The purpose of this article is to provide insight for evaluating the intact and damage stability calculations for the FSO during the wet tow condition, inclusive of longitudinal strength calculations.
To perform this task, the following scope shall be performed:
- Prepare the model of the FPSO-FSO
- Identify the lightweight data and set up the wet towing condition and Outline assumptions, calculations and results
Following codes and standards shall be followed while calculating intact and damage stability calculation of FPSO-FSO
- IMO Code on Intact Stability for all Types of Ships Covered by IMO Instruments
- MARPOL International Convention for the Prevention of Pollution from Ships
- IMO Code for the Construction and Equipment of Mobile Offshore Drilling Units
- RMRS Rules for the Classification, Construction and Equipment of Mobile Offshore Drilling Units and Fixed Offshore Platforms, Rules for the Classification
A hull model of the FPSO-FSO, including tanks and superstructure shall be created. Buoyant hull and windage area shall be defined clearly.
A loading condition to represent the towing shall be defined.
- For the tow, it is better to remove products from the cargo tanks however any residual cargo shall be estimated correctly
- If flow lines and gas lines are to be carried onboard, correct weight and CG shall be defined, If deck area is not adequate to carry these items during towing, these shall be removed from calculations.
- Weight change register maintained onboard the FSO shall be checked thoroughly. Upon discussion with the crew and comparison with the existing layout drawings weight changes shall be tracked. Lightship data shall be updated as per latest weight changes record.
- Tank loading shall be defined to assume even trim or trim by aft towing.
- Residual ballast water (normally ranges from 3% to 5%) shall be considered in each ballast tank. The inclusion of approximately of fresh water and fuel oil were to account for the vessel being manned during the tow. Slop tanks weight to represent fluids flushed from the MOPU, flow lines shall be determined.
- The water ballast shall be adjusted to obtained even trim or design trim during towing.
The lightship weight distribution was calculated using the Prohaska method, which assumes a trapezoidal distribution on top of a uniform distribution as shown below.
Longitudinal Strength Criteria
The allowable still water bending moment and shear force values are shall be taken from design midship, which will be defined in existing midship design or tanker’s existing stability booklets.
For the towing condition, the still water bending moments and shear forces are calculated by software and compared to the above allowable values.
Intact Stability Criteria
FSO intact stability is assessed against IMO A749 (18) Chapter 3 and RMRS Rules. When similar criteria were listed within the two standards, the most onerous condition was considered. A summary of the recommended general criteria is;
- Area under the GZ curve from 0 to 30 degrees shall be greater than 0.055 meter-radians;
- Area under the GZ curve from 0 to 40 degrees or the angle of flooding shall be greater than 0.09 meter-radians;
- Area under the GZ curve from 30 to 40 degrees or the angle of flooding shall be greater than 0.03 meter-radians;
- Righting lever GZ shall be greater than 0.20 meters at a heel angle equal to or greater than 30 degrees;
- Maximum righting arm should occur at a heel angle greater than 25 degrees;
- Corrected initial metacentric height shall be greater than 0.6 meters;
- Area under the righting moment curve to second intercept shall be greater that 1.4; and
- Corrected metacentric height shall be greater than 0.15m.
The intact stability of the FSO is checked for compliance with IMO Severe Wind and Rolling Criterion, as per IMO Res. A.749(18)
Damage Stability Criteria
Damage stability is assessed against the Code for the Construction and Equipment of Mobile Offshore Drilling Unites (MODU Code) and RMRS Rules for Classification, Construction and Equipment of Floating Offshore Oil and Gas Production Units. They require that the vessel have sufficient buoyancy and stability when side tank damage extends 7.2 meters horizontally, 1.5 meters transversely and without limit vertically between effective watertight bulkhead and bottom damage extends while withstanding heel due to 50 knot winds.
The following criterion are assessed for the tank damage cases
- Minimum freeboard of 1.342m;
- Angle from equilibrium to flooding after tank damage and wind heeling shall be greater than 0.0 degrees.
- The ratios of areas from equilibrium to maximum righting arm or flooding shall be greater than 1.0 inclusive of wind heeling;
- Angle at equilibrium shall be less than 7.0 degrees after the final stages of flooding;
- Maximum righting arm shall be greater than 0.3 meters; and
- Metacentric height at equilibrium shall be greater than 0.3 meters.
Compliance with the stability criteria does not ensure immunity against capsize, and under no circumstances absolves the Master from his responsibilities for the safety of the vessel. The Master should therefore exercise prudence and good seamanship at all times, having regard for the season of the year, weather forecasts and the navigational zone. When disconnected from the moorings the Master should take appropriate action as to the speed and course warranted by the prevailing conditions and circumstances.
All doorways, hatches and openings in the structure through which water can enter the hull, deckhouses are to be maintained in good condition and closed in adverse weather conditions.
Weathertight doors shall be kept closed except when necessarily opened for the working of the vessel and shall always be ready for immediate closure. Furthermore such doors are to be clearly marked to indicate that these fittings are to be kept closed except for access. Any closing devices provided for vent pipes to liquid tanks are to be secured in bad weather.
In the event of an accident which results in any compartment or compartments flooding, the operator should pay attention to trim and heel of the vessel. All compartments should then be monitored to check if continuous or progressive flooding occurs. Where possible, weathertight openings (e.g. air pipe, ventilation pipe etc.) which may cause continuous flooding are to be closed.