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www.sname.org/sname/mt October 2013 a desired position. In fact, a common test performed on DP ves- sels is a 20 m box maneuver with 90-degree rotations at each leg; this often produces deviations of less than 1 m on return to the original position. Almost every aspect of marine vessels engaged in o shore energy activities now use DP systems: drilling units, supply ves- sels, shuttle tankers, construction vessels, and so forth. Before DP, the only option for position holding over time was moorings. While this is a proven technology, moorings are limited in depth, they require handling and engineering design for each location, and are vulnerable in severe weather (in contrast to a DP vessel, which can merely terminate activities and depart). DP, while signi cantly advantageous over conventional meth- ods, is not risk free. Control systems are only as good as their design, maintenance, and operation. A failure of a DP control system or an associated ancillary in whole or in part, can result in either a failure of thrust or unwanted thrust. At the extreme, a total thrust failure (drift o ) or even worse, a failure to thrust- ing o position, (drive o ) can occur. In these circumstances, the industrial mission will almost certainly stop; or, in a worst-case scenario, damage to asset, environment, personnel, or all three may arise. In the case of a drive o , little time exists for response and there is great dependence on the action of the DP operator (DPO) to intervene correctly. Considering the operation ere are numerous scenarios in which the inability to control position can prove very costly. For example, a mobile o shore drilling unit is connected to a well by a drillstring. A signi cant position loss without disconnection will cause subsea asset dam- age and have environmental and possible safety consequences. Indeed, the potential for a major disaster arises. Similarly, an unwanted position movement in a dive boat has the potential to put divers in immediate danger. For construction vessels, it is dependent on the actual ves- sel. A DP-signi cant incident on a pipelaying vessel may cause a pipeline buckle. An event on a heavy lift may cause a loss of load control. Whatever the vessel, the outcome will more than likely result in asset damage, project delays, and possibly a cata- strophic incident. e development of IMO MSC 645 was in recognition of the growing number of DP incidents as the DP eet expanded. It de ned a set of standards upon which DP vessels are measured depending upon their robustness of design (DP1, DP 2, or DP3). is initial standard, issued in 1994, remains in e ect to this day and it is the basis upon which class rules for DP are written. IMO MSC 645 de nes equipment class as 1, 2, or 3. To para- phrase: For DP equipment class 1 (DP1), a position loss after a single failure can occur. For DP equipment class 2 (DP2), fail- ure of any active component will not result in a loss of position. For equipment class 3 (DP3), a position loss will not occur after a failure of either an active or passive component or re/ ood in a single compartment. Launched in 2007, the 280 ft. Chloe Candies is an inspection, maintenance, and repair vessel equipped with a DP2 dynamic positioning system, a large moonpool, and a 100- ton deep-sea winch. Photo courtesy of Otto Candies. 04RISK UNDERSTANDING AND RESPONSE CAPABILITY The DP operating manual is a support document that details practical arrangements for operating the vessel in line with an FMEA.