Supply Vessel Stern Mooring - A Passive System

ABSTRACT A Passive Stern Mooring System has been developed to allow supply boats and auxiliary vessels to remain in position adjacent to rigs and platforms during periods of inclement weather in order to facilitate the transfer of materials and supplies. The system uses double braided nylon ropes to absorb the mooring loads and provide the necessary restoring force to keep a vessel on station, thereby eliminating complex mechanical components and external power sources. The paper describes the mooring arrangement and system operation, provides data pertaining to the elastic characteristics of double braided nylon rope and defines the procedures used to design each mooring system. INTRODUCTION Supply boats play a continuing and vital role in the development of offshore oil and gas resources. Drilling and production rigs depend upon these vessels for the materials and equipment needed to carry out operations. In response to the increasing demands placed upon the supply boat industry, new and more efficient vessels are being built and put into service. Yet, though the supply boats themselves have become more sophisticated, the actual cargo transfer operation between boat and rig has seen only modest improvements throughout the years. A typical replenishment activity requires a supply boat to try and maintain a position. somewhere within the operational radius of a platform or rig crane while the cargo is being transferred. Since the vessel is unrestrained and free to move about in response to the prevailing winds and seas, the vessel must be maneuvered accordingly to counteract this motion. This effort can be simplified through the use of dynamic positioning equipment, but many vessels are not equipped with these units. In relatively favorable conditions this procedure is satisfactory. However, in marginal weather extreme caution is needed when lifting cargo from the boat. Sudden vessel motion can cause a load to swing abruptly, resulting in possible injury to the deck crew and/or damage to the vessel. In worsening conditions, continued operations may prove impossible due to excessive vessel movement, resulting in costly delays. As the search for oil moves farther offshore weather induced delays are likely to increase in frequency and duration. Continuation of cargo transfer operations in adverse winds and seas is contingent upon reducing the movement of the supply boat to provide a stable work platform. This in turn necessitates the use of some type of mooring arrangement. An ideal mooring system would be one that holds the supply boat in a fixed predetermined location while absorbing all mooring loads and effectively eliminating any movement. In practice, however, this is not feasible for a number of reasons. A more realistic approach is to define the acceptable general area of location for the supply boat relative to the platform and design the necessary mooring configuration to keep the vessel within this area while mitigating the effects of seas and swells.