Still-water level and currents are important parameters for the design and operation of offshore structures (Marty and Eggar, 1983; Prior-Jones, 1983). Still-water level (swl), due to tide and surge, is required for water depth and deck clearance ('air gap') studies, and it also determines the level of attack of waves on the structure. Currents contribute to the forces on the structure and may be as important as waves in coastal regions where wave heights tend to decrease (due to depth-limiting) and tidal currents are strong.

In this paper we consider the methods that are available for estimating extremes of these parameters offshore. Ideally, methods should be applied to long observational series, but there are very few North Sea sites for which good-quality long-period data are available. Therefore methods are described which are based on model simulations of tide and surge, combined with statistical analyses of onshore observations.

We have compared estimates obtained from the model with values derived directly from available observations. In particular, we have used 16 moths of sea-level data from the northern North Sea, nine years of sea level and 346 days of current data from the southern North Sea (all collected by IOS), and 714 days of current data from the central North Sea, collected by MAFF. The northern, central and southern sites are referred to as A, B and C respectively. We had intended to use five years of current data from a site in the northern North Sea but, disappointingly, they proved unsuitable for analysis because, unlike the other data sets, several different organizations were responsible for the data collection and analysis and consequently quality control has been poor (IOS Marine Information and Advisory Service, personal communication).

The model used here is the IOS continental shelf model (CSM) and it has been developed at IOS over a number of years. It was used for operational storm-surge prediction at the Meteorological Office from 1978 until replaced by an upgraded version in 1982. It is a two-dimensional model covering the whole of the European continental shelf and is based on non-linear depth-averaged hydrodynamical equations, which are solved numerically to give surface elevation and north and east components of the depth mean current (Davies and Flather, 1978; Flather, 1981; Proctor and Flather, 1983).

The paper is structured so that methods of estimating tidal, surge and total motion are presented in turn.

Tidal Motion
Basic Theory and Definitions

Tides are generated by the gravitational attraction of the moon and the sun the tidal component of the observed water level is that part coherent with the astronomical forcing. Extraction of the tide from observed levels requires a technique of analysis. The harmonic method of analysis models the astronomical tidal level as a finite number, N, of sinusoidal motions (harmonic constituents) with amplitude Hn and angular speed sn? where Vn is the initial phase at an arbitrary time origin t = O and gn is the constituent's phase lag with respect to the equilibrium tide and the Greenwich meridian.

(Formula available in full paper)

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