Extract from Y.T. Yang, H.G.Cho, K.Y. Yoon, S.S.Ha, H.S. Kang, Hyundai Heavy Industries Co., Ltd. Paper at OTC
This paper presents the design concept and operation results of load-out for the FSO (340,000 DWT Class, ELF AMENAM KPONO Project) built on the ground without dry dock facilities. It was the first attempt to build FSO completely on the ground and launch it using barges.
The major characteristics of load-out structure, which depend on FSO configuration composed of general VLCC type hull and topside structure, were its heavy weight, i.e. 53,000 metric ton and long length compared to breadth and depth, i.e. 298 meters. So, special consideration was required for safe load-out operation comparing to previous load-out of other offshore structures where the weight was less than 30,000 metric tons. The strength and longitudinal deflection of FSO were carefully analyzed with appropriate weight distribution. All facilities for load-out including ground foundation, jack-up system, pulling system and link-to-barge system were designed based on this result and friction force along the skidway. The stability of quay wall located in HHI Offshore Yard was checked against loading condition during load-out operation. Two semi-submersible barges that were connected together by rigid frame structures were used for loadout of FSO. The ballasting operation of connected double barge unit (DBU) can be controlled like one barge using load master control software (LMC) developed by HHI. Relative deflection in way of DBU connection frames was monitored during load-out operation. And DBU motion during load-out, towing to float-off site and float-off phase were analyzed and compared to model test result.
The load-out concept and methodologies described here are verified through successful load-out operation of Amenam FSO project. These concept and experience can be adopted for load-out design of extra-heavy structure which weight is more than 100,000 metric tons. Guide lines for the development of an efficient construction method for offshore and ship type structures are also suggested for future application of this concept.
Floating type offshore structures showed an overall increase in market share during the last several years in offshore construction field. Generally, the fabrication and assembly of floating offshore structures like FPSO (Floating Production Storage and Offloading system), FSO (Floating Storage and Offloading system), FPU (Floating production Unit), Rig, Jack-up etc., have been carried out in the dry dock of a shipyard by stacking unit blocks sequentially from lower to upper levels.
In case of ship type offshore structures, lower hull part fabrication and assembly are carried out in a dry dock as conventional ship construction method and topside modules are installed by heavy lifting crane near the quay side. This methodology has great dependency on dock schedule and tight fabrication process of yard. So, it was very important to match fabrication schedule between hull and topside part to meet total project schedule. In such cases, it is difficult to incorporate any design change or modification required by client
On-Ground Build Method applied to ELF AMENAM FSO project is a new construction concept for above offshore structures to reduce total construction schedule and have flexibility in fabrication phase. Hull and topside part are assembled on ground at the same time. Completed structure will be loaded-out and floated off using submersible barges. This construction method requires yard’s experience including engineering capability of load-out and float-off operation to launch complete structure.
This paper describes design concept of loadout system and operation results verified through successful operation.
2. Overview of loadout plan
2.1 Characteristic of Amenam FSO
Amenam FSO, loadout target structure, is composed of hull of 42,000 metric tons lightweight and 18EA topside modules weighing of about 12,000 metric tons. Hull midship configuration is typical single hull cargo tanker with two longitudinal bulkheads. The principal dimension of FSO is summarized as follows.
Length (overall) 298M
Breadth (mould) 62M
Depth (mould) 32.2M
Design Draft (mould) 22M
Light weight (Hull+Topside) 53,000 Mton
2.2 Yard layout and Loadout plan
Generally, the assembly location of loadout structure is decided by considering the weight of structure (line load) and water depth in way of quayside for mooring of loadout barges. Fig.1 shows general arrangement of loadout system, such as skidway, linking system, and loadout barges.
Figure. 1 AMENAM FSO Loadout Plan
3. Loadout system design basis
Various static, dynamic loads and other limitations that were considered during design of the loadout system is detailed below.
3.1 Static loads
Lightweight including 10% margin and friction force are considered as static loads for design of the loadout system.
About 1/3rd of FSO total length (124meter) is supported by hydraulic jacks arranged in four skidways in way of FSO midship area.
These hydraulic jacks distribute FSO weight evenly and act as line load to skidway and FSO bottom structures. The line load was calculated from jack loads to make balance with weight distribution considering COG variation. The maximum line load of Amenam FSO loadout was 158 ton/m.
The friction force between skidway and skidshoe can be calculated by multiplying weight and friction coefficient. 15% friction coefficient was applied for contact material. It leads to conservative design of loadout systems. 10% friction coefficient was observed at break out condition during actual operation.
3.2 Dynamic loads
Environmental forces, such as wind, wave and current influences motion of loadout barges and dynamic load to loadout system consequently. Estimation of barge motion and wave bending moment is very important to design loadout system.
Table.1 shows environmental condition for loadout operation of Amenam FSO. Table.2 is summary of motion analysis result. This result was verified through model test in towing tank.
3.3 Other limitations
There are various limitations to be considered during loadout design depending on loadout facilities, characteristics of loadout structure, and yard ground condition. Following limitations were considered in Amenam FSO loadout design.
