4.2.1 Integrated Ground Models

Essentially, the integrated ground model is a way to manage and sort information in a meaningful way so that ground risks can be captured and communicated to various stakeholders of the projects. Examples of stakeholders include investors, due diligence personnel, insurers or underwriters, certifiers, asset owners, etc. SUT (2014) defines a ground model as a database of information that includes structural geology, geomorphology, sedimentology, stratigraphy, geohazards, and geotechnical properties of the site.

The development of ground model and managing geotechnical risk is an iterative process that involves collecting relevant data and then interpreting and presenting in a meaningful way. Ground models are constantly updated as more information is available. Obtaining offshore ground data (geological, geophysical, and geotechnical) is expensive, as it involves mobilising a vessel. Therefore, ground models are not usually finalized until after the business decision of wind farm development is made. Development of the ground model is a continuous process and may begin through the use of satellite images and desk study (i.e. geological information of the site).

4.2.2 Site Information Necessary for Foundation Design

A typical offshore wind farm consists of large number turbines widely spaced and therefore change in water depth and geology may be expected. Therefore, the investigation will cover a large amount of area together with the plausible route for cables. To carry out feasibility studies as well as create a detailed design, various information and data are necessary. Table 4.1 lists the essential ones.

4.2.3 Definition of Optimised Site Characterisation

The ground engineering requirement covers a large area and there are significant costs associated with mobilisation of an offshore site investigation vessel. Therefore, at the planning stage, it is necessary to develop understanding of the project's needs and requirements so that necessary data and soil samples for the project can be obtained from one mobilisation of a vessel. For the purpose of foundation design, the main requirement is the development of design ground profile and assigning engineering parameters to each of the soil strata. This is one of the most important and critical steps that demand expertise. Section 4.3 provides typical examples of design ground profile.

4.3.1 Offshore Ground Profile from North Sea

This section provides examples of typical ground profiles from a few offshore locations. Figure 4.2 and Table 4.2 shows a design ground profile from North Sea site following Bhattacharya et al. (2009). The location of the site is provided in Figure 4.3.

Other ground profiles from European wind farms are discussed in Chapter 5.

4.3.2 Ground Profiles from Chinese Development

There are four main sea areas in Chinese water where offshore wind development is in progress: (i) Bohai Sea (enclosed sea); (ii) Yellow Sea; (iii) East China Sea; (iv) South China Sea. Generally, the ground conditions in China seas are soft and layered, in contrast to North Europe.

For example, the Bohai Sea is a low‐lying land settled by North China Plain, where the soil consists of sediments brought from mountains and Yellow River and is soft, silty clay or clayey silt. Borehole is usually a widely used method for underground geology survey. There are many records of boreholes in Bohai Sea, and a typical profile (from top to bottom) is as follows:

1. Soft clay silt (Layer 1). The depth of this layer varies but is typically 0–7.4 m in the western Bohai.
2. Mucky clay to silty clay (Layer 2). In this layer, the clay is changing from soft to stiff, with materials of mucky clay, silt, and loose sand. The depth of the clay layer varies (thickness 2.5–9.0 m in western Bohai), centre Bohai (thickness 4.0–10.0 m), Jinzhou (thickness 2.7–14.8 m) and Suizhong (thickness 0.8–5.8 m).

   

3. Combination of silty clay, clay, and loose sand (Layer 3). Again, the thickness of this layer varies − 2.5–7.0 m in western Bohai.
4. Fine sand and silt. In this layer, the main material is fine sand, combined with silty sand and clay. The stiffness is that of medium dense sand. The thickness in western Bohai is sea is 4.0–15.0 m, in centre Bohai is 4.0–13.0 m, in Jinzhou is 3.0–15.0 m, and in Suizhong is 0–14.2 m.
5. Clay. In this layer, the clay is quite stiff and hard, combined with some silty clay. The thickness in western Bohai is 2.0–18.0 m, in centre Bohai is 3.0–15.0 m, in Jinzhou is 3.0–17.0 m, and in Suizhong is 0–15.0 m.
6. Fine sand and silt

In this layer, the main material is fine sand, combined with silty clay where the layer is thin. The stiffness is quite dense. The thickness in western Bohai is 5.0–11.0 m, in centre Bohai is 6.0–15.0 m, in Jinzhou is 7.0–15.0 m, and in Suizhong is 12.0–18.0 m (see Figure 4.4).

