Presentation on theme: "TANKER DAMAGE STABILITY"— Presentation transcript:
1 TANKER DAMAGE STABILITY Paul Coley & Nick QuarmbyMaritime & Coastguard Agency
2 BackgroundThe UK first became concerned over the issue of damage stability verification on tank vessels in 2005 as a result of problems highlighted during flag in of tank vessels, port state control inspection and a survey of UK tank ship operators
3 Issues identified at Flag in Low or zero margins on stability in worst case of damageDamage cases missing from the damage stability evaluation, particularly lesser casesInsufficient consideration given to slack or partially filled tanksInsufficient consideration given to use of deck tanksApprovals based upon unrealistic conditions of loading
4 Of eleven LPG and chemical tanker stability approvals considered for endorsement by the UK at this time, seven were found to be incorrectly based and to omit critical elements to the verification. This is a rejection rate of 63%.
5 Port State Control Observations Vessels loading alternate conditions from those in the approved stability bookVessels appraising loading conditions using longitudinal strength computer with additional intact stability assessmentMasters endorsing loading conditions clearly marked “not valid for damage”
6 Survey of UK tanker operators How many vessels operated – including foreign flag vessels in their fleetsDo they load alternate conditions from the SIBDo they operate with empty or slack tanksDo they verify stability prior to departureDo they verify using intact or damage stability assessment
7 Survey Results Total of 76 ships operated 59 vessels (77%) load alternate conditions43 vessels (56%) regularly operate with slack or empty tanks69 vessels (90%) are fitted with stability assessment programs30 vessels (39%) only assess intact stability
8 ConclusionsInsufficient pre-departure checks are being made on tank vessels to ensure compliance with statutory damage stability requirements.Existing stability approvals for these vessels may not always ensure that these requirements are metThere is a compelling need for enforcement action to be undertaken to ensure that existing international instruments are being complied withRecent flag in cases for various tanker types have revealed that stability approvals for such vessels commonly exhibit no margin on stability for the sample intact loading conditions included in the stability information book. When sample loading conditions are subsequently obtained, it is common to find that these do not meet basic damage stability as the optimised intact GMf used in the initial approval cannot be sustained in practice.
9 UK ResponseTo publicise the issue amongst UK operators, IACS and international tanker operators associationsTo consult other flag states on the issue and to consider proposals for co-ordinated enforcement of existing international instrumentsWith others, to draw attention of this issue to IMO through MSC 83/25/14, proposing development of a common interpretation for these instruments to enable consistent enforcement action to be takenTo propose that guidelines be developed for tank ship stability approval in the expectation these will be taken up by IACS under the umbrella of URL5.
10 IACS Unified Requirement URL5 IACS has introduced a new requirement relating to stability computation under URL5This makes it mandatory for loading computers fitted to any IACS class vessel contracted after 1 July 2005, which incorporate a stability element, to be approved to verify all aspects of stability which apply to that vessel, including damage requirements.This entire issue has been recognised by IACS in their introduction of a new Unified Requirement URL5 for Load Line, which provides that any loading computer incorporating a stability element fitted in an IACS classed vessel contracted after 1 July 2005 should be capable of executing all stability verifications which apply to the class of vessel under consideration.The introduction of this URL leaves us with a need to address procedures on the existing vessels not covered by its provisions.
11 EnforcementGuidelines for a common interpretation of the international instruments are necessary to ensure consistent enforcement actionHowever, the need for enforcement action is clear and is not dependent upon such guidelines being developedAs a flag and port state administration the UK will seek to pursue co-ordinated enforcement action if guidelines cannot be agreed within an acceptable timescale
12 EnforcementEnforcement action shall apply only to those vessels which do not operate according to their existing stability approvalVessels which adhere closely to their approved loading conditions will not be required to provide direct means of verificationA definition of what constitutes “closely loaded” is required to verify loading is in accordance with the approved conditions
13 EnforcementWhere alternate conditions of loading are employed, verification by means of critical KG or GM data is acceptable subject to any parameters fixed to determine the critical data being verified as metIt is the opinion of the UK that such verifications should be made using a URL5 type 2 stability program to provide an auditable record for PSC inspection
14 EnforcementIt is the opinion of the UK and its co-sponsors that where vessels load alternate conditions significantly different from those in the approved stability book, these should be verified on board using a URL5 type 3 stability program
15 Enforcement - Proposal Inspection campaign to identify where vessels are loading to alternate conditions without acceptable damage verificationIn such cases an operational (ISM) defect to be raised against the ship and letter of warning to be issuedShould further cases of loading alternate conditions become apparent at subsequent inspections, detention or banning to be considered
16 Enforcement - Conclusion During any forthcoming inspection and enforcement campaign, operators must ensure that crews can produce records on board to demonstrate that damage stability has been verified.UK strongly recommends the provision of damage stability programs for this purpose on any existing vessel where adherence to the approved conditions is not practicable.All operators should be aware that in any forthcoming inspection campaign their vessels will be required to produce records which demonstrate that damage stability has been verified. In this respect, operators should take full advantage to appraise their systems prior to the commencement of any enforcement campaign.
