Methods of Preloading in Areas of High Tidal Range
By Roger Dutton – 4/3/08
We have always understood that the range of tide will have a significant impact on preloading. While dealing with a changing reference (the sea surface level), the challenges are: maintaining an air gap while keeping the rig below the maximum allowable preloading height, determining the quantity and rate of leg penetration during increasing load as well as during the static tests, and keeping track of the actual leg loads during times of buoyancy offset (hull in water).
Obviously, if we ignore these factors, the rig may find itself in a full preload condition at excessive air gap, lose it?s static test air gap, or (in the case of increased preload and buoyancy offset) an unknown final leg load during preloading.
In areas where the tidal range is extreme, these conditions are well exaggerated and we come to the conventional saying that “We live and die by the tide”.
On rigs capable of jacking with full preload, the consequences of improper planning are mitigated somewhat by the ability to follow the sea surface under a loaded condition, but most agree that due to the drastically reduced equipment life, jacking with full preload should be minimized if not eliminated. This paper will address the challenges of preloading in high tidal range areas using a rig incapable of jacking with significant preload, and it should be understood that rigs capable of jacking with full preload are more flexible in their approach. I will make notes along the way to explain when this is relevant.
Maximum Allowable Preload height (MAPH)
Rigs often use an air gap for preloading other than that in the operations manual. Most original manuals for rigs in the industry will yield a 4, 5, or 6 feet preloading air gap maximum. As we gain experience, we recognize some improvements to this method. Most people agree that the best air gap is the minimum to allow the rig mover to complete the job. “Lower is Better”. Proper risk analysis should dictate that we use different air gaps for different levels of rapid penetration risk verses wear and tear on the jacking gear, the necessity of more sea surface transitions, and time savings. I commend owners who have dictated levels of risk based on the type and quality of soil testing, rig design, and rapid penetration warning signs, and assign a recommended MAPH based on the individual location data. Example: 5 feet = little or no risk shown with good confidence in data; 3 feet with data showing no anomalies, but mediocre confidence levels; 1 foot with warning signs or poor confidence levels; draft and buoyancy offset in cases where there are multiple warning signs, or poor or no soil data.
Any time the rig maintains draft, the force of buoyancy will reduce the weight on the legs
equal to the weight of water displaced by the hull below the waterline. On many rigs, the available preload tank capacity is more than enough to load the rig to the required preload weight out of the water. By loading the tanks to more than 100% and maintaining buoyancy (by the KPI) equal to the excess load, we can achieve 100% preload while in the water – the safest place to be. During the preload procedure, we can also maintain a displacement constantly equal to the entire preload amount on board, thereby allowing us to jack the rig while we continue to load preload – even on a rig whose jacking maximum does not include preload. We will expand on the usefulness of buoyancy offset later.
The Sea Surface
We know the tide goes up and down due to gravitational pull, primarily from the effects of the Moon and the Sun. This causes an elliptical shell of water in the oceans to travel around the world from east to west roughly once every 24 hours causing two high and two low tides every day (Diurnal tide). This variable rise and fall of water level, although not completely understood, is very predictable. The mechanisms involved that create higher or lower tidal ranges, time of tide differences, tidal current variations, and even the loss of one tide during the day are very complex, but involve the inertia or momentum of the water, the hydraulics of the surrounding area, the orientation (NS or EW) of the coastline, and the phases of the Sun and Moon.
Rate of rise and fall of the tide is an important factor to consider in the planning of preload. Generally, the rate of Rise and Fall will be greater at mid-tide levels (times) and lesser at the high and low tide levels (times). We would like to think that if [for example] we have a 30 foot tide drop to experience in 6 hours we should see 5 feet an hour level drop (=5). But, in fact we may see a rate of fall (or rise) much greater than the 5 feet per hour during the mid-tide as we will see a diminishing rate around the times of high and low. Some areas will experience two tides of different range (and therefore rate) throughout a given day.
The good news is, that tides very accurately reproduce their cycles throughout the year and are therefore very predicable by regurgitating data from past experience. As rig movers, we will gain our information from tide tables, datum offsets, and on-site observations. The more accurately we can predict the times and heights of all stages of the tide, the more safely and efficiently we can preload the rig. In many cases, we can even preload faster and safer with a significant tide than we can in areas of little or no tidal range.
