Section 3 Loads

301. General

1. General

This Chapter pertains to the identification, definition and determination of the loads to which an offshore structure may be subjected during and after its transportation to site and its installation. As appropriate to the planned structure, the types of loads described in 302. are to be accounted for in design.

302. Types of Loads

1. General

Loads applied to an offshore structure are, for purposes of these Rules, categorized as follows.

2. Dead Load

Dead loads are loads which do not change during the mode of operation under consideration. Dead loads include the following.

(A) Weight in air of the structure including, as appropriate, the weight of the principal struc-

ture(e.g., jacket, tower, caissons, gravity foundation, piling), grout, module support frame, decks, modules, stiffeners, piping, helideck, skirt, columns and any other fixed structural parts.

(B) Weight of permanent ballast and the weight of permanent machinery

(C) External hydrostatic pressure and buoyancy calculated on the basis of the still water level

(D) Static earth pressure

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3. Live Load

(1) Live loads associated with the normal operation and use of the structure are loads which may change during the mode of operation considered. (Though environmental loads are live loads, they are categorized separately; see Par 5) Live loads acting after construction and installation include the following.

(A) The weight of drilling or production equipment which can be removed, such as derrick, draw works, mud pumps, mud tanks, rotating equipment, etc.

(B) The weight of crew and consumable supplies, such as mud, chemicals, water, fuel, pipe, ca-

ble, stores, drill stem, casting, etc.

(C) Liquid in the vessels and pipes during operation and testing

(D) The weight of liquids in storage and ballast tanks

(E) The forces exerted on the structure due to operations, e.g., maximum derrick reaction

(F) The forces exerted on the structure during the operation of cranes and vehicles

(G) The forces exerted on the structure by vessels moored to the structure or accidental impact consideration for a typical supply vessel that would normally service the installation

(H) The forces exerted on the structure by helicopters during take-off and landing, or while

parked on the structure

(2) Where applicable, the dynamic effects on the structure of items (1) (E) through (H) are to be

taken into account. Where equately accounted for by by the Operator, or past

appropriate, some of the items of live load listed above may be ad- designing decks, etc. to a maximum, uniform area load as specified

practice for similar conditions. Live loads occurring during trans-

portation and installation are to be determined for each specific operation involved and the dy-

namic effects of such loads are to be accounted for as necessary. (see Sec 9)

4. Deformation Loads

Deformation loads are loads due to deformations imposed on the structure. The deformation loads include those due to temperature variations(e.g., hot oil storage) leading to thermal stress in the structure and where necessary, loads due to displacements(e.g., differential settlement or lateral dis- placement) or due to deformations of adjacent structures. For concrete structures, deformation loads due to prestress, creep, shrinkage and expansion are to be taken into account.

5. Environmental Loads

Environmental loads are loads due to wind, waves, current, ice, snow, earthquake, and other envi- ronmental phenomena. The characteristic parameters defining an environmental load are to be appro-

priate to the Environmental

Environmental

installation site and in accordance with the requirements of Sec 2. Operating Loads are those loads derived from the parameters characterizing Operating Conditions(see 203. 3). The combination and severity of Design Environmental

Loads are to be in accordance with from directions producing the most

203. 2. Environmental loads are to be applied to the structure unfavorable effects on the structure, unless site-specific studies

provide evidence in support of a less stringent requirement. Directionality may be taken into ac-

count in applying the environmental criteria. Earthquake loads and loads due to accidents or rare occurrence environmental phenomena need not be combined with other environmental loads, unless

site specific conditions indicate that such combinations are appropriate.

303. Determination of Environmental Loads

1. General

(1) Model or field test data may be employed to establish environmental loads. Alternatively, envi- ronmental loads may be determined using analytical methods compatible with the data estab- lished in compliance with Sec 2.

(2) Any recognized load calculation method may be employed provided it has proven sufficiently

accurate in practice, and it is shown to be appropriate to the structure's characteristics and site conditions. The calculation methods presented herein are offered as guidance representative of current acceptable methods.

