Gus Jeans

The WAG Squall Measurement System

Squalls control design wind conditions offshore West Africa, where the wind regime is otherwise relatively benign. The West Africa Gust (WAG) Joint Industry Project was initiated in 2004 to make best use of available industry data for characterisation of squalls in engineering design. The WAG phase one report highlighted the need for further measurement to address uncertainties in the horizontal and vertical structure of squall winds. This paper describes the resulting measurement system installed on a platform offshore West Africa.Copyright

The WAG Platform Structural Effects Study

This paper describes a detailed investigation into structural effects on wind measured on an offshore platform. Squalls dominate the extreme wind regime in the West Africa region. The West Africa Gust (WAG) Joint Industry Project was initiated to provide critical new data to support engineering design. The WAG squall measurement system was carefully designed to characterise the horizontal and vertical structure of squall winds. A total of 8 anemometers were installed at various locations on a platform offshore Congo. This application required a high level of confidence in wind speed differences measured at separate locations. An intensive investigation into platform structural effects was undertaken using the in-situ data, supported by independent wind tunnel testing. This paper will explain how the wind tunnel results were effectively used to quantify structural contamination at each key anemometer location, for the full range of incident wind directions. The wind tunnel results were verified using empirical structural correction factors derived from the in-situ data. The study successfully established the required high level of confidence in the measured data, so that squall structure could be characterised for engineering design.© 2011 ASME

DEEPWATER CURRENT PROFILE DATA SOURCES FOR RISER ENGINEERING OFFSHORE BRAZIL

A variety of current profile data sources are compared for a deepwater site offshore Brazil. These data were gathered for consideration as part of the Worldwide Approximations of Current Profiles (WACUP) Joint Industry Project, described separately in OMAE2012-83348. The primary source of data for current profile characterisation is site specific full water column measurement. Sufficiently high vertical and temporal resolutions are required to capture the dominant oceanographic processes. Such in-situ data are generally expensive and time consuming to collect, so there is an increasing tendency for numerical model current data to be considered for engineering applications. In addition to being relatively inexpensive and quick to obtain, model data are also typically of much longer duration. This potentially allows inter-annual variability and rare extreme events to be captured. However, the accuracy and reliability of numerical model data remains questionable, or unproven, in many deepwater development regions. This paper explores the suitability of such models to represent a deepwater site offshore Brazil, in relation to the key oceanographic processes revealed within the in-situ data.Copyright

Deepwater Current Profile Data Sources for Riser Engineering Offshore West Africa

Reliable quantification of current profiles is required for safe and cost effective offshore exploration and field development. The current regime offshore West Africa is often considered benign, compared to some regions of oil and gas activity, but still presents challenges to reliable quantification. A key challenge to all offshore developments is acquisition of appropriate data. The primary source of data for riser design is site specific full water column measurement. Such in-situ data are generally expensive and time consuming to collect, so there is an increasing tendency for numerical model current data to be considered. Model data are often relatively quick and inexpensive to obtain, with the added benefit of a much longer duration, potentially allowing inter-annual variability and extreme events to be captured. However, the accuracy and reliability of numerical model data remains questionable, or unproven, in many deepwater development regions. This paper explores the suitability of such models to represent a deepwater site offshore West Africa, in relation to the key oceanographic processes revealed within the in-situ data.Copyright

Synergistic Use of Satellite and In-Situ Current Data to Improve the Characterisation of Seasonal Trends

The synergistic use of measured in-situ current data and altimetry derived geostrophic current data provides improved seasonal characterisation of the current regime, West of Shetland. In September 2007, considerable downtime was experienced by an offshore operator, West of Shetland, as a result of unexpectedly high currents persisting for a number of days. This downtime was unanticipated following conclusions derived from one year of in-situ measured data, which suggested a most favourable current regime during the months August to October. Ten years of altimetry derived geostrophic currents were utilised in conjunction with approximately 3 years of in-situ data to assess the validity of the reported seasonal trend. The altimetry derived geostrophic currents correlated well with the dominating long period signal extracted from the in-situ data. Seasonal comparison between the altimetry derived geostrophic currents and the total measured signal showed the previously available measurement year had a relatively benign September. Based on the 10 years of satellite data, the inter-annual variability of the current regime West of Shetland does not show any clear seasonal trend.© 2009 ASME