Representation of Height and Time of Tides
We have two means of displaying, predicting, and recording tides. They are tables and graphs. Although it is certainly possible to do all the required calculations and forecasts using tidal data from tabular form, it is much preferable to present the data in graphical form as this eliminates much of the need for interpolation and reduces the need for mass organization of tabular data. (See examples of graphical representations later).
The Goal of Preloading
We preload a rig to compress and test the soil beneath the spud cans to a condition where we have a reasonable assuredness that the applied test loads will never be exceeded during the time the rig is on location. Following this logic, if the rig remains static at these test loads, we have a reasonable assuredness that the rig will not further influence the soil at a lesser load, and the rig will remain static throughout it?s stay on that location.
To achieve this goal, we have to compress or displace soil as the leg penetrates until the leg reaction is equal to the long- term static soil strength. Keeping in mind that the rate at which we compress soil may influence the short-term static strength, we must at times allow for either extended static testing, or slower penetration (leg loading) rates.
As the options in methodology are reduced by conditions such as high tidal ranges, the availability of other pertinent information becomes more important. In other words, when we weigh the importance of such items as soil testing requirements and weather forecasting, we must impose more stringent criteria. The failure to do so will lead to increased risk and increased time to complete the process of preloading.
As an example: If we have no soil data, logically we should consider the location [unknown] as being of the highest risk level – therefore requiring a six hour static test (hold period). If, at the turn of low tide (where the rate of fall and rise is lowest – see above), the tide was predicted to drop and re-rise the range equal to the maximum allowable preload height (MAPH) within six hours then we could not safely complete a prudent static test for the location. So when we establish the desired hold period for a given location and it?s associated risk value, we must ensure the tide „envelope? will accommodate the holding time. This is one example where a rig with full preload jacking capability would have the advantage – you could jack during the static test (jacking during a static test will be discussed later).
In any case, the soil data and weather data should be closely scrutinized along with the tide data to produce a preload plan with several contingencies.
Increased Challenge of High Tidal Range
In the case of rising tide, we find that the air gap will diminish as we preload even without increasing penetration due to settling. If the range of tidal rise is greater than the MAPH, we know that we cannot complete the preload in one cycle.
Our options are:
Option 1. Delay the start of preload and follow the tide with the rig (by jacking) until we are on a falling tide and the low tide level will be less than our MAPH, where we would stop and begin the preload process.
In High Tidal Range Areas the time to preload, and complete the hold period will usually be longer than the time when the water level is below the hull. A rough calculation of the maximum range of tide would be: (No settling)
6 hour preload+2 hour hold period=8 hours.
Tide must fall 5 feet, reverse, and rise 5 feet in no less than 8 hours.
High tide to high tide generally at 12 hours, therefore 8 hrs/12 hours = 5 feet/ X hrs; Max
range of tide = 6.5 feet. (graphically solved).
To get the above chart we had to use the total height of tide at high tide as the variable. The curve was generated using a simple sine curve. In real life this is a pretty good approximation. This is why, for the sake of this paper, the definition of “High Tidal Range” is over 6.5 feet range. It is understood that in a normal rig move, there will most likely be settling, that will limit the available time to take on preload and complete the hold period. Note that we haven?t jacked the rig with preload, or had to transition into
the water or out.
Option 2. Allow the rig to gain draft, continue through the top of high tide, and regain air gap when the sea level again drops on the next falling tide. This will require the transition from air gap to draft, and back to air gap. Although this requires better (calmer) sea conditions, it is the preferred method as it gives the operator many more options on how to proceed. Full preload can be taken on board while the rig is still in the water. As the tide falls, the rig will settle (maintaining leg reactions corresponding to the preload amount minus the buoyancy) until the supporting soil can withstand the preloaded leg reaction and then the sea level will fall away from the hull. As soon as there is an air gap, the static test can commence. Normal static test procedure should be then used. If further settling is observed, the rig will be leveled and the hold period must be re-started. As long as the rate of fall does not exceed the MAPH divided by the time required to hold, this process can be done anywhere along the falling tide. For example: If we have a two hour hold time and a MAPH of 5 feet, we need to have a rate of fall less than 2.5 feet per hour (see below).
Weather Conditions and the Increased Requirement for Transition between air gap and draft.
