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This article covers introduction and analysis procedure to installation by lifting.

Table of contents

• Why lifting?
• Types of lifts
• Importance of lifting analysis
• Computer model
• Loads
• Rigging
• Boundary conditions
• Analysis
• Post-processing
• Discussion
• Appendix: Crane barge vessels list (region-wise and worldwide)

Why lifting?

• For installing offshore structures such as jackets, topsides, decks, modules, towers and appurtenances such as boat landing, riser guards, etc.
• Convenience in installation, cost–effective and clean.
• When the objects to be lifted are less than 10,000 tons.¹

Types of lifts

Types of lifts can be broadly categorized as follows:

• Onshore lifts—limited to onshore crane capacities and due to soil bearing capacity in the fabrication yards.
• Offshore lifts—relatively large lifts possible due to offshore crane barges. (Most offshore installations within the available crane capacity are lift-installed).

Topside is being lifted in an offshore lift sequence—by a heavy crane lift barge—using two hooks and spreader bar arrangement.

Importance of lifting analysis

• To assess and design the structure for installation stresses.
• Provides a better idea of structural layout in terms of weight distribution and overall weight control.
• Lifting—on many occasions—is known to be one of the critical pre–service conditions in terms of member and joint stresses.
• To capture changes in layout post-design stage tolerances.
• To capture fabrication tolerances in terms of eccentricities when finally built.

Computer model

Topside lift model—with a single hook point.

Jacket lift model—with a single hook point.

• Wireframe model acts as a fairly accurate mathematical representation of the structure to be assessed.
• Helps simulate actual conditions such as connectivity, sway, load imbalance, local stresses, etc by proper modeling.
• While using a computer software, users should remind themselves that: Junk–in is equal to junk–out. Therefore, it’s very important to have the model as accurate as possible to expect accurate results.
• Corollary to that, a model need not be necessarily detailed so long as it captures all critical aspects of a structure such as the following:
  • Member sizes, material properties
  • Effective lengths
  • Member offsets
  • Joint eccentricities, etc
  • Member releases
  • Springs
  • Applied loads (check by summation and individual COGs)

Loads

All dry weights are considered with proper contingencies (Contingencies are those multiplying factors that capture the non-modeled items in a model; they are part of the base weight, unlike a load factor such as in LRFD method of design).

The dry loads normally are:

• Self weight
• Non-modeled dead loads
• Architectural dry loads (if any)
• Equipment dry loads
• Piping dry loads
• Electrical and instrument dry loads
• Others, if any (depending upon type of structure)

In a jacket, appurtenances can be:

• Anodes
• Conductor cone plates
• Copper nickel (Piping)
• Flooding system
• Framing and walkway at seadeck
• Hydrostatic collapse rings
• Leg diaphragms
• Mooring bits
• Riser clamps
• Sling (rigging) platform
• Spacer plates
• Trunnion and trunnion rings
• Upending padeyes
• Upending slings and shackles
• Buoyancy tank(s)
• Mudmat timber
• Others (depending upon type of jacket)

All the dry loads are combined with appropriate contingencies depending upon the maturity of weight control report to form a combined load case.

Rigging

Sling Arrangement Types

© Katholieke Universiteit Leuven

Versabar, Inc specializes in delivering complex lift systems such as these (please see below) turning installation into art and providing economical, well-thought engineering solutions to unique lift problems.

© Versabar, Inc.

Hook point and Sling angles

The calculated hook point based on an industry practice minimum criteria of 60° to the horizontal needs to be checked for the crane barge limitations, if any.

In general, the preferred angle of the slings to the horizontal is between 50° and 90°, but a sling angle of 60° (±5°) is a good first choice. Whereas for heavy lifts over 3000t, a sling angle of 70° is often preferred. The higher angle implies that horizontal forces transmitted—due to horizontal force component of an inclined sling—are minimized; while implying higher tensile forces in the vertical elements. But this is beneficial in the sense that steel, as we know, is better in tension than in compression, and tensile stress limits are higher than in buckling (induced by compression).

The factors that will affect the vertical sling angle will be the lifting (or installation) contractor’s preference, the sling lengths available, and any working height restrictions of the crane vessel’s lifting hook. It is not uncommon to have a greater angle on the lower modules of a multi-stacked topsides and a smaller angle for the upper modules.

Lifting capacity of the barge varies with the hook height as depicted in the chart below. Most commercial crane barge charts are available to design engineers.

