Offshore construction and decommissioning vessel ('OCDV')

Active suction anchor is a bottom foundation able to take large vertical pulling forces; being a part of a vertical mooring system or Tie Down System (TDS) with the purpose of eliminating roll; heave and pitch motion on a floater exposed for waves. The Suction anchor consists of a cylinder with covered top. A pump is mounted on the covered top and is creating under pressure for penetration of suction anchor in the seabed and for creating a vertical holding (pulling) capacity. The suction anchor can also operate in a seabed consisting of sand. The pump then needs to run continuously in order to maintain the suction force. A filter has been developed to avoid the removal of the soil plug when running the pump. The vertical holding capacity per anchor developed in this project is some 2000 T (metric) and radius of the suction anchor is 10 meters. Computer analysis; verified by large-scale model tests of suction anchors; confirms the capacity and operability of the suction anchors. Instrumentation for monitoring of the suction anchors and operation procedures has been developed in this project.

Tie Down System (TDS) is a device consisting of tethers; adjustable in length; connected between a floater and suction anchors in the seabed; pre-tensioned and those almost eliminating vessel vertical motions for operation in the open sea. Suction anchors and tethers are deployed and recovered by apparatus on vessel. The marine vessel will prior to and during the load transfer operation of construction element in open sea (to/from fixed structures) be vertically moored and be tied down to suction anchors and thus have the motion characteristics of a tension leg platform (TLP). Heave; pitch and roll motions will almost be eliminated (only residual vessel motions from elasticity in tendons remain) which are essential during initial touch and connection between load-transfer system on the vessel and the Topside. The impact load in the Topside from kinetic energy of the vessel is; due to the above; nearly eliminated. The TDS consists of traction winches; storage reels; steel wire ropes (tethers); blocks and sheaves; hydraulic jacks and suction anchors. The hydraulic jacks of the TDS supporting the upper blocks on the vessel can work in four different modes: - Constant tension mode whilst deploying and recovery of the suction anchors. - None return valve mode during the pre-tensioning phase of the tendons. - Locked mode during load-transfer and initial lift off of Topside from Jacket. Controlled quick release mode during final separation of Topside from Jacket and thus avoid pounding between the Topside (resting on the vessel) and the substructure. During the study of the Tie Down System we have aimed at making the hydraulic system as simple as possible; and at using "proven in practice" solutions to the largest possible extent. An important feature to be mentioned is that the main operations of the cylinders do not require "active control" with the associated control equipment; software and high power flows. This approach greatly enhances the safety of the system; equipment; payload and the people. We have also made some continuous improvement of the TDS structural design through this RTD work and have ended up with a modular system for each anchor position; where each module contains two traction winches; two storage reels; one A-frame; four hydraulic cylinders; two block and tackle systems; one suction anchor and drive and control systems for the above components. Each tie down system shall be remote operated from an operation control centre and the anchors shall be deployed and recovered simultaneously in order to save operational time and decrease the required weather window. The TDS will improve for safety of personnel and material by almost eliminating the vertical motions. A simpler and less expensive hull shape is possible for load transfer of construction elements in open sea due to the feature of the TDS. The system can also be deployed on existing tonnage and Company Jumbo shipping are now evaluating to install a TDS on one of there heavy lift transportation vessels in order to be able installing offshore wind turbines and oil and gas platforms.

A Topside to be removed (or installed) is lifted from underneath by a number of load carrying points. Design criteria for the Load Transfer System (LTS): - The system should allow that the OCDV follow the natural wave frequency motion whilst performing a load transfer operation in order to avoid huge dynamic loads from vessel acting on the platform; - The system shall be adjustable able to carry different sizes of platforms - Load carrying point shall be located as close as possible to jacket leg connection to the Topside where strong point in the Topside could be found; - The system shall be easy to deploy and retract; - The system shall be constructible and easy to maintain. Typical size of dynamic forces required to fully stop the vessel motions caused by wave frequency loading is some 2-3000 T for a sea state of about Hs = 2.5 meter. This loading could cause serious damage to the platform's structural integrity and especially to the Jacket structure. Since the loading is acting on the Topside or the upper part of the Jacket; the lower part is exposed for large bending moment (some 2 - 300.000 Tm) in addition to large shear forces. Two different load transfer systems have been developed allowing for horizontal movement of the OCDV whilst performing load transfer operations at offshore locations; and those avoiding large dynamic wave loading acting on the vessel is transferred to the platform. The developed systems are the two-hinged beam system and the single hinged beam/trolley system. Two-hinged beam system: One system consists of a number of retractable main beams. Two-hinged beams are mounted on each main beam and the load carrying point is located at the tip of the outer-hinged beam. The rotation of the two-hinged beams allows the load carrying point to move in a defined area of the horizontal plane. Single hinged beam trolley system: One hinged beam and a trolley running on top of this beam on the other system developed have replaced the two-hinged beams. The load carrying point is placed on the trolley. The combination of rotation of the hinged beam and longitudinal displacement of the trolley allows the load carrying point to move in a defined area of the horizontal plane Equalizing jacks: Both systems are equipped with equalizing jacks; which are the load-carrying member. The equalizing jacks ensure an even load distribution during load transfer operations. The equalizing jacks are working in a spring mode allowing for some residual vertical motions of the OCDV without introducing large dynamic loadings in the Topside. De-ballasting the vessel and simultaneously increasing the spring stiffness in the equalizing jacks perform the load transfer operation. The equalizing jacks are also performing the lift off operation (initial separation of Topside from platform substructure). The main beams carrying the single hinged beam/trolley system (or the two hinged beam system) can be retracted in order to allow clearance whilst positioning the vessel.

