Stability and Moving

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.

A brief explanation of the above items follows:

  1. Hydrostatic Properties (Figure 1-31) are generated from the shape of the underwater portion of the rig and can be used to determine the weight of the rig and the location of the centroid, longitudinally and transversely. It has many other uses, but the ones just mentioned will be most often used when moving a rig.
  2. Cross Curves of Stability (Figure 1-32) are also generated from the underwater portion of the rig and are used by the designer to determine the amount of stability the vessel has when it is not in the upright position. Figure 1-33 shows a typical transverse section through ahull. This figure illustrates how the value of GZ is determined. As the KG value increases the GZ value decreases, or vice versa.
Cross Curve Stability
  1. Statical Stability Curves (Figure 1-34) are developed from the cross curves of stability and are curves of righting arm. They are sometimes referred to as GZ curves.
  2. Dynamic Stability Curves (Figure 1-35) are produced from the statical stability curves and calculations to determine the overturning moment caused by a wind of a given velocity. This curve is probably the most significant of all curves because it shows whether or not the rig can be towed during the forecasted weather, while remaining within the safety parameters of the regulatory bodies.

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,




\ X



displacement displacement

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.

Rig Overturning Moment
Fig. 1-33 Transverse righting arm and center of buoyancy.
  1. Motion Response Analysis is the study of the rig in a "hove to" state. This is the position when "going on location," and the results of this analysis are used to determine the stresses induced when a jack-up leg touches bottom or those caused by mooring forces on a drillship or semisub-mersible.
  2. Lightship Characteristics are probably the most used (or misused) information that should be supplied. This information is prepared either from a series of accurate weight calculations or from an inclining experiment, or both. The
Statical Stability Curve
Fig. 1-34 Statical stability curve.

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

Jack Rig Spudcan Information
Fig. 1-35 Dynamic stability curve.

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

90 kf


draft (ft)

  1. 1-36 Allowable KG curve.
  2. The prime consideration in preparing to move and while moving must be safety. Therefore, it is essential that the rig mover be completely familiar with the rig and the expected area environment during the move. He should have a basic knowledge of Naval Architecture and know how to apply it to his vessel.

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:

  1. Give notices to parties concerned.
  2. Provide overall report for Owner.
  3. Give insurance company and their representatives the following:
  • a) Distance of move, miles
  • b) Area and water depth of next location
  • c) Number, size and ownership of tugs to be used
  • d) Name of person in charge of move

C. Provide for towing company the following:

  • a) Distance of move, with initial and final positions
  • b) Number, horsepower, and type of tugs
  • c) Required towing equipment
  • dj Radiofrequency for communications
  • e) Name of person in charge of move
  • f) Request for tugs to contact rig, giving estimated time of arrival, names of captains and tugs, and which tug is lead
  1. Obtain long-range weather forecast.
  2. Finalize move schedule.
  3. Complete stability calculations for upcoming move.
  4. Develop deballasting schedule.
  5. Dispatch tugs and anchor-handling crew.
  6. Tie down all cargo.
  7. Man pump rooms,
  8. Deballast, as scheduled, to desired draft and recheck calculations.
  9. Secure pump rooms.
  10. Connect tugs.
  11. Disconnect anchors, commence tow, give U.S. Coast Guard pertinent data regarding tow and notify USCG when completed.
  12. Move into anchor pattern, connect anchors, disconnect and release tugs not being used for resetting anchors.
  13. Finalize ballast schedule to drilling draft, recalculate stability,
  14. Man pump rooms.
  15. Ballast to drilling draft (close all valves, take soundings of all tanks).
  16. Complete stability calculations at drilling draft.
  17. Pretension anchors and release to operating tensions for 1-hour static test, all activity ceased.
  18. Release remainder of tugs.
  19. Retake soundings of all tanks to make sure there are no leaks.
  20. Secure pump rooms.
  21. Authorize drilling operations to commence.

