Bulk Carrier

2.4 Additional requirements for "Bulk Carrier", "Ore Carrier" or "Bulk-Ore Carrier" Service Notation

For ships with the service notation "Bulk carrier" or "Ore carrier", there are additional features which require special attention on top of the arrangements provided in Section 2.2 concerning general cargo ships. Refer to the Rules, Part A. The traditional designs of bulk carriers and ore carriers are shown in Fig. 4 and Fig.11 respectively. Unlike general cargo ships which are traditionally built with the transverse framing system, bulk carriers are traditionally built with a combined system involving transverse side frames in the hold (to allow easy cargo trimming) and longitudinal framing in deck, bottom, and in the hopper wing tanks which are a feature of these vessels. Ore carriers are traditionally built with full depth wing tanks and full longitudinal framing as shown in Fig.11. These designs look more like a tanker than a cargo ship, except that there is a hatch and a double bottom in the centre and that they are subject to a much different service life, resulting in a lot of wear and tear, due to cargo handling unlike a tanker. Fig.4 - Typical bulk carrier midship section

Where the side framing is transverse and the deck, bottom and hopper tank framing is longitudinal the connection between the two systems is made through brackets connecting the side frames to the hopper plating and the longitudinals as highlighted in Fig.4. The longitudinal hull bending tensile and compressive stresses are highest at deck and bottom, the shear stresses due to hull bending are highest in the sides and thus there are important hull shear stresses acting in the region of the transverse framed side shell and the bracket connection system. The cargo holds are bounded by the ship side shell, the double bottom and usually by corrugated transverse bulkheads with supporting stools top and bottom, and having a large cargo hatchway (see Fig.5). The presence of the large hatchways can lead to problems from torsion of the hull as well as the usual stress concentrations at the hatchway corners. In addition, the stiffeners of transverse bulkheads may under some conditions produce stress concentrations in the deck structure between hatches. Where corrugated bulkheads are used, their lack of resistance to transverse loads may result in problems in the deck structure between hatches. In the case of single hull bulk carriers, the hold side frames are individual pieces of structure which, if rendered ineffective, will place additional load on the adjacent frames providing the possibility of progressive and rapid failure of the hold side panel. The loadings on the hold side frames and their brackets are complex and are generated in the transverse direction by cargo loading and hydrostatic loads producing rotation of the hopper and topside tanks. Cyclic loadings are induced from these load sources by the passage of waves and the motion of the ship. See Fig. 6.

a - Original design and construction Some original design features of the hull arrangement overall such as the span of the transverse frames in the hold and thus the depth of the top and bottom hopper sides and the bulkhead arrangement may have some effect on the ability of the hull to withstand operating loads and conditions and thus the life expectancy of the ship. Also the design of structural details may affect the local resistance to operating loads and behaviour under adverse conditions, this is particularly important for the side frame brackets mentioned in Section 2.4, 1.1. Coatings applied during construction have an obvious effect on the ability of the structure to withstand corrosion. This is particularly important in wing tanks of bulk carriers where salt water ballast is carried during ballast voyages and the tank remains empty but often in a "humid" condition on the loaded voyages providing an environment for corrosion. The same is valid for the coatings of holds, specially the frames, brackets and shell side. b - Operating factors Several ship operating factors may have particular adverse affects on the hull structure : b.1 - Maintenance of coatings : When paint coatings start to deteriorate and are not repaired then corrosion commences and may progress very fast in areas of coating breakdown where the uncoated areas become anodic to the area still coated. b.2 - Discharging : The use of heavy grabs (25 tonnes + unladen weight) and bulldozers as well as hydraulic hammers for cargo trimming can lead to structural damage to hold frames and brackets, tank top, hopper side plating and hold end bulkheads. b.3 - Cargo : With coal there comes the problems of high sulphur content and high temperature of the cargo coupled with condensation which forms on the comparatively cold ship's side providing an ideal corrosive environment. Because of the trimming of the cargo this corrosion tends to be concentrated by gravity on the outboard portion of the hold frame webs and the lower bracket connection to the hopper side. See Fig.7. This corrosion can sufficiently weaken the local structure to allow fractures to commence and propagate from the lower bracket to outboard or up the web connection to the side shell (leaving the bracket intact). The same effect may happen at the connection between side shell and transverse hold bulkhead. With ore cargoes the problems may be associated with loading and discharging as mentioned above but also the combined effects of corrosion, damage and cyclic loading when carrying ore cargoes is increased because the comparatively small volume of the cargo itself offers no support to the side structure in resisting the hydrostatic loads at maximum draught. The rolling motion, due to a lower centre of gravity is quicker, resulting in increased sea loads. b.4 - Repair methods : Damages caused by grabs and bulldozers to the hold frames, brackets, hopper sides, etc. will be inspected after cargo discharge by the owner. In some ports, the owner can be faced with a decision to either accept the damages as minor blemishes or to carry out temporary repairs with a view to making permanent repairs later or possibly to sail to a repair port. Note : Any significant damages should be dealt with under the attendance of a Surveyor. If a Surveyor ascertains that repairs where carried out without attendance of a Bureau Veritas Surveyor, he shall re-examine the area and request additional measures to his satisfaction.

