The selection of support vessels for offshore operations is often based on experience from similar operations in regions with similar environmental and geographical conditions. In areas with a lack of operational experience and in areas with special challenges it is important to evaluate environmental, geographical and operational challenges to establish a basis for selection of support vessels and design criteria for new vessels. In addition to information about the physical environment, it is important to have knowledge about the expected operational profile of the vessel in order to decide on values such as speed, operational sea state, loading capacity and more. The vessel’s day rate is often used as a selection criterion by major operators. In areas dominated by harsh environmental conditions and remoteness, efficiency becomes an important factor, and a dayrate comparison will in many cases not provide correct information about the optimal design. This article suggests some criteria that can be used in the process of selecting vessels to take part in offshore operations in harsh environments.
The Physical Environment
The physical environment differs significantly from region to region. Table 1 shows one-year extreme significant wave heights (Hs) for a selection of basins with offshore oil and gas activity. The design conditions vary significantly from region to region, with the harshest wave conditions being present in the areas situated around the North Atlantic. From an operational point of view, the maximum sea state is perhaps not so interesting, but it gives an indication of the areas in which the roughest wave conditions can be expected. Figure 1 shows the wave statistics presented as fraction of time (vertical scale) for which the Hs is less than the value shown on the horizontal scale for some selected offshore locations and for definition of the Det Norske Veritas (DNV) environmental regulatory numbers (ERNs).1 For the Campos Basin offshore Brazil, the Hs is <2m 90% of the time, whereas at the other locations presented in Figure 1 the sea state is <2m approximately 40–50% of the time, which is a significant difference. It is apparent that the desired maximum operational sea state varies from region to region, and thus vessel and equipment solutions and best practices also vary. The maximum operational sea state will have an impact on the design and cost of vessels and equipment, as well as on operational efficiency. Figure 2 shows that seasonal variations at a typical North Sea field can be significant. It may be very challenging to carry out operations at certain times of the year, and the core installation season may in some regions be limited to six to eight months of operation.
Waves are not the only weather phenomenon challenging offshore operations. In Arctic and sub-Arctic seas, ice features may be present to a varying degree throughout the year. Surface ice poses very special demands on vessels and equipment operating in the area. Winterisation for operations in low-temperature environments, including measures for de-icing, may also be an important aspect in the design of vessels and equipment, and ice class strengthening and winterisation may increase the vessel’s capital costs significantly. Some parts of the world are also subject to special weather phenomena such as cyclones, hurricanes and polar low atmospheric pressures, all of which will cause operations to halt and may need to be given attention in the selection of designs/methods.
Location
The development rate in regions defined as ‘remote areas’ is often low. It is hard – if not impossible – to optimise vessels for offshore operations in some less-developed areas, simply because the amount of work may be insufficient to defend a large asset investment. On the other hand, some regions viewed as remote areas contain significant amounts of planned and potential work, such as the offshore African West Coast. Some regions and certain operations involve significant amounts of transit time. The asset solution, in particular the ideal transit speed and ideal loading capacity, varies with the remoteness, transit distance and amount of local infrastructure. For operations involving large amounts of transit and transportation time, the efficiency of the transportation solution is increasingly important. In general, for remote locations faster and larger vessels will be favoured. Also, the ability to operate without needing a large number of support vessels and support functions may be an advantage in some remote areas due to the low availability of support vessels (no spot market). The quality of weather forecasts is generally poorer for remote areas due to the lack of weather observations and stations. This implies that more safety factors will need to be added to the design sea state to account for uncertainty in the weather forecasts. Alternatively, wave radars or buoys and even weather radars and a meteorologist on watch can to be included in the operation spread to increase the quality of the weather forecasts.
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