Rationale for NORSOK U-001 5th Edition Revision of Static and Cyclic Subsea Wellhead Loads

Abstract

Abstract NORSOK U-001 is a Norwegian national standard with requirements and recommendations that are supplementary to the ISO 13628 series of standards. NORSOK U-001 4th edition was issued in 2015 and carried a significant revision of design drilling riser loads applicable to subsea structures and well equipment. These maximum wellhead design loads were based on an analytical study followed by verification efforts indicating a shallow water under estimation bias of ULS and ALS design loads and overestimation bias of FLS design loads. As for cyclic design wellhead loads the NORSOK U-001 4th edition included informative design load histograms classified by BOP weight and water depth classes. As a part of development of NORSOK U-001 5th edition, a new wellhead load study has been performed to establish normative maximum and cyclic design loads for subsea wells. The objective of this paper is threefold; first to present the analysis methodology employed and explain how loads have been established. The analysis work included more than 60 000 dynamic global riser simulations of marine risers from 5 different semi-submersible drilling rigs at water depths from 80 to 1000 m. Secondly, to explain how design loads have been extracted from these results for the upcoming 5th edition. A water depth reclassification was carried out with the intent to better cater for the shallow water bias from the 4th edition. Normative cyclic loads are based on a normalized load histogram scalable by a cyclic design load value (ΔMo) determined by water depth and operational mode. Finally, to outline quantified advice on maximum and cyclic load reduction potential that can be extracted from the underlying studies.