Improved Mooring Tension Accuracy
AJ Consulting have developed new techniques for the calibration of tension meters to improve mooring safety. The techniques use tension dependent catenary characteristics such as the catenary shape, the angle subtended at the fairlead, or the elastic response to movement, to determine the actual tension delivered beyond the fairlead. The techniques have been used successfully now for over a year on rigs on the North Sea.
The Echo Sounding method
The echo sounding method simply measures the catenary from echo sounding shots and derives the tension from its observed shape. From these analyses, the tension meters can be accurately corrected. Other methods measure inside the fairlead but these techniques measure the actual tension delivered in the line.
The Visual Method
An ROV or an underwater camera can be lowered to photograph the fairlead and, after some adjustment of the image for perspective effects, the angle of the chain leaving the fairlead can be accurately measured to determine the real tension in the mooring line. In this diagram the corresponding angles for a given tension are superimposed on the image.
The Elastic Method
The elastic method uses the fact that tight moorings allow less movement than slack ones. This technique accurately measures the fairlead movements in 3D and the corresponding tension changes. If the tension change is more than expected, the tension reading is too low and vice versa.
When all three methods have been used there has been good agreement between their results.
1. These techniques calibrate in-situ without interrupting operations or contacting the chain.
2. Accurate mooring line tensions allow the master to correctly calculate vessel stability, deck loads, touchdown distances, anchor uplift, support buoy loads, drilling riser strains, payouts to stand off position, predicted drift positions after line failure etc. (HSE requirements)
3. With accurate tensions, clearances from pipelines, jacket legs, cross-moorings etc can be reliably modelled and alarmed if necessary.
4. Sharing loads on anchors in poor weather conditions is much more reliable.
Using the Elastic Method
By far the easiest method to arrange is the elastic method. No special equipment is required and the study can be carried out during normal cross tensioning procedures at any new location. All that is required is for the tensions and payouts to be recorded during the cross tensioning operation. A data sheet such as the one set out below needs to be completed.
The tension data in the example shows a session where the chain is paid in 2ft at a time on line 1 and the corresponding reactions in tension are recorded for all 8 winches. This not only allows the analysis to determine the elastic response on line 1 but the reactions on all the other lines can be analysed to determine a new position and orientation of the rig based on the elasticity of each line. From this we can work out how much the winch on line 1 moved towards or away from its anchor. As each elastic response is analysed in turn, the corrected tensions are re-used to improve the rig shift estimates.
The result is a simple graph of the expected elastic response (Tension against Payout shown in white) versus the observed values (fig 1) which is then adjusted until the tensions produced match the expectations (fig 2).
Fig 1 The theoretical curve (white) shows that the observed responses (dots) are too sluggish so the actual tensions are likely to be lower than those recorded.
Fig 2 With the tensions corrected for a 28 kip loss, the catenary reactions are now correct.
In practice, it is likely that the tension will be increased to a maximum then reduced back to working tension and often the ‘up’ curve does not match the ‘down’ curve as in this example.
However, this is also useful data as it shows the user that there is considerable friction loss at the fairlead. On the way up in tension, the winch is fighting against the friction but on the way down, the friction force is helping the tension. This is why tensions are often more accurate when paying out than paying in but if the winch response is jumpy or inconsistent, the graph shows this very clearly. When analysing this for corrections, we will usually bias towards the larger correction to ensure that tensions are over rather than underestimated by the ‘Best Fit’ solution.
The final report will give corrections to the observed tensions as a scale and/or bias which can then be applied either to the observed tensions in the control room or the control system software to display the tensions actually delivered beyond the fairlead on each line.
If any rigs require a catenary modelling package, we can provide the catenary software we use to determine the catenary characteristics. This will let you design or check your moorings and provide clearances above assets for any given tension. For safety critical moorings a real time tension output can be interfaced with our NEMO software to provide real time monitoring of catenaries, loads and clearances.