The continued development of offshore installation equipment has allowed new methods and techniques to be employed for offshore installation. This is true for both subsea developments and conventional platforms, and has enabled large-scale underwater construction to be carried out more efficiently. This chapter looks at the development trends of offshore construction vessels and their effect on subsea installations, and expands on some of the areas where heavy-lift vessels have an important role to play.
When the search for hydrocarbon resources first moved offshore, the floating construction equipment necessary for installation consisted mainly of flat-bottomed barges with relatively small cranes This was all that was required for the initial development areas of the US Gulf, and similar mild environs When more-hostile areas, such as the southern and central North Sea, were developed ship-shaped vessels of larger crane capacity proved far more capable and quickly emerged as the minimum requirement for working at such locations Crane vessel lift capacity soon increased from less than 800 tons* to over 2000 tons, as it became obvious that the bigger vessels with greater capability were the most cost-effective method of installation.
However, when very hostile waters such as the northern North Sea were encountered even the more sophisticated monohull crane vessels found it took a whole season to complete a major installation As operators could not afford these project delays, the industry started looking for ways of reducing installation tunes, extending workable weather windows and enlarging the scale of offshore operations. The semi-submersible hull form, long known as one of the most stable platforms from which to operate in hostile environments, combined with dynamic ballasting systems, proved to be ideal for heavy-crane lifting operations in harsh conditions.
Workability unproved dramatically, giving the possibility for the majority of offshore tasks to be undertaken year-round (Fig 1) and allowed more-efficient subsea operations to be performed, especially when remote or diverless operations were employed (Fig 1 is available in full paper)
Individual offshore crane capacities have increased tenfold in the past 15 years, with total vessel lifting capacities growing from 700 tonnes on monohull ships to 14 000 tonnes on SSCVs during this period (Fig 2)
Whilst this increase in capacity was aimed mainly at jacket and module installations, allied with it was the ability to maintain station accurately, to provide a very stable platform for offshore operations and to operate a large range of equipment designed specifically for deep underwater operations such as large-capacity subsea hoisting blocks, high-energy hydraulic underwater hammers and deep-dive systems These have proved invaluable for a number of underwater constructions, and most subsea templates and manifolds over 300 tons in weight have been installed with heavy-lift vessels (HLVs).
A certain amount of subsea construction has been associated with most field developments and has usually involved HLV operations This ranges from the installation of subsea items associated with conventional platforms, such as pipeline tie-ms, to construction of complete subsea production systems
