7.1 GENERAL
The barge shall be equipped with two skid beams and two rocker beams. The rocker beams shall be hinged at the stern. The position of the tracks and beams shall be adjustable to any position between 8 and 14 m from CL of the barge. Skidding will be performed by four (4) skid units, placed on separate skid rails along both sides of the skid beam. The skid rails and main skid beam are
integrated into 10m single sections that can easily be handled for (re)-positioning on deck. Each skid unit typically consists of 2 hydraulic cylinders, 1 gripper unit for clamping onto the skid rail, and an interconnecting push/pull yoke.
The Launch and Load Out system shall include the following:
• One (1) skidding mechanism, comprising four (4) skid units and one (1) HPU • Two (2) skid beams built up in sections with integrated skid rails on both sides • Two (2) rocker arms with integrated hinge shaft
• Two (2) hinge gutters, integrated in the barge’s stern structure
• Four (4) locking devices, welded onto the hinge gutters locking the hinge shaft to the barge.
• Two (2) rocker buffers, welded onto the barge’s stern
The skidding mechanism shall be capable of functioning as an autonomous (stand-alone) system.
7.2 SKIDDING MECHANISM
Reference is made to 0240 SMC 03 002 Launch – Skidding Mechanism – Specification.
The diesel-hydraulic skidding mechanism shall comprise: • Four (4) hydraulic skidding units
• One (1) diesel-hydraulic power pack
• Four (4) sets of hydraulic hoses, connecting the skidding units to the power pack
The hydraulic skidding units shall run over separate skid rails on both sides of the skid beams.
7.2.1 Skidding units
The barge is equipped with 4 skidding units of 950 ton (9,500 kN) push and pull capacity each. Each unit typically consists of 2 large hydraulic cylinders, 1 gripper
The hydraulic cylinders are connected to the yoke at the rod ends and to the gripper units at the bottom ends. The two cylinders in one skid unit are sized to – together – deliver a 950 t force both in pushing and pulling mode.
The gripper unit consists of a number of hydraulic jacks, mounted vertically in a frame which envelopes the top flange of the skid rail. By powering the jacks, the unit will grip the flange tightly and thus the push/pull force can be transferred from the jacket via the skid rail to the barge’s deck.
The yoke is a double swivel plate connected at the outer ends to the push/pull cylinder rod ends and at the center equipped with a central pin of 250 mm outer diameter. A clevis is designed accordingly. Weight of the swivel plate will be approximately 1.8 t.
The swivel is equipped with bolted-on push plates for launch assistance, which must be unbolted when a pull operation is to be performed.
The jacket needs to be equipped with push/pull supports to fit into this clevis or onto the push plate. Such jacket supports will be designed depending on the shape and size of the jacket and therefore be project-specific.
The swivel is also equipped with a gripper for locking onto the skid rail and repositioning the main gripper section. This auxiliary gripper is smaller since it only has to clamp just enough to move the skid unit’s own weight.
The skid units shall be fully manually lever-operated from a control panel at the diesel power pack. All units can be operated together or individually. Synchronization of the units shall be manual/ visual, to keep the system as simple and straight-forward as possible.
The skidding mechanism shall not be clamping right at the heaviest loaded parts of the skid beam to avoid overloading the deck frames. For the design jacket this implies that the grippers shall not clamp within the first 20 meters of the jacket and in between 30 and 55 meters. (This applies only when pulling or pushing the jacket, not when moving the skidding mechanism itself).
7.2.2 Diesel HPU
The skidding mechanism is capable of functioning as an autonomous system; therefore it is provided with its own diesel-hydraulic power pack on deck.
The diesel power pack runs in between both skid tracks to serve all skid units. It is mounted on rugged wheels and is driven by a hydro motor. It is possible to disconnect the power pack from the skid unit.
The diesel engine power shall be rated at approximately 2x 500 kW. The power pack overall dimensions are approx. 6.0x3.0x1.9m and weight is approximately 10 t. Built on fuel tank shall have a capacity 300 L. The HPU shall be functional at maximum power for trim angles up to 12 deg.
The HPU contains the skidding control panel, at least incorporating pressure gauges for the pressures of skid jacks, main grippers and swivel plate auxiliary grippers.
Flexible hoses between the power pack and the skidding units are supplied of sufficient length for on-barge operations; length shall be approximately 95m. Per skid unit the following 6 hoses are typically supplied:
• 2 pcs 1½“ for the skid jacks • 2 pcs ¾” for the main grippers
• 2 pcs ½” for the auxiliary grippers in the swivel plate The hose connections shall be of the Quick-Release type.
7.3 SKID TRACKS
Reference is made to 0240 DMC 03 001 Launch – Skid Track – Basic Construction – Plan and 0240 DMC 03 002 Launch – Skid Track Forward Section – Structural Drawing.
The skid tracks shall be executed as steel boxes with a U-shaped cross-section, opened at the bottom. They will have a typical height of 2.0 m, a width of 1.9 m at the top plate and a 2.5 m wide base. The skid beam structure is open at the bottom to allow for easy manufacturing and for weight reduction.
Each skid track has two integrated but separate skid rails to accommodate the hydraulic skid units.
