METHOD FOR PREDICTING HEAVING MOTION PARAMETERS OF SEMI-SUBMERSIBLE OFFSHORE PLATFORM BASED ON HEAVING ACCELERATION

Abstract

A method for predicting heaving motion parameters of a semi-submersible offshore platform based on heaving acceleration includes: in heaving motion of a semi-submersible offshore platform, representing heaving acceleration of the semi-submersible offshore platform based on a linear potential flow theory; considering a noise influence of a heaving motion measurement marine environment, a low-frequency influence caused by a slow change of the environment and an influence caused by a baseline drift error of an acceleration sensor, introducing a noise term, a low-frequency change term and a baseline drift error term, and uniformly representing the noise term, the low-frequency change term and the baseline drift error term by a unified Prony sequence; and removing a drift term from uniformly represented heaving acceleration, establishing a relationship between the heaving acceleration and heaving motion parameters in terms of the remaining Prony sequence with the drift term being removed, and estimating the heaving motion parameters.

Claims

1. A method for predicting heaving motion parameters of a semi-submersible offshore platform based on heaving acceleration, comprising: in heaving motion of the semi-submersible offshore platform, representing heaving acceleration of the semi-submersible offshore platform based on a linear potential flow theory without regard to a coupling influence of addition mass and radiation damping to determine a heaving acceleration theoretical value; in consideration of a noise influence of a heaving motion measurement marine environment of the semi-submersible offshore platform, a low-frequency influence caused by a slow change of the environment, and an influence caused by a baseline drift error of an acceleration sensor, introducing a noise term, a low-frequency change term, and a baseline drift error term to determine a heaving acceleration measured value; uniformly representing a heaving acceleration theoretical value term, the noise term, the low-frequency change term, and the baseline drift error term in the heaving acceleration measured value by a unified Prony sequence; and removing a drift term from an uniformly represented heaving acceleration, establishing a relationship between the heaving acceleration and the heaving motion parameters of the semi-submersible offshore platform in terms of a remaining Prony sequence with the drift term being removed, and estimating the heaving motion parameters of the semi-submersible offshore platform.

2. The method according to claim 1, wherein: in the heaving motion of the semi-submersible offshore platform, without regard to the coupling influence of the addition mass and the radiation damping and in consideration of a wave force, a restoring force, and a radiation force applied to the semi-submersible offshore platform in fluid, the heaving motion is expressed, based on the linear potential flow theory, as:
m{umlaut over (z)}.sub.0(t)=ƒ.sub.w(t)+ƒ.sub.m(t)+ƒ.sub.s(t)+ƒ.sub.r(t)  (1) wherein m represents mass of the semi-submersible offshore platform, {umlaut over (z)}.sub.0(t) represents the heaving acceleration of the semi-submersible offshore platform, ƒ.sub.w(t) represents a wave load applied to the semi-submersible offshore platform, ƒ.sub.m(t) represents a mooring force applied to the semi-submersible offshore platform, ƒ.sub.s(t) represents the restoring force applied to the semi-submersible offshore platform, and ƒ.sub.r(t) represents the radiation force applied to the semi-submersible offshore platform; wherein the restoring force ƒ.sub.s(t) is expressed as:
ƒ.sub.st=−c.sub.zz.sub.o(t)=−ρgA.sub.wz.sub.o(t)  (2) wherein z.sub.o(t) represents a vertical displacement of the semi-submersible offshore platform; c.sub.z is a restoring stiffness of the semi-submersible offshore platform in a heaving direction, which is related to an area A.sub.w of a water plane, a fluid density ρ, and gravitational acceleration g; the radiation force ƒ.sub.r(t) is expressed as:
ƒ.sub.r(t)=−m.sub.∞{umlaut over (z)}.sub.0(t)∫.sub.0.sup.tk.sub.z(t−τ)ź.sub.o(t)dr  (3) wherein ź.sub.0(t) represents a velocity of the semi-submersible offshore platform in the heaving direction, and m.sub.∞ and k.sub.z are respectively additional mass and a pulse response function at an infinite frequency in the heaving direction; in terms of formulas (1)-(3), the heaving motion of the semi-submersible offshore platform is expressed as:
(m+m.sub.∞){umlaut over (z)}.sub.0(t)=ƒ.sub.o(t)−c.sub.zz.sub.o(t)−∫.sub.0.sup.tk.sub.z(t−τ)ź.sub.o(t)dr  (4) wherein ƒ.sub.0(t)=ƒ.sub.w(t)+ƒ.sub.m(t); the heaving acceleration theoretical value of the semi-submersible offshore platform is expressed as: $\begin{array}{cc}{\stackrel{.Math.}{z}}_0\left(t\right)=\frac{1}{m+m_{\infty }}\left\{f_0\left(t\right)-c_z{z}_0\left(t\right)-{\int }_0^t{k}_z\left(t-\tau \right){\stackrel{.}{z}}_0\left(t\right)d\tau \right\},& \left(5\right)\end{array}$ theoretically, the heaving acceleration of the semi-submersible offshore platform is modeled into a group of superimposed harmonic waves, in terms of formula (5), the heaving acceleration theoretical value is represented as:
{umlaut over (z)}.sub.0(t)=Σ.sub.i=1.sup.N.sub.iA.sub.i cos(2πƒ.sub.it+θ.sub.i)=Σ.sub.i=1.sup.N.sub.iU.sub.ie.sup.v.sub.it  (6) wherein A.sub.i, ƒ.sub.i, and θ.sub.i represent an amplitude, a frequency, and a phase of an i.sup.th component in the heaving acceleration respectively, and U.sub.i and V.sub.i are parameters used for fitting the heaving acceleration theoretical value of the semi-submersible offshore platform by a Prony sequence.

