FOM 1.9	Fleet Operations Manual General Vessel Stability	Doc No.: FOM 1.9 Revision: 01 Date: 15 Oct 2024 Issued by: DPA Approved by: MD

1. APPLICATION

This document applies to Masters and Navigating Officers onboard fully managed vessels.

2. PURPOSE

The purpose of this document is to provide requirements and recommendations regarding the stability of vessels.

3. RESPONSIBILITIES

3.1 Master

• Review and approve the vessel's stability condition calculations.

3.2 Chief Officer

• Conduct vessel stability calculations as per Section 8 – Stability calculations.

4. GENERAL GUIDANCE

General guidance concerning the vessel’s stability is provided in this Section. Reference is also made to the vessel's Class approved trim and stability booklet.

An offshore supply vessel may perform complex cargo operations at sea, including loading and discharging of solid/liquid cargoes simultaneously, along with personnel transfers. The crew should have a good understanding regarding safety and adequate stability.

The following points shall be borne in mind:

• Any trim limitation and minimum freeboard at the stern should be maintained. Ballasting should be carried out to counteract the effects of loading and discharging cargo.

• Cargo should always be discharged uppermost first, i.e., discharge deck cargo prior to discharging from cargo tanks lower down in the vessel (if discharging plan permits).
• When liquid cargo is to be discharged, as soon as pumping commences a full free surface of those tanks being pumped out should be allowed for.
• When a tank is to be ballasted at sea to counteract the removal of cargo from the vessel, it should be noted that:
  • The tank should be assumed to have a full free surface as soon as ballasting commences, and;
  • The free surface will adversely affect the stability of the vessel before any benefit can be assumed from the weight of ballast water taken on board.
• Before ballasting commences, it is important that a sufficient quantity of the highest deck cargo be unloaded to lower the centre of gravity of the vessel.
• When pipes or casings are carried on deck, an allowance must be made for the volume of entrapped water regardless of whether the pipes have wooden plugs or not.
• The vessel will always be operated in compliance with the International Convention on Load Lines, 1966.

• The motion characteristics of the vessel are primarily dependent on the GM value (metacentric height), and to a lesser extent on the transverse distribution of weights which affects the roll inertia.

5. SAFE PRACTICES

• A deck loading plan or stowage plan should be received from the charterer or terminal before loading commences;

• The sequence of offloading should be agreed before operations commence.
• Any operation that involves a change in the vessel stability must be carried out in a controlled manner and within the safe working parameters laid down in the vessel’s Trim and Stability Booklet.
• The centre of gravity of the present loading condition and the final loading condition shall be checked prior to:
  • Cargo operations
  • Ballasting and de-ballasting operations

• The distribution and quantities of fuel, water, ballast, and deck-load shall be monitored and adjusted to maintain an optimum metacentric height, with respect to vessel safety and operational efficiency. The trim and stability booklet provides guidance in the form of standard loaded conditions.
• Curves for maximum allowable VCG are contained in the trim and stability booklet. Checks shall be made for various stages in the loading or ballasting operation to ensure that limits are not exceeded while loading or discharging ballast or cargo/equipment.

• Curves for maximum allowable shear forces and bending moments are contained in the Trim and Stability Booklet.

6. FREE SURFACE MOMENTS

Free surface moments can be minimized by adopting sound operating practices. Ballasting and de-ballasting shall be carefully planned to keep the number of slack tanks to a minimum during pumping.

7. OVERFLOWING OF BALLAST TANKS

The ballast tank construction and venting arrangements are designed to withstand the hydrostatic pressure of ballast water overflowing through the vent heads. However, when ballasting tanks, the soundings shall be regularly monitored to ensure that an overflow does not take place.

8. STABILITY CALCULATIONS

• Vessel stability condition calculations shall be completed weekly as a minimum, either by means of a stability computer, or by manual / spreadsheet-based calculations.

• The vessel’s draft and stability condition shall be monitored on a continuous basis during cargo, bunkering or heavy lifting operations when significant weight changes may be taking place.
• Tank gauging system(s) should be cross checked with secondary gauging method and any deviation recorded during this weekly calculation.
• The circumstances requiring a recalculation of vessel stability may include, but are not limited to, the following:
  • Docking and undocking in dry-dock
  • Pre arrival / departure from port or work site
  • Prior to loading or discharge of heavy lifts by ship / or shore crane
  • Lightening of vessel for draft limited location
  • Maintaining optimum draft and trim for DP operations by ballast adjustments
  • Above normal adjustments in project equipment
    • Prior to spooling pipe
    • Commencement and completion of pipe-lay
    • To take account of offshore supply operations to ensure sufficient intact and residual dynamic stability during the operation and consequently transit back to port
  • Change of weather conditions & the effect of heavy weather on the water-plane area and the consequent reduction in stability
  • The effect of slack tanks and the reduction in vessel stability due to free surface effect
  • Any unplanned or observed deviations from the expected condition
  • During various stages of ballast exchange if carried out
• Calculations are to take into account torsional stress, shearing force and bending moment limitations and at all times they must remain within the minimum requirements laid down in the vessel’s trim and stability booklet. The booklet covers loading, ballasting departure and arrival condition.

