7.1 Introduction

Riser is defined as the vertical or near-vertical segment of pipe connecting the facilities above water to the subsea pipeline. The riser portion extends (as a minimum) from the first above-water valve or isolation flange to a point five pipe diameters beyond the bottom elbow, based on codes. The design engineer must select the exact limits on a case-by-case basis. This may often extend the riser beyond the five diameters limit or above the isolation flange. Many operators prefer a length of 200 feet from the elbow to protect against dropped objects (i.e., heavier wall pipe). The riser design usually considers adjoining pipework segments, clamps, supports, guides, and expansion absorbing devices. These are illustrated schematically in Figure 7.1.

Figure 7.1: Typical Riser Schematic.

This chapter addresses the engineering analysis and design of conventional steel risers and riser clamps. It is written on the basis of related codes and rules in riser design. This chapter provides guideline for pre-installed and post-installed conventional steel risers, but does not address J-tube or flexible pipe risers. Risers are assumed to be of API 5L line pipe and are operated at a temperature less than 250 F. For other cases, refer to specific code allowables.

7.2 Design Procedure

For a conventional steel riser, the design procedure includes the following steps:

• Step 1: Establish the design basis.

  • Maximum wave height and period for return periods of 1 and 100 years

  • Annual significant wave height occurrence in 5-foot height intervals

  • Associated wave...