28 - w. janse nsoclc design floating lifting appliances for offshore use

8/18/2019 28 - W. Janse NSOCLC Design Floating Lifting Appliances for Offshore Use

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Vuyk Engineering Rotterdam

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The 17th North Sea OFFSHORE CRANES & LIFTING CONFERENCE

24th – 26th April 2012, Aberdeen

Design of Floating Lifting Appliances for Offshore use

Willem Janse

Vuyk Engineering RotterdamRotterdam, The Netherlands

Vuyk Engineering Rotterdam (VER) is an internationally operating engineering company

serving the maritime industry. Vuyk provides consultancy and engineering services in theareas of ship design, equipment design, marine operations and building supervision.

VER is specialized in work vessels for the dredging and offshore industries and heavy liftshipping. Aside from designing new vessels or modifications, we also have a broadexperience in engineering for installation or abandonment, transports, load-outs and salvageoperations.

www.vuykrotterdam.com




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1. Introduction

Crane vessels are usually designed for offshore lifting. Motion effects can be introduced bymeans of several design factors which are available through the Lifting Appliances designcodes of the classification societies. The paper shows how a reduction on these factors canbe accomplished by using direct motion analyses. This approach has been used for the VER

design of Rambiz 2 (Scaldis, Salvage & Marine Contractors N.V.).

The same engineering methods can be used for offshore lifts with Floating Sheerlegs,although these are usually designed for inshore lifting. The typical effects encountered whenlifting offshore have to be determined. Results are allowable loads (which may be read as areduction of lifting curves) and allowable sea conditions. An overview is given of VERexperience with offshore application of the VER designed Matador 3 (owner and operator:Bonn & Mees). It is indicated how a design of a floating sheerlegs can anticipate on offshorelifts, without compromising inshore effectiveness.

This paper presents methods to determine design loads. Also some interesting designinformation of the lifting appliances on the Rambiz 2 is included.

Crane vessel “Rambiz 2” Floating Sheerlegs “Matador 3”




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2. Using Sheerlegs Matador 3 for offshore transport and installation

The Floating Sheerlegs ‘Matador 3’ (Bonn & Mees, Rotterdam) was originally designed forinshore lifts, taking 1500 ton in the main tackles and 600 ton in the jib tackles. After a fewyears of operations, requests for sea-going lifts were received more and more. Lifting at sea,

however, results in the following additional effects:1. Dynamic amplification of the tackle load;2. Dynamic inclinations/ sidelead and offlead of the lifted load.

The first effect can be accounted for by reducing on the inshore lifting curves. Depending onthe usage of the lifting capacity there is a certain space to take into account the DynamicAmplification of the tackle loads. By mean of a motion analysis, each individual tackle isreviewed on the max tackle load. The combination of the four tackles is checked with themax load according to the lifting curve.

The second effect is more critical since the additional transverse load effects cannot simplybe included by scaling the inshore curves. The inshore curves are based on a relativelymodest lead angles in proportion to the lifted load. The operational workability offshore will

often be limited by this design value.

To establish the limits due to these additional load effects, VER applies project (lift) specificmotion analysis, using 3D diffraction software AQWA, to calculate the multi body dynamics..The figure below shows a typical AQWA model of the Matador 3 including lifted load.

Typical AQWA model of the Matador 3 with lifted load

Wave induced motions are generally determined on a short-term statistical basis, using an

appropriate wave spectrum and the ruling wave periods, depending on the operating area.Frequency and/or time domain calculations ultimately lead to most probable maximumvalues (MPM) of motions, inclinations and accelerations.

Using the design lead angles and lifting curves of the sheerlegs as limitations for theoperation in combination with other operational limitations, the workability of the operationcan be derived. A typical workability plot is shown in the figure below. Either the DynamicAmplification of the lifted loads or the lead angles are governing for the operation, but alsoother operational limitations can influence the workability.




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Typical workability plot of a jacket installation with the Matador 3

Recently, VER was requested to do the engineering for up-grading the ‘Matador 3’ to 1800ton in the main tackles and 900 ton in the jib. We took this opportunity to tune the designapproach taking into account the sea-lift operations as well.

This approach still aims for optimum in-shore lifting curves, but anticipates on sea lifts by atwo-way principle with respect to design factors:

1. Include a robust design side lead angle for the inshore curves;2. Minimize the hoisting factor for inshore lifts, within the lower bound limits set by the

classification societies.

