ABSTRACT

The paper describes the types of marine growth and the mariner in which they colonise North Sea structures The way In which marine growth is currently quantified is discussed and the shortcomings highlighted

The results of experiments undertaken both in the laboratory and offshore on both real and artificial marine growth are reviewed The Variation in hydrodynamic force coefficients for the various types and patterns of marine growth are discussed and areas where there is still considerable uncertainty are identified

Finally the practical problems of predicting a prior the effects of marine growth on ultimate strength and fatigue loading at the design stage using a global design thickness allowance are discussed and appropriate procedures for thickness allowances are suggested. Some results from a case study on a vertical conductor close to the seabed in the southern North Sea are also presented

INTRODUCTION

All structures in the North Sea either have, or will acquire, a covering of marine growth unless protected by a durable anti-fouling scheme Even the few jackets in the Norwegian sector of the North Sea that have been coated with anti-fouling are covered by light marine growth within two to three years in service and by heavier growth later in their operational life as the effectiveness of the anti-fouling reduces with tune

In most cases a significant covering is acquired within a few months The nature of the covering will depend on how and at what time of year, the structure was installed as well as its operational location At least twenty varieties of marine growth have been identified on North Sea structures and the characteristics of the most prolific species are described in Section 2 Also described in this section is the way in which marine growth is usually quantified in subsea survey reports

All types of marine growth give rise to a substantial increase in fluid loading In fact on a structure comprising relatively small tubular members (less than 1 metre in diameter) the evidence suggests that the in-line fluid loading may more than double This substantial increase arises from two sources Firstly, there is the increase in projected area of the structure, which may be as much as 30%in the wave affected zone for a small structure and the larger and commensurate increase in immersed volume (Wright 1985) Secondly, there is the substantial increase in the drag coefficient associated with all types of marine growth True the meta force is somewhat lower for hard roughness than for the equivalent smooth cylinder but for soft fouling the reverse is me In any case for most North Sea jacket structures the ultimate loading is in the drag dominated regime and the dominant fatigue loads are m the drag inertia dominated flow regime where strong vortex shedding occurs

Current offshore engineering practice tends to ignore lift forces when assessing the fluid loading on normally rigid tubular members However the indications are that these forces are both larger and have a higher degree of spanwise coherence in the presence of marine roughness

This content is only available via PDF.
You can access this article if you purchase or spend a download.