18 Basic Pre-Departure Checks Longitudinal Strength - Load LineIntact Stability - Load LineDamage StabilityOil Tankers - Marpol Annex 1, reg 25Gas Tankers - IGC Code, Chapter 2Chemical Tankers - IBC Code, Chapter 2In order to operate safely, and within statutory requirements, the following checks require to be made prior to departure. Strength is a Class requirement on all vessels exceeding 65m length, intact stability is normally assessed using an intact stability function attached to the basic loading computer program used to assess strength, but damage assessment is commonly omitted. There is a common presumption that intact stability assessment against a critical intact GMf curve is sufficient, but it is not.
19 Documentation on Board Approved Stability InformationIntact loading ConditionsDamage CalculationsCertificationOil Tankers – IOPP Certificate and Form BGas Tankers – Certificate of FitnessChemical Tankers – Certificate of FitnessStatutory documentation held on board will include the approved intact stability information, which will typically make no reference to damage stability requirements, a set of damage calculations, which verify only that the sample intact loading conditions in the approved intact stability information comply with damage, and typically with no reserve margin on the worst case, and the statutory certification applicable to the type of tank ship concerned.
20 IOPP Form B 5.7 Subdivision and Stability (regulation 25) 5.7.1 The ship is required to be constructed according to, and complies with, the requirements of regulation 255.7.2 Information and data required under regulation 25(5) have been supplied to the ship in an approved formIt may be noted that the basic requirement in the case of Marpol Annex 1 certification is for the vessel to comply with reg 25(5), and that this requires information to be provided to the master in an approved from such that the stability of the vessel may readily be established in any condition of loading. It would be the view of MCA, as the UK Maritime Administration, that this requires the stability information to be sufficiently detailed in scope to enable the loading officer to assess whether minor loading variations from the sample intact loading conditions in the approved intact stability information remain compliant with damage, or should be prohibited. As a minimum there should be information relating to empty or slack tanks, use of deck tanks where fitted, and the use of ballast. A lack of such detailed data renders the vessel unable to load minor variations from the standard loading conditions without referring these to flag or Class for prior approval, and this therefore limits the practical operation of the vessel.
21 IGC and IBC Certificates of Fitness 5 That the ship must be loaded: .1 in accordance with the loading conditions provided in the approved loading manual, stamped and dated and signed by a responsible officer of the Administration, or of an organization recognized by the Administration; or .2 in accordance with the loading limitations appended to this Certificate.Where it is required to load the ship other than in accordance with the above instruction, then the necessary calculations to justify the proposed loading conditions should be communicated to the certifying Administration who may authorize in writing the adoption of the proposed loading condition.The warning statement included on the Certificate of Fitness for IBC and IGC vessels is more stark. It clarifies the position previously given for the Marpol vessels, by including a direct reference to the need for flag or Class approval of alternate loading conditions on the Certificate. For Marpol vessels, this warning is normally included in the intact stability information and on the approval documents normally issued by Class.
22 Masters and Owners Responsibilities International Safety Management (ISM)Paragraph – The company should establish a safety and environmental policy which ensures compliance with mandatory rules and regulationsParagraph 7 - The Company should establish procedures for the preparation of plans and instructions, including checklists as appropriate, for key shipboard operations concerning the safety of the ship and the prevention of pollution. The various tasks involved should be defined and assigned to qualified personnel.Under ISM provisions it is incumbent upon companies to put procedures in place, including checklists if appropriate, to ensure that statutory requirements are met. This provision clearly extends to verification of stability compliance, including damage compliance, prior to departure from port. It is clear from inspection of UK and foreign tank vessels that company safety management systems rarely formalise this requirement and that it is common to find intact loading conditions signed off by the master which carry warnings such as “This loading condition complies with intact stability only, damage stability to be verified also”, or similar.
