Section 4 Structural Design and Analysis of the Hull

401. General

1. Design of ship type unit may generally follow the principles of design of steel ships. The design will have to account for characteristics in operation and loading of floating offshore gas installations.

2. The offshore installation design will include:

(1)

(2)

(3)

(4)

(5)

(6)

Environmental loading regime for a fixed installation location Inability to avoid severe weather

Fatigue design and details for service life

Partial filling and sloshing loads

Continuous operation and limited availability and access for inspection and repair Increased potential for cryogenic leakage

(7)

(8)

(9)

(10)

(11)

(12)

(13)

Loading in exposed locations Scaling up of existing designs Increased corrosion considerations

Increased hazard due to location of gas handling, liquefaction or regasification plant Provision of a position mooring system

Project-specific Design Accidental Loads

Different regulatory requirements

402. Design Criteria

1. Offshore operation will typically impose different requirements than those applicable for traditional LNG carrier designs. Some of the requirements will be related to safety while others will arise for

reasons of operational optimization. The design

ing permanently stationed installations and is

criteria. Offshore operation will normally imply

is based on field-specific operation, usually involv-

governed by national regulation and site specific

that the unit is permanently position-moored. A tur-

ret mooring arrangement or a spread mooring arrangement may be used. The unit will impose addi-

tional structural loads arising from topsides loads, sloshing in storage tanks, loads from ship to ship mooring during LNG transfer, and additional design accidental loads arising from activities on

board. Continuous operation offshore, typically without dry-docking, for the life of the gas field

will impose the need for increased initial quality in order to avoid the need for in-service repair or replacement. This is particularly relevant for fatigue and corrosion considerations. To minimize fa- tigue damage occurring during service, the design fatigue factors for an offshore vessel not intend- ing to dry-dock, will be stricter than for a trading carrier. The unit in benign areas with high am-

bient temperature has shown that there may be a high corrosion rate compared to oil carriers. The corrosion protection system may therefore need to meet a higher standard. Regulatory requirements applicable to unit may also impose some additional structural considerations.

2. There are a number of important key factors for a design suitable for an offshore application.

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

Design for the intended site of operation

Design life which is variable: 10-40 years usually depending on field life

Design based on limit states with the specified probability levels for environmental loads 100 year return period for Ultimate Limit States, ULS

Fatigue design for design life with increased design fatigue life Limited inspection and repair possibilities

Increased corrosion protection

Tank access and gas freeing for inspection

Additional loads from: topsides, flare, mooring system, risers, cranes, helideck

Continuous partial filling operation of the cargo tanks Different onloading and offloading pattern and berthing loads

Additional accidental load scenarios to be defined and checked in addition to prescriptive re-

quirements

Additional requirements of regulatory schemes

403. Structural design of the hull

Design of the hull is to be based on this guide 401, 402., general hull structures are to be in ac- cordance with the requirements of Pt 3 and Pt 12 of Rules for the Classification of Steel Ships.

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 are to comply with the requirements of Pt 12 of

Rules for the Classification of Steel Ships. 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 Classification of Mobile Offshore Drilling Units. In addition to the required loadings defined in Rules for Classification of Mobile Offshore Drilling Units, the structural strength of the heli- copter deck and its supporting structures are to be evaluated considering the DOC and DEC envi- ronments, 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 and ventilators are to comply with the requirements of Pt 12 of Rules for the Classification of 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.

8. Sea chests

The requirements of Sea Chests 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

the requirements of Pt 12 of Rules for the

404. Engineering analyses of the hull structure

1. General

The criteria in this Subsection relate to the analyses required to verify hull design in 403.. 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 of longitudinal stiffeners

3D FEM model. with transverses

To examine stress concentrations, such as at intersections 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.

405. Additional Application

Other various matters on the Structural Design and Analysis of the Hull are to be as specified in Pt 5, Ch4, of Rules for the Guidance for Floating Production Units.