During 2003 - 2007 eight (8) fully Duplex stainless steel chemical tankers were delivered by SSN for Norwegian owner Odfjell ASA, the one of the biggest chemical tanker operators.
The main data of these vessels are as follows:
Length o.a. - 182.88 m, Length b.p. - 175.25 m, Breadth - 32.20 m, Depth - 17.95 m, Draught - 11.50 m, Deadweight - 40 000 DWT, Cargo tanks capacity - 52 126 m3,
Number of cargo tanks - 34 + 6 /deck tanks/, Service speed - 15.5 kn,
Class - DNV.
These vessels are the biggest in the world fully Duplex stainless steel tankers with all cargo tanks / center, wing and deck tanks / made of solid Duplex stainless steel. The vessels have been designed for the niche between product and chemical tankers and as compared to standard chemical tanker have cargo tanks capacity bigger by about 15%. This allows operating the vessels in CPP market utilizing the full deadweight. From the operation point of view the vessels are very flexible thanks to cofferdam bulkheads between center and wing tanks, arrangement of center tanks and deck tanks.
As consequence of such design, building costs for such vessels are very high, mainly due to:
high lightship weight of the vessels,
amount of Duplex steel equal to 3 000 t per vessel, sophisticated propulsion system,
amount of cargo tanks and associated piping systems.
The basic design, performance and stability of the IMPROVEd chemical tanker
In 2007, with very high material cost, building cost of such vessel was on the level 140 mil. USD, that was far above market expectation.
Because the chemical tankers are considered as one of our specialization, Shipyard decided to redesign the B588- III vessel to get the building cost which could be accepted by the market.
The following alternatives have been considered:
Alternative 1
main dimensions as in original design B588-III, wing cargo tanks made of mild steel instead of
Duplex steel,
reduction of number of center cargo tanks from eighteen /18 / to twelve /12 /,
reduction of service speed to 15.0 kn, deleting of shaft generator.
Alternative 2
reduction of cargo tanks capacity to abt. 45 000 m3,
deleting of cofferdam bulkheads and replacing them by vertically corrugated bulkheads,
reduction of depth of the vessel to 15.0 m, using of Duplex steel for center tanks only, deleting of six / 6 / deck tanks,
reduction of service speed to 15.0 kn, deleting of shaft generator.
Alternative 3
As Alternative 2 except the arrangement of Duplex tanks which are arranged in the middle part of the vessel / wing and center tanks /.
Calculation of building cost done for 2007 condition shows that the most effective cost reduction is Alternative 3, and Shipyard decided to develop this design and optimize it using the IMPROVE tools.
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THE IMPROVE TANKER
3.1 Design assumptions
The IMPROVE design is based on the following assumptions:
specific gravity of sulfuric acid varies between abt.1.55 - 1.85 t/m3,
capacity of Duplex stainless steel tanks should allow to carry a/m
acid with 50% of consumables, utilizing full deadweight of the vessel,
total number of Duplex stainless steel tanks to be eighteen /18/ with different capacities
Duplex stainless steel cargo tanks to be separated from the mild steel cargo tanks by cofferdams, longitudinal bulkheads to be vertically corrugated, transverse bulkheads to be vertically or
horizontally corrugated
o Connection between longitudinal vertically corrugated bulkheads and transverse horizontally corrugated bulkheads to be subject of FEM analyses
propulsion system consists of slow speed ME driving directly FP propeller,
service speed to be 15.0 kn.
Calculation of cargo tanks capacity and arrangements for three /3/ different specific gravities of acid 1.50, 1.65, and 1.85 t/m3 has been performed and is presented in the drawing. For further optimization, cargo tanks arrangement for specific gravity 1.50 t/m3 was taken. The main target for optimization was reducing of quantity of Duplex steel due to a very high price of this material.
The following structures are subject to optimization:
scantling as shown on drawing Midship Section, see Figure 1,
transverse bulkheads, horizontally corrugated,
longitudinal bulkheads, vertically corrugated as shown on Figure 1.
3.2 The improved design
Based on the given assumptions the main particulars of IMPROVE project are as follows:
Length o.a. - 182.88 m,
Length b.p. - 175.25 m,Breadth - 32.20 m, Depth - 15.00 m,
Draught - 11.10 m, Deadweight - 40 000 mt,
Cargo tanks capacity / total / - 44 000 m3, Number of cargo tanks - 30,
Capacity of Duplex cargo tanks - 26 800 m3, Number of Duplex cargo tanks - 18,
Service speed - 15.0 kn.
The main frame is given in Figure 1 and the arrangement of the vessel is shown in Figure 2.
