Method for installation of a vessel in a seabed

Abstract

A method for installation of a vessel (10) in a seabed (30), the vessel comprising at least four legs (12a, 12b, 12c, 12d) and ballast tanks having ballast pumps and ballast valves, the method comprising the steps of: - bringing the vessel (10) into a position - lowering the at least four legs (12a, 12b, 12c, 12d) until they come in contact with the seabed (30), - locking a first set of diagonal legs (12a,12c) with locking pins - performing a first preloading step - unlocking the first set of diagonal legs (12a, 12c) - locking a second set of diagonal legs (12b,12d) with locking pins - performing a second preloading step - unlocking all legs (12a, 12b, 12c, 12d) - locking all legs (12a, 12b, 12c, 12d) and open ballast valves allowing water to enter the ballast tanks - let the ballast valves open for compensating for tides to maintain the water level constant within the ballast tanks.

Description

[0001]METHOD FOR INSTALLATION OF A VESSEL IN A SEABED INTRODUCTION The present invention concerns a method for installation of a vessel in a seabed, said vessel comprising at least four legs and ballast tanks having ballast pumps and ballast valves. BACKGROUND The installation and stabilization of vessels on the seabed can be important in offshore engineering projects. Whether for oil and gas exploration, renewable energy projects, or other marine activities, ensuring safe and secure vessel positioning may be critical. US4270877A discloses a floating apparatus for performing operations, such as dredging and drilling, on and in a bottom under water subject to violent motion. The apparatus comprises a working platform provided with legs vertically adjustable to the depth of the bottom by adjusting means. Each leg is flexibly connected at least at two vertically spaced places to parts of the apparatus formed integrally with the platform. The connection of a leg at the higher of the two places is effected by means for taking up rocking movements of the working platform, and at the lower of the two places by connecting members springing and / or dampening in two perpendicular horizontal directions, while the means adjustable in the vertical direction for connecting the leg to the working platform are also of resilient or dampening construction. EP2275341B1 discloses a jack-up barge comprising a floating platform formed by a plurality of modules that can be joined together mechanically by means of removable fixing means and a plurality of support legs, mounted orthogonally on the floating platform. Said modules comprise three or more lifting modules, each of which is mechanically associated with a leg and is provided with a support frame. The support frame having centrally fixed a guide structure, in which the leg is inserted, and having fixed a plurality of buffer panels, which define together with said guide structure, a closed box-shaped body. Said closed box-shaped body participates in the hydrostatic floating thrust of the floating platform. WO2018074977A1 discloses a reconfigurable mobile docking apparatus, which comprises: a platform having at least a longitudinal side and a transverse side, and arranged to receive an asset; a plurality of supporting legs movably coupled to the platform and configurable to be lowered to the bottom of a body of water to anchor and / or support the apparatus, wherein the platform is operable cooperatively with the lowered supporting legs to be adaptively raised or lowered with reference to a water-level; and a reconfigurable modular tank system which includes a plurality of high buoyancy ballast tanks which are removably supported by the platform and relocatable at least between a first and a second position, wherein in the first position, asset transfer to or from the platform is permitted through the longitudinal side, and wherein in the second position, asset transfer to or from the platform is permitted through the transverse side. The tanks are further capable of being offloaded into water to provide a floating structure and / or increase lift capacity. WO2017058098A1 discloses a mobile docking apparatus with a platform and supporting legs that can be lowered to the seabed for anchoring and lifting, allowing self-docking and maintenance without the need for external facilities, equipped with ballast tanks, lifting cranes, and waste management systems to minimize environmental impact. Thus, enabling on-site inspection, maintenance, and repair of maritime assets, reducing downtime and investment costs, while providing a sustainable and flexible solution for ship recycling by allowing vessels to be contained in a dry area during demolition. CN115107930B discloses a semi-floating offshore wind power construction with an installation vessel which is used to anchor to the seabed mud surface through the pile legs and utilizes the ballast water tank to increase the weight of the hull, reduce the need for