Fabrication forces are those forces imposed upon individual members, component parts of the structure, or complete units during the unloading, handling and assembly in the fabrication yard. Installation forces are those forces imposed upon the component parts of the structure during the operations of moving the components from their fabrication site or prior offshore location to the final offshore location, and installing the component parts to form the completed platform. Since installation forces involve the motion of heavy weights, the dynamic loading involved should be considered and the static forces increased by appropriate impact factors to arrive at adequate equivalent loads for design of the members affected. For those installation forces that are experienced only during transportation and launch, and which include environmental effects, basic allowable stresses for member design may be increased by one-third in keeping with provisions of 6.1.2. Also see Section 15 for comments complementary to this section.
5.4.2 Lifting Forces 5.4.2.1 General
Lifting forces are imposed on the structure by erection lifts during the fabrication and installation stages of platform construction. The magnitude of such forces should be determined through the consideration of static and dynamic forces applied to the structure during lifting and from the action of the structure itself.
Lifting forces on padeyes and on other members of the structure shall include both vertical and horizontal components, the latter occurring when lift slings are other than vertical. Vertical forces on the lift shall include buoyancy as well as forces imposed by the lifting equipment.
To compensate for any side loading on lifting eyes that may occur, in addition to the calculated horizontal and vertical components of the static load for the equilibrium lifting condition, lifting eyes and the connections to the supporting structural members shall be designed for a horizontal force of 5 % of the static sling load, applied simultaneously with the static sling load. This horizontal force shall be applied perpendicular to the padeye at the center of the pinhole.
5.4.2.2 Static Loads
When suspended, the lift will occupy a position such that the center of gravity of the lift and the centroid of all upward acting forces on the lift are in static equilibrium. The position of the lift in this state of static equilibrium should be used to determine forces in the structure and in the slings. The movement of the lift as it is picked up and set down shall be taken into account in determining critical combinations of vertical and horizontal forces at all points, including those to which lifting slings are attached.
5.4.2.3 Dynamic Load Factors
For lifts where either the lifting derrick or the structure to be lifted is on a floating vessel, the selection of the design lifting forces shall consider the impact from vessel motion. Load factors should be applied to the design forces as developed from considerations of 5.4.2.1 and 5.4.2.2.
For lifts to be made at open, exposed sea (i.e. offshore locations), padeyes and other internal members (and both end connections) framing into the joint where the padeye is attached and transmitting lifting forces within the structure shall be designed for a minimum load factor of 2.0 applied to the calculated static loads. All other structural members transmitting lifting forces shall be designed using a minimum load factor of 1.35.
For other marine situations (i.e. loadout at sheltered locations), the selection of load factors shall meet the expected local conditions but should not be less than a minimum of 1.5 and 1.15 for the two conditions listed in the previous paragraph.
API 2MOP [6] may be used to determine alternate load factors however, the load factors shall not be less than those defined above. If API 2MOP [6] is used for dynamic load factors, all the appropriate factors such as weight contingency, COG skew, etc. shall be included.
For typical fabrication yard operations where both the lifting derrick and the structure or components to be lifted are land-based, dynamic load factors may be lower than those defined above. For special procedures where unusual dynamic loads are possible, appropriate load factors should be considered.
5.4.2.4 Allowable Stresses
The lift shall be designed so that all structural steel members are proportioned for basic allowable stresses as specified in 6.1. The 6.1.2 increase in allowable stresses for short-term loads shall not be used. In addition, all critical structural connections and primary members should be designed to have adequate reserve strength to ensure structural integrity during lifting.
5.4.2.5 Effect of Tolerances
Fabrication tolerances and sling length tolerances both contribute to the distribution of forces and stresses in the lift system, which is different from those normally used for conventional design purposes.
The load factors recommended in 5.4.2.3 are intended to apply to situations where fabrication tolerances do not exceed the requirements of 14.1.5 and where the variation in length of slings does not exceed
± 0.25 % of nominal sling length, or 38 mm (1.5 in.).
The total variation from the longest to the shortest sling should not be greater than 0.5 % of the sling length or 75 mm (3 in.). If either fabrication tolerance or sling length tolerance exceeds these limits, a detailed analysis taking into account these tolerances should be performed to determine the redistribution of forces on both slings and structural members. This same type analysis should also be performed in any instances where it is anticipated that unusual deflections of particularly stiff structural systems may also affect load distribution.
5.4.2.6 Slings, Shackles, and Fittings
For normal offshore conditions, slings shall be selected to have a factor of safety of 4 for the manufacturer’s rated minimum breaking strength of the cable compared to static sling load. The static sling load should be the maximum load on any individual sling, as calculated in 5.4.2.1, 5.4.2.2, and 5.4.2.5, by taking into account all components of loading and the equilibrium position of the lift. This factor of safety should be increased when unusually severe conditions are anticipated and may be reduced to a minimum of 3 for carefully controlled conditions.
Shackles and fittings should be selected so that the manufacturer’s rated working load is equal to or greater than the static sling load, provided the manufacturer’s specifications include a minimum factor of safety of 3 compared to the minimum sling breaking strength.
5.4.3 Loadout Forces 5.4.3.1 Direct Lift
Lifting forces for a structure loaded out by direct lift onto the transportation barge should be evaluated only if the lifting arrangement differs from that to be used in the installation, since lifting in open water will impose more severe conditions.