The basic design, performance and stability of the IMPROVEd chemical tanker
Figure 1. Mainf frame of the IMPROVE tanker
Figure 2. General Arangement of the IMPROVE tanker
3.3 The Propulsion system
Proposed propulsion system consists of single diesel main engine, low speed, two stroke type,
driving directly FP propeller. Three types of main engines have been evaluated:
5S60 - MC - C7, 6S50 - ME - B9, 6S50 - ME - B8.
Main engine type 6S50 - ME -B9 is chosen for this project.
3.4 Seakeeping and stability analysis
The regular and stochastic real sea analyses of the vessel are carried out using a 2D strip theory based numerical code. In general, the vessel is expected to exhibit good seakeeping characteristics as most of the worst response modal periods are either far off from the dominant wave periods of operational area or wave headings may be adjusted to avoid severe responses. The seakeeping responses evaluated include:
Response Amplitude Operators (RAOs) Root Mean Square (RMS) Motions
Extreme (1% Probability of Exceedance in 6 hours) Motions
Motion Sickness Incidences (MSIs) Motion Induced Interruptions (MIIs) Deck Wetness Probabilities
Deck Wetness Rate
Keel Emergence Probabilities Keel Emergence Rate
Most Probable Slamming Pressure
Extreme (1% Probability of Exceedance) Slamming Pressure
RMS Horizontal Shear Force
Horizontal Shear Force Zero-Up Crossing Periods Extreme (1% Probability of Exceedance)
Horizontal Shear Force RMS Vertical Shear Force
Vertical Shear Force Zero-Up Crossing Periods Extreme (1% Probability of Exceedance) Vertical
Shear Force
RMS Torsion Bending Moment
Torsion Bending Moment Zero-Up Crossing Periods
Extreme (1% Probability of Exceedance) Torsion Bending Moment
RMS Vertical Bending Moment
Vertical Bending Moment Zero-Up Crossing Periods
Extreme (1% Probability of Exceedance) Vertical Bending Moment
RMS Horizontal Bending Moment
Horizontal Bending Moment Zero-Up Crossing Periods
Extreme (1% Probability of Exceedance) Horizontal Bending Moment
The basic design, performance and stability of the IMPROVEd chemical tanker
There are 18 sea going conditions defined in the stability booklet of the vessel (Sondaj 2008) with a range of displacements, positions of centre of gravities and trim. There is a large difference in the displacements of ballast and cargo conditions (more than 15 000 Te). In addition, a wide variation (range = 7787.2) is also existing between the cargo condition. It was, therefore, decided to carryout seakeeping analysis for at least three loading conditions corresponding to minimum, mean and maximum displacements
3.5 Hydrostatic and Hydrodynamic
For the analyses carried out, the hydrostatic and hydrodynamic features of vessels given in Table 1 were modelled in the seakeeping software. It may be noted that roll gyration radii for the three loading conditions were assumed to be around 0.35 x moulded beam, whereas, the pitch gyration radii were calculated by the seakeeping software using the detailed breakdown of loads. The later were defined from the stability booklet (Sondaj 2008) to facilitate calculation of structural load (shear forces and bending moments).
Table 1. Hydrostatic & hydrodynamic parameters of Chemical Tanker
3.6 Analysis procedure
This section of the paper briefly explains the procedure adopted for the seakeeping evaluation of IMPROVE Chemical Tanker. The overall procedure for the estimation of seakeeping characteristics of any vessel is simple two phase problem depicted in Figure 3. The first phase comprises of vessel’s response transfer functions, also called response amplitude operators, RAOs; calculations.
In the second phase, RAOs are combined with the irregular sea idealised spectra to estimate RMS, expected
extreme and extreme (1% probability of exceedance) vessel response.
Figure 3. Typical seakeeping analysis phases
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SUMMARY AND CONCLUSIONS
This paper presents the initial, yet uncompetitive design, of the chemical tanker to be improved. The improvements are presented and discussed from the shipyards point of view. As an example some technical drawings are given. This improved shipyard design serves as a basis for the optimization of the structure in the EU-IMPROVE project.
The stability and seakeeping calculations show that the IMPROVE Chemical Tanker satisfies the stability requirements of applicable rules and regulations.
REFERENCES
Sondaj, G. (2008). Improve / 042-3: Information on Stability and Longitudinal Strength - Chemical Tanker 40000 Dwt. STOCZNIA SZCZECINSKA NOWA Spółka z o.o. (Ltd). Biuro Projektowe: 112.
Description LC 003 LC 018 LC 019
Height of centre of gravity 7.24m 8.96m 9.15m Corrected Transverse Metacentric Height 8.59m 4.42m 4.01m Dry pitch radius of gyration (calculated) 48.720m 43.188m 43.810m Roll radius of gyration in water (assumed as 0.35B) 11.270m 11.270m 11.270m
EU FP6 project IMPROVE-Final Conference IMPROVE 2009, Dubrovnik, CROATIA, 17-19 Sept. 2009