23 Stability Considerations To consider the factors which may influence consideration of variations in loading.
24 Typical Stability Approval at Build Intact stability information bookletContains sample intact loading conditions.On approval, these intact loading conditions are themselves deemed to be “approved” for use.Damage stability appraisalUsually a separate submissionDemonstrates only that the approved intact loading conditions will survive the extent of damage required by the applicable Convention or Code, and achieve the minimum residual stability standard.As previously discussed, the stability approval at build typically comprises production of an intact stability information booklet which includes sample intact loading conditions, which may be selectively derived, which show compliance with damage stability requirements in a separate document. Once formal approval is issued, then the sample loading conditions themselves become “approved” for use in loading the vessel, but variations in loading, or alternate loading conditions, are specifically excluded from the approval. If the approved intact stabilty information does not contain any cohesive information on how the loading may be varied safely from the approved loading conditions, then the operability of the vessel is necessarily restricted to these loading conditions alone.
25 In this approach damage is applied directly to the approved loading conditions on a case by case basis. The results, and the approval, is therefore conditional upon the assumption that the following parameters remain unchanged in the loaded vessel, otherwise damage results may be adversely affected :Cargo SGDraught and or TrimEmpty/Part Filled Cargo TanksCargo or Ballast DistributionUse of Deck TanksIn restricting the loading to the approved conditions, there is no scope for even minor variation in loading unless the effects of such variation on damage stability compliance can be assessed. In this sense, any variation in cargo SG, Draught or trim, loading of partially filled or empty tanks, changes to the approved cargo or ballast distribution, or the use of deck tanks may make an approved condition non-compliant if guidance is not provided to assess the effects on residual stability after damage.
26 Variation in LoadingThe ability to load a vessel is restricted where approval is granted on the basis of damage appraisal of approved intact loading conditions.This may be of little consequence on a large deep sea vessel fulfilling a long term charter. But it is likely to prove problematic for a small coastal parcel tanker.It is often considered that alternate loading conditions are acceptable, particularly where these do not deviate “significantly” from the approved intact loading conditions, but there is no safe basis for this conclusion.Restricting variation in loading becomes more important as the margin on residual stability calculated in the damage appraisal decreases. In vessels with no margin on stability there is no scope to vary loading and the commonly accepted concept that the vessel may be loaded “closely to” or “not significantly different” from an approved loading condition may be erroneous.
27 Significant Variation It is the opinion of the UK and its co-sponsors that the concept of a significant deviation from an approved loading condition requires to be defined.We are presently proposing that a loading condition should be considered to deviate significantly from an approved condition if the following limits are exceededContent of cargo and ballast tanks : 1% by weightOverall condition KG or GM (corrected for FS) : ± 2cmScope to vary loading therefore depends upon the margin within which such a variation can be accommodated, and the quality of the information supplied to the master to permit such a variation to be effectively evaluated.
28 Critical KG dataIt is possible to derive critical KG data which satisfy statutory residual stability criteria, but this is normally calculated only on those classes of vessel where the damage compartments are considered to be empty, or loaded with a fixed permeability. Whilst it is theoretically possible to derive such data for a tankship, this would have to be conditional upon many initial assumptions, regarding filling level, initial draught trim and GMf, cargo SG, filling of the adjacent tanks and the damage location, each of which would have to be replicated in a live loading condition before such limiting data could be applied.
30 Vessel heels toward the statutory damage case, all damage tanks are flooded, and the critical intact GMf leading to exact compliance with the residual criteria can be readily determined.
31 Where a vessel cannot comply with statutory damage then prefilling tanks, as indicated, gives cope to reduce the overall effect of damage and to reduce the critical GMf value. However, it may be noted that application of this principle on a tank vessel will only serve to reduce cargo carrying capacity, or to reduce operational flexibility.
