Section 4 Structural Design and Analysis of the Hull

401. Structural design of the hull

Design of the hull is to be based on this guide, general hull structures are to be in accordance with the requirements of Pt 12 of Rules for the Classification of Steel Ships.

Where the conditions at the installation site are less demanding than those for unrestricted service that are the basis in hull structure of Rules for the Classification of Steel Ships, the design criteria for various components of the hull structure may be reduced, subject to the limitations indicated below to reflect these differences. However, when the site conditions produce demands that are more severe, it is mandatory that the design criteria are to be increased appropriately. In the application of the modi- fied criteria, no minimum required value of any net scantling is to be less than 85 percent of the value obtained had all the ESF Beta values been set equal to 1.0 (which is the unrestricted service condition). In view of this, for a unit converted from a vessel, where the total bending moment for unrestricted service conditions is used for determination of the minimum required value of any net

scantling, the total

bending moment should consist of the maximum still water bending moment of

the existing vessel and the wave-induced bending moment with all the beta values set equal to 1.0. The loads arising from the static tank testing condition are also to be directly considered in the design. In some instances, such conditions might control the design, especially when the overflow heights are greater than normally encountered in oil transport service, or the severity of environ- mentally-induced load components and cargo specific gravity are less than usual.

1. Hull design for additional loads and load effects

The loads addressed in this Subsection are those required in the design of an installation depending on the length of the installation. Specifically, these loads are those arising from liquid sloshing in hydrocarbon storage or ballast tanks, green water on deck, bow impact due to wave group action above the waterline, bow flare slamming during vertical entry of the bow structure into the water, bottom slamming and deck loads due to on-deck production facilities. All of these can be treated directly by reference to unit. However, when it is permitted to design for these loads and load ef- fects on a site-specific basis, reflect the introduction of the Environmental Severity Factors (ESFs-Beta- type) into the Rule criteria.

2. Superstructures and deckhouses

The designs of superstructures and deckhouses

Rules for the Classification of Steel Ships.

are to comply with the requirements of Pt 12 of The structural arrangements of Pt 12 of Rules for

the Classification of Steel Ships for forecastle decks are to be satisfied.

3. Helicopter decks

The design of the helicopter deck structure is to comply with the requirements of Rules for the Classification of Mobile Offshore Drilling Units. In addition to the required loadings defined in Rules for the Classification of Mobile Offshore Drilling Units, the structural strength of the helicopter deck and its supporting structures are to be evaluated considering the DOC and DEC en- vironments, if applicable.

4. Protection of deck openings

The machinery casings, all deck openings, hatch covers and companionway sills are to comply with the requirements of Pt 12 of Rules for the Classification of Steel Ships.

5. Bulwarks, rails, freeing ports, ventilators and portlights

Bulwarks, rails, freeing ports, portlights

12 of Rules for the Classification of

and ventilators are to comply with the requirements of Pt Steel Ships.

6. Machinery and equipment foundations

Foundations for equipment subjected to high cyclic loading, such as mooring winches, chain stop- pers and foundations for rotating process equipment, are to be analyzed to verify they provide sat-

isfactory strength and fatigue resistance. Calculations

submitted to the Bureau for review.

7. Bilge keels

The requirements of bilge keels are to comply with

Classification of Steel Ships.

and drawings showing weld details are to be

the requirements of Pt 12 of Rules for the

8. Sea chests

The requirements of Sea Chests are to comply with

Classification of Steel Ships.

the requirements of Pt

12 of Rules for the

402. Engineering analyses of the hull structure

1. General

The criteria in this Subsection relate to the analyses required to verify hull design in 401.. Depending on the specific features of the offshore

the scantlings selected in the installation, additional analy-

ses to verify and help design other portions of the hull structure will be required. Such additional

analyses include those for the deck structural components supporting deck-mounted equipment and the hull structure interface with the position mooring system. Analysis criteria for these two sit- uations are given in Section 5.

