Delft University of Technology
Faculty Mechanical, Maritime and Materials Engineering Transport Technology
L.R. Brouwer Capacity upgrade for heavy lift offshore cranes. Development of a concept design. Masters thesis, Report 2005.TL.6913, Transport Engineering and Logistics.
Problem definition and assignment (Chapter 1)
Gusto designed several upgrade systems for upgrading the lift capacity for offshore cranes. The systems that gain the most extra capacity, the so called tie-back systems, have several disadvantages. First the required installation procedures take too much time. The required time is one day till two days, as certain sheave blocks are removed from the A-frame top to a certain tie-back tackle. This results in an extra fixation of the boom top to deck, instead of originally to the A-frame top. Secondly, in the past some damages occurred during the installation procedures. For that reason an investigation is required; for establishing the existing upgrade methods and obtaining new upgrade alternatives. The most promising alternative will be fully worked out into a conceptual design.
Offshore crane concerned (Chapter 2)
The typical Heavy Lift Offshore Crane concerned can be seen in Figure 1-1.
Figure 1-1: Heavy lift offshore crane
The characteristic of an offshore crane is the crane's ability to withstand of offshore influences like sea motions, wind and vessel stability. The latter cause static and dynamic motions; acting on the load and/or crane. In Figure 1-2 the specific motions are shown.
Figure 1-2: Static and dynamic motions
The static motions heel and trim, caused by slanting of the vessel, will result in slanting of the crane. This will have consequences for the applied force of the load at the boom tip, which will not be perfectly vertical. This also occurs when the load centre of gravity is not directly beneath the crane hook, called side and off lead. The dynamic motions cause acceleration forces applied to the centre of gravity of the crane and/or load (depends on lift type) and thus causes extra lateral loading in the crane structure. This extra loading constitutes the main difference with an onshore crane. The applicable rules and regulations also have their influence in the crane design. Different classification bureaus prescribe their own design rules; to avoid damage and thus determine the vessel and crane qualifications.
Crane design (Chapter 3)
Crane designs are engineered on a turn-key base; designed according to the client demands. A client mainly prescribes the following crane specifications for the different hoist options:
Maximum hoisting capacity Work radius at maximum lift Hoisting height at work radius Maximum and minimum work radius Operating sea states
Luffing speed Slewing speed
According to the client requirements the crane can be equipped, except with a main hoist for the largest lifting objects, with one or more auxiliary and whip hoists. These hoist options can lift smaller objects with increasing lifting speeds. In this investigation the attention will be focused on the crane structure to withstand the main hoist forces.
Crane design parameters (Chapter 3) A crane design is an interaction between the following three components: the geometry
the centre of gravity the offshore factors
The final crane capacity can be displayed in a load curve that shows the accompanying lift capacities at the possible crane radii. Changing the specific crane parameters can change the load curve path. Since the load curve is established according to the (structural) strengths of the specific crane components governing at a specific radius. In Figure 1-3 the specific crane components can be seen.
Figure 1-3: Basic crane geometry
For instance, a higher A-frame will enlarge the angle between the derrick tackle and boom, which results in more lifting capacity without overloading the boom. Or a wider tub collar can reduce the bogie forces. Moving the heel point more to the front of the crane also enlarges the derrick tackle - boom angle. The lifting capacity gained is at the expense of the maximum load sizes under the boom, because the horizontal distance between the heel point and hook radius will decrease.
The composed Basic Crane Program (MathCAD calculations sheet) calculates the resulting forces in a basic crane design, caused by the crane load and own mass. Hence the various forces in the crane design can be calculated for different crane geometry dimensions. Also the effect on the vessel stability can be calculated with a special calculation sheet made in Excel. After adding the essential calculations to the Basic Crane Program, the effect of adding a particular upgrade system to a crane design can be evaluated together with its effect on vessel stability. The existing and the newly developed upgrade alternatives were investigated with these calculations sheets.
Upgrade requirements (Chapter 4)
The following main requirements of Table 1-1 were established for the upgrade design. Table 1-1: Summary requirements
Extra capacity At least 50% Installation / removing time 8 hours
Necessary workforce About 10 persons Installation procedure Simple with no risks Crane adjustments Less as possible Total mass increase Less as possible Deck space necessary Less as possible
Influence vessel stability Applicable for ship formed vessels
Cost Maximum 35% of crane value
Classification Lloyds register of Shipping
Less important but motivating criteria are:
Maintain slew motions during crane upgrade mode.
