Peter Tromans

Wave Crest Sensor Intercomparison Study: An Overview of WACSIS

The Wave Crest Sensor Intercomparison Study (WACSIS) was designed as a thorough investigation of the statistical distribution of crest heights. Measurements were made in the southern North Sea during the winter of 1997-1998 from the Meetpost Noordwijk in 18 m water depth. The platform was outfitted with several popular wave sensors, including Saab and Marex radars, an EMI laser, a Baylor wave staff and a Vlissingen step gauge. Buoys were moored nearby to obtain directional spectra. Two video cameras viewed the ocean under the wave sensors and their signals were recorded digitally. The data analysis focused on comparisons of the crest height measurements from the various sensors and comparisons of the crest height distributions derived from the sensors and from theories. Some of the sensors had greater than expected energy at high frequencies. Once the measurements were filtered at 0.64 Hz, the Baylor, EMI and Vlissingen crest height distributions match quite closely, while those from the other sensors were a few percent higher. The Baylor and EMI crest distributions agreed very well with the statistics from second order simulations, while previous parameterizations of the crest height distribution were generally too high. We conclude that crest height distributions derived from second order simulations can be used with confidence in engineering calculations. The data were also used in investigations of crest and trough shapes and the joint height/period distribution INTRODUCTION Knowledge of the statistical distribution of crest heights given the wave spectrum is central to the problem of setting deck heights. Unfortunately, there is still considerable uncertainty about the form of this distribution. The empirical evidence is confusing, since different types of instruments have tended to give different results. The theoretical problem is difficult since it is essential to account for the non-linearity of the waves. The participants at the E&P Forum (1995) Workshop on Uncertainties in the Design Process felt that there was a need to

Oceanographic and Motion Response Statistics for the Operation of a Weathervaning LNG FPSO

An oceanographic database is used to estimate the operability of a weathervaning LNG FPSO (FLNG) in multidirectional seas with swells. The oceanographic data consist of directional wave data, current profile measurements, and a wind dataset. Response functions are used to convert the oceanographic data into an equivalent dataset of vessel motion response parameters and turret mooring line tensions. The resulting response data have enabled operational type statistics to be computed, as well as the environmental conditions that induce responses with a one-year return period. In addition to the analysis of the responses of the FLNG alone, the method has been applied to a system where a LNG shuttle carrier is moored alongside the FLNG for offloading of the products. In this case, important responses are the combined roll and pitch of the two vessels and tension in the mooring lines between the shuttle carrier and the FLNG. The same method can be applied for a stern-to-bow offloading system, where the shuttle carrier is moored with a hawser or soft yoke. The diversity of the directionality of the oceanographic variables makes this a particularly interesting exercise. The analysis included significant inertial currents, a high level of background swell and frequent local-wind seas that act at an angle to the prevailing swell direction. A method for separating a sea state into a sea and swell component has been derived and is reported, along with a programme for visualising the oceanographic conditions together with the induced responses corresponding to the existing conditions.Copyright

Effect of Short-Crestedness on Extreme Wave Impact: A Summary of Findings From the Joint Industry Project “ShorTCresT”

Long-crested waves are typically used in the design of offshore structures. However, the corresponding statistics, kinematics and loading are significantly different in short-crested waves and up to date, there is no state-of-the-art methodology to apply short-crested models instead. The objective of the “ShortCresT” Joint Industry Project was to take into account short-crestedness in the design of offshore structures against extreme waves based on a good description of their spectral characteristics, statistics, kinematics, breaking and loading and to deliver (empirical) design recommendations and methods. This paper gives an overview of the findings of ShorTCresT regarding wave crest and height distributions, a comparison of basin and field data, the role of wave breaking, the most realistic directional model, hindcast models as well as the related platform loading.Copyright

Design Approach for Turret Moored Vessels in Highly Variable Squall Conditions

This paper focuses on examining the response of a large turret-moored FPSO or FLNG vessel in squall conditions and presents a novel and statistically robust response-based approach for the derivation of squall governed design loads. Turret mooring arrangements are typically used as a permanent mooring for FPSO and FLNG vessels, usually in deeper waters and in remote areas where storage capacity is required. The weather-vaning capability makes this type of mooring suitable for many types of environment; however, in tropical environments where metocean conditions are otherwise relatively benign, squalls can dominate aspects of design. Squalls are mesoscale convective systems that cause rapid increases in wind speed and are often associated with large changes in wind direction Squalls are highly variable in both their wind speed and direction profiles and typically uncorrelated with the wave and current conditions; hence the impact of squalls can be difficult to predict and a statistically robust industry standard design approach does not currently exist. Where squall events are the design drivers for mooring arrangements they require particular focus due to the industrys imperfect knowledge of squall intensity and frequency coupled with the particularly high inter-annual variation of squalls at most locations. To understand the design criteria for a squall environment the horizontal motions of the turret-moored system are modelled using a simplified time-domain approach, which makes considerable simplifying assumptions. This significantly reduces computational time, allowing the analysis to be conducted efficiently for a large range of both squall conditions and associated environmental (wave and current) conditions, including by season and direction. An extreme value analysis approach is then applied to all of the maximum values of the desired response for all combinations of squall and associated conditions, by selecting a number of bootstrap resamples, each the size of the number of squall events. The bootstrap resamples are selected on the probability of occurrence of the associated conditions. A generalised Pareto model can be automatically fitted to all bootstrap resamples and for every return period a user defined number of simulations is performed to estimate the most probable or percentile estimates of the desired return period. Finally, a back calculation of the transient design conditions at the desired return period can then be made and applied into the standard design process. This ensures statistically robust design conditions for squall design loads and deriving a few statistically robust design squalls and associated conditions minimises later analysis. Importantly the approach allows the users to propagate the remaining uncertainty estimates into subsequent robustness analyses.

FPSO Conference—Estimating Wind-Sea and Swell for FPSO Operability

The motion response of an FPSO is sensitive to the relative intensities and directions of the wind-sea and swell components in a sea state, and the operability of the FPSO is a function of the long-term variation in these components. Estimations of the operability are therefore dependent both on how the sea state is described in terms of its constituent wind-sea and swell components, and on how the long-term variability of the sea state is captured. However, there is currently no consensus on how either the sea state or its long-term variability should be described. We investigate these issues by means of a study of the responses of a typical FPSO to the wave fields at a location offshore Namibia and a location off the west coast of New Zealand. We make use of a state-of-the-art program for splitting a directional wave spectrum into wind-sea and swell components, and we examine the effect on the motion responses of allowing the spectra to be split into many swell partitions or constraining the spectral split to a maximum of two partitions, as is often assumed in response calculations. The resulting decompositions are used to examine the effects of swell on hull motions and, hence, to identify methods for generating sea state criteria for operability. In addition, one-year metocean conditions are estimated; these are relevant for analysis of the limits on operations.

Strongly directional currents in the calculation of structural reliability and response-based design conditions

Abstract In some locations, current contributes measurably to the hydrodynamic force on structures, while its amplitude may have a greater dependence on direction than does wave height. We have extended our random storms method of calculating response-based basign conditions and statistics of extreme structural response in order to resolve the influence of such strongly directional currents on directional load statistics. We have applied the extended method to a southern North Sea location where, at least by North Sea standards, currents are relatively large, though they do not dominate the drag loading on a lattice structure. We have calculated response-based design conditions and investigated the significance of the improved resolution for estimates of structural failure rate under extreme storm loading. The improved resolution of the current direction has no consequence either for this location or for other North Sea locations.