2. Wave Loads

(1) Range of Wave Parameters

A sufficient range of realistic wave periods and wave crest positions relative to the structure to be investigated to ensure an accurate determination of the maximum wave loads on

are the

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structure. Consideration should be given to other wave induced effects such as wave impact loads, dynamic amplification and fatigue of structural members. The need for analysis of these effects is to be assessed on the basis of the configuration and behavioral characteristics of the structure, the wave climate and past experience.

(2) Determination of Wave Loads

For structures composed of members having diameters which are less than 20% of the wave lengths being considered, semiempirical formulations such as (3) are considered to be an accept-

able basis for determining wave loads. For structures composed of members whose diameters are greater than 20% of the wave lengths being considered, or for structural configurations which

substantially alter the incident flow field, diffraction forces and the hydrodynamic interaction of structural members are to be accounted for in design.

(3) Load calculation

(A) The hydrodynamic force acting on a cylindrical member, as given by Morison's equation, is expressed as the sum of the force vectors indicated in the following equation.

ÁG ÁǼ ĞÁÂ

Á : hydrodynamic force vector per unit length along the member, acting normal to the axis of the member

ÁǼ :

ÁÂ :

drag force vector per unit length inertia force vector per unit length

(B) The drag force vector for a stationary, rigid member is given by the following formula.

Ǿ

ÁǼ G F JËX FǼÆǼßĢßĢ

Ǿ :

X :

Ǽ :

ÆǼ :

ß :

ĢßĢ:

weight density of water (NĤmĖ) gravitational acceleration (mĤsË)

projected width of the member in the direction of the crossflow component of ve- locity(in the case of a critical cylinder, Ǽ denotes the diameter), in m

drag coefficient (dimensionless)

component of the fluid velocity vector normal to the axis of the member, in m/s absolute value of ß , in m/s

(C) The inertia force vector for a stationary, rigid member is given by

Ǿ ŘǼ Ë

ÁÂ G F JX FFĖJFÆÀŴŸ

ÆÀ : Inertia coefficient based on the displaced mass of fluid per unit length(dimension- less)

ŴŸ : component of the fluid acceleration vector normal to the axis of the member,

mĤsË

(D) For compliant structure which exhibit substantial rigid body oscillations due to the wave ac- tion, the modified form of Morison's equation given below may be used to determine the hydrodynamic force.

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Ǿ ′ ′

Ǽ Ë Ǿ ŘǼ Ë

FǼÆ

FÃ G Ã FĢÃ

Ǿ Ř

à ĢĞ F FF J FŴ Ğ F J FF J FÆ FŴ GŴ′ F

Á G ÁǼ Ğ ÁÂ G F JËX

Ǽ Ÿ Ÿ

Ÿ G Ÿ

JX Ė

Ÿ X Ė

Ŷ Ÿ Ÿ

ß′: component of the velocity vector of the structural member normal to its axis, in m/s

ÆŶ :

Ŵ′ Ÿ :

added mass coefficient, i.e., ÆŶ G ÆÀ G Ë

component of the acceleration vector of the structural member normal to its axis, in mĤsË

(E) For structural shapes other than circular cylinders, the term ŘǼ Ë/4 in the above equation is to be replaced by the actual cross-sectional area of the shape.

(F) Values of ß and ŴŸ for use in Morison's equation are to be determined using a recognized wave theory appropriate to the wave heights, wave periods, and water depth at the in- stallation site. Values for the coefficients of drag and inertia to be used in Morison's equa- tion are to be determined on the basis of model tests, full scale measurements, or previous studies which are appropriate to the structural configuration, surface roughness, and pertinent flow parameters(e.g., Reynolds number) Generally, for pile-supported template type structures, values of ÆǼ range between 0.6 and 1.2; values of ÆÀ range between 1.5 and 2.0.

