Development of an Integrated Extreme Wind, Wave, Current, and Water Level Climatology

to Support Standards-Based Design of Offshore Wind Projects

Technology Assessment and Research Project #672

FINAL REPORT

06 February 2014

TA&R Project #672 Extreme Metocean Climatology for Offshore Wind

Executive Summary

This report describes the methodology and results of a two-year study funded by the Technology

Assessment and Research (TA&R) Program of the U.S. Bureau of Environmental Safety and

Enforcement (BSEE). The primary goal of this study was to develop and apply methodologies

for creating an extreme event climatology that characterizes standards-based design parameters

for extreme winds, waves, currents, and water levels for the offshore Mid-Atlantic region at

event return periods appropriate to the acceptable risk for safe operation and survival of the

various different components of offshore wind projects, including the turbine, tower, foundation

substructures, and accessory platforms.

The results presented herein are intended to assist BSEE regulators and Certified Verification

Agents in their review of the Design Basis for offshore wind project plans in the Mid-Atlantic

offshore Wind Energy Areas off New York, New Jersey, Delaware, Maryland, Virginia, and

northeastern North Carolina. It also will be of interest to the designers of wind turbines and

foundation substructures, and to the developers, financers, and insurers of any offshore wind

project to be sited on the Mid-Atlantic Outer Continental Shelf.

The full report is divided into five c hapters. Chapter 1 provides the standards-based context for

selection of extreme event return periods, and the meteorological and oceanographic (metocean)

parameters that the standards specify for various Design Load Cases (DLCs) and associated

structural load modeling. This chapter describes the relationship between fundamental metocean

parameters, as customarily produced by physical measurements and numerical models, and

derived metocean parameters that the standards specify for each DLC, as summarized below.

The fundamental wind parameter is the 10-minute average wind speed at the meteorological

surface elevation of 10 meters above sea level (U10). The fundamental wave parameter is the

significant wave height (HS) for an assumed 3-hour sea state duration.

All metocean parameters specified by the standards for direct application in a given DLC are

derived from one of the two fundamental metocean parameters defined above, typically by

applying a multiplier. Thus, a reference 10-minute mean wind speed at turbine hub height

(VREF) is estimated by deriving a U10 multiplier from the assumed vertical profile of wind speed.

This then becomes the basis for estimating extreme or reduced 3-second gust speeds by

applying a VREF multiplier, which is specified in the applicable standard. Likewise, various

estimates for both individual waves and the sea state as a whole, including extreme, severe

and reduced, are derived from HS multipliers, a lso specified in the applicable standard.

Chapter 2 describes the methodology our study used to estimate the fundamental metocean

parameters for the two different types of extreme storm populations that occur in our study area:

hurricanes (tropical cyclones) and noreasters (extratropical cyclones). Section 2.1 describes the

methodology and results for estimating the fundamental metocean parameters of noreasters.

Section 2.2 describes the methodology and results for estimating the fundamental metocean

parameters of hurricanes. Section 2.3 examines the relationship between these two different

storm populations and how this relationship affects the extreme probability distribution of

fundamental wind and wave parameters throughout our study region.

Final Report 1 06 Feb 2014

TA&R Project #672 Extreme Metocean Climatology for Offshore Wind

For a given threshold U10 or HS, the number of hurricane events exceeding a given threshold is

substantially less than the number of noreaster events exceeding that same threshold over any

given measurement or modeling period. For noreasters, a 20- to 30-year historical sample of

wind or wave data provides a sufficient number of events to accurately fit an extreme probability

model to the high tail end of the sample distribution, such that the model can be reliably used to

extrapolate design events having a 50- or 100-year return period.

Although the National Data Buoy Center (NDBC) has several long-lived offshore measurement

stations in our study area with record lengths exceeding 20 years, these all contain gaps that have

missed major noreaster events. Therefore, our study evaluated two 20-year hindcast databases:

the Wave Information Studies (WIS) database developed by the U.S. Army Corps of Engineers

for design of shore and harbor protection measures, and the Wavewatch III database developed

by the National Centers for Environmental Prediction (NCEP), which is the operational wave

forecast system used by the National Weather Service.

By comparison with noreasters, there are far fewer hurricane events in a 20- or 30-year sample,

which introduces much more uncertainty in extrapolating the 50- or 100-year design event.

Therefore, our study adopted the synthetic hurricane modeling approach used by the American

Society of Civil Engineers for coastal building design. This enables the Monte Carlo simulation

of thousands of synthetic storms, such that the high tail end of the sample distribution would be

more reliably represented by this much larger number of storm events.

Chapter 3 describes the derivation of specific wind and wave design parameters from the

fundamental wind and wave parameters estimated in Chapter 2. Measured wind and wave data

from a variety of platforms are used to validate the derivation multipliers that are published in

the standards. Where measurements depart from the standards-based multipliers, alternative

multipliers are recommended. This section also describes how standards-based vertical profiles

of wind speed (i.e., wind shear) compare with measured profiles as published in peer-reviewed

literature. Finally, this section describes how wave breaking alters the probability distribution of

individual wave heights in extreme sea states, and the effect this may have on various DLCs.

Chapter 4 describes the methodology and results for estimating extreme water levels, surface

current speeds, and current profiles. These are governed primarily by the same fundamental

wind and wave parameters estimated in Chapter 2, but also are influenced by astronomical tide.

Although there remain large uncertainties in the characterization of wind-driven currents and

underwater current profiles, the overturning moment contributions by wind loads on the wind

turbine rotor and wave loads on the foundation substructure are so much greater that this

uncertainty is likely to have only modest impact on the design of offshore wind facilities.

Six appendices are included with this report and can be downloaded as separate PDFs.

Final Report 2 06 Feb 2014

TA&R Project #672 Extreme Metocean Climatology for Offshore Wind

Chapter 1. Standards-Based Context

For limit state design and ultimate strength analysis, the IEC 61400-3 offshore wind turbine

design standard specifies an extreme event return period of 50 years. For hurricane-prone areas

such as the Mid-Atlantic region, the American Bureau of Shipping Guide for Building and

Classing Bottom-founded Offshore Wind Turbine Installations (ABS-BOWTI) recommends

a return period of 100 years. Both standards are otherwise identical in their specification of the

Design Load Case (DLC) 6.x and 7.x series, which consider that the turbine has been shut down

and is parked (idling) with power available from the utility grid to maintain or adjust turbine yaw

(DLC 6.x) or idling with electrical fault (DLC 7.x).

A complete table of the IEC 61400-3 Design Load Cases is included as Appendix A.

Note that the 10-minute mean wind speed at hub height is referred to as the reference wind

speed and must be vertically extrapolated from a modeled wind speed elevation, which is usually

10 m above sea level (ASL), or the elevation of a measured wind speed, which in our study area

can range from 5 m ASL at 3-meter discus buoys operated by the National Data Buoy Center

(NDBC), up to 45 m ASL on fixed platforms that are part of NDBCs Coastal and Marine

Automated Network (C-MAN). The IEC 61400-3 standard and the ABS BOWTI standard both

specify a default shear profile described by a Power Law with a Power Law exponent of 0.11.

Section 3.1 of this report evaluates the suitability of this default specification by comparing it

with measured hurricane shear profiles published in the peer-reviewed literature.

Both standards specify DLCs for combined wind and wave loading by assuming that for a given

design storm, the peak 3-second gust and the maximum individual wave height