A design lead angle for inshore lifting includes effects of static list and wind induced heel onboth the barge and the hoisting tackles. Designing for inshore lifts can be done byminimizing this angle, within the bounds set by the classification societies. This will result,however, in a narrow operating space for offshore lifts.

A balance can be made between the benefits for future offshore applications and the pricepaid with respect to the self-weight for given inshore lifting requirements. A slightly higherdesign lead angle leads to considerable more ‘design space’ for offshore lifts while the steelweight implications for inshore lifting curves are limited. More-over, the resulting robustdesign allows for several practical aspects frequently encountered when operating inshore.

With respect to the hoisting factor, there is no need to allow for more than strictly necessaryfrom a classification point of view, since nowadays drive and control systems introducesubstantially less dynamics due to operating inshore. The additional DAF at future offshoreuse can preferably be taken into account by reducing the lifting curves. Alternatively acertain amount of the DAF can be included in the Fh factor. The first method gives clarity,where the latter method will only give rise to confusion and misunderstandings: any offshoreDAF initially included is either too big or too small for a specific application and will requireadditional number processing.




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3. Design procedure Crane vessel Rambiz 2

Scaldis Salvage & Marine Contractors NV ordered Vuyk Engineering Rotterdam to providethe complete basic design for a new integrated crane vessel. The concept is based on thealready built and proven crane vessel ‘Rambiz’. The new vessel will be called ‘Rambiz 2’.

The Rambiz 2 will be a self-propelled DP2 crane vessel with two identical cranes, each witha lifting capacity of 1800 ton. The cranes can be skidded over 25 m longitudinal on the shipwhich allows the deck to be used to transport and then relocate cargo at a later stage. Thewidth of the free deck space between the cranes has been maximised and the deck loadcapacity in this area is 50 ton/m2.

The maximum lifting capacity was required in offshore condition, with significant wave heightof 1.5 meter. This requirement resulted in a different approach for specification of thehoisting factor, than for basically inshore used sheerlegs.

General Arrangement Rambiz 2

Some of the main characteristics for the lifting appliances on this vessel:- 2 identical lifting appliances- 2 operational locations for each crane- Hoisting capacity per crane 1800 ton at 16 meter aft of vessel’s stern- Length of main boom 88 meter- Provisions for additional fly jib of 35 meter- Full load capacity at Hs 1.50 meter

- Design according Lloyd’s Code for Lifting Appliances in a Marine Environment(CLAME)

Boom

BackmastBackstay




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First Rambiz, Installation substation London Array, 2011

3.1 Design loadThe CLAME specifies the hoisting factor and the lead angles which have to be taken intoaccount for the strength calculations of the structure. These values cannot be very accuratefor each specific lifting appliance, as the code covers a wide range of crane types combinedwith a wide range of vessel characteristics. The real static and dynamic behaviour of a cranevessel like Rambiz 2 will be significantly different from what has been assumed duringestablishing the code. Probably the factors and angles according any lifting appliance code

will be rather conservative, especially in the case of the very heavy lift cranes.

VER has determined realistic design loads for the Rambiz 2, instead of taking the predefinedvalues from the CLAME. Realistic design loads will result in an optimised design of thewhole lifting appliance and therefore also in an optimised design of the vessel.Determination of design loads requires good understanding of the code principles and theload effects in reality. The hoisting factor in general includes the effects of:

- Shock load at start of lifting movement;- Vertical acceleration of the load induced by the winches;- Vertical acceleration of the boom tip due to vessel motion;- Relative movement of the crane and load.

The lead angles (in longitudinal and transverse direction) are a result of- Trim and heel of the vessel;- Windpressure on the Load;- Relative horizontal displacement of load and boomtip;- Oscillation of the load due to vessel motion.

VER provided a set of motion response analyses. Different load situations had to beconsidered, because of the influence of the mass, the COG and the moment of inertia of theload. The results of the analyses, combined with the effects of the above mentionedcomponents were used to specify the design loads for the lifting appliances.




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AQWA model of the Rambiz 2 with lifted load

The study on the design loads resulted in a hoisting factor which was approx. 25% lowerthan specified by the CLAME. The difference with the CLAME values for the lead angles wassmall.