3. The method according to claim 2, wherein: in consideration of the noise influence of the heaving motion measurement marine environment of the semi-submersible offshore platform, the low-frequency influence caused by the slow change of the heaving motion measurement marine environment, and the influence caused by the baseline drift error of the acceleration sensor, the heaving acceleration measured value is determined by introducing the noise term, the low-frequency change term, and the baseline drift error term as follows:
{umlaut over (z)}.sub.0(t)={umlaut over (z)}.sub.o(t)+n(t)+v(t)+b  (7) wherein n(t) represents the noise term, v(t) represents the low-frequency change term, and b represents the baseline drift error term.

4. The method according to claim 3, wherein: the Prony sequence is introduced to represent the noise term, the low-frequency change term, and the baseline drift error term in the heaving acceleration measured value as follows:
n(t)=Σ.sub.n=1.sup.N.sub.nA.sub.ne.sup.iθ.sub.ne.sup.(−ζ.sub.n+j2πƒ.sub.n)t=Σ.sub.n=1.sup.N.sub.n  (8) wherein j=√{square root over (−1)}, =A.sub.ne.sup.iθ.sub.n, and =−ζ.sub.n+j2πƒ.sub.n, wherein A.sub.n, ƒ.sub.n, ζ.sub.n and θ.sub.n represent an amplitude, a frequency, damping, and a phase of each component in the noise term respectively;
v(t)=Σ.sub.v=1.sup.N.sub.vA.sub.v.sup.eiθv.sub.e(−ζv+j2πƒ.sub.v)t=Σ.sub.v=1.sup.N.sub.vC.sub.v.sup.eD.sub.vt  (9) wherein C.sub.v=A.sub.ve.sup.iθ.sub.v, and D.sub.v=−ζ.sub.v+j2πƒ.sub.v, wherein A.sub.v, ƒ.sub.v, ζ.sub.v and θ.sub.v, represent an amplitude, a frequency, damping and a phase of each component in the low-frequency change term respectively;
b=Ee.sup.Ft  (10). wherein E and F are parameters used for fitting the baseline drift error term; in terms of formulas (6)-(10), the heaving acceleration theoretical value term, the noise term, the low-frequency change term, and the baseline drift error term in the heaving acceleration measured value are represented by the unified Prony sequence to obtain:
{umlaut over (z)}.sub.0(t)=Σ.sub.i=1.sup.N.sub.iU.sub.ie.sup.v.sub.it+Σ.sub.n=1.sup.N.sub.n+Σ.sub.v=1.sup.N.sub.vC.sub.ve.sup.D.sub.vt+Ee.sup.Ft  (11) further, the heaving acceleration measured value is uniformly represented as:
{umlaut over (z)}.sub.0(t)=Σ.sub.p=1.sup.N.sub.p  (12) wherein N.sub.p=N.sub.i+N.sub.n+N.sub.v+1, and and Q.sub.p are Prony sequence parameters used for uniformly representing the heaving acceleration of the semi-submersible offshore platform by the Prony sequence.

