Strength Analysis of Bottom Structure of a Wind Power Installation Vessel

Abstract

Aiming at the structural safety of the offshore wind power installation during a ship’s bottom operation, taking the wind power installation in a certain sea area of the East China Sea as an example. Water was found in the pump room of the Wind Power Installation Vessel during the bottom operation. After inspection, it was found that the ship was obviously arched, the deck was deformed, the right side of the hull had cracks in the front and rear, and the left side of the hull was bulging. According to the different draught depths of the ship, three operating conditions are selected, namely, sitting on the bottom, the highest tide level on the day of incident, and at the time of the incident. Finite element analysis on hull structural strength is carried out to find out the cause of the hull rupture and water flooding in owing to the arch-in-the-ship accident. The calculation results show that due to the large area of scouring on the bottom of the ship, the bottom of the ship is not solid, and with the alternate change of the tide, there are both middle arching and sagging phenomenon on the ship. Hence, the force acting on the ship sitting on the bottom changes periodically, resulting in large local stresses in ship structures, which is the main cause of ship accidents.

1. Introduction

With the rapid development of the offshore wind power industry, the demand for wind power installation ships is increasing [1,2]. Compared with Europe, the development of offshore wind power in China is relatively backward and is currently in a stage of rapid development. Under the guidance of demand and policy, intertidal zones are the focus of offshore wind power development [3,4].The bottom-mounted wind power installation vessel adjusts its own weight through its ballast system so that the hull can sit slowly on the bottom, and uses the support force of the mud surface to keep the hull stable, so as to carry out on-site wind turbine installation operations [5].The bottom of the ship provides great convenience for the construction and shortens the offshore operation period, which has a broad application prospect.

However, with the coupling effect of wind, waves and currents, the bottom-mounted installation platform may be washed to a certain extent, and the bottom will become unstable since itis hollowed out. Serious scouring leads to the tilt of the platform, which affects the construction, and even the safety of the platform, personnel and underground [6].

In September 2016, when a bottom-mounted operation vessel in China performed the bottom-mounted installation of the fan in the sea area of Dongtai, Jiangsu, the bottom of the bow and stern was hollowed out due to scouring, the hull was severely arched, and the middle structure of the hull was seriously fractured [7]. In order to avoid or reduce the consequences of such accidents, researchers have proposed many innovative ideas and methods, and applied them to the beginning of the ship design and construction [8].

The flexural strength and ultimate strength of ship structures are important concepts to study the local and overall strength of ships. Damaged hull failures are divided into bending failures and shear failures [9,10,11,12]. There are four main methods for evaluating the response of the hull structure: simplified model analysis, model test method, real ship experiment and numerical simulation [13]. The simplified model analysis method needs the support of ideal hypothesis theory, and some secondary influencing factors are often ignored while using this method. Yu et al. [14]. used the simplified model analysis method and combined numerical simulation software to simulate the grounding performance of the web girder in the ship grounding accident. Zhang et al. [15]. selected the middle section of the hull, combined with finite element analysis software and used nonlinear finite element method to analyze the cause of the fracture accident in the middle section of the hull. Although the simulation results are consistent with the actual accident situation, it is difficult to evaluate the structural response of the whole ship. According to the actual damage situation of the ship, in this paper, the finite element analysis is used to simulate the whole ship, and the simulation results fit well with the actual accident situation. This paper focuses on the structural strength of the ship under the action of bottom scouring and the research results may provide a reference for the future operation of the ship under the bottom.

2. Overview of the Wind Power Installation Vessel

2.1. Main Data

The main technical data of the ship and the performance parameters of the steel used in the ship are shown in

Table 1. Main data.

Type	Semi-Submersible
Length overall	140 m
width	56 m
depth	8.8 m
Gross Ton/Net Ton	21,981/6594
Full load displacement in summer loadline	43,712.4 t
full load in summer loadline	17,860 t
towing area	Unlimited
Submersible working conditions	Sheltered sea areas and harbors
Elastic Modulus	2.1 × 1011 N/m2
Poisson Ratio	0.3
Shear Modulus	8.0769299 × 1010
Density	7850 kg/m3

.

