Page 421 - Pipelines and Risers

388 Chapter 20

Soil Interaction
In most deepwater fields, relatively loose clay is found on the seabed. The pipe will sink into
this clay and might be buried over time. The exact behavior of the soil is not known. The soil
uplift and sideways resistances are hence important aspects. MCR in challenging applications
in the North Sea will have a touch down bending radius close to minimum permissible. Hence
it is important to properly model riser-soil interaction effects.

Extreme Storm
The primary objective of the extreme storm analysis is to define basic geometry and assess
acceptability of response. The analysis may be conducted using FLEXCOM-3D (MCS
(1994)) and REUX (SINTEF, 1998).

A large number of analyses need to be conducted when optimizing a metallic catenary riser.
The approach is highly iterative in order to ensure that the response is optimized for all
combinations of load and vessel offset.

The severity of environmental conditions and vessel motions results in highly dynamic risers.
Tension fluctuations are large and in extreme load cases low tension or even compression can
occur near the TDP. Analysis of these arrangements is sensitive to selection of analysis
parameters and modellmesh refinement. The scatter of results produced by different software
is also greatest for these conditions with stress differences of 1040% for some of the
configurations considered.
20.3.4 Installation Studies

2H Offshore Engineering (Hatton and Willis, 1998) in the UK is co-ordinating a Joint
Industry Project (JIP) called STRIDE, sponsored by a number of oil companies, installation
contractors and regulatory bodies. The objective of this program is to establish the limitations
for design and operation of deepwater steel risers covering, SCR, wave and hybrid
configurations. This program will also identify the need for analysis improvements.

Studies focused on three methods of installation:
- Reellay
- JLay
- Towout

The use of S Lay was not investigated in detail due to excessive tension requirements in
deepwater and with large diameters. However, recent developments and new build vessels
have increased the scope of the S Lay process, and further investigations may be conducted.

20.3.5 Soil-Riser Interaction
When a pipe is placed on soil and subjected to oscillatory motion, there is complex interaction
between pipe movements, penetration into the soil and soil resistance. At the touch down