Gulf of Mexico Sub-Surface Jet Screening

There have been reports of strong submerged jet currents in the Gulf of Mexico in recent years which have implications for the design and operation of some offshore facilities. This paper describes the methodology and results of a screening study which searched for such events within the extensive body of recent data collected by the offshore oil and gas industry, made available on the NOAA National Data Buoy Center web site. This study was conducted on behalf of a consortium of industry clients with participation by relevant US government bodies. After a first phase investigation, initially well defined screening criteria were revised to avoid the numerous events triggered by clearly invalid data and the potential for missing some critical submerged events. The automatic screening criteria were replaced by manual event selection based on plots showing all available data. The identified events fall into the following broad categories: • Submerged speed peaks due to inertial period currents, most notable after the 2005 hurricanes. • A few submerged jet like events isolated in time with no clear periodicity. • Submerged speed peaks with high vertical and error velocities and often incoherent structures. • Persistent high speed currents near the limit of the ADCP range.Copyright

Worldwide Approximations of Current Profiles for Steel Riser Design: The WACUP Project

This paper describes the objectives and methodology of the WACUP (Worldwide Approximations of Current Profiles) Joint Industry Project. The project goal is to establish best practice for reducing large current profile databases into a smaller, representative set of profiles for riser design. We studied the use of the following three techniques for comparing and reducing measured in-situ databases: Empirical Orthogonal Function (EOF), Self Organizing Maps (SOM) and classical Current Profile Characterisation (CPC). We evaluated the skill of these three techniques in estimating Vortex Induced Vibration (VIV) fatigue damage to a Steel Catenary Riser (SCR). Our measure of skill was the comparison of the results from the reduced data sets with results from the original complete database. The standard techniques were modified and accuracy was improved. The role of numerical current models to complement in-situ measurements in riser design was also assessed.Copyright

The Use of Empirical Orthogonal Functions for Deriving Response Based Extreme Current Profiles

Coherent extreme current profiles are derived to reduce the over-conservatism associated with the traditional assumption that extreme currents occur at all depths through the water column simultaneously. Empirical Orthogonal Function (EOF) analysis has proven effective for derivation of coherent extreme current profiles in regions where it effectively captures the dominant characteristics of the flow regime. This is despite the questionable suitability of EOF for Current Profile Characterisation, which reduces a large current profile data set into a much smaller set of profiles for riser fatigue studies.EOF Mode 1 and 2 are used to represent six years of in-situ current profiles accounting for 97.75% of the original variance. With the assumption that depth integrated speed squared is proportional to drag on a simple riser, three sets of extreme current profiles were derived. A) Profiles associated with extreme near surface current speeds, B) Profiles associated with extreme mid-depth current speeds and C) Profiles associated with extreme drag on a riser.© 2012 ASME

Submerged Jet Currents in the Gulf of Mexico

A recent screening study of MMS NTL ADCP 2005 and 2006 data has identified two modes of submerged jet currents in the Gulf of Mexico: (1) submerged speed peaks with inertial period and (2) events isolated in time with no clear periodicity. The latter can be divided further as shallow jet events (between 150 m to 600 m) and deep jet events (deeper than 600 m). The submerged jet events can last a few hours to several days. In order to investigate the jet generation mechanisms and test the predictability of the events, representative events for each mode were studied. Satellite sea surface height (SSH), sea surface temperature (SST) data and forecast data of the Princeton Regional Ocean Forecast System (PROFS) during the selected events have been used. Mode 1 jets are mainly due to downward propagating of inertial oscillations following hurricanes, while the deep jets of mode 2 appear to be caused by deep current propagation along the 1000 m isobath. The shallow jets of mode 2 are the result of on-slope flow convergence producing a subsurface downwelling/upwelling cell and frontogenesis over the slope. The comparison of PROFS model and observations shows good predictive skill although the model has the tendency to underestimate current strengths.