2.2. Deck Equipment and Cabin Arrangement

The ship is made of steel, with a box-shaped hull, towers at four corners, arched square decks without ridges and beams, a sled-shaped bow, a round bilge, a stern bottom with beveled cut and three water-dividing heels, which can navigate in sheltered sea areas and in the harbor. Non-self-propelled unmanned deck barges for semi-submersible operations and towing in unlimited sailing areas can be carried out. The deck of the ship is equipped with a 2000 tand 650 t crawler cranes, a 75 t and 150 t small crawler cranes, a generator, a few container-type simple buildings and small equipment. Two rows of movable van blocks are laid under the tracks of the 2000 t crawler crane, and wooden blocks are placed under the tracks of the 650 t crawler crane. The detailed layout is shown in Figure 1.

There are three longitudinal and nine transverse watertight bulkheads below the load deck, dividing the main hull into thirty-two ballast water tanks(No. 1-32), two pump rooms(No. 35-36), one fuel oil tank(No. 33), and one freshwater tank(No. 34), as shown in Figure 2.

2.3. Overview of the Accident

On 14 May 2021, the ship moved to the designated position and began to press the water. It sat on the bottom on 15 May 2021 and positioned the bottom tower in the morning, and then due to weather conditions, until the early morning of 17 May 2021, there was almost no work. On the morning of 17 May 2021, the ship made a loud noise, and then it was observed whether there was a collision with other ships. No collision was found after inspection. Then, all crew members were called to increase the inspection of the hull. Additionally, then, the stern was found to collapse to a certain extent. After detailed investigation by the crew, it was successively observed that the junction of the starboard outer plate No.4 and No.5 cabins was deformed.

2.4. Accident Investigation Results

Inspection at the scene of the accident shows that the ship was arched, and the deck was deformed obviously. The highest point of the middle arch on the port side reached about the 75th frame. The outer plate of the hull on the left side was deformed and wrinkled seriously. The highest point of the middle arch on the starboard side reached about the 95th frame. The three nearby fender welds were cracked and dislocated. The deformation was obvious within 50 m of the midship. Judging from the drawings, the deformation was serious in NO.4 and NO.5 cabins. The damage to the hull is shown in Figure 3, Figure 4, Figure 5 and Figure 6 and is summarized in

Table 2. Hull damage.

Serial Number	Damaged Location	Damage
1	Portside shell (FR72-220～FR81 + 530)	Torn and deformed outer panel
2	Bottom outer panel (FR72-220～FR90 + 300)	Torn and deformed outer panel
3	Starboard shell (FR86-400～FR96 + 300)	Torn and deformed outer panel
4	Starboard ballast side tank (NO.4～NO.5)	Bend, deformation
5	Ballast tank (NO.4～NO.5)	Bend, deformation
6	Starboard ballast side tank (NO.4～NO.5)	Bend, deformation

.

3. The Effect of Changing the Support Length on the Strength of the Hull Structure

When the ship is working on the bottom, due to the change of tide, the sediment on the bottom of the ship will be carried away by the tide. At this time, the ship may not sit firmly on the bottom, resulting in a large local stress on the ship structure. In order to preliminarily obtain the variation law of the structural stress of the ship, the maximum stress under different support lengths of the ship’s bottom will be calculated by using the finite element method.

3.1. Finite Element Model

The 3D finite element model covers the whole ship structure. a right-handed coordinate system is adopted, with X axis pointing to the bow along the longitudinal direction of the hull, Y axis pointing to the port side along the width direction of the ship, and Z axis vertically upward along the hull. The model is simulated by plate elements, beam elements and rod elements. The total number of nodes is 430,879, and the total number of elements is 575,308, including 265,756 quadrilateral elements, 2670 triangular elements and 306,882 beam elements. The length and width ratio of the hull triangular and quadrilateral elements is 0.4:0.35, as shown in Figure 7.

3.2. Boundary Conditions

According to the requirements for bottom-mounted platforms in the Rules for Classification of Offshore Mobile Platforms of China Classification Society, the loss rate of the bottom-mounted area can be calculated as 20% [16], as shown in Figure 8.

