This paper illustrates the significance of lift dynamic aspects observed during two major offshore heavy lift operations performed in 1991. Extensive offshore measurements provided further knowledge related to the dynamic behaviour of heavy lift systems offshore. The contribution of lift dynamics to the overall response in the medium frequency range was found to be of similar magnitude as the response in the wave frequency range. Initial correlation studies with computer models show that this aspect was underestimated by the analyses. INTRODUCTION The large semi submersible crane vessels (SSCVS), currently active on the heavy lift market, have been specifically designed and equipped to perform very heavy lift operations at sea, even in rather severe environmental conditions. The lift records of the SSCVS show clearly the trend of increasing lift weight versus time [1,2]. This fact is not surprising as the market takes full advantage of the available lifting capacity. Simultaneously with the lift weight, the dimensions of the lifted structures have also increased significantly, resulting in minimal clearances between the load and the crane vessel. For loads which are relatively light when compared with the displacement of the crane vessel, the wave induced motion behaviour of the SSCV is almost independent of the motion behaviour of the load suspended from the cranes, Furthermore, the relative motions of the load with respect to the crane vessel, thus also the clearance may be controlled to some extent by means of control lines operated from tug winches. For relatively heavy loads, however, the wave induced motion behaviour of the SSCV can be strongly affected by the motion behaviour of the load, and vice versa [23]. The dynamic motion behaviour of both the crane vessel and the load, as well as of a cargo barge in case it is involved in the lifting operations, are in fact coupled. The crane vessel, the load and the cargo barge together form one integrated dynamic system. Considering the phenomenon described above, it becomes evident that lift dynamics play an important role in the feasibility and workability of a lift operation involving a relatively heavy load, Workability restrictions related to motions of the crane vessel and the load are therefore dependent upon the effects of lift dynamics. Furthermore, the clearance between load and crane vessel or the tension variation in the hoisting wires or the slings, can become limiting factors of a lift operation also due to the phenomenon lift dynamics. Maintaining the minimal clearance between crane vessel and lifted load throughout the lift operation has become of vital importance with respect to the overall safety. In 1985, The Netherlands, Shell Imitational Petroleum Maatschappij (SJ.PM), The Hague, and the Maritime Research Institute Netherlands (MARIN), Wageningen, commenced research work with respect to lift dynamics, which was focused on the development of a lift simulation computer program (LIFSIM) able to calculate the dynamic motion behaviour of crane vessel, load and cargo barge during the lift operation [4,5,6]. Extensive model tests in MARIN?s sea keeping basins were carried out, additionally sponsored by Heerema Engineering Service BV, Leiden, for verification and validation of the LIFSDVI program.
During the second quarter of 2002 Heerema Marine Contractors (HMC) installed the Horn Mountain Dry Tree Spar facility [ref.1], using its recently enhanced Deepwater Construction Vessel (DCV) Balder. This paper describes the most important issues of this marine installation project, including methodologies specifically developed to suit the capabilities of DCV Balder. Such features include the Worlds largest diameter Mooring Line Deployment winch and the dual crane lifting of the Spar Deck. Introduction In the late nineties of the last century major oil companies, driven by portfolio considerations and the depletion of their easily accessible reservoirs, extended their exploration activities more and more into deeper water offshore areas. As a specialized marine contractor, in order to adapt to these and future needs of operators, HMC's strategy to approach this market was based on:To remain a "Marine Contractor providing a range of solutions" with extended capability to offer the full range of its customer's future deep water needs.Maximizing and leveraging HMC's core expertise, assets and corporate values.Maintaining the highest priority on robust and safe offshore methodologies. As a result of this analysis and after extensive discussions with oil companies, the decision was taken to convert the Semi Submersible Crane Vessel (SSCV) Balder into the Deepwater Construction Vessel (DCV) Balder. Features of the DCV Balder such as the Dynamic Positioning (DP) system, the 1050mT capacity J-lay system and the world's largest Mooring Line Deployment winch, the Crane traction deepwater lowering winches in combination with its existing large heavy lifting capacity results in the following main benefits:The ability to use one vessel to execute all marine installation works for a complete Deepwater Field Architecture, thereby eliminating interface risks and multiple mobilization charges.The adaptability to change mode of operations to execute combined scopes of work such as pipelay, mooring line deployment, heavy dual crane lifting and subsea installation. DCV Balder is presently committed for the BP Southern Green Canyon/Mississippi Canyon Deepwater development in the Gulf of Mexico. Recently completed projects include the pre-lay of the Nakika FPS Moorings and installation of the BP Horn Mountain Spar. The use of DCV Balder enhanced capabilities, the revised working practices adopted by HMC and HMC's Quality, Environment, Safety and Health (QESH) Management System applied on BP Horn Mountain project will be described in this paper. Dcv Balder Capabilities DCV Balder (Fig.1) was built as Semi-Submersible Crane Vessel (SSCV) in 1978 and underwent an extensive life time extension and conversion program in 2001 [ref.2, 3, and 4]. The characteristics of the vessel are shown in table 1. Of prime importance for deepwater installation is the Dynamic Positioning (DP) capability. The DCV Balder was equipped in 2001 with seven new 3500 kW thrusters resulting in a total thrust capability of 350mT, allowing the vessel to keep position in more than 40 knot winds in DP Class 3 condition and to hold large horizontal pipelay tensions.
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