Q.3 — Reasons for Reduced Scantlings of Deck and Inner Bottom Plating of Oil Tankers in the Last Two Decades

This question refers to the well-documented industry and IACS trend, over roughly the last twenty years, of oil tanker deck and inner bottom plating being built to progressively thinner scantlings than earlier generations of tankers of comparable size, despite corrosion being a known critical failure mode. The reasons are a combination of design philosophy changes, regulatory/class rule evolution, materials technology, and economic pressure:

1. Shift from prescriptive rules to Common Structural Rules (CSR) and direct calculation methods

• Prior to 2006, individual classification societies used largely prescriptive, empirically-derived scantling formulae which tended to be conservative and included generous built-in margins.
• With the introduction of IACS Common Structural Rules for Oil Tankers (CSR-OT, 2006, later harmonized into Harmonised CSR — HCSR in 2015), scantlings began to be derived from direct strength assessment (finite element analysis) and probabilistic/rational load combination methods.
• Direct calculation allows designers to size plating much more precisely to the actual loads and stresses predicted, removing much of the “hidden” conservatism of older prescriptive rules — resulting in thinner “as-designed” (net) scantlings that are still deemed structurally adequate for the design life, but with much less spare margin beyond calculated requirements.

2. Introduction and reliance on the “corrosion addition” (net scantling) approach

• Under CSR/HCSR, ships are now designed using a net scantling approach: the required net thickness for strength is calculated first, and a standardized corrosion margin (corrosion addition, tc) is then added on top of this net figure to arrive at the “as-built” (gross) thickness.
• This is a fundamentally different philosophy from the older approach, where a single gross scantling was specified with corrosion allowance built in more informally.
• The corrosion additions specified by CSR are based on statistically-derived expected wastage over the ship’s design life (typically 25 years) rather than the more generous “just-in-case” margins used previously — meaning the total gross plate thickness (net + corrosion addition) has, in many locations, come out lower than the older prescriptive-rule gross thicknesses, especially for deck and inner bottom.

3. Improved and mandatory protective coating (PSPC) reducing assumed corrosion rates

• The IMO Performance Standard for Protective Coatings (PSPC), adopted in 2006 and mandatory for new tankers’ ballast tanks and cargo oil tank vapour spaces/deckhead from 2008 onward, requires a coating life target of 15 years (“GOOD” condition).
• Because effective coatings are now assumed as a baseline design condition (rather than an optional extra), classification societies could justify reduced statutory corrosion margins/additions built into the scantling rules, on the assumption that well-maintained PSPC-compliant coatings will keep actual metal loss close to the design assumption.
• This directly reduces the required as-built gross thickness of deck and inner bottom plating compared to the pre-PSPC era, when designers had to allow for corrosion margins assuming coatings might fail early or not be applied to the same standard.

4. Use of higher-tensile steel (HTS) in place of mild steel

• Increasing use of higher-tensile/high-strength steel (grades AH32, AH36, DH32/36, EH32/36) for deck and bottom plating allows the same strength requirement to be met with a thinner plate, since HTS has a higher yield strength than ordinary mild steel.
• While this reduces weight and increases cargo-carrying capacity/deadweight efficiency (a major commercial driver — see point 6), HTS plating, being thinner, has less absolute margin of corrosion allowance in millimetres before it reaches minimum acceptable/renewal thickness, i.e., the same percentage corrosion rate consumes a proportionately larger share of the total available thickness margin in a thinner HTS plate than in a thicker mild steel plate.
• This is a widely-noted concern: HTS improves initial strength-to-weight efficiency but can reduce the “corrosion margin buffer” in absolute terms, making these structures more sensitive to any coating breakdown or unexpectedly high in-service corrosion rate.

5. Refined load assumptions from improved hydrodynamic/structural analysis

• Modern direct calculation uses more sophisticated wave load, sloshing, and dynamic load prediction models (informed by decades of full-scale monitoring, model testing, and computational hydrodynamics) that are considered more accurate than the older simplified static/quasi-static load assumptions.
• Refined loads, particularly for double-hull tankers where the inner bottom and deck act as primary strength members of a box-girder hull, have in some cases resulted in lower calculated design bending moments and local pressures than older rules assumed, permitting thinner scantlings while still meeting the rule strength criteria.

6. Commercial/economic pressure for deadweight efficiency

• Reducing steel weight in the hull directly increases the ship’s cargo deadweight capacity for a given displacement, which is commercially significant given the capital cost of steel and the revenue impact of deadweight over a ship’s operating life.
• Shipyards and owners have pushed for scantlings at or close to the minimum permissible under class rules, rather than the more conservative “over-thickness” that was common practice in earlier decades, since steel weight above the rule minimum is regarded as an avoidable cost with no offsetting benefit if the rules are (in principle) already calibrated to be adequate.

7. Consequence — narrower margin against actual in-service corrosion

• The net effect of the above trends is that today’s tankers are built with less “spare” thickness above the calculated structural minimum than older tankers were, even though the intent of the net scantling + corrosion addition system is that the ship should still reach 25 years safely if coatings perform to standard and corrosion proceeds at or below the design assumption rate.
• This is precisely why deviations from assumed conditions — coating breakdown, unexpectedly aggressive local corrosion (pitting, vapour space attack as in Q.1/Q.2), or poor maintenance — have a proportionately larger and faster impact on structural safety margins in modern tankers than they would have on the more heavily-scantled tankers of the 1970s–1990s. This is a key reason class societies now place much greater emphasis on rigorous, more frequent close-up surveys and thickness measurement under the Enhanced Survey Programme (ESP) and CAS for tankers approaching or beyond mid-life.