Subsea structures are traditionally transported to the relevant site either on the deck of a crane vessel or a barge, depending on their size and shape. In both cases the subsea structure has to be lifted off from deck and lowered through the splash zone. The lift-off from deck is a critical phase of the operation in which there is imminent danger of large dynamic loads and collision between the structure and vessel due to relative motions. During immersion, significant wave impact forces (slamming) may also occur. Hence, these potential hazards will imply operational limits for the traditional methods of installation for subsea equipment. Another possible way of transporting and installing a subsea structure is to perform a submerged tow through the moonpool of an offshore service vessel. It may be argued that such an operation may have a wider operational window than traditional methods since offshore lifts are eliminated. However transit time during towing may reduce the profitability of this method. This paper outlines the two methods for structure installation, and states typical installation criteria based on model tests and empirical data from full scale-measurements and analysis. Also lessons learned from offshore operations are outlined and discussed.

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