise is far less weather-dependent than anchor installation.
The rig is disconnected at the old location, moved and hooked up to the preset moorings at the new one. The times for these operations depend on the number of boats available, but three or four days at each end is not unusual, especially if there are 12 lines involved. An InterMoor rig-move coordinator and a mooring engineer are present on the rig throughout the process.
The abandoned moorings are picked up. The polyester lines can be retrieved in roughly the time it took to set them; SEPLAs can be recovered in as few as four hours each. Assuming good weather, the whole pick-up generally takes one to two weeks with a single boat.
The mooring system, lines, anchors and associated hardware are taken to InterMoor’s Fourchon yard for inspection and any repair and maintenance.