By contrast, Figure 4.5 shows ground profile for the Yellow Sea, and it is evident that the ground profile is variable, which calls for detailed site investigation to minimise risks during construction. Figures 4.6 and 4.7 show the ground profiles for other two Chinese seas.

Table 4.3 shows the ground profile in South China Sea. A general observation can be made. The ground profile in Chinese waters is variable, and it shows the importance of a comprehensive site investigation ( Table 4.3). Further details can be found in Bhattacharya et al. (2017).

4.4.1 Geological Study

Chapter 1 showed that an offshore wind farm must cover a large area in order to maximise the energy yield. As shown, in some cases, the area is as large as 20 × 6.5 km. A study based on geological history of formation can be the basis of planning site investigation. Geological study is typically a desk study and is used to create the first ground model.

4.4.2 Geophysical Survey

This involves mobilising specialist vessels to map the seabed surface as well as subsea floor and the data generated can be used to update the ground model. A geophysical survey can be conducted alongside geotechnical site investigation. Table 4.4 shows the list of geophysical equipment for geophysical survey and design data/information obtained. Geophysical surveys are often carried out before the final business decision is taken.

DNV‐OS‐J101 suggests that line spacing of survey at the seabed location should be sufficient to detect all soil strata of significance for the design and installation of the wind turbine structure. It is also recommended that the survey needs to detect any buried channels with soft infill materials.

An example of an alignment chart used in a cable route investigation is given in Figure 4.8. The chart shows bathymetry, seabed features, sub‐bottom profile, magnetometer, route planning, and service chart.

4.4.3 Geotechnical Survey

The geotechnical site investigation required is governed by the foundation concept likely to be used and is, in turn, dependent on water depth, geology of the area, and the environmental conditions. In this process, a vessel is chosen depending on the requirements of the project for in‐situ testing and sample recovery from boreholes. Typically, in‐situ investigation involves pushing a cone (also known as cone penetration test, CPT) at different location to measure the soil resistance to penetration, see Figure 4.11 for an example bore log. Selection of vessel depends of various factors, and a summary of the important ones are given in Table 4.5.

Further details on the types of vessels can be found in Dean (2010), SUT (2014), and Rattley et al. (2017). Geotechnical investigation will provide the engineering parameters for the following analysis and design of: (i) foundations and its installation; (ii) inter‐array and export cables along a defined route; (iii) long‐term prediction in terms of serviceability limit state (SLS) and fatigue limit state (FLS). Some examples are: axial capacity of piles in tension and compression, p‐y curves for laterally loaded pile design, pile driveability characteristics, and bearing capacity of shallow foundations.

Typically, a geotechnical site investigation campaign involves the following:

1. CPT at many locations to a depth depending on the types of foundations. A tentative guidance on the depth of penetration is given in the next section.
2. High‐quality samples are obtained at selected locations through boreholes.
3. In‐situ measurement of small strain stiffness may involve the use of a pressuremeter, dilatometer, or seismic cone.
4. Laboratory testing (Table 4.6) provides a list of tests that are necessary. However, this is not an exhaustive list.

4.7.1 Standard/Routine Laboratory Testing

The most commonly used element testing methods are direct shear and ring shear tests, simple shear tests, triaxial tests, and consolidation tests. The readers are referred to textbooks on element testing for details and methodology. Some examples are Dean (2010), Mitchell and Soga (2005), Head (2006), and Randolph and Gourvenec (2011). Table 4.8 provides a summary on the soil parameters that can be obtained from such tests.

4.7.2 Advanced Soil Testing for Offshore Wind Turbine Applications

As discussed in Chapter 2, loads on the foundation are cyclic as well as dynamic. It is therefore necessary to understand the behaviour of soils under such loading conditions. From practical view of view, three types of apparatus readily available and widely used in engineering practice: cyclic triaxial with bender element, cyclic simple shear (CSS), and resonant column apparatus.

