Hydrodynamic Behavior During Offshore Loading and Discharge
Authors: Peters Onno , Huijsmans Rene HM , Seij Michel 
Published: April 2012

Heavy Marine Transport is a well accepted method to move large heavy offshorefloating structures around the world. Such transports are normally limited tosailing from and to sheltered locations, where loading and discharge operationof these objects are safely executed in very mild environment. As cargo andvessel sizes increase, a reduced number of inshore facilities can receive theselarger cargoes and vessels; thus demand for loading and discharge at offshorelocations is increasing. Also more production fields in remote locations drivethe need for offshore discharge operations.

As these offshore discharge operations are sensitive to environmental criteria, this has consequences for workability. Currently safe conditions are based onlong term operational experience. No proven numerical methodologies areavailable in the market today to assess the hydrodynamic behavior of submergedHTVs in rough seaway, in short Offshore Loading / Discharge; such methodologieswould improve insight in workability and understanding of safety margins.

Research and development projects in the past have dealt with many aspects ofOffshore Loading / Discharge. Both the basic design (of both HTV and systems)and hydrodynamic behavior have been investigated. Problems were encountered, especially in the area of predicting the relative behavior of floatingstructures above a HTV. Investigations have shown that inaccuracy is mainlycaused due to the narrow gap between cargo and HTV.

Development of an accurate and cost-effective method to predict hydrodynamicbehavior of a cargo floating above a HTV is in progress. This paper presentstheory and limitations of an adapted multi-body diffraction theory, based onmulti-domains with different solution methods. Difference in results withrespect to standard diffraction method is shown.

Because of analogy with the single-body floating object in shallow water withnarrow gap between body and seafloor, fundamental model tests were performed. Results of the measurements are compared with calculation results using theadapted method.

Recently, an extensive model test campaign with a HTV and a largesemi-submersible to simulate an offshore discharge operation in North WestAustralian conditions was executed. Results and comparison with calculationsare shown as well as operational findings from this test campaign.

Introduction

History

Since mid 1950's, Heavy Marine Transport (HMT) was recognized by ships withcranes being able to lift-on and lift on heavy cargo [ref. 1]. As the weightand size of cargo gradually increased, innovative solution for loading anddischarge were found, like roll-on/-off and skid-on/-off. Also, cranecapacities increased, and - to accommodate heavier cargo - stability pontoonswere developed to increase the vessel stability. Simultaneous with the evolvingheavy lift ships, the heavy transport vessels were developed.

In 1972, the first self propelled semi-submersible dock-type heavy transportvessel (HTV) name Docklift 1 was built [ref. 2]; this vessel was capable offloat-on and float-of cargo into its hold via the stern. Initially, floatingcargo like jack-up and semi-submersible rigs were wet towed all over the world, but in 1973 the first rig was dry transported on a towed barge. A couple ofyears later, the first custom-built semi-submersible barges wereintroduced.

Then, in 1979, the first self-propelled HTV with an open deck - capable oftransporting cargo with large width than the HTV itself - came in to service, being the Super Servant 1. An alternative design was introduced in 1981, beingthe so-called closed stern type; vessel with a lower cargo deck sectionamidships. With the Transshelf coming into service in 1986, the development andgrowth of heave transport vessels came to a stop for 13 years.