The skid track shall run from the rocker beams at the stern towards the bow and its total length will be 190 m. The top plates of main skid beam and skid rail must be flat with a tolerance of 5 mm relative to the bottom surface. The skid system supplier may specify additional skid rail tolerances. The skid track does not cover the remaining 4 m of length at the barge’s bow end. This area will normally not be needed for the support of the jacket.
To be able to transport protruded jackets the most forward section of the skid beam on both PS and SB side shall be strengthened in order to increase the allowable load and transfer the concentrated jacket loads into the designated strengthened area of the barge. These special skid track sections shall weigh approximately 70 t.
Each skid track is divided into longitudinal sections, each 9.97 m in length, in order to be able to easily move or remove the skid track sections from the Barge. In order to take up any barge deformations (hogging and sagging) a 30 mm play between adjacent sections is designed. Each section weighs approximately 57 t and is equipped with 4 hoist lugs for hoisting from a single hook in a 4-rope arrangement.
For the design of the skid track it is assumed that the jacket will be equipped with a hard-wooden lining and/or other means of spreading the load to the design load level as in Section 4.7.
The overall width of the skid track, together with the integrated skid rail structures and hoist lugs, will be approximately 5,800mm. The total of all skid beam sections on the barge will weigh approximately 2,170 t. The skid beams shall be manufactured out of steel D36 or equivalent.
The skid tracks can be moved laterally over (part of) the width of the barge in order to be able to accommodate various widths of jackets. After moving the skid track sections, the assembled skid track must be checked for the correct horizontal alignment and shimmed for the correct vertical alignment at the position of the large web frames. A nominal shim height of 40 mm is taken into the design. After ensuring the proper alignment, the skid beams must be welded onto the deck.
In case adjacent beam sections are in different loading conditions, the vertical alignment is ascertained by welding key plates onto the side plates of adjacent beams. In longitudinal direction these plates allow for 30 mm of barge deformation. Also the top plates are chamfered at the ends of the beam sections. A PTFE sliding track shall be placed at the top of the skid beam over its full length. Quality and details of the slide pads are, as well as the mating material at the jacket, very important to ensure the lowest possible friction coefficient and wear. For the mating material at the jacket hard wooden lining is assumed to result in the lowest possible friction. The PTFE pads shall typically be factory- bonded to steel carrier plates that must be tack welded to the skid beam’s top plate. Steel guiding strips shall be installed on both sides of the PTFE track, to ensure the lateral guiding of the jacket’s skid shoes or lining.
7.4 ROCKER ARM
Reference is made to 0240 DMC 01 001 Launch – Rocker Arm and Hinge Points – Arrangement and 0240 DMC 02 001 Launch – Rocker Arm – Basic Construction Plan.
The two (2) rocker arms shall be located at the stern of the launch barge. The hinge shaft center shall be positioned 1,350 mm from APP. The rocker arms can be laterally moved to follow the skid track width.
The rocker arms and hinge are designed to take up the forces as derived from the launch analysis. Refer to Section 4.7 for design loads.
The rocker arm shall be executed as a closed box section of 41.135 m total length and approx. 3.9 m. wide at its mid-section. The beam shall be reinforced at the mid-section where the hinge shaft will be integrated.
The rocker arm shall have tapered ends in the vertical plane; at the mid-section the maximum height shall be approximately 6.4 m, while at the forward end the height shall be approximately 2.0 m. so that it aligns with the skid beams.
Each rocker arm will weigh approximately 590 t, including the integrated hinge shaft. The rocker arm is entirely made from steel D36 and E36. Thick plates are inserted where necessary. Maximum used steel grade shall not exceed E36. Since the rocker arms must be adjustable in lateral position, the hinge part at the barge is executed as a half-round gutter section integrated into the barge’s stern structure.
7.5 ROCKER LOCKING DEVICES
Reference is made to 0240 DMC 02 005 Launch – Locking Mechanism Construction Plan – Structural Drawing.
Each rocker arm shall be laterally secured by locking devices (or retainers) on both sides. They are designed to take up any lateral and uplift forces which might occur during launching and buffering of the rocker arm.
The locking devices must be welded in position onto the barge’s stern after proper alignment of the rocker with the skid track. The locking devices shall be constructed entirely of steel D36.
7.6 HINGE POINTS
Reference is made to 0240 DMC 02 002 Launch – Hinge Point – Basic Construction Plan.
The hinge gutter on the barge is executed as a steel welded section in E36 or equivalent steel integrated into the Barge structure.
The hinge structure has approximate dimensions of 13.5x2.8x3.8m and a weight of approximately 94 t each.
To lubricate the hinge points the following grease shall be used: GRAFLOSCON A-G 1 ULTRA of Klϋber Lubrication or equivalent.
7.7 ROCKER BUFFERING
Reference is made to 0240 DMC 02 004 Launch – Buffer for Rocker Beam – Basic Construction Plan.
The rocker beams are buffered in their fully rotated positions, 85 deg rotated from their initial position.
When the jacket is launched, the rockers rotate under the influence of the jacket’s dynamic reaction forces. The rocker is slowed down before hitting the rocker buffer by the displacement of the surrounding water.
The steel buffer structure shall be designed for being welded in place onto the barge’s stern. The buffer structure shall be covered with a wooden lining and fenders/tires to dampen the impact. The structure has frames matching the horizontal stiffening in the barge stern in order to transfer the buffer forces correctly into the barge.
The buffers shall be able to be laterally positioned in any arbitrary position in a vertical line below the rocker arms.