Voith Schneider
6000
PELICAN
J CP TUNNEL
4000 HP.
000 TONS
sedco 445
FIXED KORTED
Fig. 1-104 Vital statistics of operational dynamic positioned ships.
partial listing of the advantages and disadvantages of the taut line system follow.
Advantages of taut line:
a) Simple and economical b) Visible evidence of operation c) All sophisticated equipment on deck d) Rapid deployment e) Works well in shallow water f) Immune from underwater noise
Disadvantages of taut line:
a) Mechanical taut line or tensioning system can fail b) Large heading change must be made by stepping two units c) Susceptible to error due to current drag
TABLE 1-6
Sample Tension Vs. Wire Line Size For Taut Line Unit
TABLE 1-6
Sample Tension Vs. Wire Line Size For Taut Line Unit
|
Tension for | |||||
|
Wire |
Wire |
Area |
Breaking |
32,600 |
Safety |
|
Size |
Construction |
Strength |
psi Stress |
Factor | |
|
Va |
1 xJ9 |
.012272 |
2,1 □□# |
400# |
5.0 |
|
Va |
1 x 19 |
.049087 |
8,400# |
1,600# |
5.0 |
|
% |
6x19 XIPS |
.110447 |
15,100* |
3,600# |
4.1 |
|
V2 |
6 x 19 XIPS |
.196350 |
26,600# |
6,400# |
4.1 |
|
% |
6 x 19 XIPS |
.3068 |
41,200# |
10,000# |
4.1 |
|
% |
6 x 19 XIPS |
.4418 |
58,800# |
14,400# |
4.1 |
|
% |
6 x 19 XIPS |
.601 |
79,600# |
19,600# |
4.1 |
|
1 |
6x 19 XIPS |
.785 |
103,400# |
25,600# |
4.1 |
The taut line inclinometer, normally a gravity sensing device, cannot distinguish between a true angle from the verticle and a horizontal acceleration. Thus, the effect of the horizontal accelerations accompanying a peak-to-peak surge or sway of 5 feet and a period of 12 seconds produces an oscillating apparent sway of 1.9 percent of water depth. This apparent sway subtracts from the 5 foot peak-to-peak true motion.
The 1.9 percent of water depth error is read out as feet according to the water depth. At 180 foot water depths in the above example, actual surge motion is exactly cancelled out by the surge acceleration induced effects in the inclinometer. At a greater depth than 180 feet, the acceleration induced error exceeds the actual surge motions. The problem is solved by two alternative methods: using a gyroscopically stabilized inclinometer which is insensitive to horizontal acceleration, or by simply limiting the response of the vessel only to inputs of longer periods than wave motions.
- Fig. 1-105 Counter-balanced Taut Line Unit (Courtesy of Baylor)
- BOW
- Fig. 1-106 Geometry of a Taut Wire System.
Acoustic Position Reference Systems
While the taut line system requires use of a wire to mechanically communicate position from the ocean bottom to the surface, the acoustic systems use underwater sound waves as the required communication link. Acoustic waves travel considerably faster in water than in air. The velocity for sound in water is almost 3,300 mph, about 4.4 times faster than the speed of sound in air. The short base line acoustic position reference system illustrated in Figure 1-108 has an acoustic beacon located on the bottom and a vessel with four hydrophones on the sea surface.
The beacon shown in Figure 1*109 transmits acoustic pulses at regular intervals to the hydrophone array. When the vessel is directly over the beacon (or drill hole) acoustic signals arrive at all hydrophones simultaneously. When the vessel is displaced away from the hole, the nearest hydrophone receives the acous
Fig. 1-107 Dual Taut Line installation. (Courtesy of Baylor)
tic wave front first and the furthermost hydrophone receives the acoustic wave front last. These differences in time, from the nearest hydrophone to the most remote hydrophone, are operated on by a signal processor whose output is directly proportional to the percent water depth positioning error. Two sets of hydrophones at right angles to one another give the position error in two coordinates, X and Y, corresponding to bow-stern error and port-starboaxd error. Position is displayed on the control indicator shown in Figure 1-110.
