IEC CODE

“Martha’s Vineyard loses power By Associated Press, 01/06/00

A large section of Martha’s Vineyard is without power today because of an equipment problem on the mainland. Commonwealth Electric spokesman Mike Durand said the outage is affecting about 3,500 customers in Edgartown, Tisbury and West Tisbury.

Durand said the equipment failure occurred at 9:45 this morning in Falmouth knocking out power that is fed to Martha’s Vineyard through an undersea cable. The utility is hoping to get a diesel generator up and running on the island while repairs are made on the mainland.”

[Ref: http://www.boston.com/news/daily/06/marthas_vineyard.htm].

In review of DNV-OS-J101 (2007) this standard has the same load conditions of IEC 61400-3.

In review of Germanischer Lloyd Certification of Offshore Wind Turbines 2005 the loading conditions quoted different from the IEC Standard:

• IEC Load cases 1.6 a and 1.6 b are missing;

• IEC Load cases 2.3 is missing;

• GL add cases on Temperature and Earthquake effects;

• IEC Load case 6.3 in GL are titled “Extreme oblique inflow” whereas in IEC and DNV they are called “yaw misalignment”;

• IEC Load Case 7.1a is missing from GL;

• GL has added a load case for boat impact: their 8.5.

GL has indicated that a number of the load cases were not governing and so were omitted from their standard, whereas they believe some of the other ones are required.

The issue of load cases requires further scrutiny and alignment: thus the recommendation for an FMEA to be performed on a site-specific basis, until it becomes clear the various appropriate load cases for US OCS application.

Nevertheless in the detailing of assumptions in the load cases there would be benefit to the regulatory in providing more clarity. It would also be beneficial to document for each of the load cases the critical component.

3.1.2 Foundations

3.1.2.1 IEC CODE The IEC Code 61400-3 advises that:

“Account shall be taken of the soil properties at the site, including their time variation due to seabed movement, scour and other elements of seabed instability.”

“Section 11: The foundation shall be designed to carry static and dynamic (repetitive as well as transient) actions without excessive deformation or vibrations in the structure. Special attention shall be given to the effects of repetitive and transient actions on the structural response, as well as on the strength of the supporting soils. The possibility of movement of the sea floor against foundation members shall be investigated. The loads caused by such movements, if anticipated, shall be considered in the design.”

It goes further in section 12.15 – Assessment of soil conditions:

“The soil properties at a proposed site shall be assessed by a professionally qualified geotechnical engineer.

Soil investigations shall be performed to provide adequate information to

characterise soil properties throughout the depth and area that will affect or be affected by the foundation structure. The investigations shall in general include the following:

• geological survey of the site;

• bathymetric survey of the sea floor including registration of boulders, sand waves or obstructions on the sea floor;

• geophysical investigation;

• geotechnical investigations consisting of in-situ testing and laboratory tests.

In order to develop the required foundation design parameters, data obtained during the investigations shall be considered in combination with an evaluation of the shallow geology of the region. If practical, the soil sampling and testing

program should be defined after reviewing the geophysical results.

Soil investigations shall include one or more soil borings to provide soil samples for in-situ tests and laboratory tests to determine data suitable for definition of engineering properties. The number and depths of borings required shall depend on the number and location of wind turbine foundations in the offshore wind farm, the soil variability in the vicinity of the site, the type of foundation, and the results of any preliminary geophysical investigations. Cone penetration tests (CPT) and shallow vibro-core borings may be used to supplement soil borings in the soil investigation. Site-specific soil data shall in principle be established for each foundation within the wind farm. CPTs may be used for this purpose at wind turbine locations where soil boring is not undertaken. For calibration of the CPTs, one CPT shall be performed in the close vicinity of one of the soil borings.

• data for soil classification and description of the soil;

• shear strength parameters;

• deformation properties, including consolidation parameters;

• permeability;

• stiffness and damping parameters for prediction of the dynamic properties of the wind turbine structure.

For each soil layer these engineering properties shall be thoroughly evaluated by means of appropriate in situ and laboratory testing.

The assessment of soil conditions shall also consider the potential for soil liquefaction, long term settlement and displacement of the foundation structure as well as the surrounding soil, hydraulic stability and soil stability characteristics.”