lifting capacity, and achieve 'static to static installation'. Thus, reducing the demand for lifting capacity of the lifting mechanism, reducing costs, and improving the construction efficiency – being suitable for a variety of water depths and sea conditions. WO2013045640A1 discloses a structure with pre-tensioned jack-up legs and strand jacks that are removably connected, allowing for stable lifting and lowering operations by maintaining constant tension in the strands, and a method involving towing or self-propulsion to the site, ballasting and de-ballasting, and activating strand jacks for precise placement and retraction. Thus, allowing economic, year- round operation with minimal damage from wave impacts, further allowing for easy relocation without heavy vessels, maintaining stability and reducing costs by using pre-tensioned strand jacks and ballast tanks for controlled movement. CN211849364U discloses a bottom-mounted self-elevating platform which is designed to adopt a combined structure of an upper hull, a lower floating body and pile legs. The lifting mechanism and locking mechanism are used to realize the extension and fixation of the pile legs. The lower floating body and the pile legs jointly bear the load, and the load distribution is adjusted in real time through the monitoring mechanism, which is suitable for soft geological conditions. Thus, achieving fast and stable pile insertion and extraction under soft geological conditions, avoiding the risk of platform tilt, improving operating efficiency and stability, while being suitable for softer geological conditions. EP3856980B1 discloses a method that involves preloading the legs and applying a torque to the hull by differentially loading pairs of diagonally opposed legs, allowing for increased operational margin and crane capacity without structural modifications, by adjusting load distribution to accommodate the location of cranes or cantilevers. Thus, enhancing the operational working capacity of cranes by increasing the operational margin on legs closest to the crane, allowing for heavier lifts and extended reach without jeopardizing platform stability, while maintaining safety and stability. SUMMARY OF THE INVENTION The invention relates to a method for installation of a vessel in a seabed. The vessel comprising at least four legs and ballast tanks having ballast pumps and ballast valves. The method comprises the steps of bringing the vessel into a position, lowering the at least four legs until they come in contact with the seabed, locking a first set of diagonal legs with locking pins, performing a first preloading step, unlocking the first set of legs, locking a second set of diagonal legs with locking pins, , performing a second preloading step, unlocking all legs, locking all legs and open ballast valves allowing water to enter the ballast tanks, let the ballast valves open for compensating for tides to maintain the water level constant within the ballast tanks. The method wherein the first or the second preloading step, may further comprise the steps of ballasting the vessel by obtaining a water level in the ballast tanks in order to obtain a preloading force, holding the preloading force substantially constant by adjusting the water level within the ballast tank by utilizing the ballast pumps for a preloading holding time, and removing the ballast water with the ballast pumps. BRIEF DESCRIPTION OF DRAWINGS Examples of the invention are disclosed with reference to the following drawings, wherein: Fig. 1a shows an exemplary schematic of a vessel with two cranes during deployment; Fig. 1b shows an exemplary schematic of a vessel with one crane during deployment; Fig. 2a shows an exemplary schematic of a vessel with two cranes during installation; Fig.2b shows an exemplary schematic of a vessel with one crane during installation; Fig. 3a shows an exemplary schematic of a vessel with two cranes during installation from an alternative view; and Fig.3b shows an exemplary schematic of a vessel with one crane during installation from an alternative, cross-sectional view; Fig. 4a shows an exemplary schematic of a vessel with two cranes installed in the seabed with an exemplary load on the vessel; Fig. 4b shows an exemplary schematic of a vessel with one crane installed in the seabed with an exemplary load on the vessel. DETAILED DESCRIPTION Fig.1a shows a vessel 10 during deployment comprising a barge or hull 11. The barge 11 comprises four legs 12a, 12b, 12c, 12d. The legs 12b and 12d are further equipped with up to two cranes. Respectively 14a, and 14b. The cranes 14a and 14b may be used to lower the legs 12a, 12b, 12c, 12d to the seabed, or alternatively, can be used for handling of equipment, such as loading and unloading equipment on the barge. The legs 12a, 12b, 12c, 12d can alternatively lowered using a leg handling mechanism, such as a winch. An alternative vessel 10 during deployment comprising a barge or hull 