(4) Diffraction Theory

For structural configurations which substantially alter the incident wave field, diffraction theories

of wave loading are to be employed which account for both the incident wave force(i.e., Froude-Kylov force) and the force resulting from the diffraction of the incident wave due to the presence of the structure. The hydrodynamic interaction of structural members is to be taken in- to account. For structures composed of surface piercing caissons or for installation sites where the ratio of water depth to wave length is less that 0.25, nonlinear effects of wave action are to be taken into account. This may be done by modifying linear diffraction theory to account for nonlinear effects or by performance of model tests.

3. Wind Load

(1) Wind loads and local wind pressures are to be determined on the basis of analytical methods or wind tunnel tests on a representative model of the structure.

(2) In general, the wind load on the overall structure to be combined with other design environ- mental loads is to be determined using a one-minute sustained wind speed. For installations

with negligible dynamic response to wind, a one-hour sustained wind speed may be used to cal-

culate the wind loads on the overall structure. Wind loads on broad, essentially flat structures such as living quarters, walls, enclosures, etc. are to be determined using a fifteen-second gust wind speed. Wind pressures on individual structural members, equipment on open decks, etc. are to be determined using a three second gust wind speed.

(3) For wind loads normal to flat surfaces or normal to the axis of members not having flat surfa- ces, the following relation may be used.

F F

ǾŴ Ë

ÁŽ G JËX ÆZ ÃŻ A

ÁŽ :

X :

ǾŴ :

ÆZ :

ÃŻ :

A :

wind load, in N

gravitational acceleration (mĤsË) weight density of air, in NĤm Ė shape coefficient (dimensionless) wind speed at altitude Ż, in m/s

projected area of member on a plane normal to the direction of the considered force, in mË

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(4) For any direction considered to act

of wind approach to the structure, the wind force on flat surfaces should be normal to the surface. The wind force on cylindrical objects should be as-

sumed to act in the direction of the wind.

(5) The area of open trussworks commonly used for derricks and crane booms may be approxi-

mated by taking 30 % of the projected area of both the windward and leeward sides with the

shape coefficient taken in accordance with Table 3.3.1.

Table 3.3.1 Values of shape coefficient (ÆZ )

(6) Where one structural member shields another from direct exposure to the wind, shielding may be taken into account.

(7) Generally, the two structural components are to be separated by not more that seven times the

width of the windward component for a reduction to be taken in the wind load on the leeward member.

(8) Where appropriate, dynamic effect due to the cyclical nature of gust wind and cyclic loads due to vortex induced vibration are to be investigated. Both drag and lift components of load due to

vortex induced vibration are to be taken into account.

(9) The effects of wind loading on structural members or components that would not normally be exposed to wind loads after platform installation are to be considered. This would especially ap- ply to fabrication or transportation phases.

4. Current Loads

(1) Current induced loads on immersed structural members are to be determined on the basis of an- alytical methods, model test data or full-scale measurements.

(2) When currents and waves are superimposed, the current velocity is to be added vectorially to

the wave induced particle velocity prior to computation of the total force.

(3) Current profiles used in design are to be representative of the expected conditions at the in- stallation site.

(4) Where appropriate, flutter and dynamic amplification due to vortex shedding are to be

to account.

(5) For calculation of current loads in the absence of waves, the lift force normal to flow and the drag force may be determined as follows.

taken in-

direction,

Ǿ

ÁÄ G ÆÄF JËX FÃ

Ë Ǽ

ËA Á G Æ

Ǿ

ǼFËJXF

Ë

ÃË A

ÁÄ :

ÆÄ :

Ǿ :

à :

AË :

ÁǼ :

ÆǼ :

total lift force per unit length, in N/m lift coefficient(dimensionless)

weight density of water, in NĤmĖ

local current velocity, in m/s

projected area per unit length in a plane normal to the direction of the

mËĤm

total drag force per unit length, in N/m drag coefficient

force, in

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(6) In general, lift force may become significant for long cylindrical members with large length-di- ameter ratios and should be checked in design under these conditions. The source of ÆÄ values employed is to be documented.

