Ralph G. McTaggart
ETA Engineers, Inc.
Stability is simply the ability of a rig to remain afloat. The subject of stability is further divided under the two headings of (a) Intact Stability and (b) Damaged Stability.
For every rig, the designer or builder should furnish the rig owner with a Stability Booklet which, at a very minimum, should contain (a) Hydrostatic Properties), (b) Cross Curves of Stability, [c) Statical Stability Curves, and (d) Dynamic Stability Curves, Items (c) and (d) should be sufficient to cover the normal operating range of the vessel.
Other information that many would find useful and could be included in the Stability Booklet, but which is not considered to be essential, includes (e) Allowable Dynamic Stability KG Curves, (f) Damaged Stability Calculations, (g) Motion Response Analysis, and (h) Lightship Characteristics,
Fig, 1-32 Cross Curves of stability.
e, Allowable Dynamic Stability KG Curves (Figure 1-36) are generated from the dynamic stability calculations. These curves are a growth of the dynamic stability curves, and they simplify the rig mover's job by eliminating the need to prepare a calculation of dynamic stability every time he decides on a possible tow condition.
f. Damaged Stability Calculations should be prepared for the effect of damage to the outside compartments or flooding into any compartment. These calculations should show that the vessel has sufficient reserve stability to survive either damage or flooding. If the ABS "Rules for Building and Classing Offshore Mobile Drilling Units 1973" are applied to the vessel, the ability to survive damage or flooding must be considered in association with the overturning effect of a 50-knot wind.
calculations determine the weight and center of gravity in all directions of the dry rig, i.e., no variables of any kind are included. From this information, the operator determines the condition of the rig at any time. It must be stressed here that although the shipbuilder may have gone to great lengths to determine the lightship upon completion, it is up to the owner or operator to make sure that the values are adjusted if any changes are made to the rig, i.e., equipment or structure changed, added, removed, or even relocated. The Lightship Characteristics are the foundation of all
calculations for the afloat and elevated positions, and an inaccurate number not only makes all other calculations worthless, but could also endanger the safety of the rig and personnel.
Moving is the intentional relocation of the rig for any purpose, though we think of changing drilling location when we talk of o m i o
It has not been the practice in the past for the rig mover to discuss the rig with the designer, or builder's naval architect, prior to delivery of the rig. I feel that a discussion between these people would not only increase the education of both, but would also help ensure a safer operation and, I would hope, eliminate disasters due to incompetence.
Every rig owner should receive from the builder a book entitled "Operating Book" or "Booklet of Operating Conditions." The rig mover should read and understand this book before attempting to move the rig. Each rig is like a new car, and even though you know how to drive, each car has its own peculiarities—so also does each rig, even so-called "sister ships."
Again let me stress to the rig mover: understand your rig, talk with the Naval Architect, and read the "Operating Book."
Moving comes under two basic categories; (a) Field Transit and (b) Ocean Tow. A Field Transit condition is generally considered to be a move that would require no more than a 12-hour voyage to a location where the unit could be jacked up, or to a protected location. An Ocean Tow should be considered for all other moves.
Preparation for moving a semisubmersible rig or drillship is covered by the "Operating Book," but basically it would be as follows:
C. Provide for towing company the following:
The following is a sample preparation list for floating, raising, lowering, and preloading a jack-up:
The above list is an attempt to cover a general area of moving checks. It is the responsibility of the rig mover to ensure a safe operation and to check the list given in the "Operating Book."
The hazards that might be expected during a tow are too numerous to list, but I shall cover a few. Probably the most significant hazard occurs when the rig is preparing to "go on" or "come off" location. In the case of jack-ups, consideration must be given to the sea state because of the change in floating characteristics that takes place while moving the leg up or down.
The effect of the leg striking the ocean floor must also be considered. This should have been taken care of by the designer, and the "Operating Book" should have a section related solely to allowable conditions for going on and coming off loction. For the drillsbip and the semisubmersible the sea state must also be considered, but for a different reason. Anchor handling and the effect of unequal mooring arrangement should be considered. A mooring arrangement and procedure should be included in the "Operating Book,"
Also included in the "Operating Book" should be the limits of service in both the operating and towing positions. It is false and senseless to ignore this section because to do so can only endanger the rig and personnel. If it is necessary to stray from the design criteria, a few dollars spent in engineering analysis shall serve to ease many minds and to reduce the risk potential.
One of the greatest fears that develops while at sea is that of damage which may produce flooding and, if extensive enough, the loss of the rig. It is easy to say that with a little common sense most damages and flooding can be avoided, but they still occur. The designer knows this, and should have oompartmented the rig such that flooding shall be contained within an allowable extent, which may be uncomfortable to those on board, but shall not result in a loss of the rig. On most drilling rigs presently in service, excluding drillships and self-propelled units, the probability of damage due to collision is remote. And if it takes place, it will be of such low impact velocity as to cause only a slow leak rather than a rapid flood.
The effect of the vertical center of gravity on a damaged vessel is considerable. If damage should occur, steps should be taken to lower the vertical center of gravity. This can easily be done on a jack-up by lowering the legs. On the other vessels ballasting can improve the stability. However, this is a condition that can occur suddenly, and it is not always possible to carry out a damaged stability calculation when the water is entering your cabin. Here again we see the need for preplanning, and this should be discussed between the Owner and the Naval Architect,
Problems do, of course, occur without damage. A sudden squall, a change of sea state, or a wind coming out of nowhere can cause many things to happen. Even the mightiest rig can act like a cork in the ocean on such occasions. Fortunately, we have more accurate weather forecasting today, and the rig mover sometimes can prepare for the change. Several courses of action are available: a jack-up can lower the legs, a semisubmersible can ballast, and a drillship or self-propelled vessel can alter course to avoid, or at least reduce, the effect of the squall or other problems. Once again, each vessel has its own peculiarities, and consultation with the designer combined with the experience of the rig mover can usually serve to eliminate serious conditions. If a search were made for the major contributing factor in accidents occurring during tow, it would be found that bad design and inexperience head the list. It should be remembered that the offshore drilling rigs are still new in comparison to ships. The designer can only learn from feedback, but fortunately, technology is advancing rapidly in the marine field and information is more readily available today.
In conclusion, I would again stress the importance of discussions between the designer and operator on the peculiarities of each rig. Education can only improve safety. Safety can only mean more operating time and more return on dollar investment.
Draft: Depth of submerged hull
LCB: Longitudinal center of buoyancy
VCB: Vertical center of buoyancy
LCF: Longitudinal center of floatation
KMl : Longitudinal metacentric height above keel
Righting Moment: Heeling Moment: Downflooding Angle
Vertical distance from keel to center of buoyancy
Transverse metacentric height above keel
Moment to heel one inch
Moment to trim one inch
Tons per inch immersion
Heel lever arm
Location of center of gravity
Displacement multiplied, by GZ
Overturning moment produced by wind
Angle of heel at which water will enter the hull through an opening
Second crossing of righting moment and heeling moment curves
Constant, as defined by regulatory agency.
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