11 is shown in Fig.1b. The barge 11 comprises four legs 12a, 12b, 12c, 12d. The leg 12d is further equipped with a crane 14. The crane 14 may be used to lower the legs 12a, 12b, 12c, 12d to the seabed, or alternatively, can be used for handling of equipment, such as loading and unloading equipment on the barge. The legs 12a, 12b, 12c, 12d can alternatively lowered using a leg handling mechanism, such as a winch. The vessel 10 is brought above a position above a seabed 30 as shown in Figs. 2a, 2b, 3a, 3b. The four legs 12a, 12b, 12c, 12d are lowered until they come in contact with the seabed 30. A first set of diagonal legs 12a, 12c is locked with locking pins and a first preloading step is performed. Following the first preloading step, a second set of diagonal legs 12b, 12d is locked with locking pins. The first set of legs is unlocked, and a second preloading step is performed. The first and the second preloading step comprise ballasting the vessel by obtaining a water level in the ballast tanks in order to obtain a preloading force. Subsequently, the preloading force is held substantially constant by adjusting the ballast pumps for a preloading holding time; the preloading holding time may be understood as the duration of the preloading step. Following that, the ballast water is removed with the ballast pumps. Following the second preloading step all legs are unlocked. At high tide the four legs are locked, and the ballast valves are opened to let sea water enter the ballast tanks. The ballast valves may remain open to self-adjust for tides. At low tide the four legs are locked, and the ballast valves are opened to let sea water exit the ballast tanks. The ballast valves may remain open to self-adjust the water level within the ballast tank. Figs. 4a and 4b show the installed vessel 10 in the seabed 30 with a deck load 40. The deck load 40 will have an impact on the defined preloading target and subsequent step. The first set of legs and second set of legs are interchangeable, meaning that the method can also be realised with the first set of diagonal legs being 12b, 12d and the second set of diagonal legs 12a, 12c. Example The draft in Fig. 1a and Fig. 1b is approximately 1.8 m. The total weight is 9000 t including ballast to get even keel (difference in crane weights). After the legs have been lowered, in Figs.2a, 2b, 3a, 3b, the draft is approximately 1.5 m, and the total weight is 7700 t including ballast to get even keel (difference in crane weights). In the first preloading step, the vessel is ballasted with 2.5 m in all ballast tanks to obtain a gross weight of 12500 t. If a deck load of 8000 t shall be utilized during the installation, the preloading should be increased to 17500 t. To obtain this an average water level of 3.2 m in all ballast tanks shall be obtained. When the necessary preloading force is obtained, the force is held constant by adjusting the ballast pumps. The preloading force is kept for a specified preloading holding time. The ballast water is removed with the ballast pumps and the first set of legs is unlocked. The second set of legs is locked and a preloading similar to the first preloading step is repeated for the second set of legs. All legs are unlocked, and it is ensured that the vessel keeps its position. At high tide lock the four legs and open the ballast valves to let sea water enter the ballast tanks. Let the ballast valves remain open to self-adjust for tides. The vessel may be operated within the maximum deck load, which is set before preloading.

Claims

1 P A T E N T C L A I M S 1. A method for installation of a vessel (10) in a seabed (30), the vessel comprising at least four legs (12a, 12b, 12c, 12d) and ballast tanks having ballast pumps and ballast valves, the method comprising the steps of: - bringing the vessel (10) into a position - lowering the at least four legs (12a, 12b, 12c, 12d) until they come in contact with the seabed (30), - locking a first set of diagonal legs (12a,12c) with locking pins - performing a first preloading step - unlocking the first set of diagonal legs (12a, 12c) - locking a second set of diagonal legs (12b,12d) with locking pins - performing a second preloading step - unlocking all legs (12a, 12b, 12c, 12d) - locking all legs (12a, 12b, 12c, 12d) and open ballast valves allowing water to enter the ballast tanks - let the ballast valves open for compensating for tides to maintain the water level constant within the ballast tanks.

2. The method of claim 1, wherein the first or the second preloading step comprises the steps of: - ballasting the vessel (10) by obtaining a water level in the ballast tanks in order to obtain a preloading force - holding the preloading force substantially constant by adjusting the water level within the ballast tank by utilizing the ballast pumps for a preloading holding time - removing the ballast water with the ballast pumps

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