As we can see, prudent preloading in high tidal areas will most likely involve increased transitions between air gap and draft. This can be a safer means of accomplishing the preload goal, in that the majority of the increasing load on the spud cans can take place during times where the hull is in the water and the consequences of a rapid penetration are least. The down side is that we will need better sea conditions to use this method. Forecasts must be available to allow abandonment of the preload, time to dump whatever water is on board, and jack the rig above the wave crest before the environmental jacking limits are reached.
We will work on rigs that have little or no ability to jack with preload, rigs that can jack with a significant percent of preload, and rigs that can jack with full preload.
In preload scenarios where we can jack with full preload, the intuitive thought would be to simply continue to keep the rig hull within the MAPH continually whilst preloading. Including the hold period. This is possible, but we have found that the wear and tear on the jacking gear (even for the rigs designed and advertised as capable) is so significant that this method, by itself, is rarely justified. Also, from a practical point of view, to keep track of jacking distances, air gaps, changing water depths, and corresponding penetration figures is extremely cumbersome and takes very close attention (one position (person) designated to do just that – with relief).
Jacking during the hold period is worth discussion. Although most all soil testing is done using monotonic (non-cyclic) loading, and we assume the rig to exert such load while on location, it is worth noting we will see some cyclic loading during preload. Either by repeated dumps and loads, wave action on the bottom of the hull, or by starting and stopping of the jacking system. These will all yield a dynamic load at least, if not technically cyclic (engineering-wise cyclic assumes 10,000 cycles, but for storm loading and soil, we see the number of cycles greatly reduced to at least three similar increases and decreases in load). This cyclic loading can be very dangerous – even leading to liquefaction of the soil (temporary elimination of the shear strength as the particles rearrange). So the end operational use is that we can jack during the holding period due to the increased test parameters of the soil under the cans, but we must recognize that there is an increased risk of soil failure while doing so.
On rigs that can jack with some or full preload, we do have more options during preloading in high tidal areas and the operation becomes „more forgiving?.
If we are on a rig without preload jacking capacity, we must keep the buoyancy offset (and corresponding draft) equal to the preload weight on board (By constant monitoring) and follow the tide up or down until we are ready to hold for the final penetration figure and static test. Normally, this would take place on a falling tide, where we start by estimating the expected increase in penetration between the initial and final leg penetrations. We add this expected increase to our MAPH and use the result as the point where we will stop jacking the hull down (measured from the bottom of the hull to the sea level of low tide). If the start time of taking preload yields a height of tide greater than the floating draft of the rig (as it will for large tidal variations), we must continually keep the rig draft at a point where the buoyancy offset equals the weight of preload water added so we can jack down, following the tide, until the stop point is reached (See example „a? below). Keep in mind that if the rig settles while preloading, this number must be accounted for in determining the stop position. If the margin of error in the estimated penetration difference is more than the MAPH, this procedure will have to be repeated on the next falling tide. (In some cases, the preload may be partially dumped, held at height above MAPH, and re-established through high tide for the next falling tide).
On rigs where we can jack with preload, we can “drive” the leg or legs in small increments as we get a better feel for what the penetration difference (between initial and final) will be. We can also jack down if the rig seems to not be penetrating as far as we thought it would.
On the non-preload- jacking rigs we can see this is a “one shot deal” and errors in either the tidal predictions or estimated increase in penetration can easily result in multiple preloads. It is rare that we nail it the first time. The good news is that, if using single leg preloading, we have data to “tweek” our estimates after the first leg is completed.
Experience has shown us, that the errors in estimates combine to make this a non-deductive operation. We cannot guarantee we will complete the preload on a single tide drop (even if the expected penetration difference is less than the tidal range).
Experience in the area, good tidal data, extensive soil data, consummate planning, and attentive record keeping are all necessary to safely and effectively preload in these high tidal areas.
MAPH = 5 Feet
Expected penetration difference between Initial and final = 8 feet
Note that we have gained one foot while preloading (determined by recording leg position, air gap or draft, and tide level), so we stopped lowering the rig at 7 feet above our hold point at MAPH.
Preloading in high tidal range areas obviously takes much more planning, attention to detail, and effort than preloading under „normal? circumstances but it can be done efficiently and safely. And in circumstances of large initial-to-final penetration differences, it can be faster than in areas of little or no tidal range. The important fact to remember is that we are not in the business of taking risks, so if it does not go according to the estimated schedule – STOP. Dump, jack down, re-evaluate, and start again.