5. The method according to claim 4, wherein frequencies of all components of the uniformly represented heaving acceleration are determined according to the calculated Prony sequence parameter Q.sub.p as follows: $\begin{array}{cc}{f}_p=\frac{{𝒬}_p+{𝒫}_p}{j2\pi },& \left(13\right)\end{array}$ the frequencies determined are ordered, the drift term is a minimum frequency component, and the drift term is removed from the frequencies to obtain the uniformly represented heaving acceleration measured value with the drift term being removed:
{umlaut over (z)}.sub.0(t)=Σ.sub.q=1.sup.N.sub.q  (14) wherein and Q.sub.q are Prony sequence parameters used for uniformly representing the heaving acceleration measured value with the drift term being removed by the Prony sequence.

6. The method according to claim 5, wherein: a heaving motion response is determined according to a uniformly represented heaving acceleration measured value with the drift term being removed:
ff.sub.0.sup.T{umlaut over (z)}.sub.0(t)dtdt=z.sub.0(t)+z(0)+ź(0)t  (15) wherein the relationship between the heaving acceleration and the heaving motion parameters is: $\begin{array}{cc}\int {\int }_0^T{.Math.}_{q=1}^{N_q}{𝒫}_q{e}^{{𝒬}_{q}t}\mathrm{dtdt}={.Math.}_{q=1}^{N_q}\frac{{𝒫}_q}{{𝒬}_q^2}{e}^{{𝒬}_{q}t}+{.Math.}_{q=1}^{N_q}\frac{{𝒫}_q}{{𝒬}_q^2}+{.Math.}_{q=1}^{N_q}\frac{{𝒫}_q}{{𝒬}_q}t,& \left(16\right)\end{array}$ and actual heaving motion parameters of the semi-submersible offshore platform are represented as: $\begin{array}{cc}{z}_0\left(t\right)={.Math.}_{q=1}^N\frac{{𝒫}_q}{{𝒬}_q^2}{e}^{{𝒬}_{q}t}\left[[.]\right].& \left(17\right)\end{array}$

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG. 1 is an overall flow diagram of a method for predicting heaving motion parameters of a semi-submersible offshore platform based on heaving acceleration according to the invention.

[0037] FIG. 2 is diagram of a test arrangement.

[0038] FIGS. 3A and 3B illustrate time-domain diagrams of the heaving acceleration and displacement of the semi-submersible offshore platform tested by an acceleration sensor and an optical six-degree-of-freedom instrument, wherein FIG. 3A is a time-domain diagram of the heaving acceleration, and FIG. 3B is a time-domain diagram of heaving motion parameters.

[0039] FIGS. 4A and 4B illustrate fitting results of heaving acceleration measured with Prony parameters, wherein FIG. 4A is a fitting result of the heaving acceleration obtained according to a Prony signal, and FIG. 4B is a fitting result obtained according to local acceleration signals within 100-110 s.

[0040] FIGS. 5A and 5B illustrate heaving motion parameter results of the semi-submersible offshore platform reconstructed through the method of the invention, wherein FIG. 5A is a comparison diagram of a structure displacement reconstructed through the method of the invention and a test displacement, and FIG. 5B is an estimation result obtained according to local heaving signals within 100-110 s.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0041] Specific implementations of the invention will be further described below in conjunction with the accompanying drawings.

