Milijan Mudrinic reveals the best practice with regard to offshore oil platform targets
Local reference sensors have undoubtedly played a key role in improving the safety and uptime of Dynamic Positioning (DP) operations. The CyScan (laser) and RadaScan (microwave) reference sensors are two examples commonly found on offshore supply vessels. Like all reference sensors these rely on targets being deployed on the platform to enable station keeping and initiate lift operations, which often last many hours.
Regardless of the sensor technology being used, the performance of the local reference is dependent on the number of targets deployed, their position on the platform and their condition. A poorly positioned target is one that doesn’t offer continuity of DP operation from the moment the vessel enters the platform’s 500m safety zone to the moment where supplies are being transferred. Similarly, a target that can easily be obstructed by a container being lifted onto the platform is not in an optimum position. Where the crane configuration on the platform is such that no single position offers all the necessary coverage or a relative vessel heading must be maintained (e.g. FPSO), it may be necessary to deploy multiple targets and provide clear instruction to the vessel as to which target will best suit the circumstances. The position and number of targets are the main preconditions but poorly maintained laser tube targets (most commonly installed on platforms due to their low cost) covered with dirt are of little use when they can’t be easily acquired and tracked by a sensor at the necessary range.
It should come as no surprise that the best performance and coverage is achieved when the platform takes responsibility for the deployment of targets. Only through careful planning during target installation and regular maintenance work can we ensure that Platform Supply Vessels (PSV) operations benefit from a good local reference. Guidance Marine recommends that each platform undergoes installation and regular maintenance inspections on all of the instrumentation (targets) to ensure maximum efficiency of the reference sensor.
Unfortunately, experience shows that it is often the responsibility of the vessel to deploy targets on the platform as the first step in the supply operation and to recover them at the end. The targets are not always handled with care (at best damaged, at worse lost) and their location may not be optimum or may differ between vessels, which requires repositioning during the operation. It is not unusual for platform personnel to hang targets off the railing with ropes, leaving them moving in the wind and potentially causing loss of signal or poor performance on the DP system, which could compromise the PSV station keeping position. One of Guidance Marine’s objectives for 2015 is further engagement with the oil majors and industry regulatory bodies to produce best practice guidelines for safe target installation.l
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Milijan Mudrinic is with Guidance Marine in Leicester, UK