2. Strength analysis of the hull structure

For installations of 150 m in length and above, two approaches in performing the required strength assessment of the hull structure are acceptable. One approach is based on a three cargo tank length finite element model amidships where the strength assessment is focused on the results obtained from structures in the middle tank. As an alternative, a complete hull length or full cargo block length finite element model can be used in lieu of the three cargo tank length model. Details of the required Finite Element Method (FEM) strength analysis are in accordance with Pt 12 of Rules for the Classification of Steel Ships.

When mooring and riser structures are located within the extent of the FE model, the static mass of the mooring lines and risers may be represented by a mass for which gravity and dynamic ac-

celerations can be calculated and added to the FEM model. The resulting dynamic loads shall be

compared to the mooring and riser analysis results to ensure that the dynamic effects are con- servatively assessed in the hull FE analysis.

Generally, the strength analysis is performed to determine the stress distribution in the structure. To

determine the local stress distribution in major supporting structures, particularly at intersections of two or more members, fine mesh FEM models are to be analyzed using the boundary displace- ments and load from the 3D FEM model. To examine stress concentrations, such as at intersections of longitudinal stiffeners with transverses and at cutouts, fine mesh 3D FEM models are to be analyzed. The accidental load condition, where a cargo tank is flooded, is to be assessed for longi- tudinal strength of the hull girder consistent with load cases used in damage stability calculations.

3. Three cargo tank length model

(1) Structural FE model

The three cargo tank length FE model is considered representative of cargo and ballast tanks within the 0.4L amidships. Details of the modeling are in accordance with Pt 12 of Rules for the Classification of Steel Ships.

(2) Load Conditions

The structure analysis for three Cargo Tank Length Model is applied by below load conditions.

(A) General Loading Patterns

(B) Inspection and repair conditions - Inspection and repair conditions are to be analyzed using

a minimum 1-year return period design operating condition load and a minimum specific gravity of cargo fluid of 0.9.

(C) Transit condition - The transit condition is to be analyzed using the actual tank loading pat-

terns in association with the anticipated environmental conditions based on a minimum 10-year return period to be encountered during the voyage.

(3) Load Cases

The structural responses for the still water conditions are to be calculated separately to establish reference points for assessing the wave-induced responses. Additional loading patterns may be required for special or unusual operational conditions or conditions that are not covered by the loading patterns specified in this Guidance. Topside loads are also to be included in the load cases.

4. Alternative approach – cargo block or full ship length model

(1) Structural FE Model

As an alternative to the three cargo tank length model, the finite element strength assessment can be based on a full length or cargo block length of the hull structure, including all cargo and ballast tanks. All main longitudinal and transverse structural elements are to be modeled. These include outer shell, floors and girders, transverse and vertical web frames, stringers and transverse and longitudinal bulkhead structures. All plates and stiffeners on the structure, includ- ing web stiffeners, are to be modeled. Topside stools should also be incorporated in the model. The modeling mesh and element types used should follow the principles that are described in Pt 12 of Rules for the Classification of Steel Ships.

Boundary conditions should be applied at the ends of the cargo block model for dynamic equili- brium of the structure.

Detailed local stress assessment using fine mesh models to evaluate highly stressed critical areas

are to be in accordance with Pt 12 of Rules for the Classification of Steel Ships.

(2) Loading Conditions

In the strength analyses of the cargo block or full ship length model, the static on site unit op- erating load cases are to be established to provide the most severe loading of the hull girder

and the internal tank structures. The operating load cases found in the Loading Manual and

Trim & Stability Booklet provide the most representative loading conditions to be considered for analysis. The static load cases should include as a minimum tank loading patterns resulting in

the following conditions.

(A) Ballast or minimum draft condition after offloading

(B) Partial load condition (33% full)

(C) Partial load condition (50% full)

(D) Partial load condition (67% full)

(E) Full load condition before offloading

(F) Transit load condition

(G) Inspection and repair conditions

(H) Tank testing condition ‒ during conversion and after construction (periodic survey)

The tank testing condition is to be considered as a still water condition. The static load cases

(A) to (G) will be combined with environmental loading conditions to develop static plus

namic load cases that realistically reflect the maximum loads for each component of structure.