[0042] The invention provides a method for predicting heaving motion parameters of a semi-submersible offshore platform based on heaving acceleration, which, as shown in FIG. 1, specifically comprises: [0043] (1) in heaving motion of a semi-submersible offshore platform, heaving acceleration of the semi-submersible offshore platform is represented based on a linear potential flow theory without regard to a coupling influence of addition mass and radiation damping to determine a heaving acceleration theoretical value. Specifically:

[0044] In the heaving motion of the semi-submersible offshore platform, without regard to the coupling influence of the addition mass and the radiation damping and in consideration of a wave force, a restoring force and a radiation force applied to the semi-submersible offshore platform in fluid, the heaving motion is represented, based on the linear potential flow theory, as:

m{umlaut over (z)}.sub.0(t)=ƒ.sub.w(t)+ƒ.sub.m(t)+ƒ.sub.s(t)+ƒ.sub.r(t)  (1)

[0045] In the formula, m represents the mass of the semi-submersible offshore platform, {umlaut over (z)}.sub.0(t) represents the heaving acceleration of the semi-submersible offshore platform, ƒ.sub.w(t) represents a wave load applied to the semi-submersible offshore platform, ƒ.sub.m(t) represents a mooring force applied to the semi-submersible offshore platform, ƒ.sub.s(t) represents the restoring force applied to the semi-submersible offshore platform, and ƒ.sub.r(t) represents the radiation force applied to the semi-submersible offshore platform,

[0046] wherein, the restoring force ƒ.sub.s(t) is expressed as:

ƒ.sub.st=−c.sub.zz.sub.o(t)=−ρgA.sub.wz.sub.o(t)  (2)

[0047] In the formula, z.sub.o(t) represents a vertical displacement of the semi-submersible offshore platform; c.sub.z is restoring stiffness of the semi-submersible offshore platform in the heaving direction, which is related to the area A.sub.w of a water plane, a fluid density ρ and gravitational acceleration g.

[0048] The radiation force ƒ.sub.r(t) is expressed as:

ƒ.sub.r(t)=−m.sub.∞{umlaut over (z)}.sub.0(t)∫.sub.0.sup.tk.sub.z(t−τ)ź.sub.o(t)dr  (3)

[0049] In the formula, ź.sub.o(t) represents a velocity of the semi-submersible offshore platform in the heaving direction, and m.sub.∞ and k.sub.z are additional mass and a pulse response function at an infinite frequency in the heaving direction.

[0050] In terms of formulae (1)-(3), the heaving motion of the semi-submersible offshore platform is expressed as:

(m+m.sub.∞){umlaut over (z)}.sub.0(t)=ƒ.sub.o(t)−c.sub.zz.sub.o(t)−∫.sub.0.sup.tk.sub.z(t−τ)ź.sub.o(t)dr  (4)

[0051] In the formula, ƒ.sub.0(t)=ƒ.sub.w(t)+ƒ.sub.m(t).

[0052] So, the heaving acceleration theoretical value of the semi-submersible offshore platform is expressed as:

[00005]$\begin{array}{cc}{\stackrel{.Math.}{z}}_0\left(t\right)=\frac{1}{m+m_{\infty }}\left\{f_0\left(t\right)-c_z{z}_0\left(t\right)-{\int }_0^t{k}_z\left(t-\tau \right){\stackrel{.}{z}}_0\left(t\right)d\tau \right\}.& \left(5\right)\end{array}$

[0053] Theoretically, the heaving acceleration of the semi-submersible offshore platform is modeled into a group of superimposed harmonic waves, and in terms of formula (5), the heaving acceleration theoretical value is represented as:

{umlaut over (z)}.sub.0(t)=Σ.sub.i=1.sup.N.sub.iA.sub.i cos(2πƒ.sub.it+θ.sub.i)=Σ.sub.i=1.sup.N.sub.iU.sub.ie.sup.v.sub.it  (6)

[0054] In the formula, A.sub.i, ƒ.sub.i, and θ.sub.i represent an amplitude, a frequency and a phase of an i.sup.th component in the heaving acceleration respectively, and U.sub.i and V.sub.i are parameters used for fitting the heaving acceleration theoretical value of the semi-submersible offshore platform by a Prony sequence.