5. Ice and Snow Loads

(1) At locations where structures are subject to ice and snow accumulation the following effects are to be accounted for, as appropriate to the local conditions.

(A) Weight and change in effective area of structural members due to accumulated ice and snow

(B) Incident pressures due to pack ice, pressure ridges and ice island fragments impinging on the structure

(2) For the design of structures that are to be installed in service in extreme cold weather such as

arctic regions, reference is to be made to API 2N: "Planning, Designing, and Constructing Fixed Offshore Platforms in Ice Environments"

6. Earthquake Loads

(1) For structures located in seismically active areas strength level and ductility level earthquake in- duced ground motions are to be determined on the basis of seismic data applicable to the in- stallation site.

(2) Earthquake ground motions are to be described by either applicable ground motion records or response spectra consistent with the recurrence period appropriate to the design life of the

structure.

(3) Available standardized spectra applicable to the region of the installation site are acceptable pro- vided such spectra reflect site-specific conditions affecting frequency content, energy distribution, and duration. These condition include : the type of active faults in the region, the proximity of the site to the potential source faults, the attenuation or amplification of ground motion between the faults and the site, and the soil conditions at the site.

(4) The ground motion description used in design is to consist of three components corresponding

to tow or thogonal horizontal directions and the vertical direction. All three components are to be applied to the structure simultaneously. When a standardized response spectrum, such as giv- en

in the American Petroleum Institute(API) RP 2A, is used for structural analysis, input values

of ground motion(spectral acceleration representation) to be the following.

(A) 100 % in both orthogonal horizontal directions

(B) 50 % in the vertical direction.

used are not to be less severe that

(5) When three-dimensional, site-specific ground motion accelerations are to be used.

spectra are developed, the actual directional

(6) When three-dimensional, site-specific ground motion

accelerations are to be used. If single site-specific

spectra

spectra

are

are

developed, the actual directional

developed, accelerations for the

remaining two orthogonal directions should above.

(7) If time history method is used for structural

histories are to be employed. The manner in

be applied in accordance with the factors given

analysis, at least three sets of ground motion time which the time histories are used is to account for

the potential sensitivity of the structure's response to variations in the phasing of the ground

motion records. Structural appurtenances, equipment, modules, and piping are to be designed to resist earthquake induced accelerations at their foundations. As appropriate, effects of soil lique- faction, shear failure of soft muds and loads due to acceleration of the hydrodynamic added mass by the earthquake, submarine slide, tsunamis and earthquake generated acoustic shock waves are to be taken into account.

7. Marine Growth

(1) The following effects of anticipated marine growth are to be accounted for in design.

(A) Increase in hydrodynamic diameter

(B) Increase in surface roughness in connection with the determination of hydrodynamic co- efficients(e.g., lift, drag and inertia coefficients)

(C) Increase in dead load and inertial mass

(2) The amount of accumulation assumed for design is to reflect the extent of and interval between cleaning of submerged structural parts.

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8. Sea Ice

(1) The global forces exerted by sea ice on the structure as a whole and local concentrated loads on structural elements are to be considered. The effects of rubble piles on the development of larger areas, and their forces on the structure, need to be considered.

(2) The impact effect of a sea ice feature must consider mass and hydrodynamic added mass of the ice, its velocity, direction and shape relative to the structure, the mass and size of the structure,

the added mass of water and soil accelerating with the structure, the compliance of the struc- ture- soil interaction and the failure mode of the ice-structure interaction.

(3) The dynamic response of structure to ice may be important in flexible structures.

(4) As appropriate, liquefaction of the underlying soils due to repetitive compressive failures of the

ice against the structure is to be taken into account.

9. Subsidence

The effects of subsidence should be considered in the overall foundation and structural design. This would be especially applicable to facilities where unique geotechnical conditions exist such that sig- nificant sea floor subsidence could be expected to occur as a result of depiction of the subsurface