RPD Explained What is RPD? RPD is an abbreviation of Rack Phase Difference. The term did not exist until two years after the first Modec rig left the yard. How was it discovered? The new owners of the rigs (Maersk) operated the rigs in the conventional manner. The rigs operated normally at some locations but leg and jacking system damage was noted on others. This prompted a design study by the manufacturers and Maersk’s engineering department. When the study was complete a direct link was found between leg inclination and damage. How was it measured historically? The term RPD was invented in order to define leg inclination and to make measurement easy. For these purposes customized measurement tools were designed (these tools are still used for calibration of the new system). The tool itself is very simple in design and use:- The barge must be completely level. The tool is placed into same tooth on each of the three leg cords. A straight edge projects from the tool parallel to the plain of the barge. A steel rule is then projected downwards and three measurements are obtained (one per chord). The greatest measurement is given an RPD of zero. The other two (lower) measurements are then subtracted from the measurement on the cord with zero RPD. The result of the subtraction is the RPD on the leg. An example would be:- Measurements:- Cord A - 550 mm Cord B - 525 mm Cord C - 500 mm Resulting in:- RPD Cord A - 0 Cord B - 25 Cord C - 50

How else can we determine whether a Leg has RPD? If you position yourself at each cord and look down between the rack and wear plate guide you can determine whether a leg has substantial RPD. If each cord has an equal gap then the leg has no significant RPD. If, however, one or two cords have (a) large gap(s) and the other(s) small gap(s) then the leg has significant RPD. No gap at all on any cord means that you have a very large RPD and very great care should be taken. What does RPD mean with regard to Leg inclination? As mentioned previously RPD is method of indicating inclination of the leg with respect to the level hull. Leg inclination is easily determined by using the “gap” method. Equal gaps indicate no or slight inclination. Uneven gaps indicate some kind of inclination. How can direction of inclination be determined? This is easily achieved using the gap method or by analyzing RPD on the leg. The (initial) direction or vector of inclination is always towards the cord with the largest gap or zero RPD. The second direction or vector is determined by the cord with the smallest gap or largest RPD. Taking the values given earlier we would see that the leg (looking aft):- That A cord with the large gap and zero RPD determines that the first vector is towards A (Forward). Cord C has the smallest gap and double the RPD on B this makes the second vector starboard making the overall direction or vector forward and starboard. Why did the Original Engineers determine which cord has zero RPD? Short and simple answer - no one knows. It would be pertinent to assume that they thought that it would be difficult to work with negative numbers for RPD as using the smallest measurement (with the tool) as the zero or datum would result in (using the previous values):- Cord A - -50 Cord B - -25 Cord C - 0 The following table may be of assistance in defining RPD in a physical manner:- Using the figures given earlier:-

Cord Measurement RPD Gap Loading (see later) A Highest Zero Large Highest B Middle Middle Middle Middle C Lowest Largest Smallest Lowest

You can see that the original engineering team did not use common sense when they adopted the largest measurement as zero RPD and when you take into account other factors (see later) it is even more astonishing. How does RPD damage the legs and the Jacking System Components? It is easy to imagine damage to the leg struts when a leg has substantial RPD. It is obvious that the leg will be moving through the jack house at an angle. When the angle becomes too great the leg will be pinched in and the struts will be subjected to tremendous forces. This will generally result in the struts bending and finally snapping. What is not evident is uneven pinion loading and load sharing. How does RPD affect pinion loading? Strangely enough (see earlier) the cord with least RPD takes most of the rig’s weight and is therefore subjected to greater stress. In other words your calculations of leg loading etc. are not exactly correct for a leg with large RPD. The loading differences are quite spectacular with the cord with the largest RPD taking substantially less load than the cord with zero RPD. This is the point that damage can occur to the climbing irons, motors, gear boxes, pinions or rack. Are There Mistaken Assumptions made about Pinion Loading Even without RPD Present? Yes - It has been normal practice that leg loading can be calculated by taking the maximum pinion load allowed for one pinion during a specific operation and multiplying by 18. This assumes that the loading on each pinion is equal at all times. This is certainly not the case. This can be seen visually by simply looking at the “meshing” of each set of pinions into the rack. It is obvious that they do not contact the rack at the same time and that they are “phased” to give a smooth jacking operation. How does this phasing affect pinion loads? Looking at one leg and the jack consists of three pinion pairs and taking it that each pinion has seven teeth (and 360 degrees per pinion revolution) - then each pinion pair takes most of the load at 360 / 7 degrees with a 120 degree phase shift between each pinion pair. How do we know this to True? The new jacking monitoring system allows post analysis of power consumption and current loading for each motor. The system allows traces to be configured showing current or power over a time base. When viewed these trends indicate a sinusoidal oscillation on a non standard three phase function. The function is non standard in that the peak value of current/power is not equal for the three pinion pairs. In fact the lowest pinion pair peak value is almost always higher than the middle pinion pair which in turn