5.4.3.2 Horizontal Movement onto Barge
Structures skidded onto transportation barges are subject to load conditions resulting from movement of the barge due to tidal fluctuations, nearby marine traffic, and/or change in draft, as well as from load conditions imposed by location, slope, and/or settlement of supports at all stages of the skidding operation. See API 2MOP [6] for additional guidance. Since movement is normally slow, impact need not be considered.
5.4.4 Transportation Forces 5.4.4.1 General
Transportation forces acting on templates, towers, guyed towers, minimum structures, and platform deck components shall be considered in their design, whether transported on barges or self-floating. These forces result from the way in which the structure is supported, either by barge or buoyancy, and from the response of the tow to environmental conditions encountered in route to the site. See API 2MOP [6] for additional guidance. In the subsequent paragraphs, the structure and supporting barge and the self- floating tower are referred to as the tow.
5.4.4.2 Environmental Criteria
The selection of environmental conditions to be used in determining the motions of the tow and the resulting gravitational and inertial forces acting on the tow shall consider the following:
a) previous experience along the tow route;
b) exposure time and reliability of predicted “weather windows”;
c) accessibility of safe havens;
d) seasonal weather systems;
e) appropriateness of the recurrence interval used in determining maximum design wind, wave, and current conditions and considering the characteristics of the tow, such as size, structure, sensitivity, and cost.
5.4.4.3 Determination of Forces
The tow including the structure, sea fastenings and barge shall be analyzed for the gravitational, inertial and hydro-dynamic loads resulting from the application of the environmental criteria in 5.4.4.2. The analysis should be based on model basin test results or appropriate analytical methods. Beam, head and quartering wind and seas should be considered to determine maximum transportation forces in the tow structural elements. In the case of large barge-transported structures, the structure’s stiffness may be substantially larger than the barge stiffness. This stiffness difference may have significant implications and should be considered in the structural analysis.
Where relative size of barge and jacket, magnitude of the sea states, and experience make such assumptions reasonable, tows may be analyzed based on gravitational and inertial forces resulting from the tow’s rigid body motions using appropriate period and amplitude by combining roll with heave and pitch with heave.
5.4.4.4 Other Considerations
Large jackets, templates, and compliant towers often overhang the length and/or the sides of the barge and may be subjected to partial submersion during tow. Submerged members should be investigated for slamming, buoyancy and collapse forces. Large buoyant overhanging members also may affect motions and should be considered. The effects on long slender members of wind-induced vortex shedding vibrations should be investigated. This condition may be avoided by the use of simple wire rope spoilers helically wrapped around the member.
For long transoceanic tows, repetitive member stresses may become significant to the fatigue life of certain member connections or details and should be investigated.
5.4.5 Launching Forces and Uprighting Forces 5.4.5.1 Compliant/Guyed Towers and Templates
Compliant/guyed tower and template structures that are transported by barge are usually launched at or near the installation location. The jacket is generally moved along ways, which terminate in rocker arms, on the deck of the barge. As the position of the jacket reaches a point of unstable equilibrium, the jacket rotates, causing the rocker arms at the end of the ways to rotate as the jacket continues to slide from the rocker arms. Forces supporting the jacket on the ways shall be evaluated for the full travel of the jacket.
Deflection of the rocker beam and the effect on loads throughout the jacket should be considered. In general, the most severe forces occur at the instant rotation starts. Consideration should be given to the development of dynamically induced forces resulting from launching. Horizontal forces required to initiate movement of the jacket should also be evaluated. Consideration should be given to wind, wave, current,
and dynamic forces expected on the structure and barge during launching and uprighting. See API 2MOP [6] for additional guidance.
5.4.5.2 Towers
Tower structures designed to be self-buoyant are generally launched from the fabrication yard to float with their own buoyancy for tow to the installation site. The last portion of such a tower leaving the launching ways may have localized forces imposed on it as the first portion of the tower to enter the water gains buoyancy and causes the tower to rotate from the slope of the ways. Forces shall be evaluated for the full travel of the tower down the ways.
5.4.5.3 Hook Load
Floating jackets for which lifting equipment is employed for turning to a vertical position should be designed to resist the gravitational and inertial forces required to upright the jacket.
5.4.5.4 Submergence Pressures
The submerged, nonflooded, or partially flooded members of the structure shall be designed to resist pressure-induced hoop stresses during launching and uprighting.
A member may be exposed to different values of hydrostatic pressure during installation and while in place. The integrity of the member shall be determined using the guidelines of 6.2.5 and 6.4.2.
5.4.6 Installation Foundation Loads 5.4.6.1 General
Calculated foundation loads during installation shall be conservative enough to give reasonable assurance that the structure will remain at the planned elevation and attitude until piles can be installed.
Reference should be made to appropriate paragraphs in Section 9 and Section 15.
5.4.6.2 Environmental Conditions
Consideration shall be given to effects of anticipated storm conditions during this stage of installation.
5.4.6.3 Structure Loads
Vertical and horizontal loads shall be considered taking into account changes in configuration/exposure, construction equipment, and required additional ballast for stability during storms.
5.4.7 Hydrostatic Pressure
Unflooded or partially flooded members of a structure shall be able to withstand the hydrostatic pressure acting on them caused by their location below the water surface. A member may be exposed to different values of pressure during installation and while in place. The integrity of the member shall be determined using the guidelines of 6.2.5 and 6.4.2.
5.4.8 Removal Forces
Due consideration shall be taken of removal forces such as blast loads, sudden transfer of pile weight to jacket and mudmats, lifting forces, concentrated loads during barge loading, increased weight, reduced buoyancy, and other forces that may occur. See API 2MOP [6] for additional guidance.