a - Fractures at hatchway corners An isolated case may be repaired by veeing and welding after drill stopping the crack. Careful grinding of the hatch corner radius should be carried out to remove any local stress raisers. Hatch corner cracking of a systematic nature should be repaired by partial plate renewal (insert) following all usual welding precautions in this high stress area coupled with consideration of structural modifications to reduce stress concentration i.e. additional stiffeners under deck or extensions of underdeck stiffening or hatch coamings or brackets. Welding of incipient cracks is generally not a successful repair. b - Plate panel buckling and cracks on transverse deck structure between holds b.1 - Buckling Depending on the extent of bulkling ; repairs may be by partial plate renewal or the stiffening of buckled panels ; a combination of plate partial renewals and the addition of extra transverse underdeck stiffeners; extension of hatch coaming tapering bracket in a transverse direction ; or a combination of the proposed solutions. b.2 - Cracks Refer to TNS 02, Section 2.4, 2.3. c - Fractures in hatch coamings Depending on the case, these may be repaired by veeing and welding. The important point for systematic or re-occuring fractures is to remove the stress concentration causing the fractures. Thus each case requires individual consideration. d - Fractures at connection between tank top and hopper plating These may generally be repaired by veeing and welding. However, for systematic fractures, the cause must considered and action taken to re-distribute local stresses by re-alignment of members or fitting addition stiffeners. e - Grab damage and bulldozer damage to hold side frames and brackets Grab damages, etc. are considered as for shell indents and repaired when the depth of indents exceeds acceptable limits. Fractures and splits require immediate repair. Damages to hold frames and brackets are to be treated with care as damages to these elements can seriously weaken the ship side structure. Surveyors carefully examine the connection of bulkheads to the tank top, which can be sheared by bulldozer action. This type of damage is very important because flooding of adjacent holds may lead to the loss of the ship. Similarly the connection of the bulkhead to the stool and lower wing tank shall be checked. The Surveyor should also be aware that cargo handing damages are sometimes repaired without classification survey and even by the crew. Previous repairs are checked for signs of fracture, etc. f - Grab damage to tank top, hopper sides and stools See limits of plate deformations in Mini ISM. Repairs are always requested when internal stiffeners are buckled. The Surveyor is reminded that in principle repairs by doubling can only be considered as a temporary measure (excepted in specific cases, see TNS 14), and therefore are permanently repaired during renewal surveys. g - Fractures in hold side frames and brackets

Fractures here may occur in frames without signs of damage or wastage. However damages and wastage serve to weaken the elements and lead to fractures. When fractures are detected, adjacent frames are also carefully examined for similar fractures. See Fig. 8 and Fig. 9. Systematic fracturing may be best dealt with by reducing frame spans by fitting deeper brackets or by fitting slightly increased scantling frames, etc. Fractures resulting from corrosion or damage are dealt with by renewal of wasted parts where fractured and in other similar locations where wastage is detected. h - Corrosion of frames and brackets (see Fig. 7) This should be examined by a close-up inspection. Where wastage is at the limits of acceptability the Owner may be advised to apply some form of coating. Where wastage is passed acceptable limits repairs should be requested. i - Fractures at forward and aft extremities of topside tank structures Fractures in this location in way of longitudinals may be as a result of stress concentrations, corrosion or a combination of both. j - Corrosion in topside tanks Uncoated tanks corrosion may proceed quite quickly and in areas of high stress local corrosion will be accelerated. These tanks when empty may remain damp and warm, due to the heating effect of the sun and are prone to corrosion. On uncoated tanks the effect of anodes is limited to when tanks are full and even then the upper part may not be completely protected. When tanks are coated adequately, corrosion only becomes a problem when the coating starts to fail. Unfortunately this coating failure is more likely in areas of high stress reversal thus local corrosion tends to be rapid when the coating starts to fail.

