NtS 280/1992 vRegulations on watertight closure, stability and watertight subdivision of offshore support vessels

Annex I

1 General

1.1 Definitions
For the purpose of this Notice and its corresponding Annexes the following definitions apply:
1.1.1 offshore support vessel: a ship that

1. is engaged primarily in the transport of stores, materials and equipment to offshore installations; and
2. has been designed with accommodation and bridge erections in the forward part of the vessel and an exposed cargo deck in the after part for the handling of cargo at sea;

1.1.2 offshore installation: a construction at sea for the benefit of the exploration or extraction of minerals or an equivalent installation;
1.1.3 length: the length (L) of a ship, as described in Article 2, first paragraph, of Annex I, to the Ships Decree 1965;
1.1.4 load lines: the load lines, as described in Article 2, second paragraph, of Annex I, to the Ships Decree 1965;
1.1.5 weathertight: the degree of watertightness, as described in Article 2, twelfth paragraph, of Annex I, to the Ships Decree 1965;
1.1.6 summer freeboard: the freeboard, as described in Article 40, first paragraph, of Annex I, to the Ships Decree 1965.

2 Provisions and measures against capsizing

2.1 Watertight closure and constructional precautions against capsizing
2.1.1 Air pipes and ventilators should be fitted in protected positions in order to avoid damage by cargo during operations and to minimize the possibility of flooding. Air pipes on the exposed cargo and forecastle decks should be fitted with automatic closing devices.
2.1.2 Access to the machinery space should, if possible, be arranged within the forecastle. Any access to the machinery space from the exposed cargo deck should be provided with two weathertight closures. Access to spaces below the exposed cargo deck should preferably be from a position within or above the superstructure deck.
2.1.3 Due regard should be given to the position of machinery space ventilators. Preferably they should be fitted in a position above the superstructure deck, or above an equivalent level if no superstructure deck is fitted.
2.1.4 Hatches and doors which give access to the cargo deck should be kept closed during navigation, except when necessarily opened for the working of the vessel, and should always be ready for immediate closure and be clearly marked to indicate that these fittings are to be kept closed except for access.
2.1.5 The area of the freeing ports in the side bulwarks on the cargo deck should at least meet the requirements of Article 24, of Annex I, to the Ships Decree 1965. The disposition of the freeing ports should be carefully considered to ensure the most effective drainage of water trapped in pipe deck cargoes or in recesses at the after end of the forecastle.
In vessels operating in areas where icing is likely to occur, no shutters should be fitted in the freeing ports.
2.1.6 The area provided for drainage of the pipe stowage positions should be in excess of the required freeing port area in the cargo deck bulwarks and should not be fitted with shutters.
2.1.7 A vessel engaged in towing operations should be provided with means for quick release of the towing hawser.

2.2 Operational precautions against capsizing
2.2.1 The arrangement of cargo stowed on deck should be such as to avoid any obstruction of the freeing ports or of the areas necessary for the drainage of pipe stowage positions to the freeing ports.
2.2.2 A vessel, when engaged in towing operations, should not carry deck cargo, except that a limited amount, properly secured, which would neither endanger the safe working of the crew nor impede the proper functioning of the towing equipment, may be accepted.
2.2.3 A minimum freeboard at the stern of at least 0,005 L should be maintained in all operating conditions.

