Technical Field

The present invention relates to floating structures. More specifically, the invention relates to large floating structures, having a natural period of 2 seconds or above, for which the dynamic resonance play an important role for fatigue design.

Background Art

In many areas a demand exists for large floating structures. Facilities for oil and gas production, floating windmills, floating bridges, floating quays, floating solar panel support structures, and energy islands are some examples. In many areas, land is sinking while sea level is increasing. In some areas the soil conditions are challenging. In many of the most urbanized and populated areas, large floating structures can meet urgent demands.

Large structures are built according to codes and standards approved by governments and insurance companies. Several thousands of tons of steel can be required for the building the largest structures.

For traditional floating structures, the first natural periods are always designed to be below 2 s at the feasible study phase so that hydroelastic vibrations contribute little to fatigue. Standard Quasi-static analysis method based on frequency domain is used to estimate the fatigue damage during the operation phase. The traditional method, with the floating structures designed to have natural periods below 2 s, cannot account for wave induced vibration effects, and underestimate or avoid considering the fatigue damage for floating structures with natural periods above 2 s.

Accordingly, the traditional method for structural analysis of large floating structures is based on “quasi-static” analysis in the frequency domain, in which the dynamic resonance is not accounted for. To consider the flexible vibrations of large structures, the state-of-the-art methods are to assume that the large structures can be simplified as beam structures. The hydrodynamic forces can be evaluated using the empirical Morison equation, as found in textbooks, and the Simo/Riflex method software from Sintef Ocean, Trondheim, Norway.

If the same safety as provided by following the current methods and technical steps can be ensured while saving tens or even hundreds of tons of steel, several benefits would result. Less consumption of steel means less emissions of greenhouse gases and pollution, all the way from the mines to the structure and recycling of the materials. A significant reduction in cost would also be a result. Areas having too challenging soil conditions for building quays or structures for agriculture or other use, including accommodation, may become feasible and more valuable.

The nearest prior art has been found to be represented by “Design of Havfarm 1 (Mobron, E., Torgersen, T., Zhu, S., Riis, J., Bye, M. (2022). Design of Havfarm 1. In: Piqtek, L, Lim, S.H., Wang, C.M., de Graaf-van Dinther, R. (eds) WCFS2020. Lecture Notes in Civil Engineering, vol 158. Springer, Singapore. Published: 06 August 2021 ), and the patent publication CN 114104221 A.

An objective of the invention is to provide floating structures with reduced consumption of material, be it steel or concrete or other material, while ensuring the same safety as currently provided. A further objective is that the structures and method used for building the structures, shall be approvable by existing codes and standards. Summary of invention

The invention provides a method of designing and building a floating structure with natural period of 2 seconds or above, wherein the method comprises: to provide a rigid beam model with increased stiffness, in which natural periods are greatly reduced, by extracting excitation forces, added mass and dampings via a panel model for a plurality of bodies or elements, into which the floating structure has been divided, to provide a flexible beam model with exact stiffness, by extracting excitation forces, added mass and dampings via said panel model for said plurality of bodies, whereby dynamic resonance at natural periods is captured in time domain from the beam model, to find the dynamic amplification factor by dividing the flexible beam model results on the rigid beam model results, thereby quantifying the dynamic impact and obtaining dynamic amplification factors, distinguished in that the method of designing and building a floating structure with natural period of 2 seconds or above comprises: to add the dynamic contribution to the rigid beam model excitation forces, quantified by dynamic amplification factors for the bodies, thereby finding combined forces for the bodies, to model fatigue damages, using the combined forces, to finalize the designing process, to implement the design in building models, and to build the structure on a feasible construction yard, or parts thereof on several feasible construction yards and transport and assemble said parts to the structure, wherein the resulting structure is a relatively slender structure compared to earlier method structures designed and built according to accepted codes and regulations.

The invention also provides a further method of designing and building a floating structure with natural period (eigen period, resonance vibrations, natural vibrations) of 2 seconds or above, distinguished in that it comprises the steps: to model the excitation forces by a panel model for the submerged part of the structure, prefererably, the Simo-Riflex model, wherein the structure is assumed to have increased stiffness, is established, wherein, for example, the stiffness is increased by 1000 times so that the resonance vibrations occur at periods less than 0.1 s, wherein a rigid structure model is provided, with similarities to traditional “quasi-static” analysis, whereby rigid structure model forces are extracted from the rigid structure model, to model the excitation forces of the submerged part of the structure by said panel model, but wherein the structure has the original, real stiffness and has natural resonance periods of 2 seconds or above, wherein a flexible structure model is provided, whereby flexible model forces are extracted from the flexible structure model, to find the dynamic amplification factor by dividing the flexible structure model forces on the rigid structure model forces, quantifying the dynamic amplification factor, wherein the forces of the rigid structure model have similarities to the results from traditional quasi-static analysis in the frequency domain, and when dynamic amplification factors are included and also the resonance responses are included, thereby providing combined forces, and preferably the combined forces are transformed to a beam model (approved by authorities), to estimate fatigue damage, using the combined forces, to finalize the designing process, to implement the design in building models, and to build the structure, on one or more feasible construction yards, wherein the resulting structure is a relatively slender structure compared to earlier method structures designed and built according to accepted codes and standards.