9. STABILITY LOADING COMPUTERS

• Computers and computer programs used for the calculation of vessel stability should be approved by classification society & flag state where appropriate.
• Loading computers shall be cross checked 3 monthly against class approved test data such as fixed conditions found in vessel Trim and Stability Booklet.

10. STABILITY WARNINGS

• A vessel will normally give warning signs that its stability is approaching an unacceptable condition. Should any of the warning signs be observed, an investigation into the cause is warranted, for example:
• An unexpected increase in the rolling period.
• A long slow roll with a tendency to hang at an angle of inclination (Angle of loll).
• The vessel takes on an angle of list and stays at the same angle or an increasing angle of list despite large amounts of weight being added to the high side or removed from the low side.

11. CORRECTION OF A STABILITY PROBLEM

• Recovery from a stability-related problem involves correcting the source of the problem. The most likely causes of a stability problem include, but are not limited to:
• Free surface caused by flooding;
• Off-centre weight caused by unevenly distributed ballasting;
• Insufficient freeboard or excessive draft caused by excess weight or off-centre weight distribution;
• Flooding of one or more compartments which may cause excess weight, off-centre weight distribution and free surface;
• Grounding or collision;
• High centre of gravity due to excessive top weight.
• The steps required to correct a stability problem depend on the cause of the problem. They include such measures as:
• Stopping or limiting flooding
• Correcting adverse list or trim to the extent possible
• Removing excess weight to the extent possible in such a manner as to lower the centre of gravity.

12. DAMAGE STABILITY

• The Master and Chief Officer should be able to calculate any damaged stability condition for one compartment and he should know by regular training how to counteract for each damaged stability situation to save the vessel from sinking or capsizing.
• The flooding of one compartment will drastically change:
• Draft
• Stability
• List and trim
• Reserve buoyancy
• The main rule is that the vessel will sink until the additional flooded volume equals the volume of the immersed portion.
• The Master and Chief Officer shall be trained and prepared to calculate any damaged stability condition. In this respect, they shall be familiar with:
• The watertight boundaries
• The position, amount, and volume of the watertight compartments under decks
• Position and operation stations of all watertight doors and manholes
• The operation and capabilities of the ballast system. Checks should be performed at regular intervals
• The exposed areas of the hull where damage may occur
• Damaged stability calculations and criteria
• Whatever damaged stability calculation method is used, the result is the same:
• Draft will increase combined with a considerable heel and trim
• The metacentric height will move due to change of buoyancy
• The centre of gravity might shift
• The centre of buoyancy will move up due to an increase in draft
• Free surface moment in a damaged compartment may cause additional reduction of stability
• Weather conditions may cause additional list due to wind and high waves. This will also endanger the vessel due to the increased draft and consequently less distance between main deck and waterline.

13. THIRD PARTY ASSISTANCE

• The company or vessel owners may provide a 3rd party emergency response service where all relevant stability documentation is held ashore in case of emergency. If this is the case:
• All documentation supplied shall be clearly marked, read and understood by Master and Chief Officer.
• Emergency contact details must be posted.
• Drills or communication exercises shall be carried out at pre-determined intervals.

14. WATERTIGHT INTEGRITY

• All ballast, fresh water and fuel tanks equipped with vents to the exterior of the unit are fitted with self-closing arrangements designed to prevent:
• Ingress of water into tanks during rough weather;
• Pressure or vacuum build-up inside the tank during loading or discharge.
• All tank openings will be capable of being made watertight and shall be maintained in a seaworthy condition at all times.
• Weathertight doors, bolted closures, hatches, vents, and all forms of openings on the vessel shall be maintained in a seaworthy condition at all times, capable of being closed if required.

15. WATERTIGHT DOORS

• The Master and Chief Engineer shall assess all watertight doors in accordance with the guidance provided in Part B-4 of SOLAS Regulation 22:
• Designated doors shall be clearly marked “To be kept closed at sea”.
• Doors allowed to be open shall be clearly indicated in the ship's stability information and shall always be ready to be immediately closed.

Warning:

In potentially hazardous situations such as adverse weather, restricted visibility, manoeuvring on approach to port or within port limits all watertight doors shall be closed.

Where doors are left open it is important that the means to close the vessel down in an emergency is regularly tested (e.g. as part of regular drills).

All personnel must be made aware of the correct operation of watertight doors and reminded that they must not be obstructed. Where this is unavoidable, it may only be done after being suitably risk assessed and using the Permit to Work system if required.