3 Intact stability

3.1 Drawings and documents
The drawings and documents to be submitted for determination of the stability should consist of:
3.1.1 a calculation report of the inclining test of the ship, containing:
.1 the calculation of the mass of the light ship condition;
.2 the calculation of the location of the vertical centre of gravity above baseline (KG) for the light ship condition, according to Standard NEN 3085;
.3 the calculation of the location of the longitudinal centre of gravity relative to the aft perpendicular or relative to the 1/2 L_ord (G1) for the light ship condition, according to Standard NEN 3085.
3.1.2 stability data consisting of:
.1 a capacity plan containing the necessary data concerning tanks and eventual cargo spaces. The right location and the use of the various compartments should be indicated on the plan by means of a drawing of the longitudinal cross-section of the ship, as well as the necessary horizontal cross-sections over double bottom tanks, etc. The capacity plan should mention the volumes of the different tanks and cargo spaces with the position of the corresponding vertical centres of gravity above the baseline and of the longitudinal centres of gravity relative to the aft load line or relative to 1/2 L_ord, all this according to standard NEN 3085, in the form of a table. In this table the effect of the free surfaces of liquid should also be indicated for each tank;
.2 hydrostatic curves including the curves of ordinate areas (Bonjean curves) and the curves of ordinate moments*, for checking the hydrostatic data in the calculation report of the inclining test. The calculation of the data for the hydrostatic curves should be performed in accordance with the provisions in Annex II. When the hydrostatic data in the calculation report of the inclining test have been calculated directly with the trim and the draughts, belonging to the condition at the inclining test and with software accepted by the Head of the Shipping Inspectorate, such hydrostatic curves need not be submitted;
.3 a diagram of the cross curves of static stability for angles of 5, 10, 15, 20, 25, 30, 40, 50 and 60 degrees, being the graphics of KNsinΦ, as a function of the displacement in tonnes of 1000 kg, including shell and appendages in water with a density of 1,025 t/m³ *. Here KNsinΦ is the distance from the keelpoint (K) - the intersection of the centreline of the ship with moulded frame at midship - to the line of the buoyancy force, for that particular heeling. The calculation should be performed in accordance with the provisions in Annex II;
.4 the calculation of the location of the vertical and longitudinal centres of gravity, of the fore and aft draughts, of the initial metacentric height (GM) and of the curve of the righting lever for the following loading conditions of the ship:
.4.1 vessel in fully loaded departure condition with cargo distributed below deck and with cargo specified by position and weight on deck, with full stores and fuel, corresponding to the worst service condition in which all the relevant stability criteria are met;
.4.2 vessel in fully loaded arrival condition with cargo as specified in
.4.1 and with a remainder of stores, fuel and drinking-water corresponding with 10 percent of the amounts given in .4.1;
.4.3 vessel in ballast departure condition, without cargo but with full stores, full bunkers and drinking-water tanks;
.4.4 vessel in ballast arrival condition, without cargo and with a remainder of stores, fuel and drinking-water corresponding with 10 percent of the amounts given in .4.3;
.4.5 vessel in the worst anticipated operating condition;
.4.6 other relevant intended loading conditions, taking into account the manner of use of the ship.
In the calculation of the loading conditions, mentioned in .4.1 up to .4.6, account should be taken of the effect of free surfaces of liquids in the tanks as well as the effect of the wind on the ship, as indicated in 4. respectively 5., of Annex II.
The assumptions of 3.2 should also be taken into account. In any loading condition the distribution of the cargo and the filling of the tanks should be indicated in small format in one or more longitudinal cross-sections of the ship.
.5 a table in which for a sufficient number of draughts (relative to the bottom of the keel plate), the following hydrostatic data of the ship can be read: displacement in fresh water with a density of 1,000 t/m³ including shell and appendages, in tons of 1000 kg;
displacement in fresh water with a density of 1,025 t/m³ including shell and appendages, in tons of 1000 kg;
weight, required for 1 cm change in draught in water with a density of 1.025 t/m³, in tons of 1000 kg;
moment, required for 1 cm total change of trim on the draught scales in water with a density of 1.025 t/m³, in tonmetres;
longitudinal centre of buoyancy relative to the line, as referred to in .3, in metres;
longitudinal centre of gravity relative to the line, as referred to in .3, in metres; transverse metacentre above the baseline in metres;
In order to be able to determine the various hydrostatic data accurately, the differences in draughts should not exceed 2 cm.
.6 a drawing from which the relation between the draughts read on the draught marks can be easily read, and the mean draught on 1/2 L_ord can be deduced. This drawing can be combined with the drawing 'location of draught marks';
.7 a curve of maximum allowable KG where the criteria, mentioned in 3.3, as a function of the draught in metres relative to the bottom of the keel plate and of the displacement in tons of 1000 kg including shell and appendages in water with a density of 1.025 t/m³.
Here the KG indicates the position of the centre of gravity above the baseline, after this has been corrected for the effect of free surfaces of liquids in the tanks.
In general, the trim conditions of the offshore support vessels which occur during operation will have a noticeable influence on the hydrostatic data and on the righting levers. In that case, several curves of maximum allowable KG should be submitted;
.8 information for the proper use of an eventually present device for cushioning the rolling of the ship.

