Spread Mooring Systems
Mark A. CftiJders
ODECO, Inc.
Purpose and Basic Design Criteria
In floating drilling, the purpose of the mooring system is to maintain the drilling vessel within certain horizontal limits of the centerline of the well so that drilling operations can be successfully carried out. This horizontal excursion during actual drilling operations is usually held to a maximum of 5 to 6% of water depth; however, most drilling operations are carried out within 2 to 3%. These limits are controlled by the blowout-preventer-operating-procedures.html">subsea drilling equipment such as stresses in the marine riser, angle of the lower ball joint, and the nature of the drilling operation.
During nonoperating times when the marine riseT is still connected to the blowout preventer (BOP) stack, the mooring system is usually designed to maintain the drilling vessel within approximately eight to ten percent of water depth. Once again, these limits are controlled by the subsea drilling equipment. During maximum or survival conditions when the marine riser is disconnected from the BOP stack, the amount of excursion off the hole is secondary to relieving high mooring line tensions.
Except during maximum conditions, it is necessary to think of the mooring and Bop Control Systems Well Control" href="/well-control/subsea-bop-control-systems.html">subsea system as working together. Based primarily on field experience and supported by analytical analysis, Table 1-3 gives a summary of the generally accepted design and operating criteria for mooring operations as generally discussed above.
By far, the most common type of mooring system is the spread mooring system which historically has consisted of from one to twelve mooring lines. Dynamic positioning, which has no physical connection to the ocean bottom other than the subsea drilling equipment, is also used on a limited number of vessels. At present, the station-keeping ability of the dynamically positioned ships is considerably less than a properly designed spread mooring system. The primary fortes of dynamic position mooring are ultra-deep water, high mobility and rapid well abandonment.
Spread mooring systems have been successfully used in 1,750ii fool water depths and extensive analytical analysis indicates/that they can be very successful in over 3,000 feet. However, there are some water depths, probably over 4,000 feet, where the spread mooring concept becomes impractical from an economic as well as a station-keeping standpoint.
Spread Mooring Patterns
There are many possible arrangements for spacing the mooring lines around the drilling vessel in a spread mooring pattern. The reason for the large number of different patterns stems from an effort to gain the maximum restoring force from the mooring system in combination with the environmental resistance or loading characteristics of the vessel. As may be expected, the environmental loading characteristics of ships and semisub-mersibles vary from the bow to a beam environmental attack angle. Most ships exhibit a maximum wind, wave, and cUTTent load on the beam, whereas most semisubmersibles exhibit a maximum loading condition in the vicinity of 45° off the bow or stern.
In general, there are two types of mooring patterns which can be used with any one particular type of vessel. The first is the "Omnidirectional Attack" pattern, which is arranged to take
TABLE 1-3
Design Parameters And Conditions Used For Spread Mooring Analysis
Mooring Une Marine Riser Situation
TABLE 1-3
Design Parameters And Conditions Used For Spread Mooring Analysis
Mooring Une Marine Riser Situation
|
Nominal Operation Designation |
Maximum Tension |
Leeward Line Slacking Policy |
Maximum Vessel Oftset (%) |
Condition |
Max. Lower Ball Angle (Degrees) |
Mud |
Operation |
|
Normal Drilling |
% Break |
Nominal |
Approx. 3 |
Connected |
4 |
Drilling Mud |
Drilling ahead; running casing; BOP. test tools & performing all normal operations. |
|
Drilling |
Va Break |
Nominal |
6 |
Connected |
Under 10 |
Drilling Mud |
Preparing to wait on weather, hoisting, pulling riser, setting cement or barite .plugs & critical drilling operations. |
|
Standby |
Va-Va Break |
Equivalenttoat least 2 leeward lines completely slackened |
10 |
Connected |
10 |
Displaced w/sea water as needed |
No drilling operations with riser ready to be disconnected at moments notice. Waiting on weather. |
|
Survival or Maximum |
te Break or anchor slippage |
Equivalenttoat leas! 2 leeward line completely slackened |
Unrestricted |
Disconnected |
No operations of any kind except possible mooring line manipulation. Rig may not be manned. |
environmental loads from any attack angle (0 to 360°), In areas such as the North Sea and the Gulf of Alaska where maximum winds and waves can arrive from any direction, the omnidirectional attack pattern is required.
The second type of pattern is the "Unidirectional Attack" pattern in which there is a strong prevailing environmental direction. The unidirectional attack pattern is stronger in one direction, usually the bow, than the omnidirectional pattern, but it is weaker in a direction approximately 90° from the strongest direction. The unidirectional attack pattern is advisable in such areas as the mouth of the Amazon River off Brazil, Cook Inlet in Alaska, and certain areas off South Africa where a strong prevailing current always exists in one direction or 180° from that direction.