is higher than the top pinion pair. So you can see that the phasing affect has been measured and is not hypothetical. What affect does this have on the principle of multiplying max pinion load by 18? The writer is an electrical engineer and cannot answer this but it would be pertinent to assume that the maximum pinion loads given in the marine operations manual are average pinion loads and that each pinion is capable of loads far in excess of the pinion loads stated. What is all this leading to? If you remember that the cord with zero RPD takes more load than a cord with substantial RPD then you can see that the peak values on the lower pinion pair on the cord with zero RPD will be taking “massive” loads. How can RPD be Overcome? RPD has existed and has been dealt with successfully on these rigs for a number of years without single cord jacking. Amongst the methods used to reduce RPD and it’s effect are:- Reaming the legs straight. This always works but is time consuming. It’s also self explanatory. Angling the barge to a sympathetic angle to a leg with RPD. This works when you have legs inclined in opposite directions. By angling the barge to an attitude that increases the gap between the guides on the cord or cords with high RPD you can reduce or maintain a lower RPD than keeping the barge level. In this mode you must always take into account any free surface effect of transferring load onto the lower sides of the rig. Both of these operations maintain a perpendicular (or as near to perpendicular as possible) between the hull and the leg. The only difference being that the former allows a level hull and the later does not. What is Single Cord Jacking? Single cord jacking is a new concept which is simple in the extreme. When you have digested all of the earlier descriptions of RPD and its effects it becomes evident that RPD is basically a representation of the attitude of the leg to the hull. All of its effects are directly attributable to this fact with uneven cord loading due to uneven distribution of the hull weight across the leg. Single cord jacking is an electrical means of eliminating, reducing or maintaining acceptable RPD values. When the leg is jacked normally power is applied to all 18 motors on the leg and all 18 brakes are released. With single cord jacking only the selected cord’s motors and brakes are energized.

When used correctly it moves the cord in the selected direction thereby reducing the differential between the datum (zero) and the cords with RPD. Don’t Believe it? Whether or not you believe that single cord jacking works is irrelevant. If you have not seen it then the writer would concur that the idea seems preposterous but it works! How does it work? It simply redistributes the load by distorting the leg and hull. It forces the leg to move nearer the perpendicular to the hull. Look at it like this:- Your leg (we will make it Leg # 1) is as described earlier in this document, that is:- Cord A - 0 Cord B - 25 Cord C - 50 What do I do to reduce RPD and how will I achieve it? Stop all three legs. Rotate the “Cord Selector Switch” adjacent to Leg # 1 push buttons to position “A”. Ensure that Leg # 2 & 3 “Cord Selector Switches” are at “ALL” position. Press Leg Down/Hull Up push button. Both the pressed button and the adjacent “Start Permit” push buttons should start flashing. Press “Start Permit”. Wait. The system automatically stops the single cord operation after a preset time. Look at the screen. You will see the values change after about four seconds. Rotate the “Cord Selector Switch” to position “ALL” Depending on which rig you are on (Trident 9 or Tridents 15/16/17/19) you will see different results (these are only approximations). Cord Trident 9 Trident 15/16/17/19 A 0 0 B 11 15 C 35 42 How did this happen? You pushed down cord A. This had the effect of opening the gap between the rack and the guide on cords B & C and closing the gap on cord A. This as previously described has reduced RPD. Since the spud can remains firmly stuck at the bottom of the leg you

