Improving Underbuoy Hose System Design Using Relaxed Storm Design Criteria

ABSTRACT A comparative study of two underbuoy hose systems designed for the same location, one according to the expected 10 year extremes, the other to 100 year extremes has been made. Improvements in system performance have been identified for the system designed to less stringent criteria. A comparative fatigue analysis upon full size hoses has been attempted using information derived from model tests of the two systems. INTRODUCTION Offshore mooring terminals for tanker/sea-bed crude oil transfer have now been in operation for over two decades. Design criteria for such installations are based upon observed or statistically determined environmental data such as the maximum wave, wind, current, mooring force etc. expected in, say, 100 years. Model tests and site measurements can be used to support the theoretical calculation of structural response to the environmental parameters specified and hence ensure that the mooring can withstand-all predicted loadings within the limits of the 100 year design criteria. A Widely used mooring arrangement is the Single Point Mooring (S.P.M.). The most common type of S.P.M. consists of a buoy floating on the sea surface and held in position by anchors and piles. The tanker is moored to the buoy at the bow with the oil flowing between sea-bed pipeline and buoy via flexible submarine hoses, and between buoy and tanker via a floating hose line (see Figure 1). The flexible hose link between sea-bed and buoy is designed to fit the extremes of buoy movement relative to the Pipe Line End Manifold (P.L.E.M.) on the sea-bed as dictated by the environmental design parameters. The hose system designer can determine the length of hose required to fit between the seabed and buoy manifolds such that there is sufficient when the buoy is at its highest point and maximum excursion expected. Having pre-determined the system length it then remains to optimise the system for all conditions. It is often the case that the 50 or 100 year storm design criteria are used to design the hose system as well as the mooring structure. The purpose of the study described in this paper is to show that, in some cases, by using a hose system designed to less exacting criteria than the 100 year extreme it can be made to perform better and last longer. The consequent savings in equipment cost and improvement in downtime could outweigh any possible individual losses due to ailures caused by environmental conditions occurring outside those specified by the relaxed design criteria for the hose system. STUDY OUTLINE The following approach was used in an attempt to quantify the work and provide specific results to reinforce the necessarily hypothetical nature of the concept. Decide upon a site location from which to draw the design parameters and assume some realistic but theoretical buoy arrangement based upon a known buoy either used in or planned for that area. Derive design criteria relating to the 10 year and 100 year environmental extremes, and optimise the underbuoy hose systems best suiting the two sets of design criteria.