[0055] So, in this embodiment, as for the semi-submersible offshore platform under the action of waves, a theoretical model of the heaving acceleration of the semi-submersible offshore platform is established based on the linear potential flow theory, in consideration of the wave force, restoring force and radiation force applied to the semi-submersible offshore platform in fluid and without regard to the coupling influence of the additional mass and the radiation damping in the heaving motion.

[0056] (2) In an actual operating environment of the semi-submersible offshore platform, in addition to the motion of the structure, a large quantity of noise interference will be generated by the complex marine environment and machine operation, so the measured heaving acceleration contains a large quantity of environmental noise and an effect caused by a slow change of the fluid. Moreover, due to the self-constraints of an acceleration sensor, an error will be inevitably caused by a baseline drift of the acceleration sensor. So, in consideration of a noise influence of a heaving motion measurement marine environment of the semi-submersible offshore platform, a low-frequency influence caused by a slow change of the environment and an influence caused by a baseline drift error of the acceleration sensor, a noise term, a low-frequency change term and a baseline drift error term are introduced to determine a heaving acceleration measured value:

{umlaut over (z)}.sub.0(t)={umlaut over (z)}.sub.o(t)+n(t)+v(t)+b  (7)

[0057] In the formula, n(t) represents the noise term, v(t) represents the low-frequency change term, and b represents the baseline drift error term.

[0058] In this embodiment, as for a heaving acceleration response of the semi-submersible offshore platform under the action of waves, in addition to the motion of the structure under the wave motion, the noise term caused by the marine environment and machine operation, as well as the slow change effect caused by a tidal range are taken into consideration, and a baseline drift inevitably caused by the acceleration sensor used for testing is also taken into consideration, so compared with the representation of the heaving motion parameters of the semi-submersible platform merely by superposition of harmonic waves, the representation of the heaving acceleration in this embodiment is more aligned with the operating state of the structure in the actual marine environment.

[0059] (3) A heaving acceleration theoretical value term, the noise term, the low-frequency change term and the baseline drift error term in the heaving acceleration measured value are uniformly represented by a unified Prony sequence, specifically:

[0060] The Prony sequence (7) is introduced to represent the noise term, the low-frequency change term and the baseline drift error term in the heaving acceleration measured value as follows:

n(t)=Σ.sub.n=1.sup.N.sub.nA.sub.ne.sup.iθ.sub.ne.sup.(−ζ.sub.n+j2πƒ.sub.n)t=Σ.sub.n=1.sup.N.sub.n  (8)

[0061] In the formula, j=√{square root over (−1)}, =A.sub.n.sup.eiθ.sub.n, =−ζ.sub.n+ζn+j2πƒ.sub.n, wherein A.sub.n, ƒ.sub.n, ζ.sub.n and θ.sub.n represent an amplitude, a frequency, damping and a phase of each component in the noise term respectively.

v(t)=Σ.sub.v=1.sup.N.sub.vA.sub.v.sup.eiθv.sub.e(−ζv+j2πƒ.sub.v)t=Σ.sub.v=1.sup.N.sub.vC.sub.v.sup.eD.sub.vt  (9)

[0062] In the formula, C.sub.v=A.sub.ve.sup.iθ.sub.v, D.sub.v=−.sub.v+j2πƒ.sub.v, wherein A.sub.v, ƒ.sub.v, ζ.sub.v and θ.sub.v represent an amplitude, a frequency, damping and a phase of each component in the low-frequency change term respectively.

b=Ee.sup.Ft  (10)

[0063] In the formula, E and F are parameters used for fitting the baseline drift error.

