Moorings for the Offshore Oil Industry
Authors: Atwood J.H. , Graham J.R. 
Published: December 2007

To turn a phrase around, moorings for the offshore oil industry are at the end of the beginning.

Theoretical and practical experience gained in mooring tankers, crane barges, pipeline spreads and drilling vessels has brought us to a point where mooring systems have a reasonable degree of reliability. Loads imposed by winds, currents and waves are understood and generally well handled by offshore companies. However, there is slightly less knowledge about vessel motions and movements. Wave motion on top of imposed loads creates dynamic loading in the mooring system that needy immediate technical attention. With the dynamic problem solved, the second generation of mooring can begin. The industry now must solve the problems of anchor design and holding power, soil strength, pile anchor economics, pendent vibration, fatigue corrosion, and physical wear of wire, chain and fittings. For conventional mooring methods, the problem of vessel rotation must be solved. The satisfactory solution to these problems will be necessary for the continued, confident investment of capital for production drilling and producing, and for floating storage and transfer facilities.

Introduction

Anything that floats and is supported by displacement of water or other fluids must have some freedom of motion. The amount of motion that a floating mass will have in open sea conditions is dependent on many factors. The ideal floating mass that has a minimum of motion throughout the entire range and orientation of sea states has not been invented, and probably never will be. Certain sizes and shapes of floating structures, used in various phases of the offshore drilling and transportation industry, have minimum motion in some sea states and more-than-average motion in other sea states. No one configuration or size of ship, semi-submersible, barge, or other type, has been built that does not produce undesirable motion in certain wind and sea conditions.

When considering the entire aspect of oceanic development, the problem of moorings in reasonably shallow water (less than 600 ft) is constantly present. There is hardly any endeavor at sea, whether it be erecting a platform, drilling a well, laying a pipe line, or loading a tanker, that does not employ moorings, in much the same way that foundations are required to support structures on land. Over the years certain simple, empirical formulas have been used with reasonable success to design mooring systems that provide both holding power and flexibility to permit surface motion. In all of these mooring system designs, the flexibility is provided primarily by the weighted catenary of the mooring pendent and secondarily, by elongation of the pendent itself. Obviously, if conventional chain is used for the mooring pendent, the catenary is heavier and more easily absorbs the surface-induced motion and energy. Wire line, preferred by some companies because it is somewhat easier to handle and stow, has a lighter catenary and depends principally on the stretch of the wire to absorb surface-induced motion. The combination of chain and wire should be used with care and judgment, as the varying vibratory characteristics of the two can provide severe connecting problems where the chain and wire tie together. Our recent experience has disclosed that a properly designed weight, located between the chain and the wire, can absorb the chain and wire vibrations without interaction between the two, and also improve the catenary characteristics of the system.

The offshore industry largely is responsible for the development of multipendant mooring systems in the open sea. Frequent failures of mooring systems brought engineers and designers of such equipment to the early realization that the rules for conventional ship anchoring had to be discarded. Tensioning several mooring lines, one against the other, caused problems that had not been experienced previously to any large degree by the marine industry. Attempts were made to restrict the motion of floating tenders. lay barges and other offshore floating structures by having a taut mooring system. Even today we occasionally find misinformed people who tighten up the mooring system as the weather conditions build up. This is not the right approach. It is impractical and probably impossible to restrict appreciably the motion of a floating ship or structure in storm conditions. Lateral translation can be controlled, but cyclic motions in wave action cannot. Fig. 1 shows the typical motion of a ship in wave action. Fig. 2 shows the magnitude of forces required to hold that ship motionless in the same wave action.

Winds, Currents and Waves

Forces generated against a mooring system are from three primary sources: winds, currents and waves. The wind forces usually exert a greater lateral translation force tending to physically push the moored vessel in the direction of the wind. Water currents, even when induced by winds over a long fetch, usually have considerably less translation force than does wind. For example, a 60-knot wind exerts a lateral force of approximately 15 lb/sq ft on exposed surfaces. A 2-knot current has an equivalent force of from 4 to 6 lb/sq ft on submerged areas, depending on their shape.

JPT

P. 569ˆ