Present day offshore lift operations feature the lifting of substantial loads horn a transport barge by means of large capacity semi submersible crane vessels with stem mounted dual cranes. During such operations the transport barge is moored perpendicular to the stem of the crane vessel. The motional behaviour of the crane vessel and transport barge are affected by hydro-mechanical coupling effects arising either from the hydrodynamic interaction between the two nearby vessels, or from mechanical coupling via the cranes, hoisting tie and slings. In order to investigate the hydrodynamic interaction effects a two-body diffraction analysis has been performed for a crane vessel and a nearby transport barge. The coupled equations of motion have been solved to establish the absolute and relative importance of the hydro-mechanical coupling between the two vessels. Whilst the crane vessel responses are hardly affected by hydrodynamic interaction, the transport barge motions may be significantly altered. It is demonstrated, however, that the hydrodynamic interaction effects are an order of magnitude smaller than the mechanical coupling effects and may be ignored for practical purposes. INTRODUCTION Nowadays it is generally accepted that the application of large capacity crane vessels may result in significant cost savings in offshore installation work, e.g. by adopting integrated topsides or lift able jackets 1. Recent years have seen the advent of two giant Semi Submersible Crane Vessels (SSCV?S) with twin revolving cranes mounted at the stem, the SSCV "DB102" (2×6,000 t maximum capacity) and the SSCV "M7000" (2×7,000 t). Large scale offshore lift installations feature the lifting of loads of up to 10,000 t from a transport barge by means of an SSCV. The loaded transport barge is usually moored perpendicular to the stem of the SSCV at a separation distance of only a few meters. During such operations the crane vessel and transport barge are subjected to wave induced motions, while their motional behaviour is also influenced by mechanical coupling via the cranes, hoisting wires and slings. The resulting complex dynamic behaviour of the hydro-mechanically coupled crane vessel and transport barge has been the subject of several investigations 2 which have culminated, amongst others, in the development of the LIFSIM time simulation program 3. As part of this effort a study has been performed into the hydro-mechanically coupled motions of a crane vessel and a transport barge. Common engineering practice in computational lift dynamics is to ignore hydrodynamic interaction effects. It is rc3CO@Sd, however, that the hydrodynamic interaction effects between two nearby vessels can only be analysed properly by means of a complicated two-body diffraction analysis, a major and expensive task 4. However, evidence from model tests suggests that the hydrodynamic interaction effects are at least an order of magnitude smaller than the mechanical coupling effects via the cranes, hoisting wires and slings, indicating that, for most practical applications, the hydrodynamic interaction effects can be ignored.

The course stability characteristics of two full tanker afterbodies with different rudder–propeller–skeg configurations are experimentally determined. Natural course stability characteristics are improved by adding lateral force generating devices in the afterbody.

Tanker based Floating Production, Storage and Offloading Systems (FPSO's) are moving into deeper water in the near future. 'The use of turret moored tankers for deep water applications is widely accepted to be a viable and cost effective solution. This paper will discuss the advantages of an unique asymmetric mooring system in combination with a hybrid riser system for a turret moored FPSO in water depths of 1000 m and beyond. A case study for such a mooring-riser system for a typical medium size turret moored tanker in 1000 m water depth in Gulf of Mexico environment will be discussed. 1 INTRODUCTION The use of flexible risers from the floater to the sea floor in deep water requires exceptional flexible pipe design to withstand external hydrostatic pressures and large top tensions and is therefore expected to be very expensive. Additionally, flexibles of the sizes anticipated for export lines are at the current limits of manufacturing capabilities. Moreover, flexible risers can only be installed after the FPSO is hooked-up to its anchor lines and attract therefore a penalty in terms of capital interest due to the FPSO not being able to receive well fluids. An alternative solution is the hybrid riser system (See Figures 1.1 and 1.2). This system basically consist of a large Tethered Riser Buoy (TRB) from which Steel Catenary Risers (SCR's) are suspended to the sea floor and flexible jumper hoses between the riser buoy and the turret of the FPSO. The TRB may be some 150 m below the sea level and is tethered to the seabed. The entire hybrid riser system consist of elements which are proven technology. Large diameter SCR's have already been installed as oil and gas export risers from TLP's in the Gulf of Mexico. The use of flexible risers connected to a tethered buoy in combination with turret moored tankers is also proven technology. It is cost effective to design the riser buoy so that it may accommodate as many risers as possible. Depending on the size of the risers and umbilicals, the riser buoy may be designed to accommodate up to approximately 15 risers and/or umbilicals. In case of that many risers, the horizontal subsea buoy may be long and may therefore require large free space sectors between the mooring lines to avoid riser-mooring interference. To alleviate interference, Bluewater has developed a mooring system with three groups of mooring lines in 120 degrees sectors (refer Figure 1.2). The large anchorline-free sectors in this mooring system allow easy access to the hybrid riser system for installation, maintenance and repair during the field life. In a field lay-out, the asymmetric three-bundle mooring system footprint should now not be expressed as an anchor -point radius but as anchor-point directions. Hence, in congested areas, where the anchor radius is normally kept limited, the entire three bundle mooring system may be rotated to optimise field lay out (see Figure 1.3). The hybrid riser system allows a flexible installation schedule. The Tethered Riser Buoy and Steel Catenary Risers can be installed prior to the arrival of the FPSO in the field.