The need for support vessels alongside the platform during construction programmes offshore has focused attention to the problems of mooring and maintaining station in this service. In the past, mooring systems under storm situations have been primarily designed for open ocean applications and have not catered for the requirements of operating and surviving alongside a fixed platform. In addition, anchor handling and dragging of anchors have been the primary cause of damage to submarine pipelines. This problem becomes more acute as the number of vessels around the platform increase and the number of moorings over time becomes significant enough to constitute a hazardous situation.
The Piper field, located in the 58° north sector of the North Sea, experienced these mooring difficulties compounded by poor top soil conditions, which led to the design and installation of a fixed permanent anchor pile mooring system around the platform.
Eight anchor piles are positioned symmetrically around the Piper A platform so that they provide a suitable anchor pattern for a multiple service Construction Vessel to moor up alongside the platform in 'periods of good weather and sway from the platform in periods of bad weather. Each anchor pile is approximately 4000 ft. from the centre of the platform and the connecting chain (which is 1010 ft. long) is lying on the seabed in a straight line between the anchor pile and the platform. The free end of each chain is connected to a pendant line and marker buoy which is used to Faise the free end of the chain to the surface during connecting or disconnecting operation.
The anchor pile system is designed to accommodate construction and tender vessels ranging from medium sized to the third generation semi-submersibles, with operating draft displacements in excess of 40,000 LT. This system allows for vessels to moor on any of the four sides of the platform and maintain adequate clearance of the mooring catenary from the legs of the platform jacket. The vessel anchor line is "shackled up" to the anchor pile making it a semi-permanent connection. These piles are able to take vertical and horizontal components of force, thus allowing for adequate "spring constants" throughout the vessel excursion distance even after introducing uplift forces at the anchoring point.
The mooring system consists of 8 anchor piles encircling the platform. The centre of the anchor circle is the platform centre.
A computer mooring study was conducted to determine the optimum radial distance at which to locate the piles from the platform, to estimate the mooring forces, the shape of the catenary, the forces at the anchor point and the limitations on vessel excursion during maximum fifty year storms. The results of this study were compared to a survey made of the length of chain or wire carried by vessels of potential interest to be used on this system.