32 Typical tanker cross section Consider a basic tank ship cross section.
33 Loaded with low SG cargo Suppose the vessel is loaded with a low SG cargo. What is the likely effect of damage.
34 Heels toward damageIn this case, the mass of flood water is likely to exceed that of the lost cargo and the vessel may heel toward the damage.
35 Loaded with high SG cargo Considering the same vessel with a high SG cargo.
36 Heels away from damageThe lost cargo mass now exceeds the weight of flood water, so the heel is away from the damage.
37 Vessel at deep draughtConsidering the effect of initial draught, or local trim, on damage outcome for a deep condition.
38 Heels toward damageTendency shall be for the vessel to heel toward the damage for lower SG cargoes as flooded mass will not exceed lost cargo mass.
39 Vessel at light draught However, for light draught conditions and a wider range of SG’s the opposite effect will tend to occur.
40 Heels away from damageRetained cargo mass exceeds mass of floodwater following damage.
41 Full tanksSimilar effects will be noted through variation of cargo filling. For example a vessel with full tanks.
42 Heels away after damage Will tend to heel toward the damage as lost cargo mass is maximised and may only be partially replaced by sea water.
43 Slack tanksWhen we consider the same damage case with partially full (slack) tanks, the opposite effect will tend to occur.
44 Heels toward damageFlood water mass will tend to exceed mass of lost cargo.
45 Empty tanksThe most extreme effect shall be on empty tanks, particularly in combination with empty ballast tanks outboard.
46 Severe heel toward damage With unrestricted flooding and no compensating loss of initial cargo mass, resultant heels are likely to be extreme.
47 Typical tanker layoutIf we consider a typical tank layout for a small coastal tanker.
48 Typical single SG loading pattern For a typical single SG loading pattern from the loading manual the critical damage case will be determined in the damage stability assessment.
49 Typical alternate SG loading pattern However, for a differing single SG cargo the critical damage case may change, particularly where cargo distribution is varied between loading conditions to meet damage requirements. It is also evident that the loading manual does not give adequate guidance on the carriage of single SG cargoes between the two cargo SG’s if the cargo distribution does not vary progressively.
50 Significant variation ? When one considers the possibility of the need to vary cargo distributions, and mixed SG cargoes in particular, then typical loading manuals do not consider this issue in any depth. Ultimately it may not be known where the critical damage lies or whether the condition meets damage stability requirements.
51 Significant variation ? An additional consideration is in the use of deck tanks. Again, use of these tanks are typically not covered in any depth or examined for damage stability compliance, so worst damage case may not be known, nor whether residual stability standards will be met.
52 Typical Condition ?When one moves to a typical parcel loading condition for a coastal tanker, the lack of detail in a conventional approved loading manual becomes apparent.
53 Consider variation in cargo SG Variation in cargo SG is of particular relevance when considering alternate conditions of loading, as apparently minor variations from approved loading conditions can render these non-compliant with damage stability requirements.For any loading condition the local cargo tank VCG contributes to the global VCG for the vessel.
54 Cargo and condition KG both rise However, carrying the same mass of a lower VCG cargo will raise the local cargo VCG, and hence, the global VCG of the condition. This may then render the condition non-compliant.
55 The sensitivity of approved cargo loading conditions to minor variations in cargo SG can be demonstrated if one considers a residual stability curve (indicated green) that meets the minimum requirement for residual GZ of 100mm. If the peak occurs at 40 degrees, then an increase in initial condition VCG of only 155mm will totally eliminate the residual curve.
56 This example of a damage stability appraisal for a coastal tanker shows the typical scale of peak residual GZ values for each loading condition in the final column.
57 If we consider the approved loading condition for the worst case of damage, it will be seen that the tank fillings vary between port and starboard by a significant amount, but that this was not reflected in the damage cases run.
58 Vessel is not at its marks at SG 0.63 or SG 0.74 This is another example of typical margins on stability for approved loading conditions from a chemical tanker. It may be noted that the margins are low in several instances and that this limited compliance can be compromised by filling of deck tanks (not considered for SG’s 0.74 or 1.9) or by minor reduction in cargo SG (in the case of the 1.9 SG cargo) or by the addition of extra cargo to bring the vessel to the marks (in the case of the 0.63 and 0.74 SG cargoes).Vessel is not at its marks at SG 0.63 or SG 0.74