1) Hydraulic jack capacity and maximum stroke
2) Pulling jack capacity and speed
3) Strength and deflection of FSO structure
4) Ballasting capacity and speed of loadout barges
5) Strength of loadout barges including connectors
6) Capacity of ground and quaywall
7) Other loadout facilities of yard
4. Loadout system design
This section presents design viewpoints of loadout system, required analysis items and results considering given design condition described on previous section.
4.1 Skidway on ground and barge
Skidding method, which is general method for loadout of offshore structures, was applied for Amenam FSO loadout. Four skidways on the ground and on the barges were designed to support rigid frame of FSO, i.e side shell and longitudinal bulkheads. The surface of skidway was made of special material to reduce friction force as possible.
Each skidway was supported by rigid foundation consisting of concrete mat and pile foundation for ground skidway, and grillaged beam foundation for barge skidway. These foundations should be designed not to settle down over the jack stroke limitation.
4.2 Quay wall stability analysis
Quay wall stability should be verified in early stage of loadout design by checking various failure mode considering expected load conditions. Quay wall is analyzed for the following loads.
1) Earth pressure
2) Differential water pressure
3) Line load due to weight distribution
4) Pulling force
5) Barge berthing/mooring load
Following failure modes were checked and safety margin for each failure mode was summarized in table.3.
1) Sliding failure mode
2) Over turning failure mode
3) Bearing capacity check
4) Overall stability failure mode
5) Local stability check
The safety of quay wall is mainly related to line load applied during loadout. So, loadout system should be designed to minimize line load as possible. And calculated safety margin of quay wall should be sufficient considering uncertainty.
4.3 Loadout barges and ballasting plan
Following factors are to be considered while selecting the loadout barges..
1) Dead weight considering berthing draft
2) Deck area to accommodate required activeshoe length
3) Ballasting speed considering pulling speed.
4) Longitudinal strength and deck local strength
5) Float-off stability
Two semi-submersible barges, HDB-1011/HDB-1012 of HHI Offshore Division, were selected considering above requirements. Both these barges are similar in construction and connected together with rigid frame structure at five points.
A 20-step ballasting plan was prepared for the loadout process. Ballasting status and relative deflection in way of connection frames were monitored during loadout.
4.3.1 Principal Dimension of DBU
The principal dimensions of loadout barges (HDB-1011/1012) are summarized as follows.
Length (overall) 140M
Breadth (mould) 37M
Depth (mould) 12M
Design Draft (mould) 9M
Submerged Draft 25.5M
Dead weight 33,000 Mton
Classification KR, Steel Barges, 1998
4.3.2 Two-Barge Connection
To accommodate loadout weight and required skidshoe length, both the barges were connected to each other and berthed in the longitudinal direction as shown on the Fig.1.
This arrangement of barges is concluded based on the results of case study. The arrangement selected was beneficial in efficient and safe transfer heeling moment and shear force of each ballasting step.
Five points of rigid connection frame structures were welded at the side shell of each barge and behaved as one barge. In this case, connection frames were exposed to heeling moment and shear force due to un-balanced forces caused by difference of ballasting amount of each barge and wave loads.
Connection frames were designed to have maximum strength capacity of 36000 ton-meter for heeling moment and 23,500 ton for shear force.
The control of ballasting is very important to prevent overstress in way of connection frames during loadout. The monitoring system, which is a part of LMC (Load Master Computer) developed by HHI, checked whether relative deflection between two barges exceeds allowable limit in real time during operation.
4.3.3 Ballasting control
The ballasting control should be performed to keep barge draft, trim and heel within operation limits, which is ±0.25 degree for trim, ±0.2 degree for heel.
The ballasting was divided to twenty steps for 6 meter moving distance. And also ballasting plan includes estimated ballasting time due to moving speed in relation to pulling jack speed. Actual ballasting status during loadout was monitored by measuring draft from draft guage and theodlite.
To compensate ballasting due to tide variation (about 600 mm at HHI offshore yard), external pumps independent of main ballasting system were used. The capacity of external pumps and selection of tank were decided considering tide variation rate, 140 mm/hour, and variation rate of barge displacement.
4.4 Analysis of FSO strength and deformation
The safety of loadout structure should be verified by 3-D FEM analysis. The stress ratio, plate buckling capacity, and deformation should be checked against actual line load related to hydraulic jack load.
Analysis results showed FSO structure has sufficient safety during loadout. In addition, stress ratio and deformation value were less than those of in-place condition. The maximum von-mises stress was found as 180 N/mm2 in way of side shell
plate near the midship bottom area and maximum Z-deflection was 120mm in way of supporting region of hydraulic jacks. This natural deformation should be maintained by jack stroke during loadout operation not to cause local concentration of loads. A shimming plate should be designed that can keep the deformation of FSO within limits in case of active jack failure. The deformation of loadout barges due to still water and wave bending moment should be considered in total deformation, which is covered by jack stroke.
Fig.2 shows combined stress plot and Fig.3 shows deformation during loadout condition.