The following is a sample preparation list for floating, raising, lowering, and preloading a jack-up:

  1. Check operator's console, main switchboard, and accessory equipment for corrosion, loose wiring, damage, etc. that may result in malfunction. Check all other electrical and mechanical components.
  2. Put all generators on line to check paralleling characteristics and overall engine-generator operations.
  3. Check all electrical circuits for faulty insulation, breaks, and grounds.
  4. Check all motor and brake functions and wiring if they have not been satisfactorily operated within the past 24 hours. Check and record all elevating motor torques,
  5. After checking all brakes, motors and torque readings, reinspect each motor to assure that all of the brake plugs have been reassembled and are tight. Replace motor covers.
  6. During all operations, the platform should be kept trim, Make level or equal-draft reading when approaching or leaving drilling location and check for an equal loading on each spud can when elevated and when elevating or lowering the platform. Keep a log of these conditions and record at least every 12 hours so necessary steps can be taken to maintain a trim condition.
  7. Check anchors, fairleads, anchor lines, and winches for proper operation. Check motor power and free-spooling.
  8. All service tanks should be either "pressed-up," full, or emptied of liquids. This may not always he practical for potable water and diesel-fuel systems; however, do not exempt more than one tank in each of these systems from the rule.
  9. All below waterline discharges should be checked and closed.
  10. Check diesel fuel tanks and drain off water. Check water-fuel filters and air filters.
  11. Check bilge system and all liquid-service systems. Inspect for "frozen" valve and safety devices. Voids should be clean.
  12. Periodically check derrick-holding devices.
  13. Before lowering platform, be sure jet hoses [one for each jet line) are readily available and in operating condition. Inspect deck manifolding and water-pumping arrangements to make sure they are operable.
  14. Keep emergency and repair supplies aboard at all times. Periodically inspect "stores" and note where they are kept for fast, efficient use. Rolls of heavy plastic, rope, and tape are good emergency repair items for hatches, etc. Welding, burning, and cutting equipment should be kept readily available,
  15. Tie down derrick block, swivel, and all movable items on the derrick floor.
  16. Tie down drill pipe, collars, casing, and all movable items on the deck. Check for loose boxes, crates, wires, gas bottles, etc., both on deck and within the hull. Secure cranes and booms.
  17. Close and secure hatches, doors, manholes, return lines, and all other openings between the outer and inner hull that are not in use. Station personnel near openings that are required to stay open during a move so that they may be immediately closed in an emergency. Keep the platform in this condition during all moving and preloading operations,
  18. Prepare the "sea water" tower foT operation when the platform is raised or lowered.
  19. Check adequacy of tugs and towing equipment.
  20. Check and record weather data. Use more than one source when available. Do not move when good weather and seas are estimated to be of short duration.
  21. Periodically inspect all safety, aids-to-navigation, and lifesaving equipment.
  22. Check ocean bottom data at the proposed location to determine bottom conditions.
  23. Prior to lowering spuds, check wind-direction, current-direction, and depth-recorder readings.
  24. Record spud penetration data and use as references when removing spuds from ocean floor.
  25. Know your personnel and your platform.
  26. Confine raising or lowering of platform to periods of good visibility, preferably during daylight.

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

Displacement: Weight

KMl : Longitudinal metacentric height above keel

K: Keel





Righting Moment: Heeling Moment: Downflooding Angle

Second Intercept:

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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  • Aston
    What calculations do you use when ocean drilling?
    8 years ago
  • christine
    How to determine the stability draft of submerisible?
    8 years ago
  • venerando
    What is an overturning moment?
    8 years ago
  • katrin
    7 years ago
  • Ruggero
    How do offshore rigs balance the torque generated from drilling?
    6 years ago
  • antje
    How long does takes for an oil drilling to move to another loction?
    2 years ago
    How is stability maintained on offsore oil platforms?
    1 year ago

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