Section 4 Design Loads

401. General

1. The requirements in this section define and specify load components to be considered in the overall strength analysis as well as design pressures applicable for local scantling design.

2. Design load criteria given by operational requirements requirements may be:

(1) production, workover and combinations thereof

(2) consumable re-supply procedures and frequency

(3) maintenance procedures and frequency

(4) possible load changes in most severe environmental

shall be fully considered. Examples of such

conditions.

402. Static loads

1. General

(1) The still water loads consist of the permanent and variable functional loads.

(2) Permanent functional loads relevant for offshore units are:

(A) mass of the steel of the unit including permanently installed modules and equipment, such as accommodation, helicopter deck, cranes, drilling equipment, flare and production equipment.

(B) mass of mooring lines and risers.

(3) Variable functional loads are loads that may vary in magnitude, position and direction during the period under consideration.

(4) Typical variable functional loads are:

(A) hydrostatic pressures resulting from buoyancy

(B) crude oil

(C) ballast water

(D) fuel oil

(E) consumables

(F) personnel

(G) general cargo

(H) riser tension

(I) mooring forces

(J) mud and brine

(5)

(6)

The variable functional loads utilised in structural design shall normally be taken as either the lower or upper design value, whichever gives the more unfavourable effect.

Variations in operational mass distributions (including variations in tank filling conditions) shall

be adequately accounted for in the structural design.

2. Still water hull girder loads

(1) All relevant still water load conditions shall be defined and permissible limit curves for hull girder bending moments and shear forces shall be established for transit and operating condition separately.

(2) The permissible limits for hull girder still water bending moments and hull girder still water

shear forces shall be given at least at each transverse bulkhead position and be included in the loading manual.

403. Environmental loads

1. General

Environmental loads are loads caused by environmental phenomena. Environmental loads which may contribute to structural damages shall be considered. Consideration should be given to responses re- sulting from the following listed environmental loads:

(1) current loads

(2) wind loads

(3) wave induced loads

(4) Snow and ice loads, when relevant

(5) green sea on deck

(6) sloshing in tanks

(7) slamming (e.g. on bow and bottom in fore and aft ship)

(8) vortex induced vibrations (e.g. resulting from wind loads on structural elements in a flare tower).

2. current loads

The current forces on submerged hulls, mooring lines, risers or any other submerged objects asso- ciated with the system are to be in accordance with Rules for the Classification of Mobile Offshore Drilling Units.

3. Wind loads

(1) The wind velocity for the design environmental condition is to be based on the statistical meas- urement wind data for the specific operation site or the analysis and interpretation of wind

measurement data for the specific operation site by weather consultants. It is to be in accord- ance with Rules for the Classification of Mobile Offshore Drilling Units.

(2) The wind velocity for transit, operating and survival condition should normally be not less than

the following, unless otherwise documented:

(A) Transit and operation conditions: 36 m/s (1 hour period at 10 m above sea level).

(B) Survival condition: Site specific.

4. Wave induced loads

(1) The wave loads shall be determined for the site specific environment in which the unit is in- tended to operate.

(2) The following wave induced responses shall be calculated:

(A) motions in six degrees of freedom

(B) vertical wave induced bending moment at a sufficient number of positions along the hull.

The positions shall include the areas where the maximum vertical bending moment and shear force occur and at the turret position. The vertical wave induced bending moment shall be calculated with respect to the section’s neutral axis

(C) horizontal bending moment

(D) accelerations

(E) axial forces

(F) external sea pressure distribution.

(3) The wave induced bending moments and shear forces may be calculated considering the weather vaning characteristics of the unit.

5. Green sea

(1) The forward part of the deck and areas aft of midship will be particularly exposed to green sea. Short wave periods are normally the most critical.

(2) Appropriate measures should be considered to avoid or minimize the green sea effects on the hull structure, accommodation, deckhouses, topside modules and equipment. These measures in-

clude bow shape design, bow flare, bulwarks and other protective structure. Adequate drainage

arrangements shall be provided.

(3) Structural members exposed to green sea shall be designed to withstand the induced loads.

Green sea loads are considered as local loads.

6. Sloshing loads in tanks

(1) In partly filled tanks sloshing occurs when the natural periods of the tank fluid is close to the periods of the motions of the unit. Factors governing the occurrence of sloshing are:

(A) tank dimensions

(B) tank filling level

(C) structural arrangements inside the tank (wash bulkheads, web frames etc.)

(D) transverse and longitudinal metacentric height (GM)

(E) draught

(F) natural periods of unit and cargo in roll (transverse) and pitch (longitudinal) modes.

(2) The pressures generated by sloshing of the cargo or ballast liquid and the acceptance criteria shall comply with the requirements given in the Pt 12 of Rules for the Classification of Steel Ships.

7. Bottom slamming

Bottom slamming shall comply with the requirements given in the Pt 12 of Rules for the Classification of Steel Ships.

8. Bow impact

The bow region is normally to be taken as the region forward of a position 0.1 L aft of F.P. and above the summer load waterline. The design of the bow structure exposed to impact loads shall be carried out according to the requirements given in the Pt 12 of Rules for the Classification of Steel Ships.

9. Design density of tanks

(1) The following minimum design density of tanks shall be used for the strength and fatigue anal- ysis, unless otherwise agreed by the project:

(A) Ballast tanks: 1.025 ŹĤŶĖ (seawater)

(B) Cargo tanks: 0.9 ŹĤŶĖ €

(C) Fuel oil tanks: 0.9 ŹĤŶĖ €

(D) Mud tanks: 2.5 ŹĤŶĖ

(E) Brine tanks: 2.2 ŹĤŶĖ

(F) Fresh water tanks: 1.0 ŹĤŶĖ €

* : For tank testing condition density of seawater 1.025 t/m3 shall be used both

strength check and in the cargo hold FE analysis for the harbour condition.

(2) The actual tank densities shall be stated on the tank plan.

(3) Higher design densities than given in (1) shall be used if specified by the project.

for local