4.7.2.1 Cyclic Triaxial Test

The triaxial test is the most widely used and versatile shear strength test. As the name suggests, the soil specimen is subjected to compressive stresses in three directions by fluid pressure. To fail the soil sample, additional vertical stresses are applied on the top of the sample, axially (either monotonically or cyclically) by a loading ram. Different types of tests can be carried out: drained, undrained, consolidated undrained, and multistage. The basic features of the triaxial test apparatus are presented in Figure 4.12. Figure 4.13 shows a cyclic triaxial set at SAGE (Surrey Advanced Geotechnical Engineering) laboratory. The readers are referred to textbooks for further details of triaxial test procedures. This apparatus is widely used in earthquake engineering practice to study the problem of cyclic liquefaction of saturated granular materials. The response of soils can be represented by plotting stress path graph (p′ − q) where p′ and q are the mean effective stress and deviator stress, respectively, and the definition are given below:

4.5

4.6

where, and are the maximum and minimum principal effective stress, respectively. In triaxial tests, it is assumed that the intermediate and the minimum principal stress are the same (i.e. ).

Both compression and extension cycles in the effective triaxial plane (q−p′) are usually performed. An example of a cyclic triaxial apparatus, under undrained conditions to simulate earthquake loading, is presented in Figure 4.14. The number of cycles necessary in undrained conditions for the material to reach a state of liquefaction is evaluated and the effective stress path is recorded. During the tests, both pore pressure within the sample and axial strain can be recorded (Figures 4.13 and 4.14).

Depending on the perceived imposed loading on the soil, loading stages can be generated or recreated. For example, Figure 4.15 illustrates the schematic diagram of the testing scheme adapted for the multistage soil element test to simulate an extreme case of soil liquefaction. In these tests, undrained stress‐controlled sinusoidal cyclic loading with a certain frequency and amplitude was initially applied in order to liquefy the soil sample. Once the specimen liquefied, cyclic loading was stopped and strain‐controlled monotonic load was then applied under undrained condition. This will provide the stress−strain curve of the liquefied sand. Figure 4.16 shows a result from the multistage test on Assam sand (India) to study liquefaction. Readers are referred to Rouholamin et al. (2017) and Dammala et al. (2017) for examples of multistage tests.

Bender element can be used to obtain the shear wave velocity of the sample through wave propagation technique. The cyclic triaxial text can be applied in two ways to issues related to offshore wind turbines:

1. Cyclic triaxial apparatus, together with the bender element, can be used to study the effect of an extreme storm on long‐term performance. Cyclic loading may be applied to a soil sample corresponding to a storm to recreate the stress history. At the end of the loading, monotonic load may be applied to find the residual soil stiffness and strength.
2. Based on the literature, the behaviour of standard soils such as pure clay or pure sand can be predicted with some degree of confidence. It is, however, difficult to predict the behaviour of intermediate soils, i.e. sandy‐silt or silty‐sand, or clayey‐silt, as the main question is whether the sample will behave like clay‐like material or sand‐like material. In other words, the behaviour of soils with varying amount of fines content is difficult to predict, and in such cases cyclic triaxial tests are useful. This will have a big impact on the prediction of long‐term behaviour, as it is the accumulation of pore water pressure in the soil that will dictate the behaviour due to the effects of millions of cycles of loading.

4.7.2.2 Cyclic Simple Shear Apparatus

Figure 4.17 shows a CSS apparatus together with the schematics of a sample shearing pattern taken from SAGE laboratory. The apparatus is capable of applying cyclic loads (both in vertical and horizontal direction) using two electro‐mechanical dynamic actuators. The vertical and horizontal displacements can be measured using encoders within the servo motors. Different wave forms can also be applied: sinusoidal, square, triangular, saw tooth, haversine. The apparatus is also capable of applying user‐defined waveforms. In the current context of offshore wind turbines, user‐defined functions can be used to simulate random wind and wave loading. This apparatus has been used in Nikitas et al. (2016) to study the long‐term performance of offshore wind turbine foundations.

In the study, external LVDTs (linearly varying differential transformer) were also used to record displacements and to verify the effectiveness of feedback control. Loads up to ±5kN can be applied in two directions with horizontal travel up to 25 mm and vertical travel of 15 mm. These are sufficient to study the effects of large strain levels applied to the soil. This therefore allows the study of effects of cyclic shear stress under drained and undrained conditions. The loads can be applied at frequencies of up to 5 Hz. This apparatus was used to investigate the cyclic behaviour of a silica sand (typical of North Sea) by maintaining the constant vertical consolidation stress while cycling the shear strain in horizontal direction. Typical results are presented in Figures 4.18 and 4.19.