Surveys are carried out as part of the SAP. Typical tests carried out include:

o Bathymetric surveys (sonar) locate wrecks, pipelines and other obstacles.

o Tests on the bottom material itself including Cone Penetrometer Tests (CPT) and Standard Penetration Tests (SPT), core drilling in soils and rocky materials, Menard –pressure meter tests, offshore vibrocoring and bottom sampling.

The support types for offshore oil and gas structures is not very different from the requirements laid out for offshore wind structures: the calculations, however, need to be more precise particularly for non-redundant structures such as monopiles which carry more cyclic loads with possibly severe consequences for the structure if reality is different from the calculated model in an un-conservative direction.

Most of the offshore oil and gas structures are dominated by the forces of waves, and only a steady state wind is generally used to design offshore structures and thus have not included the dynamic components necessary for offshore wind turbine structures. Wind turbine structures are in general more dynamic i.e. they oscillate more than a typical oil and gas platform as the forces are applied to them. The differences only serve to require somewhat more detail than is sometimes applied to offshore platforms, where the soil conditions may be assumed over a field. Precise data for each turbine location may be needed for individual turbines depending on the site-specific area.

Because of the oscillating loads the foundations of monopiles experience larger shears and bending moments and smaller axial loads requiring that the designer consider cyclic loading of the near-to-surface soils. This cyclic loading in the surface soils combined with the possibility of scour, particularly if the currents are reasonably large requires the designer’s attention. Because of the lack of redundancy in the monopole foundations meticulous attention has to be paid to the issue of soils, more so than an “average”

platform. Monopiles with minimum production equipment have been used in the Gulf of Mexico: many were destroyed in Hurricane Andrew probably due to lack of

consideration of dynamic loads at the time. A number of lessons were learned:

approximately 100 caisson structures were tilted during Hurricane Andrew [Ref. 3.1.30].

Guidance on piled foundation design and grouted connections is available in American Petroleum Institute, Recommended Practices for Planning Designing and Constructing Fixed Offshore Platforms (API RP2A), and NORSOK N004 Design of Steel Structures.

The piles can be drilled and cemented or driven depending on the soils.

Pile driving is usually a less weather sensitive method of installation but maintaining heading with driven piles is more difficult. Transition pieces are used to assure that any pile angle is compensated for and grouting these in place in order to assure the tower is vertical is critical to the production of electricity.

Concrete gravity based structures have been successfully used e.g. Middelgrunden, Vindeby and Tuno Knob offshore Europe. ACI 318-08 is a suitable standard for design in Concrete. ACI 357R Guide for the Design and Construction of Fixed Offshore Concrete Structures also offers useful guidance.

The natural period of the wind tower is determined by the weights and distribution of weights, the stiffness of the tower and also by the soil characteristics and stiffness of the soil-tower interface. Knowledge of the soil data particularly near the surface is critical.

Any variation within the field of turbines must also be known accurately thus sometimes requiring more boreholes than the minimum. Based on knowing the effect of the blades passing the tower, the rotor itself, the waves and wind characteristics, and the soil information it is possible to calculate the natural periods to be avoided in the design to ensure minimum possibility of resonant response.

One experience is noted “it was observed at Lely that the behaviour of two of the OWECs (Offshore Wind Energy Converters) was stiffer than predicted…It was fortunate that the exclusion period was avoided, although it must be noted that this was purely chance”.

(Ref: www.offshorewindenergy.org).

The foundation stiffness can be affected by scour which has occurred at several wind turbine sites e.g. Scroby Sands, and Prinses Amelia where granular soils and high currents were a factor.

“Following the bathymetric surveys some scour pits have been identified with a depth of 5 m and diameter of 60 m around each turbine, which were predicted by the original EIA.

We now have an improved understanding of the extent of this scour and importantly the scour tails local to each turbine and in each area of the wind farm. This information will be useful in determination of the necessity for scour protection in future years.”

[Ref. 3.1.37].

Figure 15 [Ref. 3.1.38]

Waves, as well as wind and currents affect the natural period of the structure. In shallow water there are many more cycles of low periods in a stress region where more cycles are available. As water depth increases, so the natural period increases and the stress region has less available cycles. This makes structures that have more than a single pile, and concrete structures a preferred solution as the depth of water increases. The effect of excitation and also damping is taken into account, or loads must be assessed

conservatively if sufficient data is not available. The soil is an important factor in this evaluation.