If a vessel is considered with its hydrophone array directly centered over the beacon, but with a slight roll, it is easily seen that a roil to starboard will give the starboard hydrophone a deeper submergence in the water while, at the same time, the port hydrophone is submerged to a shallower depth. If the starboard hydrophone is deeper then it is also closer to the beacon and the port hydrophone is further away from the beacon.
This change in distance of the hydrophones from the beacon
- RS-5 EQUIPMENT
Fig, 1-108 Short baseline acoustic system. (Courtesy of Honeywell Inc.)
Fig, 1-108 Short baseline acoustic system. (Courtesy of Honeywell Inc.)
due to roll makes the short base line acoustic system require a Vertical Reference Unit (VRU) to measure pitch and roll. This allows the signal processor to eliminate the pitch and roll induced errors in the hydrophone signals.
Since the acoustic beacon may not be installed over the bore hole or the hydrophone array may not have been centered under the rotary table, a means of dialing in offsets in X and Y in feet is provided. Up to 100 feet of offset compensation is usually allowed. The sum of the beacon offset and the rotary table offset also should be less than 20 percent of the vertical separation between the beacon and the hydrophones. On a large, deep submergence semi submersible rig with a tall B.O.P. stack assembly, the distance between the hydrophones and the beacon may easily be 90 feet less than the water depth. This 90 feet may result in a low separation for the hydrophone to beacon distance, even though actual water depth would not lead one to suspect such a problem.
- Fig. 1-109 Acoustic beacon. (Courtesy of Honeywell)
The ususal pitch and roll reference used is a gravity sensitive unit. This unit will respond to surge and sway accelerations to produce an acceleration error identical to one developed in a gravity sensitive inclinometer of a taut line system. As with the taut line system, the two means of dealing with the problem are to eliminate all "short" period inputs by filtering to respond only to very low frequency inputs or to use a gyroscopic vertical reference unit (VRU) which is insensitive to horizontal accelerations.
This gyroscopic vertical reference unit is quite costly and may have a life of only 20 to 50 days due to bearing life in the gyroscope. A working arrangement has been to use a conven-
Fig. 1-110 Acoustic system control indicator (Courtesy of Honeywell)
tional pendulous VRU during normal weather when motions are low. The gyroscopic VRU is then turned on to operate only during bad weather. The life of the gyroscopic VRU is conserved by using it only whenever optimum positioning performance is required.
A good, short base line acoustic positioning system can maintain a position sensing accuracy of 1 percent of water depth [1% WD) under favorable conditions. This accuracy relies on a correct hydrophone and VRU installation. Accuracy requirements on installation are in the order of magnitude of 1 degree of angular measurement and distance accurate to 0.2 percent (1 inch in 40 feet). The most stringent requirement on installation is mounting the VRU parallel to the plane of the hydrophones. The total cumulative allowable misalignment (electrical and mechanical) for the VRU reference plane to hydrophone plane alignment is± 0.1 degree for a typical system. Achieving this 1/10 of a degree alignment is simple on land, but will call for some thought and care if the vessel is afloat at the time of hydrophone installation.
In the operation of an acoustic system, many mathematical procedures are performed. For pitch and roll compensation, the output of the VRU is subtracted from the processed hydrophone signals. One degree of pitch or roll acts exactly as a 1.75 percent of water depth (l degree) change of vessel position if not compensated. Four degrees of pitch or roll would be read out as 6.98 percent of vessel position error if not properly compensated. To ease the calculation involved in determining the vessel position from the processed hydrophone signals summed up with the VRU pitch and roll signals, a very excellent small-angle approximation is made that the sine of the angle, the tangent of the angle, and the angle itself are all equal. As shown in Table 1-7,6 degrees corresponds to a vessel position of 10.47 percent of water depth, a point at which the sine approximation is 0,18 percent low and the tangent approximation is 0.36 percent high.
