As used in §§ 154.440 and 154.447:
“σY” means the minimum yield strength of the tank material, including weld metal, at room temperature.
“σB” means minimum tensile strength of the tank material, including weld metals, at room temperature.
(a) The design vapor pressure (Po) of a cargo tank must be equal to or greater than the MARVS.
(b) The Po of a cargo tank must be equal to or greater than the vapor pressure of the cargo at 45 °C (113 °F) if:
(1) The cargo tank has no temperature control for the cargo; and
(2) The vapor pressure of the cargo results solely from ambient temperature.
(c) The Po of a cargo tank may be exceeded under harbor conditions if specially approved by the Commandant (CG-ENG).
[CGD 74-289, 44 FR 26009, May 3, 1979, as amended by CGD 82-063b, 48 FR 4782, Feb. 3, 1983]
(a) Calculations must show that a cargo tank and its fixtures are designed for the following loads:
(1) Internal pressure head.
(2) External pressure load.
(3) Dynamic loads resulting from the motion of the vessel.
(4) Transient or stationary thermal loads if the design temperature is colder than −55 °C (−67 °F) or causes thermal stresses in cargo tank supports.
(5) Sloshing loads, if the cargo tank is designed for partial loads.
(6) Loads resulting from vessel's deflection.
(7) Tank weight, cargo weight, and corresponding support reaction.
(8) Insulation weight.
(9) Loads of a pipe tower and any other attachments to the cargo tank.
(10) Vapor pressure loads in harbor conditions allowed under § 154.405.
(11) Gas pressurization if the cargo tank is designed for gas pressurization as a means of cargo transfer.
(b) A cargo tank must be designed for the most unfavorable static heel angle within a 0° to 30° range without exceeding the allowable stress of the material.
(c) A hydrostatic or hydropneumatic test design load must be specially approved by the Commandant (CG-ENG).
[CGD 74-289, 44 FR 26009, May 3, 1979, as amended by CGD 82-063b, 48 FR 4782, Feb. 3, 1983; USCG-2014-0688, 79 FR 58284, Sept. 29, 2014]
(a) For the calculation required under § 154.406(a)(1) and (b), the internal pressure head (heq), must be determined from the following formula:
heq = 10 Po + (hgd)max
where:
hgd (the value of internal pressure, in meters of fresh water, resulting from the combined effects of gravity and dynamic accelerations of a full tank) = aβ Zβ Y;
where:
aβ = dimensionless acceleration relative to the acceleration of gravity resulting from gravitational and dynamic loads in the β direction (see figure 1);
Zβ = largest liquid height (m) above the point where the pressure is to be determined in the β direction (see figure 2);
Y = maximum specific weight of the cargo (t/m3) at the design temperature.
(b) The (hgd)max is determined for the β direction, on the ellipse in Figure 1, which gives the maximum value for hgd.
(c) When the longitudinal acceleration is considered in addition to the vertical transverse acceleration, an ellipsoid must be used in the calculations instead of the ellipse contained in Figure 1.
For the calculation required under § 154.406 (a)(2) and (b), the external pressure load must be the difference between the minimum internal pressure (maximum vacuum), and the maximum external pressure to which any portion of the cargo tank may be simultaneously subjected.
(a) For the calculation required under § 154.406 (a)(3) and (b), the dynamic loads must be determined from the long term distribution of vessel motions, including the effects of surge, sway, heave, roll, pitch, and yaw on irregular seas that the vessel may experience during 108 wave encounters. The speed used for this calculation may be reduced from the ship service speed if specially approved by the Commandant (CG-ENG) and if that reduced speed is used in the hull strength calculation under § 31.10-5(c) of this chapter.
(b) If the loads determined under paragraphs (c), (d), or (e) of this section result in a design stress that is lower than the allowable stress of the material under §§ 154.610, 154.615, or 154.620, the allowable stress must be reduced to that stress determined in paragraphs (c), (d), or (e).
(c) If a tank is designed to avoid plastic deformation and buckling, then acceleration components of the dynamic loads must be determined for the largest loads the vessel may experience during an operating life corresponding to the probability level of 10−8, by using one of the following methods:
(1) Method 1 is a detailed analysis of the vessel's acceleration components.
(2) Method 2 applies to vessels of 50 m (164 ft) or more in length and is an analysis by the following formulae that corresponds to a 10−8 probability level in the North Atlantic:
(d) If a cargo tank is designed to avoid fatigue, the dynamic loads determined under paragraph (a) of this section must be used to develop the dynamic spectrum.
(e) If a cargo tank is designed to avoid uncontrolled crack propagation, the dynamic loads are:
(1) Determined under paragraph (a) of this section; and
(2) For a load distribution for a period of 15 days by the method in Figure 3.
(f) When determining the accelerations for dynamic loads under paragraph (a) of this section, the accelerations acting in a cargo tank must be estimated for the cargo tank's center of gravity and include the following component accelerations:
(1) Vertical accelerations, meaning the motion acceleration of heave and pitch, and of any roll normal to the vessel base that has an effect on the component acceleration.
(2) Transverse acceleration, meaning the motion acceleration of sway, yaw and roll, and gravity component of roll.
(3) Longitudinal acceleration, meaning the motion acceleration of surge and pitch and gravity component of pitch.
[CGD 74-289, 44 FR 26009, May 3, 1979, as amended by CGD 82-063b, 48 FR 4782, Feb. 3, 1983]
(a) For the calculation required under § 154.406 (a)(5) and (b), the determined sloshing loads resulting from the accelerations under § 154.409(f) must be specially approved by the Commandant (CG-ENG).
(b) If the sloshing loads affect the cargo tank scantlings, an analysis of the effects of the sloshing loads in addition to the calculation under paragraph (a) of this section must be specially approved by the Commandant (CG-ENG).
[CGD 74-289, 44 FR 26009, May 3, 1979, as amended by CGD 82-063b, 48 FR 4782, Feb. 3, 1983]
For the calculations required under § 154.406(a)(4), the following determined loads must be specially approved by the Commandant (CG-ENG):
(a) Transient thermal loads for the cooling down periods of cargo tanks for design temperatures lower than −55 °C (−67 °F).
(b) Stationary thermal loads for cargo tanks for design temperatures lower than −55 °C (−67 °F) that cause high thermal stress.
[CGD 74-289, 44 FR 26009, May 3, 1979, as amended by CGD 82-063b, 48 FR 4782, Feb. 3, 1983]
A cargo tank must be designed with a corrosion allowance if the cargo tank:
(a) is located in a space that does not have inert gas or dry air; or
(b) carries a cargo that corrodes the tank material.
Corrosion allowance for independent tank type C is contained in § 54.01-35 of this chapter.