[0064] In terms of formulae (6)-(10), the heaving acceleration theoretical value term, the noise term, the low-frequency change term and the baseline drift error term in the heaving acceleration measured value are represented by the unified Prony sequence to obtain:

{umlaut over (z)}.sub.0(t)=Σ.sub.i=1.sup.N.sub.iU.sub.ie.sup.v.sub.it+Σ.sub.n=1.sup.N.sub.n+Σ.sub.v=1.sup.N.sub.vC.sub.ve.sup.D.sub.vt+Ee.sup.Ft  (11)

[0065] Further, the heaving acceleration measured value is uniformly represented as:

{umlaut over (z)}.sub.0(t)=Σ.sub.p=1.sup.N.sub.p  (12)

[0066] In the formula, N.sub.p=N.sub.i+N.sub.n+N.sub.v+1, and Q.sub.p are Prony sequence parameters used for uniformly representing the heaving acceleration of the semi-submersible offshore platform by the Prony sequence.

[0067] In this embodiment, based on the Prony sequence that is able to fit direct-current signals, harmonic signals and increasing (decreasing) vibration signals, the noise component, the slow change caused by tide changes and a baseline drift error term caused by the acceleration sensor in the heaving acceleration are represented respectively, so that the heaving acceleration of the semi-submersible offshore platform is uniformly represented by a Prony sequence.

[0068] (4) A drift term is removed from the uniformly represented heaving acceleration, a relationship between the heaving acceleration and heaving motion parameters of the semi-submersible offshore platform is established in terms of the remaining Prony sequence with the drift term being removed, and the heaving motion parameters of the semi-submersible offshore platform are estimated, specifically:

[0069] Frequencies of all components of the uniformly represented heaving acceleration are determined according to a calculated Prony sequence parameter Q.sub.p, that is:

[00006]$\begin{array}{cc}{f}_p=\frac{{𝒬}_p+{𝒫}_p}{j2\pi }.& \left(13\right)\end{array}$

[0070] The determined frequencies are ordered, a minimum frequency component (low-frequency noise and a baseline drift caused by the acceleration sensor), namely the drift term, is removed from the frequencies to obtain the uniformly represented heaving acceleration measured value with the drift term being removed:

{umlaut over (z)}.sub.0(t)=Σ.sub.q=1.sup.N.sub.q  (14)

[0071] In the formula, and Q.sub.q are Prony sequence parameters used for uniformly representing the heaving acceleration measured value with the drift term being removed.

[0072] The heaving motion response is determined according to the uniformly represented heaving acceleration measured value with the drift term being removed:

ff.sub.0.sup.T{umlaut over (z)}.sub.0(t)dtdt=z.sub.0(t)+z(0)+ź(0)t  (15)

[0073] That is, the relationship between the heaving acceleration and the heaving motion parameters is:

[00007]$\begin{array}{cc}\int {\int }_0^T{.Math.}_{q=1}^{N_q}{𝒫}_q{e}^{{𝒬}_{q}t}\mathrm{dtdt}={.Math.}_{q=1}^{N_q}\frac{{𝒫}_q}{{𝒬}_q^2}{e}^{{𝒬}_{q}t}+{.Math.}_{q=1}^{N_q}\frac{{𝒫}_q}{{𝒬}_q^2}+{.Math.}_{q=1}^{N_q}\frac{{𝒫}_q}{{𝒬}_q}t.& \left(16\right)\end{array}$

[0074] Actual heaving motion parameters of the semi-submersible offshore platform are represented as:

[00008]$\begin{array}{cc}{z}_0\left(t\right)={.Math.}_{q=1}^N\frac{{𝒫}_q}{{𝒬}_q^2}{e}_o^{{𝒬}_{q}t}.& \left(17\right)\end{array}$

[0075] In this embodiment, the drift term is removed from the uniformly represented heaving acceleration, that is, the low-frequency term that may cause a drift of the heaving motion parameters is removed; and then, the relationship between the heaving acceleration and the heaving motion parameters of the semi-submersible offshore platform is established according to the Prony sequence with the drift term being removed, so that the defects of traditional methods based on integration and filters are overcome.