No external help is attendant during the installing of the upgrade system, for example: no help from on board crawler crane. Crane height during transit is restricted to 57.5 meters above the water line, due to bridge passing (Bosporus bridge).
One of the restrictions is that the crane vessel will be seen as a black box. One assumes that the vessel is capable to withstand the increased crane lift capacity. Due to the capacity increase the vessel will be loaded by higher forces and bending moments.
It is emphasized that the investigation will have a technical tendency. There will be no interfering of economic or client related topics. Design method (Chapter 4) To obtain new upgrade alternatives the crane as a system will be divided in the following eight functions:
Force distribution in structure Crane balancing
Luffing Slewing
Providing vertical strength Moving crane
Heave compensation
Load lifting occurs with the main hoist; when more capacity is required the amount of reeving has to increase to accomplish this. In this manner the original winches can be still used.
Luffing; the original luffing power would probably not be able to maintain the luffing motions during increased lifting capacity. But due to the required 'good' weather conditions at heavy lift operation and the probably large sizes of the loads, luffing would not be required or even possible. Also the vessel can be moved by using the anchor lines or dynamic positioning.
Slewing; the original slewing power would probably also not be able to maintain slew motions at increased load. But slewing abilities during the upgrade mode are not required and can be taken over by the small possible vessel movements.
Moving crane; small crane movements can be done by anchor lines or dynamic positioning.
Heave compensation; heavy lift operations are restricted to 'good' weather conditions; consequently the sea motions will be small. To compensate the possible effects at lift-off (and touch-down) a certain dynamic factor will be applied to the load.
Because of the above described comments the crane upgrade possibilities lie in the bold written functions of the list. For these crane functions new alternatives will be obtained, after the evaluation of the existing upgrade methods.
Conclusion evaluation existing upgrade methods (Chapter 5)
The calculations for the described upgrade methods confirm that increasing the angle between the boom and derrick tackle (angle g in Figure 3-1) result in a significant capacity increase, with little crane reinforcements required.
Figure 3-1: Crane geometry
This principle is used at the methods Stan crane and the Tie-back systems. The other solutions are effective as well but result in smaller increase of the lift capacity. The load curves of the several upgrade methods are represented in Figure 1-4.
Figure 1-4: Load curves of upgrade methods
Obtained concepts (Chapter 6) Subsequently new upgrade alternatives will be created based on own concepts and external concepts. External concepts were gathered by having special sessions derived from brain writing and brain storming. The best promising concepts are expound in a morphological overview, arranged to the interesting crane functions. This overview can be seen in Appendix VIII. To maintain clarity in the selection procedure from this overview 12 concepts were derived. On the basis of a multi-criteria analysis these final concepts will be ordered. The four best concepts are shown in Figure 1-5.
Figure 1-5: Four best concepts in ranking 1 to 4
Finally number 2 is chosen to work out, because the 'winner' (number 1) is similar to an existing upgrade system and therefore less challenging. Number 2 also has some correspondence to an existing upgrade system, but in this configuration the installing procedure is different and much faster. When the installation time decrease is large enough this concept should be the winner, since concept number 1 needs reinforcement of the derrick tackle and A-frame. Although, the outcomes of the analysis were not that scattered, as can be seen in Table 1-2.