3.2 Structural DesignBasically, the maximum load on the specified outreach, with the applicable factors and leadangles, with the cranes in the aft position, was governing for the construction. Some otherimportant load cases for the strength analysis were

- lowering the boom into (almost) horizontal position, for maintenance or fly jibinstallation;

- sea transit in fastened condition, because the boom cannot be lowered on a boomrest.

3.3 Lifting CurvesOnce the appliance was structurally optimized for the operation point specified, lifting curveshad to be generated. This has been done very effectively by programming a ‘loop’ on theFEA design model. For each combination of operation mode and outreach the upper limit,due to any structural component can be found accurately, thereby getting the most out of thestructure over the whole range. Additionally, the hoisting factor and offlead angles had to bechecked for several points on the loadcurves, because the variation in COG and hoistingload result in different wave responses.

Basic design of Rambiz 2 in combined lift operation at Aft position




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4. Construction type A-Frame

Crane booms and A-Frames of floating lifting appliances have been made in severalconstruction types. The most frequently used construction types are:

a) Welded box girder legs, coupled with cross beams;b) Lattice construction legs, coupled with lattice cross beams;

c) Single lattice construction ;d) Tubular legs, coupled with tubular cross beams.

Floating sheerlegs ‘Cormorant’ with Floating sheerlegs ‘Matador 3’ with weldedbox girder legs tubular legs, transporting the crane boom of

The ‘Oleg Strashnov” with single latticeconstruction

Crane vessel “Rambiz” with lattice construction legs




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VER has made a comparison of construction types. The most economic solution has beendetermined, taking into account the requirements for the Rambiz 2 of Scaldis (ref. chapter 3).

4.1 Loads in the Crane boomAn important factor in selection of the construction type is the load situation in the crane

boom (see figure below). The primary load is the normal force in the boom, due to the liftedload. In case the top sheaves for lifting and luffing are not located exactly at the center line ofthe boom, the lifted load will also result in a bending moment My in the boom. Also the deadweight results in a bending moment My in the boom, depending on the angle of the boom.Side loads from the sidelead angle and wind loads cause a bending moment Mx, with amaximum at the base of the boom.

The normal force in the boom requires not only a certain cross sectional area, but also acertain bending stiffness in 2 directions to avoid global beam buckling.

Load components in Main Boom

The side loads from the sidelead angle and the wind load on the cargo, acting at the top ofthe boom are very important for the base construction, especially for offshore lifting. A widebase is required to limit the reaction forces in the main hinges on deck. However, theavailable space on deck is limited. A maximum width for the vessel Rambiz 2 was specified,and the minimum required space between the cranes resulted in a relative small base.

4.2 Weight Comparison Construction typesTo compare the weights of the different construction types for the crane boom, an analysishas been made for some different designs. The following currently most commonconstruction types have been considered:

• Welded box girder legs, coupled with cross beams.• Lattice construction legs, coupled with cross beams.

• Single lattice construction.The tubular leg variant has not been analyzed. The load capacity in combination with thelength of the boom would require large diameter tubes. These tubes are not available all

X

Z

Y

Load in Tie

Lifted Load

Side Load




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over the world. This would make the total costs of the construction very sensitive to theavailability of any supplier.

An optimized design has been made for each of these construction types, for three differentmaterial grades. The yield strengths of the considered materials are 355, 460 and690N/mm2.

The optimization of all variants has been based on the same set of requirements, taken fromthe Rambiz 2 project (ref. Chapter 3).

Welded box girder legsThe A-frame consists of two continuous rectangular box sections running from bottom to top.The 4 outer plates are stiffened by diaphragms and longitudinal profiles.

Lattice construction legsThe double lattice geometry is based on the (1st) Rambiz A-frame structure. It consists of tworectangular lattice columns. The columns are coupled with rectangular lattice cross beams.Only circular tubes are applied in this construction.




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Single lattice constructionThe single lattice construction has a tapered rectangular cross section. Only circular tubesare applied in this construction.

Basis for comparisonThe weight of the construction for each of the variants has been determined, assuming:

- Same requirements for loads, outreaches etc. for all variants;- Same distance between the main hinges on deck for all variants;- Checks made for allowable stresses with respect to yielding, beam buckling and

plate buckling;- Construction optimized for all variants, but realistic values for thicknesses and tube

diameters taken into account;- Construction details not considered.

