Page 101: of Maritime Reporter Magazine (November 2016)

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After construction, the allowable dimi- pages making it dif? cult and tedious to and future corrosion is taken into con- late current corrosion rates from multi- nution is de? ned by a percentage of the interpret the data, identify trends or draw sideration. Attentions are given to tanker ple surveys, projected corrosion on non- required thickness (called wastage al- conclusions. life, service life, future corrosion allow- gauged structures from similar corrosion lowance), usually between 20-25%, that For the corrosion processing within ance and all available corrosion data. gaugings and the location and extent of can waste away before the structure has SAGA’s FEA modeling capabilities, past These considerations are used to calcu- (Continued on page 103) to be renewed. In a conversion situation, we start with the original tanker thick- ness. Using the class minimum scantling criteria with the site speci? c environ-

DNV GL Approved

Professional mental conditions, Viking de? nes the minimum net scantlings that satisfy both

NMEA 0183 Interface

Advanced Shiphandling the global and local scantling require- in ments, and then add on the Nominal De- Manned Models sign Corrosion Value (NDCV) to de? ne

This USCG approved course meets STCW requirements the FPSO gross thickness. Depending on


the construction of the original tanker (in the instance of a conversion vessel), we often ? nd the FPSO minimum required gross thickness to be less than the origi- nal tanker as-built scantling, which pro- vides additional margin for future corro- sion.

As we move into operations, the al- lowable wastage is now based upon our

FPSO gross thickness, and we have two


minimum thicknesses to consider. Rule

KW???Y?AusIGE/YEs|sE?Au renewal thickness is the ? rst, which is sG??Gu?ŽwÐAY??&AÐsusl?^GÐ??sl?K?ÐG?

Several models – 30 years experience as thin as the structure can go before it ^dtA?sÐd?AsYsYO

Custom made ”talker/sentence” has to be replaced as dictated by class. /^


The second to consider is the thickness


Producer &A?lZG?Ð?GŽAl at substantial corrosion of hull structural


components, meaning 75% of the way

Marina Mile Business Park, Fort Lauderdale, Florida 6419 Molde, Norway

Phone: 954 583 5700 • Fax: 954 583 3699 Phone: (+47) 71 20 24 00 to the renewal thickness (i.e. 75% of the

Email: E-mail:

Contact us for a complete training schedule wastage allowance), and is the scantling dGu????????????wAsu?Ðwl?wA?sOwG?GE?

where annual special surveys are re- ????wA?sOwG?GE??Ðwl quired.

Depending on location of the corro- sion – main deck plating or structure in way of void spaces – we may be able to accommodate annual inspections with minimal impact on operations. With that being said, it is preferred to retain a ? ve year special survey schedule and keep all scantling above this threshold through the end of the service life. Based on how much service life is left for the

FPSO, we add a set amount of thickness to allow for additional future corrosion, and de? ne a minimum acceptable thick- ness to be compared against the current gauging data. If we need additional mar- gin to avoid plate renewal, then in some cases we can try to reduce the minimum required thickness by changing the still water allowables and/or adding addition- al stiffeners. However, if the scantling requirements are driven by local criteria it may not always be feasible.

Special survey reports of the hull structure typically include tabulated survey data: original and gauged thick- nesses, diminution and allowable wast- age based on original thickness, as well as drawings showing the location used to measure thickness. Comprehensive survey reports can include thousands of 101

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