3.3. Load Calculation and Loading

Design loads for hull structural strength assessment include the static load and the dynamic load. The static load contains gravity load (ship’s own weight), outboard hydrostatic pressure and tank static pressure, and dynamic load contains outboard hydrodynamic pressure, tank dynamic pressure and the inertial load of the whole ship [17], as shown in Figure 9.

3.3.1. Hull Weight

The weight of the hull includes the weight of the main hull and the first tower, which is loaded in the form of gravitational acceleration. At this time, the lifting equipment is in a ready state, and the lifting equipment and its counterweight are loaded by pressure. According to the Code for Lifting Equipment for Ships and Offshore Installations [18], the vertical load force of the crawler crane is calculated as follows:

$${\mathrm{F}}_{\mathrm{z}}=\left({\mathrm{m}}_1\mathrm{g} +{ \mathrm{m}}_2{\mathrm{g}\mathsf{\phi }}_{\mathrm{h}}\right){\mathsf{\phi }}_{\mathrm{d}}$$

(1)

3.3.2. Ballast Tank

The hull includes thirty-two ballast water tanks, two pump rooms, a fuel oil tank and a freshwater tank. According to the actual loading situation of each cabin, the load is evenly added to the base plate nodes [19].

3.3.3. Outboard Water Pressure

Hydrostatic pressure is considered in the calculation and is loaded in the form of surface pressure. According to the draft of the ship, a field function of pressure $p=\mathsf{\rho }\mathrm{g} \left(\mathrm{h}-\mathrm{z}\right)$ is established and added to the hull plate according to the water depth.

3.4. Calculation Results and Analysis

When the supporting width of the bottom plate remains constant, and change the supporting length, the variation law of the structural stress of the whole ship under different supporting lengths is obtained. Simulation results of working conditions 1 and 23, when the supporting length is the longest and shortest, are shown in Figure 10 and Figure 11. The calculation results of all working conditions are shown in

Table 3. Maximum stress under different support lengths.

Working Condition	$\mathbf{Restrictions} (\mathbf{Width} \text{×} \mathbf{Length}, {\mathbf{m}}^2)$	The Maximum Stress of the Whole Ship (MPa)
1	46.2 × 105.6	107
2	46.2 × 100.8	107
3	46.2 × 96	107
4	46.2 × 91.2	107
5	46.2 × 86.4	109
6	46.2 × 81.6	115
7	46.2 × 76.8	137
8	46.2 × 72	171
9	46.2 × 67.2	183
10	46.2 × 62.4	189
11	46.2 × 57.6	208
12	46.2 × 52.8	218
13	46.2 × 48	225
14	46.2 × 43.2	231
15	46.2 × 38.4	322
16	46.2 × 33.6	409
17	46.2 × 28.8	458
18	46.2 × 24	488
19	46.2 × 19.2	508
20	46.2 × 14.4	530
21	46.2 × 9.6	556
22	46.2 × 4.8	518
23	46.2 × 0	668

and Figure 12.

When the ship just sits on the bottom, the support length is 105.6 m and the support width is 46.2 m. At this time, the weight of the ship is supported by the whole length of the ship, and the maximum stress of the hull structure is 107 Mpa, so the ship will not be damaged. When the supporting length of the ship gradually decreases, the structural stress of the ship also increases accordingly, and when the supporting length is shortened to a certain scale, the stress reaches the ultimate breaking strength of the structure.

4. Calculation Conditions and Results under Bottom Scouring

Considering the actual scouring of the ship bottom, the Fluent module is used to simulate the seabed scouring under three working conditions, and constraints are added to the ship according to the simulation results.

4.1. Boundary Conditions

According to the actual scouring results, the stress situation of the ship in the free state is simulated, and the Z directions of numbers 1 and 2 and the X, Y and Z directions of number 3 are constrained, as shown in Figure 13.

4.2. Load Loading

The load on deck and the outboard water pressure are the same as in Section 3.3, and the load at the bottom hole is loaded with bending moment, as shown in Figure 14 and Figure 15.