The small-angle approximation for the sine shows a 1 percent accuracy up to vessel position errors of 17.5 percent which is more than adequate for any normal drilling task.
TABLE 1-7
Sin, Tan, and 9 for Small Angles Showing Closeness of Simple Approximation
TABLE 1-7
Sin, Tan, and 9 for Small Angles Showing Closeness of Simple Approximation
|
0 (Degrees) |
0 |
1 |
2 |
4 |
6 |
10 |
20 |
|
0 (Radians) |
0 |
0,0174 |
0.0349 |
0.0698 |
0.1047 |
0.1745 |
0.3490 |
|
0 (7= WD) |
0 |
1.74 |
3.49 |
6.98 |
10.47 |
17.45 |
34.90 |
|
Sin 0 (% WD) |
0 |
1.745 |
3.490 |
6.976 |
10.453 |
17.365 |
34.202 |
|
Tan © (%WD) |
0 |
1.746 |
3.492 |
6.993 |
10.510 |
17.633 |
6° = 0.1047 Radians x 180= 10.47 Sin 6" x 100 =10.45 6° = 0.1047 Radians x 180= 10.47 Sin 6" x 100 =10.45 The offsets of the center of the hydrophone array to the rotary table and that of the beacon to the subsea well bore must be subtracted from the measured position to obtain the true vessel position. As mentioned previously, the sum of these two offsets expressed as a percent of the vertical separation between the hydrophones and the beacon may be limited to 20 percent or less. In the cases where an acoustic system is to be used with an anchored semisubmersible, the effect of hydrophone and beacon offset and shallow water should be investigated by the user and the equipment supplier. This investigation is necessary
Note: The acoustic system may have a 100 percent of water depth acquire mode that operates as an exception to the above comments. It does so by sacrificing accuracy, which is not If a vessel is operated with the percentage ratio of the combined hydrophone to rotary table offset and bore hole to beacon offset exceeding 20 percent, the vertical motion of the vessel due to heave or tides which makes an effective change in water depth may yield a source of error. The acoustic system with a large percent total offset will fail to subtract the correct percent offset for the variation in water depth. The error is correctable only by feeding the true water depth, including tide and heave, into the system as a variable in the determination of the percent offset. The offset referred to is the large hydrophone and beacon offset. This water depth input, normally a constant set into the system, would have to be an onsite, real-time measurement such as a bottom echoing fathometer might provide. To the author's knowledge, at least one instance is known where this shallow water complication was an insurmountable obstacle to the desired system accuracy. The amount of error produced is closely approximated by the equation: where the % Effective Offset is the combined beacon and hydrophone offsets as a percent of water depth. For example, consider a vessel with a 20 percent effective offset in an area with a 10 foot tidal change and a 75 foot effective water depth. The tide apparent position error will be: % WD Tide Error = (20% Offset) x (It) ft./75 ft.) = 2,7% This tide or heave error is not too severe if the percent offset is kept below 20 percent. The spectrum of acoustic frequencies usable for the short base line systems probably lies between 10 kiJohertz and 100 kilohertz, The choice of frequencies in this region is not as simple as it first seems because one set of factors is optimum at the high frequencies while another set of factors is optimum at low frequencies. In the matter of wave length, for instance, at 10 kilohertzthe wave length in water is about 6 inches, while at 100 kilohertz the wave length is 0.6 inches. The hydrophones should have a very low sensitivity from ship or surface noise and have a high sensitivity directed downward iii a 10 to 30 degree cone. Since the hydrophone is analogous to an antenna, the higher frequencies result in smaller hydrophones and more easily obtainable directional characteristics. Thus, physical hydrophone design favors the higher frequencies because of the shorter wave length. This same argument holds true for the bottom located beacon and its directional characteristics. Noise levels also favor the use of high frequencies as the rig generated noise is predominantly low frequency and the amount of interfering vessel generated noise is significantly reduced at the higher frequency. This question of noise requires further investigation to define the magnitude, source, and frequency of the noise such as from the following sources:
The attenuation characteristics of water favor the use of low frequencies because as the frequency is increased the transmission losses increase strongly. It is seen that the choice of hydrophone frequency is not a simple one due to the absence of a single optimum condition. Generally speaking a less expensive, more precise high frequency system can be made for shallow to moderate water depths. For deeper systems, a more expensive low frequency system is indicated to take advantage of the lower transmission losses at the lower frequency. Other Positioning Systems Long base line acoustic systems are similar to short base line systems turned inside out or upside down. For example, consider a long base line system in which a single beacon is suspended underneath the ship and the ship is centered within a one mile square pattern of anchored hydrophones. The hydrophone signal is connected by cable to a surface buoy and radio transmitter which transmits back to the ship. Every second a short acoustic pulse is transmitted from the beacon. At the time the acoustic pulse reaches the hydrophone, the radio transmitter relays its arrival back to the ship. It is easy to see that the delay time from the start of the beacon transmission to the receipt of the signal transmitted from the hydrophone is a direct measurement of the distance to the remote hydrophone. In fact, four radio channels effectively allow measurement of the distance to each hydrophone, assuming a constant speed of sound in water. The above described system is an upside down version of the short base line system. This system seems to have great accuracy since it measures the distance to four remote subsea markers. It does not contain any pitch and roll inaccuracies except for the actual X and Y motion of the beacon as it moves in arcs formed by the pitch and roll produced by the radius of the beacon to the center of pitch and roll. The great disadvantage of this system is the need to deploy the elements of the long baseline—thesubsea hydrophones with their mechanical and electrical connection to the buoys and the general complexity of the active elements of the remote transmitters. Emplacement of a long base line system is several orders of magnitude more difficult than emplacing the single beacon used in the short base line system. The short base line is "short" out of necessity because its base line is on the vessel. Radar Radar is an obvious possibility as a positioning system, but it suffers the common shortcoming inherent of all long base line systems. If some natural target does not exist, then a system such as buoys with radar reflectors must be emplaced and the system becomes dependent on the buoys staying in place through wind, wave, and storm. Radar vessel positioning, however, can work if there are dependable base line targets. Decca, Raydist, or Loran The Decca, Raydist, and Loran systems are ail long base line systems which rely on two or more transmitters separated by a number of miles. These systems all have the capability to provide for dynamic positioning if the vessel is within the operating range of required accuracy. There is some concern over interruption of broadcasting and the dynamic positioning dependence on the remote transmitter. The patterns established by the multiple transmitters may not have constant accuracy over all portions of the pattern, and the location of use must be within the more accurate areas of the system pattern. The area of use may also be within the range of existing navigational systems (such as the North Sea). For instance, a successful dynamic positioning test using one of these systems has been conducted in the Rhine River. Inertial Systems Intertial systems, as developed for submarine navigation, undoubtedly could perform a useful role in dynamic positioning. The two elements of cost and restricted information, however, have made the inertial system unavailable. Its possible future use coupled with satellite navigation is reviewed in the following satellite navigation section. Satellite Navigation Satellite navigation entails communication with a satellite passing overhead and the determination of the latitude and longitude of the vessel at the time of the pass from information transmitted from the satellite to the vessel. Several interesting factors accompany^ the use of satellite navigation:
It is obvious that satellite navigation is much too inaccurate and "fixes" every hour or two are not frequent enough to permit satellite navigation to play a role in dynamic positioning at the present. However, with continued improvements that might be expected, the combination of satellite navigation and inertial systems may complement each other. For example, if satellite navigation could give a "fix" that is sufficiently accurate, as in very deep water, then a short-term inertial system could hold position until it is updated by another satellite pass. This system could have high reliability and yet have no necessary subsea reference points. Its use would undoubtedly be limited to deep water greater than 1,000 ft. to tolerate the system's large inaccuracy. |
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