(3) Dynamic Loading

dy-

the

In quantifying the dynamic loads, it is necessary to consider a range of wave environments and

headings at the installation site, which produce the considered critical responses. The static

and

dynamics of the position mooring and topside module loads contribution shall also be included.

Wave loads

are to be determined based on an equivalent design wave. The equivalent design

wave is defined as a regular wave that gives the same response level as the maximum design response for a specific response parameter. This maximum design response parameter or Dominant Load Parameter is to be determined for the site-specific environment with a 100-year

return dition

all of

period, transit environment with a 10-year return period, and inspection and repair con- with a 1-year return period. In selecting a specific response parameter to be maximized,

the simultaneously occurring dynamic loads induced by the wave are also derived. These

simultaneous acting dynamic load components and static loads, in addition to the quasi-static

equivalent wave loads, are applied to the cargo block model. The Dominant Load Parameters es- sentially refer to the load effects, arising from vessel motions and wave loads, that yield the

maximum structural response for critical structural members. Each set of Dominant Load

Parameters with equivalent wave and wave-induced loads represents a load case for structural FE analysis.

The wave amplitude of the equivalent design wave is to be determined from the maximum de-

sign response of a Dominant Load Parameter under consideration divided by the maximum RAO amplitude of that Dominant Load Parameter. RAOs will be calculated using a range of wave

headings and periods. The maximum RAO occurs at a specific wave frequency and wave head-

ing where the RAO has its own maximum value. The equivalent wave amplitude Dominant Load Parameter may be expressed by the following equation:

for a

Ąmax

ŴŽ G JĄAĀ

max

where,

ŴŽ

Ąmax

= equivalent wave amplitude of the Dominant Load Parameter

= maximum response of the Dominant Load Parameter

ĄAĀmax = maximum RAO amplitude of the Dominant Load Parameter

Dominant Load Parameter:

 Vertical bending moment

 Vertical shear force

 Horizontal bending moment

 Horizontal shear force

 External sea pressures

 Internal tank pressures

 Inertial accelerations

(4) Load Cases

Load cases are derived based on the above static and dynamic loading conditions, Load Parameters. For each load case, the applied loads to be developed for structural

Dominant FE analy-

sis are to include both the static and dynamic parts of each load component. The dynamic

loads represent the combined effects of a dominant load and other accompanying loads acting

simultaneously on the hull structure, deck loads and inertial loads on the the developed loads are then used in other load effects.

5. Fatigue analysis

including structural the FE

external wave pressures, internal tank pressures,

components and equipment. For each load case, analysis to determine the resulting stresses and

For all installations of 150 m and above, the extent of fatigue analysis required is indicated in Pt 3 and 12 of Rules for the Classification of Steel Ships.

For the three cargo tank length model, the fatigue assessment is to be performed applying Spectral Fatigue Assessment.

For the cargo block model, the fatigue assessment is to be performed based on spectral fatigue analysis.

The fatigue strength of welded joints and details at terminations located in highly

and in fatigue prone locations are to be assessed, especially where higher strength

stressed

steel is

areas

used.

These fatigue and/or fracture mechanics analyses, based on the combined effect of loading, material properties, and flaw characteristics are performed to predict the service life of the structure and de- termine the most effective inspection plan. Special attention is to be given to structural notches, cutouts, bracket toes, and abrupt changes of structural sections.

(1) The cumulated fatigue damage during the transit voyage from the fabrication or previous site for an existing structure to the operation site is to be included in the overall fatigue damage assessment.

(2) The stress range due to loading and unloading cycles is to be accounted for in the overall fa- tigue damage assessment.

6. Acceptance criteria

The total assessment of the structure is to be performed against the failure modes of material yield- ing, buckling, ultimate strength and fatigue. The reference acceptance criteria of each mode are giv- en in Pt 3 and 12 of Rules for the Classification of Steel Ships.