The Role of Buoyancy in Deepwater Riser Design
Authors: O&apos;Brien P.J. , O&apos;Sullivan E.J. 
Published: October 2008

As offshore exploration and production of oil and gas moves into water depths up to 10,000 ft and beyond, the demand for buoyancy on deep water production and drilling risers is increasing at near exponential rates. This demand is generally driven by the need to reduce riser top tensile stresses and floating vessel connection loads. However, buoyancy also significantly increases the riser drag-to-weight ratio, and this in turn greatly affects riser dynamic response. In many cases, the benefits to be gained by using buoyancy to reduce static top tensions can largely be wiped out by detrimental dynamic response effects. It is clear that the exact role of buoyancy in deep water riser configuration design needs to be established and this paper explores some important issues on this matter.

Introduction

This paper investigates some of the characteristic dynamic responses of compliant risers to seastate loadings and applied floating vessel motions with particular focus on the role of buoyancy in influencing that response. The paper concentrates on steel catenary risers as a comparison with deepwater flexible riser response, and identifies how buoyancy might optimally be used in developing a cost effective solution. The paper also gives some insight into the selection of buoyancy for top tensioned vertical risers.

A brief review of existing or proposed flexible and steel catenary riser design projects shows that for deep water the demand for buoyancy is increasing at a rate which is almost exponential. This can be seen clearly in Figure 1, which shows typical net buoyancy figures for a single riser in water depths from 100 metres to 2,000 metres. For one 2,000 metre development studied, the amount of distributed net buoyancy required is close to 200 tonnes. One should consider this in the context that in general the number of risers required increases with water depth: for example, it is reported in1,6 that recent and new deepwater developments have in general between 40 - 100 flexible risers connected to the floating production vessel. This greatly increases the amount of distributed buoyancy required for a single field development. Indeed, a recent deepwater study by the authors computed that the amount of buoyancy required could potentially exceed the annual capacity to two of the major buoyancy manufacturers in the UK. Hence, it is important to ask the question whether all this buoyancy is actually needed, and whether we are using it efficiently in performing riser configuration design. This paper addresses some of these issues and also investigates how the amount of buoyancy on a riser affects the stationkeeping requirements for the floating vessel.

To develop this theme, the following sections investigate the typical response of wave steel catenary risers and the role of distributed buoyancy in their response.

Wave Steel Catenary Risers

The wave steel catenary riser lifts horizontally off the seabed and ascends in the shape of a single wave to the floating production vessel. The size, length and location of the distributed buoyancy along the riser determines the shape of the wave section. Riser configuration shapes for a steel wave riser in 2,000 metre water depths at near and far vessel locations are shown in Figure 2. Note that for this design, the arch height is about 27% of the water depth, and the mean horizontal distance to touchdown is approximately 67% of the water depth. However, it would be expected that as water depth decreases down to about the 500 metre level, typical of present offshore UK Atlantic Margin developments, the mean horizontal distance to seabed touchdown can increase to as much as 250% of the water depth. This is to ensure