Monohull vessels make up by far the most wdely used hull form on the waters of the world, and not least m the field of subsea construction The workhorses of the offshore industry, the monohulls, have been designed, modified and often converted to facilitate a multitude of tasks.
It is acknowledged that the semi-submersible hull form is playing an increasing role in subsea construction, but it is the numerically superior and often more versatile monohulls upon which so much still depends. It is not the purpose of this presentation to analyse comparisons, but rather to illustrate the abilities of monohull vessels in subsea construction
Subsea construction vessels tend to fall into two categories The first are those which are designed and constructed to perform a task or tasks, with diving very much a secondary capability. The second category consists of vessels that have a major diving capability but are also able to carry out a number of other tasks.
Vessels forming the former group are the crane barges, lay barges and general-purpose construction vessels, for example, Reel Ship Apache, and the newly built DB Challenger The latter group are the diving support vessels, for example Arctic Seal and newly built Seawell One
Both categories must be able to perform to meet high specifications and the following considerations must be made during design and budding
dimensions,
power;
positioning systems,
subsea construction facilities.
Length, beam and draft make up the vital statistics which dictate the actual size of a vessel However, it is the way in which these measurements are arranged that is important.
The monohull must be full enough in the beam to provide a stable and sea-kindly work-platform, but must be an efficient profile to enable good transit speeds to be achieved and economically maintained. The draft of a vessel must be considered, not only as a component of stability but also in order to keep a wide choice of ports and harbours available, as one of a monohulls strengths is its ability to go alongside a jetty or quay for load-out and mobilization.
When considering a vessel for specific workscopes, size plays an important part in the selection process For the most part, larger vessels will be utilized for the heavier tasks and when large numbers of personnel are required. This does not by any means preclude the smaller vessels, as they come into their own for lightweight tasks and where access may be restricted Often a large vessel may carry out a single part of a project and a smaller vessel may then take its place to complete the work.
Deck and working areas are proportional to the overall dimensions of a vessel and the accommodation requirement. The scope of work will dictate how much deck is required, and therefore vessel selection. The loading capability of a deck must also be considered, as specialist construction equipment may cause problems requiring the fabrication of additional grillage and sea fastenings.
