Operational Limitations of Offshore Crane Vessels

ABSTRACT The paper analyzes the limiting conditions of crane vessel operations with special emphasis on large semisubmersibles. As reference structures a barge with 500 t crane capacity, a ship with 1,800 t capacity and a semisubmersible with two revolving 6,000 t cranes, are investigated. Model tests in irregular seas and specific wave groups demonstrate the significance of coupled resonance motions of vessel and suspended load for crane operations with semisubmersibles. 'With barges and ships resonance is of minor in1.portance due to high damping. The mass of the suspended load and the sling length are decisive parameters for the motion characteristics of crane vessels. The transfer function of the vertical load motion, which is selected as limiting quantity, has been determined experimentally for each of the three vessels. Depending on this parameter and the maximum allowable load motion the limiting wave heights for crane vessel operations are compiled. INTRODUCTION Offshore oil production in deeper and more hostile locations of the North Sea will require an investment of $15 to $20 for each barrel of future oil output. Consequently, the crude oil price collapse in 1986 - taking the value of a barrel of Brent oil down to $10 - stopped most industrial developments. The challenge, however, also provoked new ideas and concepts like floating production systems, compliant structures and light-weight platforms. A major incentive for economic field developments was the advent of giant crane vessels. With maximum load capacities of up to 14,000 tollS complete jackets, topsides, foundations, templates etc. can bc installed or removed, mostly by tandem crane operations. Installation costs are reduced up to 40 percent as the saving on steel for "slimline" jackets amounts to 20 perccnt in relation to the conventional launching method. Even greater stec1 weight savings - up to 40 percent - are being made by constructing an integrated deck rather than building a series of modules in individual shipyards [1-8]. further, offshore work requirements - costing five times more than for the same work onshore - are reduced considerably, testing and commissioning of platform decks is accomplished onshore and weather window limitations are less significant. Based on data of 2nd, 3rd and 4th-Generation North Sea platforms Vugts [9] estimates substantial cost savings by favouring an installation by crane vessc1 above traditional launching (Fig. 1). Fig. 2 presents some data of semisubmersible crane vessels (SSCV), ships or barges [10]. N'ote the dramatic increase of lifting capacity. Amazingly just the load sensitive scmisubmersibles made thc running. Of course, thcir excellent motion behaviour in the seaway is a major advantage and sheer size with options for two cranes of large load capacity, outreach and lifting height combined with a flexible ballasting system outmatches other concepts in delicate offshore lifting operations.