4.5 Hydraulic jacking system
FSO structure was assembled as supported by temporary supports during fabrication phase. Hydraulic jacks with 250 tons capacity and 250mm maximum stroke were used to jack up FSO structure before loadout.
The arrangement of hydraulic jacks should be such that calculated line loads due to jack quantity can meet other loadout system design limit, such as FSO strength, deformation, skidway strength, quay wall stability, etc. Total 404EA jacks were arranged for Amenam FSO loadout and maximum jack capacity ratio was maintained below 70% of safe working capacity.
Hydraulic jacks were arranged along the skidway and grouped as eight jack pressure groups to compensate weight distribution due to COG variation. These jack groups were used for weighing of loadout structure before the start of loadout. Fig.1 shows arrangement of jacks and Fig.4 shows typical curcuit diagram of arrangement of hydraulic jacks.
The main purpose of hydraulic jacking system is to support loadout structure without load concentration. The factors that cause concentration of load can be summarized as bellows.
1) Structural deformation at the loadout condition
2) Ground settlement
3) Variation of barge level due to trim, heeling, and environmental loads
The jack stroke can absorb above factors and keep constant line load applied to structure and skidway.
4.6 Pulling jack system
The required pulling force for loadout structure increases linearly with the increase of friction force. For the Amenam FSO loadout, the required pulling force was about 7,650 tons considering 15% friction coefficient. The pulling system should be designed to be able to control loadout direction. Typical pulling system using strand jacks was adopted on the basis of experience of previous loadout projects.
Pulling jack system has 16EA strand jacks installed at the end of hydraulic jack support, fixed anchor welded to loadout barge deck, and strand wire connecting strand jack and fixed anchor. Skidding of loadout structure is done by operation of strand jacks.
Strand jack, which has 560ton pulling capacity, 450mm stroke, and 12 meter/hour pulling speed, was used for Amenam FSO loadout. Jack capacity was designed to be below 85% of the estimated friction force to cover unexpected increase in friction. The components in loadout system design such as link beam, steel fender and skidways which are subject to pulling force should be designed considering total pulling capacity.
4.7 Linking system
4EA rigid link beams and steel fenders were used for linking each ground skidway to barges. These two linking facilities supported line load and total pulling force, respectively. Fig.5 shows arrangement of linking systems.
Link beam was designed to have sufficient strength against maximum line load. End connection of link beam was designed as hinge-sliding concept to have free movement condition due to barge motion. Link beams were supported by rigid foundation of quay wall and barge to transfer line load to rigid structure only. OTC 15313 5
Steel fenders installed between front wall of quay and transom of barges took compression force due to pulling operation. The required berthing area per fender was about 5M by 12M from quay stability analysis result. Barge transom structure was checked whether it had sufficient strength against pulling compression. Fender itself was designed to resist pulling compression and 20% of lateral force considering barge movement.
4.8 Mooring system
DBU were moored at quayside throughout the loadout. The mooring lines were designed to keep barge position against 10-year return environmental condition shown on the Table.1. Maximum mooring force was approximately 460 tons due to wide wind area that has never been experienced. The most critical condition in mooring design was the condition after loadout. In this case, the total force has to be resisted by the mooring lines only.
Emergency mooring activities were prepared considering sudden attack of typhoon based on 100-year storm condition. In this case, link beams were fastened to take a role of mooring line and also additional mooring lines were required.
4.9 Monitoring system
Concerning points to disturb operation were listed and monitored continuously during loadout. These data were compared with estimated values and verified to keep within the limit. The main items to be monitored are listed bellows.
1) Hull deflection level
2) Ground settlement
3) Loadout moving direction
4) Hydraulic jack pressure & stroke
5) Pulling jack load
6) Barge draft level & positioning
7) Relative deflection of barge connection frame
5. Concerning points for operation
All expected risky items raised in design phase through HAZOP should be carefully monitored and controlled during actual operation. Because of the characteristics of loadout structure in case of Amenam FSO, there were lots of concerning points as follows.
1) Controlling the skidding direction due to huge length of the loadout structure
2) Even distribution of excessive pulling force to quay wall.
3) Strength verification in way of barge connectors from measured data.
Excessive pulling force, barge connection, and control of DBU are highlights of Amenam FSO loadout operation.
Design methodology described in this paper contributed to successful loadout operation of world’s first on-ground build FSO. The paper suggests a new construction methodology that is different from conventional dry dock construction of floating offshore structures, such as FPU, FSO, FPSO, Drilling Rig, and Jack-up etc.
This construction concept can allow major works for commissioning to be complete on the ground. It will provide more convenient working environment than conventional construction method in view of the service of good facilities such as crane, accessibility, multi-working, etc.
This construction methodology can also be applied to conventional ship construction to reduce assembly schedule and increase efficiency of dry dock. Since this methodology shows possibility of loadout and float-off of large ship blocks to be mated in dry dock finally.
Above all, this paper will provide design basis and experience gained to develop loadout methodology of huge size structure such as MEGA FLOAT, BMP (Barge Mounted Plant), which cannot be accommodated in present dry dock facility.
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