Copyright © IF AC Control Applications in Marine Systems, Glasgow, Scotland, UK, 2001 IFAC C:UC> Pu blications www.elsevier.comllocate/ifac INERTANCE CONTROL OF UNDERWATER INSTALLATIONS Svein Ivar Sagatun *,1 Thor Inge Fossen ** Karl-Petter Lindegaard *** * Norsk Hydro Exploration and Production, Bergen, Norway, e-mail: svein.ivar.sagatun@hydro.com ** ITK, NTNU, Trondheim, e-mail: ti.f@itk.ntnu.no *** ITK, NTNU, Trondheirn, e-mail: kpl@itk.ntn·u.no Abstract: This article presents active control of offshore underwat.er installations using a modified impedance control scheme. The impedance control is augmented with acceleration feedback resulting in a concept named inertance control. The control scheme is used to tune the response of the handled object exposed to hydrodynamic loads. The proposed method is general and stability is proven. A detailed mat.hematical model of the MHS and loads and load effects on the handled object hitting the water surface and proceeding through the splash zone is included together with an illustrative simulation. Copyright ©2001 IFAC Keywords: impedance, marine systems, motion. 1. BACKGROUND There will be an increased focus on the operability of underwater equipment in the years to come. Offshore oil and gas field will be developed with all processing equipment placed on t.he seabed and in t.he production well itself. Norsk Hydro have decided to develop the Fram oil and gas field outside t.he west cost of Norway with subsea equip- ment only. Process equipment like subsea electrical multi phase pumps , subsea and down hole separa- tors, frequency converters. electrical distribution, manifolds, control and instrumentation systems are all system components which are ready for subsea production. The lower cost in using subsea equipment compared to using a floating or fixed production platform is penalized with lower avail- ability for maintenance, repair and replacement of equipment. Production stops due to component failure is costly, hence a high operability on subsea intervention is required to operate subsea fields. 1 This work is part.iall\" support .ed hy Norsk Explo- rat.ioll and Product.ion 345 High operability implies that subsea intervention must. be carried out. also during winter time, which in the North sea means underwater intervention in hash weather conditions. The objective of the marine operation is to safely install the product on the seabed. One of the critical phases during the installation is the water entry phase on which this paper focuses. It is assumed that the oper- ation is carried out by a multipurpose mono hull installation and intervention vessels equipped with a dynamic positioning system. a moonpool and a MHS (module handling system). 2. INTRODUCTION Inertance is in (Mead, 1999) defined as the com- plex harmonic force (or moment) required to pro- duce a unit real harmonic acceleration of displace- ment (or rotation) . The corresponding definition of impedance is the complex harmonic force (or moment) required to produce a unit real har- monic velocity of displacement (or rotation). A more common definition of impedance is given