For in-depth details on rigging equipment and lift systems, handling procedures and padeye design, refer to Heerema’s standard criteria.

Crane barge data: Saipem, McDermott, Heerema, NPCC.

Boundary conditions

The hook point may be fixed in all the six directions i.e., x, y, z, rx, ry and rz. The sling members shall have proper end-releases so as to accurately simulate a sling member.

Springs² to be modeled with appropriate stiffness.

As a guideline, if the spring forces are significant, it means that the hook point is not modeled above the CG. Hence, springs are activated to resist the load due to the eccentricity. Ideally speaking, for an equilibrium condition, the spring forces should be zero.

It is generally safe to input spring stiffness as 100kN/m in each direction. The springs shall be defined on joints of importance such as a main frame joint but should generally be avoided on joints of topside leg or padeye joints.

Analysis

There are at least two known types of lifting analyses that are actively performed by design engineers. They are:

1. API method—This method is as prescribed in API RP 2A and is most commonly performed.
2. 75% – 25% method—In this case, one pair of slings (diagonally opposite to each other) take up 75% of the total lift load and the other two take-up 25%, and vice versa. Structural integrity due to this “slacking” in slings is thus ascertained for adequacy.

The following procedure outlines the API method, in general. Most of this procedure also applies to the 75%-25% method, except for the forced load distribution in the latter case.

• Define springs for mathematical stability of the structure (ΣH=0, ΣV=0, ΣM=0).
• For lifting analysis, perform a linear static analysis for lifting conditions such as: (1) no–shift in COG and (2) shift in COG.

No COG shift

• This is a base case with loads as modeled.
• Spring forces are zero or negligible—this denotes that the structure is in equilibrium for a base case.

Shift in COG

Shift in COG can be of two types:

• Permanent shift in COG due to changes in layout in the post-fabricated stage. In this case, the hook point is shifted in the model when compared to the base case and counter loads are applied to capture the shift.
• Temporary shift in COG due to installation related tolerances like padeye locations, cut–sling lengths. In this case, the hook point is same as in the base case, but counter loads are applied to capture moments due to the eccentricities.
• Sum of support reactions equal to the total load sum.
• Spring forces are zero or negligible—this denotes that the structure is in equilibrium for a base case.

These criteria are intended for conventional lifts of modules and MSFs. For smaller packages and those of awkward sizes—such as flare booms, vent tripods, bridges, etc—shifts in CG is generally agreed during preparation of Structural Design Premise.

Note: A negative reaction in a sling element would mean that the sling is in compression, which in reality is not possible. The analysis therefore needs to be re–run by disabling the negative reaction experiencing sling.³

Post processing

• Check that no sling forces are negative in the base (no–COG shift) case.
• Check and ensure that spring forces are nil or negligible.
• Perform code check for primary and secondary members appropriately. API RP-2A (Page 35, Section 2.4.2c Dynamic Load Factors) recommends using an impact factor 2.0 for members connected directly to padeye, and other non-padeye connected members be checked with a factor 1.35.
• In addition—as an option—combine different conditions into one solution using ‘combine solution file’ under Utils in SACS Executive for a single Postvue and Post listing of results from both no-shift and shift COG cases.

Discussion

This article does not cover many aspects and variables involved in a typical detailed design. However, the author is of the opinion that it provides a general guideline in understanding and performing a lifting analysis. Readers are welcome to suggest errors, omissions and obvious inclusions—if any—in this article. You may write to the author here.

Disclaimer: Please be aware that the author bears no responsibility with regards to the content and use of this article.

Appendix: Crane barge vessels list (region-wise and worldwide)

Please see a list of crane barge vessels—region-wise and worldwide.

1. The weight limitation is suggestive, since the lifting infrastructural capacities are continuously challenged as capabilities improve everyday. [←]
2. Note on Springs—To simulate a mathematical stability of the structure, since the support condition is only one (physically i.e., the hook point), two pair of springs are defined one pair in single lateral directions (x, y) and one pair in (x & y). The number of springs to be specified is irrelevant as long as the spring forces are null (meaning the springs are not ‘activated’ as long as they don’t experience the force). [←]
3. A good indication where this could happen would be when the structural COG is not anywhere near the geometric center of the structure; and if attaining equilibrium is difficult or expensive, then it may be time to review the layout and make changes where possible in terms of weight distribution. This is easier said than done, but would help if weights and layouts are planned well keeping such options in mind at the concept or front end engineering stage of the project. [←]