Carefully designed and performed tests can be useful in predicting the long‐term performance. Readers are referred to Chapter 5 for further details on the soil−structure interaction. The strain levels applied to the soil under different loading conditions from the turbine can be found in Section 5.5.

4.7.2.3 Resonant Column Tests

This apparatus can be used to map the change of stiffness for a wide range of strain levels. The basic principle involved in this test is the theory of wave propagation in prismatic rods where a cylindrical soil specimen is harmonically excited until it reaches the state of resonance (i.e. peak response). Figure 4.20 shows schematic diagram of a soil sample and the apparatus. Resonant column tests can be carried out to determine shear modulus and damping ratio of soil for a wide range of strain level.

This technique was first developed in the 1930s at the University of Tokyo by Ishimoto and lida (1936) and Ishimoto and lida (1937). The resonant column only saw wider use in the 1960s with the works of authors such as Hall and Richart (1963), Hardin and Richart (1963), and Drnevich et al. (1967). In the past few decades, a number of subsequent improvements and advancements have been made in resonant column testing to broaden the range of applicability. These have included the application of anisotropic stresses to the soil element (Hardin and Music 1965; Hardin and Black 1966), adaptations to the apparatus to hollow the testing of elements (Drnevich 1967), and modifications to tests specimens at large (Anderson and Stokoe 1978; Stokoe and Lodde 1978) and at high strains (Alarcon et al. 1986; Isenhower et al. 1987; Ampadu and Tatsuoka 1993).

Figure 4.20 shows a particular type of resonant column apparatus and it uses 50 mm diameter by a 100 mm long soil sample utilising a fixed‐free configuration. This arrangement is the most widely utilised resonant column arrangement in research as the mathematical derivations required are fairly simple. In such an arrangement a cylindrical soil element is excited in torsion by a dynamic drive system connected to the top cap. A simplified schematic can be seen in Figure 4.21. Through the resonant column tests, the damping ratio (D) of the soil can be measured from the free vibration decay curve (logarithmic decrement) using the accelerometer. Figures 4.22 and 4.23 show typical results from the resonant column test.

4.7.2.4 Test on Intermediate Soils

Figures 4.24 and 4.25 show the results from resonant column tests on four types of soils with varying fines content carried out at Yamaguchi University (Japan). The graphs show as expected the variation of stiffness and damping with strain level. Figure 4.25 shows the plot of accumulated pore water pressure with strain level. It is interesting to note that for strain level corresponding to G _eq/G _max of 0.6–0.85, pore pressure starts to build and that is the onset of progressive collapse.

4.8.1 Classification of Soil Dynamics Problems

The main aim of this section is to provide a summary of the issues related to offshore wind turbine problems. Explanation of the numbers is provided in Figure 4.26:

1. Earthquake loading characterised by large strain, low number of cycles, and low frequency (0.5 to 10Hz)
2. Loading on offshore structures
3. Traffic loading
4. Represents the strain in the soil due to variation of ground water table
5. Pile driving, Soil compaction
6. Impulsive load (Blast) characterised by high frequency, very low number of cycles and small deformation

4.8.2 Important Characteristics of Soil Behaviour

All soils under moderate cyclic loads yield progressively, try to compact, generate positive pore water pressure, and soften. Collapsible soils such as loose sands can liquefy in the absence of drainage. The frequency of cyclic loading condition is therefore important, since the pore pressure generated by one full load cycle may only partially dissipate before the next load cycle.

For clays, the strength and stiffness degrades. The amount of stiffness degradation is a function of strain level and the number of cycles of loading. Extensive laboratory and field studies have clarified many aspects of the behaviour of clays under such loading; see, e.g. Vucetic and Dobry (1991). Stiffness degradation has been correlated with the plasticity index (PI) of the soil. For a particular cyclic strain in the soil, more plastic clays exhibit less degradation. Figure 4.28 shows a graph based on 16 sets of published experimental data from strain controlled tests on clays having different PIs and OCRs. In the graph, a PI of zero indicates sand and a PI of 200 represents highly plastic Mexico City clays, which show little nonlinearity up to 0.1% cyclic shear strain. G _max is the secant shear modulus at small strain. Figure 4.29 shows the effect of degradation on G/G _max for different numbers of cycles (N = 1, 10, 100, and 1000) for normally consolidated clays (OCR of 1).

