Na Kika - Deepwater Mooring and Host Installation
Authors: Paton A.K. , Smith J.D. , Newlin J.A. , Wong L.S. , Piter E.S. , van Beek C. 
Published: October 2008

The Na Kika host is moored in a water depth of approximately 1936m (6350ft) and is the deepest permanently moored floating production facility in the world. The engineering and procurement of the mooring and the installation of the mooring and host involved a focused effort over 3 ½ years. The key challenges and learnings are described in this paper.

Introduction

Engineering and planning for the mooring and installation of the Na Kika platform commenced in earnest at the beginning of 2000, shortly after the selection of the semi-submersible as the host concept, and at the time that the overall arrangement and sizing of the hull was being finalized. The execution approach adopted for the mooring was equivalent to that applied for the other parts of the project and was modeled upon that applied successfully by Shell on the recent Gulf of Mexico TLP projects. In this the Company was strongly involved in the design, and in managing a set of discrete contracts for procurement and construction with Shell also providing overall project management and interfacing between the contracts. Unlike the other parts of the project, Shell had not implemented a mooring system of this type since the Auger TLP's lateral mooring system in the early 1990's and experience in this was lacking in the Company, so a number of experienced consultants were brought in to work alongside the Shell engineers to form an in-house team to perform the work.

The first phase of the work involved firming up the mooring budget and work plans, and setting the basic configuration of the mooring system in parallel with fixing the main dimensions and displacement of the hull. Particular attention was given to installation methods for the mooring at this early stage to ensure that the selected configuration was compatible with available installation equipment. This phase of the work was concluded around mid-2000 at which point the overall project budget was established and the commitment to proceed with the project was given.

Mooring System Selection

Mooring system optimization studies evaluated alternative arrangements using polyester and steel mooring lines, and considered buoyed and un-buoyed systems with 12 and 16 mooring lines. For this application, polyester lines did not offer an overall cost advantage, nor did the use of submerged buoys in the mooring lines. Accordingly a semi-taut system with steel spiral strand wire lines was selected. Each line included a length of ground chain from above the mud line touchdown point to the anchor piles, and a short length of platform chain for the top connection to the platform (Figure 1). A twelve line mooring would have required wire and chain sizes at the limits of proven production capabilities and did not offer significant cost advantage over a sixteen line system which was selected. The system was configured with four closely spaced lines at each corner of the hull with space between the mooring groups to allow free access for riser and umbilical hook up to the hull pontoons. The field layout is such that there are more risers on the north of the platform so the south side mooring lines were made longer to balance the larger riser load to the north.