3.2 Assumptions for calculating loading conditions
The assumptions for calculating loading conditions should be as follows:
.1 if a vessel is fitted with cargo tanks, the fully loaded conditions of 3.1.2.4.1 and 3.1.2.4.2 should be modified, assuming first the cargo tanks full and then the cargo tanks empty;
.2 when an amount of water ballast is necessary in any loading condition in order to meet the stability criteria, mentioned under .4, this should be clearly indicated, and in detail, in the calculation of the loading condition involved;
.3 when deck cargo is carried the location of the longitudinal and vertical centres of gravity and a realistic stowage weight should be determined and included in the loading condition involved;
.4 where pipes are carried on deck, a quantity of trapped water equal to a certain percentage of the net volume of the pipe deck cargo should be assumed in and around the pipes. The net volume should be taken as the internal volume of the pipes, plus the volume between the pipes. This percentage should be 30 if the freeboard amidships is equal to or less than 0.015 L and 10 if the freeboard amidships is equal to or greater than 0.03 L. For intermediate values of the freeboard amidships the percentage may be obtained by linear interpolation.

3.3 Stability criteria
3.3.1 In all loading conditions the following stability criteria should be met:
.1 the area under the righting lever curve should not be less than 0.055 metre-radians up to Φ = 30° angle of heel and not less than 0.09 metre-radians up to Φ = 40° or the angle of flooding Φf* is this angle is less than 40°. Additionally, the area under the righting lever curve between the angles of heel of 30° and 40° or between 30° and Φf, if this angle is less than 40°, should not be less than 0.03 metre-radians;
.2 the righting lever GZ should be at least 0.20 m at an angle of heel equal to or greater than 30°;
.3 the maximum righting arm should occur at an angle of heel preferably exceeding 30° but not less than 25°;
.4 the initial metacentric height should not be less than 0.15 m.
3.3.2 The following equivalent criteria are recommended where a vessel's characteristics render compliance with 3.3.1 impracticable:
.1 the area under the curve of righting levers should not be less than 0.070 metre-radians up to an angle of 15° when the maximum righting lever occurs at 15° and 0.055 metre-radians up to an angle of 30° when the maximum righting lever occurs at 30° or above. Where the maximum righting lever occurs at angles between 15° and 30°, the corresponding area under the righting lever curve should be 0.055 + 0,001 (30 - Φmax) metre-radians;
.2 the area under the righting lever curve between the angles of heel of 30° and 40°, or between 30° and Φf if this angle is less than 40°, should be not less than 0.03 metre-radians;
.3 the righting lever should be at least 0.20 m at an angle of heel equal to or greater than 30°;
.4 the maximum righting lever should occur at an anlge of heel not less than 15°;
.5 the initial metacentric height should not be less than 0.15 m.
3.3.3 The angle of heel due to the wind moment (Φc) should not exceed: - 50 degrees; or - the angle of heel (Φf), where the ship is flooded, if this angle of heel is smaller than 50 degrees; or - the angle of second intercept between wind heeling lever and GZ curve when this is smaller than Φf and 50 degrees. The calculation of the effect of the wind should be in accordance with the provisions in Annex II.
3.3.4 The stability criteria mentioned in 3.3.1 and 3.3.2 should be regarded as minima. There are no maxima, but it is advisable to avoid extreme values because they can result in acceleration forces which can he harmful for the ship, the crew, the equipment or the safe carriage of the cargo.
3.3.5 When devices are used for cushioning the rolling of the ship, it should be proven that the stability criteria in 3.3.1 or 3.3.2 are complied with in case of operating devices.

