Section 3 Longitudinal Strength

301. General

1. The assessment of the longitudinal hull girder strength should be based on the following operational modes:

(1) All transit conditions

(2) All operating conditions, intact and damaged, at the design locations(s)

(3) All inspection and repair conditions

2. Changes in the design conditions of a ship-shaped unit are usually accompanied by significant changes in draught, ballast, riser connections, mooring line tension, etc. Limited variation of some of these parameters may be contained within a specific design condition.

3. The suitability of a ship-shaped unit is dependent on the environmental conditions in the areas of the intended operation. A production unit may be planned to operate at a specific site.

4. Use of finite element methodology for strength analysis of hull and cargo tank structures is primar- ily in order to obtain a better and complete understanding of the stress response when subject to wave, motion-induced loads as well as other functional loads. In practice, the structural analysis can be performed using several levels of modeling methods. The level of modeling and associated structural idealization should be based on purpose of the analysis.

302. Longitudinal hull girder strength

1. Longitudinal strength of ship type unit is to be based on Pt 3 and Pt 12 of Rules for the Classification of Steel Ships basically. The total hull girder bending moment, ÀŹ is the sum of the maximum still water bending moment for operation on site or in transit combined with the cor- responding wave-induced bending moment (ÀB) expected on-site and during transit to the in- stallation site.

2. In lieu of directly calculated wave-induced hull girder vertical bending moments and shear forces, recourse can be made to the use of the Environmental Severity Factor (ESF) approach described of this guide. The ESF approach can be applied to modify the Steel Vessel Rules wave-induced hull girder bending moment and shear force formulas. Depending on the value of the Environmental Severity Factor, ØŽẄŶ, for vertical wave-induced hull girder bending moment (see this Guide), the

minimum hull girder section modulus,

Cmi n

of unit may vary in accordance with the following.

Table 5.1 Minimum hull girder section modulus

3. Environmental Severity Factor

Environmental Severity Factors are adjustment factors for the dynamic components of loads and the expected fatigue damage that account for site-specific conditions as compared to North Atlantic un- restricted service conditions.

(1) ESFs of the Beta (Ø) Type

This type of ESF is used to introduce a comparison of the severity between the intended envi-

ronment and a base environment, which is the North Atlantic unrestricted service environment.

In the modified formulations, the Ø factors apply only to the dynamic portions of the load com-

ponents, and the load components that are considered “static” are not affected by the in- troduction of the Ø factors.

Ø G ÁZ

JÁŹ

where

ÁZ : most probable extreme value based on the intended site (100 years return period), transit (10 years return period), and repair/inspection (1 year return period)

environments for the dynamic load parameters specified in Table 5.2.

ÁŹ : most probable extreme value base on the North Atlantic environment for the dynamic load parameters specified in Table 5.2.

A Ø of 1.0 corresponds to the unrestricted service condition of a seagoing vessel. A value of Ø

less than 1.0 indicates a less severe environment than the unrestricted case.

Table 5.2 Dynamic Load Parameters

(2) ESFs of the Alpha (Ø ) Type

This type of ESF compares the fatigue damage between the specified environment and a base

environment, which is the North Atlantic environment. This type of ESF is used to adjust the

expected fatigue damage induced from the dynamic components due to environmental loadings at the installation’s site. It can be used to assess the fatigue damage accumulated during the histor- ical service either as a trading vessel or as a unit, including both the historical site(s) and his- torical transit routes.

ǼŹZ

ŊĦ ÈÈ

where

ǼŹ : annual fatigue damage based on the North Atlantic environment (unrestricted service) at the details of the hull structure

ǼZ : annual fatigue damage based on a specified environment, for historical routes, histor ical sites, transit and intended site, at the details of the hull structure

303. Hull girder ultimate strength

1. The hull girder ultimate longitudinal bending capacities are to be evaluated in accordance with Pt 12 of Rules for the Classification of Steel Ships.

2. The vertical hull girder ultimate strength for the unit design environmental condition (DEC) is to satisfy the limit state as specified below. It need only be applied within the 0.4L amidship region.

ÀÃ

ǾÅÀZŽ ĞǾBØÃAÀÀŽŽ GZŴX ≤ JǾ Ą

ÀZŽ ÀŽŽGZŴX

ÀÃ

ØÃAÀ

ǾÅ

ǾB

: permissible still-water bending moment, in kNm

: sagging vertical wave bending moment, in kNm, to be taken as the midship sagging value defined in Pt 12, Sec 7/3.4.1.1 of Rules for the Classification of Steel Ships.

: sagging vertical hull girder ultimate bending capacity, in kNm, as defined in Pt 12, Appendix A/1.1.1 of Rules for the Classification of Steel Ships.

: ESF for vertical wave-induced bending moment for DEC

: load factor for the maximum permissible still-water bending moment, but not to be taken as less than 1.0

: load factor for the wave-induced bending moment, but not to be taken as less than below for the given limits

= 1.3

= 1.2

(ÀZŽ Ĥ ŊĦËÀŹ 또는 ÀZŽ I ŊĦÈÀŹ )

(ŊĦËÀŹ ≤ÀZŽ ≤ŊĦÈÀŹ )

ÀŹ : total bending moment, in kNm

= ÀZŽ ĞØÃAÀÀŽŽ GZŴX

ǾĄ : safety factor for the vertical hull girder bending capacity, but not to be taken as less than 1.15

304. Additional Application

Other various matters on the longitudinal strength are to be as specified in Pt 5, Ch3, of Rules for the Guidance for Floating Production Units.