Determination of Mooring System Design Data - Offshore British Columbia

Abstract

Abstract An outline is presented of basic data required in the design of a mooring system for a floating drilling vessel. For offshore British Columbia, field measurements to determine the range of soil conditions, anchor performance in the various types of soil and current velocities were required to supplement the available data. Planning techniques, equipment used and procedures followed in accumulating this information are described to serve as possible guide lines for others entering new offshore areas where similar information is needed. Within the area of interest, three general classifications of soils were present:coarse gravels with clay fill,fine sands, andvery hard silts and clays with shear strengths up to 47,000 lb/sq ft immediately below the mud line. A mooring problem was not expected in the gravels and fine sands; however, applicable data on the performance of anchors in soils with shear strengths greater than 200 lb/sq ft was not available. It was necessary to establish the performance of anchors in these soils; otherwise, anchor piles would have to be used. Anchor tests proved conclusively that lightweight-type anchors in the range of weights tested could develop satisfactory holding power in all types of soil encountered. A brief summary of the meterological and oceanographic conditions off the coast of British Columbia and examples of tabular and graphic methods of presenting maximum and average conditions are included in the Appendix. Introduction Prior to the actual design of a mooring system for a floating drilling vessel, considerable data pertaining to the local environmental conditions must be established. The most significant factors may be listed as follows.Meteorological: magnitude, direction, frequency of occurrence and duration of storm winds.Oceanographic:range of water depths,normal seasonal tidal fluctuations and magnitude and frequency of occurrence of maximum storm tides,magnitude, direction, period, frequency of occurrence and duration of storm waves, andnormal and maximum storm fluctuations of current velocity and direction (for deep draft semi-submersible vessels, information on the current velocity variation with water depth should also be obtained).Miscellaneous:variations in the ocean floor soil conditions and related anchor performance must be considered; andother factors such as the proximity of dangerous rocks or reefs, floating hazards such as icebergs and the expected advance warning of severe storms should be considered in the determination of the design safety factor and the speed in which the system can be set and retrieved. In addition to these environmental factors, information on vessel reactions to wind and current at various approach angles is necessary to determine the strength of the required system. To determine flexibility of the mooring lines, vessel heave, surge, pitch, roll and yaw responses to waves of different periods, heights and approach angles must be known. These vessel characteristics are normally obtained by model tests. Regarding the specific area of interest off the coast of British Columbia, a preliminary investigation into the availability of mooring system design data showed that sufficient information was available on winds, tides and waves (Appendix). Water depths had been recorded and some soil samples had been obtained during previous seismic work. Vessel characteristic data for the Sedco 135 vessel were available from model tests.1 A review of the available information showed that individual anchor holding capacities in the order of 300,000 lb would be required to maintain the vessel on location during the maximum anticipated storm. Although a 30,000- or 40,000-lb U.S. Navy Light Weight Type (LWT) anchor is capable of developing the required holding capacity, the shear strength of the soil must be low enough to permit penetration of the anchor flukes, yet high enough to provide sufficient resistance to anchor drag.