The purpose of the HPS is to control an Offshore Construction and Decommissioning Vessel (OCDV) position and orientation relative the platform and minimise the low frequency motions of an OCDV during a load-transfer operation but still allowing for wave frequency motion in order to simplify the load-transfer system but without introducing huge dynamic forces from the vessel enter into the platform. The horizontal positioning system shall reduce the motions in three degrees of freedom (surge; sway and yaw); while the other three degrees of freedom of the vessel (roll; pitch and heave) are already restricted by means of the Tie Down System; activated before deploying the HPS. The horizontal positioning system developed in this RTD work consists of 3 hydraulic jacks connected between the OCDV and the platform. Two jacks are controlling the transverse position and orientation of the OCDV relative a platform; reducing the sway and yaw motions and a third Jack is controlling the longitudinal position; reducing the surge motions. Each HPS subsystem mainly comprises a long stroke hydraulic cylinder that connects the OCDV and the Topside. The unique with the developed system is that the hydraulic jacks are working in a spring mode with a special spring characteristics. The Jacks are very stiff in the beginning until a certain deflection and will thereafter have a �soft� spring characteristics. Results from theoretical analysis; confirmed by results from tank model tests; shows reduction in horizontal motions of some 50-60 % when HPS is deployed. A second function of the HPS is to secure the Topside to the vessel at the moment of lift off of the Topside from the Jacket. The HPS will damp and after some seconds eliminate relative motions between vessel and Topside after lift off; thus working as a temporary sea fastening. The system allows further for adjustment of Topside position on the OCDV after lift off; prior to making permanent sea fastening.

The OCDV concept is based on a floater vertically moored and tied down to active suction anchors; outfitted with a horizontal positioning system and a flexible load transfer system. The vessel is able to transport and install; or remove; heavy offshore platform structures; such as Topsides and Jacket structures; in single lift operations. Two different types of hull shapes have been developed for the "single lift" concept; a column-stabilized unit; "Master Mind" and a catamaran shaped unit; "Sea Fork One": - Main particular: "Master Mind" - Hull Length overall: l 107.8m - Hull Breadth overall: 81.0m - Hull Height: 36.2m - Length of pontoons: 104.8m - Breadth of pontoons: 16.0m - Height of pontoons: 10.2m - Operational Draught: 9-22m - Displacement: 39 000T Equipment: - Quarters: 140 persons - Helicopter deck: S 61 - Propulsion: 6*3300kW - Dynamic Positioning: IMO Class 3 - Anchor holding capacity: 4*2200T - Revolving Cranes: (main hoist)350/250T Lifting Capacity: - By de-ballasting: 16 000T - By strand jacks in moon pool: 8 000T - By ship cranes (tandem lift): 600T Main particular: "Sea Fork One" - Length pontoon: 96;57m - Breadth pontoon: 17;00m - Breadth of moon pool: 25 - 51m - Moulded draught pontoons: 27;07m - Length transverse box: 85;00m - Breadth transverse box: 26;64 m - Height transverse box: 10;27m - Light weight: 16.580T - Displacement: 31.200T - Transit draught: 5;90m - Transportation draught: 6;40 - 9;40 m - Operation draught: 6;40 - 14;00 m - Lifting capacity: 14.000 T* North Sea - Lifting capacity: 18.000 T GOM * Depending on platform geometry and elevation. The column-stabilized unit is more suited for operations in the North Sea whilst the more weather sensitive catamaran-shaped unit is better suited economically for operations in Gulf of Mexico. The catamaran-shaped units have however an acceptable operability in the North Sea during the summer season if used as a pure lifting tool. Methods and systems; as developed in this RTD work; are the same for the column-stabilized unit as for the catamaran shaped unit.

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