When analysing a soil system, three important features of soil behaviour must be considered:

1. Soil is a nonlinear material, in which the stiffness progressively decreases with increasing shear stress, until at a sufficiently high shear stress level it deforms plastically.
2. Soil is an inelastic material. Therefore, when subject to cyclic loading, it exhibits hysteretic damping (energy loss), which increases with increasing shear strain.
3. The soil properties, including strength, stiffness, and pore water pressure, may be affected by repeated cycles of load. This is particularly relevant to saturated sands and silts that may suffer a build‐up in pore water pressure with continued load application possibly resulting in liquefaction.

Five main properties (bulk density, stiffness, material damping, strength, and degradation due to cyclic loading) determine the response of a soil‐structure system under dynamic load. The bulk density affects the inertia of the soil under dynamic loading. The stiffness, which is largely defined by the shear modulus, determines the natural frequency of the system and the response. Damping is a measure of energy dissipation: the higher the damping, the greater the energy loss and therefore, the smaller the response.

4.9.1 Stiffness of Soil from Laboratory Tests

Initial stiffness of the soil (i.e. soil stiffness at small strain) is an important parameter for design. Laboratory tests suggest that the maximum shear modulus (G _max) can be expressed by Eq.

4.7

In another form, Eq. (4.7) can be expressed as G _max = 625 F(e) (OCR) ^k p_a ^0.5 (σ _M ′)^0.5 where F(e) is a function of void ratio and k is an OCR exponent, which varies with PI, as shown in Table 4.10.

Hardin (1978) proposed that F(e) = 1/(0.3 + 0.7e ²), while Jamiolkowski et al. (1991) suggested that F(e) = 1/e 1.3. Please note that G _max, p _a and σ _M ' must all be expressed in the same units. p _a is atmospheric pressure.

Ishibashi and Zhang (1993) suggest the relationship between shear modulus G and its maximum value G _max for fine grained soil:

4.8

where, σ′ _m is the mean principal effective stress, γ is the shear strain, and PI is the PI of soil.

For cohesive soils, the relationship between G ₀/S _U, PI, and OCR as summarised by Weiler (1988) can be used Table 4.11.

In addition to the theories proposed above, Seed and Idriss (1970) derived a simplified equation to describe the low strain stiffness for any granular material. As all formulations are strongly influenced by confining pressure and void ratio, only these factors were considered leading to the following derivation;

4.9

where:

The coefficient K ₂ itself has been experimentally derived from a number of different sands. It was found that under low strain conditions (γ < 10^–3%) the soil modulus coefficient only depends on the void ratio (Seed et al. 1984). Typical values for K ₂ are detailed in Table 4.12 taken from guidelines. The readers are also referred to updated correlations.

4.9.2 Practical Guidance for Cyclic Design for Clayey Soil

The salient features are:

1. Soil stratification and type of soil at the top layers. For a deep foundation, it is the top soil that deforms most.
2. What is the scour depth?
3. For soils, it is required to predict under what condition, pore water pressure will start to accumulate?

Figure 4.30 shows the variation of normalised secant stiffness with strain for fully saturated cohesive soil adapted from Vucetic (1994). It is accepted that there exists a strain level, the threshold linear shear strain, γ _tl , for which there is no stiffness degradation and the behaviour of the soil is practically linear elastic, so that there is no permanent microstructural changes of the fabric of the soil with cyclic loading. As the strain level is increased beyond γ _tl , the soil behaves nonlinearly and as a result the secant shear modulus of soil will reduce. The value of γ _tl can be estimated from the secant shear modulus reduction curve (schematically shown in Figure 4.30) assuming that linear threshold shear strain corresponding to a ratio G _sec/G _max of 0.99. There exists another value of threshold shear strain, known as volumetric threshold shear strain level (Vucetic 1994), denoted by γ _tv , beyond which permanent microstructural changes of the fabric do occur. In other words, beyond γ _tv the soil is degradable possibly due to pore pressure build‐up. In the triaxial stress space, this state corresponds to the boundary between the ‘recoverable’ and the ‘plastic zone’ (Jardine 1992). γ _tv can be assessed from the secant shear modulus reduction curve. However, two value of G _sec/G _max ratios (namely, 0.85 and 0.60) are often used, depending on the application. The value of 0.85 represents a degradation of secant shear modulus beyond which permanent deformation is expected. On the other hand, 0.6 is defined on the basis of the accumulation of excess pore pressures. These two values can represent upper and lower bound values of γ _tv for the problem in hand. The upper bound value will give the highest diameter that may be required. On the other hand, the lower bound will give the minimum diameter necessary. It is interesting to note the graphs presented in Figures 4.24 and 4.25 showing the pore water pressure build‐up. While Figure 4.30 shows the schematic representation of threshold strains, Figure 4.31 plots typical experimental data obtained from cyclic triaxial tests carried out on 10 samples of Turkish clay having a PI ranging from 9 to 40 following Okur and Ansal (2007).