Further to the preceding, it follows that surveys of bulk carriers entail checking for these known problems during the usual examinations. a - Decks Particular attention to hatch corners and coamings for fractures and signs of bucklings in transverse deck panels. Grab damages in hatch coamings. b - Holds, frames and bulkheads Particular attention to be paid to the following : - tank top for grab damages, - lower hopper side and stools for grab damages, - corrosion at lower end of the transverse bulkheads (inside/outside stools), - fractures at tank top and hopper plating connection, - fractures at bottom of bulkheads caused by bulldozers, - fractures at connection of corrugations to upper and lower wing tanks, - fractures in hopper side plating especially adjacent to bottom bracket toes, - ractures in top side tank hopper side plating, staining or actually leakages are investigated, - the hold side frames and brackets must be carefully close up inspected, from ladder, staging or mobile platform (cherry-picker) to check for corrosion and fractures and also for damages such as buckling of webs and face flats of frames or brackets, and condition of shedder plates and any tripping brackets. For side frames (Fig.10) these must be inspected along the line A B C D. For brackets, the ends A F E D are checked. The frames and brackets are sounded with a hammer and thickness measurements of the frame webs, shell in way and brackets, requested where significant corrosion is found. c - Top side tanks Particular attention to the condition of the coating, if any, and the extent of any corrosion particularly concerning deck longitudinals and the main transverse brackets and web-frame system. Thickness gaugings is requested where deemed necessary or where required by the Rules. The forward and aft extremities of these tanks are carefully examined for fractures.

Ore carriers of designs similar to that shown in Fig.11 may experience some of those problems experienced by bulk carriers and dealt with in Section 2.4, 2., i.e. fast loading, stevedore damage, corrosion in wing ballast tanks and cyclic wave loading of the ship side. When compared to typical bulk carriers, the main differences are the hull frames which are not subject to grab damages or cargo corrosion and the cyclic effect of the waves which produces a different result in these longitudinally framed vessels. In this case, the result is wastage and fractures in way of the tie beams, in the cross tie beams themselves or in the web frames, initiating at longitudinals of the shell side or longitudinal bulkhead. The combination of damaged tie beams and fractures in way of the longitudinals serves to multiply the effect and severe damage can result.

This type of problem may be very localised in the beginning and the remainder of the tank may be without signs of any deterioration. Thus it is very important for the Surveyor to specifically examine these areas during each survey.

The scope of survey is given in the Rules, Part A. The extent of survey increases with the age of the ship. This TNS does not define or limit the scope of surveys but outline the manner in which surveys are performed.

Refer to Section 2.2, 2. and Annexes 2, 5 and 06. This includes all the items required for annual surveys for all ships.

4.4.1

The examination of service spaces, machinery spaces and tanks located in way of machinery spaces is carried out as per renewal survey for all ships.

4.4.2

An overall survey of all ballast tanks and all spaces within the cargo area is carried out at each renewal survey. Sketches of typical wing tanks, cargo holds, topside tanks, double bottom tanks are used in order to note fractures, bucking, corrosion, coating failures, ultrasonic readings, etc. The Survey Report File maintained on board and the main drawings are consulted, as well as the hull condition evaluation report issued by the Society after the previous renewal survey. The aim of the overall survey is to discover any excessive corrosion, significant deformation, fracture or other structural defects, and define the extent of close-up surveys and ultrasonic thickness measurement.

4.4.3

Close-up surveys During each renewal survey, close-up surveys shall be carried out in order to assess the condition of bulkheads, shell frames, their end attachments in cargo holds and salt water ballast tanks. The minimum requirements for close-up surveys are given in the Rules, Part A. Thickness measurements of local corrosion areas in shell frames and their end attachements shall be carried out in connection with close-up surveys in order to define exactly the actual conditions and required repairs.

The requirements for thickness measurements are given in the Part A of the Rules. Provisions of Section 2.2, 6. also apply. The Surveyor's report must state a summary of findings which reflects the scope and extent of the measurements, including the percentage of thickness reduction and indicate the extent of repairs. The full measurements report taken by a certified Company shall be also sent to DNS, after having been stamped. The points which were verified by the Surveyor (at random) are clearly identified on this report.

Refer to Part A of the Rules, sub-paragraph 10.5 and to Annex.

Refer to Section 2.2, 3.5. In addition, thickness measurements shall be required as per the Rules.

Sub-para. 4.7 in Section 2.2 fully applies.

Refer to Section 2.2, 8. and Section 2.3, 10. The particular characteristics of the ship's type shall be taken into account. End of Section