Production offshore is entering deeper waters, not only in the Gulf of Mexico, i.e. Jolliet field in 533 m water depth, but also in the North Sea as shown by fields such as Snorre in 310 m of water. One aspect of such projects which is significant concerns the seabed emplacement of large subsea structures Templates continue to grow in size and complexity In 1987, Placid Oil's Green Canyon FPS template weighing approximately 1200 tonnes was successfully placed in 470 m of water The Jolliet TLWP's one-piece foundation/well template has a weight of about 1400 tonnes and sits in 533 m. The large concrete foundation templates proposed to anchor the Snorre field TLP's tethers each weigh over 4000 tonnes and will be placed in 310 m of water.

In order to meet the demands of such projects, contractors have built new-generation construction vessels Most notable are Micoperi's SSCV ‘7000’ and McDermott's ‘DB102’. Heerema has also upgraded its SSCV Hermod to 9000 tonne capacity and has proposals for a twin-crane vessel, which if built, could lift 30 000 tonnes!

In the area of ship-shape vessels, the largest such unit is McDermott's newly-built DB50 (ex-ITM Challenger) which has a single crane of 4000 tonne capacity. All of these vessels have the ability to lower objects underwater using their cranes. In most cases however, as lowering depth increases, crane capacity decreases. This is due to crane wire rope storage limitations. Thus, large structures as described above may be at or beyond the limits of many vessels with apparently large lift capabilities.

Design of the templates to incorporate buoyancy and thus decrease submerged weight is only a partial solution The added mass of the structure is still substantial. Furthermore, template designs such as skirted-type structures increase loads, particularly during setdown on the seabed The deep water in which such structures are lowered can create further problems due to significant dynamic tensions generated in the lowering system The resonant frequencies of the crane/object mass/spring system can be close to the likely seastates which would be encountered during lowering. These tension variations can be sufficient to cause snatch loadings in the crane at some stage during lowering.

In order to analyse the dynamics of this problem, a three degree of freedom system is developed and solved in the frequency domain. This model accounts for the heave and pitch of the crane vessel and the heave of the lifted load. The primary aim of this approach is to investigate resonant behaviour and the resulting dynamic amplification of hook load which may be encountered. The predictions of this analysis are compared with model tests which show reasonable agreement.

In this chapter, practical-means to prevent problems during an actual installation are suggested and recommendations made for further areas of study.

LOWERING OPTIONS

Depending upon the size of the structure being considered and the water depth, there are several established methods of installation. It is therefore appropriate to discuss these options, describing the advantages and limitations of each.

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