This paper discusses the design criteria for determining the freeboard height of artificial islands against wave run-up and overtopping. A method based on the overtopped volume of water is proposed.
Various types of artificial islands have been used recently in arctic waters for oil and gas exploration purposes because of their economic attractiveness and their ability to resist ice forces. In some regions, ice loads govern the island design freeboard height. In other areas, the design criteria for wave run-up and overtopping can be significant in the island crest height determination.
The existing methods for estimating the wave run-up and overtopping are based on experimental data for coastal structures such as seawalls and breakwaters. This paper describes a method to estimate the wave run-up and overtopping on the leeside as well as the exposed side of circular shaped artificial islands. The wave run-up and overtopping is estimated using the proposed method for three islands in water depths of 20, 40, and 60 ft at hypothetical locations in the Beaufort Sea. Both sloped and vertical sided islands were examined.
Most of the existing designs for wave run-up have been based on the criteria suggested for breakwaters and seawalls for which the structural stability is the main concern. This paper suggests the designer assesses the consequences of wave overtopping; i. e., the effect of overtopped water on the facilities and operations on the island, and the drainage requirement on the island top. Excessive wave run-up and overtopping can cause serious operational difficulties and damage to drilling or production facilities on artificial islands.
Artificial islands have been used extensively in the Beaufort Sea as oil and gas exploration platforms. Though the Beaufort Sea is covered by ice most of the time, severe storms have been recorded during the brief open water period. The effects of waves during storms on artificial islands are important considerations in the island design. The artificial islands used so far include the following types.
This island type generally has a very flat side slope, on the order of 1 vertical to 15 horizontal (1:15), and a wide beach to dissipate the wave energy as waves break on the slope and the beach. Therefore, the freeboard of a sacrificial beach island need not be very high to protect the island top surface from wave overtopping. For instance, Issungnak Island which was constructed in about 60 ft of water depth in the Canadian Beaufort Sea has a freeboard height of 15 ft.
In many instances the fill material is not readily available near the island site to permit the construction of a sacrificial beach island. Transporting the fill from a distant source is both costly and time consuming. There is therefore a need to minimize the quantity of fill for island construction, and steep sloped gravel islands have been used extensively in the Beaufort Sea wherever the foundation soil conditions permit. The steepest and most commonly constructed side slope is about 1:3.