3.4 Stability data on board
An approved stability book, containing the documents, mentioned in 3.1.2, under .1, .3, .4, .5, .6, .7 and .8, should be on board.

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* These data may also be submitted in the form of appropriate tables.
1 The angle of heel where the ship is flooded (Φf) is the angle of heel where openings in the hull, superstructures or deckhouses, not capable of watertight closure, immerse. For the purpose of this criterium, small openings through which in the opinion of the Head of the Shipping Inspectorate progressive flooding cannot take place, need not be considered as open.
2 Φmax is the angle of heel in degrees at which the righting lever curve reaches its maximum.

4 Watertight subdivision and damage stability

4.1 General
Taking into account, as initial conditions before flooding, the standard loading conditions as referred to in 3.1.2.4 and the damage assumptions in 4.2, the vessel should comply with the damage stability criteria as specified in 4.3.

4.2 Damage assumptions
4.2.1 Damage should be assumed to occur anywhere in the vessel's length between transverse watertight bulkheads.
4.2.2 The vertical extent of damage should be assumed from the underside of the cargo deck, or the continuation thereof, for the full depth of the vessel.
4.2.3 The transverse extent of damage should be assumed as 760 mm, measured inboard from the side of the vessel perpendicularly to the centreline at the level of the summer load waterline.
4.2.4 A transverse watertight bulkhead extending from the vessel's side to a distance inboard of 760 mm or more at the level of the summer load line joining longitudinal watertight bulkheads may be considered as a transverse watertight bulkhead for the purpose of the damage calculations.
4.2.5 If pipes, ducts or tunnels are situated within the assumed extent of damage, arrangements should be made to ensure that progressive flooding cannot thereby extend to compartments other than those assumed to be floodable for each case of damage.
4.2.6 If damage of a lesser extent than that specified in 4.2.2 or
4.2.3 results in a more severe condition, such lesser extent should be assumed.
4.2.7 Where a transverse watertight bulkhead is located within the transverse extent of assumed damage and is stepped in way of a double bottom or side tank by more than 3.05 m, the double bottom or side tanks adjacent to the stepped portion of the transverse watertight bulkhead should be considered as flooded simultaneously.

4.3 Damage stability criteria
4.3.1 The final waterline, taking into account sinkage, heel and trim, should be below the lower edge of any opening through which progressive flooding may take place. Such openings should include air pipes and those which are capable of being closed by means of weathertight doors or hatch covers and may exclude those openings closed by means of watertight manhole covers and flush scuttles, small watertight cargo tank hatch covers which maintain the high integrity of the deck, remotely operated watertight sliding doors, and sidescuttles of the non-opening type.
4.3.2 In the final stage of flooding, the angle of heel due to unsymmetrical flooding should not exceed 15°. This angle may be increased up to 17° if no deck immersion occurs.
4.3.3 The stability in the final stage of flooding should be investigated and may be regarded as sufficient if the righting lever curve has at least a range of 20° beyond the position of equilibrium in association with a maximum residual righting lever of at least 100 mm within this range. Unprotected openings should not become immersed at an angle of heel within the prescribed minimum range of residual stability unless the space in question has been included as a floodable space in calculations for damage stability. Within this range, immersion of any of the openings referred to in 4.3.1 and any other openings capable of being closed weathertight may be authorized.
4.3.4 The Head of the Shipping Inspectorate should be satisfied that the stability is sufficient during intermediate stages of flooding.

4.4 Assumptions for calculating damage stability
4.4.1 Compliance with 4.3 should be confirmed by calculations which take into consideration the design characteristics of the vessel, the arrangements, configuration and permeability of the damaged compartments and the distribution, specific gravities and the free surface effect of liquids.
4.4.2 The permeability of compartments assumed to be damaged should be as follows:

Spaces	Permeability
Appropriated to stores	0.60
Occupied by accommodation	0.95
Occupied by machinery	0.85
Void spaces	0.95
Intended for dry cargo	0.95

The permeability of tanks should be consistent with the amount of liquid carried, as shown in the loading conditions specified in 3.1. The permeability of empty tanks should be assumed to be not less than 0.95.
4.4.3 The free surface effect should be calculated at an angle of heel of 5 degrees for each individual compartment or the effect of free surfaces of liquids in a tank should be calculated over the range of positive residual righting arm, by assessing the shift of liquids by moment of transference calculations.
4.4.4 Free surface for each type of consumable liquid should be assumed for at least one transverse pair of tanks or a single centreline tank. The tank or tanks to be taken into account should be those where the effect of free surface is the greatest.
4.4.5 Alternatively, the actual free surface effect may be used provided the methods of calculation are acceptable to the Head of the Shipping Inspectorate.