have in fact put a slight bend into the leg. The limits of this bending moment have still to be quantified. Very good but it will only go to back to the original values very shortly Yes that’s true, but from studies and past experience of rig moves show that the RPD values do not reach their original values until much later than you would expect and that a quantity of the RPD correction is permanent. Off course you will need to perform the operation again and most probably a good few times but every time you do it you are gaining height. We have christened this type of operation “Monkying”. It is better to do this than going back into the water, pulling legs and reaming! What do I have to do to make subsequent single cord jacking operations? You have to perform a compulsory three cord jacking operation (in any direction) prior to being allowed another single cord operation. The system indicates availability of single cord jacking on the RPD Monitor page. It indicates the available mode(s). When both single and three cord jacking are allowed the screen displays “BOTH”. When only three cord jacking is allowed the screen displays “THREE CORD” in red flashing letters. The compulsory three cord jacking operation must be maintained for the time set during commissioning. This is set at:-

3 seconds automatic single cord

followed by;

4 seconds stress relief.

Are the values adjustable? Yes. They may be adjusted but only after consultation with the relevant authorities. Can I perform more than one single cord jacking operation at the same time? No. The system will not allow single cord jacking on more than one leg at a time and it will not allow a single cord operation to be performed whilst another leg is operating in three cord mode. You may perform a single cord jacking operation on all three legs on a round robin basis and then perform a joint three cord operation on all three legs. Can I jack the cord with the highest RPD up? Yes. But be very careful when using this option. When would I use this option? Let’s consider the previous example of single cord jacking. It was easy to determine which cord to jack as there was only one cord with zero RPD.

What would you do if you found two cords with zero RPD and one cord with excessive RPD? Incorrect Method If you choose one of the two zero RPD cords for single cord down then you will only see a change in that cord. The other original zero RPD cord will not have changed and likewise the cord with excessive RPD will not have changed. What do you do next? You have to perform a three cord stress relief and you still have high RPD on one cord. You are in a bind. Correct Method Select the cord with excessive RPD and jack this cord UP. This will always reduce the cord with excessive RPD. Are there any limitations to the use of single cord jacking? Yes. What are they? You have to use common sense. Look at it like this. You know that expected penetrations are about 15 M. You get RPD in excess of the limit at 4 M. You could single cord all the way. I don’t think so. It would be better to start reaming your leg(s) straight and using single cord for the last say 5 M. Remember single cording is not the answer to all inclination problems. You still have to take into account subsea conditions. In this case you have to use your own judgment. There are no hard and fast rules for every condition. What causes RPD? You may wonder why this section is placed at the end. The previous information was given to describe the phenomenon. In fact the question may well be better phrased “How did I cause the RPD”. A barge engineer once described the difference between an RPD free rig move and a nightmare as LUCK. This is entirely true given the operational, financial and client restraints. He said this as he knew that RPD was not caused by one event or condition but was caused by a combination of events or operations. Here are just a few things that induce RPD:- Uneven Sea Bed If the sea bed is uneven in height or known subsea differences are anticipated then you can expect RPD.

Current If substantial current is anticipated then you can expect RPD. Weather / Sea Conditions Only move when weather conditions allow. If you move in marginal weather then you can expect RPD at the next location. Stability If you are not completely trim then you can expect RPD. Dragging Legs at location If you drag your legs at location and you do not raise and lower them straight then you can expect RPD. Pre-Driving Legs If you deliberately preload one leg more than the others then you can expect RPD. Jacking with the hull out of trim. If you do not regularly maintain a level hull then you can expect RPD. Very Deep Water If you have very large leg extensions then you can expect RPD. Is this a definitive document? No