Results of weight comparison355 460 690

Welded box legs 161% 134% 105%

Lattice legs 177% 137% 112%

Single lattice 160% 126% 100%

Some remarkable conclusions from these results:- A single lattice is the most material efficient construction type;- A construction with 2 lattice legs is the most material in-efficient construction type;- The difference between a welded box construction and the single lattice construction

is relative small.

The results of the weight comparison turned out to be sensitive for the geometricparameters. The outcome of similar comparisons for booms with different outer dimensionscould be different from the presented values.

4.3 Selection of the preferred construction type for a crane boomMore aspects than only the weight should be taken into account at the selection of thepreferred construction type for a crane boom. Some of these aspects are:

- Fabrication costs: In general the fabrication of a lattice construction will costsignificantly more per ton than for a plate construction. However, if the boom will be




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made by a company with extensive experience in fabrication of lattices, thedifference in costs could be small;

- Material costs: tubes are more expensive than plates, higher grade steel is moreexpensive than lower grade steel;

- Fabrication facilities worldwide. More construction companies are available forwelding plated constructions than for lattice constructions;

- Inspection/ maintenance;- Visibility: A box construction will block the visibility of the load more than a lattice

construction will do;- Damage resistance: A clash of the boom with the load or any obstacle could lead to

failure of the boom. The chance for a fatal failure will be less for a box construction.

Based on the weight comparison and the arguments above, the box construction waspreferred for the lifting appliances on Rambiz 2.

Material with minimum yield of 690 N/mm2 has been selected for the main construction, toreduce the total weight. The additional costs per ton weight are less than the additional costsfor more weight and heavier equipment.

4.4 The hybrid construction of the boom for Rambiz 2In the next phase of the design process, the box concept for theconstruction of the boom has been further improved. With the double legbox concept as used frequently in cranes, the maximum sideload is ratherlimited. Sideloads cause bending in the individual legs and cross beams.Adding more cross beams will help to reduce the bending moments, butwill increase the weight significantly.

VER decided to add braces between the legs, which can transfer thesideload from the boom tip to the deck. The result is a hybrid conceptbox/lattice with only simple elements which can be fabricated at manyconstruction companies. And last but not least, the additional braces

reduce the bending moments in the legs, resulting in a lower total weight.

No space was available for braces between the legs at the lower end ofthe boom. A relative stiff crossbeam and tapered lower legs are applied tohandle the bending moments.

Hybrid box – lattice construction for boom Rambiz 2




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5. Details Rambiz 2

Some specific design features in the basic design of the Rambiz 2 from Scaldis arepresented in this part.

5.1 Flexibility in tackle configurationOne of the features to create f lexibility in lifting operations, is the possibility to allow forinclined hoisting tackles. The angle and the load of each hoisting tackle can be different, butthe horizontal components of the two hoisting tackles of each crane shall be balanced.

The two hoisting blocks of each crane can also be coupled. The hooks can be rotated in thehook blocks in this condition.

Rotation of hooks with coupled blocks (left) or rotation of tackles (right)

The upper part of the hoisting blocks can be connected directly to a spreader beam, to savethe mass of the lower part (increased lifting capacity) and to increase the available liftingheight.

5.2 Skid systemThe Booms of each crane on Rambiz 2 can be moved 25 meter in longitudinal direction.VER has made a design of a skid system, which allows moving the cranes at open sea.

Both legs of each boom are driven by an hydraulic push pull system. In case of failure of onedrive system, the remaining system at one side can bring the boom into a safe position.

5.3 Preventer

The boom of the crane will be stable in the whole range of the loadcurves. However, in caseof emergency, the preventer can hold the boom upright.




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6. Conclusions

- Sea conditions can be included in design. Detailed analysis may very well lead tolower offshore design factors, compared to those prescribed by class societies

- Including sea conditions in a design does not exclude the requirements for project-

specific motion analysis. Some cases may deviate substantially from the design loadcases foreseen.

- The design lead angle for lifting appliances which are mainly designed for inshoreuse should be chosen higher than strictly required to allow for future offshoreoperations.

- A double leg box construction with braces between the legs seems to be the mostefficient boom type for Rambiz 2. However, the available facilities at the fabricationsite could lead to different preferences. The optimal construction types for boomswith different outer dimensions may be different.

www.vuykrotterdam.com

28 - w. janse nsoclc design floating lifting appliances for offshore use

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