Preferably, the method, or the further method, further comprises one or more of the steps, in any combination: dividing the structure into a plurality of bodies, for example 50, 100, 200 or 400 bodies, modelling the added mass, damping, and excitation forces for each body, to include effects of quite high or negative added mass due to resonance modes from radiation, to reduce resonance from diffraction induced extremely high excitation forces to some extent in the damping zone, to include shielding effects from different bodies, for different bodies, at different heading angles, wherein the combined response of the elements can be modelled in the time domain, including both regular and irregular waves.

Preferably, for the method or the further method, the design results are transferred to approved models, such as flexible slender shell models including Monson elements, wherein the dynamic impact on the structure can be approved by codes and standards approved by authorities and commercial parties such as insurance companies, wherein classification and verification bodies can approve the structure and the design.

In the detailed design, preferably the radius of curvatures are large enough, transitions from one dimension to another are smooth enough, the joints are appropriately shaped and designed, to ensure a fatigue life at least as long as the design life of the overall structure, with an appropriate margin, wherein radius of curvature Ra equals or exceeds diameter D/10 and/or wall thickness t/2, of round hollow members of diameter D with wall thickness t, and similarly for other geometry members. Welded joints, at least in hot spot areas in-plane in the structure are preferably through thickness penetration butt welds.

The invention also provides a floating structure with natural period (eigen period, resonance vibrations, natural vibrations) at or above 2 seconds, such as 2 - 5 seconds, designed and built according to the method of the invention or the further method of the invention. The floating structure is distinguished in that the total consumption of structural construction material, excluding outfitting by identical utility and accommodation structures, is reduced by at least 10% compared to structures designed and built according to the state-of-the-art method for design of large dynamic structures. Preferably, in the range of natural period of 2 - 5 seconds, the reduction in steel or/and concrete utilization is 20%, 30%, 40% or even 50% compared to a state-of-the-art floating structure having been built more rigid to reduce the natural period to be below 2 seconds, or below 1 second. The phrase “excluding outfitting by identical utility and accommodation structures” means the basic hull and deck structure, excluding variable utility or accommodation structure or load.

Preferably, the maximum or average draught of the outer steel or concrete hull is 0,4, 0,3 or 0,25 times or less than the height of the outer hull, calculated as the average draught of all immersed areas of the hull, with outfitting/utilities for normal operation of the floating structure. More preferably, the maximum or average draught of the outer steel hull is 0,20 or 0, 15 or less than the height of the outer steel hull.

Preferably, the floating structure in operation is anchored to a fixed or in substance fixed position. In substance fixed position on the sea means that the anchors on the seabed are in fixed position, but the floating structure may be allowed to weather drift somewhat, as enabled by flexibility in the anchoring structure, which reduces the maximum stress levels significantly in bad weather conditions.

The invention also provides use of the method and/or the further method of the invention, for designing and building the structure of the invention.

Some distinguishing features or steps of the invention, compared to state-of-the art, are further described as follows: For finding excitation loads, a panel method is used instead of the Morison model to evaluate the hydrodynamic forces because the Monson method greatly overestimate the hydrodynamic forces at some wave periods, especially at wave periods of 2 - 5 seconds. When the hydrodynamic excitation forces at some wave periods are overestimated, the dynamic resonance can be greatly overestimated, resulting in heavier and more rigid structures than the floating structure of the invention.

With the methods and the floating structure of the invention, a new solution of the dynamic problem is provided, while enabling to conform with commercial validated code. This means that instead of using a novel software a solution is provided with a validated code, which can be verified and approved by classification societies and verification bodies.