Table 1-2: Multi-criteria analysis; judgment of criteria for all concepts
Criteria + weghing factors
Concepts Extra capacity ratio Slewing availability Estimated conversion time Boom bending moment Proven technology Additional reinforce-ments Added crane parts Help material during installation Required deck space Pre installation time Influence on vessel stability Summed results 4 1 4 1 2 3 1 2 2 1 2 A+O+T+U 4 1 3 4 4 2 3 3 2 3 2 67 B+O+T+U 4 1 3 1 2 3 3 3 2 2 4 66 C+O+T+U 4 1 2 1 4 3 4 4 1 3 3 66 D+O+T+V 4 4 3 4 3 2 3 3 2 3 1 65 E+O+R+X 4 1 3 4 4 2 2 2 2 2 2 64 F+O+T+V 3 1 4 3 3 1 4 4 3 2 2 63 G+O+T+U 3 4 4 1 1 1 4 4 4 2 2 63 H+O+T+U 3 1 4 2 2 3 3 3 3 2 1 63 I+O+T+W 4 4 2 1 2 3 3 3 2 3 2 61 J+O+T+W 4 1 1 1 2 4 1 1 3 2 3 56 K+M+T+W 4 3 1 1 2 4 1 1 2 1 3 54 K+O+P+X 4 1 1 1 3 4 1 1 1 1 3 52
Concept design parameters (Chapter 7) The upgrade design will be based on the heavy lift offshore crane vessel Stanislav Yudin with a current capacity of 2500 tons slewing. The new upgrade capacity is aimed at 3750 tons in a fixed mode (50% extra gained capacity). To accomplish the new capacity the following parameters (see Figure 1-6) are determined:
58% of guide sheaves are moved to tie-back mast
the pad-eye is located 65 meters on the crane middle line behind the crane the tie-back mast length is 26 meters
the tie-back mast angle during the lift operations is 70°
Figure 1-6: Tie-back design parameters
With these parameters the upgrade load curve in Figure 1-7 can be obtained (the tie-back mast force is governing for the upgrade load curve). The values of the upgrade parameters are chosen in accordance with the parameter study explained in paragraph 7.1.
Figure 1-7: Load curves
Design issues (Chapter 7) The idea for pulling the tie-back mast out of the A-frame, along the front legs, opposed some large obstructions as the total A-frame top has to be re-constructed. Therefore the tieback mast will be guided along the aft legs to its upward position. The tie-back mast will be hinged to a special frame; subsequently the frame will slide along the aft legs.
Final design (Chapter 7) The final design of the 'extending mast system' is shown in Figure 1-8.
Figure 1-8: Final design in crane slewing and fixed
1. bring crane in position 2. attaching fixation tackle to deck
3. pull off tie-back mast from support (handling fixation tackle winch) 4. extend tie-back mast (handling extension tackle winch)
5. alternating handle fixation and extension winch until mast is in position 6. fixate tie-back mast with hydraulic pins
7. crane is ready for the use in the fixed mode
Reinstalling occurs with the reversed version of the just described installing procedure.
Explanation and drawings of the required upgrade equipment can be found in paragraph 7.3.1 through 7.3.5.
Subsequently some additional reinforcement of the A-frame is required, because the application point of the tie-back mast force does not match the intersection of the front and aft legs neutral line. Latter is caused by following reasons: to keep the A-frame height as low as possible and to prevent the resulting extra necessary structures. Therefore this results in an extra stiffened A-frame top, a conversion to a hinged A-frame structure and relative small reinforcements at the aft legs. Maintaining a welded A-frame will cause relatively more reinforcements for both front and aft legs. This is concluded from calculations enclosed in Appendix XIX.
Accuracy during the design is very important. Due to the slide system (extension frame) and the rotating tie-back mast during the extending, certain wire ropes at the A-frame top can graze each other and/or structure parts. Therefore the course of these hauling parts (of the derrick tackle, fixation tackle and extension frame) have to be accurately investigated. In this case of the Stanislav Yudin the course of the hauling parts of the derrick tackle is affected and results in a required slot to be made in the cross beam of the aft legs.
The final results in relation to the requirements of the 'extending mast system' are: Gained capacity = 50%
Necessary workforce = 8 persons
Installing time = about 2 hours and 10 minutes Equipment mass = 396 tons
cost = 3,38 million euros Modifications mass = 64 tons
cost = 0,46 million euros Help equipment crawler crane
fork-lift truck tugger winches
It can be concluded that the upgrade system design meets the established requirements. The total mass is approximately 1145 tons with a total price estimate of € 10.296.000.
Conclusion (Chapter 8) The investigation for the different crane lift capacity upgrade possibilities leads to the result that indeed different upgrade systems are available. Most are solutions already used by Gusto. According to the calculations sheets for a basic crane design and vessel stability check, all possible and existing upgrade system could be adequately evaluated. The most promising alternatives were summarized in an overview. Depending on client demands a specific upgrade system can be chosen, which suits the specific client requirements best. Guide lines for evaluation can be found in the used selection procedure. Consequently one specific upgrade alternative was further developed. This specific design resulted in a conceptual crane lift capacity upgrade system that fulfills the established requirements. The new design promises an installation time, which is only about 13% of the comparable tie-back upgrade systems. The final price resulted in about 34% of the crane value, regarding to a new design of an offshore crane with 2500 tons capacity. Reports on Transport Engineering and Logistics (in Dutch)