Deployment of the proper mooring pattern is a very important factor in reducing mooring line loads and staying within desired horizontal displacement tolerances. A proper spread mooring pattern should coincide with the environmental wind, wave, and current loading characteristics of the drilling vessel.
For an "Omnidirectional Attack" pattern, for example, ships will "stack" or align the pattern to be strongest in the beam direction. Figure 1-74 shows six of the most commonly used types of "Omnidirectional Attack" patterns. Figure 1-74(a) shows a symmetrical nine line pattern which is used by the SEDCO 135 and ODECO "Ocean Driller" class. The symmetrical eight line system shown in Figure l-74(b) is used on such vessels as The Offshore Company's Discoverer Class and has been used on the Santa Fe Bluewater 2.
Figure l-74(c) shows the pattern used by the Pentagone 81 semisubmersible and Figure l-74(d) shows the pattern used by a number of Global Marine vessels, such as the Glomar Grand Isle. Figure l-74(e) shows the pattern being used on the Briet Engineering design semisubmersible and some shipshape vessels.
Perhaps the most common mooring pattern is that shown in Figure l-74(f) which was initially used on the LST Drilling Tenders in the Gulf of Mexico but is now used on vessels such as the ODECO "Ocean Victory" and "Ocean Queen" class.
The spread mooring system is very inefficient in that less than
Childers a) SYMMETRIC NINE LINE
- SYMMETRIC EIGHT LINE
- 45°- 90" EIGHT UNE & 45°-90" TEN LINE
- SYMMETRIC TEN LINE
- 30°- 60° EIGHT LINE
- 30° - 70° EIGHT LINE
- 1-74 Typical spread mooring patterns half of the mooring lines contribute to holding the vessel on location at any one time. Of this half, usually only one or two lines supply most of the restoring force. In fact, if the leeward lines are not manually slackened during severe conditions, they actually draw the vessel off location and contribute to higher mooring windward line tensions.
Anchors
Before the station-keeping ability of any spread mooring system can be understood or analyzed, the characteristics of the anchors, which are the main holding force of the mooring system, must be discussed. In floating drilling, the anchors most commonly used are designated "dynamic anchors" because they increase their holding power with increased horizontal pull providing there is no vertical uplifting force. To prevent an uplifting force on the anchor, sufficient mooring line must be deployed such that the catenary is tangent to the ocean floor at or before the anchor.
Tests conducted by the U.S. Navy indicate that only a nominal decrease in holding power will result with a 6° angle with horizontal; however, a significant loss in holding power will result if the angle increases to 123. For design purposes, enough mooring line should be outboard of the lower fairleader after pretensioning such that the mooring line is tangent to the ocean bottom before the anchoT.
Basically, there are three types of anchors used, the most common being the Light Weight Type (LWT), with the other two being the Stato type and Danforth, Figure 1-75 shows the nomenclature used with these types of anchors. The Stato type comes in a fabricated and cast version. Shop tests as well as field
experience indicate that the cast version with a trade name of Moorfast or Offdrill is structurally stronger and moTe durable.
All three types of anchors perform approximately the same in sand and clay. Maximum holding power is obtained when the flute angle is set at approximately 30° in sand and/or clay and 50° in soft mud. The Stato type anchor has shown superior tripping and "digging in" characteristics in soft bottoms such as the Gulf of Mexico. Tests conducted by the U.S. Navy have shown that along with soil conditions and other factors, maximum holding power varies with anchor weight.
For anchors in the 10,000 to 15,000 lb. range, a holding power of approximately 12 to 17 times their dry weight can be expected. In the range of 30,000 lb. anchors, this ratio decreases to approximately 8 to 11 times the anchor's dry weight. For anchors in the 45,000 lb. range, this factor decreases below 10, Figure 1-76 shows a log-log plot of anchor holding power vs. anchor dry weight for different soil conditions. Though the plot results in a straight line, as determined by small anchors and extrapolated to bigger anchors, some tests indicate that holding power decreases faster than indicated with increasing anchor size.
If poor holding power is experienced or expected, "piggybacking" the primary anchor in series with an additional anchor may be necessary. Before this is done in actual field practice, the anchor should be allowed to "soak" or remain relatively unloaded for a number of hours to allow the soil to consolidate and regain some of its sheer strength. Often this will result in the anchor producing the desired holding power. In any event, if "piggybacking" is necessary, calculations should be done to see that additional tension does not cause a vertical lifting force on any of the anchors.
Anchors should be properly "preloaded" in correspondence to the highest mooring line load expected, which may be as much as one-half the mooring line's rated break strength. At a minimum, the anchor should be preloaded to one-third the rated break strength, especially if design criteria such as those outlined in Table 1-3 are used.
In areas where anchor holding power is unknown, tests have shown that small anchors (approximately 3,000 lbs.) can give a
500,000-
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