In other words, the method and/or the further method of the invention comprises one, two or more of the steps as follows, in any combination:

• Divide the immersed parts of the floating structure into separate bodies or elements, for example 50, 100, 200 or 400 bodies or elements, for said panel model for traditional hydrodynamic analysis,

• Preferably, find the frequency domain effects of added mass and damping due to radiation for each body, and/or preferably, include resonance modes from radiation effects,

• Preferably, find the frequency domain excitation loads due to diffraction, incident resonance and shielding effects,

• Run wave load analysis on the slender model with increased stiffness in time domain, preferably including both regular and irregular waves, wherein the forces extracted resemble the analysis results from traditional quasi-static analysis. • Run the same wave load analysis for the flexible slender element model and extract the equivalent beam forces,

• Determine dynamic amplification factors,

• Implement the results in a design building model,

• Build the structure on a feasible construction yard, wherein the resulting structure is a relatively slender structure compared to earlier method structures designed and built according to accepted codes and regulations.

The floating structures of the invention are for example: Floating terminals, ultra large ships (for example, Ultra or Very Large Container Ships), airports, ports, fish farms, ships with multiple hulls (catamarans, trimarans etc.), structures for agriculture, aquaculture, storage, solar plants and other, wherein the structure is large enough for having a natural period, as floating in water, of 2 seconds or longer. This means that the structures are relatively large, wherein natural resonance can cause short operation life if not duly taken into the design. The normal method according to state of the art is to build stiffer structures to ensure that the natural period is below 2 seconds, preferably far below 2 seconds. If the natural period is below 2 seconds, the waves or other relevant loads are smaller and resulting harmonic waves will cause relatively less problems compared to natural periods of 2 seconds and longer, related to larger waves and loads causing higher stress levels.

Traditionally, the added mass, damping, and excitation forces are calculated for the mean wet surface for the whole panel model under still water level. In the method of the invention, the added mass, damping, and excitation forces are developed to acting over for example 200 bodies or elements (meaning that the whole structure is divided into 200 bodies or elements). The summation of excitation forces is consistent with the total excitation forces of the structure. In such way, the response between different parts can be calculated in the time domain. When the excitation forces from the panel model are used, the result may be only 20% to 50% of those calculated from the Morison beam members. Consequently, the dynamic resonance can be reduced by more than 50% in the wave period interval, and consistent natural frequency period, from 2 to 5 seconds.

As mentioned, state-of-the-art practice is to build large floating structures stiffer, which require increased utilization of building material, such as steel or concrete. A significant advantage of the present invention is enabling less utilization of building material, such as steel or concrete. The advantage is that for large floating structures tens or even hundreds of tons of steel or concrete can be saved. Less utilization or consumption of steel means less emissions of greenhouse gases and pollution, all the way from the mines to the structure and recycling of the materials. A further advantage is related to hydrodynamic behavior, in that the large floating structure, due to reduction of up to tens or hundreds or tons of steel or concrete, will float with a significantly lower draught. Roll and pitch will be significantly reduced, since the up Tightening momentum will be relatively larger.

Detailed description of the invention

As a large example structure of the invention, a typical floating structure of 300 m length and 100 m width is considered. The natural period becomes about 3,2 seconds. From the several hundreds of tons of steel required, about 50 tons can be saved. This represents a huge reduction of emissions of greenhouse gases and pollution, all the way from the mines to the structure and recycling of the materials. 50 tons reduced steel consumption reduces CO2 emissions by about 93 tons

The considered large floating structure of the invention, being relatively elastic compared to a conventional large floating structure and having very low draught and a natural period of about 3,2 seconds, behave in the sea quite differently compared to a standard rigid, heavy barge or other typical floating structure. The rigidity of prior art structures is required to decrease the natural period to below 2 seconds, to reduce the risk of serious fatigue. These distinguishing features of the large floating structure of the invention are surprising for the person skilled in the art, since increased rigidity is used to reduce the natural period to correspond to small waves and loads of low structural risk. However, due to good engineering practice and good building practice combined with the novel method of the invention, safety against fatigue can still be ensured for the floating structures of the invention, even if large enough and flexible enough to have a natural period at or longer than 2 seconds.

Surprisingly, a large floating structure of the invention, for example a large steel hull with low draught/height ratio provides a very stable structure with respect to pitch and roll caused by moving the center of gravity by movements and operation of equipment and/or loads onboard. As indicated already, this is due to a surprisingly high up Tightening momentum when any inclination results by said movements and operation. When the draught is very low, as preferable or obligatory for the large floating structure of the invention, the relative difference in buoyancy on either side of the axis for pith or roll, becomes surprisingly high for even small pitching or rolling.

Estimates indicate that large floating structures of the invention can be provided at a reduced cost compared to conventional large floating structures. The investment cost is estimated to be up to 30 % to 40 % to 50 % lower than for a traditional large floating structure according to state-of-the-art.