Figure 4.31 shows an upper and lower bound for γ _tv which corresponds to G _sec/G _max ratio of 0.85 and 0.60, respectively. The experimental results clearly show that samples with higher PI tend to have a more linear cyclic stress−strain response at small strains and to degrade less at larger strain than soils with lower PI. Therefore, clays with higher PI are characterised by higher values of volumetric threshold shear strain. For example, considering the lower bound, γ _tv increases from 0.025 to 0.15 when PI increases from 9 to 40.

Figure 4.32 collates the volumetric threshold shear strain value ( γ _tv ) for 18 types of clays having PI ranging from 9 to 100. The experimental data were collected from research published by Kim & Novak (1981), Kokusho et al. (1982), Vucetic & Dobry (1988), Georgiannou et al. (1991), Lanzo et al. (1997), Stokoe et al. (1999), Massarsch (2004), Okur & Ansal (2007), Yamada et al. (2008). In the same figure, two linear correlations for evaluating γ _tv are also suggested for the investigated range of PI (9 < PI < 100). The experimental data used for deriving the plot in Figure 4.32 are given in the Table 4.13.

4.9.3 Application to Offshore Wind Turbine Foundations

The strain level in the soil next to the monopile i.e. the strain in the mobilised shear zone can be compared to the volumetric threshold strain, γ _tv which is a fundamental property of a soil and can be obtained from soil testing. Section 5.5 in Chapter 5 presents a method to compute the soil strain and if the strain in the mobilised zone exceeds this threshold strain, degradation may be excepted. This concept is quite similar to the mobilisable strength design (MSD) concept pioneered by Osman and Bolton (2005) where the average strain in a mechanism (deformed zone or sheared zone) is linked with an element test in a soil. The above concept has been experimentally shown to be valid by Lombardi et al. (2013).

Choosing diameter of monopile for design: It will be shown in Chapter 5 that higher the diameter of the monopile, the lower is the average strain in the surrounding soil. Therefore, choosing a larger diameter pile will lower will be the degradation. The designer needs to make sure that if the strain level in the soil exceeds the γ _tv for the clayey soils, adequate allowance for stiffness reduction needs to be taken into account.

4.10.1 Typhoon‐Related Damage in the Zhejiang Province

Zhejiang Province, located on the west of East China Sea, experienced typhoon 23 times since 1997 (start time of wind farm project), causing hundreds of millions of dollars of economic loss. It is of interest to discuss the effects of 10 August 2006 Typhoon Saomai, which had 68 m s⁻¹ wind speed that devastated many wind farms. This is the most powerful typhoon to hit China in a half century. It killed 104 people and left at least 190 missing. Furthermore, it blacked out cities and smashed more than 50 000 houses in the southeast part of the country. Figure 4.33 shows the Hedingshan wind farm in Zhejiang Province before the cyclone along with the aerial photograph of the typhoon. The typhoon destroyed 28 wind turbine structures and blades, including five tower collapses (600 kW turbine), with a total a loss of about 70 million RMB (Figures 4.34–4.36).

4.10.2 Wave Conditions

Bohai Sea, Yellow Sea, East China Sea, and South China Sea are connected with each other, shaped as a bow from north to south along the China coastline. A tropical and subtropical monsoon climate also influences the China Sea waves. Thus, in Chinese offshore wind farm projects, sea wave is a main factor that influences the design. The wave height distribution in the South China Sea is variable owing to its large area of sea region. High wave height occurs northeast of South China Sea and southeast of the Indo‐China Peninsula, where waves are about 2.4–2.6 m in height. The maximum significant wave height is larger than 2.8 m west of the Luzon Strait. Comparatively, the significant wave height in the north is less than 2 m. Wave period ranges from 6 to 10 seconds, with similar distribution of wave height. Table 4.15 shows the distribution of wave height and period for the Chinese waters.