[0076] According to the method for predicting heaving motion parameters of a semi-submersible offshore platform based on heaving acceleration provided by the invention, a motion equation in the heaving direction of the structure is deduced mainly based on a linearly potential flow theory, and the mathematic relation between the heaving acceleration and the heaving motion parameters of the semi-submersible offshore platform is established through a Prony sequence. According to the method, a noise signal, a slow signal caused by a tidal range and a baseline drift component caused by an acceleration sensor are uniformly represented first; then, a low-frequency component caused by a drift is removed through frequency screening, so that the baseline drift component and low-frequency noise caused by the acceleration sensor are removed; and finally, the mathematic relation between the Prony sequence of the heaving acceleration and the heaving motion response of the semi-submersible offshore platform is deduced by means of the remaining Prony sequence with the low-frequency component being removed, so that the relationship between the heaving acceleration and the heaving motion parameters of the structure is established. Different from traditional methods based on filters, the method of the invention establishes the transformational relation between heaving acceleration and displacement of the semi-submersible offshore platform through the Prony sequence rather than correcting heaving motion parameters by traditional integration and filters, thus having higher prediction precision. Besides, the method of the invention takes into consideration the influences of many factors, including the influence of the marine environment and the influence of sensors, thus having higher practical application value.

[0077] The method is verified below with a specific test example of the semi-submersible offshore platform.

[0078] In this example, motion response data of a semi-submersible offshore platform placed in a wave tanks is used for calculation and analysis, and during a test, a wave maker is used to make waves, and an acceleration sensor is used to record heaving acceleration responses of the semi-submersible offshore platform. Besides, in order to verify the accuracy of a conversion result, an optical six-degree-of-freedom instrument is used to record heaving motion parameters of the structure. A test platform is constructed as shown in FIG. 2. During the test, the sampling frequency of a laser displacement sensor and the sampling frequency of the acceleration sensor are both set as 50 Hz.

[0079] In this example, the heaving acceleration of the semi-submersible offshore platform recorded by the acceleration sensor is analyzed, and the heaving acceleration response of the platform under the action of waves obtained during the test is shown in FIG. 3A. Meanwhile, in order to verify the accuracy of heaving motion parameters obtained by analyzing the acceleration through the method, the optical six-degree-of-freedom instrument is used to record the heaving motion parameters of the semi-submersible offshore platform during the test, and the tested heaving motion parameters are shown in FIG. 3B. As can be seen from FIGS. 3A and 3B, the semi-submersible offshore platform starts to move from a static state, and considering the influence of the initial speed and the displacement, signals within 30-180 s in FIGS. 3A and 3B are selected for later analysis.

[0080] During analysis, the heaving acceleration theoretical value term, the noise term, the low-frequency change term and the baseline drift error term in the heaving acceleration measured value are represented by a unified Prony sequence first in terms of formula (12), and a representation result and tested acceleration are shown in FIG. 4A. As shown in FIG. 4B, representation results obtained within 100-110 s are partially amplified, and it can be seen that tested acceleration signals can be well represented by the Prony sequence. Then, the Prony sequence is screened in terms of formula (13) to remove low-frequency components therefrom to finally obtain a remaining Prony sequence, as shown in formula (14). Finally, the remaining Prony sequence is substituted into formula (15) to obtain heaving motion parameters corresponding to the heaving acceleration of the structure.

[0081] Then, actual heaving motion parameters of the semi-submersible offshore platform are reconstructed by means of the remaining Prony sequence with a drift term being filtered out, and a conversion result is shown in FIG. 5A. In FIG. 5B, the reconstructed result obtained within 100-110 s is partially amplified and is compared with the test result mentioned above, and it can be seen that the heaving motion parameters of the semi-submersible offshore platform tested by means of the remaining Prony sequence and the optical six-degree-of-freedom instrument have good consistency, which proves the validity of the method of the invention.

[0082] The above description is merely used to explain preferred embodiments of the invention, and is not intended to limit other forms of the invention. Any skilled in the art can change or modify these preferred embodiments into equivalent embodiments applied to other fields based on the technical contents disclosed above. Any simple amendments and equivalent modifications and transformations made to the above embodiments according to the technical essence of the invention without departing from the contents of the technical solutions of the invention should still fall within the protection scope of the technical solutions of the invention.