An Energy Basis for the Design of Open-Ocean Single-Point Moorings
Authors: Maddox Notley Roger 
Published: October 2008

The use of tankers or large barges for permanent crude oil storage at offshore oilfield locations in deepwater, open ocean environments will require the design of single-point moorings for conditions substantially more severe than existing single-point mooring installations. Three types of single-point moorings, conventional catenary-moored SPM buoys, the newly developed single anchor-leg mooring and turret mooring, have been model tested with directional irregular waves simulating limited fetch, open ocean sea conditions. Model tests simulated loaded and ballasted tankers in the range of 28-190 Mdwt, water depths up to 600 feet, and significant wave heights up to 40 feet. A correlation of the potential energy stored by the mooring when the vessel was offset by wave action suggests that this energy is primarily a function of significant wave height and loaded storage vessel displacement and is independent of water depth and the type of single-point mooring system used. A procedure based on the energy correlation has been developed for the design of open-ocean, all-weather single-point moorings.

Introduction

As oilfields are found in more remote locations where pipelines are not technologically or economically feasible, either due to water depths or distance to the nearest shore facility, a need is generated for an alternative system for the shipment of crude oil to market. One such alternative is floating storage, which has been used with increasing frequency in mild-sea areas. Floating storage uses a vessel converted for permanent or semi-permanent mooring in the field location. Some floating storage applications use spread moorings, which have a fixed orientation. In mild environments, spreadmooring may be satisfactory; however, when storms arise, a spread-moored vessel may be subjected to quartering or beam seas. Under these conditions a large surface area is exposed to the wave action. In rough seas this leads to prohibitively high forces in the mooring and excessive vessel roll. Single-point moorings, which have been developed for use in the shallow-water, mild-sea conditions typical of crude oil terminalling operations, offer an alternative to spread mooring. A single-point mooring (SPM) is any mooring system that permits the vessel to weathervane into the prevailing seas either naturally by the actions of wind, waves, and current, or with assistance from lateral thrusters at the stern and/or bow of the vessel. The ability to adjust vessel orientation to head into storm waves, regardless of their direction, can substantially reduce mooring loads in open-ocean installations. In the most extreme open-ocean conditions, high winds, currents, and waves may put water over the deck of a fully loaded tanker. Use of single-point moorings in the open sea requires an extension of the technology developed for applications in more benign conditions.

The most common single-point mooring, called a catenary SPM, has been used in a number of ports for terminalling facilities. 1 A schematic of the system is given in Figure 1. It consists of a surface buoy moored to the ocean floor by a number of symmetrically spread chains, a riser and pipeline for crude transfer from a shore facility to the buoy.