4.3. Calculation Conditions

The draft of LSC1 (seabed sitting completed) is 4.3 m, and the draft of LCS2 (the highest water level on the day of the incident) is 8 m, and the draft of LCS3 (the time of the incident) is 3.5 m. The ballast distribution under the three working conditions is shown in Figure 16, Figure 17 and Figure 18. Green represents ballast water, red represents fuel oil, and blue represents fresh water. The darker the color, the more ballast. The distribution of each ballast tank is the same as Figure 2 and

Table 4. Load Cases.

Working Condition	Working Condition Description	Hoist	Loads Description
LCS1	When the ship is sitting	none	Hull dead weight, lifting equipment and counterweight loads, ballast water, hydrostatic pressure and buoyancy
LCS2	The water level is the highest in the day	none	Hull dead weight, lifting equipment and counterweight loads, ballast water, hydrostatic pressure and buoyancy
LCS3	At the time of the incident	none	Hull dead weight, lifting equipment and counterweight loads, ballast water, hydrostatic pressure and buoyancy

describes the load conditions under three working conditions.At this time, the reasonable ballast water weight of the ship in three conditions are as follows:

W_ballastwater = (W₀ + 6000 t) − (emptyweight + Crawler crane weight + Oil and water weight)

(2)

4.4. Calculation Results

4.4.1. LCS1

The calculation results of the stress of the bottom at the beginning of sitting without scouring is shown in Figure 19, Figure 20, Figure 21 and Figure 22 When just sitting on the bottom, since the scouring has not started, the whole weight of the ship is supported by the whole length of the ship. At this time, the ship is in a sagging state, the maximum structural stress is 133 MPa, and the structure will not be damaged, let alone fracture.

4.4.2. LCS2

Considering the actual scouring situation of the bottom of the ship, the force of the whole ship structure at the highest tide level of the day is shown in Figure 23, Figure 24, Figure 25 and Figure 26. From Figure 23, Figure 24, Figure 25 and Figure 26, it can be seen that the force of the hull structure is greatly increased compared with that when the hull has just sat on the sand, up to 429 MPa, at which time the hull undergoes plastic deformation.

4.4.3. LCS3

Considering the actual scouring of the bottom of the ship, the stress of the whole ship structure at the lowest tide level on the day of the accident is shown in Figure 27, Figure 28, Figure 29 and Figure 30. It can be seen from Figure 27, Figure 28, Figure 29 and Figure 30 that the entire hull is seriously deformed. The whole ship has obvious middle arch deformation. The deck plate and the outer plate of the bottom bilge are subjected to a large stress, and if the stress exceeds the limit breaking stress, a large deformation will occur until fracture. The outer plate of the hull suffers heavy stress and produces significant deformation. In terms of the stress, the maximum force of the hull structure of the fourth and fifth compartments is 1060 MPa, which exceeds the maximum breaking strength, and the ship will have a fracture accident.

5. Conclusions

According to the simulation results of bottom scouring, the structural strength analysis is carried out for the three working conditions of the wind power installation ship under the actual operation, and the conclusions are as follows:

• If the weight of the ship is supported by the whole length of the ship while the ship is on the bottom, the hull will not be damaged. Due to the effect of scouring, the supporting length of the ship decreases gradually, and the stress on the structure of the ship increases accordingly. When the supporting length is reduced to a certain scale, the stress reaches the ultimate breaking strength of the structure.
• If the fore and aft ends of the ship are simultaneously scoured by more serious water flow, it is easier to form a mid-arch state in which both ends are suspended and only the middle is supported. As the scouring time increases, the length of scouring becomes longer and the suspension distance also grows, resulting in the decrease of the support length in the middle of the hull and the further aggravation of the middle arch. It will eventually lead to cracks in the ship, and the cracks grow longer and longer.
• When the ship is sitting on the bottom, the length of the hull support is an important factor affecting the strength of the hull structure. However, due to the difference in the weight of ballast water in each tank, the buoyancy of the midship, bow and stern is not the same, which will make the ship in a sagging or arching state. The ship will further aggravate the damage of the hull under the alternating action of the middle sagging and arching.