Annex II

1 General
1.1 The baseline for the hydrostatic curves and for the diagram of the cross curves of static stability should be drawn parallel to the design waterline through the intersection of the moulded frame with the centreline of the ship, at 1/2 Lord; all this according to the standard NEN 3085.
1.2 The data for the hydrostatic curves and the cross curves of static stability should be calculated by software*.
1.3 The hydrostatic data and the cross curves of static stability should be performed for the trim conditions which appear in practice, taking into account the change of trim due to an occurring angle of heel. The calculation of the cross curves of static stability should be performed under the assumption that the ship can trim freely during heel.

2 Hydrostatic curves
The calculations should be performed to a draught which is significantly above the design draught and at least equal to 85 per cent of the depth to the freeboard deck.

3. Righting lever curves
3.1 A possibly present deck cover may be taken into account in the calculation of the righting lever curves.
3.2 With respect to the watertight closure of superstructures, deckhouses etc. the following applies:
.1 enclosed superstructures complying with Article 2, tenth paragraph, under b, of Annex I to the Ships Decree 1965, may be taken into account;
.2 enclosed superstructures under the second deck above the freeboard deck complying with .1, may also be taken into account;
.3 deckhouses on the freeboard deck may be taken into account, provided that they comply with the conditions for enclosed superstructures under .1;
.4 where deckhouses comply with the above conditions, except that no additional exit is provided to a deck above, such deckhouses should not be taken into account; however, any deck openings inside such deckhouses should be considered as closed even where no means of closure are provided;
.5 deckhouses, the doors of which do not comply with the requirements of Article 12 of Annex I to the Ships Decree 1965, should not be taken into account; however, any deck openings inside the deckhouse are regarded as closed where their means of closure comply with the requirements of the Articles 15, 16, 17 or 18 of Annex I to the above Decree;
.6 deckhouses on decks above the freeboard deck should not be taken into account but openings within them may be regarded as closed;
.7 superstructures and deckhouses not complying with the provisions under .1, can be taken into account up to the angle at which their openings are immersed (at this angle, the static stability curve should show one or more steps, and in subsequent computations the flooded space should be considered non-existent);
.8 in cases where the vessel would sink due to flooding through any openings, the stability curve should be cut short at the corresponding angle of flooding and the vessel should be considered to have entirely lost its stability;
.9 small openings such as those for passing wires or chains, tackle and anchors, and also holes of scuppers, discharge and sanitary pipes should not be considered as open if they submerge at an angle of inclination more than 30°. If they submerge at an angle of 30° or less, these openings should be assumed open if the Head of the Shipping Inspectorate considers them to be a source of significant flooding;

• Informatietype: Informatie voor uitvoering
• Documentsoort: Bekendmaking aan de sector
• Toegangsrechten: Extern
• Identificatie: PUC_1159_14
• Versie: 1
• Datum beschikbaarheid: 26-07-2006
• Geldigheid van: 13-02-1992
• Wijzigingsdatum: 24-11-2016
• Titel: NtS 280/1992 vRegulations on watertight closure, stability and watertight subdivision of offshore support vessels
• Organisatie: Inspectie Leefomgeving en Transport
• Taal: Engels
• Onderwerp: Veiligheid / Safety
• Hoofdtaak: Koopvaardij / Merchant Shipping
• Thema: Zeevaart / Seagoing ships / Mandatory Regulations / National Instruments (Mandatory) / Bekendmakingen aan de Scheepvaart / Notices to Shipping / Notices to Shipping